FOREWORD: At the 2004 Weak Signals expo in Toledo, Ohio,
former AMA District II vice president and current AMA Flying Site
Coordinator for the Eastern Region Joe Beshar, who is an experienced
electric-power enthusiast, made a suggestion. He mentioned to AMA
Director of Publications Rob Kurek that we badly need a standard
reference document that will allow any modeler to size the proper
electric-motor system to model aircraft of any size and weight. What
you are about to read resulted from Joe’s suggestion.
Background: When a person enters model aviation, he or she has a
choice of power sources for his or her aircraft. Years ago those
options included rubber power, hand launching (as with a glider),
towing a glider with a cord or a fueled engine, and gasoline/sparkignition
engines or later glow-fueled engines.
As time went on and the glow engine became the most popular
source of model-aircraft power, modelers quickly became adept at
selecting the right-size engine for every size and weight of model. An
engine’s cu. in. displacement was related to model weight and model
size (usually wing area expressed in square inches).
Engine classes became known as 1⁄2A, A, B, C, and even D. Each
category covered a range of engine displacements (such as—if my
memory serves me correctly—0.09-.199 cu. in. was considered Class
A). Early FF competition rules related engine displacement to weight,
so that a .19 cu. in.-engine-powered model had to weight 19 ounces,
etc.
When RC started to come of age in the 1950s, we learned (by
experience) that 1⁄2A RC models could weigh as much as 18-20
ounces, Class A RC models could weigh as much as 30 ounces, and
so on. Although these engines are rated in horsepower (and thrust), to
this day most modelers simply relate engine displacement to model
weight and size!
Enter Electric Power: Although some of the first electric-powered
flights were made in the late 1950s and early 1960s, electric power as
we know it today didn’t really come into its own until the early 1970s.
Then it became obvious that engine displacement, for identification
purposes, would not work with electric. To make an early distinction
between fuel and electric, it was decided to refer to fueled power as
“engines” and electric power as “motors.”
Bob Boucher—one of the leaders of electric-powered flight in the
USA—began referring to his motors by equivalent glow-engine
displacements. His “05” motor was supposed to be equivalent to an
.049 cu. in. glow engine, a “40” motor was equivalent to a .40 cu. in.
glow engine, etc.
While this was going on, Germany’s Graupner company was
assigning names such as Speed 400, Speed 500, and the like. The
numbers were rounded off from the Mabuchi manufacturer’s model
numbers; “RS-380” became a Speed 400, “RS-540” became a Speed
500, and so on. Surely that fact couldn’t help you select a motor for a
particular size and weight of model.
What to Do? Approximately 30 years have now gone by since that
start in electric-powered flight. Although many still have questions,
we know much more now and have developed some excellent
techniques for “sizing,” or matching the correct motor to any size and
weight of model aircraft.
When you look back at this time frame, you can compare it with
the advent of the gasoline engines in the early 1930s, which
ultimately led to the cu. in.-displacement sizing of those engines in
the 1940s. Identifying fueled and electric engines/motors has taken
time.
The main thrust of this article is to explain how to select, or size,
your motors to make them suitable for powering models in flight. I
hope to cover such things as motor identification, motor types, direct
vs. gear drive, motor power (in watts) as it relates to aircraft weight,
and many other details. I hope to make this a standard reference so
that no one will have to ask, “What motor should I use?”
Aircraft Categories:
1) Use electric-powered kits, ARF models, and published plans.
We currently enjoy the fact that many kits, ARFs, and published plans
exist for all kinds of electric-powered models. In this category, the
aircraft designer or manufacturer has selected the motor for you. You
might want to improve on the initial choice, but you can easily get in
those first few flights before you fine-tune the selection process.
2) Make a glow-fuel-powered kit electric powered. There may be a
particular glow-fueled kit or ARF that you want to build, from
inception, as an electric-powered model. You want to be able to select
a motor, the type of drive (direct or geared), the type of battery pack,
and the battery pack’s capacity, but you want to make these selections
so that you can install the electric power-system components as you
construct the model.
MA Technical Editor Bob Aberle with sport/aerobatic Acrovolt
constructed in 1995. As designed by Tom Hunt, it would have been
comfortable with 60-size glow engine but was powered by DeWALT
18-volt cordless-drill motor with belt drive. Text tells how it was
greatly improved with new brushless motor and Li-Poly batteries.
Photos courtesy the author
by Bob Aberle
Using Whattmeter to take motor data of electric power equipment
in Hobby Lobby Bonnie 20, note battery packs lined up in
foreground for test purposes. In front is TNC tachometer for
measuring propeller rpm.
AstroFlight Super Whattmeter is the heart of any motor-selection
process. When inserted between battery and motor, it indicates
motor current, voltage, power (watts), and amount of energy going
into or out of battery (in Ah).
Bonnie 20 electric-powered ARF is featured in Sport Aviator review.
It could have been powered by .20-.32 glow engine. It also makes a
wonderful electric-powered advanced RC trainer.
Hobbico SuperStar EP is intended for glow power. Bob explains
in the text how to convert it to electric power.
3) Convert a glow-fuel-powered RC model you already have to
electric power. Let’s say you have a built-up and flying glow-fueled
airplane, and you decide to remove that engine and retrofit an
electric power system. How would you go about making that
selection?
4) Update an old electric-powered model that uses old-style
technology to use all of the latest equipment. How would you make
the specific selections based on the aircraft’s current size and
weight? In this case you would be looking at a new brushless motor,
a dedicated sensorless ESC, and probably lightweight and highcapacity
Li-Poly batteries.
Abstract: My close friend, flying partner, and fellow Model
Aviation Hall of Famer Tom Hunt is probably one of the most
famous and experienced electric-power fliers in the country. He has
been my primary consultant throughout this article’s preparation. He
recently made a thought-provoking observation, which follows.
“The main problem in the selection process is that the electric
motor has a much broader operating range than an IC [internal
combustion] engine. I can have an AXI 2212/34 brushless motor fly
a 10-ounce [total weight] aircraft at 60 watts input, a 14-ounce
aircraft at 95 watts input [that’s a 50% power increase—try that with
a glow engine!], and a 22-ounce model at 120 watts input [that’s a
100% increase].”
As you can see from Tom’s examples, any specific motor will
have far more application than a comparable glow engine. Because
of this, there are many more choices in the selection process when
going to electric power. That is what mystifies most modelers, and I
hope this article will help clear things up.
Measuring Motor Input Power (Watts): Motors are not referred to
by cu. in. displacement (as glow engines are). Motors are defined by
the term “power,” which is measured in watts. All references to
power in watts is input to the motor (or output of the battery). The
motor’s output power is a function of its efficiency.
As Tom pointed out, a motor can run at 60, 95, or even 120
watts. The primary rule is that power (in watts) equals motor current
(in amperes, or amps) multiplied by the voltage.
The voltage is determined by the battery and will vary
according to battery type (Ni-Cd, NiMH, or Li-Poly) and the
number of cells employed in the particular battery pack. The
current is determined by several things, including the resistance of
the motor windings, the size of the propeller, and the type of drive
(direct, with a gear reduction, or belt-reduction drive).
So you have the variables current (amps) and voltage that yield
Sample page from ElectriCalc motor-selection computer program.
This type of program goes hand in hand with motor-manufacturer
data to assist in selection process for specific size/weight aircraft.
You must know aircraft’s exact weight to select motors. Pelouze
Model PE-5 scale (L) weighs items as heavy as 5 pounds in 0.1-
ounce increments. My Weigh digital scale (R)—from
www.goodscale.com/scale—is intended for parking lot and indoor
micro RC models.
Typical battery chargers (L-R, top and bottom): AstroFlight 109
and Peak Electronics Lithium charger intended for Li-Poly only.
AstroFlight 110 Deluxe and FMA Direct Super Nova peak-detect
chargers intended for Ni-Cd and NiMH.
The TNC tachometer is one of the best digital tachometers on the
market. It is available from Skyborn Electronics.
Page from The Great Electric Motor Test—one of many sources for
motor data. It provides such parameters as motor current, voltage,
power (watts), propeller sizes, and rpm.
power (watts). This is where the fun starts since you can vary
everything, including the propeller size, the motor drive, and the
battery. Each variable will produce different results.
With a fueled engine, you can listen to the sound and adjust the
carburetor (by ear) until you reach the correct power level. With a
motor, there is essentially no noise and therefore nothing to listen to.
So how do you obtain a figure for power (watts)?
It would be nice if the motor manufacturers had printed on the
box “100 watts (maximum short duration, 75 watts continuous),”
much as engine manufacturers indicate “.049 cu. in., or 1⁄2A.” The
variables make that type of identification impractical. Most motor
manufacturers publish excellent motor data that I will write about
later, along with computer programs that support this process.
For now, you must learn that the most important “tool” for the
electric-power flight enthusiast is the ammeter and wattmeter. We
primarily use the AstroFlight Model 101 Super Whattmeter, which
1.25 Powered sport sailplane, indoor RC airplane
you can purchase for roughly $60. It is the latest version and will
measure to 10 mA instead of 100 mA. It can also read below 4.0
volts by adding a four-cell receiver battery pack. Make sure you
purchase this new version.
The meter is self-powered by the system you are measuring. It
provides four important motor parameters: voltage (volts), current
(amps), power (watts), and the capacity going into or out of the
battery, measured in ampere-hours (Ah). A list of articles and Web
sites at the end of this article will provide the details about the
specific use of the Whattmeter.
Some modelers already own digital-type multimeters, sold by
stores such as RadioShack. Those meters are fine, but most are
somewhat limited in current range.
If you try to check a 25-amp motor with some of these meters,
you will probably end up blowing a fuse or even damaging the
device. Many of these multimeters are of the newer “auto-ranging”
type which can also get complicated during regular use. I suggest that
you stick with the AstroFlight meter for its proven ability to easily do
the job!
Aircraft Weight and Power Loading: Now that you have the motor
“identified” by power, how do you relate it to a specific-size model?
There are actually two aircraft parameters you need to be interested
in, one of which is the model’s total weight. This is generally
measured in ounces for the smaller airplanes and pounds for the
larger models.
If you are in the planning stages of selecting a motor, you may be
estimating an aircraft’s weight as a starting point. If you are dealing
with an existing model, you will have to weigh it on a scale. There
are many digital-type scales on the market that read to within 0.1
ounce. You can buy one at a stationery store such as Staples, Office
Max, and others. After constructing or assembling an ARF, you will
also have to weigh it to verify your motor selection or help you
improve performance by selecting a different motor.
Knowing the motor’s power and the model’s weight, you can
come up with one of the most important “combination parameters,”
known as watts/ounce or watts/pound. These “power loading” values
are used for judging aircraft performance.
The term watts/ounce is generally used to describe the power
loading of models that weigh as much as 1 pound. Watts/pound is a
more common term for models that weigh more than 1 pound.
Through the years, experienced electric-power enthusiasts have
come up with a series of watts/ounce and watts/pound figures that
relate to an airplane’s expected performance. Table 1 and Table 2
provide the accepted parameters that cover most aircraft types. These
numbers aren’t absolute; they are only to be used as a guide.
Wing Loading and Skill Level: After determining motor power
(watts) and aircraft weight, you must focus on airplane size, which
involves wing area and then wing loading (which takes into account
wing area and aircraft weight).
The term “power loading” that I described (watts/ounce and
watts/pound) involves motor power and model weight; they are
aircraft related. The term “wing loading” relates to the pilot’s skill
level and directly to the minimum speed the model can fly (stall
speed). Aircraft with light wing loadings will be easier to fly (for
beginners and sport fliers) than those with high wing loadings
(favored by the experienced or expert fliers).
To determine wing loading, you need to know your model’s wing
area and weight. Wing area is the wingspan multiplied by the wing’s
average width (wing chord) and is expressed in square feet (sq. ft.).
Wing area may be a given with kits, ARFs, and even published
plans. The specifications that accompany these usually include the
wing area. A pure rectangular-shaped wing is easy to figure out (span
multiplied by chord), but an elliptical-shaped wing is much more
complicated. You typically do it by estimating the average wing
chord (width).
Once you know the wing area expressed in square inches (sq. in.),
you must convert that to sq. ft. by dividing it by 144. Let’s say your
aircraft has 400 sq. in. of wing area. Divide that by 144, and you get
2.78 sq. ft. The aircraft weighs 40 ounces, so divide that by the 2.78
sq. ft. to obtain the wing loading of 14.4 ounces/sq. ft.
Table 3 provides acceptable wing loadings for various pilot-skill
levels. Values start at 5 ounces/sq. ft. and can exceed 35 ounces/sq. ft.
In the past few paragraphs you learned about motor power (watts).
Then you learned how power relates to model weight. Last, you
learned how model size and weight are related to the pilot’s skill
level. You must take all of these parameters into consideration when
selecting the proper motor for your model.
Thrust: I don’t use it because in general it is not a reliable figure.
The thrust one measures on the bench has little to do with the thrust
the propeller produces as it moves through the air.
Some modelers came up with a factor indicating that your model
can weigh as much as three or four times the rated motor thrust.
That’s fine, if you really know what that thrust is. So I avoid using
this parameter!
Selection Process Details: Before I provide some examples of motor
selection, I want to introduce you to the subject of motor data. When
you set out to make a specific motor selection, you need a large
volume of motor-parameter data at your disposal.
That information should contain such things as motor current,
voltage, and watts for a range of recommended propellers (identified
by diameter and pitch). It should also provide a range of voltages so
that you can choose a battery type and capacity to suit the application.
It would be nice to have that kind of data available for every type
of motor available on our hobby market or, better still, have in one
place. I would love to have it on one Web site, but that hasn’t
happened yet. Information is available on many Web pages, and I’ll
steer you to the major ones.
But before I do that, I want to discuss “voltage.” Much of the
motor data presented on manufacturers’ Web sites indicate numbers
of battery cells and the type of battery, but it will not state the
voltage!
When referring to numbers of cells of Ni-Cd- or NiMH-type
batteries, each cell has a nominal 1.2 volts. If the data indicates eight
NiMH cells, the nominal voltage will be eight multiplied by 1.2, or
9.6. If there are two Li-Poly cells, the voltage will be two mulitiplied
by 3.7 (that is peculiar to Li-Poly batteries), or 7.4.
Some sites are kind enough to indicate the measured voltage
under load, which is really what you want, but others will make you
use the battery cell count, and that takes extra time. Be aware!
Motor Data Web Sites: The following sites are not in any particular
order, and they are not all that are available. They are what I think
you will need as a starting point for most of your motor selections.
• www.flyingmodels.org. Webmaster Fredrik Wergeland of
Stockholm, Sweden, is responsible for this data. After getting on the
site, select “The Great Electric Motor Test,” and then you have a
choice of standard or brushless motors.
There are not many standard motors, but there are quite a few
Watts/Ounce Type of Aircraft
1.00 Model that barely gets off the ground
2.00 Parking lot/backyard flyer
3.00 Aerobatic flier
5.00 Electric 3-D model
Table 1. Watts/Ounce (Models Weighing Less Than 1 Pound)
40-50 “Sunday flier,” sport sailplane, Old-Timer
Watts/Pound Type of Aircraft
30 Model that barely gets off the ground
60-70 Mildly aerobatic model
80-100 Aggressive airplane
100 plus Ducted-fan aircraft, competition Sailplane,
electric-powered 3-D model
Table 2. Watts/Pound (Models Weighing More Than 1 Pound)
38 MODEL AVIATION
brushless including Mega, Model Motors (AXI 22 and 28 series and
Mini AC), Nippy Black, MP Jet, Kontronik, and Jeti Phasor. There is
a promise that data will be posted for Speed 280 through Speed 600
ferrite motors in the future.
This Web site has nothing to do with the popular Flying Models
magazine.
• www.aircraft-world.com/default.asp?id=18. This site—run by
Dave Radford of Air Craft Inc. (of Japan)—provides data for GWS,
Hacker, Mega, Model Motors (including the AXI 41 series), MP Jet,
and QRP motors.
• www.astroflight.com. When you access this site, you can select
brushless motors or cobalt airplane motors. In some cases only one
propeller choice is given for a particular motor/battery combination.
That should still provide a good starting point!
• www.hackerbrushless.com/motors.shtml. Hacker USA provides
data for all of its brushless motors. After accessing the site, select the
motor you are interested in, such as the “B20 Series.” Then scroll
down and click on “Click here for B20 Series Application Chart.”
You will be surprised by the extent of this data because it goes as far
as relating motor type to specific model aircraft by name!
• www.balsapr.com/catalog/motors. This is part of the Balsa Products
site. Under brushless motors you will find the new Feigao brand.
There is a category for Outrunner Brushless motors. After that is the
entire series of GWS motors, including the IPS series, LPS series,
EPS series, and more!
When you get to the particular motor series, you have to click
again to obtain the actual data. Please be patient.
• http://home.ptd.net/~rcm65/motdata.html. This is Dick Miller’s
Motor Characteristics data that he has provided as a free service to
electric-power modelers for so many years. You can sort the data by
motor name or by a typical model’s wing area.
Dick covers the following motors, many of which are the small
variety used for parking lot or indoor flying: DC 5-2.4; DC-1717;
GWS (all); AstroFlight Firefly; MTM; VL Products; Ikarus; Hi-Line
Ltd.; KP-00; Kenway; Nikko; Peck-Polymers; and the Speed 280,
300, and 400 series.
Computer Programs to Help in the Selection Process: Researching
many Web sites to obtain motor data can be a real chore. A few years
ago two software programs came onto the hobby market that were
specifically designed to aid the electric-power enthusiast in choosing
a motor. The programs have been continually upgraded through the
years to the point where they include a tremendous amount of stored
motor (and aircraft) data parameters.
When these programs were first offered, we didn’t even have
brushless motors. Now they are the most popular item in electricpowered
flight. Both programs have literally grown with our hobby,
and you don’t have to be a computer expert or highly technical to be
able to use them.
I have used Sid Kauffman’s ElectriCalc for many years. It is up to
version 2.20 and has a list price of $49.95. Software is now provided
on a CD. It is intended for PCs with operating systems of Windows
Type of Aircraft and Skill Level
15-20 Larger trainer, Sport Scale model, sport
aerobatic model
20-25
Fast sport model (usually with more than
adequate power)
25-35 Scale model, larger multiengine model
35 plus Not for the author!
5-10
Park flyer, basic trainer, powered sport
sailplane
10-15
Faster sport flier, smaller trainer,
“Sunday” flier
Table 3. Wing Loading and Skill Level
Wing Loading
(Ounces/Square Foot)
98 and up. There are no provisions for Macintosh users—sorry!
You can find the details of this product and order a copy at
www.slkelectronics.com./ecalc/index.htm. The program’s present
version has stored data for 946 motors, and that figure is constantly
increasing as new motors come onto the market. There are two ways
to use ElectriCalc.
1) Enter your aircraft’s parameters (wing area, weight, and
approximate drag coefficient, which you can obtain by using the
simple help function called “Cdrag”). Pick the battery cell type and
the number of cells you think you might need. Choose a motor from
the extensive list shown, and pick a propeller that will provide
enough power to fly the model.
2) When the program is launched, an aircraft will already be
chosen (default) from the database or the last one you looked at will
be resident in the spreadsheet. Search the “planes” database for the
aircraft you are interested in or find one similar in size and weight to
your intended model, and rename and revise the parameters to
emulate your airplane.
Option 1 requires that you have a good idea of what propulsion
system should be in the model. This is seldom the case when one is
just getting into electric-powered flight.
Option 2 allows you to “repeat” an aircraft that has already flown.
(Much of the database in ElectriCalc on “planes” has been given to
SLK Electronics by proficient electric-power modelers.)
Option 2 also allows you to experiment with an existing design to
see what other motor or motor/gearbox combinations may work.
You can do a lot of “what-iffing” without having to leave your
computer station. This can be a big time-saver.
The other popular motor-selection program is MotoCalc,
available from Stefan Vorkoetter. His product offers many features
of its own and is comparably priced. You can obtain details at
www.motocalc.com.
As an active electric-powered-flight enthusiast, you owe it to
yourself to own at least one of these fine programs. You may still use
much of the manufacturers’ data, but these programs do an excellent
job as well. Both offer demonstration packages that you can take
advantage of before making your final choice. MA
Bob Aberle
[email protected]
(Editor’s note: This concludes the first part of Bob’s article. The
second part will be published in next month’s issue.)
Manufacturers/Distributors:
Li-Poly batteries, chargers:
FMA Direct
www.fmadirect.com
Optics 6 RC system:
Hitec RCD
www.hitecrcd.com
Bonnie 20 ARF, AXI motors, radial motor mounts, Jeti controllers:
Hobby Lobby International
www.hobby-lobby.com
Acrovolt plans:
Modelair-Tech
www.modelairtech.com
Li-Poly charger:
Peak Electronics
www.siriuselectronics.com
TNC tachometer:
Skyborn Electronics
www.bktsi.com/
SuperStar EP, PT-20:
Tower Hobbies
www.towerhobbies.com/
Edition: Model Aviation - 2005/03
Page Numbers: 33,34,35,36,38
Edition: Model Aviation - 2005/03
Page Numbers: 33,34,35,36,38
FOREWORD: At the 2004 Weak Signals expo in Toledo, Ohio,
former AMA District II vice president and current AMA Flying Site
Coordinator for the Eastern Region Joe Beshar, who is an experienced
electric-power enthusiast, made a suggestion. He mentioned to AMA
Director of Publications Rob Kurek that we badly need a standard
reference document that will allow any modeler to size the proper
electric-motor system to model aircraft of any size and weight. What
you are about to read resulted from Joe’s suggestion.
Background: When a person enters model aviation, he or she has a
choice of power sources for his or her aircraft. Years ago those
options included rubber power, hand launching (as with a glider),
towing a glider with a cord or a fueled engine, and gasoline/sparkignition
engines or later glow-fueled engines.
As time went on and the glow engine became the most popular
source of model-aircraft power, modelers quickly became adept at
selecting the right-size engine for every size and weight of model. An
engine’s cu. in. displacement was related to model weight and model
size (usually wing area expressed in square inches).
Engine classes became known as 1⁄2A, A, B, C, and even D. Each
category covered a range of engine displacements (such as—if my
memory serves me correctly—0.09-.199 cu. in. was considered Class
A). Early FF competition rules related engine displacement to weight,
so that a .19 cu. in.-engine-powered model had to weight 19 ounces,
etc.
When RC started to come of age in the 1950s, we learned (by
experience) that 1⁄2A RC models could weigh as much as 18-20
ounces, Class A RC models could weigh as much as 30 ounces, and
so on. Although these engines are rated in horsepower (and thrust), to
this day most modelers simply relate engine displacement to model
weight and size!
Enter Electric Power: Although some of the first electric-powered
flights were made in the late 1950s and early 1960s, electric power as
we know it today didn’t really come into its own until the early 1970s.
Then it became obvious that engine displacement, for identification
purposes, would not work with electric. To make an early distinction
between fuel and electric, it was decided to refer to fueled power as
“engines” and electric power as “motors.”
Bob Boucher—one of the leaders of electric-powered flight in the
USA—began referring to his motors by equivalent glow-engine
displacements. His “05” motor was supposed to be equivalent to an
.049 cu. in. glow engine, a “40” motor was equivalent to a .40 cu. in.
glow engine, etc.
While this was going on, Germany’s Graupner company was
assigning names such as Speed 400, Speed 500, and the like. The
numbers were rounded off from the Mabuchi manufacturer’s model
numbers; “RS-380” became a Speed 400, “RS-540” became a Speed
500, and so on. Surely that fact couldn’t help you select a motor for a
particular size and weight of model.
What to Do? Approximately 30 years have now gone by since that
start in electric-powered flight. Although many still have questions,
we know much more now and have developed some excellent
techniques for “sizing,” or matching the correct motor to any size and
weight of model aircraft.
When you look back at this time frame, you can compare it with
the advent of the gasoline engines in the early 1930s, which
ultimately led to the cu. in.-displacement sizing of those engines in
the 1940s. Identifying fueled and electric engines/motors has taken
time.
The main thrust of this article is to explain how to select, or size,
your motors to make them suitable for powering models in flight. I
hope to cover such things as motor identification, motor types, direct
vs. gear drive, motor power (in watts) as it relates to aircraft weight,
and many other details. I hope to make this a standard reference so
that no one will have to ask, “What motor should I use?”
Aircraft Categories:
1) Use electric-powered kits, ARF models, and published plans.
We currently enjoy the fact that many kits, ARFs, and published plans
exist for all kinds of electric-powered models. In this category, the
aircraft designer or manufacturer has selected the motor for you. You
might want to improve on the initial choice, but you can easily get in
those first few flights before you fine-tune the selection process.
2) Make a glow-fuel-powered kit electric powered. There may be a
particular glow-fueled kit or ARF that you want to build, from
inception, as an electric-powered model. You want to be able to select
a motor, the type of drive (direct or geared), the type of battery pack,
and the battery pack’s capacity, but you want to make these selections
so that you can install the electric power-system components as you
construct the model.
MA Technical Editor Bob Aberle with sport/aerobatic Acrovolt
constructed in 1995. As designed by Tom Hunt, it would have been
comfortable with 60-size glow engine but was powered by DeWALT
18-volt cordless-drill motor with belt drive. Text tells how it was
greatly improved with new brushless motor and Li-Poly batteries.
Photos courtesy the author
by Bob Aberle
Using Whattmeter to take motor data of electric power equipment
in Hobby Lobby Bonnie 20, note battery packs lined up in
foreground for test purposes. In front is TNC tachometer for
measuring propeller rpm.
AstroFlight Super Whattmeter is the heart of any motor-selection
process. When inserted between battery and motor, it indicates
motor current, voltage, power (watts), and amount of energy going
into or out of battery (in Ah).
Bonnie 20 electric-powered ARF is featured in Sport Aviator review.
It could have been powered by .20-.32 glow engine. It also makes a
wonderful electric-powered advanced RC trainer.
Hobbico SuperStar EP is intended for glow power. Bob explains
in the text how to convert it to electric power.
3) Convert a glow-fuel-powered RC model you already have to
electric power. Let’s say you have a built-up and flying glow-fueled
airplane, and you decide to remove that engine and retrofit an
electric power system. How would you go about making that
selection?
4) Update an old electric-powered model that uses old-style
technology to use all of the latest equipment. How would you make
the specific selections based on the aircraft’s current size and
weight? In this case you would be looking at a new brushless motor,
a dedicated sensorless ESC, and probably lightweight and highcapacity
Li-Poly batteries.
Abstract: My close friend, flying partner, and fellow Model
Aviation Hall of Famer Tom Hunt is probably one of the most
famous and experienced electric-power fliers in the country. He has
been my primary consultant throughout this article’s preparation. He
recently made a thought-provoking observation, which follows.
“The main problem in the selection process is that the electric
motor has a much broader operating range than an IC [internal
combustion] engine. I can have an AXI 2212/34 brushless motor fly
a 10-ounce [total weight] aircraft at 60 watts input, a 14-ounce
aircraft at 95 watts input [that’s a 50% power increase—try that with
a glow engine!], and a 22-ounce model at 120 watts input [that’s a
100% increase].”
As you can see from Tom’s examples, any specific motor will
have far more application than a comparable glow engine. Because
of this, there are many more choices in the selection process when
going to electric power. That is what mystifies most modelers, and I
hope this article will help clear things up.
Measuring Motor Input Power (Watts): Motors are not referred to
by cu. in. displacement (as glow engines are). Motors are defined by
the term “power,” which is measured in watts. All references to
power in watts is input to the motor (or output of the battery). The
motor’s output power is a function of its efficiency.
As Tom pointed out, a motor can run at 60, 95, or even 120
watts. The primary rule is that power (in watts) equals motor current
(in amperes, or amps) multiplied by the voltage.
The voltage is determined by the battery and will vary
according to battery type (Ni-Cd, NiMH, or Li-Poly) and the
number of cells employed in the particular battery pack. The
current is determined by several things, including the resistance of
the motor windings, the size of the propeller, and the type of drive
(direct, with a gear reduction, or belt-reduction drive).
So you have the variables current (amps) and voltage that yield
Sample page from ElectriCalc motor-selection computer program.
This type of program goes hand in hand with motor-manufacturer
data to assist in selection process for specific size/weight aircraft.
You must know aircraft’s exact weight to select motors. Pelouze
Model PE-5 scale (L) weighs items as heavy as 5 pounds in 0.1-
ounce increments. My Weigh digital scale (R)—from
www.goodscale.com/scale—is intended for parking lot and indoor
micro RC models.
Typical battery chargers (L-R, top and bottom): AstroFlight 109
and Peak Electronics Lithium charger intended for Li-Poly only.
AstroFlight 110 Deluxe and FMA Direct Super Nova peak-detect
chargers intended for Ni-Cd and NiMH.
The TNC tachometer is one of the best digital tachometers on the
market. It is available from Skyborn Electronics.
Page from The Great Electric Motor Test—one of many sources for
motor data. It provides such parameters as motor current, voltage,
power (watts), propeller sizes, and rpm.
power (watts). This is where the fun starts since you can vary
everything, including the propeller size, the motor drive, and the
battery. Each variable will produce different results.
With a fueled engine, you can listen to the sound and adjust the
carburetor (by ear) until you reach the correct power level. With a
motor, there is essentially no noise and therefore nothing to listen to.
So how do you obtain a figure for power (watts)?
It would be nice if the motor manufacturers had printed on the
box “100 watts (maximum short duration, 75 watts continuous),”
much as engine manufacturers indicate “.049 cu. in., or 1⁄2A.” The
variables make that type of identification impractical. Most motor
manufacturers publish excellent motor data that I will write about
later, along with computer programs that support this process.
For now, you must learn that the most important “tool” for the
electric-power flight enthusiast is the ammeter and wattmeter. We
primarily use the AstroFlight Model 101 Super Whattmeter, which
1.25 Powered sport sailplane, indoor RC airplane
you can purchase for roughly $60. It is the latest version and will
measure to 10 mA instead of 100 mA. It can also read below 4.0
volts by adding a four-cell receiver battery pack. Make sure you
purchase this new version.
The meter is self-powered by the system you are measuring. It
provides four important motor parameters: voltage (volts), current
(amps), power (watts), and the capacity going into or out of the
battery, measured in ampere-hours (Ah). A list of articles and Web
sites at the end of this article will provide the details about the
specific use of the Whattmeter.
Some modelers already own digital-type multimeters, sold by
stores such as RadioShack. Those meters are fine, but most are
somewhat limited in current range.
If you try to check a 25-amp motor with some of these meters,
you will probably end up blowing a fuse or even damaging the
device. Many of these multimeters are of the newer “auto-ranging”
type which can also get complicated during regular use. I suggest that
you stick with the AstroFlight meter for its proven ability to easily do
the job!
Aircraft Weight and Power Loading: Now that you have the motor
“identified” by power, how do you relate it to a specific-size model?
There are actually two aircraft parameters you need to be interested
in, one of which is the model’s total weight. This is generally
measured in ounces for the smaller airplanes and pounds for the
larger models.
If you are in the planning stages of selecting a motor, you may be
estimating an aircraft’s weight as a starting point. If you are dealing
with an existing model, you will have to weigh it on a scale. There
are many digital-type scales on the market that read to within 0.1
ounce. You can buy one at a stationery store such as Staples, Office
Max, and others. After constructing or assembling an ARF, you will
also have to weigh it to verify your motor selection or help you
improve performance by selecting a different motor.
Knowing the motor’s power and the model’s weight, you can
come up with one of the most important “combination parameters,”
known as watts/ounce or watts/pound. These “power loading” values
are used for judging aircraft performance.
The term watts/ounce is generally used to describe the power
loading of models that weigh as much as 1 pound. Watts/pound is a
more common term for models that weigh more than 1 pound.
Through the years, experienced electric-power enthusiasts have
come up with a series of watts/ounce and watts/pound figures that
relate to an airplane’s expected performance. Table 1 and Table 2
provide the accepted parameters that cover most aircraft types. These
numbers aren’t absolute; they are only to be used as a guide.
Wing Loading and Skill Level: After determining motor power
(watts) and aircraft weight, you must focus on airplane size, which
involves wing area and then wing loading (which takes into account
wing area and aircraft weight).
The term “power loading” that I described (watts/ounce and
watts/pound) involves motor power and model weight; they are
aircraft related. The term “wing loading” relates to the pilot’s skill
level and directly to the minimum speed the model can fly (stall
speed). Aircraft with light wing loadings will be easier to fly (for
beginners and sport fliers) than those with high wing loadings
(favored by the experienced or expert fliers).
To determine wing loading, you need to know your model’s wing
area and weight. Wing area is the wingspan multiplied by the wing’s
average width (wing chord) and is expressed in square feet (sq. ft.).
Wing area may be a given with kits, ARFs, and even published
plans. The specifications that accompany these usually include the
wing area. A pure rectangular-shaped wing is easy to figure out (span
multiplied by chord), but an elliptical-shaped wing is much more
complicated. You typically do it by estimating the average wing
chord (width).
Once you know the wing area expressed in square inches (sq. in.),
you must convert that to sq. ft. by dividing it by 144. Let’s say your
aircraft has 400 sq. in. of wing area. Divide that by 144, and you get
2.78 sq. ft. The aircraft weighs 40 ounces, so divide that by the 2.78
sq. ft. to obtain the wing loading of 14.4 ounces/sq. ft.
Table 3 provides acceptable wing loadings for various pilot-skill
levels. Values start at 5 ounces/sq. ft. and can exceed 35 ounces/sq. ft.
In the past few paragraphs you learned about motor power (watts).
Then you learned how power relates to model weight. Last, you
learned how model size and weight are related to the pilot’s skill
level. You must take all of these parameters into consideration when
selecting the proper motor for your model.
Thrust: I don’t use it because in general it is not a reliable figure.
The thrust one measures on the bench has little to do with the thrust
the propeller produces as it moves through the air.
Some modelers came up with a factor indicating that your model
can weigh as much as three or four times the rated motor thrust.
That’s fine, if you really know what that thrust is. So I avoid using
this parameter!
Selection Process Details: Before I provide some examples of motor
selection, I want to introduce you to the subject of motor data. When
you set out to make a specific motor selection, you need a large
volume of motor-parameter data at your disposal.
That information should contain such things as motor current,
voltage, and watts for a range of recommended propellers (identified
by diameter and pitch). It should also provide a range of voltages so
that you can choose a battery type and capacity to suit the application.
It would be nice to have that kind of data available for every type
of motor available on our hobby market or, better still, have in one
place. I would love to have it on one Web site, but that hasn’t
happened yet. Information is available on many Web pages, and I’ll
steer you to the major ones.
But before I do that, I want to discuss “voltage.” Much of the
motor data presented on manufacturers’ Web sites indicate numbers
of battery cells and the type of battery, but it will not state the
voltage!
When referring to numbers of cells of Ni-Cd- or NiMH-type
batteries, each cell has a nominal 1.2 volts. If the data indicates eight
NiMH cells, the nominal voltage will be eight multiplied by 1.2, or
9.6. If there are two Li-Poly cells, the voltage will be two mulitiplied
by 3.7 (that is peculiar to Li-Poly batteries), or 7.4.
Some sites are kind enough to indicate the measured voltage
under load, which is really what you want, but others will make you
use the battery cell count, and that takes extra time. Be aware!
Motor Data Web Sites: The following sites are not in any particular
order, and they are not all that are available. They are what I think
you will need as a starting point for most of your motor selections.
• www.flyingmodels.org. Webmaster Fredrik Wergeland of
Stockholm, Sweden, is responsible for this data. After getting on the
site, select “The Great Electric Motor Test,” and then you have a
choice of standard or brushless motors.
There are not many standard motors, but there are quite a few
Watts/Ounce Type of Aircraft
1.00 Model that barely gets off the ground
2.00 Parking lot/backyard flyer
3.00 Aerobatic flier
5.00 Electric 3-D model
Table 1. Watts/Ounce (Models Weighing Less Than 1 Pound)
40-50 “Sunday flier,” sport sailplane, Old-Timer
Watts/Pound Type of Aircraft
30 Model that barely gets off the ground
60-70 Mildly aerobatic model
80-100 Aggressive airplane
100 plus Ducted-fan aircraft, competition Sailplane,
electric-powered 3-D model
Table 2. Watts/Pound (Models Weighing More Than 1 Pound)
38 MODEL AVIATION
brushless including Mega, Model Motors (AXI 22 and 28 series and
Mini AC), Nippy Black, MP Jet, Kontronik, and Jeti Phasor. There is
a promise that data will be posted for Speed 280 through Speed 600
ferrite motors in the future.
This Web site has nothing to do with the popular Flying Models
magazine.
• www.aircraft-world.com/default.asp?id=18. This site—run by
Dave Radford of Air Craft Inc. (of Japan)—provides data for GWS,
Hacker, Mega, Model Motors (including the AXI 41 series), MP Jet,
and QRP motors.
• www.astroflight.com. When you access this site, you can select
brushless motors or cobalt airplane motors. In some cases only one
propeller choice is given for a particular motor/battery combination.
That should still provide a good starting point!
• www.hackerbrushless.com/motors.shtml. Hacker USA provides
data for all of its brushless motors. After accessing the site, select the
motor you are interested in, such as the “B20 Series.” Then scroll
down and click on “Click here for B20 Series Application Chart.”
You will be surprised by the extent of this data because it goes as far
as relating motor type to specific model aircraft by name!
• www.balsapr.com/catalog/motors. This is part of the Balsa Products
site. Under brushless motors you will find the new Feigao brand.
There is a category for Outrunner Brushless motors. After that is the
entire series of GWS motors, including the IPS series, LPS series,
EPS series, and more!
When you get to the particular motor series, you have to click
again to obtain the actual data. Please be patient.
• http://home.ptd.net/~rcm65/motdata.html. This is Dick Miller’s
Motor Characteristics data that he has provided as a free service to
electric-power modelers for so many years. You can sort the data by
motor name or by a typical model’s wing area.
Dick covers the following motors, many of which are the small
variety used for parking lot or indoor flying: DC 5-2.4; DC-1717;
GWS (all); AstroFlight Firefly; MTM; VL Products; Ikarus; Hi-Line
Ltd.; KP-00; Kenway; Nikko; Peck-Polymers; and the Speed 280,
300, and 400 series.
Computer Programs to Help in the Selection Process: Researching
many Web sites to obtain motor data can be a real chore. A few years
ago two software programs came onto the hobby market that were
specifically designed to aid the electric-power enthusiast in choosing
a motor. The programs have been continually upgraded through the
years to the point where they include a tremendous amount of stored
motor (and aircraft) data parameters.
When these programs were first offered, we didn’t even have
brushless motors. Now they are the most popular item in electricpowered
flight. Both programs have literally grown with our hobby,
and you don’t have to be a computer expert or highly technical to be
able to use them.
I have used Sid Kauffman’s ElectriCalc for many years. It is up to
version 2.20 and has a list price of $49.95. Software is now provided
on a CD. It is intended for PCs with operating systems of Windows
Type of Aircraft and Skill Level
15-20 Larger trainer, Sport Scale model, sport
aerobatic model
20-25
Fast sport model (usually with more than
adequate power)
25-35 Scale model, larger multiengine model
35 plus Not for the author!
5-10
Park flyer, basic trainer, powered sport
sailplane
10-15
Faster sport flier, smaller trainer,
“Sunday” flier
Table 3. Wing Loading and Skill Level
Wing Loading
(Ounces/Square Foot)
98 and up. There are no provisions for Macintosh users—sorry!
You can find the details of this product and order a copy at
www.slkelectronics.com./ecalc/index.htm. The program’s present
version has stored data for 946 motors, and that figure is constantly
increasing as new motors come onto the market. There are two ways
to use ElectriCalc.
1) Enter your aircraft’s parameters (wing area, weight, and
approximate drag coefficient, which you can obtain by using the
simple help function called “Cdrag”). Pick the battery cell type and
the number of cells you think you might need. Choose a motor from
the extensive list shown, and pick a propeller that will provide
enough power to fly the model.
2) When the program is launched, an aircraft will already be
chosen (default) from the database or the last one you looked at will
be resident in the spreadsheet. Search the “planes” database for the
aircraft you are interested in or find one similar in size and weight to
your intended model, and rename and revise the parameters to
emulate your airplane.
Option 1 requires that you have a good idea of what propulsion
system should be in the model. This is seldom the case when one is
just getting into electric-powered flight.
Option 2 allows you to “repeat” an aircraft that has already flown.
(Much of the database in ElectriCalc on “planes” has been given to
SLK Electronics by proficient electric-power modelers.)
Option 2 also allows you to experiment with an existing design to
see what other motor or motor/gearbox combinations may work.
You can do a lot of “what-iffing” without having to leave your
computer station. This can be a big time-saver.
The other popular motor-selection program is MotoCalc,
available from Stefan Vorkoetter. His product offers many features
of its own and is comparably priced. You can obtain details at
www.motocalc.com.
As an active electric-powered-flight enthusiast, you owe it to
yourself to own at least one of these fine programs. You may still use
much of the manufacturers’ data, but these programs do an excellent
job as well. Both offer demonstration packages that you can take
advantage of before making your final choice. MA
Bob Aberle
[email protected]
(Editor’s note: This concludes the first part of Bob’s article. The
second part will be published in next month’s issue.)
Manufacturers/Distributors:
Li-Poly batteries, chargers:
FMA Direct
www.fmadirect.com
Optics 6 RC system:
Hitec RCD
www.hitecrcd.com
Bonnie 20 ARF, AXI motors, radial motor mounts, Jeti controllers:
Hobby Lobby International
www.hobby-lobby.com
Acrovolt plans:
Modelair-Tech
www.modelairtech.com
Li-Poly charger:
Peak Electronics
www.siriuselectronics.com
TNC tachometer:
Skyborn Electronics
www.bktsi.com/
SuperStar EP, PT-20:
Tower Hobbies
www.towerhobbies.com/
Edition: Model Aviation - 2005/03
Page Numbers: 33,34,35,36,38
FOREWORD: At the 2004 Weak Signals expo in Toledo, Ohio,
former AMA District II vice president and current AMA Flying Site
Coordinator for the Eastern Region Joe Beshar, who is an experienced
electric-power enthusiast, made a suggestion. He mentioned to AMA
Director of Publications Rob Kurek that we badly need a standard
reference document that will allow any modeler to size the proper
electric-motor system to model aircraft of any size and weight. What
you are about to read resulted from Joe’s suggestion.
Background: When a person enters model aviation, he or she has a
choice of power sources for his or her aircraft. Years ago those
options included rubber power, hand launching (as with a glider),
towing a glider with a cord or a fueled engine, and gasoline/sparkignition
engines or later glow-fueled engines.
As time went on and the glow engine became the most popular
source of model-aircraft power, modelers quickly became adept at
selecting the right-size engine for every size and weight of model. An
engine’s cu. in. displacement was related to model weight and model
size (usually wing area expressed in square inches).
Engine classes became known as 1⁄2A, A, B, C, and even D. Each
category covered a range of engine displacements (such as—if my
memory serves me correctly—0.09-.199 cu. in. was considered Class
A). Early FF competition rules related engine displacement to weight,
so that a .19 cu. in.-engine-powered model had to weight 19 ounces,
etc.
When RC started to come of age in the 1950s, we learned (by
experience) that 1⁄2A RC models could weigh as much as 18-20
ounces, Class A RC models could weigh as much as 30 ounces, and
so on. Although these engines are rated in horsepower (and thrust), to
this day most modelers simply relate engine displacement to model
weight and size!
Enter Electric Power: Although some of the first electric-powered
flights were made in the late 1950s and early 1960s, electric power as
we know it today didn’t really come into its own until the early 1970s.
Then it became obvious that engine displacement, for identification
purposes, would not work with electric. To make an early distinction
between fuel and electric, it was decided to refer to fueled power as
“engines” and electric power as “motors.”
Bob Boucher—one of the leaders of electric-powered flight in the
USA—began referring to his motors by equivalent glow-engine
displacements. His “05” motor was supposed to be equivalent to an
.049 cu. in. glow engine, a “40” motor was equivalent to a .40 cu. in.
glow engine, etc.
While this was going on, Germany’s Graupner company was
assigning names such as Speed 400, Speed 500, and the like. The
numbers were rounded off from the Mabuchi manufacturer’s model
numbers; “RS-380” became a Speed 400, “RS-540” became a Speed
500, and so on. Surely that fact couldn’t help you select a motor for a
particular size and weight of model.
What to Do? Approximately 30 years have now gone by since that
start in electric-powered flight. Although many still have questions,
we know much more now and have developed some excellent
techniques for “sizing,” or matching the correct motor to any size and
weight of model aircraft.
When you look back at this time frame, you can compare it with
the advent of the gasoline engines in the early 1930s, which
ultimately led to the cu. in.-displacement sizing of those engines in
the 1940s. Identifying fueled and electric engines/motors has taken
time.
The main thrust of this article is to explain how to select, or size,
your motors to make them suitable for powering models in flight. I
hope to cover such things as motor identification, motor types, direct
vs. gear drive, motor power (in watts) as it relates to aircraft weight,
and many other details. I hope to make this a standard reference so
that no one will have to ask, “What motor should I use?”
Aircraft Categories:
1) Use electric-powered kits, ARF models, and published plans.
We currently enjoy the fact that many kits, ARFs, and published plans
exist for all kinds of electric-powered models. In this category, the
aircraft designer or manufacturer has selected the motor for you. You
might want to improve on the initial choice, but you can easily get in
those first few flights before you fine-tune the selection process.
2) Make a glow-fuel-powered kit electric powered. There may be a
particular glow-fueled kit or ARF that you want to build, from
inception, as an electric-powered model. You want to be able to select
a motor, the type of drive (direct or geared), the type of battery pack,
and the battery pack’s capacity, but you want to make these selections
so that you can install the electric power-system components as you
construct the model.
MA Technical Editor Bob Aberle with sport/aerobatic Acrovolt
constructed in 1995. As designed by Tom Hunt, it would have been
comfortable with 60-size glow engine but was powered by DeWALT
18-volt cordless-drill motor with belt drive. Text tells how it was
greatly improved with new brushless motor and Li-Poly batteries.
Photos courtesy the author
by Bob Aberle
Using Whattmeter to take motor data of electric power equipment
in Hobby Lobby Bonnie 20, note battery packs lined up in
foreground for test purposes. In front is TNC tachometer for
measuring propeller rpm.
AstroFlight Super Whattmeter is the heart of any motor-selection
process. When inserted between battery and motor, it indicates
motor current, voltage, power (watts), and amount of energy going
into or out of battery (in Ah).
Bonnie 20 electric-powered ARF is featured in Sport Aviator review.
It could have been powered by .20-.32 glow engine. It also makes a
wonderful electric-powered advanced RC trainer.
Hobbico SuperStar EP is intended for glow power. Bob explains
in the text how to convert it to electric power.
3) Convert a glow-fuel-powered RC model you already have to
electric power. Let’s say you have a built-up and flying glow-fueled
airplane, and you decide to remove that engine and retrofit an
electric power system. How would you go about making that
selection?
4) Update an old electric-powered model that uses old-style
technology to use all of the latest equipment. How would you make
the specific selections based on the aircraft’s current size and
weight? In this case you would be looking at a new brushless motor,
a dedicated sensorless ESC, and probably lightweight and highcapacity
Li-Poly batteries.
Abstract: My close friend, flying partner, and fellow Model
Aviation Hall of Famer Tom Hunt is probably one of the most
famous and experienced electric-power fliers in the country. He has
been my primary consultant throughout this article’s preparation. He
recently made a thought-provoking observation, which follows.
“The main problem in the selection process is that the electric
motor has a much broader operating range than an IC [internal
combustion] engine. I can have an AXI 2212/34 brushless motor fly
a 10-ounce [total weight] aircraft at 60 watts input, a 14-ounce
aircraft at 95 watts input [that’s a 50% power increase—try that with
a glow engine!], and a 22-ounce model at 120 watts input [that’s a
100% increase].”
As you can see from Tom’s examples, any specific motor will
have far more application than a comparable glow engine. Because
of this, there are many more choices in the selection process when
going to electric power. That is what mystifies most modelers, and I
hope this article will help clear things up.
Measuring Motor Input Power (Watts): Motors are not referred to
by cu. in. displacement (as glow engines are). Motors are defined by
the term “power,” which is measured in watts. All references to
power in watts is input to the motor (or output of the battery). The
motor’s output power is a function of its efficiency.
As Tom pointed out, a motor can run at 60, 95, or even 120
watts. The primary rule is that power (in watts) equals motor current
(in amperes, or amps) multiplied by the voltage.
The voltage is determined by the battery and will vary
according to battery type (Ni-Cd, NiMH, or Li-Poly) and the
number of cells employed in the particular battery pack. The
current is determined by several things, including the resistance of
the motor windings, the size of the propeller, and the type of drive
(direct, with a gear reduction, or belt-reduction drive).
So you have the variables current (amps) and voltage that yield
Sample page from ElectriCalc motor-selection computer program.
This type of program goes hand in hand with motor-manufacturer
data to assist in selection process for specific size/weight aircraft.
You must know aircraft’s exact weight to select motors. Pelouze
Model PE-5 scale (L) weighs items as heavy as 5 pounds in 0.1-
ounce increments. My Weigh digital scale (R)—from
www.goodscale.com/scale—is intended for parking lot and indoor
micro RC models.
Typical battery chargers (L-R, top and bottom): AstroFlight 109
and Peak Electronics Lithium charger intended for Li-Poly only.
AstroFlight 110 Deluxe and FMA Direct Super Nova peak-detect
chargers intended for Ni-Cd and NiMH.
The TNC tachometer is one of the best digital tachometers on the
market. It is available from Skyborn Electronics.
Page from The Great Electric Motor Test—one of many sources for
motor data. It provides such parameters as motor current, voltage,
power (watts), propeller sizes, and rpm.
power (watts). This is where the fun starts since you can vary
everything, including the propeller size, the motor drive, and the
battery. Each variable will produce different results.
With a fueled engine, you can listen to the sound and adjust the
carburetor (by ear) until you reach the correct power level. With a
motor, there is essentially no noise and therefore nothing to listen to.
So how do you obtain a figure for power (watts)?
It would be nice if the motor manufacturers had printed on the
box “100 watts (maximum short duration, 75 watts continuous),”
much as engine manufacturers indicate “.049 cu. in., or 1⁄2A.” The
variables make that type of identification impractical. Most motor
manufacturers publish excellent motor data that I will write about
later, along with computer programs that support this process.
For now, you must learn that the most important “tool” for the
electric-power flight enthusiast is the ammeter and wattmeter. We
primarily use the AstroFlight Model 101 Super Whattmeter, which
1.25 Powered sport sailplane, indoor RC airplane
you can purchase for roughly $60. It is the latest version and will
measure to 10 mA instead of 100 mA. It can also read below 4.0
volts by adding a four-cell receiver battery pack. Make sure you
purchase this new version.
The meter is self-powered by the system you are measuring. It
provides four important motor parameters: voltage (volts), current
(amps), power (watts), and the capacity going into or out of the
battery, measured in ampere-hours (Ah). A list of articles and Web
sites at the end of this article will provide the details about the
specific use of the Whattmeter.
Some modelers already own digital-type multimeters, sold by
stores such as RadioShack. Those meters are fine, but most are
somewhat limited in current range.
If you try to check a 25-amp motor with some of these meters,
you will probably end up blowing a fuse or even damaging the
device. Many of these multimeters are of the newer “auto-ranging”
type which can also get complicated during regular use. I suggest that
you stick with the AstroFlight meter for its proven ability to easily do
the job!
Aircraft Weight and Power Loading: Now that you have the motor
“identified” by power, how do you relate it to a specific-size model?
There are actually two aircraft parameters you need to be interested
in, one of which is the model’s total weight. This is generally
measured in ounces for the smaller airplanes and pounds for the
larger models.
If you are in the planning stages of selecting a motor, you may be
estimating an aircraft’s weight as a starting point. If you are dealing
with an existing model, you will have to weigh it on a scale. There
are many digital-type scales on the market that read to within 0.1
ounce. You can buy one at a stationery store such as Staples, Office
Max, and others. After constructing or assembling an ARF, you will
also have to weigh it to verify your motor selection or help you
improve performance by selecting a different motor.
Knowing the motor’s power and the model’s weight, you can
come up with one of the most important “combination parameters,”
known as watts/ounce or watts/pound. These “power loading” values
are used for judging aircraft performance.
The term watts/ounce is generally used to describe the power
loading of models that weigh as much as 1 pound. Watts/pound is a
more common term for models that weigh more than 1 pound.
Through the years, experienced electric-power enthusiasts have
come up with a series of watts/ounce and watts/pound figures that
relate to an airplane’s expected performance. Table 1 and Table 2
provide the accepted parameters that cover most aircraft types. These
numbers aren’t absolute; they are only to be used as a guide.
Wing Loading and Skill Level: After determining motor power
(watts) and aircraft weight, you must focus on airplane size, which
involves wing area and then wing loading (which takes into account
wing area and aircraft weight).
The term “power loading” that I described (watts/ounce and
watts/pound) involves motor power and model weight; they are
aircraft related. The term “wing loading” relates to the pilot’s skill
level and directly to the minimum speed the model can fly (stall
speed). Aircraft with light wing loadings will be easier to fly (for
beginners and sport fliers) than those with high wing loadings
(favored by the experienced or expert fliers).
To determine wing loading, you need to know your model’s wing
area and weight. Wing area is the wingspan multiplied by the wing’s
average width (wing chord) and is expressed in square feet (sq. ft.).
Wing area may be a given with kits, ARFs, and even published
plans. The specifications that accompany these usually include the
wing area. A pure rectangular-shaped wing is easy to figure out (span
multiplied by chord), but an elliptical-shaped wing is much more
complicated. You typically do it by estimating the average wing
chord (width).
Once you know the wing area expressed in square inches (sq. in.),
you must convert that to sq. ft. by dividing it by 144. Let’s say your
aircraft has 400 sq. in. of wing area. Divide that by 144, and you get
2.78 sq. ft. The aircraft weighs 40 ounces, so divide that by the 2.78
sq. ft. to obtain the wing loading of 14.4 ounces/sq. ft.
Table 3 provides acceptable wing loadings for various pilot-skill
levels. Values start at 5 ounces/sq. ft. and can exceed 35 ounces/sq. ft.
In the past few paragraphs you learned about motor power (watts).
Then you learned how power relates to model weight. Last, you
learned how model size and weight are related to the pilot’s skill
level. You must take all of these parameters into consideration when
selecting the proper motor for your model.
Thrust: I don’t use it because in general it is not a reliable figure.
The thrust one measures on the bench has little to do with the thrust
the propeller produces as it moves through the air.
Some modelers came up with a factor indicating that your model
can weigh as much as three or four times the rated motor thrust.
That’s fine, if you really know what that thrust is. So I avoid using
this parameter!
Selection Process Details: Before I provide some examples of motor
selection, I want to introduce you to the subject of motor data. When
you set out to make a specific motor selection, you need a large
volume of motor-parameter data at your disposal.
That information should contain such things as motor current,
voltage, and watts for a range of recommended propellers (identified
by diameter and pitch). It should also provide a range of voltages so
that you can choose a battery type and capacity to suit the application.
It would be nice to have that kind of data available for every type
of motor available on our hobby market or, better still, have in one
place. I would love to have it on one Web site, but that hasn’t
happened yet. Information is available on many Web pages, and I’ll
steer you to the major ones.
But before I do that, I want to discuss “voltage.” Much of the
motor data presented on manufacturers’ Web sites indicate numbers
of battery cells and the type of battery, but it will not state the
voltage!
When referring to numbers of cells of Ni-Cd- or NiMH-type
batteries, each cell has a nominal 1.2 volts. If the data indicates eight
NiMH cells, the nominal voltage will be eight multiplied by 1.2, or
9.6. If there are two Li-Poly cells, the voltage will be two mulitiplied
by 3.7 (that is peculiar to Li-Poly batteries), or 7.4.
Some sites are kind enough to indicate the measured voltage
under load, which is really what you want, but others will make you
use the battery cell count, and that takes extra time. Be aware!
Motor Data Web Sites: The following sites are not in any particular
order, and they are not all that are available. They are what I think
you will need as a starting point for most of your motor selections.
• www.flyingmodels.org. Webmaster Fredrik Wergeland of
Stockholm, Sweden, is responsible for this data. After getting on the
site, select “The Great Electric Motor Test,” and then you have a
choice of standard or brushless motors.
There are not many standard motors, but there are quite a few
Watts/Ounce Type of Aircraft
1.00 Model that barely gets off the ground
2.00 Parking lot/backyard flyer
3.00 Aerobatic flier
5.00 Electric 3-D model
Table 1. Watts/Ounce (Models Weighing Less Than 1 Pound)
40-50 “Sunday flier,” sport sailplane, Old-Timer
Watts/Pound Type of Aircraft
30 Model that barely gets off the ground
60-70 Mildly aerobatic model
80-100 Aggressive airplane
100 plus Ducted-fan aircraft, competition Sailplane,
electric-powered 3-D model
Table 2. Watts/Pound (Models Weighing More Than 1 Pound)
38 MODEL AVIATION
brushless including Mega, Model Motors (AXI 22 and 28 series and
Mini AC), Nippy Black, MP Jet, Kontronik, and Jeti Phasor. There is
a promise that data will be posted for Speed 280 through Speed 600
ferrite motors in the future.
This Web site has nothing to do with the popular Flying Models
magazine.
• www.aircraft-world.com/default.asp?id=18. This site—run by
Dave Radford of Air Craft Inc. (of Japan)—provides data for GWS,
Hacker, Mega, Model Motors (including the AXI 41 series), MP Jet,
and QRP motors.
• www.astroflight.com. When you access this site, you can select
brushless motors or cobalt airplane motors. In some cases only one
propeller choice is given for a particular motor/battery combination.
That should still provide a good starting point!
• www.hackerbrushless.com/motors.shtml. Hacker USA provides
data for all of its brushless motors. After accessing the site, select the
motor you are interested in, such as the “B20 Series.” Then scroll
down and click on “Click here for B20 Series Application Chart.”
You will be surprised by the extent of this data because it goes as far
as relating motor type to specific model aircraft by name!
• www.balsapr.com/catalog/motors. This is part of the Balsa Products
site. Under brushless motors you will find the new Feigao brand.
There is a category for Outrunner Brushless motors. After that is the
entire series of GWS motors, including the IPS series, LPS series,
EPS series, and more!
When you get to the particular motor series, you have to click
again to obtain the actual data. Please be patient.
• http://home.ptd.net/~rcm65/motdata.html. This is Dick Miller’s
Motor Characteristics data that he has provided as a free service to
electric-power modelers for so many years. You can sort the data by
motor name or by a typical model’s wing area.
Dick covers the following motors, many of which are the small
variety used for parking lot or indoor flying: DC 5-2.4; DC-1717;
GWS (all); AstroFlight Firefly; MTM; VL Products; Ikarus; Hi-Line
Ltd.; KP-00; Kenway; Nikko; Peck-Polymers; and the Speed 280,
300, and 400 series.
Computer Programs to Help in the Selection Process: Researching
many Web sites to obtain motor data can be a real chore. A few years
ago two software programs came onto the hobby market that were
specifically designed to aid the electric-power enthusiast in choosing
a motor. The programs have been continually upgraded through the
years to the point where they include a tremendous amount of stored
motor (and aircraft) data parameters.
When these programs were first offered, we didn’t even have
brushless motors. Now they are the most popular item in electricpowered
flight. Both programs have literally grown with our hobby,
and you don’t have to be a computer expert or highly technical to be
able to use them.
I have used Sid Kauffman’s ElectriCalc for many years. It is up to
version 2.20 and has a list price of $49.95. Software is now provided
on a CD. It is intended for PCs with operating systems of Windows
Type of Aircraft and Skill Level
15-20 Larger trainer, Sport Scale model, sport
aerobatic model
20-25
Fast sport model (usually with more than
adequate power)
25-35 Scale model, larger multiengine model
35 plus Not for the author!
5-10
Park flyer, basic trainer, powered sport
sailplane
10-15
Faster sport flier, smaller trainer,
“Sunday” flier
Table 3. Wing Loading and Skill Level
Wing Loading
(Ounces/Square Foot)
98 and up. There are no provisions for Macintosh users—sorry!
You can find the details of this product and order a copy at
www.slkelectronics.com./ecalc/index.htm. The program’s present
version has stored data for 946 motors, and that figure is constantly
increasing as new motors come onto the market. There are two ways
to use ElectriCalc.
1) Enter your aircraft’s parameters (wing area, weight, and
approximate drag coefficient, which you can obtain by using the
simple help function called “Cdrag”). Pick the battery cell type and
the number of cells you think you might need. Choose a motor from
the extensive list shown, and pick a propeller that will provide
enough power to fly the model.
2) When the program is launched, an aircraft will already be
chosen (default) from the database or the last one you looked at will
be resident in the spreadsheet. Search the “planes” database for the
aircraft you are interested in or find one similar in size and weight to
your intended model, and rename and revise the parameters to
emulate your airplane.
Option 1 requires that you have a good idea of what propulsion
system should be in the model. This is seldom the case when one is
just getting into electric-powered flight.
Option 2 allows you to “repeat” an aircraft that has already flown.
(Much of the database in ElectriCalc on “planes” has been given to
SLK Electronics by proficient electric-power modelers.)
Option 2 also allows you to experiment with an existing design to
see what other motor or motor/gearbox combinations may work.
You can do a lot of “what-iffing” without having to leave your
computer station. This can be a big time-saver.
The other popular motor-selection program is MotoCalc,
available from Stefan Vorkoetter. His product offers many features
of its own and is comparably priced. You can obtain details at
www.motocalc.com.
As an active electric-powered-flight enthusiast, you owe it to
yourself to own at least one of these fine programs. You may still use
much of the manufacturers’ data, but these programs do an excellent
job as well. Both offer demonstration packages that you can take
advantage of before making your final choice. MA
Bob Aberle
[email protected]
(Editor’s note: This concludes the first part of Bob’s article. The
second part will be published in next month’s issue.)
Manufacturers/Distributors:
Li-Poly batteries, chargers:
FMA Direct
www.fmadirect.com
Optics 6 RC system:
Hitec RCD
www.hitecrcd.com
Bonnie 20 ARF, AXI motors, radial motor mounts, Jeti controllers:
Hobby Lobby International
www.hobby-lobby.com
Acrovolt plans:
Modelair-Tech
www.modelairtech.com
Li-Poly charger:
Peak Electronics
www.siriuselectronics.com
TNC tachometer:
Skyborn Electronics
www.bktsi.com/
SuperStar EP, PT-20:
Tower Hobbies
www.towerhobbies.com/
Edition: Model Aviation - 2005/03
Page Numbers: 33,34,35,36,38
FOREWORD: At the 2004 Weak Signals expo in Toledo, Ohio,
former AMA District II vice president and current AMA Flying Site
Coordinator for the Eastern Region Joe Beshar, who is an experienced
electric-power enthusiast, made a suggestion. He mentioned to AMA
Director of Publications Rob Kurek that we badly need a standard
reference document that will allow any modeler to size the proper
electric-motor system to model aircraft of any size and weight. What
you are about to read resulted from Joe’s suggestion.
Background: When a person enters model aviation, he or she has a
choice of power sources for his or her aircraft. Years ago those
options included rubber power, hand launching (as with a glider),
towing a glider with a cord or a fueled engine, and gasoline/sparkignition
engines or later glow-fueled engines.
As time went on and the glow engine became the most popular
source of model-aircraft power, modelers quickly became adept at
selecting the right-size engine for every size and weight of model. An
engine’s cu. in. displacement was related to model weight and model
size (usually wing area expressed in square inches).
Engine classes became known as 1⁄2A, A, B, C, and even D. Each
category covered a range of engine displacements (such as—if my
memory serves me correctly—0.09-.199 cu. in. was considered Class
A). Early FF competition rules related engine displacement to weight,
so that a .19 cu. in.-engine-powered model had to weight 19 ounces,
etc.
When RC started to come of age in the 1950s, we learned (by
experience) that 1⁄2A RC models could weigh as much as 18-20
ounces, Class A RC models could weigh as much as 30 ounces, and
so on. Although these engines are rated in horsepower (and thrust), to
this day most modelers simply relate engine displacement to model
weight and size!
Enter Electric Power: Although some of the first electric-powered
flights were made in the late 1950s and early 1960s, electric power as
we know it today didn’t really come into its own until the early 1970s.
Then it became obvious that engine displacement, for identification
purposes, would not work with electric. To make an early distinction
between fuel and electric, it was decided to refer to fueled power as
“engines” and electric power as “motors.”
Bob Boucher—one of the leaders of electric-powered flight in the
USA—began referring to his motors by equivalent glow-engine
displacements. His “05” motor was supposed to be equivalent to an
.049 cu. in. glow engine, a “40” motor was equivalent to a .40 cu. in.
glow engine, etc.
While this was going on, Germany’s Graupner company was
assigning names such as Speed 400, Speed 500, and the like. The
numbers were rounded off from the Mabuchi manufacturer’s model
numbers; “RS-380” became a Speed 400, “RS-540” became a Speed
500, and so on. Surely that fact couldn’t help you select a motor for a
particular size and weight of model.
What to Do? Approximately 30 years have now gone by since that
start in electric-powered flight. Although many still have questions,
we know much more now and have developed some excellent
techniques for “sizing,” or matching the correct motor to any size and
weight of model aircraft.
When you look back at this time frame, you can compare it with
the advent of the gasoline engines in the early 1930s, which
ultimately led to the cu. in.-displacement sizing of those engines in
the 1940s. Identifying fueled and electric engines/motors has taken
time.
The main thrust of this article is to explain how to select, or size,
your motors to make them suitable for powering models in flight. I
hope to cover such things as motor identification, motor types, direct
vs. gear drive, motor power (in watts) as it relates to aircraft weight,
and many other details. I hope to make this a standard reference so
that no one will have to ask, “What motor should I use?”
Aircraft Categories:
1) Use electric-powered kits, ARF models, and published plans.
We currently enjoy the fact that many kits, ARFs, and published plans
exist for all kinds of electric-powered models. In this category, the
aircraft designer or manufacturer has selected the motor for you. You
might want to improve on the initial choice, but you can easily get in
those first few flights before you fine-tune the selection process.
2) Make a glow-fuel-powered kit electric powered. There may be a
particular glow-fueled kit or ARF that you want to build, from
inception, as an electric-powered model. You want to be able to select
a motor, the type of drive (direct or geared), the type of battery pack,
and the battery pack’s capacity, but you want to make these selections
so that you can install the electric power-system components as you
construct the model.
MA Technical Editor Bob Aberle with sport/aerobatic Acrovolt
constructed in 1995. As designed by Tom Hunt, it would have been
comfortable with 60-size glow engine but was powered by DeWALT
18-volt cordless-drill motor with belt drive. Text tells how it was
greatly improved with new brushless motor and Li-Poly batteries.
Photos courtesy the author
by Bob Aberle
Using Whattmeter to take motor data of electric power equipment
in Hobby Lobby Bonnie 20, note battery packs lined up in
foreground for test purposes. In front is TNC tachometer for
measuring propeller rpm.
AstroFlight Super Whattmeter is the heart of any motor-selection
process. When inserted between battery and motor, it indicates
motor current, voltage, power (watts), and amount of energy going
into or out of battery (in Ah).
Bonnie 20 electric-powered ARF is featured in Sport Aviator review.
It could have been powered by .20-.32 glow engine. It also makes a
wonderful electric-powered advanced RC trainer.
Hobbico SuperStar EP is intended for glow power. Bob explains
in the text how to convert it to electric power.
3) Convert a glow-fuel-powered RC model you already have to
electric power. Let’s say you have a built-up and flying glow-fueled
airplane, and you decide to remove that engine and retrofit an
electric power system. How would you go about making that
selection?
4) Update an old electric-powered model that uses old-style
technology to use all of the latest equipment. How would you make
the specific selections based on the aircraft’s current size and
weight? In this case you would be looking at a new brushless motor,
a dedicated sensorless ESC, and probably lightweight and highcapacity
Li-Poly batteries.
Abstract: My close friend, flying partner, and fellow Model
Aviation Hall of Famer Tom Hunt is probably one of the most
famous and experienced electric-power fliers in the country. He has
been my primary consultant throughout this article’s preparation. He
recently made a thought-provoking observation, which follows.
“The main problem in the selection process is that the electric
motor has a much broader operating range than an IC [internal
combustion] engine. I can have an AXI 2212/34 brushless motor fly
a 10-ounce [total weight] aircraft at 60 watts input, a 14-ounce
aircraft at 95 watts input [that’s a 50% power increase—try that with
a glow engine!], and a 22-ounce model at 120 watts input [that’s a
100% increase].”
As you can see from Tom’s examples, any specific motor will
have far more application than a comparable glow engine. Because
of this, there are many more choices in the selection process when
going to electric power. That is what mystifies most modelers, and I
hope this article will help clear things up.
Measuring Motor Input Power (Watts): Motors are not referred to
by cu. in. displacement (as glow engines are). Motors are defined by
the term “power,” which is measured in watts. All references to
power in watts is input to the motor (or output of the battery). The
motor’s output power is a function of its efficiency.
As Tom pointed out, a motor can run at 60, 95, or even 120
watts. The primary rule is that power (in watts) equals motor current
(in amperes, or amps) multiplied by the voltage.
The voltage is determined by the battery and will vary
according to battery type (Ni-Cd, NiMH, or Li-Poly) and the
number of cells employed in the particular battery pack. The
current is determined by several things, including the resistance of
the motor windings, the size of the propeller, and the type of drive
(direct, with a gear reduction, or belt-reduction drive).
So you have the variables current (amps) and voltage that yield
Sample page from ElectriCalc motor-selection computer program.
This type of program goes hand in hand with motor-manufacturer
data to assist in selection process for specific size/weight aircraft.
You must know aircraft’s exact weight to select motors. Pelouze
Model PE-5 scale (L) weighs items as heavy as 5 pounds in 0.1-
ounce increments. My Weigh digital scale (R)—from
www.goodscale.com/scale—is intended for parking lot and indoor
micro RC models.
Typical battery chargers (L-R, top and bottom): AstroFlight 109
and Peak Electronics Lithium charger intended for Li-Poly only.
AstroFlight 110 Deluxe and FMA Direct Super Nova peak-detect
chargers intended for Ni-Cd and NiMH.
The TNC tachometer is one of the best digital tachometers on the
market. It is available from Skyborn Electronics.
Page from The Great Electric Motor Test—one of many sources for
motor data. It provides such parameters as motor current, voltage,
power (watts), propeller sizes, and rpm.
power (watts). This is where the fun starts since you can vary
everything, including the propeller size, the motor drive, and the
battery. Each variable will produce different results.
With a fueled engine, you can listen to the sound and adjust the
carburetor (by ear) until you reach the correct power level. With a
motor, there is essentially no noise and therefore nothing to listen to.
So how do you obtain a figure for power (watts)?
It would be nice if the motor manufacturers had printed on the
box “100 watts (maximum short duration, 75 watts continuous),”
much as engine manufacturers indicate “.049 cu. in., or 1⁄2A.” The
variables make that type of identification impractical. Most motor
manufacturers publish excellent motor data that I will write about
later, along with computer programs that support this process.
For now, you must learn that the most important “tool” for the
electric-power flight enthusiast is the ammeter and wattmeter. We
primarily use the AstroFlight Model 101 Super Whattmeter, which
1.25 Powered sport sailplane, indoor RC airplane
you can purchase for roughly $60. It is the latest version and will
measure to 10 mA instead of 100 mA. It can also read below 4.0
volts by adding a four-cell receiver battery pack. Make sure you
purchase this new version.
The meter is self-powered by the system you are measuring. It
provides four important motor parameters: voltage (volts), current
(amps), power (watts), and the capacity going into or out of the
battery, measured in ampere-hours (Ah). A list of articles and Web
sites at the end of this article will provide the details about the
specific use of the Whattmeter.
Some modelers already own digital-type multimeters, sold by
stores such as RadioShack. Those meters are fine, but most are
somewhat limited in current range.
If you try to check a 25-amp motor with some of these meters,
you will probably end up blowing a fuse or even damaging the
device. Many of these multimeters are of the newer “auto-ranging”
type which can also get complicated during regular use. I suggest that
you stick with the AstroFlight meter for its proven ability to easily do
the job!
Aircraft Weight and Power Loading: Now that you have the motor
“identified” by power, how do you relate it to a specific-size model?
There are actually two aircraft parameters you need to be interested
in, one of which is the model’s total weight. This is generally
measured in ounces for the smaller airplanes and pounds for the
larger models.
If you are in the planning stages of selecting a motor, you may be
estimating an aircraft’s weight as a starting point. If you are dealing
with an existing model, you will have to weigh it on a scale. There
are many digital-type scales on the market that read to within 0.1
ounce. You can buy one at a stationery store such as Staples, Office
Max, and others. After constructing or assembling an ARF, you will
also have to weigh it to verify your motor selection or help you
improve performance by selecting a different motor.
Knowing the motor’s power and the model’s weight, you can
come up with one of the most important “combination parameters,”
known as watts/ounce or watts/pound. These “power loading” values
are used for judging aircraft performance.
The term watts/ounce is generally used to describe the power
loading of models that weigh as much as 1 pound. Watts/pound is a
more common term for models that weigh more than 1 pound.
Through the years, experienced electric-power enthusiasts have
come up with a series of watts/ounce and watts/pound figures that
relate to an airplane’s expected performance. Table 1 and Table 2
provide the accepted parameters that cover most aircraft types. These
numbers aren’t absolute; they are only to be used as a guide.
Wing Loading and Skill Level: After determining motor power
(watts) and aircraft weight, you must focus on airplane size, which
involves wing area and then wing loading (which takes into account
wing area and aircraft weight).
The term “power loading” that I described (watts/ounce and
watts/pound) involves motor power and model weight; they are
aircraft related. The term “wing loading” relates to the pilot’s skill
level and directly to the minimum speed the model can fly (stall
speed). Aircraft with light wing loadings will be easier to fly (for
beginners and sport fliers) than those with high wing loadings
(favored by the experienced or expert fliers).
To determine wing loading, you need to know your model’s wing
area and weight. Wing area is the wingspan multiplied by the wing’s
average width (wing chord) and is expressed in square feet (sq. ft.).
Wing area may be a given with kits, ARFs, and even published
plans. The specifications that accompany these usually include the
wing area. A pure rectangular-shaped wing is easy to figure out (span
multiplied by chord), but an elliptical-shaped wing is much more
complicated. You typically do it by estimating the average wing
chord (width).
Once you know the wing area expressed in square inches (sq. in.),
you must convert that to sq. ft. by dividing it by 144. Let’s say your
aircraft has 400 sq. in. of wing area. Divide that by 144, and you get
2.78 sq. ft. The aircraft weighs 40 ounces, so divide that by the 2.78
sq. ft. to obtain the wing loading of 14.4 ounces/sq. ft.
Table 3 provides acceptable wing loadings for various pilot-skill
levels. Values start at 5 ounces/sq. ft. and can exceed 35 ounces/sq. ft.
In the past few paragraphs you learned about motor power (watts).
Then you learned how power relates to model weight. Last, you
learned how model size and weight are related to the pilot’s skill
level. You must take all of these parameters into consideration when
selecting the proper motor for your model.
Thrust: I don’t use it because in general it is not a reliable figure.
The thrust one measures on the bench has little to do with the thrust
the propeller produces as it moves through the air.
Some modelers came up with a factor indicating that your model
can weigh as much as three or four times the rated motor thrust.
That’s fine, if you really know what that thrust is. So I avoid using
this parameter!
Selection Process Details: Before I provide some examples of motor
selection, I want to introduce you to the subject of motor data. When
you set out to make a specific motor selection, you need a large
volume of motor-parameter data at your disposal.
That information should contain such things as motor current,
voltage, and watts for a range of recommended propellers (identified
by diameter and pitch). It should also provide a range of voltages so
that you can choose a battery type and capacity to suit the application.
It would be nice to have that kind of data available for every type
of motor available on our hobby market or, better still, have in one
place. I would love to have it on one Web site, but that hasn’t
happened yet. Information is available on many Web pages, and I’ll
steer you to the major ones.
But before I do that, I want to discuss “voltage.” Much of the
motor data presented on manufacturers’ Web sites indicate numbers
of battery cells and the type of battery, but it will not state the
voltage!
When referring to numbers of cells of Ni-Cd- or NiMH-type
batteries, each cell has a nominal 1.2 volts. If the data indicates eight
NiMH cells, the nominal voltage will be eight multiplied by 1.2, or
9.6. If there are two Li-Poly cells, the voltage will be two mulitiplied
by 3.7 (that is peculiar to Li-Poly batteries), or 7.4.
Some sites are kind enough to indicate the measured voltage
under load, which is really what you want, but others will make you
use the battery cell count, and that takes extra time. Be aware!
Motor Data Web Sites: The following sites are not in any particular
order, and they are not all that are available. They are what I think
you will need as a starting point for most of your motor selections.
• www.flyingmodels.org. Webmaster Fredrik Wergeland of
Stockholm, Sweden, is responsible for this data. After getting on the
site, select “The Great Electric Motor Test,” and then you have a
choice of standard or brushless motors.
There are not many standard motors, but there are quite a few
Watts/Ounce Type of Aircraft
1.00 Model that barely gets off the ground
2.00 Parking lot/backyard flyer
3.00 Aerobatic flier
5.00 Electric 3-D model
Table 1. Watts/Ounce (Models Weighing Less Than 1 Pound)
40-50 “Sunday flier,” sport sailplane, Old-Timer
Watts/Pound Type of Aircraft
30 Model that barely gets off the ground
60-70 Mildly aerobatic model
80-100 Aggressive airplane
100 plus Ducted-fan aircraft, competition Sailplane,
electric-powered 3-D model
Table 2. Watts/Pound (Models Weighing More Than 1 Pound)
38 MODEL AVIATION
brushless including Mega, Model Motors (AXI 22 and 28 series and
Mini AC), Nippy Black, MP Jet, Kontronik, and Jeti Phasor. There is
a promise that data will be posted for Speed 280 through Speed 600
ferrite motors in the future.
This Web site has nothing to do with the popular Flying Models
magazine.
• www.aircraft-world.com/default.asp?id=18. This site—run by
Dave Radford of Air Craft Inc. (of Japan)—provides data for GWS,
Hacker, Mega, Model Motors (including the AXI 41 series), MP Jet,
and QRP motors.
• www.astroflight.com. When you access this site, you can select
brushless motors or cobalt airplane motors. In some cases only one
propeller choice is given for a particular motor/battery combination.
That should still provide a good starting point!
• www.hackerbrushless.com/motors.shtml. Hacker USA provides
data for all of its brushless motors. After accessing the site, select the
motor you are interested in, such as the “B20 Series.” Then scroll
down and click on “Click here for B20 Series Application Chart.”
You will be surprised by the extent of this data because it goes as far
as relating motor type to specific model aircraft by name!
• www.balsapr.com/catalog/motors. This is part of the Balsa Products
site. Under brushless motors you will find the new Feigao brand.
There is a category for Outrunner Brushless motors. After that is the
entire series of GWS motors, including the IPS series, LPS series,
EPS series, and more!
When you get to the particular motor series, you have to click
again to obtain the actual data. Please be patient.
• http://home.ptd.net/~rcm65/motdata.html. This is Dick Miller’s
Motor Characteristics data that he has provided as a free service to
electric-power modelers for so many years. You can sort the data by
motor name or by a typical model’s wing area.
Dick covers the following motors, many of which are the small
variety used for parking lot or indoor flying: DC 5-2.4; DC-1717;
GWS (all); AstroFlight Firefly; MTM; VL Products; Ikarus; Hi-Line
Ltd.; KP-00; Kenway; Nikko; Peck-Polymers; and the Speed 280,
300, and 400 series.
Computer Programs to Help in the Selection Process: Researching
many Web sites to obtain motor data can be a real chore. A few years
ago two software programs came onto the hobby market that were
specifically designed to aid the electric-power enthusiast in choosing
a motor. The programs have been continually upgraded through the
years to the point where they include a tremendous amount of stored
motor (and aircraft) data parameters.
When these programs were first offered, we didn’t even have
brushless motors. Now they are the most popular item in electricpowered
flight. Both programs have literally grown with our hobby,
and you don’t have to be a computer expert or highly technical to be
able to use them.
I have used Sid Kauffman’s ElectriCalc for many years. It is up to
version 2.20 and has a list price of $49.95. Software is now provided
on a CD. It is intended for PCs with operating systems of Windows
Type of Aircraft and Skill Level
15-20 Larger trainer, Sport Scale model, sport
aerobatic model
20-25
Fast sport model (usually with more than
adequate power)
25-35 Scale model, larger multiengine model
35 plus Not for the author!
5-10
Park flyer, basic trainer, powered sport
sailplane
10-15
Faster sport flier, smaller trainer,
“Sunday” flier
Table 3. Wing Loading and Skill Level
Wing Loading
(Ounces/Square Foot)
98 and up. There are no provisions for Macintosh users—sorry!
You can find the details of this product and order a copy at
www.slkelectronics.com./ecalc/index.htm. The program’s present
version has stored data for 946 motors, and that figure is constantly
increasing as new motors come onto the market. There are two ways
to use ElectriCalc.
1) Enter your aircraft’s parameters (wing area, weight, and
approximate drag coefficient, which you can obtain by using the
simple help function called “Cdrag”). Pick the battery cell type and
the number of cells you think you might need. Choose a motor from
the extensive list shown, and pick a propeller that will provide
enough power to fly the model.
2) When the program is launched, an aircraft will already be
chosen (default) from the database or the last one you looked at will
be resident in the spreadsheet. Search the “planes” database for the
aircraft you are interested in or find one similar in size and weight to
your intended model, and rename and revise the parameters to
emulate your airplane.
Option 1 requires that you have a good idea of what propulsion
system should be in the model. This is seldom the case when one is
just getting into electric-powered flight.
Option 2 allows you to “repeat” an aircraft that has already flown.
(Much of the database in ElectriCalc on “planes” has been given to
SLK Electronics by proficient electric-power modelers.)
Option 2 also allows you to experiment with an existing design to
see what other motor or motor/gearbox combinations may work.
You can do a lot of “what-iffing” without having to leave your
computer station. This can be a big time-saver.
The other popular motor-selection program is MotoCalc,
available from Stefan Vorkoetter. His product offers many features
of its own and is comparably priced. You can obtain details at
www.motocalc.com.
As an active electric-powered-flight enthusiast, you owe it to
yourself to own at least one of these fine programs. You may still use
much of the manufacturers’ data, but these programs do an excellent
job as well. Both offer demonstration packages that you can take
advantage of before making your final choice. MA
Bob Aberle
[email protected]
(Editor’s note: This concludes the first part of Bob’s article. The
second part will be published in next month’s issue.)
Manufacturers/Distributors:
Li-Poly batteries, chargers:
FMA Direct
www.fmadirect.com
Optics 6 RC system:
Hitec RCD
www.hitecrcd.com
Bonnie 20 ARF, AXI motors, radial motor mounts, Jeti controllers:
Hobby Lobby International
www.hobby-lobby.com
Acrovolt plans:
Modelair-Tech
www.modelairtech.com
Li-Poly charger:
Peak Electronics
www.siriuselectronics.com
TNC tachometer:
Skyborn Electronics
www.bktsi.com/
SuperStar EP, PT-20:
Tower Hobbies
www.towerhobbies.com/
Edition: Model Aviation - 2005/03
Page Numbers: 33,34,35,36,38
FOREWORD: At the 2004 Weak Signals expo in Toledo, Ohio,
former AMA District II vice president and current AMA Flying Site
Coordinator for the Eastern Region Joe Beshar, who is an experienced
electric-power enthusiast, made a suggestion. He mentioned to AMA
Director of Publications Rob Kurek that we badly need a standard
reference document that will allow any modeler to size the proper
electric-motor system to model aircraft of any size and weight. What
you are about to read resulted from Joe’s suggestion.
Background: When a person enters model aviation, he or she has a
choice of power sources for his or her aircraft. Years ago those
options included rubber power, hand launching (as with a glider),
towing a glider with a cord or a fueled engine, and gasoline/sparkignition
engines or later glow-fueled engines.
As time went on and the glow engine became the most popular
source of model-aircraft power, modelers quickly became adept at
selecting the right-size engine for every size and weight of model. An
engine’s cu. in. displacement was related to model weight and model
size (usually wing area expressed in square inches).
Engine classes became known as 1⁄2A, A, B, C, and even D. Each
category covered a range of engine displacements (such as—if my
memory serves me correctly—0.09-.199 cu. in. was considered Class
A). Early FF competition rules related engine displacement to weight,
so that a .19 cu. in.-engine-powered model had to weight 19 ounces,
etc.
When RC started to come of age in the 1950s, we learned (by
experience) that 1⁄2A RC models could weigh as much as 18-20
ounces, Class A RC models could weigh as much as 30 ounces, and
so on. Although these engines are rated in horsepower (and thrust), to
this day most modelers simply relate engine displacement to model
weight and size!
Enter Electric Power: Although some of the first electric-powered
flights were made in the late 1950s and early 1960s, electric power as
we know it today didn’t really come into its own until the early 1970s.
Then it became obvious that engine displacement, for identification
purposes, would not work with electric. To make an early distinction
between fuel and electric, it was decided to refer to fueled power as
“engines” and electric power as “motors.”
Bob Boucher—one of the leaders of electric-powered flight in the
USA—began referring to his motors by equivalent glow-engine
displacements. His “05” motor was supposed to be equivalent to an
.049 cu. in. glow engine, a “40” motor was equivalent to a .40 cu. in.
glow engine, etc.
While this was going on, Germany’s Graupner company was
assigning names such as Speed 400, Speed 500, and the like. The
numbers were rounded off from the Mabuchi manufacturer’s model
numbers; “RS-380” became a Speed 400, “RS-540” became a Speed
500, and so on. Surely that fact couldn’t help you select a motor for a
particular size and weight of model.
What to Do? Approximately 30 years have now gone by since that
start in electric-powered flight. Although many still have questions,
we know much more now and have developed some excellent
techniques for “sizing,” or matching the correct motor to any size and
weight of model aircraft.
When you look back at this time frame, you can compare it with
the advent of the gasoline engines in the early 1930s, which
ultimately led to the cu. in.-displacement sizing of those engines in
the 1940s. Identifying fueled and electric engines/motors has taken
time.
The main thrust of this article is to explain how to select, or size,
your motors to make them suitable for powering models in flight. I
hope to cover such things as motor identification, motor types, direct
vs. gear drive, motor power (in watts) as it relates to aircraft weight,
and many other details. I hope to make this a standard reference so
that no one will have to ask, “What motor should I use?”
Aircraft Categories:
1) Use electric-powered kits, ARF models, and published plans.
We currently enjoy the fact that many kits, ARFs, and published plans
exist for all kinds of electric-powered models. In this category, the
aircraft designer or manufacturer has selected the motor for you. You
might want to improve on the initial choice, but you can easily get in
those first few flights before you fine-tune the selection process.
2) Make a glow-fuel-powered kit electric powered. There may be a
particular glow-fueled kit or ARF that you want to build, from
inception, as an electric-powered model. You want to be able to select
a motor, the type of drive (direct or geared), the type of battery pack,
and the battery pack’s capacity, but you want to make these selections
so that you can install the electric power-system components as you
construct the model.
MA Technical Editor Bob Aberle with sport/aerobatic Acrovolt
constructed in 1995. As designed by Tom Hunt, it would have been
comfortable with 60-size glow engine but was powered by DeWALT
18-volt cordless-drill motor with belt drive. Text tells how it was
greatly improved with new brushless motor and Li-Poly batteries.
Photos courtesy the author
by Bob Aberle
Using Whattmeter to take motor data of electric power equipment
in Hobby Lobby Bonnie 20, note battery packs lined up in
foreground for test purposes. In front is TNC tachometer for
measuring propeller rpm.
AstroFlight Super Whattmeter is the heart of any motor-selection
process. When inserted between battery and motor, it indicates
motor current, voltage, power (watts), and amount of energy going
into or out of battery (in Ah).
Bonnie 20 electric-powered ARF is featured in Sport Aviator review.
It could have been powered by .20-.32 glow engine. It also makes a
wonderful electric-powered advanced RC trainer.
Hobbico SuperStar EP is intended for glow power. Bob explains
in the text how to convert it to electric power.
3) Convert a glow-fuel-powered RC model you already have to
electric power. Let’s say you have a built-up and flying glow-fueled
airplane, and you decide to remove that engine and retrofit an
electric power system. How would you go about making that
selection?
4) Update an old electric-powered model that uses old-style
technology to use all of the latest equipment. How would you make
the specific selections based on the aircraft’s current size and
weight? In this case you would be looking at a new brushless motor,
a dedicated sensorless ESC, and probably lightweight and highcapacity
Li-Poly batteries.
Abstract: My close friend, flying partner, and fellow Model
Aviation Hall of Famer Tom Hunt is probably one of the most
famous and experienced electric-power fliers in the country. He has
been my primary consultant throughout this article’s preparation. He
recently made a thought-provoking observation, which follows.
“The main problem in the selection process is that the electric
motor has a much broader operating range than an IC [internal
combustion] engine. I can have an AXI 2212/34 brushless motor fly
a 10-ounce [total weight] aircraft at 60 watts input, a 14-ounce
aircraft at 95 watts input [that’s a 50% power increase—try that with
a glow engine!], and a 22-ounce model at 120 watts input [that’s a
100% increase].”
As you can see from Tom’s examples, any specific motor will
have far more application than a comparable glow engine. Because
of this, there are many more choices in the selection process when
going to electric power. That is what mystifies most modelers, and I
hope this article will help clear things up.
Measuring Motor Input Power (Watts): Motors are not referred to
by cu. in. displacement (as glow engines are). Motors are defined by
the term “power,” which is measured in watts. All references to
power in watts is input to the motor (or output of the battery). The
motor’s output power is a function of its efficiency.
As Tom pointed out, a motor can run at 60, 95, or even 120
watts. The primary rule is that power (in watts) equals motor current
(in amperes, or amps) multiplied by the voltage.
The voltage is determined by the battery and will vary
according to battery type (Ni-Cd, NiMH, or Li-Poly) and the
number of cells employed in the particular battery pack. The
current is determined by several things, including the resistance of
the motor windings, the size of the propeller, and the type of drive
(direct, with a gear reduction, or belt-reduction drive).
So you have the variables current (amps) and voltage that yield
Sample page from ElectriCalc motor-selection computer program.
This type of program goes hand in hand with motor-manufacturer
data to assist in selection process for specific size/weight aircraft.
You must know aircraft’s exact weight to select motors. Pelouze
Model PE-5 scale (L) weighs items as heavy as 5 pounds in 0.1-
ounce increments. My Weigh digital scale (R)—from
www.goodscale.com/scale—is intended for parking lot and indoor
micro RC models.
Typical battery chargers (L-R, top and bottom): AstroFlight 109
and Peak Electronics Lithium charger intended for Li-Poly only.
AstroFlight 110 Deluxe and FMA Direct Super Nova peak-detect
chargers intended for Ni-Cd and NiMH.
The TNC tachometer is one of the best digital tachometers on the
market. It is available from Skyborn Electronics.
Page from The Great Electric Motor Test—one of many sources for
motor data. It provides such parameters as motor current, voltage,
power (watts), propeller sizes, and rpm.
power (watts). This is where the fun starts since you can vary
everything, including the propeller size, the motor drive, and the
battery. Each variable will produce different results.
With a fueled engine, you can listen to the sound and adjust the
carburetor (by ear) until you reach the correct power level. With a
motor, there is essentially no noise and therefore nothing to listen to.
So how do you obtain a figure for power (watts)?
It would be nice if the motor manufacturers had printed on the
box “100 watts (maximum short duration, 75 watts continuous),”
much as engine manufacturers indicate “.049 cu. in., or 1⁄2A.” The
variables make that type of identification impractical. Most motor
manufacturers publish excellent motor data that I will write about
later, along with computer programs that support this process.
For now, you must learn that the most important “tool” for the
electric-power flight enthusiast is the ammeter and wattmeter. We
primarily use the AstroFlight Model 101 Super Whattmeter, which
1.25 Powered sport sailplane, indoor RC airplane
you can purchase for roughly $60. It is the latest version and will
measure to 10 mA instead of 100 mA. It can also read below 4.0
volts by adding a four-cell receiver battery pack. Make sure you
purchase this new version.
The meter is self-powered by the system you are measuring. It
provides four important motor parameters: voltage (volts), current
(amps), power (watts), and the capacity going into or out of the
battery, measured in ampere-hours (Ah). A list of articles and Web
sites at the end of this article will provide the details about the
specific use of the Whattmeter.
Some modelers already own digital-type multimeters, sold by
stores such as RadioShack. Those meters are fine, but most are
somewhat limited in current range.
If you try to check a 25-amp motor with some of these meters,
you will probably end up blowing a fuse or even damaging the
device. Many of these multimeters are of the newer “auto-ranging”
type which can also get complicated during regular use. I suggest that
you stick with the AstroFlight meter for its proven ability to easily do
the job!
Aircraft Weight and Power Loading: Now that you have the motor
“identified” by power, how do you relate it to a specific-size model?
There are actually two aircraft parameters you need to be interested
in, one of which is the model’s total weight. This is generally
measured in ounces for the smaller airplanes and pounds for the
larger models.
If you are in the planning stages of selecting a motor, you may be
estimating an aircraft’s weight as a starting point. If you are dealing
with an existing model, you will have to weigh it on a scale. There
are many digital-type scales on the market that read to within 0.1
ounce. You can buy one at a stationery store such as Staples, Office
Max, and others. After constructing or assembling an ARF, you will
also have to weigh it to verify your motor selection or help you
improve performance by selecting a different motor.
Knowing the motor’s power and the model’s weight, you can
come up with one of the most important “combination parameters,”
known as watts/ounce or watts/pound. These “power loading” values
are used for judging aircraft performance.
The term watts/ounce is generally used to describe the power
loading of models that weigh as much as 1 pound. Watts/pound is a
more common term for models that weigh more than 1 pound.
Through the years, experienced electric-power enthusiasts have
come up with a series of watts/ounce and watts/pound figures that
relate to an airplane’s expected performance. Table 1 and Table 2
provide the accepted parameters that cover most aircraft types. These
numbers aren’t absolute; they are only to be used as a guide.
Wing Loading and Skill Level: After determining motor power
(watts) and aircraft weight, you must focus on airplane size, which
involves wing area and then wing loading (which takes into account
wing area and aircraft weight).
The term “power loading” that I described (watts/ounce and
watts/pound) involves motor power and model weight; they are
aircraft related. The term “wing loading” relates to the pilot’s skill
level and directly to the minimum speed the model can fly (stall
speed). Aircraft with light wing loadings will be easier to fly (for
beginners and sport fliers) than those with high wing loadings
(favored by the experienced or expert fliers).
To determine wing loading, you need to know your model’s wing
area and weight. Wing area is the wingspan multiplied by the wing’s
average width (wing chord) and is expressed in square feet (sq. ft.).
Wing area may be a given with kits, ARFs, and even published
plans. The specifications that accompany these usually include the
wing area. A pure rectangular-shaped wing is easy to figure out (span
multiplied by chord), but an elliptical-shaped wing is much more
complicated. You typically do it by estimating the average wing
chord (width).
Once you know the wing area expressed in square inches (sq. in.),
you must convert that to sq. ft. by dividing it by 144. Let’s say your
aircraft has 400 sq. in. of wing area. Divide that by 144, and you get
2.78 sq. ft. The aircraft weighs 40 ounces, so divide that by the 2.78
sq. ft. to obtain the wing loading of 14.4 ounces/sq. ft.
Table 3 provides acceptable wing loadings for various pilot-skill
levels. Values start at 5 ounces/sq. ft. and can exceed 35 ounces/sq. ft.
In the past few paragraphs you learned about motor power (watts).
Then you learned how power relates to model weight. Last, you
learned how model size and weight are related to the pilot’s skill
level. You must take all of these parameters into consideration when
selecting the proper motor for your model.
Thrust: I don’t use it because in general it is not a reliable figure.
The thrust one measures on the bench has little to do with the thrust
the propeller produces as it moves through the air.
Some modelers came up with a factor indicating that your model
can weigh as much as three or four times the rated motor thrust.
That’s fine, if you really know what that thrust is. So I avoid using
this parameter!
Selection Process Details: Before I provide some examples of motor
selection, I want to introduce you to the subject of motor data. When
you set out to make a specific motor selection, you need a large
volume of motor-parameter data at your disposal.
That information should contain such things as motor current,
voltage, and watts for a range of recommended propellers (identified
by diameter and pitch). It should also provide a range of voltages so
that you can choose a battery type and capacity to suit the application.
It would be nice to have that kind of data available for every type
of motor available on our hobby market or, better still, have in one
place. I would love to have it on one Web site, but that hasn’t
happened yet. Information is available on many Web pages, and I’ll
steer you to the major ones.
But before I do that, I want to discuss “voltage.” Much of the
motor data presented on manufacturers’ Web sites indicate numbers
of battery cells and the type of battery, but it will not state the
voltage!
When referring to numbers of cells of Ni-Cd- or NiMH-type
batteries, each cell has a nominal 1.2 volts. If the data indicates eight
NiMH cells, the nominal voltage will be eight multiplied by 1.2, or
9.6. If there are two Li-Poly cells, the voltage will be two mulitiplied
by 3.7 (that is peculiar to Li-Poly batteries), or 7.4.
Some sites are kind enough to indicate the measured voltage
under load, which is really what you want, but others will make you
use the battery cell count, and that takes extra time. Be aware!
Motor Data Web Sites: The following sites are not in any particular
order, and they are not all that are available. They are what I think
you will need as a starting point for most of your motor selections.
• www.flyingmodels.org. Webmaster Fredrik Wergeland of
Stockholm, Sweden, is responsible for this data. After getting on the
site, select “The Great Electric Motor Test,” and then you have a
choice of standard or brushless motors.
There are not many standard motors, but there are quite a few
Watts/Ounce Type of Aircraft
1.00 Model that barely gets off the ground
2.00 Parking lot/backyard flyer
3.00 Aerobatic flier
5.00 Electric 3-D model
Table 1. Watts/Ounce (Models Weighing Less Than 1 Pound)
40-50 “Sunday flier,” sport sailplane, Old-Timer
Watts/Pound Type of Aircraft
30 Model that barely gets off the ground
60-70 Mildly aerobatic model
80-100 Aggressive airplane
100 plus Ducted-fan aircraft, competition Sailplane,
electric-powered 3-D model
Table 2. Watts/Pound (Models Weighing More Than 1 Pound)
38 MODEL AVIATION
brushless including Mega, Model Motors (AXI 22 and 28 series and
Mini AC), Nippy Black, MP Jet, Kontronik, and Jeti Phasor. There is
a promise that data will be posted for Speed 280 through Speed 600
ferrite motors in the future.
This Web site has nothing to do with the popular Flying Models
magazine.
• www.aircraft-world.com/default.asp?id=18. This site—run by
Dave Radford of Air Craft Inc. (of Japan)—provides data for GWS,
Hacker, Mega, Model Motors (including the AXI 41 series), MP Jet,
and QRP motors.
• www.astroflight.com. When you access this site, you can select
brushless motors or cobalt airplane motors. In some cases only one
propeller choice is given for a particular motor/battery combination.
That should still provide a good starting point!
• www.hackerbrushless.com/motors.shtml. Hacker USA provides
data for all of its brushless motors. After accessing the site, select the
motor you are interested in, such as the “B20 Series.” Then scroll
down and click on “Click here for B20 Series Application Chart.”
You will be surprised by the extent of this data because it goes as far
as relating motor type to specific model aircraft by name!
• www.balsapr.com/catalog/motors. This is part of the Balsa Products
site. Under brushless motors you will find the new Feigao brand.
There is a category for Outrunner Brushless motors. After that is the
entire series of GWS motors, including the IPS series, LPS series,
EPS series, and more!
When you get to the particular motor series, you have to click
again to obtain the actual data. Please be patient.
• http://home.ptd.net/~rcm65/motdata.html. This is Dick Miller’s
Motor Characteristics data that he has provided as a free service to
electric-power modelers for so many years. You can sort the data by
motor name or by a typical model’s wing area.
Dick covers the following motors, many of which are the small
variety used for parking lot or indoor flying: DC 5-2.4; DC-1717;
GWS (all); AstroFlight Firefly; MTM; VL Products; Ikarus; Hi-Line
Ltd.; KP-00; Kenway; Nikko; Peck-Polymers; and the Speed 280,
300, and 400 series.
Computer Programs to Help in the Selection Process: Researching
many Web sites to obtain motor data can be a real chore. A few years
ago two software programs came onto the hobby market that were
specifically designed to aid the electric-power enthusiast in choosing
a motor. The programs have been continually upgraded through the
years to the point where they include a tremendous amount of stored
motor (and aircraft) data parameters.
When these programs were first offered, we didn’t even have
brushless motors. Now they are the most popular item in electricpowered
flight. Both programs have literally grown with our hobby,
and you don’t have to be a computer expert or highly technical to be
able to use them.
I have used Sid Kauffman’s ElectriCalc for many years. It is up to
version 2.20 and has a list price of $49.95. Software is now provided
on a CD. It is intended for PCs with operating systems of Windows
Type of Aircraft and Skill Level
15-20 Larger trainer, Sport Scale model, sport
aerobatic model
20-25
Fast sport model (usually with more than
adequate power)
25-35 Scale model, larger multiengine model
35 plus Not for the author!
5-10
Park flyer, basic trainer, powered sport
sailplane
10-15
Faster sport flier, smaller trainer,
“Sunday” flier
Table 3. Wing Loading and Skill Level
Wing Loading
(Ounces/Square Foot)
98 and up. There are no provisions for Macintosh users—sorry!
You can find the details of this product and order a copy at
www.slkelectronics.com./ecalc/index.htm. The program’s present
version has stored data for 946 motors, and that figure is constantly
increasing as new motors come onto the market. There are two ways
to use ElectriCalc.
1) Enter your aircraft’s parameters (wing area, weight, and
approximate drag coefficient, which you can obtain by using the
simple help function called “Cdrag”). Pick the battery cell type and
the number of cells you think you might need. Choose a motor from
the extensive list shown, and pick a propeller that will provide
enough power to fly the model.
2) When the program is launched, an aircraft will already be
chosen (default) from the database or the last one you looked at will
be resident in the spreadsheet. Search the “planes” database for the
aircraft you are interested in or find one similar in size and weight to
your intended model, and rename and revise the parameters to
emulate your airplane.
Option 1 requires that you have a good idea of what propulsion
system should be in the model. This is seldom the case when one is
just getting into electric-powered flight.
Option 2 allows you to “repeat” an aircraft that has already flown.
(Much of the database in ElectriCalc on “planes” has been given to
SLK Electronics by proficient electric-power modelers.)
Option 2 also allows you to experiment with an existing design to
see what other motor or motor/gearbox combinations may work.
You can do a lot of “what-iffing” without having to leave your
computer station. This can be a big time-saver.
The other popular motor-selection program is MotoCalc,
available from Stefan Vorkoetter. His product offers many features
of its own and is comparably priced. You can obtain details at
www.motocalc.com.
As an active electric-powered-flight enthusiast, you owe it to
yourself to own at least one of these fine programs. You may still use
much of the manufacturers’ data, but these programs do an excellent
job as well. Both offer demonstration packages that you can take
advantage of before making your final choice. MA
Bob Aberle
[email protected]
(Editor’s note: This concludes the first part of Bob’s article. The
second part will be published in next month’s issue.)
Manufacturers/Distributors:
Li-Poly batteries, chargers:
FMA Direct
www.fmadirect.com
Optics 6 RC system:
Hitec RCD
www.hitecrcd.com
Bonnie 20 ARF, AXI motors, radial motor mounts, Jeti controllers:
Hobby Lobby International
www.hobby-lobby.com
Acrovolt plans:
Modelair-Tech
www.modelairtech.com
Li-Poly charger:
Peak Electronics
www.siriuselectronics.com
TNC tachometer:
Skyborn Electronics
www.bktsi.com/
SuperStar EP, PT-20:
Tower Hobbies
www.towerhobbies.com/