Originally Posted by
Ultralights
There is no point in comparing energy density of Batteries compared to liquid fuels,(in cars anyway) as electric motors are 5+ times more efficient,
The O-320 can deliver 75% power (120hp, about 90kW, or 90kJ/s) at 8.5 GPH (23kg/hr, 6.42g/s) according to Piper. 100LL Avgas gives 44MJ/kg, so the theoretical power available in this case is 282kJ/s. Efficiency of the O-320 at a 75% cruise is therefore just about 32%, and an extremely good electric motor might get close to 3x that efficiency.
I think that comparing the energy density does make a lot of sense. The avgas engine might be 1/3 as efficient as an electric motor, but for each kilogram of fuel (at 44MJ/kg for avgas, 900kJ/kg or 250Wh/kg for LiPo batteries) you get almost fifty times as much energy. Even taking engine efficiency into account, you're getting about 16 times as much useful energy from avgas as from batteries, per kilogram of fuel.
For small fuel loads, the relatively tiny weight of the electric motor helps a lot. However, as the amount of fuel rises and the weight of the engine becomes less significant, the advantages of avgas become unbeatable.
jack11111 - I think the idea would be that you pull the batteries out and plug those in to charge, then stick your spare set of batteries in the plane for the next flight.
It might seem expensive to have a second set of batteries, but if that means that each set is only doing three cycles per day rather than six then your battery life has just risen from about 100 days to about 200 days.
At 60 minutes to charge after a 60 minute flight, two sets should give you more-or-less continuous usage.
mcoates - a quick question: how's the heater? Plenty of places get cold enough in winter that a heater is desired, even just for circuits, and conventional electric systems (as a direct result of their efficiency) tend not to produce much waste heat.