PPRuNe Forums - View Single Post - Modern Transport Aircraft Stability Question
Old 20th Jul 2009, 22:03
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john_tullamarine
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I'd say simple. I adjust pitch up or down to maintain my assigned altitude, and adjust the throttle to maintain my assigned speed.

If we introduce thrust changes, we start to complicate the basics a little due to pitching moments as indicated by Genghis. However, again I would ask you to have a coffee and contemplate just how you intend to exercise any sort of control with a very low gradient. From an on speed position, any inadvertent control input (or turbulence, whatever) will send you off on a speed variation. With little or no tactile feedback (the human system is not terribly good at figuring displacement but is reasonably fine with force) the aircraft speed and flightpath will wander hither and thither unless you are constantly directing cognitive effort to the flying task. If the gradient reverses, the workload goes up dramatically.

I go back to my analogy of trying to sit on a big beach ball out in a moderate sea ... a bit beyond my meagre gymnastic capabilities, I fear. With a decent stick force gradient, it becomes a bit like putting down a keel on a yacht.

I'm also aware that this was a problem with the prototype F-16 which had no movement at-all.

If I recall correctly from a course many years ago, the F16 initial problem was put down to having too low a stick break out force. Once this was addressed the twitchiness resolved itself. A bit like trying to hold your hand out steadily with or without some external support.

I'm unfamiliar with the specific accident

They got back on the ground fine as far as I am aware .. just a bit less than impressed with the situation. My understanding was that the main problem was associated with the reversed force gradient and conditioned responses. Genghis almost certainly would know the TP concerned.

Contrary to what Mr Tullamarine may assert

My comment was intended to be tongue in cheek .. knowing Genghis' background, I defer to his far greater knowledge and experience in this matter.
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