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Comment by mikepavone | Hacker News Reader

This and most recent articles on the demonstrator are frustratingly light on details. Found some more info on Wikipedia's article on hybrid electric aircraft [0] which cites an Aviation Week article [1] that is unfortunately behind a paywall

> One 2,150 hp (1,600 kW) PW121 turboprop will be replaced by a 1 MW (1,300 hp) gas turbine joined with an electric motor of the same rating, powered by off-the-shelf lithium-ion batteries for takeoff and climb. The turbine is used alone in cruise and drives the motor-generator to recharge the batteries in descent. The downsized engine operates at its optimum for 30% fuel savings over 200–250 nmi (370–460 km). Range is reduced from 1,000 to 600 nmi (1,900 to 1,100 km) due to the higher empty weight and 50% lower fuel capacity.

[0] https://en.wikipedia.org/wiki/Hybrid_electric_aircraft

[1] https://aviationweek.com/aerospace/utcs-dash-8-hybrid-electr...

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I've been disdaining the idea of hybrid aircraft for years, thinking there was no way to capture the energy of deceleration (as you can in a hybrid car). Here, though, the rationale is different; you're storing turbine power during descent for use at the next ascent.

I wonder if you could indeed capture the deceleration energy by having the air stream drive the prop and thus the motor/generator. Planes have multiple systems for wasting energy and slowing down (flaps, air brakes, even dropping the landing gear); perhaps this wasted energy could be put into the batteries?

Energy recovery is absolutely possible.

On a smaller scale, all jet airliners have a RAT (Ram Air Turbine), which is just a propeller that drops into the airstream if both engines fail. It provides hydraulic pressure and electrical power during an emergency.

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.

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.

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?

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.

When they handle the low hanging fruit of having an electric motor to propel the aircraft from the gate to the runway (bonus points for being able to pushback under their own power) it might become worth while.

Well, part of the reason you start them at the gate is also to test them and warm them up before you get to the most critical part of the flight where you absolutely need to be sure they're going to run: takeoff and initial climb. You don't want to be doing that procedure on the runway for operational reasons.

And some airplanes are already capable of pushing back under their own power. The reason we don't do that is not the plane -- it's the airport. The blast can damage the airport, ground equipment, and endanger people.

In context, a jet engine uses a significant percentage of cruise fuel consumption just idling on the ground. Many airports aircraft can taxi for well over 30 minutes just to reach the runway.

Some record setting flights have been conducted where the aircraft was brimmed on the runway and the manufacturers set minimum times for stabilising the engines before takeoff, circa 30s if I remember correctly.

Issues of blast damage go away if it's propelled by electric power. My understanding is one of the main reasons it isn't often used is that there is a real danger of tipping the aircraft over backwards if the wheel brakes are applied while reversing.

If you want to hybridise an aircraft then starting with electrifying the time spent on the ground is the low hanging fruit.

There's no ingestion risk if it's propelled by electric power, but there's still FOD risk. As I understand, gates in the US have to be specifically designated for powerbacks because of this risk. No matter the method of creating the thrust, if you can move a plane with it, you can blow some object into a person or the airport windows.

It certainly could be done but it would require changes to airports to accommodate it.

Sorry, it might not have been obvious, I meant by mechanically driving a wheel rather than projecting any kind of thrust.

Oh that makes sense. I was thinking in terms of the prop-driven flight discussed above.

Perhaps the reason they need to recharge in descent is less to save energy for the next flight, but more the need to have reasonably well charged batteries in case they need to perform a go-around. Rate of climb when the batteries are discharged is likely to be pretty low. So maybe they will need to operate the turbine at full power while descending to achieve this, whereas normally the engines on a turboprop would be nearer idle in the descent.

Most likely it's about running the turbine at a more efficient power setting, which would be a higher power setting.

> The turbine is used alone in cruise

If I understand this right, they're still driving the prop mechanically through a gearbox during cruise, rather than using turbo-electric transmission?

That's my understanding. Given that the turbine is used in parallel during takeoff/climb and drives the electric motor as a generator during descent (while presumably still driving the prop) there would need to be a mechanical linkage. Since the linkage is already there, you wouldn't save on weight by using a serial hybrid setup for cruise and the turbine is already sized to be running in an efficient power band for cruise flight.

Loss in range? What’s the point now?

30% fuel savings on short haul trips less than whatever 60% of the standard range would be.

These prop airplanes are used for flights nowhere near close to the airplane's maximum range (although most major airlines are retiring them).

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