cornutt
Topic Author
Posts: 333
Joined: Sun Jan 20, 2013 6:57 am

Limits To Current High-bypass Engine Designs?

Sat Feb 16, 2013 3:54 pm

How much farther do you think manufacturers can go with the current big-fan high bypass engine designs? Will someone be able to build an engine of this design pattern that will reach 150,000 lbs thrust? I wonder how much bigger fans can get... at some point, fan blade tip speed has to become a problem, not to mention fitting a giant-fan engine under the wings without the nacelle dragging on the ground.

I've wondered about the possibility of building an engine that has two or three fans side by side. The idea is that one fan is driven by the engine core in the conventional fashion, and then there are one or two additional fans in parallel that are either driven by mechanical linkage off of the low-pressure spool, or via a low-pressure turbine on the additional fan's shaft that is driven via an exhaust bleed from the core. Is this at all practical, or a crazy idea? Has anyone done any work on such an engine?
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Limits To Current High-bypass Engine Designs?

Sun Feb 17, 2013 10:43 am

Quoting cornutt (Thread starter):
How much farther do you think manufacturers can go with the current big-fan high bypass engine designs? Will someone be able to build an engine of this design pattern that will reach 150,000 lbs thrust?

Sure. If they can figure out how to make money from such a beast. FWIW, the GE-90-115B has been run at 127,900lbs.

Quoting cornutt (Thread starter):
I wonder how much bigger fans can get... at some point, fan blade tip speed has to become a problem,

No, the bigger the fan, the slower it rotates. Tip speed is approximately constant somewhere around Mach 1.2. One problem with slower fan rotation is the increasing mismatch between the speed at which LP turbines are efficient and the fan speed. So a GTF design might be particularly attractive at such power levels.

Quoting cornutt (Thread starter):
not to mention fitting a giant-fan engine under the wings without the nacelle dragging on the ground.

Given what Boeing has accomplished to stuff large diameter engines under the 737 wing, I'd say there was a lot of slack. But on you'd not be putting that big an engine on any current aircraft, and a new aircraft you would design with enough ground clearance.

Quoting cornutt (Thread starter):
I've wondered about the possibility of building an engine that has two or three fans side by side. The idea is that one fan is driven by the engine core in the conventional fashion, and then there are one or two additional fans in parallel that are either driven by mechanical linkage off of the low-pressure spool, or via a low-pressure turbine on the additional fan's shaft that is driven via an exhaust bleed from the core. Is this at all practical, or a crazy idea? Has anyone done any work on such an engine?

Technically there is no conceptual problem driving several fans. Just consider your engine a turboshaft, hang the appropriate transmission in front of the thing, and away you go. Consider the lift fan on the STOVL F-35B. The lift fan is mounted just behind the cockpit, and is driven off a long shaft back to the engine - it's exactly a second fan hung off the engine, although it's mounted at right angles to the application you're proposing. Similarly the rotors on helicopters (or V-22) usually are interconnected in such a way that you could consider them as two "fans" driven of one engine (of cluster of engines).

OTOH, if you had *any* choice, you'd avoid such a mechanical nightmare. Just look how hard Pratt has been working to get the (rather simpler) GTF ready. Just make the fan a bit bigger if you can. A pair of fans would be less efficient aerodynamically too - even if only in wetted area.

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