Originally Posted by
hawk37
Are we perhaps talking different definitions of stick force gradients? In post 4 I referred to "pitch force gradient" after which I apologized and in post 9 suggested I should use the term "stick force gradient", which I then clarified to mean "change in stick force with a change in airspeed". Other than that, I've been very careful to indicate the gradient is with respect to speed.
I see this from FAR 25..
"(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots"
Does my post 9 make sense now?
If not, then I offer the following:
Genghis, when I said thrust is reduced, I meant that the aircraft is to maintain level flight. I said this in para 1, but did not repeat it in para 2.
That was my understanding of your post, yes.
By level I meant maintain altitude, though I realize that I did not state that. And hence airspeed must decrease. I was trying to take a simple case.
Again, yes, my understanding also.
But as to why it is necessary (other than the certification requirements) to have a 1 lb per 6 knot stick force change I can't understand.
In order to give the pilot a resolution of changes in speed with stick force which is reasonably perceptible. An aeroplane without that sort of gradient is still flyable, but pilot workload to maintain fine speed control goes up, and ability to fly by attitude alone goes down.
Surely the aircraft would be very easy to fly without this stick force gradient with respect to speed (maintain altitude, very slowly increase pitch as the speed drops, then maintain this new attitude for the assigned altitude, as the speed settles in at about 240 kias).
The odds are that without a stick force gradient, you'd lose the trimmed airspeed with change in attitude and pilot workload to re-establish this would be significant. In "steady" flight the relationship between attitude and airspeed is fairly direct - so a lb/kn problem would also be a lb/deg problem.
MFS mentions the Greek Falcon 900 accident, and though I haven't read it again recently, my memory tells me this was more of a stick force per inch of travel type of problem,
I've seen several (prototype light) aircraft over my career with a stick force per inch problem - my personal working rule has become to look for a minimum gradient of 0.1mm/kn. I'm also aware that this was a problem with the prototype F-16 which had no movement at-all. However, in each case it's been the lack of enough movement to allow the pilot, through tactile feedback, to accurately resolve forces - not the shallow movement gradient in itself.
or stick force per G. Not a changing of stick force per knot.
The odds are that these are closely related. Whilst I've said that the two characteristics are unrelated, this isn't strictly true. In *most* cases, an aircraft with poor lb/kn will also have poor lb/g, and vice versa.
I believe the falcon was at something like 330 kias, and if the speed was decreasing at the time, it was not the decreasing speed that gave the pilot difficulty.
No, it seems to have been the problem in maintaining fixed pitch attitude - the relationship between that and lb/kn is a strong one, if not linear.
Mr Tullarmarine, you ask "but how are you intending to exercise any sort of control over what the aircraft is doing .. ie other than just being along on a roller coaster ride ?"
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 I'm asked to slow from 250 to 240 kias, I pull the levers back a tad and crosscheck. I don't see why I can't do this very easily with a zero stick force per knot gradient. Sounds sort of nice, like an auto trim system engaged.
What is this business about adjusting throttle to change airspeed? Whilst there will always be a secondary pitching moment related effect of power on speed, it's pitch attitude / elevator angle / elevator trim tab angle which affect(s) airspeed, whilst power affects rates of climb and/or descent.
Are you possibly flying an aircraft in altitude hold engaged - so when you change the power, the AFCS modifies pitch to maintain altitude - creating a reduction in airspeed.
You also mention the TP incident, but I do not see how stick force per knot is a player here. Sounds like control was difficult even while at a constant speed, so a changing stick force per knot should not have been the root of his problem (which was admittedly that the loading was outside the envelope.)
I'm unfamiliar with the specific accident but if pitch control is problematic at constant speed, that indicates probably something like a poorly damped longitudinal SPO? That is consistent with being out of aft CG, but so is poor lb/kn- so it all ties together without the lb/kn being the specific problem.
"Don't feel too bad about this stuff confusing you a tad". Yep, I'm confused
Contrary to what Mr Tullamarine may assert, despite 20ish years of learning and using this stuff, I still struggle regularly and get the textbooks out on a regular basis. It ain't simple, and don't let anybody pretend to you that it is.
G