Our aeromodeling skills serve us in areas besides aircraft
February 2008 73
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• The difference between wet
and dry power
The contrarotating propeller shown is by Michael Ramel. Electric contrarotators have
exciting applications in Scale and Aerobatics. The EF-503 unit is not yet for sale in the US.
IF IT FLIES, other people are interested in
it too! It tickles me when I see all sorts of
fliers in the hobby/sport who, like me, seem
unable to stay between the lines. They
dabble, at the very least, in more than one
segment of aeromodeling. That failure to
“stay between the lines” is something we
share, whether we recognize it or not.
If we really stayed between the
proverbial lines, we probably wouldn’t be
building or flying model airplanes at all. It’s
not something people you meet every day
do. What percentage of the general
population do you figure has anything to do e
02sig3.QXD 12/20/07 12:18 PM Page 73with model airplanes? Maybe one in 1,000?
I’ll bet it isn’t even that many.
We’re a bit different—“plane” crazy,
you could say! We stand out from the
general population not just because we fly
model airplanes, but because we build
them—and rebuild them! We’re different
because some of us design them and because
many of us teach others to fly them.
This “wrapping our arms around” the
many skills and bits of expertise that make
up aeromodeling is uncommon these days.
They include many useful things, and the
more different corners of aeromodeling you
look into, the more of these “outside-thelines”
skills you will accumulate.
The other day my neighbor was edging
his lawn. As I wrestled with my Autumntime
foe, the swimming pool cover, I could
hear the edger in the next yard bogging
down lean. That sound normally bugs me
because it means a spoiled flight and an
early landing. I can identify it from 500 feet
away, and you can too if you are an
experienced wet-power flier. Right?
This time that sound meant I had an
excuse to grab a screwdriver, walk next
door, motion to my ear-protector-clad
neighbor as he stopped to greet me, and
deftly twist the high-speed mixture screw
out one-quarter turn. I watched and listened
only long enough to verify that the job was
done, and then I returned to the back yard
smug, satisfied, and ready for a rematch
with my big, green fabric nemesis.
That is a skill any reasonably successful
wet flier takes for granted! Normal people
would have taken that power tool to a smallengine
and mower-repair shop and spent a
meaningful amount of money to have it
“fixed.” Forget that!
That’s right; we aeromodelers are
different that way, but for some reason we
also stand out as being different from each
other.
“Hey Dean, you fly wet and electric
both, right?” my clubmate, Dave T., asked a
couple months ago. “Maybe you should
write something about the differences
between wet and electric flight.”
Dave’s suggestion was helpful, and I’ll
get to it shortly, but as I started to
contemplate that subject, a shoe wedged
itself between the gears in my noggin. I did
not think being heavily involved with both
wet and electric power was such a
distinction, but, in fact, I had gone through
quite a learning curve in the last couple
years!
At least in the clubs I belonged to, until
just a few years ago electric fliers were a
distinct group; but that isn’t the case
anymore. Now it seems like almost every
wet flier has a flat foamie or park flyer in
the hangar, although relatively few of those
fliers mess with larger, high-performance
electrics.
Meanwhile, there are fliers who have
spent years in the sport, becoming experts,
without ever owning a glow or gas engine.
Could you have imagined that just 10 or so
years ago? I didn’t.
The Pedagogical Problem: There must be
people looking to make that transition from
wet power to high-performance electric, and,
even more interestingly, there are
accomplished fliers who have only a vague
idea of how to properly set up a glow engine.
I’ll describe that shoe in the mental gears I
mentioned. It’s the problem you run into
when you try to explain something basic to
someone who is already accomplished at
something else.
It’s easy to insult someone without
meaning to. When we step out of our
personal comfort zone, or area of expertise,
we become beginners. It can either be a pain
in the neck or an opportunity for discovery
and fun.
So I’ll take Dave’s suggestion and look at
the differences between setup and technique
for electric and glow flight. If it gets basic,
please think of it as a reminder of all the
many useful things you’ve mastered without
thinking much about it.
I’m going to write a bit about the big
picture. I’ll start with a discussion about how
wet and electric power have different
characteristics, which has a great deal to do
with how their torque vs. rpm characteristics
are shaped.
Later we can get bogged down in all sorts
of specific stuff, making comparisons
between electric and wet power as they apply
74 MODEL AVIATION
02sig3.QXD 12/20/07 12:18 PM Page 74to many styles of flying. There’s a lot of it, so
I’ll probably chop the subject up into little
bits from month to month.
I’ll start by describing a glow engine’s
torque characteristics. Within their preferred
operating rpm range, glow engines make
more horsepower as they turn faster. When
you put slightly less propeller on them and let
them rev up some, you get more
performance.
That’s true only as long as the propeller is
fairly well matched to the airplane. If you
overdo this propeller reduction, you get noise
with no added performance.
The funny thing is that more noise fools
many pilots into thinking they have more
performance. This touches base with the
human-factors part of the discussion months
ago about making your airplanes quiet. That
loose end is still dangling.
A glow engine typically has a horsepower
peak at roughly 15,000 rpm. Depending on
the engine and muffler design, it will have a
torque peak at a much lower rpm; maybe it
happens at roughly two-thirds of the
horsepower peak rpm. Quieter mufflers often
bring the peak power rpm down a bunch,
while affecting the torque peak rpm
somewhat less.
The “trick” for getting a friendly engine
setup is to use propeller size that has enough
pitch to fly at the desired speed when the rpm
is in the middle of the engine’s rpm sweet
spot and enough diameter to load the engine
76 MODEL AVIATION
down to just above the torque peak at a
standstill. Loaded that way, takeoff
performance will be strong and the engine
will unload into that sweet spot between
maximum torque and maximum
horsepower while flying around.
An accompanying diagram shows this
relationship, but it is just a picture until
we explore what it means in comparison
to the same picture for an electric motor.
Electrics have an entirely different
torque vs. rpm curve. Once you have
chosen how many cells in series you will
use, if you want more power from an
electric you must lower the rpm by
putting a bigger propeller load on it! If
you are familiar with electric power, this
comes as no surprise. But think of the
huge change in thinking this represents
for many of us.
Revving up has always meant more
power; it’s almost programmed into our
DNA. But with electrics you have to grunt
to make more power. (There’s a Tim
Taylor joke in there somewhere.)
When you look at the electric torque
vs. rpm curve, you see that it’s not a curve
at all, but an almost perfect straight line.
Torque and current draw fall linearly with
increased rpm.
Horsepower is not a straight line,
though. The horsepower curve is a nice,
neat parabola, with a peak value that is
measured in watts. There is no magic to
that; there are 746 watts per horsepower,
so watts and horsepower are really the
same thing.
(When you see “Ps” in the specifications
for a model engine in an advertisement,
that’s the symbol for a metric horsepower,
or one Pferdestarke. It’s approximately 735
watts, so the difference between a US
horsepower and a metric horsepower is
only a bit less than 1.5%.)
The rpm where the horsepower peak
would occur is at half the no-load rpm.
That’s true with a motor, but operating
there may be impractical. The no-load rpm,
at the right side of the diagram, is the same
as the Kv of the motor multiplied by the
battery voltage.
The motor constant Kv is expressed in
rpm per volt—not kilovolts! I don’t know
how many times I see that boo-boo in
advertisements and catalogs.
Operating Within the Happy Zone: Every
engine design has an rpm range within
which it will run happily. Yes, “happily.” I
realize that isn’t a precise term, but it will
have to do for now. There are engines
intended for normal sport use and engines
designed for high-rpm use, such as racing.
With all the necessary emphasis on
flying-site retention these days, there are
more engines optimized to make gobs of
torque at low and moderate rpm, for the
purpose of turning large propellers quietly.
That’s a good thing, and the side benefit
is that many fliers will learn that larger
propellers fly better. There are notable
exceptions to this general rule, and that
02sig3.QXD 12/20/07 12:18 PM Page 76means we have yet another loose end to tend
to some other time.
Look at the torque curve presented for a
muffled engine. It is mostly flat or gently
sloping downward with higher rpm for a
healthy portion of the engine’s usable rpm
range. That’s the part of the range between
the torque peak and the horsepower peak.
This is the rpm range where the engine is
going to be most forgiving. Stable torque
readings mean the engine is breathing
properly.
Because any propeller changes load
characteristics as the airplane’s speed
changes, glow (and gas) engines will see a
substantial rpm change between running on
the ground, climbing, or in a high-speed
pass down the runway. The rpm can change
as much as 20% and sometimes more.
That’s because the load the propeller
presents to the engine drops as the airplane
goes faster. (Yes, there are exceptions to the
last statement!)
How do you keep the engine running in
the sweet spot? By changing the propeller’s
pitch and diameter. If the propeller has too
little pitch for the airspeed at which your
airplane is intended to fly, the load will drop
off dramatically during a high-speed flyby.
As a result, the rpm will rise substantially—
maybe even past the peak horsepower rpm.
That’s not terribly useful and it makes a
bunch of noise. This is actually how many
competition fun-fly and 3-D airplanes are
set up intentionally, but they are never flown
at high speed because they tend to flutter
and explode!
When selecting a propeller for one of
these 3-D setups, a very low pitch is chosen
and then the diameter is adjusted to load the
engine to near the torque peak. Why?
Because torque provides the grunt needed
for hovering.
For flying, as opposed to hovering, start
by picking a pitch that will fly at the hopedfor
airspeed when the rpm is maybe 10%
higher than the ground rpm. Most glow
engines intended for sport have torque peaks
of roughly 10,000-11,000 rpm, so we tend to
load them to 11,000 or 12,000 rpm. That’s
approximately 200 revolutions per second.
Let’s say you want to fly at roughly 85
mph, which works out to 110 feet per
second. The pitch needs to be nearly 6 or 7
inches. The arithmetic is (110 feet per
second divided by 200 revs per second)
multiplied by (12 inches per foot). I got 6.6
inches of pitch. I’d start with a 7-inch pitch
and then add as much diameter as I can
before the engine “lugs,” or bogs, at full
throttle on the ground.
With an electric setup the propeller
selection is done similarly, but first you
have to figure out what your motor’s
running rpm will be. Estimate the voltage of
your batteries. Let’s say you are using a 6S,
or six Li-Poly cells in series. That works out
to 22.2 volts.
Reduce that figure by approximately
10% to account for the voltage lost across
the winding resistance of the motor. We
could calculate it more accurately, but that is
for another day. Let’s say we have 18 volts
left.
Multiply that by the Kv of the motor, and
that will be the running rpm, give or take a
few percent. Now pick the pitch the same as
in the preceding. But here comes the
difference.
Assuming you selected the correct pitch
for the desired flying speed and your
motor’s realistic full-throttle rpm, you must
pick the diameter to get the desired current
draw. If the diameter is too big, the current
draw goes way up. If the diameter is too
small, the current draw (or horsepower, or
watts, or Pferdestarkes, or whatever!) will
be too low to fly the airplane properly. How
do you choose how much current draw you
want?
From a practical point of view, you want
only as much as is needed to get satisfactory
performance. You would probably start
slightly small on diameter and work your
way up until the climb performance is
acceptable.
There are good online electric-flight
calculators to help you make a good first
guess at propeller size. Many motor
manufacturers have them on their Web sites,
and there are computer programs you can
buy. There are other considerations when
choosing how high of a current to run with
electric power.
The diagram for the motor has an extra
78 MODEL AVIATION
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curve on it, labeled efficiency. We
typically choose full-throttle rpm so the
current and efficiency are within
reasonable bounds.
In this case you might choose a
maximum current based on how long you
want to be able to fly without using more
than roughly three-quarters of the battery,
or you might choose the maximum current
based on your batteries’ “C rating.” You
also need to look at your motor’s
maximum current rating. Exceeding any of
these will shorten the life of your
equipment.
If you are interested in building an
electric model to break a duration record,
you will aim for the current and rpm that
give the best efficiency. If you look at the
electric diagram, you will see that this
happens at relatively low current.
If you are building an airplane for CL
electric Speed, where less than a half
minute at full throttle is needed, you will
probably want to crank the current all the
way up to the C rating of your battery or
the short-term maximum current rating of
your motor.
Overload characteristics, or what
happens if you overload the system with
too much propeller? If the glow engine is
overloaded, it will turn at much less than
the torque peak rpm. This is on the left side
of the diagram.
The drop in torque happens because the
engine can’t breathe properly at such a low
speed. That’s a long subject, and I will
probably try to draft an expert to write
about it someday.
Because the engine is not breathing
properly, the fuel-to-air mixture becomes
inconsistent. The symptom is that the
needle valve becomes difficult to set, and
the setting often changes dramatically as
the engine unloads in the air. If you do get
the right mixture, takeoff and climb
performance will suffer. Throttle response
usually suffers in this case, especially
when you punch the throttle for a goaround
on landing.
Overloaded electrics are easy to spot—
by the smell. It’s the smell of melting
motor windings, blown ESCs, and
overheated batteries. There’s no need to
discuss this further!
In the next column I will describe how the
running characteristics of wet and electrics
affect their use in a variety of applications
from CL Stunt, to RC sport, even to FF.
There may not be an inherent advantage to
one or the other, but they have their strong
points and vulnerabilities, and different
applications highlight them well.
Until next time, go fly an airplane—any
kind of airplane! MA
Sources:
Michael Ramel
www.f3a-e-factor.de/eng/
DIVERSIFIED SOLUTIONS, LLC.
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80 MODEL AVIATION
02sig3.QXD 12/20/07 12:20 PM Page 80
Edition: Model Aviation - 2008/02
Page Numbers: 73,74,76,78,80
Edition: Model Aviation - 2008/02
Page Numbers: 73,74,76,78,80
Our aeromodeling skills serve us in areas besides aircraft
February 2008 73
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• The difference between wet
and dry power
The contrarotating propeller shown is by Michael Ramel. Electric contrarotators have
exciting applications in Scale and Aerobatics. The EF-503 unit is not yet for sale in the US.
IF IT FLIES, other people are interested in
it too! It tickles me when I see all sorts of
fliers in the hobby/sport who, like me, seem
unable to stay between the lines. They
dabble, at the very least, in more than one
segment of aeromodeling. That failure to
“stay between the lines” is something we
share, whether we recognize it or not.
If we really stayed between the
proverbial lines, we probably wouldn’t be
building or flying model airplanes at all. It’s
not something people you meet every day
do. What percentage of the general
population do you figure has anything to do e
02sig3.QXD 12/20/07 12:18 PM Page 73with model airplanes? Maybe one in 1,000?
I’ll bet it isn’t even that many.
We’re a bit different—“plane” crazy,
you could say! We stand out from the
general population not just because we fly
model airplanes, but because we build
them—and rebuild them! We’re different
because some of us design them and because
many of us teach others to fly them.
This “wrapping our arms around” the
many skills and bits of expertise that make
up aeromodeling is uncommon these days.
They include many useful things, and the
more different corners of aeromodeling you
look into, the more of these “outside-thelines”
skills you will accumulate.
The other day my neighbor was edging
his lawn. As I wrestled with my Autumntime
foe, the swimming pool cover, I could
hear the edger in the next yard bogging
down lean. That sound normally bugs me
because it means a spoiled flight and an
early landing. I can identify it from 500 feet
away, and you can too if you are an
experienced wet-power flier. Right?
This time that sound meant I had an
excuse to grab a screwdriver, walk next
door, motion to my ear-protector-clad
neighbor as he stopped to greet me, and
deftly twist the high-speed mixture screw
out one-quarter turn. I watched and listened
only long enough to verify that the job was
done, and then I returned to the back yard
smug, satisfied, and ready for a rematch
with my big, green fabric nemesis.
That is a skill any reasonably successful
wet flier takes for granted! Normal people
would have taken that power tool to a smallengine
and mower-repair shop and spent a
meaningful amount of money to have it
“fixed.” Forget that!
That’s right; we aeromodelers are
different that way, but for some reason we
also stand out as being different from each
other.
“Hey Dean, you fly wet and electric
both, right?” my clubmate, Dave T., asked a
couple months ago. “Maybe you should
write something about the differences
between wet and electric flight.”
Dave’s suggestion was helpful, and I’ll
get to it shortly, but as I started to
contemplate that subject, a shoe wedged
itself between the gears in my noggin. I did
not think being heavily involved with both
wet and electric power was such a
distinction, but, in fact, I had gone through
quite a learning curve in the last couple
years!
At least in the clubs I belonged to, until
just a few years ago electric fliers were a
distinct group; but that isn’t the case
anymore. Now it seems like almost every
wet flier has a flat foamie or park flyer in
the hangar, although relatively few of those
fliers mess with larger, high-performance
electrics.
Meanwhile, there are fliers who have
spent years in the sport, becoming experts,
without ever owning a glow or gas engine.
Could you have imagined that just 10 or so
years ago? I didn’t.
The Pedagogical Problem: There must be
people looking to make that transition from
wet power to high-performance electric, and,
even more interestingly, there are
accomplished fliers who have only a vague
idea of how to properly set up a glow engine.
I’ll describe that shoe in the mental gears I
mentioned. It’s the problem you run into
when you try to explain something basic to
someone who is already accomplished at
something else.
It’s easy to insult someone without
meaning to. When we step out of our
personal comfort zone, or area of expertise,
we become beginners. It can either be a pain
in the neck or an opportunity for discovery
and fun.
So I’ll take Dave’s suggestion and look at
the differences between setup and technique
for electric and glow flight. If it gets basic,
please think of it as a reminder of all the
many useful things you’ve mastered without
thinking much about it.
I’m going to write a bit about the big
picture. I’ll start with a discussion about how
wet and electric power have different
characteristics, which has a great deal to do
with how their torque vs. rpm characteristics
are shaped.
Later we can get bogged down in all sorts
of specific stuff, making comparisons
between electric and wet power as they apply
74 MODEL AVIATION
02sig3.QXD 12/20/07 12:18 PM Page 74to many styles of flying. There’s a lot of it, so
I’ll probably chop the subject up into little
bits from month to month.
I’ll start by describing a glow engine’s
torque characteristics. Within their preferred
operating rpm range, glow engines make
more horsepower as they turn faster. When
you put slightly less propeller on them and let
them rev up some, you get more
performance.
That’s true only as long as the propeller is
fairly well matched to the airplane. If you
overdo this propeller reduction, you get noise
with no added performance.
The funny thing is that more noise fools
many pilots into thinking they have more
performance. This touches base with the
human-factors part of the discussion months
ago about making your airplanes quiet. That
loose end is still dangling.
A glow engine typically has a horsepower
peak at roughly 15,000 rpm. Depending on
the engine and muffler design, it will have a
torque peak at a much lower rpm; maybe it
happens at roughly two-thirds of the
horsepower peak rpm. Quieter mufflers often
bring the peak power rpm down a bunch,
while affecting the torque peak rpm
somewhat less.
The “trick” for getting a friendly engine
setup is to use propeller size that has enough
pitch to fly at the desired speed when the rpm
is in the middle of the engine’s rpm sweet
spot and enough diameter to load the engine
76 MODEL AVIATION
down to just above the torque peak at a
standstill. Loaded that way, takeoff
performance will be strong and the engine
will unload into that sweet spot between
maximum torque and maximum
horsepower while flying around.
An accompanying diagram shows this
relationship, but it is just a picture until
we explore what it means in comparison
to the same picture for an electric motor.
Electrics have an entirely different
torque vs. rpm curve. Once you have
chosen how many cells in series you will
use, if you want more power from an
electric you must lower the rpm by
putting a bigger propeller load on it! If
you are familiar with electric power, this
comes as no surprise. But think of the
huge change in thinking this represents
for many of us.
Revving up has always meant more
power; it’s almost programmed into our
DNA. But with electrics you have to grunt
to make more power. (There’s a Tim
Taylor joke in there somewhere.)
When you look at the electric torque
vs. rpm curve, you see that it’s not a curve
at all, but an almost perfect straight line.
Torque and current draw fall linearly with
increased rpm.
Horsepower is not a straight line,
though. The horsepower curve is a nice,
neat parabola, with a peak value that is
measured in watts. There is no magic to
that; there are 746 watts per horsepower,
so watts and horsepower are really the
same thing.
(When you see “Ps” in the specifications
for a model engine in an advertisement,
that’s the symbol for a metric horsepower,
or one Pferdestarke. It’s approximately 735
watts, so the difference between a US
horsepower and a metric horsepower is
only a bit less than 1.5%.)
The rpm where the horsepower peak
would occur is at half the no-load rpm.
That’s true with a motor, but operating
there may be impractical. The no-load rpm,
at the right side of the diagram, is the same
as the Kv of the motor multiplied by the
battery voltage.
The motor constant Kv is expressed in
rpm per volt—not kilovolts! I don’t know
how many times I see that boo-boo in
advertisements and catalogs.
Operating Within the Happy Zone: Every
engine design has an rpm range within
which it will run happily. Yes, “happily.” I
realize that isn’t a precise term, but it will
have to do for now. There are engines
intended for normal sport use and engines
designed for high-rpm use, such as racing.
With all the necessary emphasis on
flying-site retention these days, there are
more engines optimized to make gobs of
torque at low and moderate rpm, for the
purpose of turning large propellers quietly.
That’s a good thing, and the side benefit
is that many fliers will learn that larger
propellers fly better. There are notable
exceptions to this general rule, and that
02sig3.QXD 12/20/07 12:18 PM Page 76means we have yet another loose end to tend
to some other time.
Look at the torque curve presented for a
muffled engine. It is mostly flat or gently
sloping downward with higher rpm for a
healthy portion of the engine’s usable rpm
range. That’s the part of the range between
the torque peak and the horsepower peak.
This is the rpm range where the engine is
going to be most forgiving. Stable torque
readings mean the engine is breathing
properly.
Because any propeller changes load
characteristics as the airplane’s speed
changes, glow (and gas) engines will see a
substantial rpm change between running on
the ground, climbing, or in a high-speed
pass down the runway. The rpm can change
as much as 20% and sometimes more.
That’s because the load the propeller
presents to the engine drops as the airplane
goes faster. (Yes, there are exceptions to the
last statement!)
How do you keep the engine running in
the sweet spot? By changing the propeller’s
pitch and diameter. If the propeller has too
little pitch for the airspeed at which your
airplane is intended to fly, the load will drop
off dramatically during a high-speed flyby.
As a result, the rpm will rise substantially—
maybe even past the peak horsepower rpm.
That’s not terribly useful and it makes a
bunch of noise. This is actually how many
competition fun-fly and 3-D airplanes are
set up intentionally, but they are never flown
at high speed because they tend to flutter
and explode!
When selecting a propeller for one of
these 3-D setups, a very low pitch is chosen
and then the diameter is adjusted to load the
engine to near the torque peak. Why?
Because torque provides the grunt needed
for hovering.
For flying, as opposed to hovering, start
by picking a pitch that will fly at the hopedfor
airspeed when the rpm is maybe 10%
higher than the ground rpm. Most glow
engines intended for sport have torque peaks
of roughly 10,000-11,000 rpm, so we tend to
load them to 11,000 or 12,000 rpm. That’s
approximately 200 revolutions per second.
Let’s say you want to fly at roughly 85
mph, which works out to 110 feet per
second. The pitch needs to be nearly 6 or 7
inches. The arithmetic is (110 feet per
second divided by 200 revs per second)
multiplied by (12 inches per foot). I got 6.6
inches of pitch. I’d start with a 7-inch pitch
and then add as much diameter as I can
before the engine “lugs,” or bogs, at full
throttle on the ground.
With an electric setup the propeller
selection is done similarly, but first you
have to figure out what your motor’s
running rpm will be. Estimate the voltage of
your batteries. Let’s say you are using a 6S,
or six Li-Poly cells in series. That works out
to 22.2 volts.
Reduce that figure by approximately
10% to account for the voltage lost across
the winding resistance of the motor. We
could calculate it more accurately, but that is
for another day. Let’s say we have 18 volts
left.
Multiply that by the Kv of the motor, and
that will be the running rpm, give or take a
few percent. Now pick the pitch the same as
in the preceding. But here comes the
difference.
Assuming you selected the correct pitch
for the desired flying speed and your
motor’s realistic full-throttle rpm, you must
pick the diameter to get the desired current
draw. If the diameter is too big, the current
draw goes way up. If the diameter is too
small, the current draw (or horsepower, or
watts, or Pferdestarkes, or whatever!) will
be too low to fly the airplane properly. How
do you choose how much current draw you
want?
From a practical point of view, you want
only as much as is needed to get satisfactory
performance. You would probably start
slightly small on diameter and work your
way up until the climb performance is
acceptable.
There are good online electric-flight
calculators to help you make a good first
guess at propeller size. Many motor
manufacturers have them on their Web sites,
and there are computer programs you can
buy. There are other considerations when
choosing how high of a current to run with
electric power.
The diagram for the motor has an extra
78 MODEL AVIATION
02sig3.QXD 12/20/07 1:10 PM Page 78Introduces NEW!
SUPER COOL Plug with
Hi Temp Insulator
In addition to . . .
● The FIREBALL R/C IDLE BAR plug
only $3.20
● Hot & Standard Non-Idle Bar plugs
still only $2.85
only $3.20
Swanson
Associates
P.O. Box 151
Wayne, NJ
07470
Since 1948
curve on it, labeled efficiency. We
typically choose full-throttle rpm so the
current and efficiency are within
reasonable bounds.
In this case you might choose a
maximum current based on how long you
want to be able to fly without using more
than roughly three-quarters of the battery,
or you might choose the maximum current
based on your batteries’ “C rating.” You
also need to look at your motor’s
maximum current rating. Exceeding any of
these will shorten the life of your
equipment.
If you are interested in building an
electric model to break a duration record,
you will aim for the current and rpm that
give the best efficiency. If you look at the
electric diagram, you will see that this
happens at relatively low current.
If you are building an airplane for CL
electric Speed, where less than a half
minute at full throttle is needed, you will
probably want to crank the current all the
way up to the C rating of your battery or
the short-term maximum current rating of
your motor.
Overload characteristics, or what
happens if you overload the system with
too much propeller? If the glow engine is
overloaded, it will turn at much less than
the torque peak rpm. This is on the left side
of the diagram.
The drop in torque happens because the
engine can’t breathe properly at such a low
speed. That’s a long subject, and I will
probably try to draft an expert to write
about it someday.
Because the engine is not breathing
properly, the fuel-to-air mixture becomes
inconsistent. The symptom is that the
needle valve becomes difficult to set, and
the setting often changes dramatically as
the engine unloads in the air. If you do get
the right mixture, takeoff and climb
performance will suffer. Throttle response
usually suffers in this case, especially
when you punch the throttle for a goaround
on landing.
Overloaded electrics are easy to spot—
by the smell. It’s the smell of melting
motor windings, blown ESCs, and
overheated batteries. There’s no need to
discuss this further!
In the next column I will describe how the
running characteristics of wet and electrics
affect their use in a variety of applications
from CL Stunt, to RC sport, even to FF.
There may not be an inherent advantage to
one or the other, but they have their strong
points and vulnerabilities, and different
applications highlight them well.
Until next time, go fly an airplane—any
kind of airplane! MA
Sources:
Michael Ramel
www.f3a-e-factor.de/eng/
DIVERSIFIED SOLUTIONS, LLC.
5932 Chicago Ave. South, Minneapolis, MN 55417
Ph: 1-612-243-1234 Fax: 1-612-243-8950
Email: [email protected] • Web: www.klasskote.com
For Color Chart and Information, Send SASE
Don’t Delay – Order Yours Today!
You Built the Best Model, So Use The Best Paint!
“Superior
Quality”
Epoxy Paint
System
Available in
Colors, Clear
& Primer.
38 Years of Extensive Field
Performance Provides
Outstanding Adhesion & Protection Against Many RC Model Fuels
80 MODEL AVIATION
02sig3.QXD 12/20/07 12:20 PM Page 80
Edition: Model Aviation - 2008/02
Page Numbers: 73,74,76,78,80
Our aeromodeling skills serve us in areas besides aircraft
February 2008 73
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• The difference between wet
and dry power
The contrarotating propeller shown is by Michael Ramel. Electric contrarotators have
exciting applications in Scale and Aerobatics. The EF-503 unit is not yet for sale in the US.
IF IT FLIES, other people are interested in
it too! It tickles me when I see all sorts of
fliers in the hobby/sport who, like me, seem
unable to stay between the lines. They
dabble, at the very least, in more than one
segment of aeromodeling. That failure to
“stay between the lines” is something we
share, whether we recognize it or not.
If we really stayed between the
proverbial lines, we probably wouldn’t be
building or flying model airplanes at all. It’s
not something people you meet every day
do. What percentage of the general
population do you figure has anything to do e
02sig3.QXD 12/20/07 12:18 PM Page 73with model airplanes? Maybe one in 1,000?
I’ll bet it isn’t even that many.
We’re a bit different—“plane” crazy,
you could say! We stand out from the
general population not just because we fly
model airplanes, but because we build
them—and rebuild them! We’re different
because some of us design them and because
many of us teach others to fly them.
This “wrapping our arms around” the
many skills and bits of expertise that make
up aeromodeling is uncommon these days.
They include many useful things, and the
more different corners of aeromodeling you
look into, the more of these “outside-thelines”
skills you will accumulate.
The other day my neighbor was edging
his lawn. As I wrestled with my Autumntime
foe, the swimming pool cover, I could
hear the edger in the next yard bogging
down lean. That sound normally bugs me
because it means a spoiled flight and an
early landing. I can identify it from 500 feet
away, and you can too if you are an
experienced wet-power flier. Right?
This time that sound meant I had an
excuse to grab a screwdriver, walk next
door, motion to my ear-protector-clad
neighbor as he stopped to greet me, and
deftly twist the high-speed mixture screw
out one-quarter turn. I watched and listened
only long enough to verify that the job was
done, and then I returned to the back yard
smug, satisfied, and ready for a rematch
with my big, green fabric nemesis.
That is a skill any reasonably successful
wet flier takes for granted! Normal people
would have taken that power tool to a smallengine
and mower-repair shop and spent a
meaningful amount of money to have it
“fixed.” Forget that!
That’s right; we aeromodelers are
different that way, but for some reason we
also stand out as being different from each
other.
“Hey Dean, you fly wet and electric
both, right?” my clubmate, Dave T., asked a
couple months ago. “Maybe you should
write something about the differences
between wet and electric flight.”
Dave’s suggestion was helpful, and I’ll
get to it shortly, but as I started to
contemplate that subject, a shoe wedged
itself between the gears in my noggin. I did
not think being heavily involved with both
wet and electric power was such a
distinction, but, in fact, I had gone through
quite a learning curve in the last couple
years!
At least in the clubs I belonged to, until
just a few years ago electric fliers were a
distinct group; but that isn’t the case
anymore. Now it seems like almost every
wet flier has a flat foamie or park flyer in
the hangar, although relatively few of those
fliers mess with larger, high-performance
electrics.
Meanwhile, there are fliers who have
spent years in the sport, becoming experts,
without ever owning a glow or gas engine.
Could you have imagined that just 10 or so
years ago? I didn’t.
The Pedagogical Problem: There must be
people looking to make that transition from
wet power to high-performance electric, and,
even more interestingly, there are
accomplished fliers who have only a vague
idea of how to properly set up a glow engine.
I’ll describe that shoe in the mental gears I
mentioned. It’s the problem you run into
when you try to explain something basic to
someone who is already accomplished at
something else.
It’s easy to insult someone without
meaning to. When we step out of our
personal comfort zone, or area of expertise,
we become beginners. It can either be a pain
in the neck or an opportunity for discovery
and fun.
So I’ll take Dave’s suggestion and look at
the differences between setup and technique
for electric and glow flight. If it gets basic,
please think of it as a reminder of all the
many useful things you’ve mastered without
thinking much about it.
I’m going to write a bit about the big
picture. I’ll start with a discussion about how
wet and electric power have different
characteristics, which has a great deal to do
with how their torque vs. rpm characteristics
are shaped.
Later we can get bogged down in all sorts
of specific stuff, making comparisons
between electric and wet power as they apply
74 MODEL AVIATION
02sig3.QXD 12/20/07 12:18 PM Page 74to many styles of flying. There’s a lot of it, so
I’ll probably chop the subject up into little
bits from month to month.
I’ll start by describing a glow engine’s
torque characteristics. Within their preferred
operating rpm range, glow engines make
more horsepower as they turn faster. When
you put slightly less propeller on them and let
them rev up some, you get more
performance.
That’s true only as long as the propeller is
fairly well matched to the airplane. If you
overdo this propeller reduction, you get noise
with no added performance.
The funny thing is that more noise fools
many pilots into thinking they have more
performance. This touches base with the
human-factors part of the discussion months
ago about making your airplanes quiet. That
loose end is still dangling.
A glow engine typically has a horsepower
peak at roughly 15,000 rpm. Depending on
the engine and muffler design, it will have a
torque peak at a much lower rpm; maybe it
happens at roughly two-thirds of the
horsepower peak rpm. Quieter mufflers often
bring the peak power rpm down a bunch,
while affecting the torque peak rpm
somewhat less.
The “trick” for getting a friendly engine
setup is to use propeller size that has enough
pitch to fly at the desired speed when the rpm
is in the middle of the engine’s rpm sweet
spot and enough diameter to load the engine
76 MODEL AVIATION
down to just above the torque peak at a
standstill. Loaded that way, takeoff
performance will be strong and the engine
will unload into that sweet spot between
maximum torque and maximum
horsepower while flying around.
An accompanying diagram shows this
relationship, but it is just a picture until
we explore what it means in comparison
to the same picture for an electric motor.
Electrics have an entirely different
torque vs. rpm curve. Once you have
chosen how many cells in series you will
use, if you want more power from an
electric you must lower the rpm by
putting a bigger propeller load on it! If
you are familiar with electric power, this
comes as no surprise. But think of the
huge change in thinking this represents
for many of us.
Revving up has always meant more
power; it’s almost programmed into our
DNA. But with electrics you have to grunt
to make more power. (There’s a Tim
Taylor joke in there somewhere.)
When you look at the electric torque
vs. rpm curve, you see that it’s not a curve
at all, but an almost perfect straight line.
Torque and current draw fall linearly with
increased rpm.
Horsepower is not a straight line,
though. The horsepower curve is a nice,
neat parabola, with a peak value that is
measured in watts. There is no magic to
that; there are 746 watts per horsepower,
so watts and horsepower are really the
same thing.
(When you see “Ps” in the specifications
for a model engine in an advertisement,
that’s the symbol for a metric horsepower,
or one Pferdestarke. It’s approximately 735
watts, so the difference between a US
horsepower and a metric horsepower is
only a bit less than 1.5%.)
The rpm where the horsepower peak
would occur is at half the no-load rpm.
That’s true with a motor, but operating
there may be impractical. The no-load rpm,
at the right side of the diagram, is the same
as the Kv of the motor multiplied by the
battery voltage.
The motor constant Kv is expressed in
rpm per volt—not kilovolts! I don’t know
how many times I see that boo-boo in
advertisements and catalogs.
Operating Within the Happy Zone: Every
engine design has an rpm range within
which it will run happily. Yes, “happily.” I
realize that isn’t a precise term, but it will
have to do for now. There are engines
intended for normal sport use and engines
designed for high-rpm use, such as racing.
With all the necessary emphasis on
flying-site retention these days, there are
more engines optimized to make gobs of
torque at low and moderate rpm, for the
purpose of turning large propellers quietly.
That’s a good thing, and the side benefit
is that many fliers will learn that larger
propellers fly better. There are notable
exceptions to this general rule, and that
02sig3.QXD 12/20/07 12:18 PM Page 76means we have yet another loose end to tend
to some other time.
Look at the torque curve presented for a
muffled engine. It is mostly flat or gently
sloping downward with higher rpm for a
healthy portion of the engine’s usable rpm
range. That’s the part of the range between
the torque peak and the horsepower peak.
This is the rpm range where the engine is
going to be most forgiving. Stable torque
readings mean the engine is breathing
properly.
Because any propeller changes load
characteristics as the airplane’s speed
changes, glow (and gas) engines will see a
substantial rpm change between running on
the ground, climbing, or in a high-speed
pass down the runway. The rpm can change
as much as 20% and sometimes more.
That’s because the load the propeller
presents to the engine drops as the airplane
goes faster. (Yes, there are exceptions to the
last statement!)
How do you keep the engine running in
the sweet spot? By changing the propeller’s
pitch and diameter. If the propeller has too
little pitch for the airspeed at which your
airplane is intended to fly, the load will drop
off dramatically during a high-speed flyby.
As a result, the rpm will rise substantially—
maybe even past the peak horsepower rpm.
That’s not terribly useful and it makes a
bunch of noise. This is actually how many
competition fun-fly and 3-D airplanes are
set up intentionally, but they are never flown
at high speed because they tend to flutter
and explode!
When selecting a propeller for one of
these 3-D setups, a very low pitch is chosen
and then the diameter is adjusted to load the
engine to near the torque peak. Why?
Because torque provides the grunt needed
for hovering.
For flying, as opposed to hovering, start
by picking a pitch that will fly at the hopedfor
airspeed when the rpm is maybe 10%
higher than the ground rpm. Most glow
engines intended for sport have torque peaks
of roughly 10,000-11,000 rpm, so we tend to
load them to 11,000 or 12,000 rpm. That’s
approximately 200 revolutions per second.
Let’s say you want to fly at roughly 85
mph, which works out to 110 feet per
second. The pitch needs to be nearly 6 or 7
inches. The arithmetic is (110 feet per
second divided by 200 revs per second)
multiplied by (12 inches per foot). I got 6.6
inches of pitch. I’d start with a 7-inch pitch
and then add as much diameter as I can
before the engine “lugs,” or bogs, at full
throttle on the ground.
With an electric setup the propeller
selection is done similarly, but first you
have to figure out what your motor’s
running rpm will be. Estimate the voltage of
your batteries. Let’s say you are using a 6S,
or six Li-Poly cells in series. That works out
to 22.2 volts.
Reduce that figure by approximately
10% to account for the voltage lost across
the winding resistance of the motor. We
could calculate it more accurately, but that is
for another day. Let’s say we have 18 volts
left.
Multiply that by the Kv of the motor, and
that will be the running rpm, give or take a
few percent. Now pick the pitch the same as
in the preceding. But here comes the
difference.
Assuming you selected the correct pitch
for the desired flying speed and your
motor’s realistic full-throttle rpm, you must
pick the diameter to get the desired current
draw. If the diameter is too big, the current
draw goes way up. If the diameter is too
small, the current draw (or horsepower, or
watts, or Pferdestarkes, or whatever!) will
be too low to fly the airplane properly. How
do you choose how much current draw you
want?
From a practical point of view, you want
only as much as is needed to get satisfactory
performance. You would probably start
slightly small on diameter and work your
way up until the climb performance is
acceptable.
There are good online electric-flight
calculators to help you make a good first
guess at propeller size. Many motor
manufacturers have them on their Web sites,
and there are computer programs you can
buy. There are other considerations when
choosing how high of a current to run with
electric power.
The diagram for the motor has an extra
78 MODEL AVIATION
02sig3.QXD 12/20/07 1:10 PM Page 78Introduces NEW!
SUPER COOL Plug with
Hi Temp Insulator
In addition to . . .
● The FIREBALL R/C IDLE BAR plug
only $3.20
● Hot & Standard Non-Idle Bar plugs
still only $2.85
only $3.20
Swanson
Associates
P.O. Box 151
Wayne, NJ
07470
Since 1948
curve on it, labeled efficiency. We
typically choose full-throttle rpm so the
current and efficiency are within
reasonable bounds.
In this case you might choose a
maximum current based on how long you
want to be able to fly without using more
than roughly three-quarters of the battery,
or you might choose the maximum current
based on your batteries’ “C rating.” You
also need to look at your motor’s
maximum current rating. Exceeding any of
these will shorten the life of your
equipment.
If you are interested in building an
electric model to break a duration record,
you will aim for the current and rpm that
give the best efficiency. If you look at the
electric diagram, you will see that this
happens at relatively low current.
If you are building an airplane for CL
electric Speed, where less than a half
minute at full throttle is needed, you will
probably want to crank the current all the
way up to the C rating of your battery or
the short-term maximum current rating of
your motor.
Overload characteristics, or what
happens if you overload the system with
too much propeller? If the glow engine is
overloaded, it will turn at much less than
the torque peak rpm. This is on the left side
of the diagram.
The drop in torque happens because the
engine can’t breathe properly at such a low
speed. That’s a long subject, and I will
probably try to draft an expert to write
about it someday.
Because the engine is not breathing
properly, the fuel-to-air mixture becomes
inconsistent. The symptom is that the
needle valve becomes difficult to set, and
the setting often changes dramatically as
the engine unloads in the air. If you do get
the right mixture, takeoff and climb
performance will suffer. Throttle response
usually suffers in this case, especially
when you punch the throttle for a goaround
on landing.
Overloaded electrics are easy to spot—
by the smell. It’s the smell of melting
motor windings, blown ESCs, and
overheated batteries. There’s no need to
discuss this further!
In the next column I will describe how the
running characteristics of wet and electrics
affect their use in a variety of applications
from CL Stunt, to RC sport, even to FF.
There may not be an inherent advantage to
one or the other, but they have their strong
points and vulnerabilities, and different
applications highlight them well.
Until next time, go fly an airplane—any
kind of airplane! MA
Sources:
Michael Ramel
www.f3a-e-factor.de/eng/
DIVERSIFIED SOLUTIONS, LLC.
5932 Chicago Ave. South, Minneapolis, MN 55417
Ph: 1-612-243-1234 Fax: 1-612-243-8950
Email: [email protected] • Web: www.klasskote.com
For Color Chart and Information, Send SASE
Don’t Delay – Order Yours Today!
You Built the Best Model, So Use The Best Paint!
“Superior
Quality”
Epoxy Paint
System
Available in
Colors, Clear
& Primer.
38 Years of Extensive Field
Performance Provides
Outstanding Adhesion & Protection Against Many RC Model Fuels
80 MODEL AVIATION
02sig3.QXD 12/20/07 12:20 PM Page 80
Edition: Model Aviation - 2008/02
Page Numbers: 73,74,76,78,80
Our aeromodeling skills serve us in areas besides aircraft
February 2008 73
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• The difference between wet
and dry power
The contrarotating propeller shown is by Michael Ramel. Electric contrarotators have
exciting applications in Scale and Aerobatics. The EF-503 unit is not yet for sale in the US.
IF IT FLIES, other people are interested in
it too! It tickles me when I see all sorts of
fliers in the hobby/sport who, like me, seem
unable to stay between the lines. They
dabble, at the very least, in more than one
segment of aeromodeling. That failure to
“stay between the lines” is something we
share, whether we recognize it or not.
If we really stayed between the
proverbial lines, we probably wouldn’t be
building or flying model airplanes at all. It’s
not something people you meet every day
do. What percentage of the general
population do you figure has anything to do e
02sig3.QXD 12/20/07 12:18 PM Page 73with model airplanes? Maybe one in 1,000?
I’ll bet it isn’t even that many.
We’re a bit different—“plane” crazy,
you could say! We stand out from the
general population not just because we fly
model airplanes, but because we build
them—and rebuild them! We’re different
because some of us design them and because
many of us teach others to fly them.
This “wrapping our arms around” the
many skills and bits of expertise that make
up aeromodeling is uncommon these days.
They include many useful things, and the
more different corners of aeromodeling you
look into, the more of these “outside-thelines”
skills you will accumulate.
The other day my neighbor was edging
his lawn. As I wrestled with my Autumntime
foe, the swimming pool cover, I could
hear the edger in the next yard bogging
down lean. That sound normally bugs me
because it means a spoiled flight and an
early landing. I can identify it from 500 feet
away, and you can too if you are an
experienced wet-power flier. Right?
This time that sound meant I had an
excuse to grab a screwdriver, walk next
door, motion to my ear-protector-clad
neighbor as he stopped to greet me, and
deftly twist the high-speed mixture screw
out one-quarter turn. I watched and listened
only long enough to verify that the job was
done, and then I returned to the back yard
smug, satisfied, and ready for a rematch
with my big, green fabric nemesis.
That is a skill any reasonably successful
wet flier takes for granted! Normal people
would have taken that power tool to a smallengine
and mower-repair shop and spent a
meaningful amount of money to have it
“fixed.” Forget that!
That’s right; we aeromodelers are
different that way, but for some reason we
also stand out as being different from each
other.
“Hey Dean, you fly wet and electric
both, right?” my clubmate, Dave T., asked a
couple months ago. “Maybe you should
write something about the differences
between wet and electric flight.”
Dave’s suggestion was helpful, and I’ll
get to it shortly, but as I started to
contemplate that subject, a shoe wedged
itself between the gears in my noggin. I did
not think being heavily involved with both
wet and electric power was such a
distinction, but, in fact, I had gone through
quite a learning curve in the last couple
years!
At least in the clubs I belonged to, until
just a few years ago electric fliers were a
distinct group; but that isn’t the case
anymore. Now it seems like almost every
wet flier has a flat foamie or park flyer in
the hangar, although relatively few of those
fliers mess with larger, high-performance
electrics.
Meanwhile, there are fliers who have
spent years in the sport, becoming experts,
without ever owning a glow or gas engine.
Could you have imagined that just 10 or so
years ago? I didn’t.
The Pedagogical Problem: There must be
people looking to make that transition from
wet power to high-performance electric, and,
even more interestingly, there are
accomplished fliers who have only a vague
idea of how to properly set up a glow engine.
I’ll describe that shoe in the mental gears I
mentioned. It’s the problem you run into
when you try to explain something basic to
someone who is already accomplished at
something else.
It’s easy to insult someone without
meaning to. When we step out of our
personal comfort zone, or area of expertise,
we become beginners. It can either be a pain
in the neck or an opportunity for discovery
and fun.
So I’ll take Dave’s suggestion and look at
the differences between setup and technique
for electric and glow flight. If it gets basic,
please think of it as a reminder of all the
many useful things you’ve mastered without
thinking much about it.
I’m going to write a bit about the big
picture. I’ll start with a discussion about how
wet and electric power have different
characteristics, which has a great deal to do
with how their torque vs. rpm characteristics
are shaped.
Later we can get bogged down in all sorts
of specific stuff, making comparisons
between electric and wet power as they apply
74 MODEL AVIATION
02sig3.QXD 12/20/07 12:18 PM Page 74to many styles of flying. There’s a lot of it, so
I’ll probably chop the subject up into little
bits from month to month.
I’ll start by describing a glow engine’s
torque characteristics. Within their preferred
operating rpm range, glow engines make
more horsepower as they turn faster. When
you put slightly less propeller on them and let
them rev up some, you get more
performance.
That’s true only as long as the propeller is
fairly well matched to the airplane. If you
overdo this propeller reduction, you get noise
with no added performance.
The funny thing is that more noise fools
many pilots into thinking they have more
performance. This touches base with the
human-factors part of the discussion months
ago about making your airplanes quiet. That
loose end is still dangling.
A glow engine typically has a horsepower
peak at roughly 15,000 rpm. Depending on
the engine and muffler design, it will have a
torque peak at a much lower rpm; maybe it
happens at roughly two-thirds of the
horsepower peak rpm. Quieter mufflers often
bring the peak power rpm down a bunch,
while affecting the torque peak rpm
somewhat less.
The “trick” for getting a friendly engine
setup is to use propeller size that has enough
pitch to fly at the desired speed when the rpm
is in the middle of the engine’s rpm sweet
spot and enough diameter to load the engine
76 MODEL AVIATION
down to just above the torque peak at a
standstill. Loaded that way, takeoff
performance will be strong and the engine
will unload into that sweet spot between
maximum torque and maximum
horsepower while flying around.
An accompanying diagram shows this
relationship, but it is just a picture until
we explore what it means in comparison
to the same picture for an electric motor.
Electrics have an entirely different
torque vs. rpm curve. Once you have
chosen how many cells in series you will
use, if you want more power from an
electric you must lower the rpm by
putting a bigger propeller load on it! If
you are familiar with electric power, this
comes as no surprise. But think of the
huge change in thinking this represents
for many of us.
Revving up has always meant more
power; it’s almost programmed into our
DNA. But with electrics you have to grunt
to make more power. (There’s a Tim
Taylor joke in there somewhere.)
When you look at the electric torque
vs. rpm curve, you see that it’s not a curve
at all, but an almost perfect straight line.
Torque and current draw fall linearly with
increased rpm.
Horsepower is not a straight line,
though. The horsepower curve is a nice,
neat parabola, with a peak value that is
measured in watts. There is no magic to
that; there are 746 watts per horsepower,
so watts and horsepower are really the
same thing.
(When you see “Ps” in the specifications
for a model engine in an advertisement,
that’s the symbol for a metric horsepower,
or one Pferdestarke. It’s approximately 735
watts, so the difference between a US
horsepower and a metric horsepower is
only a bit less than 1.5%.)
The rpm where the horsepower peak
would occur is at half the no-load rpm.
That’s true with a motor, but operating
there may be impractical. The no-load rpm,
at the right side of the diagram, is the same
as the Kv of the motor multiplied by the
battery voltage.
The motor constant Kv is expressed in
rpm per volt—not kilovolts! I don’t know
how many times I see that boo-boo in
advertisements and catalogs.
Operating Within the Happy Zone: Every
engine design has an rpm range within
which it will run happily. Yes, “happily.” I
realize that isn’t a precise term, but it will
have to do for now. There are engines
intended for normal sport use and engines
designed for high-rpm use, such as racing.
With all the necessary emphasis on
flying-site retention these days, there are
more engines optimized to make gobs of
torque at low and moderate rpm, for the
purpose of turning large propellers quietly.
That’s a good thing, and the side benefit
is that many fliers will learn that larger
propellers fly better. There are notable
exceptions to this general rule, and that
02sig3.QXD 12/20/07 12:18 PM Page 76means we have yet another loose end to tend
to some other time.
Look at the torque curve presented for a
muffled engine. It is mostly flat or gently
sloping downward with higher rpm for a
healthy portion of the engine’s usable rpm
range. That’s the part of the range between
the torque peak and the horsepower peak.
This is the rpm range where the engine is
going to be most forgiving. Stable torque
readings mean the engine is breathing
properly.
Because any propeller changes load
characteristics as the airplane’s speed
changes, glow (and gas) engines will see a
substantial rpm change between running on
the ground, climbing, or in a high-speed
pass down the runway. The rpm can change
as much as 20% and sometimes more.
That’s because the load the propeller
presents to the engine drops as the airplane
goes faster. (Yes, there are exceptions to the
last statement!)
How do you keep the engine running in
the sweet spot? By changing the propeller’s
pitch and diameter. If the propeller has too
little pitch for the airspeed at which your
airplane is intended to fly, the load will drop
off dramatically during a high-speed flyby.
As a result, the rpm will rise substantially—
maybe even past the peak horsepower rpm.
That’s not terribly useful and it makes a
bunch of noise. This is actually how many
competition fun-fly and 3-D airplanes are
set up intentionally, but they are never flown
at high speed because they tend to flutter
and explode!
When selecting a propeller for one of
these 3-D setups, a very low pitch is chosen
and then the diameter is adjusted to load the
engine to near the torque peak. Why?
Because torque provides the grunt needed
for hovering.
For flying, as opposed to hovering, start
by picking a pitch that will fly at the hopedfor
airspeed when the rpm is maybe 10%
higher than the ground rpm. Most glow
engines intended for sport have torque peaks
of roughly 10,000-11,000 rpm, so we tend to
load them to 11,000 or 12,000 rpm. That’s
approximately 200 revolutions per second.
Let’s say you want to fly at roughly 85
mph, which works out to 110 feet per
second. The pitch needs to be nearly 6 or 7
inches. The arithmetic is (110 feet per
second divided by 200 revs per second)
multiplied by (12 inches per foot). I got 6.6
inches of pitch. I’d start with a 7-inch pitch
and then add as much diameter as I can
before the engine “lugs,” or bogs, at full
throttle on the ground.
With an electric setup the propeller
selection is done similarly, but first you
have to figure out what your motor’s
running rpm will be. Estimate the voltage of
your batteries. Let’s say you are using a 6S,
or six Li-Poly cells in series. That works out
to 22.2 volts.
Reduce that figure by approximately
10% to account for the voltage lost across
the winding resistance of the motor. We
could calculate it more accurately, but that is
for another day. Let’s say we have 18 volts
left.
Multiply that by the Kv of the motor, and
that will be the running rpm, give or take a
few percent. Now pick the pitch the same as
in the preceding. But here comes the
difference.
Assuming you selected the correct pitch
for the desired flying speed and your
motor’s realistic full-throttle rpm, you must
pick the diameter to get the desired current
draw. If the diameter is too big, the current
draw goes way up. If the diameter is too
small, the current draw (or horsepower, or
watts, or Pferdestarkes, or whatever!) will
be too low to fly the airplane properly. How
do you choose how much current draw you
want?
From a practical point of view, you want
only as much as is needed to get satisfactory
performance. You would probably start
slightly small on diameter and work your
way up until the climb performance is
acceptable.
There are good online electric-flight
calculators to help you make a good first
guess at propeller size. Many motor
manufacturers have them on their Web sites,
and there are computer programs you can
buy. There are other considerations when
choosing how high of a current to run with
electric power.
The diagram for the motor has an extra
78 MODEL AVIATION
02sig3.QXD 12/20/07 1:10 PM Page 78Introduces NEW!
SUPER COOL Plug with
Hi Temp Insulator
In addition to . . .
● The FIREBALL R/C IDLE BAR plug
only $3.20
● Hot & Standard Non-Idle Bar plugs
still only $2.85
only $3.20
Swanson
Associates
P.O. Box 151
Wayne, NJ
07470
Since 1948
curve on it, labeled efficiency. We
typically choose full-throttle rpm so the
current and efficiency are within
reasonable bounds.
In this case you might choose a
maximum current based on how long you
want to be able to fly without using more
than roughly three-quarters of the battery,
or you might choose the maximum current
based on your batteries’ “C rating.” You
also need to look at your motor’s
maximum current rating. Exceeding any of
these will shorten the life of your
equipment.
If you are interested in building an
electric model to break a duration record,
you will aim for the current and rpm that
give the best efficiency. If you look at the
electric diagram, you will see that this
happens at relatively low current.
If you are building an airplane for CL
electric Speed, where less than a half
minute at full throttle is needed, you will
probably want to crank the current all the
way up to the C rating of your battery or
the short-term maximum current rating of
your motor.
Overload characteristics, or what
happens if you overload the system with
too much propeller? If the glow engine is
overloaded, it will turn at much less than
the torque peak rpm. This is on the left side
of the diagram.
The drop in torque happens because the
engine can’t breathe properly at such a low
speed. That’s a long subject, and I will
probably try to draft an expert to write
about it someday.
Because the engine is not breathing
properly, the fuel-to-air mixture becomes
inconsistent. The symptom is that the
needle valve becomes difficult to set, and
the setting often changes dramatically as
the engine unloads in the air. If you do get
the right mixture, takeoff and climb
performance will suffer. Throttle response
usually suffers in this case, especially
when you punch the throttle for a goaround
on landing.
Overloaded electrics are easy to spot—
by the smell. It’s the smell of melting
motor windings, blown ESCs, and
overheated batteries. There’s no need to
discuss this further!
In the next column I will describe how the
running characteristics of wet and electrics
affect their use in a variety of applications
from CL Stunt, to RC sport, even to FF.
There may not be an inherent advantage to
one or the other, but they have their strong
points and vulnerabilities, and different
applications highlight them well.
Until next time, go fly an airplane—any
kind of airplane! MA
Sources:
Michael Ramel
www.f3a-e-factor.de/eng/
DIVERSIFIED SOLUTIONS, LLC.
5932 Chicago Ave. South, Minneapolis, MN 55417
Ph: 1-612-243-1234 Fax: 1-612-243-8950
Email: [email protected] • Web: www.klasskote.com
For Color Chart and Information, Send SASE
Don’t Delay – Order Yours Today!
You Built the Best Model, So Use The Best Paint!
“Superior
Quality”
Epoxy Paint
System
Available in
Colors, Clear
& Primer.
38 Years of Extensive Field
Performance Provides
Outstanding Adhesion & Protection Against Many RC Model Fuels
80 MODEL AVIATION
02sig3.QXD 12/20/07 12:20 PM Page 80
Edition: Model Aviation - 2008/02
Page Numbers: 73,74,76,78,80
Our aeromodeling skills serve us in areas besides aircraft
February 2008 73
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• The difference between wet
and dry power
The contrarotating propeller shown is by Michael Ramel. Electric contrarotators have
exciting applications in Scale and Aerobatics. The EF-503 unit is not yet for sale in the US.
IF IT FLIES, other people are interested in
it too! It tickles me when I see all sorts of
fliers in the hobby/sport who, like me, seem
unable to stay between the lines. They
dabble, at the very least, in more than one
segment of aeromodeling. That failure to
“stay between the lines” is something we
share, whether we recognize it or not.
If we really stayed between the
proverbial lines, we probably wouldn’t be
building or flying model airplanes at all. It’s
not something people you meet every day
do. What percentage of the general
population do you figure has anything to do e
02sig3.QXD 12/20/07 12:18 PM Page 73with model airplanes? Maybe one in 1,000?
I’ll bet it isn’t even that many.
We’re a bit different—“plane” crazy,
you could say! We stand out from the
general population not just because we fly
model airplanes, but because we build
them—and rebuild them! We’re different
because some of us design them and because
many of us teach others to fly them.
This “wrapping our arms around” the
many skills and bits of expertise that make
up aeromodeling is uncommon these days.
They include many useful things, and the
more different corners of aeromodeling you
look into, the more of these “outside-thelines”
skills you will accumulate.
The other day my neighbor was edging
his lawn. As I wrestled with my Autumntime
foe, the swimming pool cover, I could
hear the edger in the next yard bogging
down lean. That sound normally bugs me
because it means a spoiled flight and an
early landing. I can identify it from 500 feet
away, and you can too if you are an
experienced wet-power flier. Right?
This time that sound meant I had an
excuse to grab a screwdriver, walk next
door, motion to my ear-protector-clad
neighbor as he stopped to greet me, and
deftly twist the high-speed mixture screw
out one-quarter turn. I watched and listened
only long enough to verify that the job was
done, and then I returned to the back yard
smug, satisfied, and ready for a rematch
with my big, green fabric nemesis.
That is a skill any reasonably successful
wet flier takes for granted! Normal people
would have taken that power tool to a smallengine
and mower-repair shop and spent a
meaningful amount of money to have it
“fixed.” Forget that!
That’s right; we aeromodelers are
different that way, but for some reason we
also stand out as being different from each
other.
“Hey Dean, you fly wet and electric
both, right?” my clubmate, Dave T., asked a
couple months ago. “Maybe you should
write something about the differences
between wet and electric flight.”
Dave’s suggestion was helpful, and I’ll
get to it shortly, but as I started to
contemplate that subject, a shoe wedged
itself between the gears in my noggin. I did
not think being heavily involved with both
wet and electric power was such a
distinction, but, in fact, I had gone through
quite a learning curve in the last couple
years!
At least in the clubs I belonged to, until
just a few years ago electric fliers were a
distinct group; but that isn’t the case
anymore. Now it seems like almost every
wet flier has a flat foamie or park flyer in
the hangar, although relatively few of those
fliers mess with larger, high-performance
electrics.
Meanwhile, there are fliers who have
spent years in the sport, becoming experts,
without ever owning a glow or gas engine.
Could you have imagined that just 10 or so
years ago? I didn’t.
The Pedagogical Problem: There must be
people looking to make that transition from
wet power to high-performance electric, and,
even more interestingly, there are
accomplished fliers who have only a vague
idea of how to properly set up a glow engine.
I’ll describe that shoe in the mental gears I
mentioned. It’s the problem you run into
when you try to explain something basic to
someone who is already accomplished at
something else.
It’s easy to insult someone without
meaning to. When we step out of our
personal comfort zone, or area of expertise,
we become beginners. It can either be a pain
in the neck or an opportunity for discovery
and fun.
So I’ll take Dave’s suggestion and look at
the differences between setup and technique
for electric and glow flight. If it gets basic,
please think of it as a reminder of all the
many useful things you’ve mastered without
thinking much about it.
I’m going to write a bit about the big
picture. I’ll start with a discussion about how
wet and electric power have different
characteristics, which has a great deal to do
with how their torque vs. rpm characteristics
are shaped.
Later we can get bogged down in all sorts
of specific stuff, making comparisons
between electric and wet power as they apply
74 MODEL AVIATION
02sig3.QXD 12/20/07 12:18 PM Page 74to many styles of flying. There’s a lot of it, so
I’ll probably chop the subject up into little
bits from month to month.
I’ll start by describing a glow engine’s
torque characteristics. Within their preferred
operating rpm range, glow engines make
more horsepower as they turn faster. When
you put slightly less propeller on them and let
them rev up some, you get more
performance.
That’s true only as long as the propeller is
fairly well matched to the airplane. If you
overdo this propeller reduction, you get noise
with no added performance.
The funny thing is that more noise fools
many pilots into thinking they have more
performance. This touches base with the
human-factors part of the discussion months
ago about making your airplanes quiet. That
loose end is still dangling.
A glow engine typically has a horsepower
peak at roughly 15,000 rpm. Depending on
the engine and muffler design, it will have a
torque peak at a much lower rpm; maybe it
happens at roughly two-thirds of the
horsepower peak rpm. Quieter mufflers often
bring the peak power rpm down a bunch,
while affecting the torque peak rpm
somewhat less.
The “trick” for getting a friendly engine
setup is to use propeller size that has enough
pitch to fly at the desired speed when the rpm
is in the middle of the engine’s rpm sweet
spot and enough diameter to load the engine
76 MODEL AVIATION
down to just above the torque peak at a
standstill. Loaded that way, takeoff
performance will be strong and the engine
will unload into that sweet spot between
maximum torque and maximum
horsepower while flying around.
An accompanying diagram shows this
relationship, but it is just a picture until
we explore what it means in comparison
to the same picture for an electric motor.
Electrics have an entirely different
torque vs. rpm curve. Once you have
chosen how many cells in series you will
use, if you want more power from an
electric you must lower the rpm by
putting a bigger propeller load on it! If
you are familiar with electric power, this
comes as no surprise. But think of the
huge change in thinking this represents
for many of us.
Revving up has always meant more
power; it’s almost programmed into our
DNA. But with electrics you have to grunt
to make more power. (There’s a Tim
Taylor joke in there somewhere.)
When you look at the electric torque
vs. rpm curve, you see that it’s not a curve
at all, but an almost perfect straight line.
Torque and current draw fall linearly with
increased rpm.
Horsepower is not a straight line,
though. The horsepower curve is a nice,
neat parabola, with a peak value that is
measured in watts. There is no magic to
that; there are 746 watts per horsepower,
so watts and horsepower are really the
same thing.
(When you see “Ps” in the specifications
for a model engine in an advertisement,
that’s the symbol for a metric horsepower,
or one Pferdestarke. It’s approximately 735
watts, so the difference between a US
horsepower and a metric horsepower is
only a bit less than 1.5%.)
The rpm where the horsepower peak
would occur is at half the no-load rpm.
That’s true with a motor, but operating
there may be impractical. The no-load rpm,
at the right side of the diagram, is the same
as the Kv of the motor multiplied by the
battery voltage.
The motor constant Kv is expressed in
rpm per volt—not kilovolts! I don’t know
how many times I see that boo-boo in
advertisements and catalogs.
Operating Within the Happy Zone: Every
engine design has an rpm range within
which it will run happily. Yes, “happily.” I
realize that isn’t a precise term, but it will
have to do for now. There are engines
intended for normal sport use and engines
designed for high-rpm use, such as racing.
With all the necessary emphasis on
flying-site retention these days, there are
more engines optimized to make gobs of
torque at low and moderate rpm, for the
purpose of turning large propellers quietly.
That’s a good thing, and the side benefit
is that many fliers will learn that larger
propellers fly better. There are notable
exceptions to this general rule, and that
02sig3.QXD 12/20/07 12:18 PM Page 76means we have yet another loose end to tend
to some other time.
Look at the torque curve presented for a
muffled engine. It is mostly flat or gently
sloping downward with higher rpm for a
healthy portion of the engine’s usable rpm
range. That’s the part of the range between
the torque peak and the horsepower peak.
This is the rpm range where the engine is
going to be most forgiving. Stable torque
readings mean the engine is breathing
properly.
Because any propeller changes load
characteristics as the airplane’s speed
changes, glow (and gas) engines will see a
substantial rpm change between running on
the ground, climbing, or in a high-speed
pass down the runway. The rpm can change
as much as 20% and sometimes more.
That’s because the load the propeller
presents to the engine drops as the airplane
goes faster. (Yes, there are exceptions to the
last statement!)
How do you keep the engine running in
the sweet spot? By changing the propeller’s
pitch and diameter. If the propeller has too
little pitch for the airspeed at which your
airplane is intended to fly, the load will drop
off dramatically during a high-speed flyby.
As a result, the rpm will rise substantially—
maybe even past the peak horsepower rpm.
That’s not terribly useful and it makes a
bunch of noise. This is actually how many
competition fun-fly and 3-D airplanes are
set up intentionally, but they are never flown
at high speed because they tend to flutter
and explode!
When selecting a propeller for one of
these 3-D setups, a very low pitch is chosen
and then the diameter is adjusted to load the
engine to near the torque peak. Why?
Because torque provides the grunt needed
for hovering.
For flying, as opposed to hovering, start
by picking a pitch that will fly at the hopedfor
airspeed when the rpm is maybe 10%
higher than the ground rpm. Most glow
engines intended for sport have torque peaks
of roughly 10,000-11,000 rpm, so we tend to
load them to 11,000 or 12,000 rpm. That’s
approximately 200 revolutions per second.
Let’s say you want to fly at roughly 85
mph, which works out to 110 feet per
second. The pitch needs to be nearly 6 or 7
inches. The arithmetic is (110 feet per
second divided by 200 revs per second)
multiplied by (12 inches per foot). I got 6.6
inches of pitch. I’d start with a 7-inch pitch
and then add as much diameter as I can
before the engine “lugs,” or bogs, at full
throttle on the ground.
With an electric setup the propeller
selection is done similarly, but first you
have to figure out what your motor’s
running rpm will be. Estimate the voltage of
your batteries. Let’s say you are using a 6S,
or six Li-Poly cells in series. That works out
to 22.2 volts.
Reduce that figure by approximately
10% to account for the voltage lost across
the winding resistance of the motor. We
could calculate it more accurately, but that is
for another day. Let’s say we have 18 volts
left.
Multiply that by the Kv of the motor, and
that will be the running rpm, give or take a
few percent. Now pick the pitch the same as
in the preceding. But here comes the
difference.
Assuming you selected the correct pitch
for the desired flying speed and your
motor’s realistic full-throttle rpm, you must
pick the diameter to get the desired current
draw. If the diameter is too big, the current
draw goes way up. If the diameter is too
small, the current draw (or horsepower, or
watts, or Pferdestarkes, or whatever!) will
be too low to fly the airplane properly. How
do you choose how much current draw you
want?
From a practical point of view, you want
only as much as is needed to get satisfactory
performance. You would probably start
slightly small on diameter and work your
way up until the climb performance is
acceptable.
There are good online electric-flight
calculators to help you make a good first
guess at propeller size. Many motor
manufacturers have them on their Web sites,
and there are computer programs you can
buy. There are other considerations when
choosing how high of a current to run with
electric power.
The diagram for the motor has an extra
78 MODEL AVIATION
02sig3.QXD 12/20/07 1:10 PM Page 78Introduces NEW!
SUPER COOL Plug with
Hi Temp Insulator
In addition to . . .
● The FIREBALL R/C IDLE BAR plug
only $3.20
● Hot & Standard Non-Idle Bar plugs
still only $2.85
only $3.20
Swanson
Associates
P.O. Box 151
Wayne, NJ
07470
Since 1948
curve on it, labeled efficiency. We
typically choose full-throttle rpm so the
current and efficiency are within
reasonable bounds.
In this case you might choose a
maximum current based on how long you
want to be able to fly without using more
than roughly three-quarters of the battery,
or you might choose the maximum current
based on your batteries’ “C rating.” You
also need to look at your motor’s
maximum current rating. Exceeding any of
these will shorten the life of your
equipment.
If you are interested in building an
electric model to break a duration record,
you will aim for the current and rpm that
give the best efficiency. If you look at the
electric diagram, you will see that this
happens at relatively low current.
If you are building an airplane for CL
electric Speed, where less than a half
minute at full throttle is needed, you will
probably want to crank the current all the
way up to the C rating of your battery or
the short-term maximum current rating of
your motor.
Overload characteristics, or what
happens if you overload the system with
too much propeller? If the glow engine is
overloaded, it will turn at much less than
the torque peak rpm. This is on the left side
of the diagram.
The drop in torque happens because the
engine can’t breathe properly at such a low
speed. That’s a long subject, and I will
probably try to draft an expert to write
about it someday.
Because the engine is not breathing
properly, the fuel-to-air mixture becomes
inconsistent. The symptom is that the
needle valve becomes difficult to set, and
the setting often changes dramatically as
the engine unloads in the air. If you do get
the right mixture, takeoff and climb
performance will suffer. Throttle response
usually suffers in this case, especially
when you punch the throttle for a goaround
on landing.
Overloaded electrics are easy to spot—
by the smell. It’s the smell of melting
motor windings, blown ESCs, and
overheated batteries. There’s no need to
discuss this further!
In the next column I will describe how the
running characteristics of wet and electrics
affect their use in a variety of applications
from CL Stunt, to RC sport, even to FF.
There may not be an inherent advantage to
one or the other, but they have their strong
points and vulnerabilities, and different
applications highlight them well.
Until next time, go fly an airplane—any
kind of airplane! MA
Sources:
Michael Ramel
www.f3a-e-factor.de/eng/
DIVERSIFIED SOLUTIONS, LLC.
5932 Chicago Ave. South, Minneapolis, MN 55417
Ph: 1-612-243-1234 Fax: 1-612-243-8950
Email: [email protected] • Web: www.klasskote.com
For Color Chart and Information, Send SASE
Don’t Delay – Order Yours Today!
You Built the Best Model, So Use The Best Paint!
“Superior
Quality”
Epoxy Paint
System
Available in
Colors, Clear
& Primer.
38 Years of Extensive Field
Performance Provides
Outstanding Adhesion & Protection Against Many RC Model Fuels
80 MODEL AVIATION
02sig3.QXD 12/20/07 12:20 PM Page 80
