All of my $$ planes have two power sources for the servos. Some have
two battery packs, and some have a BEC and a battery pack. They have a
right and left wiring harness and these harnesses both plug into the RX.
Each wiring harness has an input connector for power so that the power
need not go thru the RX to get to the servos, unless there one of the
power sources fails. A separate switch harness is used for each side.
I have not done a Reliability Analysis on this arrangement but I believe
it to quite good due to the reduced number of components between the
servos and the power source. It has already saved one $$ plane.
On 31 August 03 my 4.23m HKM ASW-27 was saved by having a redundant
power supply and a Picolario telemetry system. It was the first $$ plan
saved by the redundant power system. It was the third $$ plane saved by
a Picolario telemetry system. The other two had Volz Micromax servo
failures (thin metal brushes worn and welding to the commutator. The
Picolario warned me of a voltage drop due to the impending failure of
the servos. I've since replaced all with Micromax X motors which have
carbon brushes. I digress.
My HKM ASW-27 has a nose-mounted electric motor. It was the first
flight with a Kontronik Fun 500 motor with 5.2:1 gearbox (later replaced
with a Hacker), 14 x 7 folding prop, and 16 x RC2400 pack. The power
circuit from the motor to speed controller to battery is straight and
the wires are just long enough.
An ACT "Powerbooster 24" BEC was spliced into the drive battery side of
the speed controller to provide power to the Rx and servos. A 4-cell
500AR backup battery was connected to the Rx and servos through a
separate circuit.
During the second flight with this motor/prop/battery combo, a couple of
climbs to altitude went normally. A few radio glitches hit the Rx
but they were intermittent and did not seem to be life-threatening.
Glitches cause the telemetry to make a unique sound and can cause a
momentary low-voltage warning (did I mention the benefits of telemetry?).
Following the minor glitches, the telemetry indicated that the voltage
on the Rx was continuing down, where I would expect it to stay constant
due to the BEC. I brought the plane lower for observation, then tried
to run the motor, but it was wouldnt run. Landed safely.
Post-landing, I found that the motor had unscrewed from the gearbox,
twisting the motor and controller wires, and had UNPLUGGING the drive
battery. This disabled the motor and the BEC, so the Rx and servos were
powered only by the 500mah backup battery. I can email photos if anyone
wants them.
The Kontronik Fun 500 did not have mechanical lock on the gearbox
thread. With a normal folding tractor prop, the torque of the motor
tends to unscrew the gearbox. The torque which tends to unscrew the
motor from the gearbox is the sum of the motor shaft output torque and
the reaction caused by accelerating the commutator. The gearbox was
relatively dry, so maybe the lack of grease caused extra vibration which
helped to loosen it. The motor/gearbox was noisier than I would expect.
Lessons Learned: Lock threaded gearboxes. Picolario telemetry and
independently-wired backup batteries saves airplanes
Regards,
Jimmy
Douglas, Brent wrote:
Has anyone here ever set up a redundant battery and switch for a scale
ship? Was it worth the trouble?
I'm working out my servo / rx / battery tray, and I'm torn between
duplicating everything or just using a "Y" on two batteries.
The extreme would be having two RXs, splitting the plane, either left /
right, maybe one for the wings the other for everything else. Without a
split elevator, that's probably not worth pursuing (I probably couldn't
land it if if I lost the elevator).
In the end, I'm just looking for reliable power for a lot of servos.
Any experiences you've had would be much appreciated.
Thanks,
Brent
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