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Fly Baby - A Glow to Electric Conversion

by Stuart Warne

Article by Andrew Gibbs

Electric Power Fly Baby

This beautiful quarter scale model of a Bowers Fly Baby comes from Stuart Warne of the UK. The model was built from a Jim Pipino plan, and spans 84 inches (2,130 mm). Stuart bought the model as he was looking for a large model, but being a busy man, he wanted to save the effort of building a complete airframe.

The model was originally flown with a 0.70 4-stroke, which was still installed at the time of purchase. Stuart removed the engine and sold it on. He then stripped the airframe of all its covering, and began converting the model to electric power. Modifications included fitting an electric motor, installing a battery tray and making provision for cooling of the power system components.

Fly Baby electric RC model

The Fly baby before conversion to electric power. The glow engine has already been removed

Fly Baby Motor and ESC installation

The ESC is situated along with the motor in the cowl area of the model. This is a good place for the ESC - it keeps it away from the RC system, and this position also makes it easy to ensure adequate cooling.

Power system
Stuart contacted 4-Max for his power system, and selected a 380 Kv motor, 6S 5,000 mAh LiPo and a 16 x 9 propeller. With this combination the motor draws 45 Amps, equating to just over 1,000 Watts of power. This is roughly equivalent in power to the previous 4-stroke (4-cycle) glow engine. As an i.c. model it weighed 13½ lbs, but in electric format it's lighter, tipping the scales at just 11 lbs.

Power loading
The power loading is just under 100 Watts per lb, which is a generous figure for a scale light aircraft. The battery provides flights of around 15-20 minutes, indicating that the model spends most of its time throttled well back - if one can use such a term for an electric motor!

Electric RC model battery tray

Stuart added this angled battery tray. Velcro helps to secure the flight battery in position. I have no doubt that Stuart has also included a strap or a similarly secure method of holding the battery in place on the finished model. This is wise because ‘G’ loads and turbulence, plus vibration during grass take offs and landings on even fairly smooth ground can exert surprisingly strong forces on a model and its battery.

Electric RC model battery tray

Another view of the angled battery tray to accommodate the 6S pack. The tray can just be seen through the short stringers of the uncovered nose area. The tray is installed at an angle, and the battery is installed through the cockpit opening.

Fly Baby model pilot

The Fly Baby’s pilot, patiently waiting in his detailed cockpit.

Fly Baby large electric RC model

The Fly Baby in action. The designer of the full size machine, Pete Bowers, was an aeromodeller in his youth.

Kv, propeller rpm and pitch speed
Stuart’s choice of a 380 Kv motor (i.e. a design rpm per volt of 380) means that the on a 6S battery, which has a nominal voltage of around 22 Volts, the no-load rpm would be around 8,300 rpm (22 x 380 = 8,300). In practice, with the load of a propeller, the rpm will be about 15% less, giving an actual prop rpm around 7,100.

This means that the pitch speed of the 16 x 9 prop, which is the forward speed it would achieve if there were no airframe drag, is around 63 mph (data taken from a table in the Gibbs Guide to Power Systems). I estimate that the model probably flies around 40 - 45 mph. When choosing a power system, for best efficiency I usually suggest that the pitch speed should be somewhere in the region of about 25 % higher than the model’s maximum level flight speed. In this case then, a pitch speed of around 50 – 56 mph would be appropriate, not far off the 63 mph just mentioned. The model flies very well and has long flight times so that makes the power system a successful one!

Choosing props is never an exact science; the best we can do is make a sensible estimate of what is likely to work well, and see how the chosen prop works out, perhaps also trying out a couple of alternatives.

Click here for more information on props and Electric Power Systems

Fly baby electric RC model landing

Stuart Warne’s fine electric powered Bowers Fly Baby on final approach to land.

Fly baby RC model landing flare

About to make a very gentle flare. This shot makes it easy to see how the bracing wires do their job of supporting the wings.

Fly Baby electric RC model

Lovely! Stuart has made a find job of refurbishing this model and converting it to electric power. Stuart’s rendition of this aircraft has all the character of the full sized aircraft.

Electric motor and ESC in nose

The ESC is situated along with the motor in the cowl area of the model. This is a good place for the ESC; it keeps it away from the RC electrics, and this position makes it easy to ensure adequate cooling.

Fly Baby large electric RC model

The Fly Baby has elegant proportions, and what looks like a close to scale sized propeller.

Stuart Warne and Fly baby

Stuart is justifiably looking well pleased with his fine Fly Baby.

Flying the Fly Baby
The model has plenty of power, but Stuart told me that he had a problem with the model’s aileron response, saying that it would not turn easily. This made me suspect that the problem may be adverse yaw, which occurs when the down going aileron produces an increase in drag, such that the model has a tendency to yaw in the opposite direction to the intended direction of turn. An effective solution for this is to adjust the aileron travel so that the down going aileron has a reduced travel compared to the up going one. This is known as differential aileron and it works because the additional drag caused by the down going aileron is reduced.

Adverse yaw and affects full size aircraft as well, and differential aileron is particularly useful with high aspect ratio wings, such as found on gliders and certain light aircraft (model and full size) such as Piper Cubs. Setting up ailerons in this way carries an additional bonus in that tip stalling becomes less likely on approach, making for safer flying. I almost always set my own models up with differential aileron travel.

Fly Baby power system data
6S 5,000mAh LiPo
  RPM (approx) Voltage Current
Pitch speed
Full throttle 7,100 22.2 45 1,000 n/a
Cruising flight TBA TBA TBA TBA n/a


Anyway, I questioned Stuart about the way the model was set up, and he told me that the ailerons had an equal throw in both directions. I suggested that he change the travel so that the down going aileron had perhaps half the travel of the up going surface. He duly made this change, and reported that it did indeed help, but that the model also required rudder input to make turns. This was no surprise to me; plenty of high aspect ratio aircraft need to be turned using a combination of rudder and aileron. Indeed, some aircraft need a lot more rudder than aileron to enter a turn, and Stuart later told me this was indeed the case with his Fly Baby. He later said “I’ve found now that if I use the rudder to enter into every turn it ‘holds’ the turn in a much more controlled fashion. In fact now I find that I turn mostly with the rudder, only using the ailerons when needed”

Thank for sharing your lovely model here, Stuart.


Installing battery in large RC model

The Fly Baby’s battery is fitted through the cockpit aperture. The pilot is removable for this purpose.

pilot in electric RC model

Stuart has given his pilot a neat instrument panel. Full sized homebuilt aircraft are typically fitted with a fairly sparse collection of instrumentation.

RC model bracing wires

These bracing wires are called landing wires, as their purpose is to support the weight of the wing after landing. Interesting details like this really bring the models to life and give it character.

RC model landing wires

This is the other end of one of the wires seen in the photo just to the left. This photo shows how the landing wires join the wing, using a simple metal bracket.

electric RC model in flight

The FlyBaby makes a low pass for the camera, bathed in summer sunlight.

Fly Baby electric RC model in flight

Another shot of the Fly Baby in its natural element. In my view, this is one of the prettiest homebuilt aircraft ever designed.


Fly Baby technical data
Span 2,160 mm 85 inches
Length 1,420 mm 56 inches
Flying weight
4,990 g 11 lb 0 oz
(176 oz)
Wing Area 0.74 sq m 1,150 sq in
Wing Loading 00 g/dm 22 oz/sq ft
Battery 6S 5,000mAh LiPo
Motor PPO5065-380 7 turn
Propeller 16x9
Max Power 1,000 W
Power Loading (max power) 59 W/kg 130 W/lb
Power Loading (average power) TBA
Control functions Ailerons, elevator, rudder & throttle.

Note: All the power system themes touched on here such as power loading, pitch speed and much more are explained clearly in the 3-part series on Electric Power Systems

Click here for more information on Electric Power Systems

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