I wonder though if you could do "regenerative braking" in electric planes, i.e. spin up a flywheel anytime you descend, and then tap that energy next time you want to ascend. Doesn't really make sense for planes that cruise at altitude, but could make sense for hobby craft.

This very same airplane mentioned in the top article does in fact capture energy from descent, and it's only viable because the application is assumed to be training, where the point of the airplane is to do a lot of takeoffs and landings.

Another reference: https://newsline.kitplanes.com/2017/07/26/alpha-electro-an-e...

I think there's plenty of viability for recovering energy during a descent. Any aircraft that has spoilers for use during descent could instead use the energy extraction from the prop windmilling to reduce the need for spoilers and thus get some "free" energy during descent. It isn't going to be a ton of energy, but it might be enough to get a "free" taxi to the gate/hangar/tie-down out of the deal.

It's not quite that direct. Many aicraft use flaps to boost lift during descent (better low-speed control) and steepen the fuselage pitch relative to the direction of flight (better visibility of the ground). Putting that energy into the propeller doesn't help with either of those. Also, some of the speed brakes are deployed so that the engine can stay warm by producing power instead of being "shock cooled". Otherwise stress builds in the engine and results in the engine wearing out earlier than it otherwise would if it were consistently cooled slowly. So this also doesn't directly translate; the pilot of the electric plane simply pulls back all the power and is coasting, which for many airplanes (not gliders!) results in a satisfactory sink rate and no need for additional braking. The pilot could engage more braking, but then the sink rate would be "emergency" class.

Flaps and speed brakes are separate. You wouldn't be using regenerative braking for the same purposes as speed brakes/spoilers. You use spoilers specifically when you want to kill speed, or more often when you want to quickly lose altitude. Sucking potential (and kinetic) energy up and putting it in the battery would accomplish a lot of the same goals.

You're not likely to have shock cooling issues in an electrically powered aircraft. Shock cooling is a piston aircraft thing, and is mostly an issue on turbocharged aircraft. You're not worried about shock-cooling the engine, you're worried about shock-cooling the hot side of the turbocharger. So in the use-case of spoilers being used to keep the engine spooled up, you're right that it wouldn't be necessary here.

And as far as pulling the throttles to idle and more-or-less gliding during the descent, that's the ideal, but in the real world spoilers are used to drop more quickly all the time for non-emergency reasons. If you have favorable winds up high, you might choose to stay up high until absolutely necessary and then quickly descend to make better time. In trainer class aircraft you don't typically have spoilers to help this, so you end up cross controlling the aircraft to increase the sink rate. If I could have a big ol windmill in front of me in a 172 to do that, it'd be pretty darn nice.

The aircraft in the article uses the propeller as a windmill during descents to charge the battery.

It's actually precisely the opposit. Altitude is your battery.

When power is surplus, climb to your service ceiling. When not, descend. Utilise battery when at minimum altitude.

This was the technique used by Solar Impulse 2 on its round-the-world (though not nonstop) flight. At a cruse speed of 70 kph (43 mph).

https://en.wikipedia.org/wiki/Solar_Impulse

The problem is that you'd have to descend rather steeply for any sort of "braking" to work. In practice, you're unlikely to get more range by descending steeply and then using the power gained from that to fly level than you would get by descending at best glide angle.

The slower your plane move, the more difference those would make.

As people already commented, it wouldn't make much difference for the usual models, but you can design a plane that benefits from it. It may have some good uses (agriculture comes to mind), but probably won't be good for transportation.

Not for substantial cargo or human transport, the areal energy density is simply too low.

For surveillance, comms, and very small-scale payload delivery (possibly drugs, far more likely munitions), ultralight drones with battery + solar could offer loiter / time-in-air and / or modest speed longer-distance capabilities.

It probably helps, since solar impulse works with that. But if it makes a difference for heavy planes .. good question.

Was about to say this:

https://en.wikipedia.org/wiki/Solar_Impulse

:)

Not in any meaningful way with just the sun's light hitting the panels.

On the other hands, if you were to beam a few hundred kilowatts by (carefully!!) aiming at the panels with a massive laser, you could power it this way, yes.

Bonus if you are doing it from a satellite: you can power a plane even across an ocean and don't need many ground stations. On the other hand you just built an orbital death ray so expect protests from, well, everybody.

It might. Wings have a lot of surface area, so it makes more sense than putting solar on cars. Also, if a plane isn't being constantly used it could be useful to have it charge while parked (which is also a good argument for putting solar on cars even if it doesn't noticeably improve range).

It would also provide a charging option if you have to land at an airstrip that doesn't have an electrical outlet handy.

People are experimenting with this https://www.facebook.com/groups/138721826332626/permalink/66...

They would not meaningfully contribute.

Quick back of the envelope calculation:

Wing area of a Cessna 172 (representative GA aircraft) is 16 m^2.

Best-case insolation on a sunny day with no clouds at high noon is around 1000 W/m^2.

Best commercially available solar cells are the triple-junction cells used on commercial satellites, at about 30% efficient. They are horrendously expensive (around $50-100k per square meter), but we will ignore economics for now.

16 m^2 * 1000 W m^2 * 0.3 = 4.8 kW.

Powerplant for a Cessna 173 is a 160 hp piston engine, or 120 kW. So, we're about an order of magnitude and a half off. The cells also aren't massless, so they will add weight and reduce range... you'd need to do a cost/benefit there.

There are a couple solar UAVs in existence (example: https://en.wikipedia.org/wiki/Qinetiq_Zephyr) which use super lightweight materials and have very large wing areas to support solar cells. Even then, they are really on the hairy edge of where physics works in your favor.

Does it use the full 160hp while cruising though? I’d imagine most of that peak power is for take off.

Most of it is still used while cruising. 100hp+ depending on cruise speed. The excess power for climb is very small, which is why weight/loading is so critical in aviation.

> The excess power for climb is very small, which is why weight/loading is so critical in aviation.

This is one of the reasons in favor hybrid electric aircraft. The marginal hp to weigh ratio of an electric motor is about 3 hp/lb or ~5.5 kw/kg. And partial load efficiency is high. Means you potentially have a lot more hp available on take off.

For commecial passenger aircraft hybrid turbofans wouldn't have spooling up lag like straight turbofans. Turbine engines take seconds to spool up. This big big problem with jet aircraft during landing. If you hit wind shear and/or need to go around you need to apply more power and sometimes the lag is fatal.

Aviation is an expensive market, so in some future world where a 75% efficiency of solar cells was available, it would be possible. And maybe even economically feasible at some point. Not any time soon though.

And the great thing about flying is that it's pretty much always a sunny day when you're in cruise.