Electric / hybrid aircraft propulsion (NOT cars)

One hour of circuit does not use much energy, much less than one hour of cruise.

Huh?? Ever topped off the thanks of an a/c after an hour’s circuit bashing and then after an hour’s cruise flight? Big difference – cruise uses a lot less.

But that’s not really the point. I think these airplanes could be very useful indeed. Here’s why:

a) noise: the noise signature of any airfield with a significant amount of flight training would drop dramatically; of course, this would also mean, that the ‘normal’ a/c stand out much more, so will appear ‘louder’ than they really are and thus attract more opposition. Law of unintended consequences…..

b) cost: from an FTO (or ATO or whatever they are called now) perspective, the cost per hour of training should go down considerably

As for the different ‘sound and feel’, that shouldn’t be too hard. You could easily replicate the sounds of an IC engine by feeding the appropriate sounds into the headset of the stude. As for the feel, no idea, but my guess would be that it’s not too different from a C152.

Let’s assume you put a cruise propeller on, designed to go fast. Now your takeoff roll and climb will suffer. How would you mitigate that without adding a variable prop? You’d overpower it, so that even with a cruise prop, you’d get good takeoff performance. The penalty for overpowering in an electric world is very small.

You over-torque it, not overpower it. Assuming a fixed pitch propeller, when the load curve of prop torque versus rpm is steeper due to lower airspeed (high blade AoA) you can deliver the same propulsion power by lowering rpm and increasing torque. That means the motor is over sized relative to a fixed speed motor, and also (typically) that the motor drive has to be sized larger to deliver more current. The energy storage setup is unchanged, but the overall system weight increases to accommodate the lack of a variable pitch propeller. Not all that much different a trade-off than with an IC engine.

Peak power for circuits is 100 % or some sizable fraction of 100% depending on the type of propeller (or similar as described above), but average power is typically low relative to cruise, because average speed is close to best L/D. So the energy (fuel burn) per hour is lower from first principles. A Luscombe for instance will burn 3.5 gph in doing touch and goes, versus 4.5 gph in cruise. Other aircraft may pay a penalty, but its because their engines and fuel systems were optimized for steady state operation, and are highly non-optimum for circuits..

Circuits done by others in my C172M were 35l/h,

It’s been ages since I topped up after doing circuits. But the Pawnee (O540) towing gliders used about 50 l/h. Rather exactly 300 ft/l towing height at average. I guess the C-172 is a very inefficient climber

An IO540 (250HP) with the RSA-5AD1 fuel servo, adjusted correctly within its specified +/-2% band, burns 87 litres/hr at max rated power, ISA conditions.

The only way circuits are more efficient than cruise flight is if you spend 50% of the time taxing and waiting for traffic on the ground. This is the only way I can see that an 85kW motor will give a 1.5hrs with a 17kWh battery. That for me is stretching the numbers.

But again I’m in no way an expert and might be missing something…

I was referring to the RSA5AD1 fuel servo which is set to (don’t recall the precise value) 23 USG/hr at a given airflow corresponding to max rated power in the IO540-C4 engine. I discussed this in detail with the specialist in the USA who built my current fuel servo. It’s not an illegal setting. I do actually burn just over 23 USG/hr during initial climb, 2575 rpm.

Anyway, my point is that there is no way a 540 engine could be climbing especially while towing a glider and burning just 13.2 USG/hr (50L/hr) – unless the climb is well throttled back, to maybe 70% of max rated power, which would be strange for glider towing.

Different engine – TCM. The fuel flow spec on mine is a lot tighter. I asked the guy to set the servo for something like 24.5 but he refused, and gave me the specs on it. He’s a top specialist so he should know.