Airbus E-Fan X ? 2

[rb1957]

found at Flight Global ...
opinions on this ?

To me, I don't see the sense in having an engine to generate electricity to power a motor to drive a fan.
More steps, more power conversions, less efficiency.

Unless the point is to develop the motor side of the technology to have something ready when the electrical side develops.

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[IRstuff]

The only reason that it makes remote sense is the same reason electric motors powered by gas engines kind of make sense; such a configuration divorces maximum torque from power efficiency

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[WKTaylor]

Airbus E-Fan X? or the BAE146 EF test-bed?

The BAE146 test-bed will have 3-TFEs [for safety and EE-power gen] + 1-EF [mounted on an IB strut] for RDTE.
Airbus E-Fan X First Flight to Occur in 2020

http://www.aviationtoday.com/2017/11/28/airbus-e-fan-x-first-flight-occur-2020/

Comment, RE recent discussion of ambient-air ducted fan power [thread

http://www.eng-tips.com/viewthread.cfm?qid=432780

].

GregLocock mentions thrust limits due to MACH chocking. Turbine engines run at such high internal temperatures that the speed of sound dramatically increases within the heated exit-airflow so that MACH chock occurs at much higher stream velocities... which allows much higher mass-flow-rate per engine size... hence thrust... at the cost of burned fuel. This is where electric fan-power challenge resides.

Perhaps 'prop-fan' [Unducted-Fan] technology may be the key???

https://en.wikipedia.org/wiki/General_Electric_GE36

Regards, Wil Taylor

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[rb1957]

yes, I meant the BAe146 test-bed, as we know little of the "real" proposal.

The concept is, as I understand it, to have a turboprop (without the prop) ... a turbine driving a shaft to drive a generator to power a motor to drive a fan. Sure you may be able to gain something, but I can't see it being more than you'll lose with the power conversions.

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[GregLocock]

I was idly thinking about turboprops the other day, when that ducked fan (I think that ought to be the new word for them) question came up. Given the the figure of merit for them does not seem to exceed 0.6, yet a marine propeller can be 0.8, why on earth do people bother with ducked fans rather than propellers? OK there are practical considerations, obviously an unducted prop has to operate in chilly rarefied air and is in danger of exceeding the speed of sound at the tip, even for reasonably subsonic aircraft speeds. A ducked fan can have a shaped inlet to decelerate the intake air before it accelerates again in the thrust stage.

And then I went and did a google search and found that the above statement may be wrong. Sadly the best papers are on NTRS and dtic.mil which now appears to be inaccessible.

here's a nice graph contradicting the above, unfortunately it is CFD not real. t/h is the tip clearance

But in real tests my point remains

http://4hv.org/e107_files/public/1386100085_2431_FT0_1353043533_2431_ft145838_duct_pdf.pdf

figure 7 in particular., here it is

That's a bit off topic.

So having got that out of my rapidly disappearing hair, I agree that they are getting ready for a breakthrough in battery technology, but given that the efficiency of the hot part of a gas turbine is probably quite poor, particularly when cruising at lower speeds, ahha, maybe that's it. An aircraft needs a lot of thrust to take off and climb, but for optimum cruising efficiency you'd actually prefer to fly more slowly than M 0.92, but for a conventional jet there is no point in doing so since the engine efficiency drops off, so what you gain aerodynamically you lose in the engine. Other engine types, such as diesels, are quite happy to operate at part load.

I have thought of another way of achieving the same thing - switch an engine off when cruising. You'd need some way of blanking off the intake etc of the redundant engine, but that is not beyond the wit of man.

Cheers

Greg Locock


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[rb1957]

why ducked fans ? ... like winglets ... they're new and "sexy".

with CFD you can show anything good, or bad ...

I thought jet engines were working at only a %age of power during cruise, whereas props are max'd out. It's one of the trade-offs between them ... maintenance ... jets are working hard for only a portion of the flight (t/o and landing), turboprops are max'd out for most of the flight.

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[GregLocock]

One slight proviso. The efficiency of 60% I quoted is from a NASA windtunnel test, the only one I've seen. However you'll find many papers quoting up to 80% for a DF, based on CFD, and a chat with a real DF engineer suggests that this may be reasonable. The NASA DF had a parallel bore for the duct, which is a surprisingly poor choice.

Cheers

Greg Locock


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[rb1957]

I think a major reason for ducted fans is reduced noise, but there are many plus/minus features in the design ... I think Boeing claim/show that the added drag and weight of the nacelle out-weighs the propulsive advantages. Boeing tried an open rotor in the 90s (7E7), found (I thnk) a problem with the interaction of the wing wake on the prop disc ... and the idea died.

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