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We build and test an electric
ELECTRIC FLI
Electric powered model aircraft are becoming very common
– but they do present pitfalls, not just for the beginner but
for the experienced modeller as well! In this article we
review the electric scene and build and test an electricpower Piper Cub. How did it go? Read on.
M
uch has changed in the world
of electric-powered, radio-controlled aircraft since SILICON CHIP last
visited the subject back in 1992.
If readers may pardon the play on
words, electric flight is undergoing
a quiet revolution; a revolution so
radical that the economic viability
of internal combustion (IC) motor
manufacturers must surely be under
threat.
This startling transformation has
come about as a result of a number of
electric flight technologies coming of
age almost simultaneously.
Of these developments by far the
most important are:
12 Silicon Chip
[1] Application of Rare Earth magnets
to brush motors
[2] High power, low-on-resistance
FETs
[3] Microprocessor-controlled smart
speed controllers and smart
chargers.
[4] Brushless motors
[5] Battery technology improvement.
Prior to 1992 electric flight was in
the hands of a small group of dedicated contest flyers. Today’s electric
flyers owe this group a huge debt of
gratitude, for without them electric
flight would be nowhere near as
advanced.
It was this group and in particular,
Peter Blomart of Belgium, that established the internationally recognised
class of F3E competitions in 1986.
Since those early days of primitive hand made soft start switches
and analog electronic speed control
(ESC), progress has been staggering, to the point where the modern
microprocessor speed controller can
now distinguish between brush and
brushless motors and configure itself
accordingly.
International R/C aerobatic competitions have long held their place as
the most prestigious R/C events. While
traditionally dominated by IC motors
siliconchip.com.au
Piper Cub
IGHT
By BOB YOUNG
of ever larger capacity and power, it is
increasingly common for electric powered models to snatch places from the
IC brigade in these showcase events
and it is here that the real threat to IC
motor manufacturers is developing
most rapidly.
The radio-controlled boat boys have
also been hard at it. Currently the fastest R/C boat in the world is electric,
with a speed of 120.7 mph.
In Australia, Ray Cooper of Victoria
set a world record for electric-powered
models in the distance to goal and
return class with a flight of 54.3km
(108.6km total), lasting 1 hour 22
minutes.
siliconchip.com.au
Across the world, R/C flyers are
scrambling to emulate their international heroes.
Manufacturers of electric motors
are springing up like mushrooms and
battery manufacturers are continuing
to confound, with batteries that are
lighter, with more capacity and higher
cell voltages. Manufacturers of the
chargers for these batteries are hardpressed to meet demand and so the
bandwagon has been set in motion.
With all this going on, the time has
come to review this wonderful world
of ultra mobile electrons in the most
practical way possible: building and
flying an electric powered model suitable for park flying on those quiet,
wind-free evenings that are an R/C
modeller’s special delight.
World Models Piper J-3 Cub EP
The kit chosen was selected for several reasons. Small enough to qualify
as a park flyer, it is simple to build and
fly and is reasonably priced.
The kit includes a geared, brush
motor (Speed 400) and thus can be
controlled by a simple and economic
ESC. It also uses a genuine lightweight
4-channel R/C system providing four
proportional channels.
However the real reason for the
choice of this kit was that I have had a
soft spot for Piper Cubs for a very long
time. It is a very pretty aircraft, easy
to fly and is one of the nicest aircraft
for take-offs, landings and especially
“touch and goes”. If the reader loves
to watch a graceful aircraft land and
takeoff then there is no better model
than the Cub.
This emotional approach to the kit
purchase was to have a dramatic effect
on the ultimate outcome of this whole
project but more of that later.
The Cub does have one small vice
and that is adverse yaw during an
aileron-only turn. This is largely as a
result of the flat bottom wing section
(Clark Y) and poor aileron design.
To turn an aircraft, the aileron on
the inside wing must be raised at the
trailing edge and the outside aileron
depressed. This reduces the lift on the
inside wing tip and the wing starts to
fall due to the unbalanced lift distribution.
However with the reduction in
lift, the drag at the inside tip is also
reduced. Conversely, when the aileron
on the outside wing goes down to lift
that wing tip, the lift shoots up dramatically, as does the drag.
The result is that the aircraft rolls in
the direction of the turn but the nose
is pulled around in the opposite direction by the badly unbalanced drag
forces at each wing tip. This gives rise
to a very awkward situation known as
“adverse yaw”.
However, there are several tricks
that will help improve the flying
characteristics of the Clark Y type
wing. These include heavy differential aileron movement (more up than
down) and coupled aileron/rudder
mixing on either the transmitter or in
the model (ie, turning using rudder
and aileron together).
Full size designers may resort to
differential movement and designs
such as the Frieze Aileron, which
uses a complex hinge that allows the
leading edge of the up-going aileron
to protrude into the slipstream underneath the wing, thereby increasing the
drag on the inside wing and balancing
overall drag.
However do not lose sight of the
fact that these are only patches and
they introduce other problems such
The kit as she comes, straight out of the box. All of the difficult model work of
past years – wing and body shaping, etc – is already done for you!
February 2006 13
11.4V, 1800mah 3 cell Li-PO battery left rear. 3 cell ESC left
front and two sizes of brushless motors.
Two of the ultra mini servos fitted to the Cub. They’re
significantly smaller than the servos you’re used to . . .
as spoiling the rolling characteristics and reducing aerodynamic efficiency. As with all fixes, the real answer is in
the initial design of the aircraft. The prime rule should be
no flat bottom wings with simple centre-line aileron hinges
on sport aircraft.
Scale aircraft are a different matter.
Why kit designers insist on using a flat bottom wing with
simple centre-line aileron hinges on training and sports
models is absolutely beyond me. They must subscribe to
the theory that if you can fly this sort of trainer you can fly
anything. They really are unpleasant to fly if the necessary
precautions are not taken.
Even a wing with a moderately curved underside (Semisymmetrical, eg, NACA 2415) will completely transform
the flying characteristics of any model with a flat bottom
wing section and may almost completely eliminate adverse
yaw.
While this is a diversion from the main topic it has been
covered in detail because it is important for all tyro R/C
modellers to understand the effects of flat bottom wings.
It is the only factor that may spoil the delight of flying this
really nice model.
Nowhere in the Cub instructions does it warn of adverse
yaw or mention the above precautions which is a pity as
otherwise the Cub is a good value kit.
The above applies to all R/C model aircraft, so do yourself
a favour when purchasing your next trainer or sport model.
Look for a model with a symmetrical or semi-symmetrical
wing section.
ings. No mention is made of the type of servos the kit was
designed around and so the servo trays fitted in the wing
were unsuitable for the servos used in this model. They
had to be cut away and new ones fitted.
Watch out for the cross-brace at the bottom of the servo
well. This is actually a little pull out handle attached to
the fine cotton pull-through used to pull the servo lead
through the wing tunnel to the wing root.
It is best to remove the servo connector and solder a
3-core ribbon cable extension lead long enough to reach
the wing root plus an extra 75mm. When the wing halves
are joined then splice the two 3-core cables together with a
servo lead about 100mm long. This single servo lead then
plugs into the receiver aileron socket. There is virtually
no way that a servo connector can be threaded through
the wing tunnel so conventional servo extensions and “Y”
leads cannot be used.
Remember here to set the aileron servos in such as manner that ailerons move up the recommended 10mm but
contrary to the instructions only go down about 2mm, not
10mm. This can be achieved mechanically by offsetting
the servo arm as shown in the relevant photo or by using
the transmitter settings in computer radios.
The rest of the wing assembly is straightforward enough
with the exception of one final point. The wing uses a
straight spar to join the two halves. It is a good idea when
gluing the two halves together to place a moderate weight
on the centre, giving the wing a very small amount of dihedral. A dead straight wing on a high wing model tends to
Assembly
The Cub is one of the new breed of kits called ARF
(Almost Ready to Fly). This means that the manufacturer
has already done all of the hard work. The model is almost
completely built and fully covered in plastic shrink film
when it comes out of the box.
All that remains is some detail work and the installation
of radio and motor.
Thus for the enthusiastic tyro, building will typically
take around 10-15 hours. The following is not meant as a
detailed how-to of assembly, merely a guide to point out
some of the difficulties and shortcomings in the kit and to
help anyone building this model avoid the pitfalls.
Assembly begins with the preparation of the two wing
halves and it is here that the only real problem in assembly was encountered. The instructions are very poor and
consist merely of photo sequences and a few odd draw14 Silicon Chip
Engine Room with cowl removed showing the Speed 400
motor and gearbox as well as the folding prop.
siliconchip.com.au
look as if the wing tips are drooping and the slight dihedral
eliminates the droopy look.
The remaining assembly is routine but with two points
of concern. The hinges used are sheet Mylar. Glue only
one side into the model using super glue. Do not allow
superglue to get across the hinge line, as this will make the
hinge stiff and brittle. Do not attempt to coat the hinges on
the control surfaces with super glue and then slide them
into place.
The glue goes off immediately it touches balsa. Instead,
fit the control surfaces and then drill vertical holes down
through the control surface and hinge ready to accept a
normal household pin. Wick super glue down the hole and
then slide in the pin. Nip off the pin flush to the underside
of the control surface.
Hinges should always be pinned into place. Do not rely
on the glue to hold them. Many a good model has been
lost in this manner.
Lastly, the elevator and rudder pushrods are blued steel
and they slide into long Mylar tubes, giving a fit quite high
in friction. To avoid this, before inserting the pushrods
into the tubes, use a piece of sandpaper to remove all of
the blueing to take the surface back to bright steel. Then
coat the pushrods with graphite lubricant before finally
inserting them into the tubes.
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As stated previously the Cub is supplied with a geared
Speed 400 motor and a folding prop. Fitting the motor and
associated electronics is very straightforward. The folding
prop is a curiosity, as few people would attempt to soar this
sort of model. The folding prop is designed to reduce drag
during gliding flight when the motor stops. An excellent
range of wooden e-props designed especially for electric
motors is now available and this model would no doubt
benefit from one of them.
The folding prop does have one advantage though: resistance to breakage in bad landings!
The motor is already fitted with suppression capacitors,
100nF (0.1uF) across the two motor terminals and 100nF
from each terminal to the motor case.
A simple, economical speed controller (ESC) was chosen and wired in to place. Be sure to follow the polarity
instructions carefully or on the first take-off you will be
run over by an aeroplane flying in reverse!
The modern breed of speed controllers, designed for use
in conjunction with lithium-polymer batteries, are very
interesting. They feature a mandatory low voltage cutoff to prevent the Li-POs falling below 2.4V per cell and
destroying themselves. They also feature a BEC (Battery
Elimination Circuit) to provide receiver and servo power.
Be sure when choosing a speed controller (the kit does
not provide the ESC) to specify the number of servos to be
operated from the BEC. The Cub uses four servos.
The ESC uses a microprocessor to control all of these
functions. One additional feature of micro-controlled ESCs
is a degree of input pulse monitoring. This checks the in-
Engine Room with cowl removed showing the Speed 400
motor and gearbox as well as the folding prop.
Radio Room. Note servos mounted but push-rods not yet
fitted.
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February 2006 15
coming signal for noise and should the input pulse count
exceed the specified number or change pattern dramatically
then the ESC shuts down the motor drive.
Another very good safety feature is that the ESC cannot
operate accidentally when turned on. To function correctly
the ESC must first be set to LOW throttle and then the ESC
is armed and ready for use. Be careful on the first powerup, as it may be that the throttle channel is reversed with
low at the top of the transmitter (TX). In this case use the
TX channel reversing function to set LOW throttle with
the stick down towards the bottom of the TX.
Now we come to the electronically juicy part of this
saga.
Range reduction
Our experience with the analog speed controller presented in the 1992 articles indicated that there will be
a loss of usable transmitter range with an electric motor
running, as against motor stopped. Our testing in 1992
confirmed that this range reduction would be in the order
of 10-12%, due to the noise produced by the commutator
and brushes of the motor.
So we decided that anything in excess 85% of the engineoff range is an acceptable figure, with the ranges available
from modern receivers.
So imagine our surprise at the field one perfect spring
morning for test flying, when we were confronted with 15%
of the radio range on the throttle control. All of the flying
Fuselage complete except for decals. Note the folding prop.
16 Silicon Chip
No, it wasn’t a hard landing which tore the wings off . . .
here the Cub is almost complete: motor, servos and radio
installed and wings ready for fitting.
controls worked well at the normal range but the throttle
would run up to speed and cut off at about 15% of normal
range. Obviously it was back to the drawing board. What
had happened?
Firstly in spite of my many years experience in R/C flying,
the most fundamental rule of all had been ignored. That is,
test everything at home before leaving for the field and that
includes a retracted TX antenna range check with motor
running and motor stopped, even though the motor running test is difficult to do on your own. This is especially
true when preparing for any model’s first flight.
So what had happened? Firstly, I’m an old power hound
who likes models with only two speeds, stop and go very
fast, and who believes that too much power is better than
too little (you can always throttle back), so the Cub was
fitted with an 11.4V 3 cell Li-PO battery. Thus the rating
of the 8.4V Speed 400 motor was exceeded by the extra
3V. This is OK if one is prepared to accept abnormal motor wear.
However the Speed 400 motor does not have good brush
design and it arcs quite badly unless tuned properly. An
old trick here was to time the motor in a darkened room
for minimum arcing on the brushes. Clearly then the extra
3V was elevating the motor noise and this was getting into
the speed controller and shutting it down via the pulse
counting safety circuit.
So we fitted a reverse-biased Schottky diode and a 40V
varistor across the motor brushes as recommended in the
1992 articles but with little improvement. Where to from
here?
Change the receiver perhaps? The noise path was most
likely coming down the receiver antenna and then through
the receiver. Ultimately four brands of FM receivers were
tried and located as far from the ESC and motor as possible;
three imported brands and the original Silvertone. Three
brands of TX were tried as well. These gave very little
improvement and the results were still not acceptable for
successful flying.
It was then decided to change the operating voltage,
reducing it to a 2-cell Li-PO battery delivering 7.2V. Unfortunately this introduced a complication in that the
economical little speed controllers are set for the number
of cells to be used. There is no built in facility for cut-off
voltage adjustments. So a change in battery voltage called
siliconchip.com.au
Close-up of tailplane assembly showing control horns,
pushrod connections and steerable tailwheel.
Close-up of fin assembly showing the steerable tailwheel
anchor/bearing plate.
siliconchip.com.au
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and visibility problems, the last thing needed is a glitch
or two to add to the misery. The result may very well be
a smashed or lost model.
I have four much flown and much cherished models
approaching or exceeding 30 years old. One even has
30-year-old servos still fitted and functioning. The key to
this sort of longevity is constant and effective servicing
coupled with well-built airframes and a rigid pre-flight
test procedure. This includes not accepting any shortfalls
in performance. Models take too long to build and are too
expensive to treat in an off-hand manner.
Even the modern ARF still takes a lot of time to prepare
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for a new ESC with a low voltage cut-off exceeding 4.8V.
After fitting the new ESC and battery the same tests
were conducted with the FM receivers. The range was a
lot better (50 –60% depending upon the FM receiver used)
but still not up to the 85% figure adopted originally as the
acceptable standard.
Finally in desperation a Silvertone AM receiver was
fitted and this gave the desired range. The model was at
long last ready for flying.
In conclusion, then, what had caused all of the problems
and what had fixed them?
Firstly, had I used my head instead of my heart and
purchased a kit fitted with a brushless motor and a more
advanced ESC perhaps things may have been different.
That is another story of course and we may yet see the Cub
fitted with such a system.
However, the exercise was to learn about the modern
electric flight systems and how to make them work in spite
of whatever shortfalls there were in that equipment. People
do buy with their heart and especially with their pocket
in mind. The Speed 400 is a common motor in kits and
the economical little ESCs are very attractive to beginners
feeling their way into electric flight.
The problems began when the recommended motor voltage was exceeded. Not clearly understanding the operation
of the safety circuit in the ESC compounded the matter.
It took time for us to realise that it was the safety circuit
cutting off the motor.
The FM receiver performing poorly against the AM
receiver is easily understood. The powerful AGC on the
AM receiver keeps the receiver in a less sensitive state for
approximately 80% of its range. Thus noise-induced spikes
may be much reduced in AM receivers, depending upon
the nature of the noise. AM receivers in spark ignition and
electric models can often give the best results. Again, this
is a trial-and-error process.
Perhaps re-timing the motor would have reduced the
spark noise, but that is not a job for inexperienced modellers. It was dropping the voltage that reduced the interference most dramatically. Maybe 50-60% of the range is
acceptable for some modellers. These are, after all, small
models and cannot been seen clearly at long ranges.
However, beginners often let their models get out of
hand at times and they can very quickly be blown down
wind a great distance. For a beginner struggling with wind
February 2006 17
Aileron servo mounted in wing. Note the offset on the
servo arm at neutral to provide differential movement of
the ailerons (for non-computer radio control systems).
for flight. Too much to just squander with a casual attitude.
Beside this, there is the safety issue to consider. An out-ofcontrol aircraft may be a health hazard. So think carefully
about your decisions to fly or not fly. You can always come
back another day – if you have a model to fly that is!
Flying the Cub
As expected, the model flew just like a Piper Cub, looking as pretty as a picture. Also as expected, despite the
differential aileron built in during assembly there was still
an excessive amount of adverse yaw during aileron turns.
In the course of trimming the model, Coupled Aileron/
Rudder (CAR) will be called up in the TX program with a
switch to enable/disable the CAR in flight.
CAR is a function in the TX program whereby a small
amount of rudder control is mixed into the aileron control
to help initiate the turn and hold out the adverse yaw. The
switch is desirable for aerobatics, to switch off the CAR in
flight during rolls etc.
The radio worked perfectly with no sign of interference
from the motor in flight. Take-off power with the 2-cell
Li-PO battery was marginal but once airborne there was
ample power for climb and cruise. There would be no hope
of getting off even a smooth grass strip. We used a tarred
road for take-off. Take-offs on grass and aerobatics would
definitely call for a 3-cell Li-PO battery to be fitted.
The real surprise was the lack of down thrust. The model
climbed on full power and dived when the throttle was cut.
At least another 3 degrees of down thrust will be required
to correct this effect. This will be a real pain to retrofit as
the motor slides snugly into holes in two bulkheads.
To use a flat bottom wing-section is a tragic error but to
provide a pre-built fuselage with the incorrect thrustline
is unforgivable. Why do kit manufacturers do this sort of
thing? How on earth are beginners expected to fight their
way through a maze of annoying little problems? These
things are not all that serious and relatively easily fixed,
certainly in the kit building stage but they make potentially nice models unpleasant to fly. Why spoil what is
essentially a really nice kit with lack of attention to some
of the fine detail?
There are several ways to handle the lack of down thrust.
First you can carve out the bulkheads and set the motor at
the correct angle. This is the right way to do it aerodynamically. Or you can mix some elevator trim in with throttle so
that when the throttle is opened the elevator moves down to
compensate for the climb. Finally and for the experienced
flyer only, try moving the CG back until the full aerobatic
position is established. (Not for beginners.)
However the nicest part of this whole story is being able
to drive for five minutes to a large clear local area and fly
without annoying people nearby. The model was as quiet
as a church-mouse in flight. This is another priceless legacy
of technological progress.
So there you have it, a true, warts-and-all introduction
to electric flight.
Ready for take-off! The Piper Cub complete with decals (they’re all supplied in the kit) and ready to fly. The “bendy”
propellor, so disconcerting to some, is clearly visible in this shot – it straightens up once it starts pushing air!
18 Silicon Chip
siliconchip.com.au
PRECAUTIONS WHEN USING Li-PO BATTERIES
Li-PO batteries contain volatile and toxic chemicals.
For your safety please read the following carefully.
• NEVER leave batteries on charge unattended! It is
also a good idea to place the battery in a steel or ceramic
dish while charging and keep well away from inflammable
materials.
• Never leave the battery connected to the speed control
as these units have a small leakage current. Do not allow
batteries to fall below 2.4V either in use or by self-discharge.
Always recharge batteries at least once every 3 months.
Batteries that fall below 2.4V are ruined and will never
work again.
• IMMEDIATELY remove a Li-PO battery
from a model if it is
involved in a crash. Carefully inspect the battery
for even the smallest
of dents, cracks, splits,
punctures or damage to
the wiring and connectors. CAUTION! Cells
may be hot!
DO NOT allow the battery’s internal electrolyte
to get in the eyes or on
skin – wash affected areas immediately if they
come in contact with the
electrolyte.
A Li-PO battery might
not appear to be damaged after a crash but it could smoulder over a short amount
of time and suddenly catch fire unexpectedly. If in doubt,
place the battery in a fireproof location indefinitely.
• Disconnect the battery IMMEDIATELY from the charger
if it begins to swell, emits smoke or is warm to the touch!
Place warm or hot batteries in a fire-safe location, such as
a container made of metal (such as an empty ammunition
box) or ceramic. Always monitor the area with a smoke or
fire alarm, and have an “ABC type” fire extinguisher available at all times.
• DO NOT set the battery charge rate to a value greater
than the battery’s 1C value as permanent damage could
result. Do not exceed a 9C discharge rate.
• DO NOT allow LiPO cells to overheat at any time! Cells
which reach greater than 140°‑F (60°C) can and USUALLY
WILL become damaged physically and could possibly
catch fire! Always inspect a battery which has previously
overheated for potential damage and do not re-use if you
suspect it has been damaged in any way. Do not leave a
battery near a heat source above 80 °C (Stove, heater etc).
siliconchip.com.au
Leaking batteries must be kept away from naked flames.
Keep the battery as cool as possible at all times, particularly
when charging.
• Always provide adequate ventilation around Li-PO batteries during charge, discharge, while in use and during
storage. If a battery becomes overheated, remove it from
the charger immediately and place it in a fireproof location
until it cools.
• Use a charge lead that is directly compatible with the
“charge” connector on the Li-Po battery. It is strongly recommended to use pre-assembled charge leads. These can
be found at most hobby retailers.
• Do not use automotive
chargers to power Li-PO
chargers.
• It is preferable to
charge individual cells
for best results (parallel
charging). If series charging is used, do not attempt
to charge more cells in
series than the charger is
designed for.
• Always disconnect
chargers from the input
power source when not
in use.
• Keep out of reach of
children.
• Dispose of discarded batteries responsibly!
WARNING!!
You MUST NOT care for lithium-polymer (Li-PO) cells in
the same way as other battery types!! It is very important to
have a good understanding of the operating characteristics
of Li-Po batteries – especially their exact rated voltage and
maximum acceptable charge current.
Always read the specifications printed on the label of your
Li-Po battery prior to use. Failure to follow the care and handling instructions can quickly result in permanent damage
to the batteries and its surroundings and even start a FIRE!
Do not mistake lithium-polymer cells for other lithiumbased cell types (such as lithium-metal, lithium-phosphate,
etc.), as other lithium hybrids have different care and handling characteristics .
It is strongly recommended to use packs that have been
assembled with built-in charge protection circuits. Such
circuits help to regulate the maximum voltage per cell in
the pack to ensure that that they do not accidentally become
overcharged.
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