Hybrid gas turbine-electric aircraft, is it feasible?

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SmithW6079

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Greetings everyone,

I've been mulling over an idea lately and thought I'd check with the board's collected wisdom. Is it feasible to build an aircraft which uses a turbine to power a generator, which would in turn drive an electric motor to propel the aircraft (through a prop or ducted fan)? I was particularly thinking of VTOL applications, would the weight saved by electric transmission be an improvement on mechanical transmission systems?

Would throttle response be a problem? If you added a battery pack to improve throttle response, would its weight and those of the electric motors offset any possible savings from the transmission? I'm envisioning a tilt-rotor or tilt-wing aircraft, likely using ducted fans instead of prop rotors.

Any ideas, thoughts, and criticisms are welcome...
 
There is a big weight problem with it.

If you're looking to the marine turbines: a RR Trent based MT30 36MW Turbine has a weight of 22t. The generator weights 55t. And you need an electric motor (weighting around 50t).

You don't need this big amount of power, but I think you can scale it down:

A F-135 weights around 2t --> generator: 5t, motor: 5t. A mechanical power distribution system is much lighter.
 
SmithW6079 said:
Greetings everyone,

I've been mulling over an idea lately and thought I'd check with the board's collected wisdom. Is it feasible to build an aircraft which uses a turbine to power a generator, which would in turn drive an electric motor to propel the aircraft (through a prop or ducted fan)? I was particularly thinking of VTOL applications, would the weight saved by electric transmission be an improvement on mechanical transmission systems?

Would throttle response be a problem? If you added a battery pack to improve throttle response, would its weight and those of the electric motors offset any possible savings from the transmission? I'm envisioning a tilt-rotor or tilt-wing aircraft, likely using ducted fans instead of prop rotors.

Any ideas, thoughts, and criticisms are welcome...
Click to expand...

The lift fan of the F-35 and your average APU could be considered oblique applications of this, but for direct power, probably not. For a STOL application, you can just remove the electric motor period and directly connect the lift fan to the engine ala F-35, and for forward propulsion, once again you're better off just using the engine directly. It works in ground applications (ships, tanks and trains) because expelling hot gasses to incur a reaction force is an inefficient way to overcome rolling or sliding friction.
 
I must admit that at one point I thought that a X-22A type VTOL with electric drive to the duct fans would be a wonderful idea. Multiple paths for providing power to the ducts to overcome battle damage and needing all that messy cross-shafting. I asked a few rotorcraft engineers, they smiled, patted me on the head and told me that there would need to be some metallurgy breakthroughs to overcome the weight and potential heat issues. That was about ten years ago. Maybe I should ask again. :-\
 
Racer said:
There is a big weight problem with it.

If you're looking to the marine turbines: a RR Trent based MT30 36MW Turbine has a weight of 22t. The generator weights 55t. And you need an electric motor (weighting around 50t).

You don't need this big amount of power, but I think you can scale it down:

A F-135 weights around 2t --> generator: 5t, motor: 5t. A mechanical power distribution system is much lighter.
Click to expand...

Back in the 80s when they were working on SDI stuff they got a 20 MW generator down to 3000 lbs. (This was from the Wright Propulsion site)
 

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Some thoughts about electric driven liftfans were made in France, too, as I found
in Icare 1976(?). There were drawings to show the idea of a VTOL passenger a/c, using
quite a number of stowable electric liftfans. Not a project, of course, just a debatable
proposal !
 

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There's enough info about all electric aircraft around. Seems they focus on superconducting electric motors as what has been mentioned already conventional electric motors are usually just too heavy. Trouble is cooling ofcourse. Just one example:

http://www.sciencedaily.com/releases/2007/06/070619183236.htm
 
Aren't they playing around with a superconducting generator to power the ATL (advanced tactical laser) project? Though admittedly, that is using a whole C-130 for what is a very non-continuous duty cycle.
 
sferrin said:
Back in the 80s when they were working on SDI stuff they got a 20 MW generator down to 3000 lbs. (This was from the Wright Propulsion site)
Click to expand...

Interesting stuff. Incredible expensive though?
 
AeroVironment's Global Observer UAV has a central generating plant (either a microturbine/alternator or fuel cell) and multiple electrically powered propellers.
Without looking into this in too much detail, I've wondered from time to time whether turbine/electric drive would have a payoff. Would there be an advantage to a variable-speed, fixed-pitch propulsor? What about a light airplane with a regenerative microturbine aft of the cabin, driving ducted or open rotors? One of the problems with small turbine/prop designs is the high RPM of a small turbine and consequently heavy reduction gear.
For a UAV, too, you could slow to loiter speed and power and use the residual volts to drive the AESA.
 
Looking through the posts, the consensus seems to be that a hybrid electric aircraft is theoretically feasible and has even been built on a smaller scale (primarily UAV), but a V-22 sized aircraft is not possible with the current technology. It will be interesting to see what the future holds.
 
It looks as though other people are considering hybrid-electric propulsion for aircraft. Falx Air has two projects on their website, a Very Light Hybrid Transport (VLHT) and a small VTOL. Performance figures are posted but the site does not go into any details about the engines and neither aircraft has been built or tested yet. But given time (and more importantly, money)...



The official website is here:
http://www.falxair.com/

More photos can be found here:
http://www.gizmag.com/falx-air-hybrid-aircraft/9535/gallery/
http://www.gizmag.com/falx-to-debut-hybrid-electric-tilt-rotor-aircraft-with-inbuilt-solar-c/9234/gallery/
 

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The turbo-electric aircraft is quite an old idea, although I have severe doubts about
the feasibilty of the shown Giroz Hélicoplane, a kind of tilt-wing VTOL:
(from Science & Vie 1949)
 

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I spoke to Falx. They are bench testing a hybrid electric engine made by an unnamed company that is also under evaluation by several car manufacturers, they say. Once they have real performance data from bench testing - and have found 10 million pounds from somewhere - they plan to start work on a three-seat tiltrotor. The hybrid propulsion system does sound interesting.
 
Without getting into details, the feasibility of hybrid electric hinges on the specific power of the generators and motors. Those things are, as of today, still very heavy. I'm talking about state-of-the-art machines.
Maybe in ten-twenty years...
 
Hybrid cars work because the internal combustion engine of a car mostly works at low (but very variable) fractions of its full power. This makes the ICE inefficient. The car also frequently has to brake, which offers an opportunity to recover some of the energy.

An aircraft has neither. Its engine runs at a constant power and speed setting, so the engine can be optimized for that setting. Energy recovery would only happen at the end of a flight, so you'd have to carry (heavy) batteries that would only get one power cycle per flight.
So I don't really see the point of a hybrid aircraft.
 
Hobbes said:
... so you'd have to carry (heavy) batteries that would only get one power cycle per flight.
So I don't really see the point of a hybrid aircraft.
Click to expand...

I think, the discussion was just about turbo-electric VTOL aircraft without storage of electric energy
via a battery, So, the term "hybrid" is used in a different sense here, as nowadays for cars.
 
In that case the question is even simpler: Does the electric motor plus generator weigh more than a drive shaft and gearing?
 
I can answer that - YES.
Unless you start doing weird things and need multiple long segmented shafts and gearboxes.
Otherwise, it's hard to beat the power transmission efficiency of a shaft.
Think about the equivalent electric powertrain and its efficiency:


-gearbox efficiency to run generator at optimum speed
-generator efficiency
-power losses in wires
-power conditioning
-motor efficiency


I doubt you get better than 85%. OTOH a simple shaft plus gearbox is probably in the mid nineties.
 
Shaft based systems start to lose when faced with a distributed propulsion requirement, such as embedded spanwise propulsors in a BWB. Though to be fair, a similar arrangement could be achieved just by having more smaller engines, avoiding the shafting. During some reworking of NASA N+2/N+3 study results, a 767 class non-superconducting generator with electric propulsor combo was found to have a closing design, featuring a wing inboard span fitted with a thick wing with center embedded electric propulors.
 
The aircraft engine industry are taking small steps.
 
In some concepts high temp fuel cells like SOFC (> 1000° C) with electrical efficiencies > 60% act as combustor in a gas turbine (conventional combustion chamber is replaced by the SOFC) which could lead to a very high efficiency propulsion system.
 
If you increase the efficiency of the turbine you might better result but weight would still be a problem.
 
publiusr said:
I wonder if a turbo compound would have a hybrid comeback.
Click to expand...
Yes is could. See Honeywell electric turbo mounted in AMG cars. The electric turbo has a net zero balance when it comes to electricity and can be used to extend the built-in battery (car is an hybrid, ICE and EV).
The electric engine inside the turbo can speen the turbine wheel up to 200k rpm to suppress any discernable lag. And be used as a regenerator when engine rpm is decreased (exhaust gases speen the turbine that spin the turbo electric engine to generate recharge).

I expect a similar setup to be used in aerospace. Imagine a 21st century Baron with such turbo. Most of the cruise flight will generate electricity that could be stored for landing phases, either for VTOL (if you are really light) or decrease noise emissions.
 
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I would think another issue would be motor speed. You'd need to use DC motors (heavier than AC) to get a good variable speed. You can vary AC motors but it's really inefficient comparatively. Another limitation is obviously the same as with other prop engines, you are limited in top speed by aerodynamics of the propeller.
 
A megawatt motor is being looked at
techxplore.com

Megawatt electrical motor could help electrify aviation

Aviation's huge carbon footprint could shrink significantly with electrification. To date, however, only small all-electric planes have gotten off the ground. Their electric motors generate hundreds of kilowatts of power. To electrify larger, heavier jets, such as commercial airliners...
techxplore.com techxplore.com

As designed, the MIT electric motor and power electronics are each about the size of a checked suitcase weighing less than an adult passenger.

And gas turbines can now be quieter
techxplore.com

Engine treatment can slash jet noise

There's a song anyone who lives near an airport or directly under the flight path of incoming and departing jets daily wishes they could play: Simon and Garfunkel's "The Sound of Silence."
techxplore.com techxplore.com

But what is really going to help aviation is this:
phys.org

Superlubricity coating could reduce economic losses from friction and wear

Scientists at the Department of Energy's Oak Ridge National Laboratory have invented a coating that could dramatically reduce friction in common load-bearing systems with moving parts, from vehicle drive trains to wind and hydroelectric turbines. It reduces the friction of steel rubbing on steel...
phys.org phys.org
 

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