On Thu, 6 Jan 2005 at 09:35:46 in message
, Peter Duniho
wrote:

It's a little of both. It won't be as bad as the aforementioned station
wagon, because the rudder isn't capable of producing as much of a turn. In
fact, this is why it's so important to use aileron to bank the plane too for
a turn. But a turn does still happen to a lesser degree, and the passengers
would still feel the apparent force pushing them to the outside of the turn.

I have no special theory abut the sad crash - I just don't know enough
about it - but the comments about why the loading cases are 'not as high
as they should be' caused me to start writing.

I don't know how much steady yaw 'g' can be generated by an airliner. I
do know that there are smaller high powered aircraft that can fly 'knife
edge' and must therefore be capable of generating a steady one 'g' yaw
'acceleration'. I have doubts as to whether a station wagon can generate
much more than one 'g' in a turn.

But my point is this. Higher loads and angles of yaw can be generated by
control inputs that are in phase with the natural yaw frequency. A
single application of full rudder will generate a deflection in yaw
which may overshoot the final steady state yaw deflection which will
then be damped out. However if control inputs are added so as to
increase the yaw rate as the aircraft oscillates from one side to the
other then it is theoretically possible to generate very large loads
indeed. The maximum will be generated dependent on the amount of yaw
damping present and the energy inputs. An operating yaw damper does the
opposite; it operates so as to reduce the amplitude of yaw oscillations.

The same sort of thing can occur in pitch but conventional aircraft are
likely to have much better damping in pitch than yaw. The classic
phugoid oscillation in pitch is not often mentioned, the ones I am
referring to here are the short period stability oscillations.

I believe this driving up of the deflections might be described as
'pilot induced oscillations'.

I have not the data to answer the questions posed by this but the
possibility may be there. The fact that yaw dampers are often fitted to
assist the pilot (human or auto) seems to suggest that at some parts of
the flight envelope large deflections of yaw angle need to be reduced.
('Yaw dampers' may also be part of a system to produce automatic
balanced turns but that is not the effect I am alluding to.

Why can flutter be so destructive? Because at certain speeds the input
of aerodynamic forces changes in such a way as to drive the structure to
deflect beyond its limits in an oscillatory way.

Divergence means that a single structural defection will increase the
load on the structure so much that it bends enough to reinforce the
aerodynamic force and this can lead to catastrophic failure. It is
however rare in aircraft!

Peter:- believe me it was only after I had written this that I realise I
had entered quoting your post. Several posts before that made me want to
react. I am not reacting directly because of your post. I am not
following you about - honest!
--
David CL Francis