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Comment by antattack | Hacker News Reader

Do longer/larger prop blades translate to better efficacy?

I wonder if we would be better off with array of smaller electric motors rather than pursuing 1:1 replacement of ICE engines.

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Depends on flight speeds and prop rotation (tip speed specifically), but the propellers have for a long time had a 90+% efficiency already. Ideally, a long and narrow one-bladed propeller would have best performance, somewhat similar to sailplane's wing. For practical reasons (ease of balancing and limited diameter) we use two or more shorter blades, even if it causes minor efficiency loss.

A compounding concern is that of a propeller tip speed - when it approaches speed of sound, the physics of airflow become complex and it's very hard to maintain efficiency - and near impossible to maintain reasonable noise levels. This necessitates keeping blades of high speed propellers rather short. The Tu-114 was exceptionally loud for a plane, both due to propeller speed concerns, and also due to contra-rotating propellers inherently causing increased turbulence.

There is a good four-part series on propeller theory and practice:

https://www.youtube.com/watch?v=SgoPPg8oVt8&list=PLD2EcpzcvT...

If all else is equal (in reality it isn’t), you always get more efficiency by increasing blade length.

You want to maximise the momentum of the air your moving in any unit time for a fixed energy cost. If you double blade area you double both momentum and energy, if you halve the blade speed you halve the momentum and quarter the energy, so if you quadrupled blade length while halving the speed you get double the force for the same power.

This is of course a gross oversimplification, but it is why the human-powered helicopter a while back had such massive blades: https://en.wikipedia.org/wiki/AeroVelo_Atlas

all of those options have been studied for years; how many motors, what they weigh, where they are mounted, blades on the prop, rpm on the prop, any gear reduction. there is a room of aircraft designers that will show you why the current design is the "sweet spot", and the technical reasons that drove them there.

my point being this was all analyzed ahead of time, in modeling. no-one just said, "lets go with 4 foot props"..

Electric motors are more compact, need less cooling, work well at all altitudes, have likely more mounting options, can spin fast, offer different power curve, probably easier to service and diagnose.

I think there's enough of differentiating factors that could move the "sweet spot" when compared to ICE motors.

This NASA project seems to be pursuing that line if thinking

https://www.nasa.gov/specials/X57/

I think lots of little motors are the approach lilium is taking. https://lilium.com/ I’m a firmware engineer not an aviation engineer so I have no idea what the efficiency would be.

the original article shows ten electric engines on their planned plane.

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