[skykooler]

The interesting thing about rotary-wing craft is that they actually work sort of independent of altitude (as long as everything stays subsonic.) As air pressure decreases, lift at a given rotor RPM decreases - but the drag on the rotor also decreases, so the RPM increases and it ends up providing the same amount of thrust. The reason helicopters on Earth haven't flown very high is not because of the rotors, but rather the engines, which require more air than is present at high altitude. For electric motors, that is not a problem.

[nanomonkey]

I wonder if a sterling engine would work well for high altitude rotary-wing craft. The higher you go up the cooler the air, causing the sterling engine to be more efficient.

[skykooler]

The big problem with stirling engines is the power-to-weight ratio; they are much heavier than internal combustion engines for the same power output, too heavy for any practical use in a flying machine.

[nanomonkey]

I'm not sure that this is inherent to Stirling engines, or just the past engineering implementations. For one thing, unlike internal combustion engines, there is no need to contain an explosion (otto cycle) or flame front (diesel) so the pistons and cylinder walls can be made of lighter materials and don't need lubricant filters. In fact self lubricating graphite pistons are often used modern Stirling engines. Secondly a higher efficiency implies that more power is extracted from the fuel which indicates that the combined fuel tank and engine can be smaller and that more power will be put to doing work. Lastly, a Stirling engine usually falls short in torque, and that can be compensated for with a hybrid system that utilizes electric motors for instantaneous changes speed, where added torque is required.