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Fly Baby - A Glow to Electric Conversion
by Stuart Warne
Article by Andrew Gibbs
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.
The Fly baby before conversion
to electric power. The glow engine has already been
removed
|
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!
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. |
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. |
The Fly Baby’s pilot,
patiently waiting in his detailed cockpit. |
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
Stuart Warne’s fine
electric powered Bowers Fly Baby on final approach
to land. |
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. |
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. |
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. |
The Fly Baby has elegant
proportions, and what looks like a close to scale
sized propeller. |
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
(Amps)
|
Power
(Watts)
|
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.
The Fly Baby’s battery
is fitted through the cockpit aperture. The pilot
is removable for this purpose. |
Stuart has given his pilot
a neat instrument panel. Full sized homebuilt aircraft
are typically fitted with a fairly sparse collection
of instrumentation. |
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. |
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. |
The FlyBaby makes a low
pass for the camera, bathed in summer sunlight. |
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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