I stupidly imagined it might be quiet

No mention of what will be supplying power? A 560kW generator is not tiny, even if it is a gas turbine, and batteries will only go so long... The article is incomplete without this basic information.

This one has potential.

I wonder if prop pitch still needs to be continuously variable, since the power delivery curve of the motor is so flat over rpm - anyone know?

Des M'Dromic: This shows how much noise comes from the prop itself.

Quiet props probably are a tradeoff for max thrust. The Navy has been designing quiet props for submarines for decades. A Japanese company was barred for some years from contracting with the DOD because it failed to protect such designs. Interior fans in cars are an example of complete disregard for that. Turboprop aircraft generally fly with constant-speed jet engines, around 10k rpm, and they are loud. 2K rpm should be relatively quiet; no dB data in the story. E.g., the C-130 which has been around since the 1950s, on up to the really big cargo jobs and the commercial turbos. They all have variable-pitch props. It is very mature technology. The big question for this aircraft will be battery weight, which is not in the story. Aerial refueling is SOP for military jets. Haven't heard of aerial re-charging yet (other than with solar panels), but supercapacitors could make that a reality.

McDesign: Variable pitch propellers (constant speed) are more to allow optimisation of the thrust at different airspeeds, not because of the motor torque / power curve. - note that a propeller designed to turn at 2000rpm won't provide much thrust at all at 500..

Aircraft need to start at Zero airspeed (with high thrust reaction) and then fly "efficiently" at fairly high speeds (higher power - in high performance aircraft). All the while the propeller needs to be turning at a high speed - tip speed needs to be limited to avoid compressive effects at high subsonic speeds (sometimes transonic around the blade tips, and this is not desirable.)

Fixed pitch aircraft are relegated to fairly low top-end airspeed, OR have very poor takeoff and climb performance. The variable pitch prop gives the best of both worlds. High thrust at low speeds, good climb at moderate speeds and efficient cruise at "high" speed.

Des M'Dromic: Anyone who has played with large electric RC aircraft will concur that the propeller makes a lot of the noise in flight.
- have a look at any number of videos online, Electric flight is NOT silent. (electric-ducted-fans make an absolute racket).
The noise a Propeller makes is a trade-off with the allowable diameter of the propeller and the work needed to be done. Large slowly turning propellers with deep (wide) blades waste less energy with noise, but are difficult to package (need a longer ladder to enter and exit aircraft, higher hangers etc), therefore are not widely used in light aircraft unless they need to absorb a lot of power - and then they aren't going to be turning slowly - tip velocity approaching high subsonic speeds.

The new CO2 Lithium battery will make this a practical solution for an electric general aviation aircraft. Not bad things are looking up

MD - thanks for that great explanation. I guess this is the reason for the "it's more efficient to accelerate a lot of a air a little than a little air a lot" idea, coupled with the need for high thrust to take off and climb?

@jinpa For ariel refuelling of electric planes. Battery drones could fly to and attach to the plane. Releasing after completing the energy transfer.