Dennis W Zeuch <DZTOPS@aol.com> wrote:
Subject: Re: Letter?...What Letter?
> BraniffIntl@aol.com writes:
><< rapid rudder movement in flight could result in serious >repurcussion=
s.
>>
>If its not safe (to the point of ripping the rudder off the acft) why
>is it possible to do it? Seems the controls could be adjusted to
>prohibit dangerous excess?
Well, a rudder has to be large and powerful by the nature of the
beast. On a twin, its main aim in life is the capability to maintain
equilibrium in an engine-inoperative situation at low airspeed. If
you look at the planform of the A-300, the tail arm is roughly three time=
s
the distance between the engine and the centre line. With 60000
lbs of thrust on one wing, a couple of thousand pounds of windmilling dra=
g
on the other, you need roughly 20000 lbs of side-force on the
fin to maintain equilibrium.
With the rudder sized for a low-speed condition, it is indeed
overly powerful at high speed. That explains why all high-speed
aircraft have some system to limit rudder deflection as a function
of airspeed.
But that is not the issue here. Every aircraft has a =
manoeuvering speed, and common wisdom explains this as the speed
below which you cannot overstress the airplane by sudden and maximum
control deflections. In the aftermath of earlier accidents, much
emphasis was placed during pilot training in recent years on
recovery from unusual attitudes. This included the admonishment
not to be shy of using large control deflections when you need them.
Now it would seem that this 'guarantee' of not being able to
overstress the aircraft below the manoeuvering speed, was accompanied by
some small print which stated 'not valid during sudden reversals of
large rudder deflections'. This limitation is of course not new
(and affects all modern airliners), but it was certainly not widely
known until the AMR crash.
Large opposite rudder deflections did occur during the AMR A-300
climb out. What is not yet known (at least as far as I have seen),
is whether these rudder movements were applied by the pilots (possibly
in response to wake turbulence) or by some mechanical malfunction.
Just before anyone asks why a single-engined aircraft has a rudder
at all: the fin and rudder is sized either by the engine-inoperative
case or by the cross-wind landing (sideslip) case. For twins the
former is usually limiting, for single-engined aircraft obviously
the latter.
Kees de Lezenne Coulander
-- =
C.M. de Lezenne Coulander
Aircraft Development and Systems Engineering B.V.
Hoofddorp, The Netherlands
=