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In this blog I read about the Quiet Flight Initiative, which - if I understand correctly - will be quieter than a comparable carbon fueled plane at least in part because it is electric. It's an engineering challenge associated with Embry-Riddle Aeronautical University.

What aspects of the plane design - when switching from combustion engine to electric motor - will help reduce noise? I had thought that most external aircraft noise heard on the ground (in this case, National Parks) is associated with the propellor, and so noise is closely linked to performance, not the particular method of torque generation.

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  • $\begingroup$ @ymb1 Considering that batteries are roughly 50 to 100X heavier than fuel for an equivalent amount of useful work, it may be tough to do that. I don't see why it being electric would make that easier, except perhaps re-start reliability. $\endgroup$ – uhoh Sep 24 '16 at 4:23
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    $\begingroup$ Check this en.wikipedia.org/wiki/Motor_glider#Retractable_propeller — Link also lists some electric gliders. $\endgroup$ – ymb1 Sep 24 '16 at 4:23
  • $\begingroup$ @ymb1 I see! I noticed for electric power: "Greatly reduced noise, allowing take-off from locations where other powered aircraft are not permitted" also says "[citation needed]" but it looks like there may indeed be an answer there. But I still don't understand why the electrification reduces noise there either - doesn't it come from the propellors? $\endgroup$ – uhoh Sep 24 '16 at 4:59
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You are right, propeller noise is dominant in case of GA aircraft, but not all propellers make the same noise. Much depends on blade loading and tip speed - making the propeller larger and letting it turn more slowly will improve efficiency and reduce noise. This has worked very convincingly for model aircraft over the last 30 years. GA engines are normally ungeared, so their propeller speed and diameter are the same. Switching to a geared IC engine like a Rotax 914 not only makes the engine lighter but also the propeller less noisy. Switching to an electric motor, which can produce high torque at low speed, will allow to exploit this effect to the fullest.

Other sources of noise will profit from electrification, too. The exhaust noise will be completely avoided, and airframe vibrations will be greatly reduced. Radiator noise will fall away, too. It is a small source of noise, but every bit counts.

What will be left is the noise of the slower propeller(s) and aerodynamic noise. Now landing gear retraction would be the next big step in noise reduction.

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