72 MODEL AVIATION
Simple questions can sometimes lead to complicated answers
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• Compare and contrast:
electric and glow
The firewall was cut away and an
electron wall was added to mount the
Axi 2826/10 motor. The battery is a fourcell
Thunder Power Pro Lite 4200 mAh
pack; the ESC is a Phoenix-45.
The Genesis started with a glow Ro-Jett .40 with carbon-fiber tuned pipe. It provided
excellent power and table manners. The package weighed just less than 19 ounces
without fuel.
Bob Hunt’s Genesis Extreme CL Stunt
model lets us compare electric and glow
power. It gained roughly 1 ounce
compared with the piped glow engine,
and electric’s predictability made a
winning combination.
WHAT IS THE mission of “If It Flies ... ”?
From where I sit, it is to help tie together the
big picture in aeromodeling, helping us all
benefit from the knowledge, experience, and
techniques used in other corners of “toy
airplane-dom.”
To garner that benefit we need to know
just enough about some flying event or
some particular type of flying to be able to
have a discussion. Sometimes just knowing
the jargon is enough.
For example, if they look like ailerons on
an RC model, they are probably the same
things that are called “maneuvering flaps”
on a CL aircraft. If you see the same things
on a competition FF Gas model, they are
probably called “variable-camber flaps” or
something similar.
Once we get past the problems posed by
the mere mechanics of talking to each other,
we aeromodelers tend to enjoy each other’s
company as much as we like finding out
about different kinds of flying machines.
Sure, some will scoff at whatever is
different, but that’s just because they don’t
understand it.
For instance, lots (and I mean lots!) of
fliers have never used anything other than
heat-shrink covering. When most of them
look at the elbow-deep finish on a tissue
paper-and-dope-finished CL Aerobatics
(Stunt) model or the mirrorlike epoxy finish
on a Pylon racer, their responses range from
“I could never do that!” to “Wow!” and
eventually to “I have to learn how to do that!”
That’s our mission: to help connect each
of us to other fliers in many remote corners of
the aeromodeling world. You have to see the
many different things other people do,
whether it’s something you think is
interesting, useful, or just cool. Then the
eager research staff at “If It Flies ... ” finds
people who know how to do that thing well,
and we suck the information out of their
brains and present it in these pages!
To carry out this grandiose mission, the
research staff (that’s me, myself, and I) need
your questions! What do you want to know
about?
What little snippet of strange (to you)
aeromodeling have you seen that needs an
explanation? Have you seen something at a
flying field or in any magazine that needed
much more explanation than the little caption
underneath the tiny picture?
I’m betting there’s something you want to
know about, but you didn’t know whom to
ask. Finding out more about it might make
your flying better or more enjoyable.
I was surfing through a handful of the
many model-flying forums on the Internet
(this particular one was predominantly
04sig3.QXD 2/25/08 12:21 PM Page 72
74 MODEL AVIATION
devoted to electric flight), where a flier
wrote that he thought it was a shame that
“nobody flew Control-Line anymore.”
Approximately a zillion helpful souls
pointed out to him that he wasn’t looking in
the right circles and that there is plenty of
CL activity all over the world.
Yes, that was an utterly shameless play
on words, but the neat bit about this episode
was what followed. Nobody took the guy to
task; instead the thread exploded.
There were those who thought much
like the person who started the thread.
They found out too! Still others
materialized from cyberspace and admitted
that they, too, were looking to pick up a
handle again. Many of them started
reminiscing because nostalgia can be fun.
Another group responded with helpful
hints for where to find clubs and flying
buddies in various parts of the country, and a
whole other group started to tell this writer
about the “leading edge” stuff going on in
electric-powered CL, including the
availability of new electric ARFs and the
recent adoption of E-power by some
previous World Champions in competitive
Stunt. (Remember that this was on a forum
with a great deal of electric-related activity.)
The collision between nostalgia and the
top of the state of the art was a neat byproduct
of a simple question. Just like some
of the greatest scientific discoveries, it was a
happy accident.
Sullivan, The Fuel Systems Experts.
www.sullivanproducts.com
New!
Heavy Duty Aluminum
Stopper Assembly
Double Screen “Crap
Trap” Fuel Filters Aluminum Check Valve
It’s a Fact: Sullivan Is The
Leader In Fuel System Components
Fuel Clamps For Tubing
Fuel Filler Valve
Gasoline Conversion Kit
Universal Fuel Stopper
Super Klunk
One North Haven St.
Baltimore, MD. 21224
Wrapping Our Arms Around the Big-
Picture Differences Between Electric and
Gas/Glow Flying: When my flying buddy,
Dave T., suggested taking a look at the
differences between electric and gas/glow
power, he was opening a can of worms! The
last time we got together, rather than diving
into the differences in equipment, airframebuilding
requirements, or even airplane
types, being overly ambitious I decided to
shoot for the moon.
All those would have been good things
to write about, but instead I dragged out
some typical torque and power vs. rpm
curves for gas and DC motors, and then I set
out to explain the very real difference in
“feel” between the two. Maybe this “from
first principles” approach was, as I
mentioned, a bit too ambitious, but there
was a method to my madness. So please
allow me to draw a quick outline around the
subject.
• Electric power and gas have fundamentally
different “grunt vs. go” behaviors.
• The gas engine can change its personality
fairly substantially, depending on how
heavily you load it down and how the
propeller pitch and diameter play with the
airplane’s airspeed and drag.
A gas/glow engine typically gains
horsepower with rpm over an extremely
wide range of rpm, but the torque curve is
almost flat over an exceptionally wide range
and falls off at both high and low rpm.
When loaded with a propeller-and-model
combination that keeps the rpm in the nearly
level part of the torque curve, whether on
the ground or in high-speed flight, the
engine’s handling characteristics and
lifetime will be maximized. Large propellers
that load the engine into the drop at the lowrpm
end of the torque plateau create all sorts
of bad handling characteristics.
Finally, operation near the fast-revving
power peak will make considerable racket
and accelerate engine wear, as well as
reducing propeller efficiency and real flying
power in most cases.
• The electriclike characteristic of torque
and power increasing with added load can
be replicated in gas/glow by using an
intentionally mistuned exhaust system.
Although useful, this produces less than
optimal horsepower. This is also known as
the CLPA tuned-pipe setup.
• The motor displays much less flexibility in
its personality, but that basic personality is
suitable to almost everyone except the speed
merchants.
• The motor makes more of torque and
power when you load it down with a bigger
or higher-pitch propeller, as long as the
voltage or “throttle” is unchanged. This
means that the electric power plant has an
inherent tendency to try to fly at the same
speed, whether climbing or diving, as long
as the throttle is unmoved. This is handy in
sport and all manner of aerobatics.
• The process of picking the correct
propeller for both types of power plants is
similar in some respects but still different
04sig3.QXD 2/25/08 12:11 PM Page 74
enough to give you fits, depending on the
type of airplane you want to fly. The flat
torque curve of the gas/glow engine, and
rpm rise during flight, forgives a large range
of pitch choices on a particular airplane,
while the electric setup will unload only a
handful of percent in rpm and requires more
careful pitch selection if you want a
particular flying speed.
In addition to this difference in the flavor
of the horsepower, each power-plant type
has its own interesting list of implementation
details and headaches.
For wet power these include such
subjects as proper fuel-system setup, adding
fuel-pumping systems, bearing and overall
maintenance, tuned exhaust systems, and
noise abatement. Propeller selection can be
magic too, transforming an airplane’s flying
and sound characteristics. There are other
factors, of course.
Electric power has its own techniques
and tricks. Wiring methods and connector
selection aren’t all that difficult, but you’d
think it was rocket science judging by the
amount of discussion!
Battery maintenance is as big of a
subject, probably because of the expense of
the fantastic battery technology these days.
After all, mistreating expensive equipment is
dumb.
Matching propellers to motors and
gearbox ratios seems like black magic to
many people, so I will eventually spend
some time writing about that. Even choosing
a motor is a subject unto itself.
Many of these subjects will wait for
another time, but for now I’ll cover some
specific differences between electric and
gas/glow. I’ll bet that when my buddy, Dave,
suggested a comparison of wet and electric
power, this is what he meant.
I’m going to put aside, for now, any
discussion of park flyers and indoor
airplanes that weigh just a few ounces;
this discussion is about medium- and
high-performance airplanes. Fliers who
were raised on fuel and the others who
were introduced to the hobby/sport with
small electric stuff are both looking at a
major learning curve when they decide to
change from one type of power plant to
the other.
For starters, I’ll dispel a myth. Electric
airplanes do indeed stay cleaner, but they
still require cleaning at the end of the flying
day. For those of us who fly from grass, that
green chlorophyll stain the propeller kicked
up looks the same. Hey! This is important
stuff.
After changing to electric power for his
competitive efforts, Bob Hunt, MA’s editor
emeritus, discovered that he still had to
carefully wipe down his self-designed
Genesis with Windex. He blamed it on all
the oily fingerprints of his wet-flying
buddies who satisfy their still-endless
curiosity by picking up the airplane and
carefully looking at it.
Compare and Contrast: The nowelectrified
CL Genesis’s conversion
highlighted something a few of us have
observed in RC. Even when the horsepower
available in an electric setup is slightly less
than with glow or gas, electric propulsion’s
consistency and controllability can often
help make up for it.
This controllability and reliability factor
is of great importance to some. Many 3-D
and Stunt pilots have learned that electric
power’s time lag-free throttle response is a
great convenience. It makes throttle
management easier.
Complicated multiengine projects are
flown with the proverbial sword of
Damocles hanging above them. For all the
cool factor of multiple engines, the prospect
of an engine failure demands extra diligence
in engine setup and maintenance.
There is the increased probability of an
engine failure; and successful landings with
an engine out are still a less than 50/50
proposition. This is especially true of
airplanes with heavy wing loadings, such as
many Scale subjects.
Here is one of the few cases in which one
power plant has a clear advantage over the
other. Add to that the ease with which
present-day brushless motors can be set to
run in the opposite direction, and counterrotating
twins are a cinch. Still, there is the
issue of excess weight.
Airframes destined for electric power
have historically had to be built more lightly,
76 MODEL AVIATION
04sig3.QXD 2/25/08 12:23 PM Page 76
just to be able to fly well. However, the
advent of high-discharge-rate Lithium-based
battery chemistries has changed all that.
The rule used to be that for lively
performance you needed an airframe and
radio that weighed no more than the
batteries and motor put together. If the
airframe with radio weighed as little as 1.5
times the power plant, the performance
would barely be adequate for a slow-flying
trainer or an electric glider.
Today’s batteries allow you to fly 10-
minute, high-energy aerobatic routines with
long vertical climbs at the same 1.5:1 weight
ratio. With Li-Polys the motor battery
combinations can typically weigh 1-11/4
times as much as an equivalent engine,
throttle servo, and full tank of fuel.
No kidding. Remember to do the
comparison with a full, or at least half full,
tank.
Still, if your airframe is decidedly on the
heavy side—and suffers from a bit of extra
weight, even with a glow engine—
performance is definitely going to suffer
after you add the weight of enough battery.
If you tend to build really heavy, stick with
gas!
The problem becomes a sort of “domino
theory of weight.” Let’s look at some good
cases and the power-to-weight loadings they
produce.
To start out, the horsepower-to-weight
ratio that is necessary to fly with a desired
performance level is the same with electrics
as with gas. It’s just that in one case we refer
to horsepower (usually the manufacturer’s
claim that is measured at impractically high
rpm), and in the other case we refer to watts.
(These are watts measured coming out of the
battery—not out of the motor.) This means
we have to do a bit of decoding before we
can make a comparison.
Let’s do that using a 5-pound airplane as
an example. If it is a trainer, a glow .25
engine will be just enough power as long as
the grass runway has been mowed recently.
A .40 will be necessary for decent
performance, and a .60 will yield sprightly
performance.
A typical .25 puts out maybe threequarters
horsepower at some ridiculously
high rpm, such as 18,000. That probably
translates to maybe 70% power at 12,000
rpm, so we get approximately 0.5 horse.
Multiply by 746 watts per horse and we get
maybe 400 watts, give or take 10% or 20%.
On a 5-pound airplane, that works out to 80
watts per pound.
Derate that by maybe 10% for the
motor’s efficiency to get the actual power at
the propeller, and we are down near 70 watts
per pound. That puts us at the middle of the
trainerlike power bracket, as defined by the
time-honored rule of thumb that has been
attributed to longtime electric-flight guru,
Keith Shaw.
The old rule specifies 50-85 watts per
pound for trainers that need to take off rather
than be hand-launched. The 50-watt-perpound
figure is marginal for grass, but it will
do nicely for pavement and hand launch.
Most electric FF designs I am familiar with
use a relatively low power loading in this
neighborhood.
Now we move up to a .40 or maybe a .46
engine. They range all over the place, but
let’s assume 1.5 horsepower at something
such as 15,000 or 16,000 rpm as a
manufacturer’s claim.
Derating to a more sensible 12,000 rpm
and converting to watts, we get maybe 600
or 800. That would work out to 125-150
watts per pound for the same 5-pound
airplane. Subtract maybe 10% for the motor
efficiency, and we are in line with the old
rule of thumb saying that 100-150 watts
provides “sport” performance.
If you managed to build a 5-pounder,
with a .60 in it, you would have a tiger by
the tail! That muffled .60 would yield just
less than 1.5 horses, and that would translate
to close to 1,000 watts. A figure of 200 watts
per pound is the same power loading as
competitive RC Aerobatics and IMAC
(International Miniature Aerobatic Club) 3-
D models have. I like it!
Wow! We are out of space for this
installment, but I promise to pick up next
time with the “domino theory of airplane
weight” and include notes about converting
an airframe that is intended for wet power to
electric and vice versa.
Until then, go fly something! MA
WW-II bomber crews remember that it
was the P-51B Mustang that turned the
tide of daylight bombing. Previously,
fighter escorts such as the P-47 did not
have the required range and had to
turn back, leaving the bombers to
make their bombing runs with no
fighter protection. North American
Aviation went to work delivering the
first P-51 prototype in just 102 days and
it was a remarkable step forward in
fighter performance. The P-51B could
climb to 20,000 feet in under 6
minutes and fly 440 mph at altitude -
faster than any other fighter plane at
the time. Most importantly, it was the
range of 1080 miles (a whopping 2600
mile range with drop tanks), which
enabled the P-51B to escort the
bombers all the way to Berlin.
The SIG P-51B is loaded - flaps, retracts,
complete "Shangri-La" decal set,
painted fiberglass cowl, and molded
canopy. The model is hand built from
balsa and plywood and expertly
covered in SIG AeroKote®.
SIG Manufacturing Company, Inc.
P. O. Box 520 • Montezuma, Iowa
www.sigmfg.com • 641-623-5154
P-51B Mustang ARF
SPECIFICATIONS:
Wingspan: 66.9 inches
Length: 55.9 inches
Engine Required: .90 - 1.00 cu. in.
2-stroke or 4-stroke
Radio Required: 6 Channel, 8 servos
Order No. SIGRC103ARF
* $319.99 at Participating Dealers
04sig3.QXD 2/25/08 12:23 PM Page 78
Edition: Model Aviation - 2008/04
Page Numbers: 72,74,76,78
Edition: Model Aviation - 2008/04
Page Numbers: 72,74,76,78
72 MODEL AVIATION
Simple questions can sometimes lead to complicated answers
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• Compare and contrast:
electric and glow
The firewall was cut away and an
electron wall was added to mount the
Axi 2826/10 motor. The battery is a fourcell
Thunder Power Pro Lite 4200 mAh
pack; the ESC is a Phoenix-45.
The Genesis started with a glow Ro-Jett .40 with carbon-fiber tuned pipe. It provided
excellent power and table manners. The package weighed just less than 19 ounces
without fuel.
Bob Hunt’s Genesis Extreme CL Stunt
model lets us compare electric and glow
power. It gained roughly 1 ounce
compared with the piped glow engine,
and electric’s predictability made a
winning combination.
WHAT IS THE mission of “If It Flies ... ”?
From where I sit, it is to help tie together the
big picture in aeromodeling, helping us all
benefit from the knowledge, experience, and
techniques used in other corners of “toy
airplane-dom.”
To garner that benefit we need to know
just enough about some flying event or
some particular type of flying to be able to
have a discussion. Sometimes just knowing
the jargon is enough.
For example, if they look like ailerons on
an RC model, they are probably the same
things that are called “maneuvering flaps”
on a CL aircraft. If you see the same things
on a competition FF Gas model, they are
probably called “variable-camber flaps” or
something similar.
Once we get past the problems posed by
the mere mechanics of talking to each other,
we aeromodelers tend to enjoy each other’s
company as much as we like finding out
about different kinds of flying machines.
Sure, some will scoff at whatever is
different, but that’s just because they don’t
understand it.
For instance, lots (and I mean lots!) of
fliers have never used anything other than
heat-shrink covering. When most of them
look at the elbow-deep finish on a tissue
paper-and-dope-finished CL Aerobatics
(Stunt) model or the mirrorlike epoxy finish
on a Pylon racer, their responses range from
“I could never do that!” to “Wow!” and
eventually to “I have to learn how to do that!”
That’s our mission: to help connect each
of us to other fliers in many remote corners of
the aeromodeling world. You have to see the
many different things other people do,
whether it’s something you think is
interesting, useful, or just cool. Then the
eager research staff at “If It Flies ... ” finds
people who know how to do that thing well,
and we suck the information out of their
brains and present it in these pages!
To carry out this grandiose mission, the
research staff (that’s me, myself, and I) need
your questions! What do you want to know
about?
What little snippet of strange (to you)
aeromodeling have you seen that needs an
explanation? Have you seen something at a
flying field or in any magazine that needed
much more explanation than the little caption
underneath the tiny picture?
I’m betting there’s something you want to
know about, but you didn’t know whom to
ask. Finding out more about it might make
your flying better or more enjoyable.
I was surfing through a handful of the
many model-flying forums on the Internet
(this particular one was predominantly
04sig3.QXD 2/25/08 12:21 PM Page 72
74 MODEL AVIATION
devoted to electric flight), where a flier
wrote that he thought it was a shame that
“nobody flew Control-Line anymore.”
Approximately a zillion helpful souls
pointed out to him that he wasn’t looking in
the right circles and that there is plenty of
CL activity all over the world.
Yes, that was an utterly shameless play
on words, but the neat bit about this episode
was what followed. Nobody took the guy to
task; instead the thread exploded.
There were those who thought much
like the person who started the thread.
They found out too! Still others
materialized from cyberspace and admitted
that they, too, were looking to pick up a
handle again. Many of them started
reminiscing because nostalgia can be fun.
Another group responded with helpful
hints for where to find clubs and flying
buddies in various parts of the country, and a
whole other group started to tell this writer
about the “leading edge” stuff going on in
electric-powered CL, including the
availability of new electric ARFs and the
recent adoption of E-power by some
previous World Champions in competitive
Stunt. (Remember that this was on a forum
with a great deal of electric-related activity.)
The collision between nostalgia and the
top of the state of the art was a neat byproduct
of a simple question. Just like some
of the greatest scientific discoveries, it was a
happy accident.
Sullivan, The Fuel Systems Experts.
www.sullivanproducts.com
New!
Heavy Duty Aluminum
Stopper Assembly
Double Screen “Crap
Trap” Fuel Filters Aluminum Check Valve
It’s a Fact: Sullivan Is The
Leader In Fuel System Components
Fuel Clamps For Tubing
Fuel Filler Valve
Gasoline Conversion Kit
Universal Fuel Stopper
Super Klunk
One North Haven St.
Baltimore, MD. 21224
Wrapping Our Arms Around the Big-
Picture Differences Between Electric and
Gas/Glow Flying: When my flying buddy,
Dave T., suggested taking a look at the
differences between electric and gas/glow
power, he was opening a can of worms! The
last time we got together, rather than diving
into the differences in equipment, airframebuilding
requirements, or even airplane
types, being overly ambitious I decided to
shoot for the moon.
All those would have been good things
to write about, but instead I dragged out
some typical torque and power vs. rpm
curves for gas and DC motors, and then I set
out to explain the very real difference in
“feel” between the two. Maybe this “from
first principles” approach was, as I
mentioned, a bit too ambitious, but there
was a method to my madness. So please
allow me to draw a quick outline around the
subject.
• Electric power and gas have fundamentally
different “grunt vs. go” behaviors.
• The gas engine can change its personality
fairly substantially, depending on how
heavily you load it down and how the
propeller pitch and diameter play with the
airplane’s airspeed and drag.
A gas/glow engine typically gains
horsepower with rpm over an extremely
wide range of rpm, but the torque curve is
almost flat over an exceptionally wide range
and falls off at both high and low rpm.
When loaded with a propeller-and-model
combination that keeps the rpm in the nearly
level part of the torque curve, whether on
the ground or in high-speed flight, the
engine’s handling characteristics and
lifetime will be maximized. Large propellers
that load the engine into the drop at the lowrpm
end of the torque plateau create all sorts
of bad handling characteristics.
Finally, operation near the fast-revving
power peak will make considerable racket
and accelerate engine wear, as well as
reducing propeller efficiency and real flying
power in most cases.
• The electriclike characteristic of torque
and power increasing with added load can
be replicated in gas/glow by using an
intentionally mistuned exhaust system.
Although useful, this produces less than
optimal horsepower. This is also known as
the CLPA tuned-pipe setup.
• The motor displays much less flexibility in
its personality, but that basic personality is
suitable to almost everyone except the speed
merchants.
• The motor makes more of torque and
power when you load it down with a bigger
or higher-pitch propeller, as long as the
voltage or “throttle” is unchanged. This
means that the electric power plant has an
inherent tendency to try to fly at the same
speed, whether climbing or diving, as long
as the throttle is unmoved. This is handy in
sport and all manner of aerobatics.
• The process of picking the correct
propeller for both types of power plants is
similar in some respects but still different
04sig3.QXD 2/25/08 12:11 PM Page 74
enough to give you fits, depending on the
type of airplane you want to fly. The flat
torque curve of the gas/glow engine, and
rpm rise during flight, forgives a large range
of pitch choices on a particular airplane,
while the electric setup will unload only a
handful of percent in rpm and requires more
careful pitch selection if you want a
particular flying speed.
In addition to this difference in the flavor
of the horsepower, each power-plant type
has its own interesting list of implementation
details and headaches.
For wet power these include such
subjects as proper fuel-system setup, adding
fuel-pumping systems, bearing and overall
maintenance, tuned exhaust systems, and
noise abatement. Propeller selection can be
magic too, transforming an airplane’s flying
and sound characteristics. There are other
factors, of course.
Electric power has its own techniques
and tricks. Wiring methods and connector
selection aren’t all that difficult, but you’d
think it was rocket science judging by the
amount of discussion!
Battery maintenance is as big of a
subject, probably because of the expense of
the fantastic battery technology these days.
After all, mistreating expensive equipment is
dumb.
Matching propellers to motors and
gearbox ratios seems like black magic to
many people, so I will eventually spend
some time writing about that. Even choosing
a motor is a subject unto itself.
Many of these subjects will wait for
another time, but for now I’ll cover some
specific differences between electric and
gas/glow. I’ll bet that when my buddy, Dave,
suggested a comparison of wet and electric
power, this is what he meant.
I’m going to put aside, for now, any
discussion of park flyers and indoor
airplanes that weigh just a few ounces;
this discussion is about medium- and
high-performance airplanes. Fliers who
were raised on fuel and the others who
were introduced to the hobby/sport with
small electric stuff are both looking at a
major learning curve when they decide to
change from one type of power plant to
the other.
For starters, I’ll dispel a myth. Electric
airplanes do indeed stay cleaner, but they
still require cleaning at the end of the flying
day. For those of us who fly from grass, that
green chlorophyll stain the propeller kicked
up looks the same. Hey! This is important
stuff.
After changing to electric power for his
competitive efforts, Bob Hunt, MA’s editor
emeritus, discovered that he still had to
carefully wipe down his self-designed
Genesis with Windex. He blamed it on all
the oily fingerprints of his wet-flying
buddies who satisfy their still-endless
curiosity by picking up the airplane and
carefully looking at it.
Compare and Contrast: The nowelectrified
CL Genesis’s conversion
highlighted something a few of us have
observed in RC. Even when the horsepower
available in an electric setup is slightly less
than with glow or gas, electric propulsion’s
consistency and controllability can often
help make up for it.
This controllability and reliability factor
is of great importance to some. Many 3-D
and Stunt pilots have learned that electric
power’s time lag-free throttle response is a
great convenience. It makes throttle
management easier.
Complicated multiengine projects are
flown with the proverbial sword of
Damocles hanging above them. For all the
cool factor of multiple engines, the prospect
of an engine failure demands extra diligence
in engine setup and maintenance.
There is the increased probability of an
engine failure; and successful landings with
an engine out are still a less than 50/50
proposition. This is especially true of
airplanes with heavy wing loadings, such as
many Scale subjects.
Here is one of the few cases in which one
power plant has a clear advantage over the
other. Add to that the ease with which
present-day brushless motors can be set to
run in the opposite direction, and counterrotating
twins are a cinch. Still, there is the
issue of excess weight.
Airframes destined for electric power
have historically had to be built more lightly,
76 MODEL AVIATION
04sig3.QXD 2/25/08 12:23 PM Page 76
just to be able to fly well. However, the
advent of high-discharge-rate Lithium-based
battery chemistries has changed all that.
The rule used to be that for lively
performance you needed an airframe and
radio that weighed no more than the
batteries and motor put together. If the
airframe with radio weighed as little as 1.5
times the power plant, the performance
would barely be adequate for a slow-flying
trainer or an electric glider.
Today’s batteries allow you to fly 10-
minute, high-energy aerobatic routines with
long vertical climbs at the same 1.5:1 weight
ratio. With Li-Polys the motor battery
combinations can typically weigh 1-11/4
times as much as an equivalent engine,
throttle servo, and full tank of fuel.
No kidding. Remember to do the
comparison with a full, or at least half full,
tank.
Still, if your airframe is decidedly on the
heavy side—and suffers from a bit of extra
weight, even with a glow engine—
performance is definitely going to suffer
after you add the weight of enough battery.
If you tend to build really heavy, stick with
gas!
The problem becomes a sort of “domino
theory of weight.” Let’s look at some good
cases and the power-to-weight loadings they
produce.
To start out, the horsepower-to-weight
ratio that is necessary to fly with a desired
performance level is the same with electrics
as with gas. It’s just that in one case we refer
to horsepower (usually the manufacturer’s
claim that is measured at impractically high
rpm), and in the other case we refer to watts.
(These are watts measured coming out of the
battery—not out of the motor.) This means
we have to do a bit of decoding before we
can make a comparison.
Let’s do that using a 5-pound airplane as
an example. If it is a trainer, a glow .25
engine will be just enough power as long as
the grass runway has been mowed recently.
A .40 will be necessary for decent
performance, and a .60 will yield sprightly
performance.
A typical .25 puts out maybe threequarters
horsepower at some ridiculously
high rpm, such as 18,000. That probably
translates to maybe 70% power at 12,000
rpm, so we get approximately 0.5 horse.
Multiply by 746 watts per horse and we get
maybe 400 watts, give or take 10% or 20%.
On a 5-pound airplane, that works out to 80
watts per pound.
Derate that by maybe 10% for the
motor’s efficiency to get the actual power at
the propeller, and we are down near 70 watts
per pound. That puts us at the middle of the
trainerlike power bracket, as defined by the
time-honored rule of thumb that has been
attributed to longtime electric-flight guru,
Keith Shaw.
The old rule specifies 50-85 watts per
pound for trainers that need to take off rather
than be hand-launched. The 50-watt-perpound
figure is marginal for grass, but it will
do nicely for pavement and hand launch.
Most electric FF designs I am familiar with
use a relatively low power loading in this
neighborhood.
Now we move up to a .40 or maybe a .46
engine. They range all over the place, but
let’s assume 1.5 horsepower at something
such as 15,000 or 16,000 rpm as a
manufacturer’s claim.
Derating to a more sensible 12,000 rpm
and converting to watts, we get maybe 600
or 800. That would work out to 125-150
watts per pound for the same 5-pound
airplane. Subtract maybe 10% for the motor
efficiency, and we are in line with the old
rule of thumb saying that 100-150 watts
provides “sport” performance.
If you managed to build a 5-pounder,
with a .60 in it, you would have a tiger by
the tail! That muffled .60 would yield just
less than 1.5 horses, and that would translate
to close to 1,000 watts. A figure of 200 watts
per pound is the same power loading as
competitive RC Aerobatics and IMAC
(International Miniature Aerobatic Club) 3-
D models have. I like it!
Wow! We are out of space for this
installment, but I promise to pick up next
time with the “domino theory of airplane
weight” and include notes about converting
an airframe that is intended for wet power to
electric and vice versa.
Until then, go fly something! MA
WW-II bomber crews remember that it
was the P-51B Mustang that turned the
tide of daylight bombing. Previously,
fighter escorts such as the P-47 did not
have the required range and had to
turn back, leaving the bombers to
make their bombing runs with no
fighter protection. North American
Aviation went to work delivering the
first P-51 prototype in just 102 days and
it was a remarkable step forward in
fighter performance. The P-51B could
climb to 20,000 feet in under 6
minutes and fly 440 mph at altitude -
faster than any other fighter plane at
the time. Most importantly, it was the
range of 1080 miles (a whopping 2600
mile range with drop tanks), which
enabled the P-51B to escort the
bombers all the way to Berlin.
The SIG P-51B is loaded - flaps, retracts,
complete "Shangri-La" decal set,
painted fiberglass cowl, and molded
canopy. The model is hand built from
balsa and plywood and expertly
covered in SIG AeroKote®.
SIG Manufacturing Company, Inc.
P. O. Box 520 • Montezuma, Iowa
www.sigmfg.com • 641-623-5154
P-51B Mustang ARF
SPECIFICATIONS:
Wingspan: 66.9 inches
Length: 55.9 inches
Engine Required: .90 - 1.00 cu. in.
2-stroke or 4-stroke
Radio Required: 6 Channel, 8 servos
Order No. SIGRC103ARF
* $319.99 at Participating Dealers
04sig3.QXD 2/25/08 12:23 PM Page 78
Edition: Model Aviation - 2008/04
Page Numbers: 72,74,76,78
72 MODEL AVIATION
Simple questions can sometimes lead to complicated answers
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• Compare and contrast:
electric and glow
The firewall was cut away and an
electron wall was added to mount the
Axi 2826/10 motor. The battery is a fourcell
Thunder Power Pro Lite 4200 mAh
pack; the ESC is a Phoenix-45.
The Genesis started with a glow Ro-Jett .40 with carbon-fiber tuned pipe. It provided
excellent power and table manners. The package weighed just less than 19 ounces
without fuel.
Bob Hunt’s Genesis Extreme CL Stunt
model lets us compare electric and glow
power. It gained roughly 1 ounce
compared with the piped glow engine,
and electric’s predictability made a
winning combination.
WHAT IS THE mission of “If It Flies ... ”?
From where I sit, it is to help tie together the
big picture in aeromodeling, helping us all
benefit from the knowledge, experience, and
techniques used in other corners of “toy
airplane-dom.”
To garner that benefit we need to know
just enough about some flying event or
some particular type of flying to be able to
have a discussion. Sometimes just knowing
the jargon is enough.
For example, if they look like ailerons on
an RC model, they are probably the same
things that are called “maneuvering flaps”
on a CL aircraft. If you see the same things
on a competition FF Gas model, they are
probably called “variable-camber flaps” or
something similar.
Once we get past the problems posed by
the mere mechanics of talking to each other,
we aeromodelers tend to enjoy each other’s
company as much as we like finding out
about different kinds of flying machines.
Sure, some will scoff at whatever is
different, but that’s just because they don’t
understand it.
For instance, lots (and I mean lots!) of
fliers have never used anything other than
heat-shrink covering. When most of them
look at the elbow-deep finish on a tissue
paper-and-dope-finished CL Aerobatics
(Stunt) model or the mirrorlike epoxy finish
on a Pylon racer, their responses range from
“I could never do that!” to “Wow!” and
eventually to “I have to learn how to do that!”
That’s our mission: to help connect each
of us to other fliers in many remote corners of
the aeromodeling world. You have to see the
many different things other people do,
whether it’s something you think is
interesting, useful, or just cool. Then the
eager research staff at “If It Flies ... ” finds
people who know how to do that thing well,
and we suck the information out of their
brains and present it in these pages!
To carry out this grandiose mission, the
research staff (that’s me, myself, and I) need
your questions! What do you want to know
about?
What little snippet of strange (to you)
aeromodeling have you seen that needs an
explanation? Have you seen something at a
flying field or in any magazine that needed
much more explanation than the little caption
underneath the tiny picture?
I’m betting there’s something you want to
know about, but you didn’t know whom to
ask. Finding out more about it might make
your flying better or more enjoyable.
I was surfing through a handful of the
many model-flying forums on the Internet
(this particular one was predominantly
04sig3.QXD 2/25/08 12:21 PM Page 72
74 MODEL AVIATION
devoted to electric flight), where a flier
wrote that he thought it was a shame that
“nobody flew Control-Line anymore.”
Approximately a zillion helpful souls
pointed out to him that he wasn’t looking in
the right circles and that there is plenty of
CL activity all over the world.
Yes, that was an utterly shameless play
on words, but the neat bit about this episode
was what followed. Nobody took the guy to
task; instead the thread exploded.
There were those who thought much
like the person who started the thread.
They found out too! Still others
materialized from cyberspace and admitted
that they, too, were looking to pick up a
handle again. Many of them started
reminiscing because nostalgia can be fun.
Another group responded with helpful
hints for where to find clubs and flying
buddies in various parts of the country, and a
whole other group started to tell this writer
about the “leading edge” stuff going on in
electric-powered CL, including the
availability of new electric ARFs and the
recent adoption of E-power by some
previous World Champions in competitive
Stunt. (Remember that this was on a forum
with a great deal of electric-related activity.)
The collision between nostalgia and the
top of the state of the art was a neat byproduct
of a simple question. Just like some
of the greatest scientific discoveries, it was a
happy accident.
Sullivan, The Fuel Systems Experts.
www.sullivanproducts.com
New!
Heavy Duty Aluminum
Stopper Assembly
Double Screen “Crap
Trap” Fuel Filters Aluminum Check Valve
It’s a Fact: Sullivan Is The
Leader In Fuel System Components
Fuel Clamps For Tubing
Fuel Filler Valve
Gasoline Conversion Kit
Universal Fuel Stopper
Super Klunk
One North Haven St.
Baltimore, MD. 21224
Wrapping Our Arms Around the Big-
Picture Differences Between Electric and
Gas/Glow Flying: When my flying buddy,
Dave T., suggested taking a look at the
differences between electric and gas/glow
power, he was opening a can of worms! The
last time we got together, rather than diving
into the differences in equipment, airframebuilding
requirements, or even airplane
types, being overly ambitious I decided to
shoot for the moon.
All those would have been good things
to write about, but instead I dragged out
some typical torque and power vs. rpm
curves for gas and DC motors, and then I set
out to explain the very real difference in
“feel” between the two. Maybe this “from
first principles” approach was, as I
mentioned, a bit too ambitious, but there
was a method to my madness. So please
allow me to draw a quick outline around the
subject.
• Electric power and gas have fundamentally
different “grunt vs. go” behaviors.
• The gas engine can change its personality
fairly substantially, depending on how
heavily you load it down and how the
propeller pitch and diameter play with the
airplane’s airspeed and drag.
A gas/glow engine typically gains
horsepower with rpm over an extremely
wide range of rpm, but the torque curve is
almost flat over an exceptionally wide range
and falls off at both high and low rpm.
When loaded with a propeller-and-model
combination that keeps the rpm in the nearly
level part of the torque curve, whether on
the ground or in high-speed flight, the
engine’s handling characteristics and
lifetime will be maximized. Large propellers
that load the engine into the drop at the lowrpm
end of the torque plateau create all sorts
of bad handling characteristics.
Finally, operation near the fast-revving
power peak will make considerable racket
and accelerate engine wear, as well as
reducing propeller efficiency and real flying
power in most cases.
• The electriclike characteristic of torque
and power increasing with added load can
be replicated in gas/glow by using an
intentionally mistuned exhaust system.
Although useful, this produces less than
optimal horsepower. This is also known as
the CLPA tuned-pipe setup.
• The motor displays much less flexibility in
its personality, but that basic personality is
suitable to almost everyone except the speed
merchants.
• The motor makes more of torque and
power when you load it down with a bigger
or higher-pitch propeller, as long as the
voltage or “throttle” is unchanged. This
means that the electric power plant has an
inherent tendency to try to fly at the same
speed, whether climbing or diving, as long
as the throttle is unmoved. This is handy in
sport and all manner of aerobatics.
• The process of picking the correct
propeller for both types of power plants is
similar in some respects but still different
04sig3.QXD 2/25/08 12:11 PM Page 74
enough to give you fits, depending on the
type of airplane you want to fly. The flat
torque curve of the gas/glow engine, and
rpm rise during flight, forgives a large range
of pitch choices on a particular airplane,
while the electric setup will unload only a
handful of percent in rpm and requires more
careful pitch selection if you want a
particular flying speed.
In addition to this difference in the flavor
of the horsepower, each power-plant type
has its own interesting list of implementation
details and headaches.
For wet power these include such
subjects as proper fuel-system setup, adding
fuel-pumping systems, bearing and overall
maintenance, tuned exhaust systems, and
noise abatement. Propeller selection can be
magic too, transforming an airplane’s flying
and sound characteristics. There are other
factors, of course.
Electric power has its own techniques
and tricks. Wiring methods and connector
selection aren’t all that difficult, but you’d
think it was rocket science judging by the
amount of discussion!
Battery maintenance is as big of a
subject, probably because of the expense of
the fantastic battery technology these days.
After all, mistreating expensive equipment is
dumb.
Matching propellers to motors and
gearbox ratios seems like black magic to
many people, so I will eventually spend
some time writing about that. Even choosing
a motor is a subject unto itself.
Many of these subjects will wait for
another time, but for now I’ll cover some
specific differences between electric and
gas/glow. I’ll bet that when my buddy, Dave,
suggested a comparison of wet and electric
power, this is what he meant.
I’m going to put aside, for now, any
discussion of park flyers and indoor
airplanes that weigh just a few ounces;
this discussion is about medium- and
high-performance airplanes. Fliers who
were raised on fuel and the others who
were introduced to the hobby/sport with
small electric stuff are both looking at a
major learning curve when they decide to
change from one type of power plant to
the other.
For starters, I’ll dispel a myth. Electric
airplanes do indeed stay cleaner, but they
still require cleaning at the end of the flying
day. For those of us who fly from grass, that
green chlorophyll stain the propeller kicked
up looks the same. Hey! This is important
stuff.
After changing to electric power for his
competitive efforts, Bob Hunt, MA’s editor
emeritus, discovered that he still had to
carefully wipe down his self-designed
Genesis with Windex. He blamed it on all
the oily fingerprints of his wet-flying
buddies who satisfy their still-endless
curiosity by picking up the airplane and
carefully looking at it.
Compare and Contrast: The nowelectrified
CL Genesis’s conversion
highlighted something a few of us have
observed in RC. Even when the horsepower
available in an electric setup is slightly less
than with glow or gas, electric propulsion’s
consistency and controllability can often
help make up for it.
This controllability and reliability factor
is of great importance to some. Many 3-D
and Stunt pilots have learned that electric
power’s time lag-free throttle response is a
great convenience. It makes throttle
management easier.
Complicated multiengine projects are
flown with the proverbial sword of
Damocles hanging above them. For all the
cool factor of multiple engines, the prospect
of an engine failure demands extra diligence
in engine setup and maintenance.
There is the increased probability of an
engine failure; and successful landings with
an engine out are still a less than 50/50
proposition. This is especially true of
airplanes with heavy wing loadings, such as
many Scale subjects.
Here is one of the few cases in which one
power plant has a clear advantage over the
other. Add to that the ease with which
present-day brushless motors can be set to
run in the opposite direction, and counterrotating
twins are a cinch. Still, there is the
issue of excess weight.
Airframes destined for electric power
have historically had to be built more lightly,
76 MODEL AVIATION
04sig3.QXD 2/25/08 12:23 PM Page 76
just to be able to fly well. However, the
advent of high-discharge-rate Lithium-based
battery chemistries has changed all that.
The rule used to be that for lively
performance you needed an airframe and
radio that weighed no more than the
batteries and motor put together. If the
airframe with radio weighed as little as 1.5
times the power plant, the performance
would barely be adequate for a slow-flying
trainer or an electric glider.
Today’s batteries allow you to fly 10-
minute, high-energy aerobatic routines with
long vertical climbs at the same 1.5:1 weight
ratio. With Li-Polys the motor battery
combinations can typically weigh 1-11/4
times as much as an equivalent engine,
throttle servo, and full tank of fuel.
No kidding. Remember to do the
comparison with a full, or at least half full,
tank.
Still, if your airframe is decidedly on the
heavy side—and suffers from a bit of extra
weight, even with a glow engine—
performance is definitely going to suffer
after you add the weight of enough battery.
If you tend to build really heavy, stick with
gas!
The problem becomes a sort of “domino
theory of weight.” Let’s look at some good
cases and the power-to-weight loadings they
produce.
To start out, the horsepower-to-weight
ratio that is necessary to fly with a desired
performance level is the same with electrics
as with gas. It’s just that in one case we refer
to horsepower (usually the manufacturer’s
claim that is measured at impractically high
rpm), and in the other case we refer to watts.
(These are watts measured coming out of the
battery—not out of the motor.) This means
we have to do a bit of decoding before we
can make a comparison.
Let’s do that using a 5-pound airplane as
an example. If it is a trainer, a glow .25
engine will be just enough power as long as
the grass runway has been mowed recently.
A .40 will be necessary for decent
performance, and a .60 will yield sprightly
performance.
A typical .25 puts out maybe threequarters
horsepower at some ridiculously
high rpm, such as 18,000. That probably
translates to maybe 70% power at 12,000
rpm, so we get approximately 0.5 horse.
Multiply by 746 watts per horse and we get
maybe 400 watts, give or take 10% or 20%.
On a 5-pound airplane, that works out to 80
watts per pound.
Derate that by maybe 10% for the
motor’s efficiency to get the actual power at
the propeller, and we are down near 70 watts
per pound. That puts us at the middle of the
trainerlike power bracket, as defined by the
time-honored rule of thumb that has been
attributed to longtime electric-flight guru,
Keith Shaw.
The old rule specifies 50-85 watts per
pound for trainers that need to take off rather
than be hand-launched. The 50-watt-perpound
figure is marginal for grass, but it will
do nicely for pavement and hand launch.
Most electric FF designs I am familiar with
use a relatively low power loading in this
neighborhood.
Now we move up to a .40 or maybe a .46
engine. They range all over the place, but
let’s assume 1.5 horsepower at something
such as 15,000 or 16,000 rpm as a
manufacturer’s claim.
Derating to a more sensible 12,000 rpm
and converting to watts, we get maybe 600
or 800. That would work out to 125-150
watts per pound for the same 5-pound
airplane. Subtract maybe 10% for the motor
efficiency, and we are in line with the old
rule of thumb saying that 100-150 watts
provides “sport” performance.
If you managed to build a 5-pounder,
with a .60 in it, you would have a tiger by
the tail! That muffled .60 would yield just
less than 1.5 horses, and that would translate
to close to 1,000 watts. A figure of 200 watts
per pound is the same power loading as
competitive RC Aerobatics and IMAC
(International Miniature Aerobatic Club) 3-
D models have. I like it!
Wow! We are out of space for this
installment, but I promise to pick up next
time with the “domino theory of airplane
weight” and include notes about converting
an airframe that is intended for wet power to
electric and vice versa.
Until then, go fly something! MA
WW-II bomber crews remember that it
was the P-51B Mustang that turned the
tide of daylight bombing. Previously,
fighter escorts such as the P-47 did not
have the required range and had to
turn back, leaving the bombers to
make their bombing runs with no
fighter protection. North American
Aviation went to work delivering the
first P-51 prototype in just 102 days and
it was a remarkable step forward in
fighter performance. The P-51B could
climb to 20,000 feet in under 6
minutes and fly 440 mph at altitude -
faster than any other fighter plane at
the time. Most importantly, it was the
range of 1080 miles (a whopping 2600
mile range with drop tanks), which
enabled the P-51B to escort the
bombers all the way to Berlin.
The SIG P-51B is loaded - flaps, retracts,
complete "Shangri-La" decal set,
painted fiberglass cowl, and molded
canopy. The model is hand built from
balsa and plywood and expertly
covered in SIG AeroKote®.
SIG Manufacturing Company, Inc.
P. O. Box 520 • Montezuma, Iowa
www.sigmfg.com • 641-623-5154
P-51B Mustang ARF
SPECIFICATIONS:
Wingspan: 66.9 inches
Length: 55.9 inches
Engine Required: .90 - 1.00 cu. in.
2-stroke or 4-stroke
Radio Required: 6 Channel, 8 servos
Order No. SIGRC103ARF
* $319.99 at Participating Dealers
04sig3.QXD 2/25/08 12:23 PM Page 78
Edition: Model Aviation - 2008/04
Page Numbers: 72,74,76,78
72 MODEL AVIATION
Simple questions can sometimes lead to complicated answers
If It Flies ... Dean Pappas | [email protected]
Also included in this column:
• Compare and contrast:
electric and glow
The firewall was cut away and an
electron wall was added to mount the
Axi 2826/10 motor. The battery is a fourcell
Thunder Power Pro Lite 4200 mAh
pack; the ESC is a Phoenix-45.
The Genesis started with a glow Ro-Jett .40 with carbon-fiber tuned pipe. It provided
excellent power and table manners. The package weighed just less than 19 ounces
without fuel.
Bob Hunt’s Genesis Extreme CL Stunt
model lets us compare electric and glow
power. It gained roughly 1 ounce
compared with the piped glow engine,
and electric’s predictability made a
winning combination.
WHAT IS THE mission of “If It Flies ... ”?
From where I sit, it is to help tie together the
big picture in aeromodeling, helping us all
benefit from the knowledge, experience, and
techniques used in other corners of “toy
airplane-dom.”
To garner that benefit we need to know
just enough about some flying event or
some particular type of flying to be able to
have a discussion. Sometimes just knowing
the jargon is enough.
For example, if they look like ailerons on
an RC model, they are probably the same
things that are called “maneuvering flaps”
on a CL aircraft. If you see the same things
on a competition FF Gas model, they are
probably called “variable-camber flaps” or
something similar.
Once we get past the problems posed by
the mere mechanics of talking to each other,
we aeromodelers tend to enjoy each other’s
company as much as we like finding out
about different kinds of flying machines.
Sure, some will scoff at whatever is
different, but that’s just because they don’t
understand it.
For instance, lots (and I mean lots!) of
fliers have never used anything other than
heat-shrink covering. When most of them
look at the elbow-deep finish on a tissue
paper-and-dope-finished CL Aerobatics
(Stunt) model or the mirrorlike epoxy finish
on a Pylon racer, their responses range from
“I could never do that!” to “Wow!” and
eventually to “I have to learn how to do that!”
That’s our mission: to help connect each
of us to other fliers in many remote corners of
the aeromodeling world. You have to see the
many different things other people do,
whether it’s something you think is
interesting, useful, or just cool. Then the
eager research staff at “If It Flies ... ” finds
people who know how to do that thing well,
and we suck the information out of their
brains and present it in these pages!
To carry out this grandiose mission, the
research staff (that’s me, myself, and I) need
your questions! What do you want to know
about?
What little snippet of strange (to you)
aeromodeling have you seen that needs an
explanation? Have you seen something at a
flying field or in any magazine that needed
much more explanation than the little caption
underneath the tiny picture?
I’m betting there’s something you want to
know about, but you didn’t know whom to
ask. Finding out more about it might make
your flying better or more enjoyable.
I was surfing through a handful of the
many model-flying forums on the Internet
(this particular one was predominantly
04sig3.QXD 2/25/08 12:21 PM Page 72
74 MODEL AVIATION
devoted to electric flight), where a flier
wrote that he thought it was a shame that
“nobody flew Control-Line anymore.”
Approximately a zillion helpful souls
pointed out to him that he wasn’t looking in
the right circles and that there is plenty of
CL activity all over the world.
Yes, that was an utterly shameless play
on words, but the neat bit about this episode
was what followed. Nobody took the guy to
task; instead the thread exploded.
There were those who thought much
like the person who started the thread.
They found out too! Still others
materialized from cyberspace and admitted
that they, too, were looking to pick up a
handle again. Many of them started
reminiscing because nostalgia can be fun.
Another group responded with helpful
hints for where to find clubs and flying
buddies in various parts of the country, and a
whole other group started to tell this writer
about the “leading edge” stuff going on in
electric-powered CL, including the
availability of new electric ARFs and the
recent adoption of E-power by some
previous World Champions in competitive
Stunt. (Remember that this was on a forum
with a great deal of electric-related activity.)
The collision between nostalgia and the
top of the state of the art was a neat byproduct
of a simple question. Just like some
of the greatest scientific discoveries, it was a
happy accident.
Sullivan, The Fuel Systems Experts.
www.sullivanproducts.com
New!
Heavy Duty Aluminum
Stopper Assembly
Double Screen “Crap
Trap” Fuel Filters Aluminum Check Valve
It’s a Fact: Sullivan Is The
Leader In Fuel System Components
Fuel Clamps For Tubing
Fuel Filler Valve
Gasoline Conversion Kit
Universal Fuel Stopper
Super Klunk
One North Haven St.
Baltimore, MD. 21224
Wrapping Our Arms Around the Big-
Picture Differences Between Electric and
Gas/Glow Flying: When my flying buddy,
Dave T., suggested taking a look at the
differences between electric and gas/glow
power, he was opening a can of worms! The
last time we got together, rather than diving
into the differences in equipment, airframebuilding
requirements, or even airplane
types, being overly ambitious I decided to
shoot for the moon.
All those would have been good things
to write about, but instead I dragged out
some typical torque and power vs. rpm
curves for gas and DC motors, and then I set
out to explain the very real difference in
“feel” between the two. Maybe this “from
first principles” approach was, as I
mentioned, a bit too ambitious, but there
was a method to my madness. So please
allow me to draw a quick outline around the
subject.
• Electric power and gas have fundamentally
different “grunt vs. go” behaviors.
• The gas engine can change its personality
fairly substantially, depending on how
heavily you load it down and how the
propeller pitch and diameter play with the
airplane’s airspeed and drag.
A gas/glow engine typically gains
horsepower with rpm over an extremely
wide range of rpm, but the torque curve is
almost flat over an exceptionally wide range
and falls off at both high and low rpm.
When loaded with a propeller-and-model
combination that keeps the rpm in the nearly
level part of the torque curve, whether on
the ground or in high-speed flight, the
engine’s handling characteristics and
lifetime will be maximized. Large propellers
that load the engine into the drop at the lowrpm
end of the torque plateau create all sorts
of bad handling characteristics.
Finally, operation near the fast-revving
power peak will make considerable racket
and accelerate engine wear, as well as
reducing propeller efficiency and real flying
power in most cases.
• The electriclike characteristic of torque
and power increasing with added load can
be replicated in gas/glow by using an
intentionally mistuned exhaust system.
Although useful, this produces less than
optimal horsepower. This is also known as
the CLPA tuned-pipe setup.
• The motor displays much less flexibility in
its personality, but that basic personality is
suitable to almost everyone except the speed
merchants.
• The motor makes more of torque and
power when you load it down with a bigger
or higher-pitch propeller, as long as the
voltage or “throttle” is unchanged. This
means that the electric power plant has an
inherent tendency to try to fly at the same
speed, whether climbing or diving, as long
as the throttle is unmoved. This is handy in
sport and all manner of aerobatics.
• The process of picking the correct
propeller for both types of power plants is
similar in some respects but still different
04sig3.QXD 2/25/08 12:11 PM Page 74
enough to give you fits, depending on the
type of airplane you want to fly. The flat
torque curve of the gas/glow engine, and
rpm rise during flight, forgives a large range
of pitch choices on a particular airplane,
while the electric setup will unload only a
handful of percent in rpm and requires more
careful pitch selection if you want a
particular flying speed.
In addition to this difference in the flavor
of the horsepower, each power-plant type
has its own interesting list of implementation
details and headaches.
For wet power these include such
subjects as proper fuel-system setup, adding
fuel-pumping systems, bearing and overall
maintenance, tuned exhaust systems, and
noise abatement. Propeller selection can be
magic too, transforming an airplane’s flying
and sound characteristics. There are other
factors, of course.
Electric power has its own techniques
and tricks. Wiring methods and connector
selection aren’t all that difficult, but you’d
think it was rocket science judging by the
amount of discussion!
Battery maintenance is as big of a
subject, probably because of the expense of
the fantastic battery technology these days.
After all, mistreating expensive equipment is
dumb.
Matching propellers to motors and
gearbox ratios seems like black magic to
many people, so I will eventually spend
some time writing about that. Even choosing
a motor is a subject unto itself.
Many of these subjects will wait for
another time, but for now I’ll cover some
specific differences between electric and
gas/glow. I’ll bet that when my buddy, Dave,
suggested a comparison of wet and electric
power, this is what he meant.
I’m going to put aside, for now, any
discussion of park flyers and indoor
airplanes that weigh just a few ounces;
this discussion is about medium- and
high-performance airplanes. Fliers who
were raised on fuel and the others who
were introduced to the hobby/sport with
small electric stuff are both looking at a
major learning curve when they decide to
change from one type of power plant to
the other.
For starters, I’ll dispel a myth. Electric
airplanes do indeed stay cleaner, but they
still require cleaning at the end of the flying
day. For those of us who fly from grass, that
green chlorophyll stain the propeller kicked
up looks the same. Hey! This is important
stuff.
After changing to electric power for his
competitive efforts, Bob Hunt, MA’s editor
emeritus, discovered that he still had to
carefully wipe down his self-designed
Genesis with Windex. He blamed it on all
the oily fingerprints of his wet-flying
buddies who satisfy their still-endless
curiosity by picking up the airplane and
carefully looking at it.
Compare and Contrast: The nowelectrified
CL Genesis’s conversion
highlighted something a few of us have
observed in RC. Even when the horsepower
available in an electric setup is slightly less
than with glow or gas, electric propulsion’s
consistency and controllability can often
help make up for it.
This controllability and reliability factor
is of great importance to some. Many 3-D
and Stunt pilots have learned that electric
power’s time lag-free throttle response is a
great convenience. It makes throttle
management easier.
Complicated multiengine projects are
flown with the proverbial sword of
Damocles hanging above them. For all the
cool factor of multiple engines, the prospect
of an engine failure demands extra diligence
in engine setup and maintenance.
There is the increased probability of an
engine failure; and successful landings with
an engine out are still a less than 50/50
proposition. This is especially true of
airplanes with heavy wing loadings, such as
many Scale subjects.
Here is one of the few cases in which one
power plant has a clear advantage over the
other. Add to that the ease with which
present-day brushless motors can be set to
run in the opposite direction, and counterrotating
twins are a cinch. Still, there is the
issue of excess weight.
Airframes destined for electric power
have historically had to be built more lightly,
76 MODEL AVIATION
04sig3.QXD 2/25/08 12:23 PM Page 76
just to be able to fly well. However, the
advent of high-discharge-rate Lithium-based
battery chemistries has changed all that.
The rule used to be that for lively
performance you needed an airframe and
radio that weighed no more than the
batteries and motor put together. If the
airframe with radio weighed as little as 1.5
times the power plant, the performance
would barely be adequate for a slow-flying
trainer or an electric glider.
Today’s batteries allow you to fly 10-
minute, high-energy aerobatic routines with
long vertical climbs at the same 1.5:1 weight
ratio. With Li-Polys the motor battery
combinations can typically weigh 1-11/4
times as much as an equivalent engine,
throttle servo, and full tank of fuel.
No kidding. Remember to do the
comparison with a full, or at least half full,
tank.
Still, if your airframe is decidedly on the
heavy side—and suffers from a bit of extra
weight, even with a glow engine—
performance is definitely going to suffer
after you add the weight of enough battery.
If you tend to build really heavy, stick with
gas!
The problem becomes a sort of “domino
theory of weight.” Let’s look at some good
cases and the power-to-weight loadings they
produce.
To start out, the horsepower-to-weight
ratio that is necessary to fly with a desired
performance level is the same with electrics
as with gas. It’s just that in one case we refer
to horsepower (usually the manufacturer’s
claim that is measured at impractically high
rpm), and in the other case we refer to watts.
(These are watts measured coming out of the
battery—not out of the motor.) This means
we have to do a bit of decoding before we
can make a comparison.
Let’s do that using a 5-pound airplane as
an example. If it is a trainer, a glow .25
engine will be just enough power as long as
the grass runway has been mowed recently.
A .40 will be necessary for decent
performance, and a .60 will yield sprightly
performance.
A typical .25 puts out maybe threequarters
horsepower at some ridiculously
high rpm, such as 18,000. That probably
translates to maybe 70% power at 12,000
rpm, so we get approximately 0.5 horse.
Multiply by 746 watts per horse and we get
maybe 400 watts, give or take 10% or 20%.
On a 5-pound airplane, that works out to 80
watts per pound.
Derate that by maybe 10% for the
motor’s efficiency to get the actual power at
the propeller, and we are down near 70 watts
per pound. That puts us at the middle of the
trainerlike power bracket, as defined by the
time-honored rule of thumb that has been
attributed to longtime electric-flight guru,
Keith Shaw.
The old rule specifies 50-85 watts per
pound for trainers that need to take off rather
than be hand-launched. The 50-watt-perpound
figure is marginal for grass, but it will
do nicely for pavement and hand launch.
Most electric FF designs I am familiar with
use a relatively low power loading in this
neighborhood.
Now we move up to a .40 or maybe a .46
engine. They range all over the place, but
let’s assume 1.5 horsepower at something
such as 15,000 or 16,000 rpm as a
manufacturer’s claim.
Derating to a more sensible 12,000 rpm
and converting to watts, we get maybe 600
or 800. That would work out to 125-150
watts per pound for the same 5-pound
airplane. Subtract maybe 10% for the motor
efficiency, and we are in line with the old
rule of thumb saying that 100-150 watts
provides “sport” performance.
If you managed to build a 5-pounder,
with a .60 in it, you would have a tiger by
the tail! That muffled .60 would yield just
less than 1.5 horses, and that would translate
to close to 1,000 watts. A figure of 200 watts
per pound is the same power loading as
competitive RC Aerobatics and IMAC
(International Miniature Aerobatic Club) 3-
D models have. I like it!
Wow! We are out of space for this
installment, but I promise to pick up next
time with the “domino theory of airplane
weight” and include notes about converting
an airframe that is intended for wet power to
electric and vice versa.
Until then, go fly something! MA
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04sig3.QXD 2/25/08 12:23 PM Page 78