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Old 24th Aug 2012, 03:37
  #84 (permalink)  
Join Date: Jun 2009
Location: florida
Age: 76
Posts: 1,061
FBW interference with "natural" law

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Well, I shall comment. I know that Doze will bring up operational requirements and such, but I maintain I can contribute to the discussion.

It comes down to the control law implementation and then the natural stability of the jet.

Ours was negative static stability until about 0.95 mach. So the FBW system kept the pointy end forward. This is not the case with the 'bus. The thing has positive static stability unless the fuel transfer system goes awry. But HAL tries to provide a neutral speed stability ( no regard for AoA) and also corrects the basic gee command for pitch attitude. So the BEA comment is correct, with the exception that it should mention that the 'bus has inherent longitudinal static stability. In other words, go to "direct" law and the thing flies like most planes we have all flown ( not the Viper, which would be impossible without HAL).

Without some sort of stick pressure corresponding to the commanded gee as the jet slows down, then it becomes unclear as to what is happening. FOR THE ONE THOUSANDTH TIME....AoA is very important. AoA is what keeps the plane flying. So our primitive FBW system ( for the benefit of Doze, heh heh) had a neat AoA limit that kept you from stalling unless......... If you climbed at a steep attitude and reduced power, you could reach a speed where those air molecules and full nose down horizontal tail positions were not sufficient to prevent entering the stall. A true "deep stall", it was, due to our cee gee and pitch moment at 40 degrees AoA or so. Does this scenario sound familiar?

I disagree with the BEA terminology with respect to stability. The basic aero of the 'bus is fine. It's the FBW interference that makes the jet seem like it has neutral speed stability and static longitudinal stability. Go to "direct law" and you have what all planes have had since Wilbur and Orville flew the first one. Could be "touchy", but the sucker would try to achieve the trimmed AoA, and AoA is what planes use to provide those liftie doofers.

I do not recommend that the 'bus go from full laws to "direct law" willy nilly. But seems to me that a few practice sessions in the real jet in "direct law" would provide a lotta confidence to the crews and also show them the inherent characteristics of the jet.
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