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Comment by inamberclad | Hacker News Reader

Everyone is talking energy density but nobody is talking about Urban Air Mobility.

Electric is the name of the game for a VTOL plane that will take you from SFO to downtown or Santa Cruz. They don't have to have all the performance in the world, they just need to have enough performance to do their job.

Also, pilots will appreciate the operating costs and simplicity of these aircraft. Student pilots will love a plane that costs $10/hr instead of $100/hr in the Bay Area. 90 minutes of flight time (1 hour lesson + 30 minute VFR 'fuel' reserve) is all it needs.

Neighbors will appreciate higher torque motors that turn modern props at 1500 RPM instead of 2200 for the noise reduction.

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> nobody is talking about Urban Air Mobility.

Because flying cars have always been just behind fusion.

Five years away, my friend. Five years. I know it's true because I've been reading the same figure since about 1972, and much earlier if you count old copies of Popular Science I bought at garage sales.

Except there are only a few rooftops in cities where you could land such a plane, and that real estate is very expensive. It doesn't scale.

It's not the scale that I'd be worried about. I see enough idiots in land-based vehicles, last thing we want is a load of air-based vehicles flying around. Learners trying to hover over your house and youths joy-flying.

And let's not neglect that those rooftops are going to be plastered in solar panels anyway...

How much of a prop plane's noise is from its propeller vs its engine?

Depends on the size of the propeller. The main issue is the tip of the propeller breaking the sound barrier. This limits the RPM. Bigger propellers mean more noise at lower RPM. Smaller propellers enable higher RPM.

So the noise profile of a plane with smaller, faster propellers and engines is going to be very different from a big noisy helicopter where you can clearly hear the engine over the already substantial noise of the very loud propeller. Faster also means the noise is of a higher frequency and carries less far.

Comparatively the engine noise for electrical engines is not going to be a factor. Think of the noise level of your vacuum cleaner; most of which is in fact the spinning blade inside hitting a few thousand rpm; 3-5K is pretty common for vacuum cleaners. Dysons apparently go way beyond that. A typical Cessna would max out at around 2700 RPM and be cruising at around 2200.

This varies extremely widely depending on engine selection and propeller design. Urban air mobility vehicles are also very likely to be loudest during vertical take-off and landing, which is not comparable to a conventional small aircraft.

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