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When the center of pressure (lift) moves, stick
forces will be required to keep the nose at the desired attitude. How much this
center of pressure travels will partly determine stick forces. The effectiveness
of the elevator will also determine stick forces. Recall the DC-9 with it's
"flying the tabs" for aileron and elevator control, very little stick force
required. Very low stick forces contribute to Pilot Induced Oscilation for those
pilots that are not used to it.
The end result of a particular airframe can be
measured and documented easy enough. What I'm interested in is what all causes
these stick forces.
Wolfgang
The CAFE report on the small tail 320 mentions the very low "stick force
gradient" as
a problem. The stick force was found to be almost neutral at
the rear CG limit.
As I understand it the gradient is much higher in the
Legacy,
and about halfway in between on the large tail 360.
I
seem to remember
Greg Heinze of HPAT telling me the SFG numbers for the three
models
on the phone...but I have since forgotten the
values he
quoted.
I understand that a low gradient is a problem since the pilot has
no feel that
he is pulling hard just before stall. Perhaps worse, in the
stall, just releasing the stick pressure does not
unstall the A/C.Rather, the
pilot must calmly place the stick forward....preferably without inducing PIO,
despite
the adrenaline load.
I don't know much aerodynamics but I am
familiar with second order differential equations.
Having tuned servo
systems, I would be wary of nuetral stability. Wouldn't INstability be
right around the corner?
Am I right about this?
The recent long debate
on CG ranges has not so far discussed this. What is
the role of stick force
gradient in this debate?
--
Jeff
Peterson
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