ray foley

Hi Blake:

There are two design philosophies regarding epower. The first is low voltage - high current, the second is high voltage - low current. The first scenario is aimed at high performance, the second scenario is aimed at long duration.

So the higher you go in voltage, the lower will be the current for a given power output. All batteries are charge storage devices which have a specific maximum storage capacity. So the lower the average current for a SS flight regimen the longer will be the flight time. The motors within reason don't care what voltage the battery is when it is being chopped for part throttle operation. So if you can limit the maximum part throttle duty cycle so that you don't overspeed the motor and break things, then you can go higher in voltage for the given capacity of the cells and attain longer flight times.

example: Assume 1 Kilowatt SS power requirement for the airframe, motor, and prop (that is a 20lb plane and 50 watts per lb power loading). If you use 10s lipo and need 30 amps at about 50% duty cycle to maintain best l/d cruise speed, then, if you have 15s lipo you can expect 20 amps at about 33% duty cycle to attain the same average power. Hence your battery lasts 50% longer at that SS flight condition. You will have to limit the maximum percentage duty cycle and hence RPM to whatever is necessary for reasonable takeoff and climb rate but that is pretty easy all in all, and of course never use full power settings.

So if you go to 20s lipo you would need 15 amp average current draw and even lower %duty cycle from the ESC to the motor. Longer flight time results.

There are ESCs which are good for 12s lipos and prototypes which may be available on loan or evaluation from CC with up to 20s capability.

if two hours is the goal, and 20s lipo and 15 amps is the SS average draw, then 30 amphours capacity is required for continuous operation. That is a big battery pack

If the assumed power requirement is higher than really needed, adjust the calculations and capacity accordingly. However a 20 lb plane at 50 watts per pound (1 kwpower) is fairly low power loading. The airplane will take off a hard runway or closely clipped grass (fairway quality) at this loading. Do not expect aerobatics but it will fly in a scale like manner. Use of aileron to rudder coupling will be necessary and watch the bank angle during turns (20* or less initially).

You must absolutely insure that the ESC, motor, and batteries are properly cooled during operation, adequate air inlet and outlets as well as processor type cooling fans may be necessary. Duct work for moving the air is probably necessary.

Interesting project though

ciao -rjf

ps: I have seen a twenty pound electric glider plane maintain SS altitude at a mere 500 watts power. Takeoff power was around 850 watts. It was a fairly long takeoff run. YMMV -r