Today I flew my big 1/4 scale Tiger Moth.
Because it's a big airplane, I use a separate battery for the on board radio.
When I was assembling the plane and getting it ready for its first flight, I didn't have a voltage regulator nor a separate BEC, so I used what I had on had; a 40 amp E-Flite speed control with BEC and a 3-cell lipo battery.
Basically, I used an electric speed control and battery as a separate power supply for the radio, not connected to any motor.
Can anyone think of a downside or a problem with this approach?
What your basically doing is running a separate BEC. You could have even used a 2S battery as it would have been less voltage to regulate. An alternative is to use a LiFe pack (2S) as it can be run straight to the rx and servos leaving one less thing to fail.
This separation is a good idea with such a large plane. If the esc ever fried or the flight battery you would still have control. This is all even more important with spectrums troubles with voltage fluctuations.
As for using the BEC of the esc and not using the remainder I see no troubles except most esc's have limited output power. Such a large plane I would like to have at least 10amps of power available.
No down-side that I can see as long as you insulate the motor wires of course. That's an expensive BEC. :)