SOMETIMES WE become so
involved with the various types of
model engines, performance
ratings, and displacements that we
overlook something important. Just
as a fully race-prepared car engine
is useless until its power is
transmitted to the ground, a model
engine is useless until its power is transmitted to
the air. Most often, model aircraft use propellers
for this job. Our engines also need to burn fuel to
produce power, and they need to ignite that fuel.
These functions, which are directly connected to
the engine, are this month’s subject.
Propellers: A model’s engine is only as good as
its propeller. The propeller’s size, shape, and
composition determine how much of the engine’s
power is transmitted to the air and the manner in
which the aircraft can best use that power. The
best combination of propeller characteristics for a
particular model is a compromise.
The pilot must choose a propeller that
produces the best performance based on the
aircraft’s mission (training, racing, aerobatics,
combat, etc.), the engine’s power range, and the
flying-field conditions.
A racing airplane would do best if its
propeller were designed solely to produce high
airspeeds while rotating at the same rpm at which
the engine produces maximum horsepower. This
is the right choice even if durability, climb, and
acceleration rates are sacrificed.
Choosing the right propeller requires
understanding and a few “prop tips,” one of
which is that a propeller’s blade rigidity is
important. A propeller is nothing more than a
rotating wing. All propellers have airfoil shapes
and direct their lift in a horizontal path, called
thrust, instead of a vertical direction, as does the
aircraft’s main wing. Thrust pulls the aircraft
forward.
Imagine how much of your aircraft’s wing lift
would be lost if the outer third of the wing were
to flex enough that its incidence—its angle of
attack (AOA) to the oncoming airstream—
significantly decreased during every turn or
climb. In the same way, a propeller in which the
34 MODEL AVIATION
The Rest of the Engine by Frank Granelli
The 2.5-inch spinner reduces propeller drag while streamlining the model’s
front end. Removing the spinner reduces the engine’s top rpm by 450—a 4%
power loss.
09sig2.QXD 6/24/04 9:03 am Page 34
tips flex does “flatten out,” reducing its
incidence during acceleration and climb,
thereby losing thrust when it is most needed.
Unlike a wing, which develops lift along
almost its entire span, a rotating propeller
produces the majority of its thrust centered
around the 75% point of each blade’s length.
This makes the thrust lost caused by tip
flexing even more critical.
Stand slightly behind and to the side of the
spinning propeller and watch the tips. If they
follow a wavy path, that signals excessive
pitch loss (lower propeller AOA), which
results in power lost transferring the engine’s
energy to the air.
The first 20% of a propeller blade’s
length—its span—produces much drag but
little thrust. This section is the area where the
propeller’s round center—the hub—tapers
into the working “wing” of the blade, which
does all the work. There is little “wing area”
here.
This area also moves the slowest through
the air since it is closest to the center of the
“disc” formed by the rotating propeller.
However, this inner section does rotate and
therefore produces air drag. This is why
spinners make propellers more efficient.
The next 50% of the blade’s span is the
area where the LE-to-TE width—the chord—
increases to maximum and the airfoil becomes
fully developed. Some thrust is lost until the
blade is fully formed, and more is lost because
the center-section rotates more slowly than
the remaining outer blade area. Since a wing’s
total lift depends, in part, on its airspeed, the
lift produced by different blade sections
depends a great deal on their rotational
speeds.
How different are these rotational speeds?
The blade section 1 inch out from the hub of
an 11-inch-diameter model propeller rotating
at 11,000 rpm has an “airspeed” of just 96 feet
per second (fps), or 60 mph. The middle of
the blade is rotating through the air at 260 fps,
or 180 mph, and the 75% point is moving at
396 fps, or 264 mph.
Even though the blade’s area near the tip
(90%) is much less than that near the middle,
it is moving nearly twice as fast, at 475 fps, or
317 mph, and is therefore producing more
thrust than the center-section is.
Please study that last rotational speed. The
tip itself is moving at 530 fps, which is
approximately the same speed as some .45-
caliber bullets. If you want to know what
happens if you are careless enough to put a
hand into a spinning model propeller’s arc,
envision pointing a Colt .45 at your hand and
pulling the trigger! Not an attractive image.
Please be careful.
Tune your engine
while standing behind
the propeller, never
stand directly to the
side of a spinning
propeller, and keep
children away from
your engine at all
times.
Since rigidity is important
to propeller performance, a
major factor to consider when
choosing a propeller is its
construction. Today they are
usually made from one of four
basic materials: fiberglassfilled
nylon composite,
fiberglass-reinforced nylon,
wood, or carbon fiber (CF).
Pure nylon propellers were
once manufactured, but for
the most part they have been
replaced by nylon composite
construction. The fiberglassfilled
nylon propellers are
safer and stiffer than the old
nylon-only variety, but they
remain the most flexible kind.
Most fiberglass-filled
nylon propellers have large blade areas to
improve their performance. They produce
excellent thrust for a given rpm but tend to
rotate more slowly than same-size propellers
of different construction. These propellers
suffer the most thrust loss as the airplane
climbs steeply since the outer blade areas flex
the most under stress.
However, this flexibility is a major
advantage for newer model pilots. The blades
bend well on those poor landings—those that
bend the nose wheel back nearly far enough to
touch the fuselage bottom. Fiberglass-filled
nylon propellers bend backward and usually
do not break in those situations. They also last
the longest when flying from paved runways.
This durability saves money and keeps
newer pilots flying on those days when they
would have exhausted their supply of more
rigid propellers. Most RTF trainers are
equipped with the fiberglass-filled nylon
variety for exactly these reasons.
Fiberglass-reinforced propellers are stiffer
and sometimes feature undercambered
(concave-bottom) airfoils. They have tips with
a small area but quickly widen to large chords
just short of the tip. The tiny tip area helps the
engine stay quiet and increases the propeller’s
efficiency. One of the most efficient wings
ever designed employs elliptical wingtips that
reduce drag by reducing wingtip vortices; just
ask any Spitfire pilot.
Fiberglass-reinforced propellers have
wingtip designs that most closely resemble
the elliptical wing shape. The reduced tip drag
allows the propeller to accelerate quickly and
to reach a higher top speed. That combined
with the more rigid blade make fiberglass
propellers famous for excellent climb
performance. The middle areas of many
reinforced blades are usually the largest in
their respective size classes. This helps
increase overall thrust, again adding to the
aircraft’s climbing ability.
However, these stiffer fiberglassreinforced
blades still flex a bit under load and
are easy to break during hard landings. Paved
runways are rough on them since the tip area
is small and may be destroyed with one
contact, even if the propeller is not rotating.
Wood propellers are more rigid than
fiberglass-filled and fiberglass-reinforced
nylon types. Some wood propellers have
special tip designs to produce increased thrust
and rpm. Most have roughly the same blade
area as fiberglass-filled propellers that are the
same size.However, wood propellers must be
carved—not molded—and therefore do not
usually feature the more exotic blade designs
that are so common in some molded model
propellers. Depending on their design, wood
propellers produce excellent top speeds and
quick acceleration because they are light and
stiff.
Wood propellers break easily with any
ground contact, and prolonged use on
grass runways results in excessive blade
September 2004 35
Right: Fiberglass-filled nylon propellers have flexible blades
that resist damage but lower propeller efficiency.
L-R: Idle-bar glow plug ensures reliable idle
even when engine is mounted inverted.
Standard glow plug works well in most other
RC applications.
Four major propeller types are (top to bottom) solid
carbon fiber, wood (one shown is balanced; note
partially removed writing on right blade), fiberglassreinforced
nylon, and fiberglass-filled nylon.
Photos by the author
09sig2.QXD 6/24/04 9:03 am Page 35
36 MODEL AVIATION
wear. They also require the most balancing
effort because density and water content
may vary in a single propeller.
You know that all propellers must be
balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major
problems. First, the engine’s bearings wear
quickly. Second, the onboard radio
components, especially servos, suffer
excessive wear and can fail early. Whenever a
servo quits in flight, much of the fun of flying
RC models is diminished.
Third, an unbalanced propeller will cause
a 3%-4% rpm loss. An engine that would
have turned a balanced propeller at 11,000
rpm turns an unbalanced propeller at only
10,600 rpm. Climb rate and aerobatic
performance are reduced.
Many different propeller balancers are
available. I will cover these in the last edition
of this segment of the series.
CF propellers are the ultimate in rigidity;
they have almost no detectable flex. They can
assume any airfoil shape and blade area as
they are molded. Some are solid and others
are hollow. CF propellers are light, allowing
for the fastest engine acceleration possible,
and hollow ones accelerate even more
quickly. The solid and hollow kind feature
excellent performance across the entire
aerobatic spectrum. You can even purchase
them prebalanced.
However, despite their superior
performance, few modelers use CF propellers.
There are two good reasons for this, the first
of which is cost. CF propellers vary from
nearly $30 to $120 for the larger sizes. A
modeler can buy an abundance of wood,
fiberglass-filled, or fiberglass-reinforced
propellers for $30.
Second, trainers and many lowerperformance
sport models are unable to take
full advantage of the performance increase
that such a propeller provides. From level
flight, a 40-size trainer may be able to
perform a 100-foot vertical climb. If a CF
propeller provides a 15% climb increase, that
trainer will perform a 115-foot vertical climb.
It’s not that noticeable of a difference for the
money.
But install that propeller on a 40-size
Pattern airplane, and its normal 250-foot
vertical climb stretches to nearly 300 feet with
enough remaining airspeed to provide
excellent control.
After construction, the next important
factor in picking the right propeller is size.
Two numbers label their dimensions, and the
first is diameter in inches. The second is pitch,
which represents the distance in inches the
propeller would travel forward in one
revolution if there were no friction, drag, or
other limiting factors. This is the propeller’s
AOA, or incidence.
The numbers are separated by the usual
“by” designation: “x.” An 11-inch-diameter
propeller with a 6-inch pitch is called an “11 x
6.”
Understanding both numbers’
performance implications is critical. They
interact in a complicated dance of airflow,
engine performance, thrust, and geometry.
Fortunately the dance becomes easy to
understand once you know the few simple
steps. Well, step one, which follows, is not all
that simple but is easy to understand with a
little geometry.
The propeller’s efficiency for a given task
is determined by the amount of air it moves
per revolution and its speed. On a sport
airplane, if a propeller can move a huge
amount of air, but only at a slower speed, that
is better than moving small amounts of air at
high speeds.
The diameter of the disc that the rotating
propeller produces has more effect on the
power transmitted than a speed increase does
because the disc area increases by the square
of the radius. Therefore, an increase in
diameter (radius) moves additional amounts
of air by the square of the radius. This is a
large force multiplier.
Without tripping over the math, airstream
speed increase has an even less than linear
effect increasing the amount of power applied
to the air. This is a small force multiplier. The
idea is that as long as you have enough pitch
to fly at the speed you need, diameter is king,
offering faster acceleration, better climb, and
shorter takeoff runs.
The larger the propeller disc, the more
engine power can be applied to the air. An
additional factor in model aircraft is that the
center of the propeller disc area is located
only inches ahead of the fuselage and/or
cowling, which produces airstream
interference and drag that lowers propeller
efficiency. The larger the disc area outside the
cowling, the more efficient the propeller is.
Step two: The pitch determines airspeed
only in combination with the airframe. A 20-
inch-pitch propeller sounds fast. But if the
airframe has a high level of aerodynamic
drag, fixed landing gear, and straight wings,
this drag prevents the airplane from ever
reaching the propeller’s theoretical top speed.
The result is that the propeller cannot reach its
maximum rpm because the extra airframe
drag increases the propeller’s air load.
For step three, the best propeller size for a
given engine, in a 40- to 60-size, high-drag
sport RC model, is that one compromise
between the largest diameter and highest pitch
that still allows the engine’s maximum ground
rpm to be roughly 1,000-1,500 rpm higher
than its high-torque (maximum twisting
power) rpm. (This figure is after the highspeed
mixture has been adjusted to be 500
rpm less than absolute peak.)
Why this rpm? Once the airplane is flying,
there is an average increase of 500 rpm. This
happens because the aircraft’s forward speed
acts to decrease the propeller’s AOA. Another
way to picture this is that the air “flowing”
into the propeller from the front is helping the
engine turn it. It isn’t, of course, but it is
decreasing the engine’s workload by reducing
the effective AOA.
An airplane stops flying faster when the
propeller’s AOA nears zero. But once the
aircraft’s nose is pointed skyward, more of its
weight is placed on the propeller and therefore
on the engine’s turning ability. The airspeed
decays, and those 500 “free” rpm disappear as
propeller drag increases with the escalating
AOA.
Increasing the fuel’s nitro content from 15% to 20% (using the same fuel with
additional nitro) raised the engine’s output by only 300 rpm.
Replacing idle-bar glow plug with same manufacturer’s standard plug
increased engine’s top rpm by 300—same gain as 5% nitro increase. But
standard plug does not increase operating costs or cause additional
engine wear.
09sig2.QXD 6/24/04 9:04 am Page 36
The stress of pulling the aircraft upward
increases the power demands on the engine. It
responds by turning more slowly, just as a
car’s engine does when going up a steep hill
until extra energy, in the form of stepping on
the gas, is applied. But the model engine is
already at full power; there is no extra “gas”
to give.
In fact, the engine’s rpm will drop until it
reaches its high-torque rpm. If the engine is
the right size for the airplane and the climb
angle is not steeper than what the
engine/airframe combination was designed to
maintain (usually at least 45°), the rpm
reduction stops here and the airplane
maintains a constant climb rate.
Why not use a propeller that allows the
engine to rotate at its peak horsepower rpm?
The horsepower ratings for most .40-.60 twostroke
engines are usually at so high an rpm
that they are nearly unusable for sport
applications. Most reach peak horsepower
well in excess of 13,000 rpm.
At this number, model pilots do not need
to worry about their airplanes’ performance
because most clubs won’t let them fly such
loud models. Even if they can fly them, the
propeller disc must be so small—7-9 inches—
that little thrust can be applied to the air (step
one). The result is an inefficient propeller, an
airplane flying roughly 35 mph, and a
screaming engine trying to tear itself apart.
Experienced RC modelers have known
this “great truth” for years, many times
without even knowing they know it, but they
have had no data to support their intuitive
propeller choices. So I set out to prove this
last step.
I used a relatively new tool to gather the
needed data. The RC Flight Data Recorder
manufactured by Eagle Tree Systems records
the airspeed, rate of climb, climb angle,
altitude, and servo performance during flight.
It can also record in-flight engine rpm and
temperatures, but these systems were not
installed on the test aircraft.
The recorder correlates flight data and
transmitter inputs and time. This lets the pilot
know what was happening and when. I have
been using this instrument for sometime when
evaluating aircraft for MA’s Sport Aviator
online magazine (www.masportaviator.com)
and have become familiar with interpreting
the reported data. I used my trusty, manyyear-
old SuperStar 40 trainer equipped with
an even older .45 engine.
This engine reaches its maximum
horsepower, 1.35 when new but now much
less, at roughly 15,000 rpm. Its torque curve
peaks near 10,000 rpm. I tested identicaldesign,
fiberglass-reinforced propellers in 10
x 6, 10 x 7, 11 x 6, and 11 x 7 sizes. The
Flight Test Results chart summarizes the tests.
38 MODEL AVIATION
Propeller Size Ground rpm Top Speed Takeoff Climbout 45 Degree Climb Approach Speed
Performance* Performance*
10 x 6 11,940 55 mph 1,100@28 1,200@6 33 mph
10 x 7 11,220 64 mph 1,400@32 1,500@31 35 mph
11 x 6 10,920 60 mph 1,500@25 1,800@28 31 mph
11 x 7 10,140 51 mph 1,200@27 1,200@26 24 mph
*Feet per minute climb at mph climb speed
Flight Test Results
Recognizing those who have defended,
protected, and supported the sport of
aeromodeling through their personal
contributions and devotion. Their
advocacy has long been the foundation
upon which the growth and advances in
the sport have been based, and for this
they are presented as true champions!
Ted Teach
A charter member of the Legion of Champions
and a continuing supporter
Legion of Champions
09sig2.QXD 6/24/04 12:13 pm Page 38
As shown, the highest ground rpm does
not translate into the fastest airspeed or the
best climb rate. Under the demands of a
climb, the smaller 10-inch propeller discs
could not transfer the engine’s power to the
air as effectively as the 11-inch discs could.
An inch may not seem to be a big
difference. However, the 11-inch propeller
has an effective area of 95 square inches
versus the 10-inch propeller’s 79 square
inches. The engine’s “force area” is 20%
larger using the 11-inch propeller.
The larger disc is the reason why the 11 x
6 propeller produced a 20% better climb rate
than the 10 x 7, despite the slower climb
speed. The extra power required to turn the 11
x 7 propeller when climbing proved more
than the ol’ engine possessed. Climb and top
speed suffered, but landing speed was the
slowest, probably because the idle speed was
less than 2,100 rpm. The 11 x 7 might cause
engine overheating in hot weather.
The 11 x 6 allowed the aircraft to leave the
ground in the shortest time at the slowest
airspeed, reducing airframe wear. It produced
a climb rate up to 50% higher, and its top
speed was only 6% less than the highest but
up to 18% higher than the remaining
propellers’. During your next visit to the
flying field, check the propellers on most .45
two-stroke engines. Most will be various
types of 11 x 6s.
Choose the propeller-and-glow-plug
combination that permits the engine to turn
the largest-diameter propeller approximately
1,000-1,500 rpm higher than its peak torque
speed on the ground. Start with the largest
diameter and lowest pitch recommended for
your engine. If the engine will not turn this
propeller fast enough, drop to the next smaller
diameter, again with the lowest pitch.
Increase the pitch if the engine turns too fast.
Continue until you find the right size
combination.
The .30-size engines will usually use a 9-
inch-diameter propeller, .40s will use up to
10.5 inches, .45s will use 11 inches, and .60s
work best with a 12-inch propeller.
Remember that this is for sport models only.
Glow Plugs: Did I also mention the glow
plug? It causes the fuel to burn and release its
energy. Fortunately there are only two types
of glow plugs that newer sport-model pilots
need to know about.
The idle-bar plug was once the only
design that provided a reliable idle. The metal
bar protected the glow element from unburnt
liquid fuel that otherwise cooled the element
to lower than the fuel’s ignition temperature
when at idle speeds. The bar itself became
extremely hot, adding protection to the glow
element’s idle temperature.
But today’s more powerful sport engines
are equipped with mufflers that preserve the
chamber’s heat at idle. Modern carburetors
allow finer adjustment of the fuel/air mixture,
reducing the amount of liquid, unburnt fuel
that enters the chamber at idle speeds.
Therefore, idle-bar glow plugs are not always
required on newer sport engines.
Photos show a non-idle-bar glow plug’s
performance advantage. All settings and
equipment remain the same, and the photos
were taken only minutes apart. The non-idlebar
plug produced a 300 rpm gain.
However, an idle-bar glow plug is a good
idea for two-strokes mounted with the
cylinder head pointed downward—called
inverted mounting—for engines that have
difficulty idling and for older, well-worn
engines that may need the extra heat to keep
the glow element hot. Consider using nonidle-
bar glow plugs for all other sport
applications.
Fuel: Choosing the right fuel is the last
critical factor to ensure that your new twostroke
engine gets the best performance and
longest life. The “right” fuel is also one of the
most controversial, opinion-rich, and
individualistic subjects in model aviation. But
there are some useful guidelines to remember.
Two-stroke fuel has three major
ingredients: methanol, castor oil, and
nitromenthane (nitro). Methanol comprises
60%-75% of most fuels. It burns completely
and adds to the fuel’s total energy output.
The lubricating oil comprises roughly
20% of the fuel. Most two-stroke fuels
contain two types of oil; 4%-8% is usually
castor and the remaining 12%-16% is a
synthetic that varies by manufacturer. Oils
typically do not burn completely, and what
small percentage of the oil that does burn
does so at lower energy levels than methanol.
Castor oil is used because it maintains a
lubricating film at higher temperatures than
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40 MODEL AVIATION
09sig2.QXD 6/24/04 9:04 am Page 40
most synthetic oils do. If the engine’s highspeed
fuel/air ratio is too lean—too much
air—the engine will run at high temperatures.
Castor oil will maintain lubrication in an
overheated engine; most synthetic oils burn
away.
Castor oil also helps remove heat from
the combustion chamber better than most
synthetics. Castor oil leaves a film residue in
the engine that offers some rust protection.
Most synthetic oils do not.
However, too much castor oil causes
excessive residue buildup that can diminish
an engine’s performance. Because castor oil
does not burn, it reduces the fuel’s total
energy output. For these reasons, the castor
oil percentage is usually kept at less than 8%.
Synthetic oils do burn, but not well or
completely. Little synthetic oil residue is left
inside an engine. Synthetics also offer
excellent engine lubrication when operating
at normal engine temperatures.
Because high oil content detracts from the
fuel’s total energy output, the easiest way to
increase an engine’s apparent power output
is to reduce the fuel’s oil content. However,
oil contents much less than 18% can cause
long-term wear problems in .40-.60 sport
engines.
Fuel manufacturers are studying new oils
that produce more power and offer better
protection with quantities as low as 16%. But
for now, consider using fuels with 18%-20%
oil content in newer or sophisticated
(expensive?) .40-.60 engines.
The third fuel component is nitromethane.
It burns at a higher energy level than
methanol. However, it also produces higher
combustion-chamber temperatures and
therefore needs to be limited. Most sport
fuels contain 5%-25% nitro. It prolongs the
combustion event. The burning process takes
longer, and that also produces more energy.
Many pilots overrate the power increase
obtained by “upping the nitro” in their sport
engine’s fuel. Photos show how little effect
higher nitro content has on sport engines.
Raising its content by 5% in a fuel produced
only a 300 rpm gain. But nitro does improve
an engine’s idling ability, permitting a lower
reliable idle speed. Therefore, consider nitro
contents in the 10%-15% range as a good
sport-flying compromise.
There is such a thing as too much nitro
content for a given engine. If the combustion
event becomes too prolonged, detonation
may occur. You may hear pinging, a sound
like frying eggs, or the exhaust note may
become very loose. Unless operating at high
altitudes—exceeding 5,000 feet—there is no
reason, and little gain, to use fuels with nitro
contents higher than 20%.
Next month I’ll cover some of the
differences in and advantages of four-stroke
engines. Since model engines do not run for
long periods without an onboard fuel supply,
I’ll cover fuel-tank choices and installation
requirements as well. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
42 MODEL AVIATION
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SINGLE
ISOLATOR
09sig2.QXD 6/24/04 9:04 am Page 42
Edition: Model Aviation - 2004/09
Page Numbers: 34,35,36,38,40,42
Edition: Model Aviation - 2004/09
Page Numbers: 34,35,36,38,40,42
SOMETIMES WE become so
involved with the various types of
model engines, performance
ratings, and displacements that we
overlook something important. Just
as a fully race-prepared car engine
is useless until its power is
transmitted to the ground, a model
engine is useless until its power is transmitted to
the air. Most often, model aircraft use propellers
for this job. Our engines also need to burn fuel to
produce power, and they need to ignite that fuel.
These functions, which are directly connected to
the engine, are this month’s subject.
Propellers: A model’s engine is only as good as
its propeller. The propeller’s size, shape, and
composition determine how much of the engine’s
power is transmitted to the air and the manner in
which the aircraft can best use that power. The
best combination of propeller characteristics for a
particular model is a compromise.
The pilot must choose a propeller that
produces the best performance based on the
aircraft’s mission (training, racing, aerobatics,
combat, etc.), the engine’s power range, and the
flying-field conditions.
A racing airplane would do best if its
propeller were designed solely to produce high
airspeeds while rotating at the same rpm at which
the engine produces maximum horsepower. This
is the right choice even if durability, climb, and
acceleration rates are sacrificed.
Choosing the right propeller requires
understanding and a few “prop tips,” one of
which is that a propeller’s blade rigidity is
important. A propeller is nothing more than a
rotating wing. All propellers have airfoil shapes
and direct their lift in a horizontal path, called
thrust, instead of a vertical direction, as does the
aircraft’s main wing. Thrust pulls the aircraft
forward.
Imagine how much of your aircraft’s wing lift
would be lost if the outer third of the wing were
to flex enough that its incidence—its angle of
attack (AOA) to the oncoming airstream—
significantly decreased during every turn or
climb. In the same way, a propeller in which the
34 MODEL AVIATION
The Rest of the Engine by Frank Granelli
The 2.5-inch spinner reduces propeller drag while streamlining the model’s
front end. Removing the spinner reduces the engine’s top rpm by 450—a 4%
power loss.
09sig2.QXD 6/24/04 9:03 am Page 34
tips flex does “flatten out,” reducing its
incidence during acceleration and climb,
thereby losing thrust when it is most needed.
Unlike a wing, which develops lift along
almost its entire span, a rotating propeller
produces the majority of its thrust centered
around the 75% point of each blade’s length.
This makes the thrust lost caused by tip
flexing even more critical.
Stand slightly behind and to the side of the
spinning propeller and watch the tips. If they
follow a wavy path, that signals excessive
pitch loss (lower propeller AOA), which
results in power lost transferring the engine’s
energy to the air.
The first 20% of a propeller blade’s
length—its span—produces much drag but
little thrust. This section is the area where the
propeller’s round center—the hub—tapers
into the working “wing” of the blade, which
does all the work. There is little “wing area”
here.
This area also moves the slowest through
the air since it is closest to the center of the
“disc” formed by the rotating propeller.
However, this inner section does rotate and
therefore produces air drag. This is why
spinners make propellers more efficient.
The next 50% of the blade’s span is the
area where the LE-to-TE width—the chord—
increases to maximum and the airfoil becomes
fully developed. Some thrust is lost until the
blade is fully formed, and more is lost because
the center-section rotates more slowly than
the remaining outer blade area. Since a wing’s
total lift depends, in part, on its airspeed, the
lift produced by different blade sections
depends a great deal on their rotational
speeds.
How different are these rotational speeds?
The blade section 1 inch out from the hub of
an 11-inch-diameter model propeller rotating
at 11,000 rpm has an “airspeed” of just 96 feet
per second (fps), or 60 mph. The middle of
the blade is rotating through the air at 260 fps,
or 180 mph, and the 75% point is moving at
396 fps, or 264 mph.
Even though the blade’s area near the tip
(90%) is much less than that near the middle,
it is moving nearly twice as fast, at 475 fps, or
317 mph, and is therefore producing more
thrust than the center-section is.
Please study that last rotational speed. The
tip itself is moving at 530 fps, which is
approximately the same speed as some .45-
caliber bullets. If you want to know what
happens if you are careless enough to put a
hand into a spinning model propeller’s arc,
envision pointing a Colt .45 at your hand and
pulling the trigger! Not an attractive image.
Please be careful.
Tune your engine
while standing behind
the propeller, never
stand directly to the
side of a spinning
propeller, and keep
children away from
your engine at all
times.
Since rigidity is important
to propeller performance, a
major factor to consider when
choosing a propeller is its
construction. Today they are
usually made from one of four
basic materials: fiberglassfilled
nylon composite,
fiberglass-reinforced nylon,
wood, or carbon fiber (CF).
Pure nylon propellers were
once manufactured, but for
the most part they have been
replaced by nylon composite
construction. The fiberglassfilled
nylon propellers are
safer and stiffer than the old
nylon-only variety, but they
remain the most flexible kind.
Most fiberglass-filled
nylon propellers have large blade areas to
improve their performance. They produce
excellent thrust for a given rpm but tend to
rotate more slowly than same-size propellers
of different construction. These propellers
suffer the most thrust loss as the airplane
climbs steeply since the outer blade areas flex
the most under stress.
However, this flexibility is a major
advantage for newer model pilots. The blades
bend well on those poor landings—those that
bend the nose wheel back nearly far enough to
touch the fuselage bottom. Fiberglass-filled
nylon propellers bend backward and usually
do not break in those situations. They also last
the longest when flying from paved runways.
This durability saves money and keeps
newer pilots flying on those days when they
would have exhausted their supply of more
rigid propellers. Most RTF trainers are
equipped with the fiberglass-filled nylon
variety for exactly these reasons.
Fiberglass-reinforced propellers are stiffer
and sometimes feature undercambered
(concave-bottom) airfoils. They have tips with
a small area but quickly widen to large chords
just short of the tip. The tiny tip area helps the
engine stay quiet and increases the propeller’s
efficiency. One of the most efficient wings
ever designed employs elliptical wingtips that
reduce drag by reducing wingtip vortices; just
ask any Spitfire pilot.
Fiberglass-reinforced propellers have
wingtip designs that most closely resemble
the elliptical wing shape. The reduced tip drag
allows the propeller to accelerate quickly and
to reach a higher top speed. That combined
with the more rigid blade make fiberglass
propellers famous for excellent climb
performance. The middle areas of many
reinforced blades are usually the largest in
their respective size classes. This helps
increase overall thrust, again adding to the
aircraft’s climbing ability.
However, these stiffer fiberglassreinforced
blades still flex a bit under load and
are easy to break during hard landings. Paved
runways are rough on them since the tip area
is small and may be destroyed with one
contact, even if the propeller is not rotating.
Wood propellers are more rigid than
fiberglass-filled and fiberglass-reinforced
nylon types. Some wood propellers have
special tip designs to produce increased thrust
and rpm. Most have roughly the same blade
area as fiberglass-filled propellers that are the
same size.However, wood propellers must be
carved—not molded—and therefore do not
usually feature the more exotic blade designs
that are so common in some molded model
propellers. Depending on their design, wood
propellers produce excellent top speeds and
quick acceleration because they are light and
stiff.
Wood propellers break easily with any
ground contact, and prolonged use on
grass runways results in excessive blade
September 2004 35
Right: Fiberglass-filled nylon propellers have flexible blades
that resist damage but lower propeller efficiency.
L-R: Idle-bar glow plug ensures reliable idle
even when engine is mounted inverted.
Standard glow plug works well in most other
RC applications.
Four major propeller types are (top to bottom) solid
carbon fiber, wood (one shown is balanced; note
partially removed writing on right blade), fiberglassreinforced
nylon, and fiberglass-filled nylon.
Photos by the author
09sig2.QXD 6/24/04 9:03 am Page 35
36 MODEL AVIATION
wear. They also require the most balancing
effort because density and water content
may vary in a single propeller.
You know that all propellers must be
balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major
problems. First, the engine’s bearings wear
quickly. Second, the onboard radio
components, especially servos, suffer
excessive wear and can fail early. Whenever a
servo quits in flight, much of the fun of flying
RC models is diminished.
Third, an unbalanced propeller will cause
a 3%-4% rpm loss. An engine that would
have turned a balanced propeller at 11,000
rpm turns an unbalanced propeller at only
10,600 rpm. Climb rate and aerobatic
performance are reduced.
Many different propeller balancers are
available. I will cover these in the last edition
of this segment of the series.
CF propellers are the ultimate in rigidity;
they have almost no detectable flex. They can
assume any airfoil shape and blade area as
they are molded. Some are solid and others
are hollow. CF propellers are light, allowing
for the fastest engine acceleration possible,
and hollow ones accelerate even more
quickly. The solid and hollow kind feature
excellent performance across the entire
aerobatic spectrum. You can even purchase
them prebalanced.
However, despite their superior
performance, few modelers use CF propellers.
There are two good reasons for this, the first
of which is cost. CF propellers vary from
nearly $30 to $120 for the larger sizes. A
modeler can buy an abundance of wood,
fiberglass-filled, or fiberglass-reinforced
propellers for $30.
Second, trainers and many lowerperformance
sport models are unable to take
full advantage of the performance increase
that such a propeller provides. From level
flight, a 40-size trainer may be able to
perform a 100-foot vertical climb. If a CF
propeller provides a 15% climb increase, that
trainer will perform a 115-foot vertical climb.
It’s not that noticeable of a difference for the
money.
But install that propeller on a 40-size
Pattern airplane, and its normal 250-foot
vertical climb stretches to nearly 300 feet with
enough remaining airspeed to provide
excellent control.
After construction, the next important
factor in picking the right propeller is size.
Two numbers label their dimensions, and the
first is diameter in inches. The second is pitch,
which represents the distance in inches the
propeller would travel forward in one
revolution if there were no friction, drag, or
other limiting factors. This is the propeller’s
AOA, or incidence.
The numbers are separated by the usual
“by” designation: “x.” An 11-inch-diameter
propeller with a 6-inch pitch is called an “11 x
6.”
Understanding both numbers’
performance implications is critical. They
interact in a complicated dance of airflow,
engine performance, thrust, and geometry.
Fortunately the dance becomes easy to
understand once you know the few simple
steps. Well, step one, which follows, is not all
that simple but is easy to understand with a
little geometry.
The propeller’s efficiency for a given task
is determined by the amount of air it moves
per revolution and its speed. On a sport
airplane, if a propeller can move a huge
amount of air, but only at a slower speed, that
is better than moving small amounts of air at
high speeds.
The diameter of the disc that the rotating
propeller produces has more effect on the
power transmitted than a speed increase does
because the disc area increases by the square
of the radius. Therefore, an increase in
diameter (radius) moves additional amounts
of air by the square of the radius. This is a
large force multiplier.
Without tripping over the math, airstream
speed increase has an even less than linear
effect increasing the amount of power applied
to the air. This is a small force multiplier. The
idea is that as long as you have enough pitch
to fly at the speed you need, diameter is king,
offering faster acceleration, better climb, and
shorter takeoff runs.
The larger the propeller disc, the more
engine power can be applied to the air. An
additional factor in model aircraft is that the
center of the propeller disc area is located
only inches ahead of the fuselage and/or
cowling, which produces airstream
interference and drag that lowers propeller
efficiency. The larger the disc area outside the
cowling, the more efficient the propeller is.
Step two: The pitch determines airspeed
only in combination with the airframe. A 20-
inch-pitch propeller sounds fast. But if the
airframe has a high level of aerodynamic
drag, fixed landing gear, and straight wings,
this drag prevents the airplane from ever
reaching the propeller’s theoretical top speed.
The result is that the propeller cannot reach its
maximum rpm because the extra airframe
drag increases the propeller’s air load.
For step three, the best propeller size for a
given engine, in a 40- to 60-size, high-drag
sport RC model, is that one compromise
between the largest diameter and highest pitch
that still allows the engine’s maximum ground
rpm to be roughly 1,000-1,500 rpm higher
than its high-torque (maximum twisting
power) rpm. (This figure is after the highspeed
mixture has been adjusted to be 500
rpm less than absolute peak.)
Why this rpm? Once the airplane is flying,
there is an average increase of 500 rpm. This
happens because the aircraft’s forward speed
acts to decrease the propeller’s AOA. Another
way to picture this is that the air “flowing”
into the propeller from the front is helping the
engine turn it. It isn’t, of course, but it is
decreasing the engine’s workload by reducing
the effective AOA.
An airplane stops flying faster when the
propeller’s AOA nears zero. But once the
aircraft’s nose is pointed skyward, more of its
weight is placed on the propeller and therefore
on the engine’s turning ability. The airspeed
decays, and those 500 “free” rpm disappear as
propeller drag increases with the escalating
AOA.
Increasing the fuel’s nitro content from 15% to 20% (using the same fuel with
additional nitro) raised the engine’s output by only 300 rpm.
Replacing idle-bar glow plug with same manufacturer’s standard plug
increased engine’s top rpm by 300—same gain as 5% nitro increase. But
standard plug does not increase operating costs or cause additional
engine wear.
09sig2.QXD 6/24/04 9:04 am Page 36
The stress of pulling the aircraft upward
increases the power demands on the engine. It
responds by turning more slowly, just as a
car’s engine does when going up a steep hill
until extra energy, in the form of stepping on
the gas, is applied. But the model engine is
already at full power; there is no extra “gas”
to give.
In fact, the engine’s rpm will drop until it
reaches its high-torque rpm. If the engine is
the right size for the airplane and the climb
angle is not steeper than what the
engine/airframe combination was designed to
maintain (usually at least 45°), the rpm
reduction stops here and the airplane
maintains a constant climb rate.
Why not use a propeller that allows the
engine to rotate at its peak horsepower rpm?
The horsepower ratings for most .40-.60 twostroke
engines are usually at so high an rpm
that they are nearly unusable for sport
applications. Most reach peak horsepower
well in excess of 13,000 rpm.
At this number, model pilots do not need
to worry about their airplanes’ performance
because most clubs won’t let them fly such
loud models. Even if they can fly them, the
propeller disc must be so small—7-9 inches—
that little thrust can be applied to the air (step
one). The result is an inefficient propeller, an
airplane flying roughly 35 mph, and a
screaming engine trying to tear itself apart.
Experienced RC modelers have known
this “great truth” for years, many times
without even knowing they know it, but they
have had no data to support their intuitive
propeller choices. So I set out to prove this
last step.
I used a relatively new tool to gather the
needed data. The RC Flight Data Recorder
manufactured by Eagle Tree Systems records
the airspeed, rate of climb, climb angle,
altitude, and servo performance during flight.
It can also record in-flight engine rpm and
temperatures, but these systems were not
installed on the test aircraft.
The recorder correlates flight data and
transmitter inputs and time. This lets the pilot
know what was happening and when. I have
been using this instrument for sometime when
evaluating aircraft for MA’s Sport Aviator
online magazine (www.masportaviator.com)
and have become familiar with interpreting
the reported data. I used my trusty, manyyear-
old SuperStar 40 trainer equipped with
an even older .45 engine.
This engine reaches its maximum
horsepower, 1.35 when new but now much
less, at roughly 15,000 rpm. Its torque curve
peaks near 10,000 rpm. I tested identicaldesign,
fiberglass-reinforced propellers in 10
x 6, 10 x 7, 11 x 6, and 11 x 7 sizes. The
Flight Test Results chart summarizes the tests.
38 MODEL AVIATION
Propeller Size Ground rpm Top Speed Takeoff Climbout 45 Degree Climb Approach Speed
Performance* Performance*
10 x 6 11,940 55 mph 1,100@28 1,200@6 33 mph
10 x 7 11,220 64 mph 1,400@32 1,500@31 35 mph
11 x 6 10,920 60 mph 1,500@25 1,800@28 31 mph
11 x 7 10,140 51 mph 1,200@27 1,200@26 24 mph
*Feet per minute climb at mph climb speed
Flight Test Results
Recognizing those who have defended,
protected, and supported the sport of
aeromodeling through their personal
contributions and devotion. Their
advocacy has long been the foundation
upon which the growth and advances in
the sport have been based, and for this
they are presented as true champions!
Ted Teach
A charter member of the Legion of Champions
and a continuing supporter
Legion of Champions
09sig2.QXD 6/24/04 12:13 pm Page 38
As shown, the highest ground rpm does
not translate into the fastest airspeed or the
best climb rate. Under the demands of a
climb, the smaller 10-inch propeller discs
could not transfer the engine’s power to the
air as effectively as the 11-inch discs could.
An inch may not seem to be a big
difference. However, the 11-inch propeller
has an effective area of 95 square inches
versus the 10-inch propeller’s 79 square
inches. The engine’s “force area” is 20%
larger using the 11-inch propeller.
The larger disc is the reason why the 11 x
6 propeller produced a 20% better climb rate
than the 10 x 7, despite the slower climb
speed. The extra power required to turn the 11
x 7 propeller when climbing proved more
than the ol’ engine possessed. Climb and top
speed suffered, but landing speed was the
slowest, probably because the idle speed was
less than 2,100 rpm. The 11 x 7 might cause
engine overheating in hot weather.
The 11 x 6 allowed the aircraft to leave the
ground in the shortest time at the slowest
airspeed, reducing airframe wear. It produced
a climb rate up to 50% higher, and its top
speed was only 6% less than the highest but
up to 18% higher than the remaining
propellers’. During your next visit to the
flying field, check the propellers on most .45
two-stroke engines. Most will be various
types of 11 x 6s.
Choose the propeller-and-glow-plug
combination that permits the engine to turn
the largest-diameter propeller approximately
1,000-1,500 rpm higher than its peak torque
speed on the ground. Start with the largest
diameter and lowest pitch recommended for
your engine. If the engine will not turn this
propeller fast enough, drop to the next smaller
diameter, again with the lowest pitch.
Increase the pitch if the engine turns too fast.
Continue until you find the right size
combination.
The .30-size engines will usually use a 9-
inch-diameter propeller, .40s will use up to
10.5 inches, .45s will use 11 inches, and .60s
work best with a 12-inch propeller.
Remember that this is for sport models only.
Glow Plugs: Did I also mention the glow
plug? It causes the fuel to burn and release its
energy. Fortunately there are only two types
of glow plugs that newer sport-model pilots
need to know about.
The idle-bar plug was once the only
design that provided a reliable idle. The metal
bar protected the glow element from unburnt
liquid fuel that otherwise cooled the element
to lower than the fuel’s ignition temperature
when at idle speeds. The bar itself became
extremely hot, adding protection to the glow
element’s idle temperature.
But today’s more powerful sport engines
are equipped with mufflers that preserve the
chamber’s heat at idle. Modern carburetors
allow finer adjustment of the fuel/air mixture,
reducing the amount of liquid, unburnt fuel
that enters the chamber at idle speeds.
Therefore, idle-bar glow plugs are not always
required on newer sport engines.
Photos show a non-idle-bar glow plug’s
performance advantage. All settings and
equipment remain the same, and the photos
were taken only minutes apart. The non-idlebar
plug produced a 300 rpm gain.
However, an idle-bar glow plug is a good
idea for two-strokes mounted with the
cylinder head pointed downward—called
inverted mounting—for engines that have
difficulty idling and for older, well-worn
engines that may need the extra heat to keep
the glow element hot. Consider using nonidle-
bar glow plugs for all other sport
applications.
Fuel: Choosing the right fuel is the last
critical factor to ensure that your new twostroke
engine gets the best performance and
longest life. The “right” fuel is also one of the
most controversial, opinion-rich, and
individualistic subjects in model aviation. But
there are some useful guidelines to remember.
Two-stroke fuel has three major
ingredients: methanol, castor oil, and
nitromenthane (nitro). Methanol comprises
60%-75% of most fuels. It burns completely
and adds to the fuel’s total energy output.
The lubricating oil comprises roughly
20% of the fuel. Most two-stroke fuels
contain two types of oil; 4%-8% is usually
castor and the remaining 12%-16% is a
synthetic that varies by manufacturer. Oils
typically do not burn completely, and what
small percentage of the oil that does burn
does so at lower energy levels than methanol.
Castor oil is used because it maintains a
lubricating film at higher temperatures than
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40 MODEL AVIATION
09sig2.QXD 6/24/04 9:04 am Page 40
most synthetic oils do. If the engine’s highspeed
fuel/air ratio is too lean—too much
air—the engine will run at high temperatures.
Castor oil will maintain lubrication in an
overheated engine; most synthetic oils burn
away.
Castor oil also helps remove heat from
the combustion chamber better than most
synthetics. Castor oil leaves a film residue in
the engine that offers some rust protection.
Most synthetic oils do not.
However, too much castor oil causes
excessive residue buildup that can diminish
an engine’s performance. Because castor oil
does not burn, it reduces the fuel’s total
energy output. For these reasons, the castor
oil percentage is usually kept at less than 8%.
Synthetic oils do burn, but not well or
completely. Little synthetic oil residue is left
inside an engine. Synthetics also offer
excellent engine lubrication when operating
at normal engine temperatures.
Because high oil content detracts from the
fuel’s total energy output, the easiest way to
increase an engine’s apparent power output
is to reduce the fuel’s oil content. However,
oil contents much less than 18% can cause
long-term wear problems in .40-.60 sport
engines.
Fuel manufacturers are studying new oils
that produce more power and offer better
protection with quantities as low as 16%. But
for now, consider using fuels with 18%-20%
oil content in newer or sophisticated
(expensive?) .40-.60 engines.
The third fuel component is nitromethane.
It burns at a higher energy level than
methanol. However, it also produces higher
combustion-chamber temperatures and
therefore needs to be limited. Most sport
fuels contain 5%-25% nitro. It prolongs the
combustion event. The burning process takes
longer, and that also produces more energy.
Many pilots overrate the power increase
obtained by “upping the nitro” in their sport
engine’s fuel. Photos show how little effect
higher nitro content has on sport engines.
Raising its content by 5% in a fuel produced
only a 300 rpm gain. But nitro does improve
an engine’s idling ability, permitting a lower
reliable idle speed. Therefore, consider nitro
contents in the 10%-15% range as a good
sport-flying compromise.
There is such a thing as too much nitro
content for a given engine. If the combustion
event becomes too prolonged, detonation
may occur. You may hear pinging, a sound
like frying eggs, or the exhaust note may
become very loose. Unless operating at high
altitudes—exceeding 5,000 feet—there is no
reason, and little gain, to use fuels with nitro
contents higher than 20%.
Next month I’ll cover some of the
differences in and advantages of four-stroke
engines. Since model engines do not run for
long periods without an onboard fuel supply,
I’ll cover fuel-tank choices and installation
requirements as well. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
42 MODEL AVIATION
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09sig2.QXD 6/24/04 9:04 am Page 42
Edition: Model Aviation - 2004/09
Page Numbers: 34,35,36,38,40,42
SOMETIMES WE become so
involved with the various types of
model engines, performance
ratings, and displacements that we
overlook something important. Just
as a fully race-prepared car engine
is useless until its power is
transmitted to the ground, a model
engine is useless until its power is transmitted to
the air. Most often, model aircraft use propellers
for this job. Our engines also need to burn fuel to
produce power, and they need to ignite that fuel.
These functions, which are directly connected to
the engine, are this month’s subject.
Propellers: A model’s engine is only as good as
its propeller. The propeller’s size, shape, and
composition determine how much of the engine’s
power is transmitted to the air and the manner in
which the aircraft can best use that power. The
best combination of propeller characteristics for a
particular model is a compromise.
The pilot must choose a propeller that
produces the best performance based on the
aircraft’s mission (training, racing, aerobatics,
combat, etc.), the engine’s power range, and the
flying-field conditions.
A racing airplane would do best if its
propeller were designed solely to produce high
airspeeds while rotating at the same rpm at which
the engine produces maximum horsepower. This
is the right choice even if durability, climb, and
acceleration rates are sacrificed.
Choosing the right propeller requires
understanding and a few “prop tips,” one of
which is that a propeller’s blade rigidity is
important. A propeller is nothing more than a
rotating wing. All propellers have airfoil shapes
and direct their lift in a horizontal path, called
thrust, instead of a vertical direction, as does the
aircraft’s main wing. Thrust pulls the aircraft
forward.
Imagine how much of your aircraft’s wing lift
would be lost if the outer third of the wing were
to flex enough that its incidence—its angle of
attack (AOA) to the oncoming airstream—
significantly decreased during every turn or
climb. In the same way, a propeller in which the
34 MODEL AVIATION
The Rest of the Engine by Frank Granelli
The 2.5-inch spinner reduces propeller drag while streamlining the model’s
front end. Removing the spinner reduces the engine’s top rpm by 450—a 4%
power loss.
09sig2.QXD 6/24/04 9:03 am Page 34
tips flex does “flatten out,” reducing its
incidence during acceleration and climb,
thereby losing thrust when it is most needed.
Unlike a wing, which develops lift along
almost its entire span, a rotating propeller
produces the majority of its thrust centered
around the 75% point of each blade’s length.
This makes the thrust lost caused by tip
flexing even more critical.
Stand slightly behind and to the side of the
spinning propeller and watch the tips. If they
follow a wavy path, that signals excessive
pitch loss (lower propeller AOA), which
results in power lost transferring the engine’s
energy to the air.
The first 20% of a propeller blade’s
length—its span—produces much drag but
little thrust. This section is the area where the
propeller’s round center—the hub—tapers
into the working “wing” of the blade, which
does all the work. There is little “wing area”
here.
This area also moves the slowest through
the air since it is closest to the center of the
“disc” formed by the rotating propeller.
However, this inner section does rotate and
therefore produces air drag. This is why
spinners make propellers more efficient.
The next 50% of the blade’s span is the
area where the LE-to-TE width—the chord—
increases to maximum and the airfoil becomes
fully developed. Some thrust is lost until the
blade is fully formed, and more is lost because
the center-section rotates more slowly than
the remaining outer blade area. Since a wing’s
total lift depends, in part, on its airspeed, the
lift produced by different blade sections
depends a great deal on their rotational
speeds.
How different are these rotational speeds?
The blade section 1 inch out from the hub of
an 11-inch-diameter model propeller rotating
at 11,000 rpm has an “airspeed” of just 96 feet
per second (fps), or 60 mph. The middle of
the blade is rotating through the air at 260 fps,
or 180 mph, and the 75% point is moving at
396 fps, or 264 mph.
Even though the blade’s area near the tip
(90%) is much less than that near the middle,
it is moving nearly twice as fast, at 475 fps, or
317 mph, and is therefore producing more
thrust than the center-section is.
Please study that last rotational speed. The
tip itself is moving at 530 fps, which is
approximately the same speed as some .45-
caliber bullets. If you want to know what
happens if you are careless enough to put a
hand into a spinning model propeller’s arc,
envision pointing a Colt .45 at your hand and
pulling the trigger! Not an attractive image.
Please be careful.
Tune your engine
while standing behind
the propeller, never
stand directly to the
side of a spinning
propeller, and keep
children away from
your engine at all
times.
Since rigidity is important
to propeller performance, a
major factor to consider when
choosing a propeller is its
construction. Today they are
usually made from one of four
basic materials: fiberglassfilled
nylon composite,
fiberglass-reinforced nylon,
wood, or carbon fiber (CF).
Pure nylon propellers were
once manufactured, but for
the most part they have been
replaced by nylon composite
construction. The fiberglassfilled
nylon propellers are
safer and stiffer than the old
nylon-only variety, but they
remain the most flexible kind.
Most fiberglass-filled
nylon propellers have large blade areas to
improve their performance. They produce
excellent thrust for a given rpm but tend to
rotate more slowly than same-size propellers
of different construction. These propellers
suffer the most thrust loss as the airplane
climbs steeply since the outer blade areas flex
the most under stress.
However, this flexibility is a major
advantage for newer model pilots. The blades
bend well on those poor landings—those that
bend the nose wheel back nearly far enough to
touch the fuselage bottom. Fiberglass-filled
nylon propellers bend backward and usually
do not break in those situations. They also last
the longest when flying from paved runways.
This durability saves money and keeps
newer pilots flying on those days when they
would have exhausted their supply of more
rigid propellers. Most RTF trainers are
equipped with the fiberglass-filled nylon
variety for exactly these reasons.
Fiberglass-reinforced propellers are stiffer
and sometimes feature undercambered
(concave-bottom) airfoils. They have tips with
a small area but quickly widen to large chords
just short of the tip. The tiny tip area helps the
engine stay quiet and increases the propeller’s
efficiency. One of the most efficient wings
ever designed employs elliptical wingtips that
reduce drag by reducing wingtip vortices; just
ask any Spitfire pilot.
Fiberglass-reinforced propellers have
wingtip designs that most closely resemble
the elliptical wing shape. The reduced tip drag
allows the propeller to accelerate quickly and
to reach a higher top speed. That combined
with the more rigid blade make fiberglass
propellers famous for excellent climb
performance. The middle areas of many
reinforced blades are usually the largest in
their respective size classes. This helps
increase overall thrust, again adding to the
aircraft’s climbing ability.
However, these stiffer fiberglassreinforced
blades still flex a bit under load and
are easy to break during hard landings. Paved
runways are rough on them since the tip area
is small and may be destroyed with one
contact, even if the propeller is not rotating.
Wood propellers are more rigid than
fiberglass-filled and fiberglass-reinforced
nylon types. Some wood propellers have
special tip designs to produce increased thrust
and rpm. Most have roughly the same blade
area as fiberglass-filled propellers that are the
same size.However, wood propellers must be
carved—not molded—and therefore do not
usually feature the more exotic blade designs
that are so common in some molded model
propellers. Depending on their design, wood
propellers produce excellent top speeds and
quick acceleration because they are light and
stiff.
Wood propellers break easily with any
ground contact, and prolonged use on
grass runways results in excessive blade
September 2004 35
Right: Fiberglass-filled nylon propellers have flexible blades
that resist damage but lower propeller efficiency.
L-R: Idle-bar glow plug ensures reliable idle
even when engine is mounted inverted.
Standard glow plug works well in most other
RC applications.
Four major propeller types are (top to bottom) solid
carbon fiber, wood (one shown is balanced; note
partially removed writing on right blade), fiberglassreinforced
nylon, and fiberglass-filled nylon.
Photos by the author
09sig2.QXD 6/24/04 9:03 am Page 35
36 MODEL AVIATION
wear. They also require the most balancing
effort because density and water content
may vary in a single propeller.
You know that all propellers must be
balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major
problems. First, the engine’s bearings wear
quickly. Second, the onboard radio
components, especially servos, suffer
excessive wear and can fail early. Whenever a
servo quits in flight, much of the fun of flying
RC models is diminished.
Third, an unbalanced propeller will cause
a 3%-4% rpm loss. An engine that would
have turned a balanced propeller at 11,000
rpm turns an unbalanced propeller at only
10,600 rpm. Climb rate and aerobatic
performance are reduced.
Many different propeller balancers are
available. I will cover these in the last edition
of this segment of the series.
CF propellers are the ultimate in rigidity;
they have almost no detectable flex. They can
assume any airfoil shape and blade area as
they are molded. Some are solid and others
are hollow. CF propellers are light, allowing
for the fastest engine acceleration possible,
and hollow ones accelerate even more
quickly. The solid and hollow kind feature
excellent performance across the entire
aerobatic spectrum. You can even purchase
them prebalanced.
However, despite their superior
performance, few modelers use CF propellers.
There are two good reasons for this, the first
of which is cost. CF propellers vary from
nearly $30 to $120 for the larger sizes. A
modeler can buy an abundance of wood,
fiberglass-filled, or fiberglass-reinforced
propellers for $30.
Second, trainers and many lowerperformance
sport models are unable to take
full advantage of the performance increase
that such a propeller provides. From level
flight, a 40-size trainer may be able to
perform a 100-foot vertical climb. If a CF
propeller provides a 15% climb increase, that
trainer will perform a 115-foot vertical climb.
It’s not that noticeable of a difference for the
money.
But install that propeller on a 40-size
Pattern airplane, and its normal 250-foot
vertical climb stretches to nearly 300 feet with
enough remaining airspeed to provide
excellent control.
After construction, the next important
factor in picking the right propeller is size.
Two numbers label their dimensions, and the
first is diameter in inches. The second is pitch,
which represents the distance in inches the
propeller would travel forward in one
revolution if there were no friction, drag, or
other limiting factors. This is the propeller’s
AOA, or incidence.
The numbers are separated by the usual
“by” designation: “x.” An 11-inch-diameter
propeller with a 6-inch pitch is called an “11 x
6.”
Understanding both numbers’
performance implications is critical. They
interact in a complicated dance of airflow,
engine performance, thrust, and geometry.
Fortunately the dance becomes easy to
understand once you know the few simple
steps. Well, step one, which follows, is not all
that simple but is easy to understand with a
little geometry.
The propeller’s efficiency for a given task
is determined by the amount of air it moves
per revolution and its speed. On a sport
airplane, if a propeller can move a huge
amount of air, but only at a slower speed, that
is better than moving small amounts of air at
high speeds.
The diameter of the disc that the rotating
propeller produces has more effect on the
power transmitted than a speed increase does
because the disc area increases by the square
of the radius. Therefore, an increase in
diameter (radius) moves additional amounts
of air by the square of the radius. This is a
large force multiplier.
Without tripping over the math, airstream
speed increase has an even less than linear
effect increasing the amount of power applied
to the air. This is a small force multiplier. The
idea is that as long as you have enough pitch
to fly at the speed you need, diameter is king,
offering faster acceleration, better climb, and
shorter takeoff runs.
The larger the propeller disc, the more
engine power can be applied to the air. An
additional factor in model aircraft is that the
center of the propeller disc area is located
only inches ahead of the fuselage and/or
cowling, which produces airstream
interference and drag that lowers propeller
efficiency. The larger the disc area outside the
cowling, the more efficient the propeller is.
Step two: The pitch determines airspeed
only in combination with the airframe. A 20-
inch-pitch propeller sounds fast. But if the
airframe has a high level of aerodynamic
drag, fixed landing gear, and straight wings,
this drag prevents the airplane from ever
reaching the propeller’s theoretical top speed.
The result is that the propeller cannot reach its
maximum rpm because the extra airframe
drag increases the propeller’s air load.
For step three, the best propeller size for a
given engine, in a 40- to 60-size, high-drag
sport RC model, is that one compromise
between the largest diameter and highest pitch
that still allows the engine’s maximum ground
rpm to be roughly 1,000-1,500 rpm higher
than its high-torque (maximum twisting
power) rpm. (This figure is after the highspeed
mixture has been adjusted to be 500
rpm less than absolute peak.)
Why this rpm? Once the airplane is flying,
there is an average increase of 500 rpm. This
happens because the aircraft’s forward speed
acts to decrease the propeller’s AOA. Another
way to picture this is that the air “flowing”
into the propeller from the front is helping the
engine turn it. It isn’t, of course, but it is
decreasing the engine’s workload by reducing
the effective AOA.
An airplane stops flying faster when the
propeller’s AOA nears zero. But once the
aircraft’s nose is pointed skyward, more of its
weight is placed on the propeller and therefore
on the engine’s turning ability. The airspeed
decays, and those 500 “free” rpm disappear as
propeller drag increases with the escalating
AOA.
Increasing the fuel’s nitro content from 15% to 20% (using the same fuel with
additional nitro) raised the engine’s output by only 300 rpm.
Replacing idle-bar glow plug with same manufacturer’s standard plug
increased engine’s top rpm by 300—same gain as 5% nitro increase. But
standard plug does not increase operating costs or cause additional
engine wear.
09sig2.QXD 6/24/04 9:04 am Page 36
The stress of pulling the aircraft upward
increases the power demands on the engine. It
responds by turning more slowly, just as a
car’s engine does when going up a steep hill
until extra energy, in the form of stepping on
the gas, is applied. But the model engine is
already at full power; there is no extra “gas”
to give.
In fact, the engine’s rpm will drop until it
reaches its high-torque rpm. If the engine is
the right size for the airplane and the climb
angle is not steeper than what the
engine/airframe combination was designed to
maintain (usually at least 45°), the rpm
reduction stops here and the airplane
maintains a constant climb rate.
Why not use a propeller that allows the
engine to rotate at its peak horsepower rpm?
The horsepower ratings for most .40-.60 twostroke
engines are usually at so high an rpm
that they are nearly unusable for sport
applications. Most reach peak horsepower
well in excess of 13,000 rpm.
At this number, model pilots do not need
to worry about their airplanes’ performance
because most clubs won’t let them fly such
loud models. Even if they can fly them, the
propeller disc must be so small—7-9 inches—
that little thrust can be applied to the air (step
one). The result is an inefficient propeller, an
airplane flying roughly 35 mph, and a
screaming engine trying to tear itself apart.
Experienced RC modelers have known
this “great truth” for years, many times
without even knowing they know it, but they
have had no data to support their intuitive
propeller choices. So I set out to prove this
last step.
I used a relatively new tool to gather the
needed data. The RC Flight Data Recorder
manufactured by Eagle Tree Systems records
the airspeed, rate of climb, climb angle,
altitude, and servo performance during flight.
It can also record in-flight engine rpm and
temperatures, but these systems were not
installed on the test aircraft.
The recorder correlates flight data and
transmitter inputs and time. This lets the pilot
know what was happening and when. I have
been using this instrument for sometime when
evaluating aircraft for MA’s Sport Aviator
online magazine (www.masportaviator.com)
and have become familiar with interpreting
the reported data. I used my trusty, manyyear-
old SuperStar 40 trainer equipped with
an even older .45 engine.
This engine reaches its maximum
horsepower, 1.35 when new but now much
less, at roughly 15,000 rpm. Its torque curve
peaks near 10,000 rpm. I tested identicaldesign,
fiberglass-reinforced propellers in 10
x 6, 10 x 7, 11 x 6, and 11 x 7 sizes. The
Flight Test Results chart summarizes the tests.
38 MODEL AVIATION
Propeller Size Ground rpm Top Speed Takeoff Climbout 45 Degree Climb Approach Speed
Performance* Performance*
10 x 6 11,940 55 mph 1,100@28 1,200@6 33 mph
10 x 7 11,220 64 mph 1,400@32 1,500@31 35 mph
11 x 6 10,920 60 mph 1,500@25 1,800@28 31 mph
11 x 7 10,140 51 mph 1,200@27 1,200@26 24 mph
*Feet per minute climb at mph climb speed
Flight Test Results
Recognizing those who have defended,
protected, and supported the sport of
aeromodeling through their personal
contributions and devotion. Their
advocacy has long been the foundation
upon which the growth and advances in
the sport have been based, and for this
they are presented as true champions!
Ted Teach
A charter member of the Legion of Champions
and a continuing supporter
Legion of Champions
09sig2.QXD 6/24/04 12:13 pm Page 38
As shown, the highest ground rpm does
not translate into the fastest airspeed or the
best climb rate. Under the demands of a
climb, the smaller 10-inch propeller discs
could not transfer the engine’s power to the
air as effectively as the 11-inch discs could.
An inch may not seem to be a big
difference. However, the 11-inch propeller
has an effective area of 95 square inches
versus the 10-inch propeller’s 79 square
inches. The engine’s “force area” is 20%
larger using the 11-inch propeller.
The larger disc is the reason why the 11 x
6 propeller produced a 20% better climb rate
than the 10 x 7, despite the slower climb
speed. The extra power required to turn the 11
x 7 propeller when climbing proved more
than the ol’ engine possessed. Climb and top
speed suffered, but landing speed was the
slowest, probably because the idle speed was
less than 2,100 rpm. The 11 x 7 might cause
engine overheating in hot weather.
The 11 x 6 allowed the aircraft to leave the
ground in the shortest time at the slowest
airspeed, reducing airframe wear. It produced
a climb rate up to 50% higher, and its top
speed was only 6% less than the highest but
up to 18% higher than the remaining
propellers’. During your next visit to the
flying field, check the propellers on most .45
two-stroke engines. Most will be various
types of 11 x 6s.
Choose the propeller-and-glow-plug
combination that permits the engine to turn
the largest-diameter propeller approximately
1,000-1,500 rpm higher than its peak torque
speed on the ground. Start with the largest
diameter and lowest pitch recommended for
your engine. If the engine will not turn this
propeller fast enough, drop to the next smaller
diameter, again with the lowest pitch.
Increase the pitch if the engine turns too fast.
Continue until you find the right size
combination.
The .30-size engines will usually use a 9-
inch-diameter propeller, .40s will use up to
10.5 inches, .45s will use 11 inches, and .60s
work best with a 12-inch propeller.
Remember that this is for sport models only.
Glow Plugs: Did I also mention the glow
plug? It causes the fuel to burn and release its
energy. Fortunately there are only two types
of glow plugs that newer sport-model pilots
need to know about.
The idle-bar plug was once the only
design that provided a reliable idle. The metal
bar protected the glow element from unburnt
liquid fuel that otherwise cooled the element
to lower than the fuel’s ignition temperature
when at idle speeds. The bar itself became
extremely hot, adding protection to the glow
element’s idle temperature.
But today’s more powerful sport engines
are equipped with mufflers that preserve the
chamber’s heat at idle. Modern carburetors
allow finer adjustment of the fuel/air mixture,
reducing the amount of liquid, unburnt fuel
that enters the chamber at idle speeds.
Therefore, idle-bar glow plugs are not always
required on newer sport engines.
Photos show a non-idle-bar glow plug’s
performance advantage. All settings and
equipment remain the same, and the photos
were taken only minutes apart. The non-idlebar
plug produced a 300 rpm gain.
However, an idle-bar glow plug is a good
idea for two-strokes mounted with the
cylinder head pointed downward—called
inverted mounting—for engines that have
difficulty idling and for older, well-worn
engines that may need the extra heat to keep
the glow element hot. Consider using nonidle-
bar glow plugs for all other sport
applications.
Fuel: Choosing the right fuel is the last
critical factor to ensure that your new twostroke
engine gets the best performance and
longest life. The “right” fuel is also one of the
most controversial, opinion-rich, and
individualistic subjects in model aviation. But
there are some useful guidelines to remember.
Two-stroke fuel has three major
ingredients: methanol, castor oil, and
nitromenthane (nitro). Methanol comprises
60%-75% of most fuels. It burns completely
and adds to the fuel’s total energy output.
The lubricating oil comprises roughly
20% of the fuel. Most two-stroke fuels
contain two types of oil; 4%-8% is usually
castor and the remaining 12%-16% is a
synthetic that varies by manufacturer. Oils
typically do not burn completely, and what
small percentage of the oil that does burn
does so at lower energy levels than methanol.
Castor oil is used because it maintains a
lubricating film at higher temperatures than
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40 MODEL AVIATION
09sig2.QXD 6/24/04 9:04 am Page 40
most synthetic oils do. If the engine’s highspeed
fuel/air ratio is too lean—too much
air—the engine will run at high temperatures.
Castor oil will maintain lubrication in an
overheated engine; most synthetic oils burn
away.
Castor oil also helps remove heat from
the combustion chamber better than most
synthetics. Castor oil leaves a film residue in
the engine that offers some rust protection.
Most synthetic oils do not.
However, too much castor oil causes
excessive residue buildup that can diminish
an engine’s performance. Because castor oil
does not burn, it reduces the fuel’s total
energy output. For these reasons, the castor
oil percentage is usually kept at less than 8%.
Synthetic oils do burn, but not well or
completely. Little synthetic oil residue is left
inside an engine. Synthetics also offer
excellent engine lubrication when operating
at normal engine temperatures.
Because high oil content detracts from the
fuel’s total energy output, the easiest way to
increase an engine’s apparent power output
is to reduce the fuel’s oil content. However,
oil contents much less than 18% can cause
long-term wear problems in .40-.60 sport
engines.
Fuel manufacturers are studying new oils
that produce more power and offer better
protection with quantities as low as 16%. But
for now, consider using fuels with 18%-20%
oil content in newer or sophisticated
(expensive?) .40-.60 engines.
The third fuel component is nitromethane.
It burns at a higher energy level than
methanol. However, it also produces higher
combustion-chamber temperatures and
therefore needs to be limited. Most sport
fuels contain 5%-25% nitro. It prolongs the
combustion event. The burning process takes
longer, and that also produces more energy.
Many pilots overrate the power increase
obtained by “upping the nitro” in their sport
engine’s fuel. Photos show how little effect
higher nitro content has on sport engines.
Raising its content by 5% in a fuel produced
only a 300 rpm gain. But nitro does improve
an engine’s idling ability, permitting a lower
reliable idle speed. Therefore, consider nitro
contents in the 10%-15% range as a good
sport-flying compromise.
There is such a thing as too much nitro
content for a given engine. If the combustion
event becomes too prolonged, detonation
may occur. You may hear pinging, a sound
like frying eggs, or the exhaust note may
become very loose. Unless operating at high
altitudes—exceeding 5,000 feet—there is no
reason, and little gain, to use fuels with nitro
contents higher than 20%.
Next month I’ll cover some of the
differences in and advantages of four-stroke
engines. Since model engines do not run for
long periods without an onboard fuel supply,
I’ll cover fuel-tank choices and installation
requirements as well. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
42 MODEL AVIATION
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09sig2.QXD 6/24/04 9:04 am Page 42
Edition: Model Aviation - 2004/09
Page Numbers: 34,35,36,38,40,42
SOMETIMES WE become so
involved with the various types of
model engines, performance
ratings, and displacements that we
overlook something important. Just
as a fully race-prepared car engine
is useless until its power is
transmitted to the ground, a model
engine is useless until its power is transmitted to
the air. Most often, model aircraft use propellers
for this job. Our engines also need to burn fuel to
produce power, and they need to ignite that fuel.
These functions, which are directly connected to
the engine, are this month’s subject.
Propellers: A model’s engine is only as good as
its propeller. The propeller’s size, shape, and
composition determine how much of the engine’s
power is transmitted to the air and the manner in
which the aircraft can best use that power. The
best combination of propeller characteristics for a
particular model is a compromise.
The pilot must choose a propeller that
produces the best performance based on the
aircraft’s mission (training, racing, aerobatics,
combat, etc.), the engine’s power range, and the
flying-field conditions.
A racing airplane would do best if its
propeller were designed solely to produce high
airspeeds while rotating at the same rpm at which
the engine produces maximum horsepower. This
is the right choice even if durability, climb, and
acceleration rates are sacrificed.
Choosing the right propeller requires
understanding and a few “prop tips,” one of
which is that a propeller’s blade rigidity is
important. A propeller is nothing more than a
rotating wing. All propellers have airfoil shapes
and direct their lift in a horizontal path, called
thrust, instead of a vertical direction, as does the
aircraft’s main wing. Thrust pulls the aircraft
forward.
Imagine how much of your aircraft’s wing lift
would be lost if the outer third of the wing were
to flex enough that its incidence—its angle of
attack (AOA) to the oncoming airstream—
significantly decreased during every turn or
climb. In the same way, a propeller in which the
34 MODEL AVIATION
The Rest of the Engine by Frank Granelli
The 2.5-inch spinner reduces propeller drag while streamlining the model’s
front end. Removing the spinner reduces the engine’s top rpm by 450—a 4%
power loss.
09sig2.QXD 6/24/04 9:03 am Page 34
tips flex does “flatten out,” reducing its
incidence during acceleration and climb,
thereby losing thrust when it is most needed.
Unlike a wing, which develops lift along
almost its entire span, a rotating propeller
produces the majority of its thrust centered
around the 75% point of each blade’s length.
This makes the thrust lost caused by tip
flexing even more critical.
Stand slightly behind and to the side of the
spinning propeller and watch the tips. If they
follow a wavy path, that signals excessive
pitch loss (lower propeller AOA), which
results in power lost transferring the engine’s
energy to the air.
The first 20% of a propeller blade’s
length—its span—produces much drag but
little thrust. This section is the area where the
propeller’s round center—the hub—tapers
into the working “wing” of the blade, which
does all the work. There is little “wing area”
here.
This area also moves the slowest through
the air since it is closest to the center of the
“disc” formed by the rotating propeller.
However, this inner section does rotate and
therefore produces air drag. This is why
spinners make propellers more efficient.
The next 50% of the blade’s span is the
area where the LE-to-TE width—the chord—
increases to maximum and the airfoil becomes
fully developed. Some thrust is lost until the
blade is fully formed, and more is lost because
the center-section rotates more slowly than
the remaining outer blade area. Since a wing’s
total lift depends, in part, on its airspeed, the
lift produced by different blade sections
depends a great deal on their rotational
speeds.
How different are these rotational speeds?
The blade section 1 inch out from the hub of
an 11-inch-diameter model propeller rotating
at 11,000 rpm has an “airspeed” of just 96 feet
per second (fps), or 60 mph. The middle of
the blade is rotating through the air at 260 fps,
or 180 mph, and the 75% point is moving at
396 fps, or 264 mph.
Even though the blade’s area near the tip
(90%) is much less than that near the middle,
it is moving nearly twice as fast, at 475 fps, or
317 mph, and is therefore producing more
thrust than the center-section is.
Please study that last rotational speed. The
tip itself is moving at 530 fps, which is
approximately the same speed as some .45-
caliber bullets. If you want to know what
happens if you are careless enough to put a
hand into a spinning model propeller’s arc,
envision pointing a Colt .45 at your hand and
pulling the trigger! Not an attractive image.
Please be careful.
Tune your engine
while standing behind
the propeller, never
stand directly to the
side of a spinning
propeller, and keep
children away from
your engine at all
times.
Since rigidity is important
to propeller performance, a
major factor to consider when
choosing a propeller is its
construction. Today they are
usually made from one of four
basic materials: fiberglassfilled
nylon composite,
fiberglass-reinforced nylon,
wood, or carbon fiber (CF).
Pure nylon propellers were
once manufactured, but for
the most part they have been
replaced by nylon composite
construction. The fiberglassfilled
nylon propellers are
safer and stiffer than the old
nylon-only variety, but they
remain the most flexible kind.
Most fiberglass-filled
nylon propellers have large blade areas to
improve their performance. They produce
excellent thrust for a given rpm but tend to
rotate more slowly than same-size propellers
of different construction. These propellers
suffer the most thrust loss as the airplane
climbs steeply since the outer blade areas flex
the most under stress.
However, this flexibility is a major
advantage for newer model pilots. The blades
bend well on those poor landings—those that
bend the nose wheel back nearly far enough to
touch the fuselage bottom. Fiberglass-filled
nylon propellers bend backward and usually
do not break in those situations. They also last
the longest when flying from paved runways.
This durability saves money and keeps
newer pilots flying on those days when they
would have exhausted their supply of more
rigid propellers. Most RTF trainers are
equipped with the fiberglass-filled nylon
variety for exactly these reasons.
Fiberglass-reinforced propellers are stiffer
and sometimes feature undercambered
(concave-bottom) airfoils. They have tips with
a small area but quickly widen to large chords
just short of the tip. The tiny tip area helps the
engine stay quiet and increases the propeller’s
efficiency. One of the most efficient wings
ever designed employs elliptical wingtips that
reduce drag by reducing wingtip vortices; just
ask any Spitfire pilot.
Fiberglass-reinforced propellers have
wingtip designs that most closely resemble
the elliptical wing shape. The reduced tip drag
allows the propeller to accelerate quickly and
to reach a higher top speed. That combined
with the more rigid blade make fiberglass
propellers famous for excellent climb
performance. The middle areas of many
reinforced blades are usually the largest in
their respective size classes. This helps
increase overall thrust, again adding to the
aircraft’s climbing ability.
However, these stiffer fiberglassreinforced
blades still flex a bit under load and
are easy to break during hard landings. Paved
runways are rough on them since the tip area
is small and may be destroyed with one
contact, even if the propeller is not rotating.
Wood propellers are more rigid than
fiberglass-filled and fiberglass-reinforced
nylon types. Some wood propellers have
special tip designs to produce increased thrust
and rpm. Most have roughly the same blade
area as fiberglass-filled propellers that are the
same size.However, wood propellers must be
carved—not molded—and therefore do not
usually feature the more exotic blade designs
that are so common in some molded model
propellers. Depending on their design, wood
propellers produce excellent top speeds and
quick acceleration because they are light and
stiff.
Wood propellers break easily with any
ground contact, and prolonged use on
grass runways results in excessive blade
September 2004 35
Right: Fiberglass-filled nylon propellers have flexible blades
that resist damage but lower propeller efficiency.
L-R: Idle-bar glow plug ensures reliable idle
even when engine is mounted inverted.
Standard glow plug works well in most other
RC applications.
Four major propeller types are (top to bottom) solid
carbon fiber, wood (one shown is balanced; note
partially removed writing on right blade), fiberglassreinforced
nylon, and fiberglass-filled nylon.
Photos by the author
09sig2.QXD 6/24/04 9:03 am Page 35
36 MODEL AVIATION
wear. They also require the most balancing
effort because density and water content
may vary in a single propeller.
You know that all propellers must be
balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major
problems. First, the engine’s bearings wear
quickly. Second, the onboard radio
components, especially servos, suffer
excessive wear and can fail early. Whenever a
servo quits in flight, much of the fun of flying
RC models is diminished.
Third, an unbalanced propeller will cause
a 3%-4% rpm loss. An engine that would
have turned a balanced propeller at 11,000
rpm turns an unbalanced propeller at only
10,600 rpm. Climb rate and aerobatic
performance are reduced.
Many different propeller balancers are
available. I will cover these in the last edition
of this segment of the series.
CF propellers are the ultimate in rigidity;
they have almost no detectable flex. They can
assume any airfoil shape and blade area as
they are molded. Some are solid and others
are hollow. CF propellers are light, allowing
for the fastest engine acceleration possible,
and hollow ones accelerate even more
quickly. The solid and hollow kind feature
excellent performance across the entire
aerobatic spectrum. You can even purchase
them prebalanced.
However, despite their superior
performance, few modelers use CF propellers.
There are two good reasons for this, the first
of which is cost. CF propellers vary from
nearly $30 to $120 for the larger sizes. A
modeler can buy an abundance of wood,
fiberglass-filled, or fiberglass-reinforced
propellers for $30.
Second, trainers and many lowerperformance
sport models are unable to take
full advantage of the performance increase
that such a propeller provides. From level
flight, a 40-size trainer may be able to
perform a 100-foot vertical climb. If a CF
propeller provides a 15% climb increase, that
trainer will perform a 115-foot vertical climb.
It’s not that noticeable of a difference for the
money.
But install that propeller on a 40-size
Pattern airplane, and its normal 250-foot
vertical climb stretches to nearly 300 feet with
enough remaining airspeed to provide
excellent control.
After construction, the next important
factor in picking the right propeller is size.
Two numbers label their dimensions, and the
first is diameter in inches. The second is pitch,
which represents the distance in inches the
propeller would travel forward in one
revolution if there were no friction, drag, or
other limiting factors. This is the propeller’s
AOA, or incidence.
The numbers are separated by the usual
“by” designation: “x.” An 11-inch-diameter
propeller with a 6-inch pitch is called an “11 x
6.”
Understanding both numbers’
performance implications is critical. They
interact in a complicated dance of airflow,
engine performance, thrust, and geometry.
Fortunately the dance becomes easy to
understand once you know the few simple
steps. Well, step one, which follows, is not all
that simple but is easy to understand with a
little geometry.
The propeller’s efficiency for a given task
is determined by the amount of air it moves
per revolution and its speed. On a sport
airplane, if a propeller can move a huge
amount of air, but only at a slower speed, that
is better than moving small amounts of air at
high speeds.
The diameter of the disc that the rotating
propeller produces has more effect on the
power transmitted than a speed increase does
because the disc area increases by the square
of the radius. Therefore, an increase in
diameter (radius) moves additional amounts
of air by the square of the radius. This is a
large force multiplier.
Without tripping over the math, airstream
speed increase has an even less than linear
effect increasing the amount of power applied
to the air. This is a small force multiplier. The
idea is that as long as you have enough pitch
to fly at the speed you need, diameter is king,
offering faster acceleration, better climb, and
shorter takeoff runs.
The larger the propeller disc, the more
engine power can be applied to the air. An
additional factor in model aircraft is that the
center of the propeller disc area is located
only inches ahead of the fuselage and/or
cowling, which produces airstream
interference and drag that lowers propeller
efficiency. The larger the disc area outside the
cowling, the more efficient the propeller is.
Step two: The pitch determines airspeed
only in combination with the airframe. A 20-
inch-pitch propeller sounds fast. But if the
airframe has a high level of aerodynamic
drag, fixed landing gear, and straight wings,
this drag prevents the airplane from ever
reaching the propeller’s theoretical top speed.
The result is that the propeller cannot reach its
maximum rpm because the extra airframe
drag increases the propeller’s air load.
For step three, the best propeller size for a
given engine, in a 40- to 60-size, high-drag
sport RC model, is that one compromise
between the largest diameter and highest pitch
that still allows the engine’s maximum ground
rpm to be roughly 1,000-1,500 rpm higher
than its high-torque (maximum twisting
power) rpm. (This figure is after the highspeed
mixture has been adjusted to be 500
rpm less than absolute peak.)
Why this rpm? Once the airplane is flying,
there is an average increase of 500 rpm. This
happens because the aircraft’s forward speed
acts to decrease the propeller’s AOA. Another
way to picture this is that the air “flowing”
into the propeller from the front is helping the
engine turn it. It isn’t, of course, but it is
decreasing the engine’s workload by reducing
the effective AOA.
An airplane stops flying faster when the
propeller’s AOA nears zero. But once the
aircraft’s nose is pointed skyward, more of its
weight is placed on the propeller and therefore
on the engine’s turning ability. The airspeed
decays, and those 500 “free” rpm disappear as
propeller drag increases with the escalating
AOA.
Increasing the fuel’s nitro content from 15% to 20% (using the same fuel with
additional nitro) raised the engine’s output by only 300 rpm.
Replacing idle-bar glow plug with same manufacturer’s standard plug
increased engine’s top rpm by 300—same gain as 5% nitro increase. But
standard plug does not increase operating costs or cause additional
engine wear.
09sig2.QXD 6/24/04 9:04 am Page 36
The stress of pulling the aircraft upward
increases the power demands on the engine. It
responds by turning more slowly, just as a
car’s engine does when going up a steep hill
until extra energy, in the form of stepping on
the gas, is applied. But the model engine is
already at full power; there is no extra “gas”
to give.
In fact, the engine’s rpm will drop until it
reaches its high-torque rpm. If the engine is
the right size for the airplane and the climb
angle is not steeper than what the
engine/airframe combination was designed to
maintain (usually at least 45°), the rpm
reduction stops here and the airplane
maintains a constant climb rate.
Why not use a propeller that allows the
engine to rotate at its peak horsepower rpm?
The horsepower ratings for most .40-.60 twostroke
engines are usually at so high an rpm
that they are nearly unusable for sport
applications. Most reach peak horsepower
well in excess of 13,000 rpm.
At this number, model pilots do not need
to worry about their airplanes’ performance
because most clubs won’t let them fly such
loud models. Even if they can fly them, the
propeller disc must be so small—7-9 inches—
that little thrust can be applied to the air (step
one). The result is an inefficient propeller, an
airplane flying roughly 35 mph, and a
screaming engine trying to tear itself apart.
Experienced RC modelers have known
this “great truth” for years, many times
without even knowing they know it, but they
have had no data to support their intuitive
propeller choices. So I set out to prove this
last step.
I used a relatively new tool to gather the
needed data. The RC Flight Data Recorder
manufactured by Eagle Tree Systems records
the airspeed, rate of climb, climb angle,
altitude, and servo performance during flight.
It can also record in-flight engine rpm and
temperatures, but these systems were not
installed on the test aircraft.
The recorder correlates flight data and
transmitter inputs and time. This lets the pilot
know what was happening and when. I have
been using this instrument for sometime when
evaluating aircraft for MA’s Sport Aviator
online magazine (www.masportaviator.com)
and have become familiar with interpreting
the reported data. I used my trusty, manyyear-
old SuperStar 40 trainer equipped with
an even older .45 engine.
This engine reaches its maximum
horsepower, 1.35 when new but now much
less, at roughly 15,000 rpm. Its torque curve
peaks near 10,000 rpm. I tested identicaldesign,
fiberglass-reinforced propellers in 10
x 6, 10 x 7, 11 x 6, and 11 x 7 sizes. The
Flight Test Results chart summarizes the tests.
38 MODEL AVIATION
Propeller Size Ground rpm Top Speed Takeoff Climbout 45 Degree Climb Approach Speed
Performance* Performance*
10 x 6 11,940 55 mph 1,100@28 1,200@6 33 mph
10 x 7 11,220 64 mph 1,400@32 1,500@31 35 mph
11 x 6 10,920 60 mph 1,500@25 1,800@28 31 mph
11 x 7 10,140 51 mph 1,200@27 1,200@26 24 mph
*Feet per minute climb at mph climb speed
Flight Test Results
Recognizing those who have defended,
protected, and supported the sport of
aeromodeling through their personal
contributions and devotion. Their
advocacy has long been the foundation
upon which the growth and advances in
the sport have been based, and for this
they are presented as true champions!
Ted Teach
A charter member of the Legion of Champions
and a continuing supporter
Legion of Champions
09sig2.QXD 6/24/04 12:13 pm Page 38
As shown, the highest ground rpm does
not translate into the fastest airspeed or the
best climb rate. Under the demands of a
climb, the smaller 10-inch propeller discs
could not transfer the engine’s power to the
air as effectively as the 11-inch discs could.
An inch may not seem to be a big
difference. However, the 11-inch propeller
has an effective area of 95 square inches
versus the 10-inch propeller’s 79 square
inches. The engine’s “force area” is 20%
larger using the 11-inch propeller.
The larger disc is the reason why the 11 x
6 propeller produced a 20% better climb rate
than the 10 x 7, despite the slower climb
speed. The extra power required to turn the 11
x 7 propeller when climbing proved more
than the ol’ engine possessed. Climb and top
speed suffered, but landing speed was the
slowest, probably because the idle speed was
less than 2,100 rpm. The 11 x 7 might cause
engine overheating in hot weather.
The 11 x 6 allowed the aircraft to leave the
ground in the shortest time at the slowest
airspeed, reducing airframe wear. It produced
a climb rate up to 50% higher, and its top
speed was only 6% less than the highest but
up to 18% higher than the remaining
propellers’. During your next visit to the
flying field, check the propellers on most .45
two-stroke engines. Most will be various
types of 11 x 6s.
Choose the propeller-and-glow-plug
combination that permits the engine to turn
the largest-diameter propeller approximately
1,000-1,500 rpm higher than its peak torque
speed on the ground. Start with the largest
diameter and lowest pitch recommended for
your engine. If the engine will not turn this
propeller fast enough, drop to the next smaller
diameter, again with the lowest pitch.
Increase the pitch if the engine turns too fast.
Continue until you find the right size
combination.
The .30-size engines will usually use a 9-
inch-diameter propeller, .40s will use up to
10.5 inches, .45s will use 11 inches, and .60s
work best with a 12-inch propeller.
Remember that this is for sport models only.
Glow Plugs: Did I also mention the glow
plug? It causes the fuel to burn and release its
energy. Fortunately there are only two types
of glow plugs that newer sport-model pilots
need to know about.
The idle-bar plug was once the only
design that provided a reliable idle. The metal
bar protected the glow element from unburnt
liquid fuel that otherwise cooled the element
to lower than the fuel’s ignition temperature
when at idle speeds. The bar itself became
extremely hot, adding protection to the glow
element’s idle temperature.
But today’s more powerful sport engines
are equipped with mufflers that preserve the
chamber’s heat at idle. Modern carburetors
allow finer adjustment of the fuel/air mixture,
reducing the amount of liquid, unburnt fuel
that enters the chamber at idle speeds.
Therefore, idle-bar glow plugs are not always
required on newer sport engines.
Photos show a non-idle-bar glow plug’s
performance advantage. All settings and
equipment remain the same, and the photos
were taken only minutes apart. The non-idlebar
plug produced a 300 rpm gain.
However, an idle-bar glow plug is a good
idea for two-strokes mounted with the
cylinder head pointed downward—called
inverted mounting—for engines that have
difficulty idling and for older, well-worn
engines that may need the extra heat to keep
the glow element hot. Consider using nonidle-
bar glow plugs for all other sport
applications.
Fuel: Choosing the right fuel is the last
critical factor to ensure that your new twostroke
engine gets the best performance and
longest life. The “right” fuel is also one of the
most controversial, opinion-rich, and
individualistic subjects in model aviation. But
there are some useful guidelines to remember.
Two-stroke fuel has three major
ingredients: methanol, castor oil, and
nitromenthane (nitro). Methanol comprises
60%-75% of most fuels. It burns completely
and adds to the fuel’s total energy output.
The lubricating oil comprises roughly
20% of the fuel. Most two-stroke fuels
contain two types of oil; 4%-8% is usually
castor and the remaining 12%-16% is a
synthetic that varies by manufacturer. Oils
typically do not burn completely, and what
small percentage of the oil that does burn
does so at lower energy levels than methanol.
Castor oil is used because it maintains a
lubricating film at higher temperatures than
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40 MODEL AVIATION
09sig2.QXD 6/24/04 9:04 am Page 40
most synthetic oils do. If the engine’s highspeed
fuel/air ratio is too lean—too much
air—the engine will run at high temperatures.
Castor oil will maintain lubrication in an
overheated engine; most synthetic oils burn
away.
Castor oil also helps remove heat from
the combustion chamber better than most
synthetics. Castor oil leaves a film residue in
the engine that offers some rust protection.
Most synthetic oils do not.
However, too much castor oil causes
excessive residue buildup that can diminish
an engine’s performance. Because castor oil
does not burn, it reduces the fuel’s total
energy output. For these reasons, the castor
oil percentage is usually kept at less than 8%.
Synthetic oils do burn, but not well or
completely. Little synthetic oil residue is left
inside an engine. Synthetics also offer
excellent engine lubrication when operating
at normal engine temperatures.
Because high oil content detracts from the
fuel’s total energy output, the easiest way to
increase an engine’s apparent power output
is to reduce the fuel’s oil content. However,
oil contents much less than 18% can cause
long-term wear problems in .40-.60 sport
engines.
Fuel manufacturers are studying new oils
that produce more power and offer better
protection with quantities as low as 16%. But
for now, consider using fuels with 18%-20%
oil content in newer or sophisticated
(expensive?) .40-.60 engines.
The third fuel component is nitromethane.
It burns at a higher energy level than
methanol. However, it also produces higher
combustion-chamber temperatures and
therefore needs to be limited. Most sport
fuels contain 5%-25% nitro. It prolongs the
combustion event. The burning process takes
longer, and that also produces more energy.
Many pilots overrate the power increase
obtained by “upping the nitro” in their sport
engine’s fuel. Photos show how little effect
higher nitro content has on sport engines.
Raising its content by 5% in a fuel produced
only a 300 rpm gain. But nitro does improve
an engine’s idling ability, permitting a lower
reliable idle speed. Therefore, consider nitro
contents in the 10%-15% range as a good
sport-flying compromise.
There is such a thing as too much nitro
content for a given engine. If the combustion
event becomes too prolonged, detonation
may occur. You may hear pinging, a sound
like frying eggs, or the exhaust note may
become very loose. Unless operating at high
altitudes—exceeding 5,000 feet—there is no
reason, and little gain, to use fuels with nitro
contents higher than 20%.
Next month I’ll cover some of the
differences in and advantages of four-stroke
engines. Since model engines do not run for
long periods without an onboard fuel supply,
I’ll cover fuel-tank choices and installation
requirements as well. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
42 MODEL AVIATION
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• Enjoy 500-1500 flights on servos, pots, gears,
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ISOLATOR
09sig2.QXD 6/24/04 9:04 am Page 42
Edition: Model Aviation - 2004/09
Page Numbers: 34,35,36,38,40,42
SOMETIMES WE become so
involved with the various types of
model engines, performance
ratings, and displacements that we
overlook something important. Just
as a fully race-prepared car engine
is useless until its power is
transmitted to the ground, a model
engine is useless until its power is transmitted to
the air. Most often, model aircraft use propellers
for this job. Our engines also need to burn fuel to
produce power, and they need to ignite that fuel.
These functions, which are directly connected to
the engine, are this month’s subject.
Propellers: A model’s engine is only as good as
its propeller. The propeller’s size, shape, and
composition determine how much of the engine’s
power is transmitted to the air and the manner in
which the aircraft can best use that power. The
best combination of propeller characteristics for a
particular model is a compromise.
The pilot must choose a propeller that
produces the best performance based on the
aircraft’s mission (training, racing, aerobatics,
combat, etc.), the engine’s power range, and the
flying-field conditions.
A racing airplane would do best if its
propeller were designed solely to produce high
airspeeds while rotating at the same rpm at which
the engine produces maximum horsepower. This
is the right choice even if durability, climb, and
acceleration rates are sacrificed.
Choosing the right propeller requires
understanding and a few “prop tips,” one of
which is that a propeller’s blade rigidity is
important. A propeller is nothing more than a
rotating wing. All propellers have airfoil shapes
and direct their lift in a horizontal path, called
thrust, instead of a vertical direction, as does the
aircraft’s main wing. Thrust pulls the aircraft
forward.
Imagine how much of your aircraft’s wing lift
would be lost if the outer third of the wing were
to flex enough that its incidence—its angle of
attack (AOA) to the oncoming airstream—
significantly decreased during every turn or
climb. In the same way, a propeller in which the
34 MODEL AVIATION
The Rest of the Engine by Frank Granelli
The 2.5-inch spinner reduces propeller drag while streamlining the model’s
front end. Removing the spinner reduces the engine’s top rpm by 450—a 4%
power loss.
09sig2.QXD 6/24/04 9:03 am Page 34
tips flex does “flatten out,” reducing its
incidence during acceleration and climb,
thereby losing thrust when it is most needed.
Unlike a wing, which develops lift along
almost its entire span, a rotating propeller
produces the majority of its thrust centered
around the 75% point of each blade’s length.
This makes the thrust lost caused by tip
flexing even more critical.
Stand slightly behind and to the side of the
spinning propeller and watch the tips. If they
follow a wavy path, that signals excessive
pitch loss (lower propeller AOA), which
results in power lost transferring the engine’s
energy to the air.
The first 20% of a propeller blade’s
length—its span—produces much drag but
little thrust. This section is the area where the
propeller’s round center—the hub—tapers
into the working “wing” of the blade, which
does all the work. There is little “wing area”
here.
This area also moves the slowest through
the air since it is closest to the center of the
“disc” formed by the rotating propeller.
However, this inner section does rotate and
therefore produces air drag. This is why
spinners make propellers more efficient.
The next 50% of the blade’s span is the
area where the LE-to-TE width—the chord—
increases to maximum and the airfoil becomes
fully developed. Some thrust is lost until the
blade is fully formed, and more is lost because
the center-section rotates more slowly than
the remaining outer blade area. Since a wing’s
total lift depends, in part, on its airspeed, the
lift produced by different blade sections
depends a great deal on their rotational
speeds.
How different are these rotational speeds?
The blade section 1 inch out from the hub of
an 11-inch-diameter model propeller rotating
at 11,000 rpm has an “airspeed” of just 96 feet
per second (fps), or 60 mph. The middle of
the blade is rotating through the air at 260 fps,
or 180 mph, and the 75% point is moving at
396 fps, or 264 mph.
Even though the blade’s area near the tip
(90%) is much less than that near the middle,
it is moving nearly twice as fast, at 475 fps, or
317 mph, and is therefore producing more
thrust than the center-section is.
Please study that last rotational speed. The
tip itself is moving at 530 fps, which is
approximately the same speed as some .45-
caliber bullets. If you want to know what
happens if you are careless enough to put a
hand into a spinning model propeller’s arc,
envision pointing a Colt .45 at your hand and
pulling the trigger! Not an attractive image.
Please be careful.
Tune your engine
while standing behind
the propeller, never
stand directly to the
side of a spinning
propeller, and keep
children away from
your engine at all
times.
Since rigidity is important
to propeller performance, a
major factor to consider when
choosing a propeller is its
construction. Today they are
usually made from one of four
basic materials: fiberglassfilled
nylon composite,
fiberglass-reinforced nylon,
wood, or carbon fiber (CF).
Pure nylon propellers were
once manufactured, but for
the most part they have been
replaced by nylon composite
construction. The fiberglassfilled
nylon propellers are
safer and stiffer than the old
nylon-only variety, but they
remain the most flexible kind.
Most fiberglass-filled
nylon propellers have large blade areas to
improve their performance. They produce
excellent thrust for a given rpm but tend to
rotate more slowly than same-size propellers
of different construction. These propellers
suffer the most thrust loss as the airplane
climbs steeply since the outer blade areas flex
the most under stress.
However, this flexibility is a major
advantage for newer model pilots. The blades
bend well on those poor landings—those that
bend the nose wheel back nearly far enough to
touch the fuselage bottom. Fiberglass-filled
nylon propellers bend backward and usually
do not break in those situations. They also last
the longest when flying from paved runways.
This durability saves money and keeps
newer pilots flying on those days when they
would have exhausted their supply of more
rigid propellers. Most RTF trainers are
equipped with the fiberglass-filled nylon
variety for exactly these reasons.
Fiberglass-reinforced propellers are stiffer
and sometimes feature undercambered
(concave-bottom) airfoils. They have tips with
a small area but quickly widen to large chords
just short of the tip. The tiny tip area helps the
engine stay quiet and increases the propeller’s
efficiency. One of the most efficient wings
ever designed employs elliptical wingtips that
reduce drag by reducing wingtip vortices; just
ask any Spitfire pilot.
Fiberglass-reinforced propellers have
wingtip designs that most closely resemble
the elliptical wing shape. The reduced tip drag
allows the propeller to accelerate quickly and
to reach a higher top speed. That combined
with the more rigid blade make fiberglass
propellers famous for excellent climb
performance. The middle areas of many
reinforced blades are usually the largest in
their respective size classes. This helps
increase overall thrust, again adding to the
aircraft’s climbing ability.
However, these stiffer fiberglassreinforced
blades still flex a bit under load and
are easy to break during hard landings. Paved
runways are rough on them since the tip area
is small and may be destroyed with one
contact, even if the propeller is not rotating.
Wood propellers are more rigid than
fiberglass-filled and fiberglass-reinforced
nylon types. Some wood propellers have
special tip designs to produce increased thrust
and rpm. Most have roughly the same blade
area as fiberglass-filled propellers that are the
same size.However, wood propellers must be
carved—not molded—and therefore do not
usually feature the more exotic blade designs
that are so common in some molded model
propellers. Depending on their design, wood
propellers produce excellent top speeds and
quick acceleration because they are light and
stiff.
Wood propellers break easily with any
ground contact, and prolonged use on
grass runways results in excessive blade
September 2004 35
Right: Fiberglass-filled nylon propellers have flexible blades
that resist damage but lower propeller efficiency.
L-R: Idle-bar glow plug ensures reliable idle
even when engine is mounted inverted.
Standard glow plug works well in most other
RC applications.
Four major propeller types are (top to bottom) solid
carbon fiber, wood (one shown is balanced; note
partially removed writing on right blade), fiberglassreinforced
nylon, and fiberglass-filled nylon.
Photos by the author
09sig2.QXD 6/24/04 9:03 am Page 35
36 MODEL AVIATION
wear. They also require the most balancing
effort because density and water content
may vary in a single propeller.
You know that all propellers must be
balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major
problems. First, the engine’s bearings wear
quickly. Second, the onboard radio
components, especially servos, suffer
excessive wear and can fail early. Whenever a
servo quits in flight, much of the fun of flying
RC models is diminished.
Third, an unbalanced propeller will cause
a 3%-4% rpm loss. An engine that would
have turned a balanced propeller at 11,000
rpm turns an unbalanced propeller at only
10,600 rpm. Climb rate and aerobatic
performance are reduced.
Many different propeller balancers are
available. I will cover these in the last edition
of this segment of the series.
CF propellers are the ultimate in rigidity;
they have almost no detectable flex. They can
assume any airfoil shape and blade area as
they are molded. Some are solid and others
are hollow. CF propellers are light, allowing
for the fastest engine acceleration possible,
and hollow ones accelerate even more
quickly. The solid and hollow kind feature
excellent performance across the entire
aerobatic spectrum. You can even purchase
them prebalanced.
However, despite their superior
performance, few modelers use CF propellers.
There are two good reasons for this, the first
of which is cost. CF propellers vary from
nearly $30 to $120 for the larger sizes. A
modeler can buy an abundance of wood,
fiberglass-filled, or fiberglass-reinforced
propellers for $30.
Second, trainers and many lowerperformance
sport models are unable to take
full advantage of the performance increase
that such a propeller provides. From level
flight, a 40-size trainer may be able to
perform a 100-foot vertical climb. If a CF
propeller provides a 15% climb increase, that
trainer will perform a 115-foot vertical climb.
It’s not that noticeable of a difference for the
money.
But install that propeller on a 40-size
Pattern airplane, and its normal 250-foot
vertical climb stretches to nearly 300 feet with
enough remaining airspeed to provide
excellent control.
After construction, the next important
factor in picking the right propeller is size.
Two numbers label their dimensions, and the
first is diameter in inches. The second is pitch,
which represents the distance in inches the
propeller would travel forward in one
revolution if there were no friction, drag, or
other limiting factors. This is the propeller’s
AOA, or incidence.
The numbers are separated by the usual
“by” designation: “x.” An 11-inch-diameter
propeller with a 6-inch pitch is called an “11 x
6.”
Understanding both numbers’
performance implications is critical. They
interact in a complicated dance of airflow,
engine performance, thrust, and geometry.
Fortunately the dance becomes easy to
understand once you know the few simple
steps. Well, step one, which follows, is not all
that simple but is easy to understand with a
little geometry.
The propeller’s efficiency for a given task
is determined by the amount of air it moves
per revolution and its speed. On a sport
airplane, if a propeller can move a huge
amount of air, but only at a slower speed, that
is better than moving small amounts of air at
high speeds.
The diameter of the disc that the rotating
propeller produces has more effect on the
power transmitted than a speed increase does
because the disc area increases by the square
of the radius. Therefore, an increase in
diameter (radius) moves additional amounts
of air by the square of the radius. This is a
large force multiplier.
Without tripping over the math, airstream
speed increase has an even less than linear
effect increasing the amount of power applied
to the air. This is a small force multiplier. The
idea is that as long as you have enough pitch
to fly at the speed you need, diameter is king,
offering faster acceleration, better climb, and
shorter takeoff runs.
The larger the propeller disc, the more
engine power can be applied to the air. An
additional factor in model aircraft is that the
center of the propeller disc area is located
only inches ahead of the fuselage and/or
cowling, which produces airstream
interference and drag that lowers propeller
efficiency. The larger the disc area outside the
cowling, the more efficient the propeller is.
Step two: The pitch determines airspeed
only in combination with the airframe. A 20-
inch-pitch propeller sounds fast. But if the
airframe has a high level of aerodynamic
drag, fixed landing gear, and straight wings,
this drag prevents the airplane from ever
reaching the propeller’s theoretical top speed.
The result is that the propeller cannot reach its
maximum rpm because the extra airframe
drag increases the propeller’s air load.
For step three, the best propeller size for a
given engine, in a 40- to 60-size, high-drag
sport RC model, is that one compromise
between the largest diameter and highest pitch
that still allows the engine’s maximum ground
rpm to be roughly 1,000-1,500 rpm higher
than its high-torque (maximum twisting
power) rpm. (This figure is after the highspeed
mixture has been adjusted to be 500
rpm less than absolute peak.)
Why this rpm? Once the airplane is flying,
there is an average increase of 500 rpm. This
happens because the aircraft’s forward speed
acts to decrease the propeller’s AOA. Another
way to picture this is that the air “flowing”
into the propeller from the front is helping the
engine turn it. It isn’t, of course, but it is
decreasing the engine’s workload by reducing
the effective AOA.
An airplane stops flying faster when the
propeller’s AOA nears zero. But once the
aircraft’s nose is pointed skyward, more of its
weight is placed on the propeller and therefore
on the engine’s turning ability. The airspeed
decays, and those 500 “free” rpm disappear as
propeller drag increases with the escalating
AOA.
Increasing the fuel’s nitro content from 15% to 20% (using the same fuel with
additional nitro) raised the engine’s output by only 300 rpm.
Replacing idle-bar glow plug with same manufacturer’s standard plug
increased engine’s top rpm by 300—same gain as 5% nitro increase. But
standard plug does not increase operating costs or cause additional
engine wear.
09sig2.QXD 6/24/04 9:04 am Page 36
The stress of pulling the aircraft upward
increases the power demands on the engine. It
responds by turning more slowly, just as a
car’s engine does when going up a steep hill
until extra energy, in the form of stepping on
the gas, is applied. But the model engine is
already at full power; there is no extra “gas”
to give.
In fact, the engine’s rpm will drop until it
reaches its high-torque rpm. If the engine is
the right size for the airplane and the climb
angle is not steeper than what the
engine/airframe combination was designed to
maintain (usually at least 45°), the rpm
reduction stops here and the airplane
maintains a constant climb rate.
Why not use a propeller that allows the
engine to rotate at its peak horsepower rpm?
The horsepower ratings for most .40-.60 twostroke
engines are usually at so high an rpm
that they are nearly unusable for sport
applications. Most reach peak horsepower
well in excess of 13,000 rpm.
At this number, model pilots do not need
to worry about their airplanes’ performance
because most clubs won’t let them fly such
loud models. Even if they can fly them, the
propeller disc must be so small—7-9 inches—
that little thrust can be applied to the air (step
one). The result is an inefficient propeller, an
airplane flying roughly 35 mph, and a
screaming engine trying to tear itself apart.
Experienced RC modelers have known
this “great truth” for years, many times
without even knowing they know it, but they
have had no data to support their intuitive
propeller choices. So I set out to prove this
last step.
I used a relatively new tool to gather the
needed data. The RC Flight Data Recorder
manufactured by Eagle Tree Systems records
the airspeed, rate of climb, climb angle,
altitude, and servo performance during flight.
It can also record in-flight engine rpm and
temperatures, but these systems were not
installed on the test aircraft.
The recorder correlates flight data and
transmitter inputs and time. This lets the pilot
know what was happening and when. I have
been using this instrument for sometime when
evaluating aircraft for MA’s Sport Aviator
online magazine (www.masportaviator.com)
and have become familiar with interpreting
the reported data. I used my trusty, manyyear-
old SuperStar 40 trainer equipped with
an even older .45 engine.
This engine reaches its maximum
horsepower, 1.35 when new but now much
less, at roughly 15,000 rpm. Its torque curve
peaks near 10,000 rpm. I tested identicaldesign,
fiberglass-reinforced propellers in 10
x 6, 10 x 7, 11 x 6, and 11 x 7 sizes. The
Flight Test Results chart summarizes the tests.
38 MODEL AVIATION
Propeller Size Ground rpm Top Speed Takeoff Climbout 45 Degree Climb Approach Speed
Performance* Performance*
10 x 6 11,940 55 mph 1,100@28 1,200@6 33 mph
10 x 7 11,220 64 mph 1,400@32 1,500@31 35 mph
11 x 6 10,920 60 mph 1,500@25 1,800@28 31 mph
11 x 7 10,140 51 mph 1,200@27 1,200@26 24 mph
*Feet per minute climb at mph climb speed
Flight Test Results
Recognizing those who have defended,
protected, and supported the sport of
aeromodeling through their personal
contributions and devotion. Their
advocacy has long been the foundation
upon which the growth and advances in
the sport have been based, and for this
they are presented as true champions!
Ted Teach
A charter member of the Legion of Champions
and a continuing supporter
Legion of Champions
09sig2.QXD 6/24/04 12:13 pm Page 38
As shown, the highest ground rpm does
not translate into the fastest airspeed or the
best climb rate. Under the demands of a
climb, the smaller 10-inch propeller discs
could not transfer the engine’s power to the
air as effectively as the 11-inch discs could.
An inch may not seem to be a big
difference. However, the 11-inch propeller
has an effective area of 95 square inches
versus the 10-inch propeller’s 79 square
inches. The engine’s “force area” is 20%
larger using the 11-inch propeller.
The larger disc is the reason why the 11 x
6 propeller produced a 20% better climb rate
than the 10 x 7, despite the slower climb
speed. The extra power required to turn the 11
x 7 propeller when climbing proved more
than the ol’ engine possessed. Climb and top
speed suffered, but landing speed was the
slowest, probably because the idle speed was
less than 2,100 rpm. The 11 x 7 might cause
engine overheating in hot weather.
The 11 x 6 allowed the aircraft to leave the
ground in the shortest time at the slowest
airspeed, reducing airframe wear. It produced
a climb rate up to 50% higher, and its top
speed was only 6% less than the highest but
up to 18% higher than the remaining
propellers’. During your next visit to the
flying field, check the propellers on most .45
two-stroke engines. Most will be various
types of 11 x 6s.
Choose the propeller-and-glow-plug
combination that permits the engine to turn
the largest-diameter propeller approximately
1,000-1,500 rpm higher than its peak torque
speed on the ground. Start with the largest
diameter and lowest pitch recommended for
your engine. If the engine will not turn this
propeller fast enough, drop to the next smaller
diameter, again with the lowest pitch.
Increase the pitch if the engine turns too fast.
Continue until you find the right size
combination.
The .30-size engines will usually use a 9-
inch-diameter propeller, .40s will use up to
10.5 inches, .45s will use 11 inches, and .60s
work best with a 12-inch propeller.
Remember that this is for sport models only.
Glow Plugs: Did I also mention the glow
plug? It causes the fuel to burn and release its
energy. Fortunately there are only two types
of glow plugs that newer sport-model pilots
need to know about.
The idle-bar plug was once the only
design that provided a reliable idle. The metal
bar protected the glow element from unburnt
liquid fuel that otherwise cooled the element
to lower than the fuel’s ignition temperature
when at idle speeds. The bar itself became
extremely hot, adding protection to the glow
element’s idle temperature.
But today’s more powerful sport engines
are equipped with mufflers that preserve the
chamber’s heat at idle. Modern carburetors
allow finer adjustment of the fuel/air mixture,
reducing the amount of liquid, unburnt fuel
that enters the chamber at idle speeds.
Therefore, idle-bar glow plugs are not always
required on newer sport engines.
Photos show a non-idle-bar glow plug’s
performance advantage. All settings and
equipment remain the same, and the photos
were taken only minutes apart. The non-idlebar
plug produced a 300 rpm gain.
However, an idle-bar glow plug is a good
idea for two-strokes mounted with the
cylinder head pointed downward—called
inverted mounting—for engines that have
difficulty idling and for older, well-worn
engines that may need the extra heat to keep
the glow element hot. Consider using nonidle-
bar glow plugs for all other sport
applications.
Fuel: Choosing the right fuel is the last
critical factor to ensure that your new twostroke
engine gets the best performance and
longest life. The “right” fuel is also one of the
most controversial, opinion-rich, and
individualistic subjects in model aviation. But
there are some useful guidelines to remember.
Two-stroke fuel has three major
ingredients: methanol, castor oil, and
nitromenthane (nitro). Methanol comprises
60%-75% of most fuels. It burns completely
and adds to the fuel’s total energy output.
The lubricating oil comprises roughly
20% of the fuel. Most two-stroke fuels
contain two types of oil; 4%-8% is usually
castor and the remaining 12%-16% is a
synthetic that varies by manufacturer. Oils
typically do not burn completely, and what
small percentage of the oil that does burn
does so at lower energy levels than methanol.
Castor oil is used because it maintains a
lubricating film at higher temperatures than
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40 MODEL AVIATION
09sig2.QXD 6/24/04 9:04 am Page 40
most synthetic oils do. If the engine’s highspeed
fuel/air ratio is too lean—too much
air—the engine will run at high temperatures.
Castor oil will maintain lubrication in an
overheated engine; most synthetic oils burn
away.
Castor oil also helps remove heat from
the combustion chamber better than most
synthetics. Castor oil leaves a film residue in
the engine that offers some rust protection.
Most synthetic oils do not.
However, too much castor oil causes
excessive residue buildup that can diminish
an engine’s performance. Because castor oil
does not burn, it reduces the fuel’s total
energy output. For these reasons, the castor
oil percentage is usually kept at less than 8%.
Synthetic oils do burn, but not well or
completely. Little synthetic oil residue is left
inside an engine. Synthetics also offer
excellent engine lubrication when operating
at normal engine temperatures.
Because high oil content detracts from the
fuel’s total energy output, the easiest way to
increase an engine’s apparent power output
is to reduce the fuel’s oil content. However,
oil contents much less than 18% can cause
long-term wear problems in .40-.60 sport
engines.
Fuel manufacturers are studying new oils
that produce more power and offer better
protection with quantities as low as 16%. But
for now, consider using fuels with 18%-20%
oil content in newer or sophisticated
(expensive?) .40-.60 engines.
The third fuel component is nitromethane.
It burns at a higher energy level than
methanol. However, it also produces higher
combustion-chamber temperatures and
therefore needs to be limited. Most sport
fuels contain 5%-25% nitro. It prolongs the
combustion event. The burning process takes
longer, and that also produces more energy.
Many pilots overrate the power increase
obtained by “upping the nitro” in their sport
engine’s fuel. Photos show how little effect
higher nitro content has on sport engines.
Raising its content by 5% in a fuel produced
only a 300 rpm gain. But nitro does improve
an engine’s idling ability, permitting a lower
reliable idle speed. Therefore, consider nitro
contents in the 10%-15% range as a good
sport-flying compromise.
There is such a thing as too much nitro
content for a given engine. If the combustion
event becomes too prolonged, detonation
may occur. You may hear pinging, a sound
like frying eggs, or the exhaust note may
become very loose. Unless operating at high
altitudes—exceeding 5,000 feet—there is no
reason, and little gain, to use fuels with nitro
contents higher than 20%.
Next month I’ll cover some of the
differences in and advantages of four-stroke
engines. Since model engines do not run for
long periods without an onboard fuel supply,
I’ll cover fuel-tank choices and installation
requirements as well. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
42 MODEL AVIATION
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PATENTED
SINGLE
ISOLATOR
09sig2.QXD 6/24/04 9:04 am Page 42
Edition: Model Aviation - 2004/09
Page Numbers: 34,35,36,38,40,42
SOMETIMES WE become so
involved with the various types of
model engines, performance
ratings, and displacements that we
overlook something important. Just
as a fully race-prepared car engine
is useless until its power is
transmitted to the ground, a model
engine is useless until its power is transmitted to
the air. Most often, model aircraft use propellers
for this job. Our engines also need to burn fuel to
produce power, and they need to ignite that fuel.
These functions, which are directly connected to
the engine, are this month’s subject.
Propellers: A model’s engine is only as good as
its propeller. The propeller’s size, shape, and
composition determine how much of the engine’s
power is transmitted to the air and the manner in
which the aircraft can best use that power. The
best combination of propeller characteristics for a
particular model is a compromise.
The pilot must choose a propeller that
produces the best performance based on the
aircraft’s mission (training, racing, aerobatics,
combat, etc.), the engine’s power range, and the
flying-field conditions.
A racing airplane would do best if its
propeller were designed solely to produce high
airspeeds while rotating at the same rpm at which
the engine produces maximum horsepower. This
is the right choice even if durability, climb, and
acceleration rates are sacrificed.
Choosing the right propeller requires
understanding and a few “prop tips,” one of
which is that a propeller’s blade rigidity is
important. A propeller is nothing more than a
rotating wing. All propellers have airfoil shapes
and direct their lift in a horizontal path, called
thrust, instead of a vertical direction, as does the
aircraft’s main wing. Thrust pulls the aircraft
forward.
Imagine how much of your aircraft’s wing lift
would be lost if the outer third of the wing were
to flex enough that its incidence—its angle of
attack (AOA) to the oncoming airstream—
significantly decreased during every turn or
climb. In the same way, a propeller in which the
34 MODEL AVIATION
The Rest of the Engine by Frank Granelli
The 2.5-inch spinner reduces propeller drag while streamlining the model’s
front end. Removing the spinner reduces the engine’s top rpm by 450—a 4%
power loss.
09sig2.QXD 6/24/04 9:03 am Page 34
tips flex does “flatten out,” reducing its
incidence during acceleration and climb,
thereby losing thrust when it is most needed.
Unlike a wing, which develops lift along
almost its entire span, a rotating propeller
produces the majority of its thrust centered
around the 75% point of each blade’s length.
This makes the thrust lost caused by tip
flexing even more critical.
Stand slightly behind and to the side of the
spinning propeller and watch the tips. If they
follow a wavy path, that signals excessive
pitch loss (lower propeller AOA), which
results in power lost transferring the engine’s
energy to the air.
The first 20% of a propeller blade’s
length—its span—produces much drag but
little thrust. This section is the area where the
propeller’s round center—the hub—tapers
into the working “wing” of the blade, which
does all the work. There is little “wing area”
here.
This area also moves the slowest through
the air since it is closest to the center of the
“disc” formed by the rotating propeller.
However, this inner section does rotate and
therefore produces air drag. This is why
spinners make propellers more efficient.
The next 50% of the blade’s span is the
area where the LE-to-TE width—the chord—
increases to maximum and the airfoil becomes
fully developed. Some thrust is lost until the
blade is fully formed, and more is lost because
the center-section rotates more slowly than
the remaining outer blade area. Since a wing’s
total lift depends, in part, on its airspeed, the
lift produced by different blade sections
depends a great deal on their rotational
speeds.
How different are these rotational speeds?
The blade section 1 inch out from the hub of
an 11-inch-diameter model propeller rotating
at 11,000 rpm has an “airspeed” of just 96 feet
per second (fps), or 60 mph. The middle of
the blade is rotating through the air at 260 fps,
or 180 mph, and the 75% point is moving at
396 fps, or 264 mph.
Even though the blade’s area near the tip
(90%) is much less than that near the middle,
it is moving nearly twice as fast, at 475 fps, or
317 mph, and is therefore producing more
thrust than the center-section is.
Please study that last rotational speed. The
tip itself is moving at 530 fps, which is
approximately the same speed as some .45-
caliber bullets. If you want to know what
happens if you are careless enough to put a
hand into a spinning model propeller’s arc,
envision pointing a Colt .45 at your hand and
pulling the trigger! Not an attractive image.
Please be careful.
Tune your engine
while standing behind
the propeller, never
stand directly to the
side of a spinning
propeller, and keep
children away from
your engine at all
times.
Since rigidity is important
to propeller performance, a
major factor to consider when
choosing a propeller is its
construction. Today they are
usually made from one of four
basic materials: fiberglassfilled
nylon composite,
fiberglass-reinforced nylon,
wood, or carbon fiber (CF).
Pure nylon propellers were
once manufactured, but for
the most part they have been
replaced by nylon composite
construction. The fiberglassfilled
nylon propellers are
safer and stiffer than the old
nylon-only variety, but they
remain the most flexible kind.
Most fiberglass-filled
nylon propellers have large blade areas to
improve their performance. They produce
excellent thrust for a given rpm but tend to
rotate more slowly than same-size propellers
of different construction. These propellers
suffer the most thrust loss as the airplane
climbs steeply since the outer blade areas flex
the most under stress.
However, this flexibility is a major
advantage for newer model pilots. The blades
bend well on those poor landings—those that
bend the nose wheel back nearly far enough to
touch the fuselage bottom. Fiberglass-filled
nylon propellers bend backward and usually
do not break in those situations. They also last
the longest when flying from paved runways.
This durability saves money and keeps
newer pilots flying on those days when they
would have exhausted their supply of more
rigid propellers. Most RTF trainers are
equipped with the fiberglass-filled nylon
variety for exactly these reasons.
Fiberglass-reinforced propellers are stiffer
and sometimes feature undercambered
(concave-bottom) airfoils. They have tips with
a small area but quickly widen to large chords
just short of the tip. The tiny tip area helps the
engine stay quiet and increases the propeller’s
efficiency. One of the most efficient wings
ever designed employs elliptical wingtips that
reduce drag by reducing wingtip vortices; just
ask any Spitfire pilot.
Fiberglass-reinforced propellers have
wingtip designs that most closely resemble
the elliptical wing shape. The reduced tip drag
allows the propeller to accelerate quickly and
to reach a higher top speed. That combined
with the more rigid blade make fiberglass
propellers famous for excellent climb
performance. The middle areas of many
reinforced blades are usually the largest in
their respective size classes. This helps
increase overall thrust, again adding to the
aircraft’s climbing ability.
However, these stiffer fiberglassreinforced
blades still flex a bit under load and
are easy to break during hard landings. Paved
runways are rough on them since the tip area
is small and may be destroyed with one
contact, even if the propeller is not rotating.
Wood propellers are more rigid than
fiberglass-filled and fiberglass-reinforced
nylon types. Some wood propellers have
special tip designs to produce increased thrust
and rpm. Most have roughly the same blade
area as fiberglass-filled propellers that are the
same size.However, wood propellers must be
carved—not molded—and therefore do not
usually feature the more exotic blade designs
that are so common in some molded model
propellers. Depending on their design, wood
propellers produce excellent top speeds and
quick acceleration because they are light and
stiff.
Wood propellers break easily with any
ground contact, and prolonged use on
grass runways results in excessive blade
September 2004 35
Right: Fiberglass-filled nylon propellers have flexible blades
that resist damage but lower propeller efficiency.
L-R: Idle-bar glow plug ensures reliable idle
even when engine is mounted inverted.
Standard glow plug works well in most other
RC applications.
Four major propeller types are (top to bottom) solid
carbon fiber, wood (one shown is balanced; note
partially removed writing on right blade), fiberglassreinforced
nylon, and fiberglass-filled nylon.
Photos by the author
09sig2.QXD 6/24/04 9:03 am Page 35
36 MODEL AVIATION
wear. They also require the most balancing
effort because density and water content
may vary in a single propeller.
You know that all propellers must be
balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major
problems. First, the engine’s bearings wear
quickly. Second, the onboard radio
components, especially servos, suffer
excessive wear and can fail early. Whenever a
servo quits in flight, much of the fun of flying
RC models is diminished.
Third, an unbalanced propeller will cause
a 3%-4% rpm loss. An engine that would
have turned a balanced propeller at 11,000
rpm turns an unbalanced propeller at only
10,600 rpm. Climb rate and aerobatic
performance are reduced.
Many different propeller balancers are
available. I will cover these in the last edition
of this segment of the series.
CF propellers are the ultimate in rigidity;
they have almost no detectable flex. They can
assume any airfoil shape and blade area as
they are molded. Some are solid and others
are hollow. CF propellers are light, allowing
for the fastest engine acceleration possible,
and hollow ones accelerate even more
quickly. The solid and hollow kind feature
excellent performance across the entire
aerobatic spectrum. You can even purchase
them prebalanced.
However, despite their superior
performance, few modelers use CF propellers.
There are two good reasons for this, the first
of which is cost. CF propellers vary from
nearly $30 to $120 for the larger sizes. A
modeler can buy an abundance of wood,
fiberglass-filled, or fiberglass-reinforced
propellers for $30.
Second, trainers and many lowerperformance
sport models are unable to take
full advantage of the performance increase
that such a propeller provides. From level
flight, a 40-size trainer may be able to
perform a 100-foot vertical climb. If a CF
propeller provides a 15% climb increase, that
trainer will perform a 115-foot vertical climb.
It’s not that noticeable of a difference for the
money.
But install that propeller on a 40-size
Pattern airplane, and its normal 250-foot
vertical climb stretches to nearly 300 feet with
enough remaining airspeed to provide
excellent control.
After construction, the next important
factor in picking the right propeller is size.
Two numbers label their dimensions, and the
first is diameter in inches. The second is pitch,
which represents the distance in inches the
propeller would travel forward in one
revolution if there were no friction, drag, or
other limiting factors. This is the propeller’s
AOA, or incidence.
The numbers are separated by the usual
“by” designation: “x.” An 11-inch-diameter
propeller with a 6-inch pitch is called an “11 x
6.”
Understanding both numbers’
performance implications is critical. They
interact in a complicated dance of airflow,
engine performance, thrust, and geometry.
Fortunately the dance becomes easy to
understand once you know the few simple
steps. Well, step one, which follows, is not all
that simple but is easy to understand with a
little geometry.
The propeller’s efficiency for a given task
is determined by the amount of air it moves
per revolution and its speed. On a sport
airplane, if a propeller can move a huge
amount of air, but only at a slower speed, that
is better than moving small amounts of air at
high speeds.
The diameter of the disc that the rotating
propeller produces has more effect on the
power transmitted than a speed increase does
because the disc area increases by the square
of the radius. Therefore, an increase in
diameter (radius) moves additional amounts
of air by the square of the radius. This is a
large force multiplier.
Without tripping over the math, airstream
speed increase has an even less than linear
effect increasing the amount of power applied
to the air. This is a small force multiplier. The
idea is that as long as you have enough pitch
to fly at the speed you need, diameter is king,
offering faster acceleration, better climb, and
shorter takeoff runs.
The larger the propeller disc, the more
engine power can be applied to the air. An
additional factor in model aircraft is that the
center of the propeller disc area is located
only inches ahead of the fuselage and/or
cowling, which produces airstream
interference and drag that lowers propeller
efficiency. The larger the disc area outside the
cowling, the more efficient the propeller is.
Step two: The pitch determines airspeed
only in combination with the airframe. A 20-
inch-pitch propeller sounds fast. But if the
airframe has a high level of aerodynamic
drag, fixed landing gear, and straight wings,
this drag prevents the airplane from ever
reaching the propeller’s theoretical top speed.
The result is that the propeller cannot reach its
maximum rpm because the extra airframe
drag increases the propeller’s air load.
For step three, the best propeller size for a
given engine, in a 40- to 60-size, high-drag
sport RC model, is that one compromise
between the largest diameter and highest pitch
that still allows the engine’s maximum ground
rpm to be roughly 1,000-1,500 rpm higher
than its high-torque (maximum twisting
power) rpm. (This figure is after the highspeed
mixture has been adjusted to be 500
rpm less than absolute peak.)
Why this rpm? Once the airplane is flying,
there is an average increase of 500 rpm. This
happens because the aircraft’s forward speed
acts to decrease the propeller’s AOA. Another
way to picture this is that the air “flowing”
into the propeller from the front is helping the
engine turn it. It isn’t, of course, but it is
decreasing the engine’s workload by reducing
the effective AOA.
An airplane stops flying faster when the
propeller’s AOA nears zero. But once the
aircraft’s nose is pointed skyward, more of its
weight is placed on the propeller and therefore
on the engine’s turning ability. The airspeed
decays, and those 500 “free” rpm disappear as
propeller drag increases with the escalating
AOA.
Increasing the fuel’s nitro content from 15% to 20% (using the same fuel with
additional nitro) raised the engine’s output by only 300 rpm.
Replacing idle-bar glow plug with same manufacturer’s standard plug
increased engine’s top rpm by 300—same gain as 5% nitro increase. But
standard plug does not increase operating costs or cause additional
engine wear.
09sig2.QXD 6/24/04 9:04 am Page 36
The stress of pulling the aircraft upward
increases the power demands on the engine. It
responds by turning more slowly, just as a
car’s engine does when going up a steep hill
until extra energy, in the form of stepping on
the gas, is applied. But the model engine is
already at full power; there is no extra “gas”
to give.
In fact, the engine’s rpm will drop until it
reaches its high-torque rpm. If the engine is
the right size for the airplane and the climb
angle is not steeper than what the
engine/airframe combination was designed to
maintain (usually at least 45°), the rpm
reduction stops here and the airplane
maintains a constant climb rate.
Why not use a propeller that allows the
engine to rotate at its peak horsepower rpm?
The horsepower ratings for most .40-.60 twostroke
engines are usually at so high an rpm
that they are nearly unusable for sport
applications. Most reach peak horsepower
well in excess of 13,000 rpm.
At this number, model pilots do not need
to worry about their airplanes’ performance
because most clubs won’t let them fly such
loud models. Even if they can fly them, the
propeller disc must be so small—7-9 inches—
that little thrust can be applied to the air (step
one). The result is an inefficient propeller, an
airplane flying roughly 35 mph, and a
screaming engine trying to tear itself apart.
Experienced RC modelers have known
this “great truth” for years, many times
without even knowing they know it, but they
have had no data to support their intuitive
propeller choices. So I set out to prove this
last step.
I used a relatively new tool to gather the
needed data. The RC Flight Data Recorder
manufactured by Eagle Tree Systems records
the airspeed, rate of climb, climb angle,
altitude, and servo performance during flight.
It can also record in-flight engine rpm and
temperatures, but these systems were not
installed on the test aircraft.
The recorder correlates flight data and
transmitter inputs and time. This lets the pilot
know what was happening and when. I have
been using this instrument for sometime when
evaluating aircraft for MA’s Sport Aviator
online magazine (www.masportaviator.com)
and have become familiar with interpreting
the reported data. I used my trusty, manyyear-
old SuperStar 40 trainer equipped with
an even older .45 engine.
This engine reaches its maximum
horsepower, 1.35 when new but now much
less, at roughly 15,000 rpm. Its torque curve
peaks near 10,000 rpm. I tested identicaldesign,
fiberglass-reinforced propellers in 10
x 6, 10 x 7, 11 x 6, and 11 x 7 sizes. The
Flight Test Results chart summarizes the tests.
38 MODEL AVIATION
Propeller Size Ground rpm Top Speed Takeoff Climbout 45 Degree Climb Approach Speed
Performance* Performance*
10 x 6 11,940 55 mph 1,100@28 1,200@6 33 mph
10 x 7 11,220 64 mph 1,400@32 1,500@31 35 mph
11 x 6 10,920 60 mph 1,500@25 1,800@28 31 mph
11 x 7 10,140 51 mph 1,200@27 1,200@26 24 mph
*Feet per minute climb at mph climb speed
Flight Test Results
Recognizing those who have defended,
protected, and supported the sport of
aeromodeling through their personal
contributions and devotion. Their
advocacy has long been the foundation
upon which the growth and advances in
the sport have been based, and for this
they are presented as true champions!
Ted Teach
A charter member of the Legion of Champions
and a continuing supporter
Legion of Champions
09sig2.QXD 6/24/04 12:13 pm Page 38
As shown, the highest ground rpm does
not translate into the fastest airspeed or the
best climb rate. Under the demands of a
climb, the smaller 10-inch propeller discs
could not transfer the engine’s power to the
air as effectively as the 11-inch discs could.
An inch may not seem to be a big
difference. However, the 11-inch propeller
has an effective area of 95 square inches
versus the 10-inch propeller’s 79 square
inches. The engine’s “force area” is 20%
larger using the 11-inch propeller.
The larger disc is the reason why the 11 x
6 propeller produced a 20% better climb rate
than the 10 x 7, despite the slower climb
speed. The extra power required to turn the 11
x 7 propeller when climbing proved more
than the ol’ engine possessed. Climb and top
speed suffered, but landing speed was the
slowest, probably because the idle speed was
less than 2,100 rpm. The 11 x 7 might cause
engine overheating in hot weather.
The 11 x 6 allowed the aircraft to leave the
ground in the shortest time at the slowest
airspeed, reducing airframe wear. It produced
a climb rate up to 50% higher, and its top
speed was only 6% less than the highest but
up to 18% higher than the remaining
propellers’. During your next visit to the
flying field, check the propellers on most .45
two-stroke engines. Most will be various
types of 11 x 6s.
Choose the propeller-and-glow-plug
combination that permits the engine to turn
the largest-diameter propeller approximately
1,000-1,500 rpm higher than its peak torque
speed on the ground. Start with the largest
diameter and lowest pitch recommended for
your engine. If the engine will not turn this
propeller fast enough, drop to the next smaller
diameter, again with the lowest pitch.
Increase the pitch if the engine turns too fast.
Continue until you find the right size
combination.
The .30-size engines will usually use a 9-
inch-diameter propeller, .40s will use up to
10.5 inches, .45s will use 11 inches, and .60s
work best with a 12-inch propeller.
Remember that this is for sport models only.
Glow Plugs: Did I also mention the glow
plug? It causes the fuel to burn and release its
energy. Fortunately there are only two types
of glow plugs that newer sport-model pilots
need to know about.
The idle-bar plug was once the only
design that provided a reliable idle. The metal
bar protected the glow element from unburnt
liquid fuel that otherwise cooled the element
to lower than the fuel’s ignition temperature
when at idle speeds. The bar itself became
extremely hot, adding protection to the glow
element’s idle temperature.
But today’s more powerful sport engines
are equipped with mufflers that preserve the
chamber’s heat at idle. Modern carburetors
allow finer adjustment of the fuel/air mixture,
reducing the amount of liquid, unburnt fuel
that enters the chamber at idle speeds.
Therefore, idle-bar glow plugs are not always
required on newer sport engines.
Photos show a non-idle-bar glow plug’s
performance advantage. All settings and
equipment remain the same, and the photos
were taken only minutes apart. The non-idlebar
plug produced a 300 rpm gain.
However, an idle-bar glow plug is a good
idea for two-strokes mounted with the
cylinder head pointed downward—called
inverted mounting—for engines that have
difficulty idling and for older, well-worn
engines that may need the extra heat to keep
the glow element hot. Consider using nonidle-
bar glow plugs for all other sport
applications.
Fuel: Choosing the right fuel is the last
critical factor to ensure that your new twostroke
engine gets the best performance and
longest life. The “right” fuel is also one of the
most controversial, opinion-rich, and
individualistic subjects in model aviation. But
there are some useful guidelines to remember.
Two-stroke fuel has three major
ingredients: methanol, castor oil, and
nitromenthane (nitro). Methanol comprises
60%-75% of most fuels. It burns completely
and adds to the fuel’s total energy output.
The lubricating oil comprises roughly
20% of the fuel. Most two-stroke fuels
contain two types of oil; 4%-8% is usually
castor and the remaining 12%-16% is a
synthetic that varies by manufacturer. Oils
typically do not burn completely, and what
small percentage of the oil that does burn
does so at lower energy levels than methanol.
Castor oil is used because it maintains a
lubricating film at higher temperatures than
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40 MODEL AVIATION
09sig2.QXD 6/24/04 9:04 am Page 40
most synthetic oils do. If the engine’s highspeed
fuel/air ratio is too lean—too much
air—the engine will run at high temperatures.
Castor oil will maintain lubrication in an
overheated engine; most synthetic oils burn
away.
Castor oil also helps remove heat from
the combustion chamber better than most
synthetics. Castor oil leaves a film residue in
the engine that offers some rust protection.
Most synthetic oils do not.
However, too much castor oil causes
excessive residue buildup that can diminish
an engine’s performance. Because castor oil
does not burn, it reduces the fuel’s total
energy output. For these reasons, the castor
oil percentage is usually kept at less than 8%.
Synthetic oils do burn, but not well or
completely. Little synthetic oil residue is left
inside an engine. Synthetics also offer
excellent engine lubrication when operating
at normal engine temperatures.
Because high oil content detracts from the
fuel’s total energy output, the easiest way to
increase an engine’s apparent power output
is to reduce the fuel’s oil content. However,
oil contents much less than 18% can cause
long-term wear problems in .40-.60 sport
engines.
Fuel manufacturers are studying new oils
that produce more power and offer better
protection with quantities as low as 16%. But
for now, consider using fuels with 18%-20%
oil content in newer or sophisticated
(expensive?) .40-.60 engines.
The third fuel component is nitromethane.
It burns at a higher energy level than
methanol. However, it also produces higher
combustion-chamber temperatures and
therefore needs to be limited. Most sport
fuels contain 5%-25% nitro. It prolongs the
combustion event. The burning process takes
longer, and that also produces more energy.
Many pilots overrate the power increase
obtained by “upping the nitro” in their sport
engine’s fuel. Photos show how little effect
higher nitro content has on sport engines.
Raising its content by 5% in a fuel produced
only a 300 rpm gain. But nitro does improve
an engine’s idling ability, permitting a lower
reliable idle speed. Therefore, consider nitro
contents in the 10%-15% range as a good
sport-flying compromise.
There is such a thing as too much nitro
content for a given engine. If the combustion
event becomes too prolonged, detonation
may occur. You may hear pinging, a sound
like frying eggs, or the exhaust note may
become very loose. Unless operating at high
altitudes—exceeding 5,000 feet—there is no
reason, and little gain, to use fuels with nitro
contents higher than 20%.
Next month I’ll cover some of the
differences in and advantages of four-stroke
engines. Since model engines do not run for
long periods without an onboard fuel supply,
I’ll cover fuel-tank choices and installation
requirements as well. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
42 MODEL AVIATION
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mounts.
• Double to triple flight time per charge.
**(see page 52, RCM, April 04)
• Proven isolator life approx. 7000 flights.
Only 25-75 is common with ALL others.
• Initial cost comparison of a Hyde Mount to value of
equipment saved. There really is no comparison!!!!
• In the Winners Circle over 2000 times.
* Simply return within one year with verifiable test data that a
Hyde Mount is not the best overall for major components of
your plane or engine.
** Referenced article clearly links high current drain to vibration
that results in equipment failure.
$64.95 - $284.95 + $7.00 S&H.
Orders/info: Merle Hyde, 3 Golf View
Drive, Henderson, NV 89074
Ph/fax: 702-269-7829 or e-mail:
[email protected]
3 Years/3000 flights complete
satisfaction money back guarantee
(a 3 year trial offer)
*plus, double refund trial offer
Over 132 types, styles, and sizes available
for all engines .049-20.0 cu. in.
SPECIAL: 40-70% off
.20/.30 - $39.95
.40/.50 - $49.95
.60/.70 - $59.95
$99.95
any large engine
backplate mount
PATENTED
SINGLE
ISOLATOR
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