Re: Interresting thread in comp.lang.eiffel

["Ken Garlington" <Ken.Garlington@computer.org>]

"David Gillon" <david.gillon@baesystems.com> wrote in message
news:397C29CA.7B445687@baesystems.com...
>
>
> Ken Garlington wrote:
>
> >
> > > > The gains changed abruptly when the aircraft transitioned from
> > > > takeoff-and-landing to up-and-away mode. This produced a sudden
> > transient,
> > > > triggering the start of the PIO. The transition now occurs more
> > gradually.
> > >
> > >    That fixes the primary cause, but not the actual failure mode.
Does
> > > the stick now
> > > have tactile feedback?
>
> > No -- you don't often get significant tactile feedback from a
force-sensor
> > side stick connected to a FBW system, such as that on the F-16 or F-22.
(In
> > fact, the original F-16 stick didn't move at all!)
>
> IIRC, the YF-16 had a PIO incident during the fast taxi trial and was
> only saved by the test pilot lifting off before the oscillation about
> the roll axis became severe enough to contact the runway with the
> wingtips.

Actually, the wingtips DID contact the runway, although the damage wasn't so
severe as to make the aircraft unflyable.

> He managed to dampen it out once in the air, did a circuit and
> landed without further incident. The solution lay in the (analogue) FCS,
> not the fixed stick.

Correct, although the stick was later changed to move slightly with the
pilot's hand, to give it a better "feel". It doesn't provide any feedback
from the airframe, however.

> There is an undeniable advantage to a moving stick, in that it
> immediately demonstrates the last control input if the pilot is
> hands-off or in a two crew cockpit.

IMO, It tends to be more defensible with respect to throttle, if the FCS can
independently select thrust (autothrottle).

> However I don't believe tactile
> feedback as a solution to PIO is a workable argument for mandating one.
> (Stick-shaker operation as a stall warner is a completely separate
> issue).

Particularly for a relaxed stability aircraft, the whole idea of feedback
gets interesting. Should it move in response to the surfaces, which aren't
necessaily moving in a way the pilot "expects"? Should the FCS -- which is
in the PIO loop -- be synthesizing some sort of artificial feedback? What
does "feedback" mean in the presence of thrust vectoring?