David--

Here's a little bit of math for helping you wrap your head around this. The post is long, I apologize, but in my experience it's best to have the right numbers in hand before squaring off against the laws of physics.

Efficiency: for a well chosen system it's going to be in the 60-70% range. For example, 94% motor * 98% ESC * 75% propeller = 70%. Note that this doesn't include the battery, since defining efficiency for the battery is somewhat dependent on charging cycles and that's not really important in the air.

Heat: heat is the hardest part of this conversion. Gas engines have it easy, they're big and can safely get very hot. Motors really shouldn't go past 80C for continuous operation, and that's hard when you're in Texas and the ground OAT is 35C. From experience with high-power motors in aviation, dumping heat is so much harder than anyone thinks, and there are very few strategies to radically improve heat transfer when it's not doing very well. Most solutions are bandaids in wait of a white-sheet redesign. DON'T NEGLECT MOTOR COOLING.

Power: it's always wise to specify which power is being referred to, as there are 3 notably different definitions of power in an electric aircraft: Electrical power, shaft power, and propeller output power.

- Propeller output is the easiest to calculate, it's simply force * airspeed (to make sure you don't make unit conversion mistakes and wind up crashing into Mars, do everything in metric. Trust me on this one.)
- Shaft power is the propeller power divided by the props's efficiency at that RPM and airspeed. You calculate this by torque * rotational speed (again, metric for everything). You know RPM, and you get torque from the propeller manufacturer.
- Electrical input power is the shaft power divided by the motor's efficiency. This is the figure you'll see when you're shopping for motors, so be *very* careful not to confuse it with shaft power, which for some motors can be 10-20% less.

Drag: an earlier poster is exactly right, drag is simply the aircraft's weight divided by the L/D. However, don't neglect to consider the drag you care about is when the motor is operational. On my AC-5M, which is around 35:1 with the engine retracted, it is only 20:1 with the engine deployed. (The engine really does almost double the airplane's drag!) So in my case, at 660lbs MTOW, that's 33lbs. Converting to standard units, we get 20kg * 9.8 = 196N. At 25m/s (~49kts), that's 25*196 = 4.9kW of drag for level flight.

Climb power: Climb power is NOT level flight plus climb rate. This is because an inclined plane requires less lift (imagine an aerobatic plane doing a prop hang. The wings are producing 0 lift in this case). Formally-- I can provide the analysis to show this upon request--, it's F = W*(sin(theta)+alpha*cos(theta)), where theta is your climb angle. For a glider climbing at 2m/s (~400fpm) it's around 5 degrees. In my case, that means 380N of force and 9.5kW of propeller output power. For that, I will need a motor between 13kW and 18kW, depending on system efficiency.

Hope this helps!

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Hank-- The ASW-24E would be a poor choice, IMHO. It's paper empty weight is 275kg, a full 88kg more than my AC-5M's true weight. 50% heavier means 50% more everything, and costly mistakes are all too easy to make at these power levels. (Everything else about it is a great plane, though.)

I would especially like to generally caution against considering 120V systems. Unless you know what you are doing and are extremely comfortable around deadly voltages, don't go higher than 48V. We ran 1kV at our drone company, from experience I can say it takes a lot more than a backyard engineering project to do anything 60V safely. Even 60V is dicey, but at least it's not very likely to kill you, only somewhat likely.

The three advantages to 100V voltage are cheaper motor controllers, lower cabling weight, and reduced radio interference. For a one-off prototype the motor controller cost is high no matter what you do. The actual cable weight in a glider is negligible, and can be halved by moving to aluminum, so there are better, safer ways to keep resistive losses low. Lastly, we don't need to worry about radio interference when working on experimental, non-consumer, NORAD aircraft.

Please note that motor efficiency is, surprisingly, not dependent on motor voltage. I can go into this on a separate post, but I've already gone on really long as it is...

On Sunday, February 7, 2021 at 6:30:16 PM UTC-5, wrote:
David,

He already gave you the power consumption from the electric side. That is already taking the efficiency in count.
Rule of thumb, 50% overall efficiency.
So, if you are modifying something similar to the 24, you will need component rates for that power needs (always over rate).
The batteries won't modify all the others component. If in the future there are better cells, you can change the packs only and save weight.

Regards

Emir