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divemaster08
Topic Author
Posts: 197
Joined: Fri Jul 18, 2008 1:45 pm

APU "Electric" aircraft

Wed Apr 07, 2021 1:34 pm

Hello all.

I was listening to the FR24 podcast (March 26th 2021) in which they have an interview with the CEO of the last Fokker 50 operator in Europe. The discussion about how long they can operate the aircraft came up and it was stated that there really isnt a true replacement for the F50s. They talked about the lack of ATR 42s/Q300s/Saab 2000s being too old to be replacement aircraft. They also mentioned how Wideroe were looking at electric aircraft as a future aircraft, but talked about its limitations also with current infrastructure and technology.


With that all in mind, I can only think that for the F50s, Q300s, ATRs and all other 30-50 seat turbo props, there needs to be a new aircraft developed for this market. I cannot see anything really in the future coming for this area, as Embraer's new Turboprop looks more like a E175 replacement and may not be suitable for unpaved runways (low wing profile).

It also got me questioning the following......

Railways have used for a while the diesel-electric system, where a diesel generator creates electricity for the electric motors, rather than just for the drivetrain. Is there the possibility of an "APU" style of electric aircraft ever been thought of?

Example. Instead of 2 Turboprops, there are 2 APUs there which drive 1-2 generators per side, which then provide the power for electric motors to power the aircraft. During turnarounds its possible to just need to run 1/2 of one APU to keep power to the aircraft. It wouldn't need to then have heavy batteries or recharge stations at every stop, and could just be topped with Jet fuel at the airports.

Would the APUs be able to be smaller than the size of the Turboprops?
Would it also then be more fuel efficient than current Turboprops?
Is it too heavy/complicated a system compared to just having a Turboprop and all its gearing?
Maybe there isnt an electric motor available to fit into a 50 seat size aircraft thats viable?
Would it be more fuel efficient?

Would be interested in hearing any responses if any.

Cheers,
 
GalaxyFlyer
Posts: 8584
Joined: Fri Jan 01, 2016 4:44 am

Re: APU "Electric" aircraft

Wed Apr 07, 2021 1:50 pm

There’s nowhere near enough power in an APU to power a Fokker 50. That and APUs aren’t really very efficient, fuel burn per power developed.
 
mxaxai
Posts: 2811
Joined: Sat Jun 18, 2016 7:29 am

Re: APU "Electric" aircraft

Wed Apr 07, 2021 2:01 pm

It's a concept that's heavily studied in current (academic) research. Compare the aircraft on the lower left in this study with a single large "APU" in the tailcone: https://horizon-magazine.eu/article/how ... ravel.html
Image
 
N965UW
Posts: 264
Joined: Wed Jul 29, 2020 11:31 pm

Re: APU "Electric" aircraft

Wed Apr 07, 2021 4:08 pm

As GalaxyFlyer says, a present-day APU would not provide enough power. But this could conceivably work with a purpose-built "APU" or auxiliary engine that's designed to efficiently provide large amounts of electrical power. If the weight of that engine plus fuel and electric motors isn't too prohibitive, this could be viable.
 
VSMUT
Posts: 5497
Joined: Mon Aug 08, 2016 11:40 am

Re: APU "Electric" aircraft

Wed Apr 07, 2021 5:41 pm

That was part of the concept behind the Airbus E-Fan X demonstrator, which was cancelled a year ago:
https://en.wikipedia.org/wiki/Airbus_E-Fan_X

divemaster08 wrote:
The discussion about how long they can operate the aircraft came up and it was stated that there really isnt a true replacement for the F50s. They talked about the lack of ATR 42s/Q300s/Saab 2000s being too old to be replacement aircraft. They also mentioned how Wideroe were looking at electric aircraft as a future aircraft, but talked about its limitations also with current infrastructure and technology.


Let me translate that for you:

"We are too cheap to buy new aircraft from ATR, and can't find any used aircraft on the market for a price we want to pay"...

He isn't going to buy a hybrid or electric aircraft, even if it existed.
 
divemaster08
Topic Author
Posts: 197
Joined: Fri Jul 18, 2008 1:45 pm

Re: APU "Electric" aircraft

Wed Apr 07, 2021 6:21 pm

Thanks for responses.

While I said APU, I wasn't really meaning APU size as we know them. It was more its just a turbine that power generators instead that could maybe be more efficient.

It seems that this idea had some momentum, but not really being pursued.
 
gloom
Posts: 580
Joined: Thu Jun 30, 2016 4:24 pm

Re: APU "Electric" aircraft

Thu Apr 08, 2021 8:37 pm

divemaster08 wrote:
It seems that this idea had some momentum, but not really being pursued.


For other transports that use this kind of solution (hybrid cars, diesel-electric locos), there's a benefit of running high-low profile. You accelerate, then cruise with low to no power. This means you can adjust all electric system to store/release power and reduce power, since you deliver stored power and energy generator, reducing requirements for engine power/size. And it's also more efficient, than engine/brakes only.

In a plane, though, you don't just cruise, you need to deliver power constantly (more or less). You could use some takeoff energy (very much like 50s/60s rocket takeoff, where it was used to accelerate out of runway), but battery to do that is quite heavy, and also the power level is quite high. In flight, until descend phase, plane needs quite a power, and extra step in power transfer (from engine to prop now, from engine to electric, to prop then) reduces effectivity. That is in my understanding the reason, why it is so difficult to employ turbine-electric-prop on an airplane.

Cheers,
Adam
 
kemala
Posts: 5
Joined: Mon Apr 05, 2021 8:07 pm

Re: APU "Electric" aircraft

Thu Apr 08, 2021 8:54 pm

divemaster08 wrote:
Railways have used for a while the diesel-electric system, where a diesel generator creates electricity for the electric motors, rather than just for the drivetrain. Is there the possibility of an "APU" style of electric aircraft ever been thought of?

Example. Instead of 2 Turboprops, there are 2 APUs there which drive 1-2 generators per side, which then provide the power for electric motors to power the aircraft. During turnarounds its possible to just need to run 1/2 of one APU to keep power to the aircraft. It wouldn't need to then have heavy batteries or recharge stations at every stop, and could just be topped with Jet fuel at the airports.

First law of thermodynamics explains why this can't be possible. I won't explain the law here but simply I can say this. You need a rotating fan blade to produce thrust so you will burn fuel, produce electricity, using electricity, will create thrust; instead of burning fuel and creating thrust. According to law, you will lose some energy on every step as heat caused by friction. So, you can never acquire enough thrust by that conversion to carry a huge load. It will never be efficient.
 
dtw2hyd
Posts: 9100
Joined: Wed Jan 09, 2013 12:11 pm

Re: APU "Electric" aircraft

Thu Apr 08, 2021 10:13 pm

VSMUT already stated E-Fan-X.
https://metroairways.net/inside-metro/i ... ry-update/
The dropped concept had 2MW generator inside, that would work for a test bed but not for production. I thought Siemens was quiet positive about weight thrust ratios, not sure what happened.

UK's EAG has a slightly matured concept. inboard engine nacelles house generators, outboard are just electric motors. This has potential to scale up.
https://www.flightglobal.com/air-transp ... 41.article
 
ZaphodHarkonnen
Posts: 1160
Joined: Sun Jan 04, 2015 10:20 am

Re: APU "Electric" aircraft

Mon Apr 12, 2021 2:35 am

kemala wrote:
divemaster08 wrote:
Railways have used for a while the diesel-electric system, where a diesel generator creates electricity for the electric motors, rather than just for the drivetrain. Is there the possibility of an "APU" style of electric aircraft ever been thought of?

Example. Instead of 2 Turboprops, there are 2 APUs there which drive 1-2 generators per side, which then provide the power for electric motors to power the aircraft. During turnarounds its possible to just need to run 1/2 of one APU to keep power to the aircraft. It wouldn't need to then have heavy batteries or recharge stations at every stop, and could just be topped with Jet fuel at the airports.

First law of thermodynamics explains why this can't be possible. I won't explain the law here but simply I can say this. You need a rotating fan blade to produce thrust so you will burn fuel, produce electricity, using electricity, will create thrust; instead of burning fuel and creating thrust. According to law, you will lose some energy on every step as heat caused by friction. So, you can never acquire enough thrust by that conversion to carry a huge load. It will never be efficient.


This is the core reason it won't happen in production. If you're going to burn fuel to spin a fan or prop then it's always going to be more efficient to make the engine as big as possible and be directly coupled. Now an APU feeding a electric battery system might be useful for a boost phase but at that point you now have two systems that have to interact near perfectly.

Most likely what we'll see is as mentioned in the podcast. Shorter range to begin with for regional/commuter aircraft that are doing a few hops a day and can charge up from a high capacity source like a national grid. Then you might see small jetliner equivalents like the B737/A320 size for the 2-3 hour hops. I think it unlikely we'll find a replacement for the true long haul any time soon. Liquid fuels just offer too many advantages that electric batteries can't beat.
 
RJMAZ
Posts: 2495
Joined: Sat Jul 09, 2016 2:54 am

Re: APU "Electric" aircraft

Mon Apr 12, 2021 9:40 pm

I think we will first see a hybrid electric aircraft.

The APU in the 787 provides approximately 500kw of electrical power with only 250kg of weight.

Electric motors in the wings are connected to batteries in the wings. The small APU in the tail charges the battery, but is sized to a fraction of the power needed for takeoff. This allows for multiple modes.

On a short flight up to 1 hour the aircraft can fly fully electric. On a short flight the APU is designed for emergency if the aircraft has to hold or divert the APU can be started. The electricity from the APU allows the aircraft to loitor or divert at a slow speed.

On a long flight the APU is running the entire time. This extra electricity coming into the battery reduces the rate the battery depletes in flight. This could double the range up to 2-3 hours.

This would cover 90+% of flights operated today. The aircraft would be big and heavy. Think of a C-130 Hercules wing fitted with a small ATR72 fuselage.

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