Are fully electric helicopters suitable for military operations?

Question

I read this question about diesel-electric (DE) helicopters, which sparked the following question:

They are more silent, and electric motors are lighter than combustion engines. Would the only drawback be battery weight?

@CGCampbell Two of them: the transmission won't be necessary either. — Koyovis, Dec 27 '17 at 21:36
I'd be very interested in that electric motor which is lighter than a combustion engine (which in helicopters is very often a gas turbine). Where can I get one? — Peter Kämpf, Feb 25 '24 at 07:54

John Dallman · Answer 1 · 2024-02-26T14:55:32.990

Battery weight is a huge negative. Liquid hydrocarbon fuels have energy densities of about 45MJ/Kg, and the significant advantage that as you use them, their weight stops needing to be transported.

A lithium-air battery is theoretically capable of getting close to the energy density of hydrocarbon fuels, but the best that's been achieved so far in the lab is about 6MJ/Kg. The energy density of lithium-ion batteries, which are the best that's deployable at present. are more like 1MJ/kg, at the cell level, or rather worse at the pack level. If you want to drop used batteries, your operating costs go way up.

And while electric motors might be lighter than piston engines, they are heavier than gas turbines, which are the dominant form of power for helicopters, especially military models.

An electric helicopter could be built, but its flight duration would be very short. Just like quadcopter drones.

Also, the question starts with a wrong proposition: Electric engines are heavier than comparable combustion engines; in that case gas turbines. — Peter Kämpf, Feb 25 '24 at 07:56

score 3 · Accepted Answer · edited Feb 26 '24 at 16:33

Fully electric helicopters would be much more silent indeed: turbines are incredibly noisy, and the gearbox is another source of noise that would not be present since electric motors don't require one. Use a many bladed rotor with swept tips and high aspect ratio blades, and rotor noise would be reduced as well.

They would also have a short range and a small payload capacity, since a lot of it is used for lugging batteries around. Weight is a serious issue for aircraft in general and helicopters in particular: vertical flight can use very little aerodynamic L/D leverage effect. In vertical flight, thrust = weight.

A hybrid helicopter would reduce battery size: a turbine plus generator provides electricity for the electric motor and charges the battery. When within range of the target, the turbine is shut down and the helicopter continues forward flight and landing on the batteries. Extra weight compared to conventional would be the electric motor + generator + batteries, weight saving = transmission. Eject the spent batteries on the way home for increased range.

From the perspective of someone on the ground, the engine & gearbox don't seem to contribute a significant amount of noise. It's almost all the distinctive "thwop thwop thwop" of the rotor blades. Also, in vertical flight, thrust > weight. I once had an early toy electric helicopter which would fly ok in near sea level San Jose, but just sink at my ~5000 ft home. — jamesqf, Dec 28 '17 at 03:05

Are fully electric helicopters suitable for military operations?

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