Photos courtesy the author
by Bob Aberle
Part 2
The Acrovolt lives up to its name by performing spirited
aerobatics in an effortless manner.
selected 50 watts/pound. From that I multiplied 3.93 pounds (63
ounces) by 50 and obtained 197 watts. Since power is amps multiplied
by volts, you can work backward using the power (watts) and the
estimated motor current.
That motor-current figure will always prove to be the tricky part. It
will be an estimate that is a compromise between how long a motor run
you desire and how much current your specific motor can tolerate
(known as maximum continuous current in motor specifications).
This is where the ElectriCalc and MotoCalc motor-selection programs
can really help you. For this application I decided on a range of 20.0-
25.0 amps and 22.5 as an average. Since power (watts) equals current
(amps) multiplied by volts, you can divide 197 watts by 22.5 amps and
obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an
eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between
two and three Li-Poly cells because the characteristic voltage is 3.7
volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives
you a ballpark figure.
From this point you can look up your motor data on one of the six
Web sites I listed last month. I generally go for the AXI brushless
outrunner motors because they are available in many sizes, and I have
found them to be extremely reliable.
For the average-size AXI motor I use “The Great Electric Motor
Test,” at www.flyingmodels.org. I searched through the data looking at
motor current, power (watts), and propeller sizes. You have to be
patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor.
On 8.0 volts and with an APC 10 x 7E propeller, it would have a
current of 23.0 amps and 176 watts of power. The 10-inch propeller
can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should
work. The Ni-Cd cells will likely produce slightly higher voltage,
current, and wattage. A recommendation is an eight-cell Sanyo 1950
The author selected Tom Hunt’s successful original-design
Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background
of how power sources for modeling are rated and specifically how
electric motors have been related in power output to glow engines. He
covered how to go about sizing, or matching, a specific-size motor to a
given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained
how to measure motor input power, aircraft weight and power-loading
considerations, wing loading and how it relates to flying experience
and skill, and thrust factors. He included details of the motor-selection
process, a comprehensive listing of Web sites containing helpful data,
and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection
process.)
IT’S TIME TO put all you’ve learned together with several examples
of motor selections. Let’s make a glow kit or glow ARF electricpowered.
Many popular glow-engine-powered full kits and ARF “kits” on
the market can easily be constructed and/or assembled from scratch
with electric power in mind. One such model is Tower Hobbies’
“Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended
for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of
wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of
16-20 ounces/square foot (sq. ft.).
As a start, take into account that the wing area is 515 square inches
(3.6 sq. ft.). Pick a wing loading for the intended skill level. Using
Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft.
That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing
loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up
with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading.
Wanting a bit more in performance (with some reserve), I specifically
AXI 4120/18 motor with special radial mount plate at right. An
“outrunner” motor’s entire outer casing rotates.
L-R: Older, larger, heavier DeWALT motor with H-1000 belt
drive and replacement Hobby Lobby AXI 4120/18 brushless
outrunner motor. It weighs 11 ounces!
Acrovolt went from 7 pounds to 5 pounds, 5 ounces. Motor run
time was increased from six to eight minutes to more than 20
minutes. This was accomplished with new brushless motor
and new Li-Poly batteries.
Original Acrovolt motor system, consisting of DeWALT 18-volt
cordless-drill motor and Modelair-Tech H-1000 belt reduction drive.
Motor and belt drive weighed 21 ounces!
A page from Bob’s logbook. Record data for each new or revised
aircraft, and then you have something to compare to when visiting
new, similar-size/weight aircraft.
mAh NiMH pack, which will get me slightly over my target but allow
me to do some throttling back during a flight. That will stretch the run
time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As
you progress, feel free to “what-if” your parameters in either of the two
computer motor-selection programs. You can even work up to Li-Poly
batteries by carefully selecting a propeller size that will allow you to
use a three-cell pack, but not at excessive motor current.
Converting/Updating: Almost 10 years ago, Tom Hunt designed a
large, aerobatic/Pattern, electric-powered model called the Acrovolt. It
was inspired by Art Schroeder’s Eyeball design from the 1960s.
I flew Art’s Eyeball back then. I remember that it weighed 6 pounds
with an Enya 60 glow engine. Tom designed his Acrovolt around the
use of his then-new Modelair-Tech H-1000 belt drive and either a
Speed 700 ferrite motor or a DeWALT 18-volt cordless-drill motor.
In 1995 the Acrovolt was powered by a 16-cell Ni-Cd battery pack,
which was later increased to an 18-cell 3000 mAh NiMH pack. The
weight at the time was roughly 7 pounds. The wing loading was high,
How the Acrovolt’s crowded battery compartment looked when
Bob had to employ upward of 18 3000 mAh NiMH cells.
Closer look at AXI 4120/18 motor from rear shows accessory
radial mounting plate which Bob highly recommends. Collar on
shafts moves up flush with rear of motor.
The same battery area today, after switching to Li-Poly!
New square Kokam Li-Poly battery pack is mounted ahead of
wing LE and before firewall. Because of lighter motor weight, Li-
Poly pack’s forward position proved to be necessary to maintain
proper CG position.
Top: The 18-cell 3000 mAh NiMH pack as used years ago to power
the Acrovolt weighed more than 40 ounces. Bottom: New Kokam
5S2P Li-Poly pack weighs just 20 ounces. The motor run went from
six to eight minutes to more than 20 minutes.
and the motor run time was short—six minutes average with an
occasional seven to eight minutes with some throttling! But at that
weight, throttling almost got the model back on the ground!
That airplane stayed idle in my shop until recently, when a thought
came to me while I was preparing this article. Here I had a .60 cu. in.
glow-powered model (40-size by today’s standards) that was converted
to electric power using the technology that was available 10 years ago.
How would the same airplane perform today with a modern and
efficient brushless motor, an equally modern brushless/sensorless ESC,
and, best of all, a lighter-weight/high-capacity Li-Poly battery pack?
The sequence of the selection process is the same as in the
preceding. The 600-square-inch wing area is the same as 4.17 square
feet. Referring to Table 3 I selected the skill-level category of the fast
sport model (usually with more than adequate power), which calls for
20-25 ounces/sq. ft. of wing loading.
This was roughly the range I experienced with my Eyeball using a
60 glow engine at 6 pounds total weight. The older electric version—at
7 pounds—had a wing loading of 27 ounces/sq. ft., which I knew was
way too high.
Recognizing that the new motor and battery would save me a great
deal of weight, I selected 20 ounces/sq. ft. as my target wing loading. If
I multiplied that 20.0 by the 4.17 sq. ft. wing area, I ended up with a
target weight of 83.4 ounces (slightly heavier than 5 pounds).
Looking at Table 2 I chose 80-100 watts/pound (for aggressive
airplanes) as the power loading. Not wanting to go absolutely crazy, I
Upper view shows added plywood motor box and battery-pack
position. Jeti brushless ESC is also in battery compartment.
After removing DeWALT motor and belt drive, 3⁄16 plywood box
was made to hold new AXI 4120/18 brushless motor at roughly
same prop position.
Clear plastic cowl shows new AXI brushless-motor installation.
Bob painted inside of molded cowl after photos were taken.
After being refitted with a modern power system, the Acrovolt is
more spirited than ever in flight!
Bob replaced old 2-ounce ACE RC Smart ESC (top) with new 1-
ounce Jeti Advance brushless ESC (bottom). New ESC senses
battery voltage at every start and automatically sets proper cutoff
voltage point. This is crucial when using Li-Poly batteries.
selected the midrange number of 90 watts/pound. From that I
multiplied 5.2 pounds (83.4 ounces) by 90 and obtained 468 watts.
Since power is amps multiplied by volts, you can work backward
using the power (watts) and the estimated motor current. Again, the
estimate of motor current is all-important and requires your knowledge
and judgment.
For this application I settled on approximately 27.0 amps. With the
older motor 10 years ago I ran 30.0-32.0 amps. I wanted to use less
current this time because my airplane will weigh much less.
Since power (watts) equals current (amps) multiplied by volts, you
can divide 468.0 watts by 27.0 amps and obtain 17.3 volts. For this
new application I wanted to use a large Li-Poly pack. Since each of
these cells has a nominal 3.7 volts, the closest I could get to 17.3 was
five Li-Poly cells (3.7 x 5 = 18.5 volts). I’ll get into the Li-Poly
capacity selection in a bit.
Having collected all of this information, I needed to select a
brushless motor for my application. I mentioned that I favor the AXIs,
so I went to their larger-motor data. The Web site containing The Great
Electric Motor Test did not list the larger 41-series AXIs, but Dave
Radford of Air Craft Inc. provided the data at www.aircraftworld.
com/default.asp?id=18. After going through his AXI 41-series
information, I settled on the AXI 4120-18 brushless outrunner.
The data specifically called for the use of an APC 12 x 8E propeller
and 16 Ni-Cd or NiMH cells (1.2 x 16 = 19.2 volts). The motor current
given was 24.5 amps, and the power was 434
watts. For my purpose this was close enough
for a starting point. To my surprise, the actual
numbers came out closer to my estimate than
the published data.
The photos accompanying this article
show how easy it was to swap out the much
larger ferrite motor for the brushless motor.
Since the Li-Poly battery was much lighter, I
was forced to move it farther forward (just
behind the firewall) to maintain the proper CG
location.
Flying the “revised” Acrovolt was
wonderful. The final model weight was 85.3
ounces (5 pounds, 5.3 ounces), yielding a
wing loading of 20.5 ounces/sq. ft. The actual
motor parameters worked out to be higher
than my predictions, but they were in the
direction I wanted so I left them that way for
my initial flight tests. The motor current was
28.5 amps, voltage under load was 18.0 volts,
wattage was 511.0, the watts/pound figure
was 95.8, and rpm was 8,600.
The Kokam 5S2P battery pack that I used
consisted of 2.0-ampere-hour (Ah)-rated cells
(15C-load capable). With two sets in parallel
(2P), the actual capacity is 4.0 Ah. Total
flying time obtained with this battery pack
was approximately 20 minutes, with some
throttling management. That means you don’t
run the motor all the time at full throttle.
In fact, the Acrovolt—now at an
extremely light weight and with a highperformance
power system—is capable of
doing loops from level flight at half throttle.
When the model landed each time at this
power level, the motor, ESC, and battery pack
were hardly warm. This was a perfect powersystem
choice in every regard! I ended up
with a model that flew better than it did with a
glow engine or the older ferrite electric motor.
Estimating Motor Run Time: When you near
the end of your motor-selection process, you
need to decide what capacity of battery you
want to use, regardless of whether it’s Ni-Cd,
NiMH, or Li-Poly.
Battery capacity is usually rated in mAh or
Ah. Your battery’s weight is directly related
to its rated capacity. The more capacity there
is in the battery, the more it will weigh. The
motor-selection programs can help you
considerably when making this choice.
For a quick estimate, I use a simple
formula. I multiply 60 by the battery’s
capacity rating (in Ah) and divide that by the
motor current (in amps). The result is an
estimated motor run time in minutes.
As an example, let’s go back to when we
were building a glow-powered kit as an
electric-powered model. The battery had a
1950 mAh capacity rating, and the motor
current was 23.0 amps. So multiply 60 by
1.95 Ah (the same as 1950 mAh), and divide
that by the 23.0 amps. The answer is 5.09
minutes.
Keep in mind that these are static figures.
When the model is in the air and the propeller
unloads, the current will be less. Also, it is
assumed that you will not fly the entire flight
at full throttle. As I pointed out earlier, this
model’s actual flight time was approximately
eight minutes. Nevertheless, this formula
provides a rough starting point.
Keeping Records: One of the best ways to
put your electric-power experience to good
use is by keeping accurate records of your
various aircraft. With new airplanes
especially, I make dated entries in a bound
logbook about the aircraft; electric parameters
such as motor current, voltage, watts,
watts/ounce or watts/pound, rpm, and run
time; and flight performance.
As you progress with electric power, these
records will provide practical comparisons
when you choose power systems for new
aircraft.
Where to Get Help: When you get stuck and
can’t find that elusive electric power system,
you will need some help. One place to find it
is a model forum on the Internet that
specializes in electric power, such as RC
Groups, E-Digest, RC Universe, and SFRC.
Hopefully you will reach an “expert” when
you send out your question.
Contacting local modeling clubs that
specialize in electric power is another way to
learn. But if you live in a remote area, that
may not be easy.
In the end, there is a group of many hobby
distributors who will work with you,
including Kirk Massey at New Creations RC
([936] 856-4630), Dave Thacker at Radical
RC (www.radicalrc.com), Dave and Bob Peru
at Balsa Products (www.balsapr.com), Sal
DeFrancesco at Northeast Sailplane Products
(www.nesail.com), Helmut Goestl of Dymond
Modelsport (www.rc-dymond.com), and Tom
Hunt at Modelair-Tech (www.modelair
tech.com).
References: In the past two years I have
authored a series of articles specifically about
electric-powered flight that have been
published in MA, and they are as follows.
(You can find the “From the Ground Up”
articles on the MA Web site at www.modelair
craft.org/mag/index.htm.)
• “State of the Sport: Electric-Powered
Flight”: April 2002, pages 18-40.
• “From the Ground Up: Introduction to
Electric Power”: July 2003, pages 56-64.
• “From the Ground Up: Battery Basics”:
October 2003, pages 54-62.
• “Introduction to Lithium-Polymer Batteries:
May 2004, pages 44-58.
I have also written articles that have been
posted on MA’s Sport Aviator online
magazine. They are:
• “The SuperStar-EP Electric ARF” at
www.masportaviator.com/ah.asp?CatID=1&I
D=16. This is a review. SuperStar EP is
similar to the Tower Hobbies PT-20 that I
mentioned in this article.
• “Bonnie 20 ARF Electric Trainer” at
www.masportaviator.com/ah.asp?CatID=1&I
D=39. This is also a review.
• “Bonnie 20—Adapting to Li-Poly Batteries”
at www.masportaviator.com/ah.asp?CatID
=2&ID=43. This shows in depth how to
convert a Bonnie from NiMH/Ni-Cd batteries
to the new Li-Poly variety.
In addition, I wrote the book Getting
Started in Backyard Flying, which is available
from AMA. Pages 58 and 59 explain the
process of measuring motor current using an
AstroFlight Super Whattmeter (part 101).
The process of selecting a motor system for a
particular aircraft may seem overwhelming at
first, but I assure you it’s not! Follow the
logical steps I have presented here; they work!
I would love to be able to reference a single
page or table and say, “That’s the answer.” Joe
Beshar thought that was possible when he
made his suggestion, but there are far too
many variables with electric power.
Buy that meter and take your own
parameter measurements. Buy a scale and
weigh your aircraft; don’t guess! Buy a
tachometer and really see if you improved
things! Buy one of the two recommended
computer programs; you’ll be amazed by how
helpful they are.
If you hit a snag, please write or E-mail
me. I will not only try to answer your detailed
questions, but I will include those inquiries/
answers in MA’s “Frequently Asked
Questions” column if it will benefit other
readers. Don’t give up on electric power; get
more involved! MA
Bob Aberle
[email protected]
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 - 2005/04
Page Numbers: 53,54,55,56,58,60
Edition: Model Aviation - 2005/04
Page Numbers: 53,54,55,56,58,60
Photos courtesy the author
by Bob Aberle
Part 2
The Acrovolt lives up to its name by performing spirited
aerobatics in an effortless manner.
selected 50 watts/pound. From that I multiplied 3.93 pounds (63
ounces) by 50 and obtained 197 watts. Since power is amps multiplied
by volts, you can work backward using the power (watts) and the
estimated motor current.
That motor-current figure will always prove to be the tricky part. It
will be an estimate that is a compromise between how long a motor run
you desire and how much current your specific motor can tolerate
(known as maximum continuous current in motor specifications).
This is where the ElectriCalc and MotoCalc motor-selection programs
can really help you. For this application I decided on a range of 20.0-
25.0 amps and 22.5 as an average. Since power (watts) equals current
(amps) multiplied by volts, you can divide 197 watts by 22.5 amps and
obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an
eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between
two and three Li-Poly cells because the characteristic voltage is 3.7
volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives
you a ballpark figure.
From this point you can look up your motor data on one of the six
Web sites I listed last month. I generally go for the AXI brushless
outrunner motors because they are available in many sizes, and I have
found them to be extremely reliable.
For the average-size AXI motor I use “The Great Electric Motor
Test,” at www.flyingmodels.org. I searched through the data looking at
motor current, power (watts), and propeller sizes. You have to be
patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor.
On 8.0 volts and with an APC 10 x 7E propeller, it would have a
current of 23.0 amps and 176 watts of power. The 10-inch propeller
can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should
work. The Ni-Cd cells will likely produce slightly higher voltage,
current, and wattage. A recommendation is an eight-cell Sanyo 1950
The author selected Tom Hunt’s successful original-design
Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background
of how power sources for modeling are rated and specifically how
electric motors have been related in power output to glow engines. He
covered how to go about sizing, or matching, a specific-size motor to a
given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained
how to measure motor input power, aircraft weight and power-loading
considerations, wing loading and how it relates to flying experience
and skill, and thrust factors. He included details of the motor-selection
process, a comprehensive listing of Web sites containing helpful data,
and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection
process.)
IT’S TIME TO put all you’ve learned together with several examples
of motor selections. Let’s make a glow kit or glow ARF electricpowered.
Many popular glow-engine-powered full kits and ARF “kits” on
the market can easily be constructed and/or assembled from scratch
with electric power in mind. One such model is Tower Hobbies’
“Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended
for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of
wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of
16-20 ounces/square foot (sq. ft.).
As a start, take into account that the wing area is 515 square inches
(3.6 sq. ft.). Pick a wing loading for the intended skill level. Using
Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft.
That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing
loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up
with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading.
Wanting a bit more in performance (with some reserve), I specifically
AXI 4120/18 motor with special radial mount plate at right. An
“outrunner” motor’s entire outer casing rotates.
L-R: Older, larger, heavier DeWALT motor with H-1000 belt
drive and replacement Hobby Lobby AXI 4120/18 brushless
outrunner motor. It weighs 11 ounces!
Acrovolt went from 7 pounds to 5 pounds, 5 ounces. Motor run
time was increased from six to eight minutes to more than 20
minutes. This was accomplished with new brushless motor
and new Li-Poly batteries.
Original Acrovolt motor system, consisting of DeWALT 18-volt
cordless-drill motor and Modelair-Tech H-1000 belt reduction drive.
Motor and belt drive weighed 21 ounces!
A page from Bob’s logbook. Record data for each new or revised
aircraft, and then you have something to compare to when visiting
new, similar-size/weight aircraft.
mAh NiMH pack, which will get me slightly over my target but allow
me to do some throttling back during a flight. That will stretch the run
time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As
you progress, feel free to “what-if” your parameters in either of the two
computer motor-selection programs. You can even work up to Li-Poly
batteries by carefully selecting a propeller size that will allow you to
use a three-cell pack, but not at excessive motor current.
Converting/Updating: Almost 10 years ago, Tom Hunt designed a
large, aerobatic/Pattern, electric-powered model called the Acrovolt. It
was inspired by Art Schroeder’s Eyeball design from the 1960s.
I flew Art’s Eyeball back then. I remember that it weighed 6 pounds
with an Enya 60 glow engine. Tom designed his Acrovolt around the
use of his then-new Modelair-Tech H-1000 belt drive and either a
Speed 700 ferrite motor or a DeWALT 18-volt cordless-drill motor.
In 1995 the Acrovolt was powered by a 16-cell Ni-Cd battery pack,
which was later increased to an 18-cell 3000 mAh NiMH pack. The
weight at the time was roughly 7 pounds. The wing loading was high,
How the Acrovolt’s crowded battery compartment looked when
Bob had to employ upward of 18 3000 mAh NiMH cells.
Closer look at AXI 4120/18 motor from rear shows accessory
radial mounting plate which Bob highly recommends. Collar on
shafts moves up flush with rear of motor.
The same battery area today, after switching to Li-Poly!
New square Kokam Li-Poly battery pack is mounted ahead of
wing LE and before firewall. Because of lighter motor weight, Li-
Poly pack’s forward position proved to be necessary to maintain
proper CG position.
Top: The 18-cell 3000 mAh NiMH pack as used years ago to power
the Acrovolt weighed more than 40 ounces. Bottom: New Kokam
5S2P Li-Poly pack weighs just 20 ounces. The motor run went from
six to eight minutes to more than 20 minutes.
and the motor run time was short—six minutes average with an
occasional seven to eight minutes with some throttling! But at that
weight, throttling almost got the model back on the ground!
That airplane stayed idle in my shop until recently, when a thought
came to me while I was preparing this article. Here I had a .60 cu. in.
glow-powered model (40-size by today’s standards) that was converted
to electric power using the technology that was available 10 years ago.
How would the same airplane perform today with a modern and
efficient brushless motor, an equally modern brushless/sensorless ESC,
and, best of all, a lighter-weight/high-capacity Li-Poly battery pack?
The sequence of the selection process is the same as in the
preceding. The 600-square-inch wing area is the same as 4.17 square
feet. Referring to Table 3 I selected the skill-level category of the fast
sport model (usually with more than adequate power), which calls for
20-25 ounces/sq. ft. of wing loading.
This was roughly the range I experienced with my Eyeball using a
60 glow engine at 6 pounds total weight. The older electric version—at
7 pounds—had a wing loading of 27 ounces/sq. ft., which I knew was
way too high.
Recognizing that the new motor and battery would save me a great
deal of weight, I selected 20 ounces/sq. ft. as my target wing loading. If
I multiplied that 20.0 by the 4.17 sq. ft. wing area, I ended up with a
target weight of 83.4 ounces (slightly heavier than 5 pounds).
Looking at Table 2 I chose 80-100 watts/pound (for aggressive
airplanes) as the power loading. Not wanting to go absolutely crazy, I
Upper view shows added plywood motor box and battery-pack
position. Jeti brushless ESC is also in battery compartment.
After removing DeWALT motor and belt drive, 3⁄16 plywood box
was made to hold new AXI 4120/18 brushless motor at roughly
same prop position.
Clear plastic cowl shows new AXI brushless-motor installation.
Bob painted inside of molded cowl after photos were taken.
After being refitted with a modern power system, the Acrovolt is
more spirited than ever in flight!
Bob replaced old 2-ounce ACE RC Smart ESC (top) with new 1-
ounce Jeti Advance brushless ESC (bottom). New ESC senses
battery voltage at every start and automatically sets proper cutoff
voltage point. This is crucial when using Li-Poly batteries.
selected the midrange number of 90 watts/pound. From that I
multiplied 5.2 pounds (83.4 ounces) by 90 and obtained 468 watts.
Since power is amps multiplied by volts, you can work backward
using the power (watts) and the estimated motor current. Again, the
estimate of motor current is all-important and requires your knowledge
and judgment.
For this application I settled on approximately 27.0 amps. With the
older motor 10 years ago I ran 30.0-32.0 amps. I wanted to use less
current this time because my airplane will weigh much less.
Since power (watts) equals current (amps) multiplied by volts, you
can divide 468.0 watts by 27.0 amps and obtain 17.3 volts. For this
new application I wanted to use a large Li-Poly pack. Since each of
these cells has a nominal 3.7 volts, the closest I could get to 17.3 was
five Li-Poly cells (3.7 x 5 = 18.5 volts). I’ll get into the Li-Poly
capacity selection in a bit.
Having collected all of this information, I needed to select a
brushless motor for my application. I mentioned that I favor the AXIs,
so I went to their larger-motor data. The Web site containing The Great
Electric Motor Test did not list the larger 41-series AXIs, but Dave
Radford of Air Craft Inc. provided the data at www.aircraftworld.
com/default.asp?id=18. After going through his AXI 41-series
information, I settled on the AXI 4120-18 brushless outrunner.
The data specifically called for the use of an APC 12 x 8E propeller
and 16 Ni-Cd or NiMH cells (1.2 x 16 = 19.2 volts). The motor current
given was 24.5 amps, and the power was 434
watts. For my purpose this was close enough
for a starting point. To my surprise, the actual
numbers came out closer to my estimate than
the published data.
The photos accompanying this article
show how easy it was to swap out the much
larger ferrite motor for the brushless motor.
Since the Li-Poly battery was much lighter, I
was forced to move it farther forward (just
behind the firewall) to maintain the proper CG
location.
Flying the “revised” Acrovolt was
wonderful. The final model weight was 85.3
ounces (5 pounds, 5.3 ounces), yielding a
wing loading of 20.5 ounces/sq. ft. The actual
motor parameters worked out to be higher
than my predictions, but they were in the
direction I wanted so I left them that way for
my initial flight tests. The motor current was
28.5 amps, voltage under load was 18.0 volts,
wattage was 511.0, the watts/pound figure
was 95.8, and rpm was 8,600.
The Kokam 5S2P battery pack that I used
consisted of 2.0-ampere-hour (Ah)-rated cells
(15C-load capable). With two sets in parallel
(2P), the actual capacity is 4.0 Ah. Total
flying time obtained with this battery pack
was approximately 20 minutes, with some
throttling management. That means you don’t
run the motor all the time at full throttle.
In fact, the Acrovolt—now at an
extremely light weight and with a highperformance
power system—is capable of
doing loops from level flight at half throttle.
When the model landed each time at this
power level, the motor, ESC, and battery pack
were hardly warm. This was a perfect powersystem
choice in every regard! I ended up
with a model that flew better than it did with a
glow engine or the older ferrite electric motor.
Estimating Motor Run Time: When you near
the end of your motor-selection process, you
need to decide what capacity of battery you
want to use, regardless of whether it’s Ni-Cd,
NiMH, or Li-Poly.
Battery capacity is usually rated in mAh or
Ah. Your battery’s weight is directly related
to its rated capacity. The more capacity there
is in the battery, the more it will weigh. The
motor-selection programs can help you
considerably when making this choice.
For a quick estimate, I use a simple
formula. I multiply 60 by the battery’s
capacity rating (in Ah) and divide that by the
motor current (in amps). The result is an
estimated motor run time in minutes.
As an example, let’s go back to when we
were building a glow-powered kit as an
electric-powered model. The battery had a
1950 mAh capacity rating, and the motor
current was 23.0 amps. So multiply 60 by
1.95 Ah (the same as 1950 mAh), and divide
that by the 23.0 amps. The answer is 5.09
minutes.
Keep in mind that these are static figures.
When the model is in the air and the propeller
unloads, the current will be less. Also, it is
assumed that you will not fly the entire flight
at full throttle. As I pointed out earlier, this
model’s actual flight time was approximately
eight minutes. Nevertheless, this formula
provides a rough starting point.
Keeping Records: One of the best ways to
put your electric-power experience to good
use is by keeping accurate records of your
various aircraft. With new airplanes
especially, I make dated entries in a bound
logbook about the aircraft; electric parameters
such as motor current, voltage, watts,
watts/ounce or watts/pound, rpm, and run
time; and flight performance.
As you progress with electric power, these
records will provide practical comparisons
when you choose power systems for new
aircraft.
Where to Get Help: When you get stuck and
can’t find that elusive electric power system,
you will need some help. One place to find it
is a model forum on the Internet that
specializes in electric power, such as RC
Groups, E-Digest, RC Universe, and SFRC.
Hopefully you will reach an “expert” when
you send out your question.
Contacting local modeling clubs that
specialize in electric power is another way to
learn. But if you live in a remote area, that
may not be easy.
In the end, there is a group of many hobby
distributors who will work with you,
including Kirk Massey at New Creations RC
([936] 856-4630), Dave Thacker at Radical
RC (www.radicalrc.com), Dave and Bob Peru
at Balsa Products (www.balsapr.com), Sal
DeFrancesco at Northeast Sailplane Products
(www.nesail.com), Helmut Goestl of Dymond
Modelsport (www.rc-dymond.com), and Tom
Hunt at Modelair-Tech (www.modelair
tech.com).
References: In the past two years I have
authored a series of articles specifically about
electric-powered flight that have been
published in MA, and they are as follows.
(You can find the “From the Ground Up”
articles on the MA Web site at www.modelair
craft.org/mag/index.htm.)
• “State of the Sport: Electric-Powered
Flight”: April 2002, pages 18-40.
• “From the Ground Up: Introduction to
Electric Power”: July 2003, pages 56-64.
• “From the Ground Up: Battery Basics”:
October 2003, pages 54-62.
• “Introduction to Lithium-Polymer Batteries:
May 2004, pages 44-58.
I have also written articles that have been
posted on MA’s Sport Aviator online
magazine. They are:
• “The SuperStar-EP Electric ARF” at
www.masportaviator.com/ah.asp?CatID=1&I
D=16. This is a review. SuperStar EP is
similar to the Tower Hobbies PT-20 that I
mentioned in this article.
• “Bonnie 20 ARF Electric Trainer” at
www.masportaviator.com/ah.asp?CatID=1&I
D=39. This is also a review.
• “Bonnie 20—Adapting to Li-Poly Batteries”
at www.masportaviator.com/ah.asp?CatID
=2&ID=43. This shows in depth how to
convert a Bonnie from NiMH/Ni-Cd batteries
to the new Li-Poly variety.
In addition, I wrote the book Getting
Started in Backyard Flying, which is available
from AMA. Pages 58 and 59 explain the
process of measuring motor current using an
AstroFlight Super Whattmeter (part 101).
The process of selecting a motor system for a
particular aircraft may seem overwhelming at
first, but I assure you it’s not! Follow the
logical steps I have presented here; they work!
I would love to be able to reference a single
page or table and say, “That’s the answer.” Joe
Beshar thought that was possible when he
made his suggestion, but there are far too
many variables with electric power.
Buy that meter and take your own
parameter measurements. Buy a scale and
weigh your aircraft; don’t guess! Buy a
tachometer and really see if you improved
things! Buy one of the two recommended
computer programs; you’ll be amazed by how
helpful they are.
If you hit a snag, please write or E-mail
me. I will not only try to answer your detailed
questions, but I will include those inquiries/
answers in MA’s “Frequently Asked
Questions” column if it will benefit other
readers. Don’t give up on electric power; get
more involved! MA
Bob Aberle
[email protected]
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 - 2005/04
Page Numbers: 53,54,55,56,58,60
Photos courtesy the author
by Bob Aberle
Part 2
The Acrovolt lives up to its name by performing spirited
aerobatics in an effortless manner.
selected 50 watts/pound. From that I multiplied 3.93 pounds (63
ounces) by 50 and obtained 197 watts. Since power is amps multiplied
by volts, you can work backward using the power (watts) and the
estimated motor current.
That motor-current figure will always prove to be the tricky part. It
will be an estimate that is a compromise between how long a motor run
you desire and how much current your specific motor can tolerate
(known as maximum continuous current in motor specifications).
This is where the ElectriCalc and MotoCalc motor-selection programs
can really help you. For this application I decided on a range of 20.0-
25.0 amps and 22.5 as an average. Since power (watts) equals current
(amps) multiplied by volts, you can divide 197 watts by 22.5 amps and
obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an
eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between
two and three Li-Poly cells because the characteristic voltage is 3.7
volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives
you a ballpark figure.
From this point you can look up your motor data on one of the six
Web sites I listed last month. I generally go for the AXI brushless
outrunner motors because they are available in many sizes, and I have
found them to be extremely reliable.
For the average-size AXI motor I use “The Great Electric Motor
Test,” at www.flyingmodels.org. I searched through the data looking at
motor current, power (watts), and propeller sizes. You have to be
patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor.
On 8.0 volts and with an APC 10 x 7E propeller, it would have a
current of 23.0 amps and 176 watts of power. The 10-inch propeller
can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should
work. The Ni-Cd cells will likely produce slightly higher voltage,
current, and wattage. A recommendation is an eight-cell Sanyo 1950
The author selected Tom Hunt’s successful original-design
Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background
of how power sources for modeling are rated and specifically how
electric motors have been related in power output to glow engines. He
covered how to go about sizing, or matching, a specific-size motor to a
given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained
how to measure motor input power, aircraft weight and power-loading
considerations, wing loading and how it relates to flying experience
and skill, and thrust factors. He included details of the motor-selection
process, a comprehensive listing of Web sites containing helpful data,
and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection
process.)
IT’S TIME TO put all you’ve learned together with several examples
of motor selections. Let’s make a glow kit or glow ARF electricpowered.
Many popular glow-engine-powered full kits and ARF “kits” on
the market can easily be constructed and/or assembled from scratch
with electric power in mind. One such model is Tower Hobbies’
“Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended
for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of
wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of
16-20 ounces/square foot (sq. ft.).
As a start, take into account that the wing area is 515 square inches
(3.6 sq. ft.). Pick a wing loading for the intended skill level. Using
Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft.
That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing
loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up
with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading.
Wanting a bit more in performance (with some reserve), I specifically
AXI 4120/18 motor with special radial mount plate at right. An
“outrunner” motor’s entire outer casing rotates.
L-R: Older, larger, heavier DeWALT motor with H-1000 belt
drive and replacement Hobby Lobby AXI 4120/18 brushless
outrunner motor. It weighs 11 ounces!
Acrovolt went from 7 pounds to 5 pounds, 5 ounces. Motor run
time was increased from six to eight minutes to more than 20
minutes. This was accomplished with new brushless motor
and new Li-Poly batteries.
Original Acrovolt motor system, consisting of DeWALT 18-volt
cordless-drill motor and Modelair-Tech H-1000 belt reduction drive.
Motor and belt drive weighed 21 ounces!
A page from Bob’s logbook. Record data for each new or revised
aircraft, and then you have something to compare to when visiting
new, similar-size/weight aircraft.
mAh NiMH pack, which will get me slightly over my target but allow
me to do some throttling back during a flight. That will stretch the run
time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As
you progress, feel free to “what-if” your parameters in either of the two
computer motor-selection programs. You can even work up to Li-Poly
batteries by carefully selecting a propeller size that will allow you to
use a three-cell pack, but not at excessive motor current.
Converting/Updating: Almost 10 years ago, Tom Hunt designed a
large, aerobatic/Pattern, electric-powered model called the Acrovolt. It
was inspired by Art Schroeder’s Eyeball design from the 1960s.
I flew Art’s Eyeball back then. I remember that it weighed 6 pounds
with an Enya 60 glow engine. Tom designed his Acrovolt around the
use of his then-new Modelair-Tech H-1000 belt drive and either a
Speed 700 ferrite motor or a DeWALT 18-volt cordless-drill motor.
In 1995 the Acrovolt was powered by a 16-cell Ni-Cd battery pack,
which was later increased to an 18-cell 3000 mAh NiMH pack. The
weight at the time was roughly 7 pounds. The wing loading was high,
How the Acrovolt’s crowded battery compartment looked when
Bob had to employ upward of 18 3000 mAh NiMH cells.
Closer look at AXI 4120/18 motor from rear shows accessory
radial mounting plate which Bob highly recommends. Collar on
shafts moves up flush with rear of motor.
The same battery area today, after switching to Li-Poly!
New square Kokam Li-Poly battery pack is mounted ahead of
wing LE and before firewall. Because of lighter motor weight, Li-
Poly pack’s forward position proved to be necessary to maintain
proper CG position.
Top: The 18-cell 3000 mAh NiMH pack as used years ago to power
the Acrovolt weighed more than 40 ounces. Bottom: New Kokam
5S2P Li-Poly pack weighs just 20 ounces. The motor run went from
six to eight minutes to more than 20 minutes.
and the motor run time was short—six minutes average with an
occasional seven to eight minutes with some throttling! But at that
weight, throttling almost got the model back on the ground!
That airplane stayed idle in my shop until recently, when a thought
came to me while I was preparing this article. Here I had a .60 cu. in.
glow-powered model (40-size by today’s standards) that was converted
to electric power using the technology that was available 10 years ago.
How would the same airplane perform today with a modern and
efficient brushless motor, an equally modern brushless/sensorless ESC,
and, best of all, a lighter-weight/high-capacity Li-Poly battery pack?
The sequence of the selection process is the same as in the
preceding. The 600-square-inch wing area is the same as 4.17 square
feet. Referring to Table 3 I selected the skill-level category of the fast
sport model (usually with more than adequate power), which calls for
20-25 ounces/sq. ft. of wing loading.
This was roughly the range I experienced with my Eyeball using a
60 glow engine at 6 pounds total weight. The older electric version—at
7 pounds—had a wing loading of 27 ounces/sq. ft., which I knew was
way too high.
Recognizing that the new motor and battery would save me a great
deal of weight, I selected 20 ounces/sq. ft. as my target wing loading. If
I multiplied that 20.0 by the 4.17 sq. ft. wing area, I ended up with a
target weight of 83.4 ounces (slightly heavier than 5 pounds).
Looking at Table 2 I chose 80-100 watts/pound (for aggressive
airplanes) as the power loading. Not wanting to go absolutely crazy, I
Upper view shows added plywood motor box and battery-pack
position. Jeti brushless ESC is also in battery compartment.
After removing DeWALT motor and belt drive, 3⁄16 plywood box
was made to hold new AXI 4120/18 brushless motor at roughly
same prop position.
Clear plastic cowl shows new AXI brushless-motor installation.
Bob painted inside of molded cowl after photos were taken.
After being refitted with a modern power system, the Acrovolt is
more spirited than ever in flight!
Bob replaced old 2-ounce ACE RC Smart ESC (top) with new 1-
ounce Jeti Advance brushless ESC (bottom). New ESC senses
battery voltage at every start and automatically sets proper cutoff
voltage point. This is crucial when using Li-Poly batteries.
selected the midrange number of 90 watts/pound. From that I
multiplied 5.2 pounds (83.4 ounces) by 90 and obtained 468 watts.
Since power is amps multiplied by volts, you can work backward
using the power (watts) and the estimated motor current. Again, the
estimate of motor current is all-important and requires your knowledge
and judgment.
For this application I settled on approximately 27.0 amps. With the
older motor 10 years ago I ran 30.0-32.0 amps. I wanted to use less
current this time because my airplane will weigh much less.
Since power (watts) equals current (amps) multiplied by volts, you
can divide 468.0 watts by 27.0 amps and obtain 17.3 volts. For this
new application I wanted to use a large Li-Poly pack. Since each of
these cells has a nominal 3.7 volts, the closest I could get to 17.3 was
five Li-Poly cells (3.7 x 5 = 18.5 volts). I’ll get into the Li-Poly
capacity selection in a bit.
Having collected all of this information, I needed to select a
brushless motor for my application. I mentioned that I favor the AXIs,
so I went to their larger-motor data. The Web site containing The Great
Electric Motor Test did not list the larger 41-series AXIs, but Dave
Radford of Air Craft Inc. provided the data at www.aircraftworld.
com/default.asp?id=18. After going through his AXI 41-series
information, I settled on the AXI 4120-18 brushless outrunner.
The data specifically called for the use of an APC 12 x 8E propeller
and 16 Ni-Cd or NiMH cells (1.2 x 16 = 19.2 volts). The motor current
given was 24.5 amps, and the power was 434
watts. For my purpose this was close enough
for a starting point. To my surprise, the actual
numbers came out closer to my estimate than
the published data.
The photos accompanying this article
show how easy it was to swap out the much
larger ferrite motor for the brushless motor.
Since the Li-Poly battery was much lighter, I
was forced to move it farther forward (just
behind the firewall) to maintain the proper CG
location.
Flying the “revised” Acrovolt was
wonderful. The final model weight was 85.3
ounces (5 pounds, 5.3 ounces), yielding a
wing loading of 20.5 ounces/sq. ft. The actual
motor parameters worked out to be higher
than my predictions, but they were in the
direction I wanted so I left them that way for
my initial flight tests. The motor current was
28.5 amps, voltage under load was 18.0 volts,
wattage was 511.0, the watts/pound figure
was 95.8, and rpm was 8,600.
The Kokam 5S2P battery pack that I used
consisted of 2.0-ampere-hour (Ah)-rated cells
(15C-load capable). With two sets in parallel
(2P), the actual capacity is 4.0 Ah. Total
flying time obtained with this battery pack
was approximately 20 minutes, with some
throttling management. That means you don’t
run the motor all the time at full throttle.
In fact, the Acrovolt—now at an
extremely light weight and with a highperformance
power system—is capable of
doing loops from level flight at half throttle.
When the model landed each time at this
power level, the motor, ESC, and battery pack
were hardly warm. This was a perfect powersystem
choice in every regard! I ended up
with a model that flew better than it did with a
glow engine or the older ferrite electric motor.
Estimating Motor Run Time: When you near
the end of your motor-selection process, you
need to decide what capacity of battery you
want to use, regardless of whether it’s Ni-Cd,
NiMH, or Li-Poly.
Battery capacity is usually rated in mAh or
Ah. Your battery’s weight is directly related
to its rated capacity. The more capacity there
is in the battery, the more it will weigh. The
motor-selection programs can help you
considerably when making this choice.
For a quick estimate, I use a simple
formula. I multiply 60 by the battery’s
capacity rating (in Ah) and divide that by the
motor current (in amps). The result is an
estimated motor run time in minutes.
As an example, let’s go back to when we
were building a glow-powered kit as an
electric-powered model. The battery had a
1950 mAh capacity rating, and the motor
current was 23.0 amps. So multiply 60 by
1.95 Ah (the same as 1950 mAh), and divide
that by the 23.0 amps. The answer is 5.09
minutes.
Keep in mind that these are static figures.
When the model is in the air and the propeller
unloads, the current will be less. Also, it is
assumed that you will not fly the entire flight
at full throttle. As I pointed out earlier, this
model’s actual flight time was approximately
eight minutes. Nevertheless, this formula
provides a rough starting point.
Keeping Records: One of the best ways to
put your electric-power experience to good
use is by keeping accurate records of your
various aircraft. With new airplanes
especially, I make dated entries in a bound
logbook about the aircraft; electric parameters
such as motor current, voltage, watts,
watts/ounce or watts/pound, rpm, and run
time; and flight performance.
As you progress with electric power, these
records will provide practical comparisons
when you choose power systems for new
aircraft.
Where to Get Help: When you get stuck and
can’t find that elusive electric power system,
you will need some help. One place to find it
is a model forum on the Internet that
specializes in electric power, such as RC
Groups, E-Digest, RC Universe, and SFRC.
Hopefully you will reach an “expert” when
you send out your question.
Contacting local modeling clubs that
specialize in electric power is another way to
learn. But if you live in a remote area, that
may not be easy.
In the end, there is a group of many hobby
distributors who will work with you,
including Kirk Massey at New Creations RC
([936] 856-4630), Dave Thacker at Radical
RC (www.radicalrc.com), Dave and Bob Peru
at Balsa Products (www.balsapr.com), Sal
DeFrancesco at Northeast Sailplane Products
(www.nesail.com), Helmut Goestl of Dymond
Modelsport (www.rc-dymond.com), and Tom
Hunt at Modelair-Tech (www.modelair
tech.com).
References: In the past two years I have
authored a series of articles specifically about
electric-powered flight that have been
published in MA, and they are as follows.
(You can find the “From the Ground Up”
articles on the MA Web site at www.modelair
craft.org/mag/index.htm.)
• “State of the Sport: Electric-Powered
Flight”: April 2002, pages 18-40.
• “From the Ground Up: Introduction to
Electric Power”: July 2003, pages 56-64.
• “From the Ground Up: Battery Basics”:
October 2003, pages 54-62.
• “Introduction to Lithium-Polymer Batteries:
May 2004, pages 44-58.
I have also written articles that have been
posted on MA’s Sport Aviator online
magazine. They are:
• “The SuperStar-EP Electric ARF” at
www.masportaviator.com/ah.asp?CatID=1&I
D=16. This is a review. SuperStar EP is
similar to the Tower Hobbies PT-20 that I
mentioned in this article.
• “Bonnie 20 ARF Electric Trainer” at
www.masportaviator.com/ah.asp?CatID=1&I
D=39. This is also a review.
• “Bonnie 20—Adapting to Li-Poly Batteries”
at www.masportaviator.com/ah.asp?CatID
=2&ID=43. This shows in depth how to
convert a Bonnie from NiMH/Ni-Cd batteries
to the new Li-Poly variety.
In addition, I wrote the book Getting
Started in Backyard Flying, which is available
from AMA. Pages 58 and 59 explain the
process of measuring motor current using an
AstroFlight Super Whattmeter (part 101).
The process of selecting a motor system for a
particular aircraft may seem overwhelming at
first, but I assure you it’s not! Follow the
logical steps I have presented here; they work!
I would love to be able to reference a single
page or table and say, “That’s the answer.” Joe
Beshar thought that was possible when he
made his suggestion, but there are far too
many variables with electric power.
Buy that meter and take your own
parameter measurements. Buy a scale and
weigh your aircraft; don’t guess! Buy a
tachometer and really see if you improved
things! Buy one of the two recommended
computer programs; you’ll be amazed by how
helpful they are.
If you hit a snag, please write or E-mail
me. I will not only try to answer your detailed
questions, but I will include those inquiries/
answers in MA’s “Frequently Asked
Questions” column if it will benefit other
readers. Don’t give up on electric power; get
more involved! MA
Bob Aberle
[email protected]
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 - 2005/04
Page Numbers: 53,54,55,56,58,60
Photos courtesy the author
by Bob Aberle
Part 2
The Acrovolt lives up to its name by performing spirited
aerobatics in an effortless manner.
selected 50 watts/pound. From that I multiplied 3.93 pounds (63
ounces) by 50 and obtained 197 watts. Since power is amps multiplied
by volts, you can work backward using the power (watts) and the
estimated motor current.
That motor-current figure will always prove to be the tricky part. It
will be an estimate that is a compromise between how long a motor run
you desire and how much current your specific motor can tolerate
(known as maximum continuous current in motor specifications).
This is where the ElectriCalc and MotoCalc motor-selection programs
can really help you. For this application I decided on a range of 20.0-
25.0 amps and 22.5 as an average. Since power (watts) equals current
(amps) multiplied by volts, you can divide 197 watts by 22.5 amps and
obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an
eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between
two and three Li-Poly cells because the characteristic voltage is 3.7
volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives
you a ballpark figure.
From this point you can look up your motor data on one of the six
Web sites I listed last month. I generally go for the AXI brushless
outrunner motors because they are available in many sizes, and I have
found them to be extremely reliable.
For the average-size AXI motor I use “The Great Electric Motor
Test,” at www.flyingmodels.org. I searched through the data looking at
motor current, power (watts), and propeller sizes. You have to be
patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor.
On 8.0 volts and with an APC 10 x 7E propeller, it would have a
current of 23.0 amps and 176 watts of power. The 10-inch propeller
can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should
work. The Ni-Cd cells will likely produce slightly higher voltage,
current, and wattage. A recommendation is an eight-cell Sanyo 1950
The author selected Tom Hunt’s successful original-design
Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background
of how power sources for modeling are rated and specifically how
electric motors have been related in power output to glow engines. He
covered how to go about sizing, or matching, a specific-size motor to a
given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained
how to measure motor input power, aircraft weight and power-loading
considerations, wing loading and how it relates to flying experience
and skill, and thrust factors. He included details of the motor-selection
process, a comprehensive listing of Web sites containing helpful data,
and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection
process.)
IT’S TIME TO put all you’ve learned together with several examples
of motor selections. Let’s make a glow kit or glow ARF electricpowered.
Many popular glow-engine-powered full kits and ARF “kits” on
the market can easily be constructed and/or assembled from scratch
with electric power in mind. One such model is Tower Hobbies’
“Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended
for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of
wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of
16-20 ounces/square foot (sq. ft.).
As a start, take into account that the wing area is 515 square inches
(3.6 sq. ft.). Pick a wing loading for the intended skill level. Using
Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft.
That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing
loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up
with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading.
Wanting a bit more in performance (with some reserve), I specifically
AXI 4120/18 motor with special radial mount plate at right. An
“outrunner” motor’s entire outer casing rotates.
L-R: Older, larger, heavier DeWALT motor with H-1000 belt
drive and replacement Hobby Lobby AXI 4120/18 brushless
outrunner motor. It weighs 11 ounces!
Acrovolt went from 7 pounds to 5 pounds, 5 ounces. Motor run
time was increased from six to eight minutes to more than 20
minutes. This was accomplished with new brushless motor
and new Li-Poly batteries.
Original Acrovolt motor system, consisting of DeWALT 18-volt
cordless-drill motor and Modelair-Tech H-1000 belt reduction drive.
Motor and belt drive weighed 21 ounces!
A page from Bob’s logbook. Record data for each new or revised
aircraft, and then you have something to compare to when visiting
new, similar-size/weight aircraft.
mAh NiMH pack, which will get me slightly over my target but allow
me to do some throttling back during a flight. That will stretch the run
time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As
you progress, feel free to “what-if” your parameters in either of the two
computer motor-selection programs. You can even work up to Li-Poly
batteries by carefully selecting a propeller size that will allow you to
use a three-cell pack, but not at excessive motor current.
Converting/Updating: Almost 10 years ago, Tom Hunt designed a
large, aerobatic/Pattern, electric-powered model called the Acrovolt. It
was inspired by Art Schroeder’s Eyeball design from the 1960s.
I flew Art’s Eyeball back then. I remember that it weighed 6 pounds
with an Enya 60 glow engine. Tom designed his Acrovolt around the
use of his then-new Modelair-Tech H-1000 belt drive and either a
Speed 700 ferrite motor or a DeWALT 18-volt cordless-drill motor.
In 1995 the Acrovolt was powered by a 16-cell Ni-Cd battery pack,
which was later increased to an 18-cell 3000 mAh NiMH pack. The
weight at the time was roughly 7 pounds. The wing loading was high,
How the Acrovolt’s crowded battery compartment looked when
Bob had to employ upward of 18 3000 mAh NiMH cells.
Closer look at AXI 4120/18 motor from rear shows accessory
radial mounting plate which Bob highly recommends. Collar on
shafts moves up flush with rear of motor.
The same battery area today, after switching to Li-Poly!
New square Kokam Li-Poly battery pack is mounted ahead of
wing LE and before firewall. Because of lighter motor weight, Li-
Poly pack’s forward position proved to be necessary to maintain
proper CG position.
Top: The 18-cell 3000 mAh NiMH pack as used years ago to power
the Acrovolt weighed more than 40 ounces. Bottom: New Kokam
5S2P Li-Poly pack weighs just 20 ounces. The motor run went from
six to eight minutes to more than 20 minutes.
and the motor run time was short—six minutes average with an
occasional seven to eight minutes with some throttling! But at that
weight, throttling almost got the model back on the ground!
That airplane stayed idle in my shop until recently, when a thought
came to me while I was preparing this article. Here I had a .60 cu. in.
glow-powered model (40-size by today’s standards) that was converted
to electric power using the technology that was available 10 years ago.
How would the same airplane perform today with a modern and
efficient brushless motor, an equally modern brushless/sensorless ESC,
and, best of all, a lighter-weight/high-capacity Li-Poly battery pack?
The sequence of the selection process is the same as in the
preceding. The 600-square-inch wing area is the same as 4.17 square
feet. Referring to Table 3 I selected the skill-level category of the fast
sport model (usually with more than adequate power), which calls for
20-25 ounces/sq. ft. of wing loading.
This was roughly the range I experienced with my Eyeball using a
60 glow engine at 6 pounds total weight. The older electric version—at
7 pounds—had a wing loading of 27 ounces/sq. ft., which I knew was
way too high.
Recognizing that the new motor and battery would save me a great
deal of weight, I selected 20 ounces/sq. ft. as my target wing loading. If
I multiplied that 20.0 by the 4.17 sq. ft. wing area, I ended up with a
target weight of 83.4 ounces (slightly heavier than 5 pounds).
Looking at Table 2 I chose 80-100 watts/pound (for aggressive
airplanes) as the power loading. Not wanting to go absolutely crazy, I
Upper view shows added plywood motor box and battery-pack
position. Jeti brushless ESC is also in battery compartment.
After removing DeWALT motor and belt drive, 3⁄16 plywood box
was made to hold new AXI 4120/18 brushless motor at roughly
same prop position.
Clear plastic cowl shows new AXI brushless-motor installation.
Bob painted inside of molded cowl after photos were taken.
After being refitted with a modern power system, the Acrovolt is
more spirited than ever in flight!
Bob replaced old 2-ounce ACE RC Smart ESC (top) with new 1-
ounce Jeti Advance brushless ESC (bottom). New ESC senses
battery voltage at every start and automatically sets proper cutoff
voltage point. This is crucial when using Li-Poly batteries.
selected the midrange number of 90 watts/pound. From that I
multiplied 5.2 pounds (83.4 ounces) by 90 and obtained 468 watts.
Since power is amps multiplied by volts, you can work backward
using the power (watts) and the estimated motor current. Again, the
estimate of motor current is all-important and requires your knowledge
and judgment.
For this application I settled on approximately 27.0 amps. With the
older motor 10 years ago I ran 30.0-32.0 amps. I wanted to use less
current this time because my airplane will weigh much less.
Since power (watts) equals current (amps) multiplied by volts, you
can divide 468.0 watts by 27.0 amps and obtain 17.3 volts. For this
new application I wanted to use a large Li-Poly pack. Since each of
these cells has a nominal 3.7 volts, the closest I could get to 17.3 was
five Li-Poly cells (3.7 x 5 = 18.5 volts). I’ll get into the Li-Poly
capacity selection in a bit.
Having collected all of this information, I needed to select a
brushless motor for my application. I mentioned that I favor the AXIs,
so I went to their larger-motor data. The Web site containing The Great
Electric Motor Test did not list the larger 41-series AXIs, but Dave
Radford of Air Craft Inc. provided the data at www.aircraftworld.
com/default.asp?id=18. After going through his AXI 41-series
information, I settled on the AXI 4120-18 brushless outrunner.
The data specifically called for the use of an APC 12 x 8E propeller
and 16 Ni-Cd or NiMH cells (1.2 x 16 = 19.2 volts). The motor current
given was 24.5 amps, and the power was 434
watts. For my purpose this was close enough
for a starting point. To my surprise, the actual
numbers came out closer to my estimate than
the published data.
The photos accompanying this article
show how easy it was to swap out the much
larger ferrite motor for the brushless motor.
Since the Li-Poly battery was much lighter, I
was forced to move it farther forward (just
behind the firewall) to maintain the proper CG
location.
Flying the “revised” Acrovolt was
wonderful. The final model weight was 85.3
ounces (5 pounds, 5.3 ounces), yielding a
wing loading of 20.5 ounces/sq. ft. The actual
motor parameters worked out to be higher
than my predictions, but they were in the
direction I wanted so I left them that way for
my initial flight tests. The motor current was
28.5 amps, voltage under load was 18.0 volts,
wattage was 511.0, the watts/pound figure
was 95.8, and rpm was 8,600.
The Kokam 5S2P battery pack that I used
consisted of 2.0-ampere-hour (Ah)-rated cells
(15C-load capable). With two sets in parallel
(2P), the actual capacity is 4.0 Ah. Total
flying time obtained with this battery pack
was approximately 20 minutes, with some
throttling management. That means you don’t
run the motor all the time at full throttle.
In fact, the Acrovolt—now at an
extremely light weight and with a highperformance
power system—is capable of
doing loops from level flight at half throttle.
When the model landed each time at this
power level, the motor, ESC, and battery pack
were hardly warm. This was a perfect powersystem
choice in every regard! I ended up
with a model that flew better than it did with a
glow engine or the older ferrite electric motor.
Estimating Motor Run Time: When you near
the end of your motor-selection process, you
need to decide what capacity of battery you
want to use, regardless of whether it’s Ni-Cd,
NiMH, or Li-Poly.
Battery capacity is usually rated in mAh or
Ah. Your battery’s weight is directly related
to its rated capacity. The more capacity there
is in the battery, the more it will weigh. The
motor-selection programs can help you
considerably when making this choice.
For a quick estimate, I use a simple
formula. I multiply 60 by the battery’s
capacity rating (in Ah) and divide that by the
motor current (in amps). The result is an
estimated motor run time in minutes.
As an example, let’s go back to when we
were building a glow-powered kit as an
electric-powered model. The battery had a
1950 mAh capacity rating, and the motor
current was 23.0 amps. So multiply 60 by
1.95 Ah (the same as 1950 mAh), and divide
that by the 23.0 amps. The answer is 5.09
minutes.
Keep in mind that these are static figures.
When the model is in the air and the propeller
unloads, the current will be less. Also, it is
assumed that you will not fly the entire flight
at full throttle. As I pointed out earlier, this
model’s actual flight time was approximately
eight minutes. Nevertheless, this formula
provides a rough starting point.
Keeping Records: One of the best ways to
put your electric-power experience to good
use is by keeping accurate records of your
various aircraft. With new airplanes
especially, I make dated entries in a bound
logbook about the aircraft; electric parameters
such as motor current, voltage, watts,
watts/ounce or watts/pound, rpm, and run
time; and flight performance.
As you progress with electric power, these
records will provide practical comparisons
when you choose power systems for new
aircraft.
Where to Get Help: When you get stuck and
can’t find that elusive electric power system,
you will need some help. One place to find it
is a model forum on the Internet that
specializes in electric power, such as RC
Groups, E-Digest, RC Universe, and SFRC.
Hopefully you will reach an “expert” when
you send out your question.
Contacting local modeling clubs that
specialize in electric power is another way to
learn. But if you live in a remote area, that
may not be easy.
In the end, there is a group of many hobby
distributors who will work with you,
including Kirk Massey at New Creations RC
([936] 856-4630), Dave Thacker at Radical
RC (www.radicalrc.com), Dave and Bob Peru
at Balsa Products (www.balsapr.com), Sal
DeFrancesco at Northeast Sailplane Products
(www.nesail.com), Helmut Goestl of Dymond
Modelsport (www.rc-dymond.com), and Tom
Hunt at Modelair-Tech (www.modelair
tech.com).
References: In the past two years I have
authored a series of articles specifically about
electric-powered flight that have been
published in MA, and they are as follows.
(You can find the “From the Ground Up”
articles on the MA Web site at www.modelair
craft.org/mag/index.htm.)
• “State of the Sport: Electric-Powered
Flight”: April 2002, pages 18-40.
• “From the Ground Up: Introduction to
Electric Power”: July 2003, pages 56-64.
• “From the Ground Up: Battery Basics”:
October 2003, pages 54-62.
• “Introduction to Lithium-Polymer Batteries:
May 2004, pages 44-58.
I have also written articles that have been
posted on MA’s Sport Aviator online
magazine. They are:
• “The SuperStar-EP Electric ARF” at
www.masportaviator.com/ah.asp?CatID=1&I
D=16. This is a review. SuperStar EP is
similar to the Tower Hobbies PT-20 that I
mentioned in this article.
• “Bonnie 20 ARF Electric Trainer” at
www.masportaviator.com/ah.asp?CatID=1&I
D=39. This is also a review.
• “Bonnie 20—Adapting to Li-Poly Batteries”
at www.masportaviator.com/ah.asp?CatID
=2&ID=43. This shows in depth how to
convert a Bonnie from NiMH/Ni-Cd batteries
to the new Li-Poly variety.
In addition, I wrote the book Getting
Started in Backyard Flying, which is available
from AMA. Pages 58 and 59 explain the
process of measuring motor current using an
AstroFlight Super Whattmeter (part 101).
The process of selecting a motor system for a
particular aircraft may seem overwhelming at
first, but I assure you it’s not! Follow the
logical steps I have presented here; they work!
I would love to be able to reference a single
page or table and say, “That’s the answer.” Joe
Beshar thought that was possible when he
made his suggestion, but there are far too
many variables with electric power.
Buy that meter and take your own
parameter measurements. Buy a scale and
weigh your aircraft; don’t guess! Buy a
tachometer and really see if you improved
things! Buy one of the two recommended
computer programs; you’ll be amazed by how
helpful they are.
If you hit a snag, please write or E-mail
me. I will not only try to answer your detailed
questions, but I will include those inquiries/
answers in MA’s “Frequently Asked
Questions” column if it will benefit other
readers. Don’t give up on electric power; get
more involved! MA
Bob Aberle
[email protected]
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 - 2005/04
Page Numbers: 53,54,55,56,58,60
Photos courtesy the author
by Bob Aberle
Part 2
The Acrovolt lives up to its name by performing spirited
aerobatics in an effortless manner.
selected 50 watts/pound. From that I multiplied 3.93 pounds (63
ounces) by 50 and obtained 197 watts. Since power is amps multiplied
by volts, you can work backward using the power (watts) and the
estimated motor current.
That motor-current figure will always prove to be the tricky part. It
will be an estimate that is a compromise between how long a motor run
you desire and how much current your specific motor can tolerate
(known as maximum continuous current in motor specifications).
This is where the ElectriCalc and MotoCalc motor-selection programs
can really help you. For this application I decided on a range of 20.0-
25.0 amps and 22.5 as an average. Since power (watts) equals current
(amps) multiplied by volts, you can divide 197 watts by 22.5 amps and
obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an
eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between
two and three Li-Poly cells because the characteristic voltage is 3.7
volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives
you a ballpark figure.
From this point you can look up your motor data on one of the six
Web sites I listed last month. I generally go for the AXI brushless
outrunner motors because they are available in many sizes, and I have
found them to be extremely reliable.
For the average-size AXI motor I use “The Great Electric Motor
Test,” at www.flyingmodels.org. I searched through the data looking at
motor current, power (watts), and propeller sizes. You have to be
patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor.
On 8.0 volts and with an APC 10 x 7E propeller, it would have a
current of 23.0 amps and 176 watts of power. The 10-inch propeller
can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should
work. The Ni-Cd cells will likely produce slightly higher voltage,
current, and wattage. A recommendation is an eight-cell Sanyo 1950
The author selected Tom Hunt’s successful original-design
Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background
of how power sources for modeling are rated and specifically how
electric motors have been related in power output to glow engines. He
covered how to go about sizing, or matching, a specific-size motor to a
given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained
how to measure motor input power, aircraft weight and power-loading
considerations, wing loading and how it relates to flying experience
and skill, and thrust factors. He included details of the motor-selection
process, a comprehensive listing of Web sites containing helpful data,
and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection
process.)
IT’S TIME TO put all you’ve learned together with several examples
of motor selections. Let’s make a glow kit or glow ARF electricpowered.
Many popular glow-engine-powered full kits and ARF “kits” on
the market can easily be constructed and/or assembled from scratch
with electric power in mind. One such model is Tower Hobbies’
“Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended
for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of
wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of
16-20 ounces/square foot (sq. ft.).
As a start, take into account that the wing area is 515 square inches
(3.6 sq. ft.). Pick a wing loading for the intended skill level. Using
Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft.
That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing
loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up
with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading.
Wanting a bit more in performance (with some reserve), I specifically
AXI 4120/18 motor with special radial mount plate at right. An
“outrunner” motor’s entire outer casing rotates.
L-R: Older, larger, heavier DeWALT motor with H-1000 belt
drive and replacement Hobby Lobby AXI 4120/18 brushless
outrunner motor. It weighs 11 ounces!
Acrovolt went from 7 pounds to 5 pounds, 5 ounces. Motor run
time was increased from six to eight minutes to more than 20
minutes. This was accomplished with new brushless motor
and new Li-Poly batteries.
Original Acrovolt motor system, consisting of DeWALT 18-volt
cordless-drill motor and Modelair-Tech H-1000 belt reduction drive.
Motor and belt drive weighed 21 ounces!
A page from Bob’s logbook. Record data for each new or revised
aircraft, and then you have something to compare to when visiting
new, similar-size/weight aircraft.
mAh NiMH pack, which will get me slightly over my target but allow
me to do some throttling back during a flight. That will stretch the run
time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As
you progress, feel free to “what-if” your parameters in either of the two
computer motor-selection programs. You can even work up to Li-Poly
batteries by carefully selecting a propeller size that will allow you to
use a three-cell pack, but not at excessive motor current.
Converting/Updating: Almost 10 years ago, Tom Hunt designed a
large, aerobatic/Pattern, electric-powered model called the Acrovolt. It
was inspired by Art Schroeder’s Eyeball design from the 1960s.
I flew Art’s Eyeball back then. I remember that it weighed 6 pounds
with an Enya 60 glow engine. Tom designed his Acrovolt around the
use of his then-new Modelair-Tech H-1000 belt drive and either a
Speed 700 ferrite motor or a DeWALT 18-volt cordless-drill motor.
In 1995 the Acrovolt was powered by a 16-cell Ni-Cd battery pack,
which was later increased to an 18-cell 3000 mAh NiMH pack. The
weight at the time was roughly 7 pounds. The wing loading was high,
How the Acrovolt’s crowded battery compartment looked when
Bob had to employ upward of 18 3000 mAh NiMH cells.
Closer look at AXI 4120/18 motor from rear shows accessory
radial mounting plate which Bob highly recommends. Collar on
shafts moves up flush with rear of motor.
The same battery area today, after switching to Li-Poly!
New square Kokam Li-Poly battery pack is mounted ahead of
wing LE and before firewall. Because of lighter motor weight, Li-
Poly pack’s forward position proved to be necessary to maintain
proper CG position.
Top: The 18-cell 3000 mAh NiMH pack as used years ago to power
the Acrovolt weighed more than 40 ounces. Bottom: New Kokam
5S2P Li-Poly pack weighs just 20 ounces. The motor run went from
six to eight minutes to more than 20 minutes.
and the motor run time was short—six minutes average with an
occasional seven to eight minutes with some throttling! But at that
weight, throttling almost got the model back on the ground!
That airplane stayed idle in my shop until recently, when a thought
came to me while I was preparing this article. Here I had a .60 cu. in.
glow-powered model (40-size by today’s standards) that was converted
to electric power using the technology that was available 10 years ago.
How would the same airplane perform today with a modern and
efficient brushless motor, an equally modern brushless/sensorless ESC,
and, best of all, a lighter-weight/high-capacity Li-Poly battery pack?
The sequence of the selection process is the same as in the
preceding. The 600-square-inch wing area is the same as 4.17 square
feet. Referring to Table 3 I selected the skill-level category of the fast
sport model (usually with more than adequate power), which calls for
20-25 ounces/sq. ft. of wing loading.
This was roughly the range I experienced with my Eyeball using a
60 glow engine at 6 pounds total weight. The older electric version—at
7 pounds—had a wing loading of 27 ounces/sq. ft., which I knew was
way too high.
Recognizing that the new motor and battery would save me a great
deal of weight, I selected 20 ounces/sq. ft. as my target wing loading. If
I multiplied that 20.0 by the 4.17 sq. ft. wing area, I ended up with a
target weight of 83.4 ounces (slightly heavier than 5 pounds).
Looking at Table 2 I chose 80-100 watts/pound (for aggressive
airplanes) as the power loading. Not wanting to go absolutely crazy, I
Upper view shows added plywood motor box and battery-pack
position. Jeti brushless ESC is also in battery compartment.
After removing DeWALT motor and belt drive, 3⁄16 plywood box
was made to hold new AXI 4120/18 brushless motor at roughly
same prop position.
Clear plastic cowl shows new AXI brushless-motor installation.
Bob painted inside of molded cowl after photos were taken.
After being refitted with a modern power system, the Acrovolt is
more spirited than ever in flight!
Bob replaced old 2-ounce ACE RC Smart ESC (top) with new 1-
ounce Jeti Advance brushless ESC (bottom). New ESC senses
battery voltage at every start and automatically sets proper cutoff
voltage point. This is crucial when using Li-Poly batteries.
selected the midrange number of 90 watts/pound. From that I
multiplied 5.2 pounds (83.4 ounces) by 90 and obtained 468 watts.
Since power is amps multiplied by volts, you can work backward
using the power (watts) and the estimated motor current. Again, the
estimate of motor current is all-important and requires your knowledge
and judgment.
For this application I settled on approximately 27.0 amps. With the
older motor 10 years ago I ran 30.0-32.0 amps. I wanted to use less
current this time because my airplane will weigh much less.
Since power (watts) equals current (amps) multiplied by volts, you
can divide 468.0 watts by 27.0 amps and obtain 17.3 volts. For this
new application I wanted to use a large Li-Poly pack. Since each of
these cells has a nominal 3.7 volts, the closest I could get to 17.3 was
five Li-Poly cells (3.7 x 5 = 18.5 volts). I’ll get into the Li-Poly
capacity selection in a bit.
Having collected all of this information, I needed to select a
brushless motor for my application. I mentioned that I favor the AXIs,
so I went to their larger-motor data. The Web site containing The Great
Electric Motor Test did not list the larger 41-series AXIs, but Dave
Radford of Air Craft Inc. provided the data at www.aircraftworld.
com/default.asp?id=18. After going through his AXI 41-series
information, I settled on the AXI 4120-18 brushless outrunner.
The data specifically called for the use of an APC 12 x 8E propeller
and 16 Ni-Cd or NiMH cells (1.2 x 16 = 19.2 volts). The motor current
given was 24.5 amps, and the power was 434
watts. For my purpose this was close enough
for a starting point. To my surprise, the actual
numbers came out closer to my estimate than
the published data.
The photos accompanying this article
show how easy it was to swap out the much
larger ferrite motor for the brushless motor.
Since the Li-Poly battery was much lighter, I
was forced to move it farther forward (just
behind the firewall) to maintain the proper CG
location.
Flying the “revised” Acrovolt was
wonderful. The final model weight was 85.3
ounces (5 pounds, 5.3 ounces), yielding a
wing loading of 20.5 ounces/sq. ft. The actual
motor parameters worked out to be higher
than my predictions, but they were in the
direction I wanted so I left them that way for
my initial flight tests. The motor current was
28.5 amps, voltage under load was 18.0 volts,
wattage was 511.0, the watts/pound figure
was 95.8, and rpm was 8,600.
The Kokam 5S2P battery pack that I used
consisted of 2.0-ampere-hour (Ah)-rated cells
(15C-load capable). With two sets in parallel
(2P), the actual capacity is 4.0 Ah. Total
flying time obtained with this battery pack
was approximately 20 minutes, with some
throttling management. That means you don’t
run the motor all the time at full throttle.
In fact, the Acrovolt—now at an
extremely light weight and with a highperformance
power system—is capable of
doing loops from level flight at half throttle.
When the model landed each time at this
power level, the motor, ESC, and battery pack
were hardly warm. This was a perfect powersystem
choice in every regard! I ended up
with a model that flew better than it did with a
glow engine or the older ferrite electric motor.
Estimating Motor Run Time: When you near
the end of your motor-selection process, you
need to decide what capacity of battery you
want to use, regardless of whether it’s Ni-Cd,
NiMH, or Li-Poly.
Battery capacity is usually rated in mAh or
Ah. Your battery’s weight is directly related
to its rated capacity. The more capacity there
is in the battery, the more it will weigh. The
motor-selection programs can help you
considerably when making this choice.
For a quick estimate, I use a simple
formula. I multiply 60 by the battery’s
capacity rating (in Ah) and divide that by the
motor current (in amps). The result is an
estimated motor run time in minutes.
As an example, let’s go back to when we
were building a glow-powered kit as an
electric-powered model. The battery had a
1950 mAh capacity rating, and the motor
current was 23.0 amps. So multiply 60 by
1.95 Ah (the same as 1950 mAh), and divide
that by the 23.0 amps. The answer is 5.09
minutes.
Keep in mind that these are static figures.
When the model is in the air and the propeller
unloads, the current will be less. Also, it is
assumed that you will not fly the entire flight
at full throttle. As I pointed out earlier, this
model’s actual flight time was approximately
eight minutes. Nevertheless, this formula
provides a rough starting point.
Keeping Records: One of the best ways to
put your electric-power experience to good
use is by keeping accurate records of your
various aircraft. With new airplanes
especially, I make dated entries in a bound
logbook about the aircraft; electric parameters
such as motor current, voltage, watts,
watts/ounce or watts/pound, rpm, and run
time; and flight performance.
As you progress with electric power, these
records will provide practical comparisons
when you choose power systems for new
aircraft.
Where to Get Help: When you get stuck and
can’t find that elusive electric power system,
you will need some help. One place to find it
is a model forum on the Internet that
specializes in electric power, such as RC
Groups, E-Digest, RC Universe, and SFRC.
Hopefully you will reach an “expert” when
you send out your question.
Contacting local modeling clubs that
specialize in electric power is another way to
learn. But if you live in a remote area, that
may not be easy.
In the end, there is a group of many hobby
distributors who will work with you,
including Kirk Massey at New Creations RC
([936] 856-4630), Dave Thacker at Radical
RC (www.radicalrc.com), Dave and Bob Peru
at Balsa Products (www.balsapr.com), Sal
DeFrancesco at Northeast Sailplane Products
(www.nesail.com), Helmut Goestl of Dymond
Modelsport (www.rc-dymond.com), and Tom
Hunt at Modelair-Tech (www.modelair
tech.com).
References: In the past two years I have
authored a series of articles specifically about
electric-powered flight that have been
published in MA, and they are as follows.
(You can find the “From the Ground Up”
articles on the MA Web site at www.modelair
craft.org/mag/index.htm.)
• “State of the Sport: Electric-Powered
Flight”: April 2002, pages 18-40.
• “From the Ground Up: Introduction to
Electric Power”: July 2003, pages 56-64.
• “From the Ground Up: Battery Basics”:
October 2003, pages 54-62.
• “Introduction to Lithium-Polymer Batteries:
May 2004, pages 44-58.
I have also written articles that have been
posted on MA’s Sport Aviator online
magazine. They are:
• “The SuperStar-EP Electric ARF” at
www.masportaviator.com/ah.asp?CatID=1&I
D=16. This is a review. SuperStar EP is
similar to the Tower Hobbies PT-20 that I
mentioned in this article.
• “Bonnie 20 ARF Electric Trainer” at
www.masportaviator.com/ah.asp?CatID=1&I
D=39. This is also a review.
• “Bonnie 20—Adapting to Li-Poly Batteries”
at www.masportaviator.com/ah.asp?CatID
=2&ID=43. This shows in depth how to
convert a Bonnie from NiMH/Ni-Cd batteries
to the new Li-Poly variety.
In addition, I wrote the book Getting
Started in Backyard Flying, which is available
from AMA. Pages 58 and 59 explain the
process of measuring motor current using an
AstroFlight Super Whattmeter (part 101).
The process of selecting a motor system for a
particular aircraft may seem overwhelming at
first, but I assure you it’s not! Follow the
logical steps I have presented here; they work!
I would love to be able to reference a single
page or table and say, “That’s the answer.” Joe
Beshar thought that was possible when he
made his suggestion, but there are far too
many variables with electric power.
Buy that meter and take your own
parameter measurements. Buy a scale and
weigh your aircraft; don’t guess! Buy a
tachometer and really see if you improved
things! Buy one of the two recommended
computer programs; you’ll be amazed by how
helpful they are.
If you hit a snag, please write or E-mail
me. I will not only try to answer your detailed
questions, but I will include those inquiries/
answers in MA’s “Frequently Asked
Questions” column if it will benefit other
readers. Don’t give up on electric power; get
more involved! MA
Bob Aberle
[email protected]
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 - 2005/04
Page Numbers: 53,54,55,56,58,60
Photos courtesy the author
by Bob Aberle
Part 2
The Acrovolt lives up to its name by performing spirited
aerobatics in an effortless manner.
selected 50 watts/pound. From that I multiplied 3.93 pounds (63
ounces) by 50 and obtained 197 watts. Since power is amps multiplied
by volts, you can work backward using the power (watts) and the
estimated motor current.
That motor-current figure will always prove to be the tricky part. It
will be an estimate that is a compromise between how long a motor run
you desire and how much current your specific motor can tolerate
(known as maximum continuous current in motor specifications).
This is where the ElectriCalc and MotoCalc motor-selection programs
can really help you. For this application I decided on a range of 20.0-
25.0 amps and 22.5 as an average. Since power (watts) equals current
(amps) multiplied by volts, you can divide 197 watts by 22.5 amps and
obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an
eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between
two and three Li-Poly cells because the characteristic voltage is 3.7
volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives
you a ballpark figure.
From this point you can look up your motor data on one of the six
Web sites I listed last month. I generally go for the AXI brushless
outrunner motors because they are available in many sizes, and I have
found them to be extremely reliable.
For the average-size AXI motor I use “The Great Electric Motor
Test,” at www.flyingmodels.org. I searched through the data looking at
motor current, power (watts), and propeller sizes. You have to be
patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor.
On 8.0 volts and with an APC 10 x 7E propeller, it would have a
current of 23.0 amps and 176 watts of power. The 10-inch propeller
can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should
work. The Ni-Cd cells will likely produce slightly higher voltage,
current, and wattage. A recommendation is an eight-cell Sanyo 1950
The author selected Tom Hunt’s successful original-design
Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background
of how power sources for modeling are rated and specifically how
electric motors have been related in power output to glow engines. He
covered how to go about sizing, or matching, a specific-size motor to a
given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained
how to measure motor input power, aircraft weight and power-loading
considerations, wing loading and how it relates to flying experience
and skill, and thrust factors. He included details of the motor-selection
process, a comprehensive listing of Web sites containing helpful data,
and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection
process.)
IT’S TIME TO put all you’ve learned together with several examples
of motor selections. Let’s make a glow kit or glow ARF electricpowered.
Many popular glow-engine-powered full kits and ARF “kits” on
the market can easily be constructed and/or assembled from scratch
with electric power in mind. One such model is Tower Hobbies’
“Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended
for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of
wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of
16-20 ounces/square foot (sq. ft.).
As a start, take into account that the wing area is 515 square inches
(3.6 sq. ft.). Pick a wing loading for the intended skill level. Using
Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft.
That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing
loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up
with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading.
Wanting a bit more in performance (with some reserve), I specifically
AXI 4120/18 motor with special radial mount plate at right. An
“outrunner” motor’s entire outer casing rotates.
L-R: Older, larger, heavier DeWALT motor with H-1000 belt
drive and replacement Hobby Lobby AXI 4120/18 brushless
outrunner motor. It weighs 11 ounces!
Acrovolt went from 7 pounds to 5 pounds, 5 ounces. Motor run
time was increased from six to eight minutes to more than 20
minutes. This was accomplished with new brushless motor
and new Li-Poly batteries.
Original Acrovolt motor system, consisting of DeWALT 18-volt
cordless-drill motor and Modelair-Tech H-1000 belt reduction drive.
Motor and belt drive weighed 21 ounces!
A page from Bob’s logbook. Record data for each new or revised
aircraft, and then you have something to compare to when visiting
new, similar-size/weight aircraft.
mAh NiMH pack, which will get me slightly over my target but allow
me to do some throttling back during a flight. That will stretch the run
time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As
you progress, feel free to “what-if” your parameters in either of the two
computer motor-selection programs. You can even work up to Li-Poly
batteries by carefully selecting a propeller size that will allow you to
use a three-cell pack, but not at excessive motor current.
Converting/Updating: Almost 10 years ago, Tom Hunt designed a
large, aerobatic/Pattern, electric-powered model called the Acrovolt. It
was inspired by Art Schroeder’s Eyeball design from the 1960s.
I flew Art’s Eyeball back then. I remember that it weighed 6 pounds
with an Enya 60 glow engine. Tom designed his Acrovolt around the
use of his then-new Modelair-Tech H-1000 belt drive and either a
Speed 700 ferrite motor or a DeWALT 18-volt cordless-drill motor.
In 1995 the Acrovolt was powered by a 16-cell Ni-Cd battery pack,
which was later increased to an 18-cell 3000 mAh NiMH pack. The
weight at the time was roughly 7 pounds. The wing loading was high,
How the Acrovolt’s crowded battery compartment looked when
Bob had to employ upward of 18 3000 mAh NiMH cells.
Closer look at AXI 4120/18 motor from rear shows accessory
radial mounting plate which Bob highly recommends. Collar on
shafts moves up flush with rear of motor.
The same battery area today, after switching to Li-Poly!
New square Kokam Li-Poly battery pack is mounted ahead of
wing LE and before firewall. Because of lighter motor weight, Li-
Poly pack’s forward position proved to be necessary to maintain
proper CG position.
Top: The 18-cell 3000 mAh NiMH pack as used years ago to power
the Acrovolt weighed more than 40 ounces. Bottom: New Kokam
5S2P Li-Poly pack weighs just 20 ounces. The motor run went from
six to eight minutes to more than 20 minutes.
and the motor run time was short—six minutes average with an
occasional seven to eight minutes with some throttling! But at that
weight, throttling almost got the model back on the ground!
That airplane stayed idle in my shop until recently, when a thought
came to me while I was preparing this article. Here I had a .60 cu. in.
glow-powered model (40-size by today’s standards) that was converted
to electric power using the technology that was available 10 years ago.
How would the same airplane perform today with a modern and
efficient brushless motor, an equally modern brushless/sensorless ESC,
and, best of all, a lighter-weight/high-capacity Li-Poly battery pack?
The sequence of the selection process is the same as in the
preceding. The 600-square-inch wing area is the same as 4.17 square
feet. Referring to Table 3 I selected the skill-level category of the fast
sport model (usually with more than adequate power), which calls for
20-25 ounces/sq. ft. of wing loading.
This was roughly the range I experienced with my Eyeball using a
60 glow engine at 6 pounds total weight. The older electric version—at
7 pounds—had a wing loading of 27 ounces/sq. ft., which I knew was
way too high.
Recognizing that the new motor and battery would save me a great
deal of weight, I selected 20 ounces/sq. ft. as my target wing loading. If
I multiplied that 20.0 by the 4.17 sq. ft. wing area, I ended up with a
target weight of 83.4 ounces (slightly heavier than 5 pounds).
Looking at Table 2 I chose 80-100 watts/pound (for aggressive
airplanes) as the power loading. Not wanting to go absolutely crazy, I
Upper view shows added plywood motor box and battery-pack
position. Jeti brushless ESC is also in battery compartment.
After removing DeWALT motor and belt drive, 3⁄16 plywood box
was made to hold new AXI 4120/18 brushless motor at roughly
same prop position.
Clear plastic cowl shows new AXI brushless-motor installation.
Bob painted inside of molded cowl after photos were taken.
After being refitted with a modern power system, the Acrovolt is
more spirited than ever in flight!
Bob replaced old 2-ounce ACE RC Smart ESC (top) with new 1-
ounce Jeti Advance brushless ESC (bottom). New ESC senses
battery voltage at every start and automatically sets proper cutoff
voltage point. This is crucial when using Li-Poly batteries.
selected the midrange number of 90 watts/pound. From that I
multiplied 5.2 pounds (83.4 ounces) by 90 and obtained 468 watts.
Since power is amps multiplied by volts, you can work backward
using the power (watts) and the estimated motor current. Again, the
estimate of motor current is all-important and requires your knowledge
and judgment.
For this application I settled on approximately 27.0 amps. With the
older motor 10 years ago I ran 30.0-32.0 amps. I wanted to use less
current this time because my airplane will weigh much less.
Since power (watts) equals current (amps) multiplied by volts, you
can divide 468.0 watts by 27.0 amps and obtain 17.3 volts. For this
new application I wanted to use a large Li-Poly pack. Since each of
these cells has a nominal 3.7 volts, the closest I could get to 17.3 was
five Li-Poly cells (3.7 x 5 = 18.5 volts). I’ll get into the Li-Poly
capacity selection in a bit.
Having collected all of this information, I needed to select a
brushless motor for my application. I mentioned that I favor the AXIs,
so I went to their larger-motor data. The Web site containing The Great
Electric Motor Test did not list the larger 41-series AXIs, but Dave
Radford of Air Craft Inc. provided the data at www.aircraftworld.
com/default.asp?id=18. After going through his AXI 41-series
information, I settled on the AXI 4120-18 brushless outrunner.
The data specifically called for the use of an APC 12 x 8E propeller
and 16 Ni-Cd or NiMH cells (1.2 x 16 = 19.2 volts). The motor current
given was 24.5 amps, and the power was 434
watts. For my purpose this was close enough
for a starting point. To my surprise, the actual
numbers came out closer to my estimate than
the published data.
The photos accompanying this article
show how easy it was to swap out the much
larger ferrite motor for the brushless motor.
Since the Li-Poly battery was much lighter, I
was forced to move it farther forward (just
behind the firewall) to maintain the proper CG
location.
Flying the “revised” Acrovolt was
wonderful. The final model weight was 85.3
ounces (5 pounds, 5.3 ounces), yielding a
wing loading of 20.5 ounces/sq. ft. The actual
motor parameters worked out to be higher
than my predictions, but they were in the
direction I wanted so I left them that way for
my initial flight tests. The motor current was
28.5 amps, voltage under load was 18.0 volts,
wattage was 511.0, the watts/pound figure
was 95.8, and rpm was 8,600.
The Kokam 5S2P battery pack that I used
consisted of 2.0-ampere-hour (Ah)-rated cells
(15C-load capable). With two sets in parallel
(2P), the actual capacity is 4.0 Ah. Total
flying time obtained with this battery pack
was approximately 20 minutes, with some
throttling management. That means you don’t
run the motor all the time at full throttle.
In fact, the Acrovolt—now at an
extremely light weight and with a highperformance
power system—is capable of
doing loops from level flight at half throttle.
When the model landed each time at this
power level, the motor, ESC, and battery pack
were hardly warm. This was a perfect powersystem
choice in every regard! I ended up
with a model that flew better than it did with a
glow engine or the older ferrite electric motor.
Estimating Motor Run Time: When you near
the end of your motor-selection process, you
need to decide what capacity of battery you
want to use, regardless of whether it’s Ni-Cd,
NiMH, or Li-Poly.
Battery capacity is usually rated in mAh or
Ah. Your battery’s weight is directly related
to its rated capacity. The more capacity there
is in the battery, the more it will weigh. The
motor-selection programs can help you
considerably when making this choice.
For a quick estimate, I use a simple
formula. I multiply 60 by the battery’s
capacity rating (in Ah) and divide that by the
motor current (in amps). The result is an
estimated motor run time in minutes.
As an example, let’s go back to when we
were building a glow-powered kit as an
electric-powered model. The battery had a
1950 mAh capacity rating, and the motor
current was 23.0 amps. So multiply 60 by
1.95 Ah (the same as 1950 mAh), and divide
that by the 23.0 amps. The answer is 5.09
minutes.
Keep in mind that these are static figures.
When the model is in the air and the propeller
unloads, the current will be less. Also, it is
assumed that you will not fly the entire flight
at full throttle. As I pointed out earlier, this
model’s actual flight time was approximately
eight minutes. Nevertheless, this formula
provides a rough starting point.
Keeping Records: One of the best ways to
put your electric-power experience to good
use is by keeping accurate records of your
various aircraft. With new airplanes
especially, I make dated entries in a bound
logbook about the aircraft; electric parameters
such as motor current, voltage, watts,
watts/ounce or watts/pound, rpm, and run
time; and flight performance.
As you progress with electric power, these
records will provide practical comparisons
when you choose power systems for new
aircraft.
Where to Get Help: When you get stuck and
can’t find that elusive electric power system,
you will need some help. One place to find it
is a model forum on the Internet that
specializes in electric power, such as RC
Groups, E-Digest, RC Universe, and SFRC.
Hopefully you will reach an “expert” when
you send out your question.
Contacting local modeling clubs that
specialize in electric power is another way to
learn. But if you live in a remote area, that
may not be easy.
In the end, there is a group of many hobby
distributors who will work with you,
including Kirk Massey at New Creations RC
([936] 856-4630), Dave Thacker at Radical
RC (www.radicalrc.com), Dave and Bob Peru
at Balsa Products (www.balsapr.com), Sal
DeFrancesco at Northeast Sailplane Products
(www.nesail.com), Helmut Goestl of Dymond
Modelsport (www.rc-dymond.com), and Tom
Hunt at Modelair-Tech (www.modelair
tech.com).
References: In the past two years I have
authored a series of articles specifically about
electric-powered flight that have been
published in MA, and they are as follows.
(You can find the “From the Ground Up”
articles on the MA Web site at www.modelair
craft.org/mag/index.htm.)
• “State of the Sport: Electric-Powered
Flight”: April 2002, pages 18-40.
• “From the Ground Up: Introduction to
Electric Power”: July 2003, pages 56-64.
• “From the Ground Up: Battery Basics”:
October 2003, pages 54-62.
• “Introduction to Lithium-Polymer Batteries:
May 2004, pages 44-58.
I have also written articles that have been
posted on MA’s Sport Aviator online
magazine. They are:
• “The SuperStar-EP Electric ARF” at
www.masportaviator.com/ah.asp?CatID=1&I
D=16. This is a review. SuperStar EP is
similar to the Tower Hobbies PT-20 that I
mentioned in this article.
• “Bonnie 20 ARF Electric Trainer” at
www.masportaviator.com/ah.asp?CatID=1&I
D=39. This is also a review.
• “Bonnie 20—Adapting to Li-Poly Batteries”
at www.masportaviator.com/ah.asp?CatID
=2&ID=43. This shows in depth how to
convert a Bonnie from NiMH/Ni-Cd batteries
to the new Li-Poly variety.
In addition, I wrote the book Getting
Started in Backyard Flying, which is available
from AMA. Pages 58 and 59 explain the
process of measuring motor current using an
AstroFlight Super Whattmeter (part 101).
The process of selecting a motor system for a
particular aircraft may seem overwhelming at
first, but I assure you it’s not! Follow the
logical steps I have presented here; they work!
I would love to be able to reference a single
page or table and say, “That’s the answer.” Joe
Beshar thought that was possible when he
made his suggestion, but there are far too
many variables with electric power.
Buy that meter and take your own
parameter measurements. Buy a scale and
weigh your aircraft; don’t guess! Buy a
tachometer and really see if you improved
things! Buy one of the two recommended
computer programs; you’ll be amazed by how
helpful they are.
If you hit a snag, please write or E-mail
me. I will not only try to answer your detailed
questions, but I will include those inquiries/
answers in MA’s “Frequently Asked
Questions” column if it will benefit other
readers. Don’t give up on electric power; get
more involved! MA
Bob Aberle
[email protected]
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/