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0 pointsmarvin3y ago0 comments

I don't understand how developing tech for energy recovery makes sense. Why not just idle and descend at a reasonable speed for a reasonable L/D, the way practically all airlines do for practically all routes today? This is energy recovery; turning your altitude into distance traversed.

Hard to believe that putting a vortex-generating turbine and a generator into the airstream will improve efficiency against that.

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hedgehog3y ago· 2 in thread

I don't think that's what the Raytheon prototype is doing. It sounds like they're capturing some of the mechanical energy from the engine during descent and putting that into the battery for later use. Hybrid cars do that, when the driver requests an amount of power too far from the most efficient band they make up the difference by charging/discharging the battery. Knowing the target usage lets the engineers make more aggressive tradeoffs to optimize efficiency for that band.

fomine33y ago

Hybrid cars' energy regeneration make sense because we must stop/slow down for many reason like crossroad. GP says it's not the case for airplanes. Possibly is military use exception?

hedgehog3y ago

It's a common misconception that regenerative braking is the only way hybrid cars charge their batteries. The other common situation is when the driver requests less output from the engine than it can produce efficiently. The engine has a minimum operating speed (say 1000 RPM) and at that speed requires a certain amount of energy just to keep it turning (friction, pumping, etc), in turn making it inefficient at low loads (say 20HP). It turns out that round-tripping energy through the battery is efficient enough to make it worthwhile keep running the engine at close to peak efficiency (say 50HP), send the driver's requested power to the wheels, and store the rest in the battery. Later if the battery is getting full and the driver is requesting little power to the wheels the car can stop the engine and switch to electric until the battery is back to the ideal (around 50%). Knowing this is how the drivetrain behaves the engineers are free aggressively optimize the engine for efficiency only at the speed/torque sweet spot. It's not very intuitive but this chart and Quora article go into more detail:

https://qph.cf2.quoracdn.net/main-qimg-ffde81e7feb628b80088a...

https://qr.ae/pGjufl

I think the Raytheon engineers did something similar. You can't turn off the engine during flight so during landing (when the pilot doesn't need so much thrust) they keep running the engine at the sweet spot and store the extra energy in the battery for later use (takeoff). Then optimize the engine design for that sweet spot. There's no braking/deceleration, it's just a way to keep the engine running at close to peak efficiency the whole flight.

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