ThiS moNTh’S column will cover a
popular E-supplier business change, a new
E-catalog, a new pinion press, some
common power system questions, and some
E-conversion considerations.
modelair-Tech (MAT) is a popular supplier
of E-products, including a unique line of
belt drives and “stick” kits. I have and I fly
several MAT products, and they show up in
large numbers at meets.
Tom Hunt, head modeler at MAT,
recently advised that his products will no
longer be available direct, as they have in
the past. MAT goodies will only be retailed
by familiar names, such as New Creations
R/C and others to be named.
Tom made this change to allow more
time for new product design and
development, which is something he does
extremely well. However, MAT will
continue to offer assistance in product
selection for a given application.
Contact MAT at Box 1467, Lake Grove
NY 11755-0867; Tel./Fax: (631) 981-0372;
E-mail: [email protected]. You can
also check out www.modelairtech.com for
more information on this announcement and
complete product catalog information.
Speaking of newcreations, respected head
modeler Kirk Massey has released his latest
catalog—number 30—and ya gotta have it! You
can request one from New Creations R/C, Box
496, Willis TX 77378, or call (936) 856-4630.
New Creations R/C offers in-depth
inventory at discounted pricing, with
friendly, speedy service and free, sound
advice if you need it!
One photo shows a new pinion press by
New Creations R/C. This is an upgrade of an
earlier version, and it is very nice to use. It
will press the appropriate pinion gear on any
motor shaft with ease and in complete safety.
It can do so by virtue of its wide range of
adjustability and multiple shaft adaptors.
You can check out the pinion press and a
huge number of other E-goodies in that new
catalog, and do tell ’em Bob sent ya!
Electric power system matters can be
confusing for some, and it’s common to hear
recurring questions. Following are a few
topics—in no particular order—that may
help you better understand and apply
motors, propellers, and batteries.
The wire windings on a typical brushedmotor
armature (the part that rotates) are
normally described by the number of turns
and the wire gauge.
For a given motor design and applied
voltage, the fewer the turns, the higher the
rpm, and the lower the torque. Conversely,
more turns result in reduced rpm.
Fewer turns permit a larger wire size to be
used in the same winding space, so the armature
resistance is lower. The lower this resistance, the
lower the (power) losses and associated heating.
If all this makes it sound as though all
motors should be made with only one turn
of very heavy wire, they shouldn’t! The few
turns/high-rpm combination is not good for
direct drive, because it dictates too small of
a propeller just to keep the current demand
reasonable. And a small prop turning fast is
rarely effective in the air.
In the limit, as the prop gets smaller, the
airplane is apt to act more like a static motor
test block than a flyable craft! This is why
most “car motors” are not useful for directdrive
E-aero applications. One cannot get a
prop small enough to keep the current
reasonable and still fly most models well.
However, fast-turning motors work well
with speed reducers, such as gear and belt
drives. This permits the use of an efficient highrpm
motor to turn a larger, more-effective prop
at a speed better suited to most models. I’ve seen
“car motors” applied very well this way.
Not unlike the “turns” issue above, the
motor magnets and associated magnetic
Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
Gus Janger’s (Philadelphia PA) enlarged REVOLT! (11/94 MA) is majestic in flight.
Spans 90 inches, weighs eight pounds, has geared Astro 40 on 18 2000s, trike gear!
Third in a series, new, larger MAT Elipstik 750 was test-flown at 2000 Lehigh Valley (PA)
meet. Flies well on 05 “can” motor direct drive to folder, seven 2000s.
102 M ODEL AVIATION
field strength inside the motor housing have
their own considerations.
For any armature residing and rotating
in such a field, the torque will increase
and the current drain and rpm will
decrease as the strength of the magnets
(and field) increase.
Another perspective is that very strong
magnets permit fewer armature turns to attain
the same shaft rpm, and fewer turns allows
room for heavier wire—all “good stuff.”
Along this line, many readers have asked
what a “flux ring” is. It is typically an iron
band that fits tightly over the outside of a
metal motor housing, for the purpose of
thickening the metal surrounding the internal
magnets. This results in less magnetic field
loss (leakage) outside the case, i.e., more
magnetic lines of force stay inside.
As above, this ring results in greater
torque at lower current and rpm for a given
voltage and prop; this may be a more
suitable operating condition for a particular
model. Flux rings are most commonly found
on economy “can” motors.
Motors are typically designed with a
particular operating voltage, rpm, and
current range in mind. For a given motor
and prop, a specific applied voltage results
in specific rpm and specific input current.
Varying the voltage—as with a speed
control—causes the rpm and input current
quantities to vary. Thus, one normally
chooses a motor/prop/battery combination to
have a preferred maximum or peak operating
power, then adjusts that level downward with
an Electronic Speed Control (ESC) for much
of the flight. Then power could be invoked as
needed for the flight task of the moment.
One might push the throttle stick forward
to pull through that outside loop, then back
off after exiting the maneuver. And one
might lower the throttle while positioning
the airplane for the next maneuver—no need
to race around the field just for that.
Playing the throttle in this manner
throughout a flight will give you more air
time per charge with appropriate motor power
input—and pack drain—throughout the flight.
New E-fliers often claim “short flight
time”—before they catch on to the practiced
use of throttle control!
Motor current is most often the guiding
consideration in power-system makeup, for
several reasons.
Current is what “empties the tank,” so
flight time is dictated by the battery capacity
and current drain. Thus, for some desired
operating power level, one might add cells
(increase voltage) rather than prop for
higher current.
Additionally, “high current” is a common
contributor to motor wear and tear. Relatively
speaking, “high” current means relatively
high brush and commutator temperature, and
associated wear and shortened life.
However, “high current” is a relative
term that is usually product dependent.
Much sport-flying is done below 30 amps,
and most of that is closer to 20 amps with
many popular products, such as Astro
motors. They work very well, and have
good life at these levels.
The increasingly popular “small” motors
have lower operating current
recommendations. Speed 400 motors are
known to die quickly at much past roughly
10 amps. Very small power systems, as in
Indoor and park fliers, have even lower
tolerable operating currents.
Increasing motor-input power by
increasing voltage (more cells) and holding
the current at more desirable levels usually
requires smaller prop size or speed reducers.
But do note that there is also a practical
upper voltage value for each motor.
As voltage increases, armature rpm
increases, and this “shaft” rpm must be held
to a reasonable value for mechanical
reasons. It is possible to run at an rpm so
high that the armature windings and
commutator segments are physically thrown
off, thereby destroying the motor.
Just knowing such considerations exist is
very helpful, because it allows you to pursue
a question you didn’t know you had!
One modeler asked me if it was okay to
run the popular low-cost Magnetic Meyhem
on 14 cells, as long as he geared and
propped to maintain a 20-amp current.
The answer is no! The current is okay,
but this motor will throw windings past 10
cells, and some do it at 10.
However, one local flier runs his Astro
05 geared on 10 cells with a 10 x 5 prop,
and this combo works very well—despite
the common usage of the Astro 05 with
seven cells.
At the Northeast Electric Aircraft
Technology fair (NEAT) in September, I
spoke with one modeler who ran an Astro
15/Superbox combination on 22 cells, and
that motor is normally used with 12 cells.
The armature was spinning at nearly twice
the recommended rpm. I think the only
reason it worked was because Astro
armatures are very well made.
All armatures have limits to the rpm they
can survive, and sometimes one has to learn
what they are the hard way.
All this may sound daunting, largely
because there are so many variables, and
sometimes unknowns, to consider.
However, once E-aeromodelers get
some firsthand experience with E-power,
most of them quickly get into motor,
gear, battery, and prop selection details
with insight and understanding, using to
advantage the variety of E-products that
exist today.
It’s important to keep operating current
and maximum applied voltage within limits
for any motor, to avoid damage and/or
reduced motor life while still powering your
airplane for good performance and long
flight time.
If you get “stuck” at this point, ask!
i’ve been getting an unusually large
number of reader questions about Electric
conversions.
Although I know almost any gaspowered
design can be flown Electric when
the conversion is done properly, I’m not
familiar with all the designs out there.
Therefore, it’s not always easy for me to
provide a detailed power system “shopping
list” for a specific inquiry.
Following is a way for you to ease the
task for both of us.
An Electric power system (motor,
battery) makes up roughly 50% of a
complete Electric’s weight (an Electric rule
of thumb). That is not a rigid 50%, but a
“target” or nominal number; a range of 40-
60% is okay for most purposes.
Suppose you had a finished, ready-to-fly
Electric that weighed five pounds (80
ounces). The guideline allows 32-48 ounces
of this for the power system, and the
remaining 48-32 ounces for the finished
airplane and radio.
Gas kits (thus reader input to me) often
quote the advertized model weight (range)
with a suitable gas engine selection. You
New Creations R/C’s pinion press works well on broad range of
motor sizes, and has adaptors to fit all motor shaft sizes.
Great-working Dymond peak charger for small packs. Selectable
0.3-, 0.5-, and 1.0-amp outputs for four to eight cells.
104 M ODEL AVIATION
March 2001 105
need to estimate the airplane’s finished
weight with radio, but without the engine,
tank, etc. Even better is the same estimate
including any weight savings you think
you can make to the model.
This information yields the weight
budget for the power system. Then
other rules of thumb then come in to
assist, such as the “watts per pound”
guideline—and it’s here I can be most
helpful with your inquiries.
You can usually be sure that your
selected conversion will fly well when the
right power-system choices are made.
Some E-modelers enjoy the challenge
of electrifying “stock” wet designs to
prove E-flyability, and they take no
special care. However, wet designs are
usually overbuilt, so “losing weight” in
them is a worthy pursuit. Lighter does fly
better, and I encourage this approach.
Please go through these
considerations before writing me about
a specific conversion. That will give
me the best shot at specific powersystem
recommendations, to get you in
the air most expediently.
I may need additional information for
certain cases, but the preceding
considerations are a good starting point. I’ll
always do the best I can with this (or any)
kind of inquiry—even if it means referring
you to a third person, who is more suited to
handle your particular case.
So ends one more column. Please enclose
an SASE with any correspondence for
which you’d like a reply.
Remember: E-power makes being an
“all-season flier” easier, because there’s
no balky engine or sticky goo to deal
with in cold months. Beyond that, one
can fly E-indoors almost any time! (I
wonder if that counts.) MA
