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Comment by raquo | Hacker News Reader

Power-to-weight ratio. It's a major weakness of electric power with current technology, and a single engine with a big prop is more efficient than any other configuration.

Sometimes you see electric aircraft have multiple smaller engines and props, but that is mostly driven by structural or design concerns of their esoteric missions like needing to distribute the weight along a huge wing (Sunseeker) or needing to land in tight spots (various VTOL designs). Conventional planform electric aircraft don't have such extreme limitations so there is no point in sacrificing performance.

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Power to weight ratio's of electric motors are higher than piston engines. But the usable energy to weight ratio of batteries is 5-10 times worse than a gasoline tank.

I seen some stuff that indicates that you can use small props at the wing tips to reduce drag. Works by canceling out the wing tip vortexes. Hasn't been done for a lot of reasons but mostly because of structural concerns.

I think one prop is more efficient than two for the same thrust. Multi prop airplanes are potentially tricky to fly.

Given the fuel density problem it's interesting we haven't seen any hybrid designs that generate electricity with an ICE engine (like the range-extenders in some PE cars like the Volt or the i3). I assume the added weight of the engine would cancel out some of the benefit.

I think Airbus and others are poking at hybrid turbo fan designs. One reason is that high bypass turbo fans are hitting limits due to fan diameter. If you use electric motors to drive the fans you can have multiple fans per engines. Like a 2:4 configuration, 2 turbines feeding 4 fans. Some advantages to this.

1. Much higher bypass ratios. Maybe close to double.

2. Ability to shift power around to keep the aircraft thrust balanced in case of an engine failure.

If you have a battery then you also get.

3. Potentially much faster throttle response. The slow throttle response of turbine engines is a real issue that kills people.

4. Ability perhaps to take off and land with minimal turbine power. Which reduces noise.

Thing is all this stuff has only recently gotten cheap enough and commercial aircraft probably take 10-15 years to develop and roll out.

There are hybrid designs, and you can also do tricks like charging the batteries from running a prop backwards when descending... similar to regenerative braking in electric and hybrid cars.

You'll note that hybrid cars aren't exactly winning in the market.

There isn't much point to a hybrid gas/electric aircraft. Most of the value in cars is converting kinetic energy back to electricity when stopping. There's a bit from running the engine at peak efficiency as often as possible - if that produces an excess of energy you charge the battery for a while and then run in pure electric mode for a bit (engine off). But aircraft tend to run at high power all the time and don't stop much. Same thing with boats.

There are some (for the Octocopter from Workhorse)

http://workhorse.com/surefly

This is a good idea though, it gives you some flexibility in trading off range and payload. Probably maximum take off weight is the limiting factor here. But if it's just me, and I have 200 lbs of extra payload, why not load that up with 20 gallons of fuel and a tiny ~25hp generator.

Apparently these guys are doing wing tip propellers for their electric plane. They are expected to make a first flight later this year, if I recall correctly.

https://www.eviation.co/

I was under the impression wing tip vortices were an inevitable result of having air deflected downward under the wing. Do wing tip props just change the behavior near the center of rotation, or eliminate vortices completely?

I think you can look at the problem two ways.

One way is that the coefficient of lift drops as you approach the wing tips. The other is that energy used to generate/contained in the wing tip vortexes shows up as a decrease in efficiency.

A wing tip prop that rotates in the opposite direction cancels the vortexes. And the pressure produced by the prop increases the Cl of the upwash side of the wing. Negating the loss of Cl due to spill over at the wing tips.

This is from two papers I read one from 1967 and the other more recently. The result of looking at this thing.

https://www.designboom.com/technology/eviation-alice-electri...

I know that, for fixed winglets, there are a couple of (equivalent) ways of looking at them. The first way to look at them is that they resist the tendency of higher pressure air under the wing to leak around the wing tip to the lower pressure air on the top. The second way to look at them is that the winglets are flying in a circulating flow, and a portion of their lift is in the direction of travel of the airplane. (That one is trickier to see, for some.)

My educated guess is that tip propellers would operate counter to the vortex direction, so that their vorticity would tend to cancel the trailing vortices. Notionally, you could think of them as throwing air back under the wing, though that's just a heuristic I made up to help imagine the direction they'd spin.

Tl;dr: It's complicated!

This might be an amusing read (from 1968)

https://crgis.ndc.nasa.gov/crgis/images/5/51/Wingtip-mounted...

As well this

https://crgis.ndc.nasa.gov/crgis/images/e/e0/Vortex_Attenuat...

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