Which engine type would allow a propeller aircraft to operate silently?
closed as off-topic by a CVn♦, Vincent, L.Dutch♦, sphennings, Azuaron Jul 20 '17 at 18:16
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Electric engines can be very quiet, but it's not the engine, it's propellers that would give away the most noise. So that would be the most challenging part. Check out Silent Drone
As Alexander noted in his answer, the engine itself is the smaller part of your problem. A spinning propeller or tiltrotor will be noisy.
- While they are not exactly a propeller aircraft, consider what is known about stealth helicopters used by the US. In addition to radar stealth, they have sound baffling measures.
- A propeller aircraft may be designed with a good glide ratio and turn the engine off on the final approach. Think of something like a motor glider. This will reduce the payload and make vertical takeoff and landing impossible, of course, but the landing distance may be low. Did you ever watch the movie Escape from New York?