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Old 1st Jan 2011, 11:30
  #47 (permalink)  
Join Date: Nov 2007
Location: United Kingdom
Age: 66
Posts: 710
I too am having difficulty in getting my head around this whole "inlet drag" scenario; however, I "think" I'm getting closer to the crux of the matter.

As my previous NASA clip suggests, a jet engine requires to slow down the gasses as they pass through the compressor; add fuel to the compressed gasses, big bang, and then accelerate those gasses, via the turbine, out through the back end as thrust. In a high bypass engine the fan (just like a propellor) accelerates the air through the bypass stage... as thrust.

In order to produce, say, 20 Klb of Net thrust, the engine needs to generate, say, 100 Klb Gross thrust, because 80% of that work is absorbed by the compressor. Now, here's the crux of the matter; and also the area where I find it difficult to assimilate; if you remove the thrust, you're left with the compressor absorbing all of the energy as inlet drag. Obviously, you cannot simply remove the thrust; however, you can (by use of the thrust reverser) redirect the thrust-line to anywhere but useful (in producing rearwards thrust). When you now increase engine power (with reversers deployed) there is no useful rearwards thrust, but that the compressor is now absorbing more work and bringing the inlet towards a choked condition (but not fully; due to the operation of compressor bleed valves/offload valves); hence the increase in inlet drag.

I'm not totally convinced... but I'm open to further discussion.

The "Barn-Door" principle is much more appealing to me; however, that wall of "reverse thrust air" isn't fixed to the aeroplane, but it does (perhaps) go a long way in explaining why reverse thrust is much more effective at higher speed.

Whatever the answer might be; thrust reversers are here to stay and... they do work as published!

Happy New Year to you all.

TheChitterneFlyer is offline