WingFlaps wrote in
:

On Dec 26, 12:15 am, Bertie the Bunyip wrote:
WingFlaps wrote in
news:797747ee-e7ec-4c72-b61b-
:



The bottom line is that If the COG of the surface is behind it's
center of lift flutter is promoted. That's why most control
surfaces add weights (depleted uranium!?) to bring the COG forward.
On the preflight you check that that balance weights are there and
fixed. Flutter will destroy the surface, structure and it's hinges
pretty quickly.

Actually, that's not quite correct. the CG of the surface has to be
at the hinge line, not it's Cl. to eliminate this tendency. It's down
to the rididity of the hinge in space, though. The less rigid the
greater the need for balancing. Most wings and stab surfaces are
quite flexible, though, so at least some balance, if not 100% (which
would put it at the hinge line) is reguired for most airplanes.


Err, I didn's say the Cl was at the hinge line (rather that COG must
be forward of control surface lift).


I know. I understood what you said, but the Cl of the surface is
irrelevant to flutter.


But I agree, if the COG _can_ be
put at the the hinge line then not just control surface flutter is
reduced but also flying surface flutter (in this case fin + rudder).

They're th esame thing, really. Although you can have eiter, they're
caused by the same situation, the surface as a whole, whether or not it
has a seperate flying surface or not, is producing and flexing the
surface.
The control surface element is caused by the rudder or whatever, lagging
behind the moving fin due to inertia. This causes displacement of the
rudder and creates a rudder input which provokes the movement of the fin
even further. the combined inertia stretches the elastic fin and when it
gets as far as it;'s going to go in that direction, the elastic property
of the construction material flips it back the other way, leaving the
rudder behind and causing another rudder input in the opposite direction
and so on, increasing in amplitude until something fails, be it the
hinge, the spar, or whatever reahes it's limit first. Usually it's the
spar.

The trouble with the lightt structures in a plane is that balancing
one end (e.g. with a leaded horn) may lead to tortional problems..

Huh?
Torsional flutter is kinda outside the scope of this explanation, but
the amount required if you put it at the tip would be lower anyway if
you're not shooting for 100%, so that's why it is placed at the end if
that's the solution required. There are a few types where massive lumps
are mounted at the outboard ends of the ailerons. Others, like Cessnas,
just have loooong strips inside the hinge gap.

It's done all the time, but it doesn;t have to be at one end and it
doesn't have to be a horn. It also doesn't have to be a 100% balance if
the performance doesn't neccesitate it or if the stab or wing is rigid
enough for flutter not to be an issue within the speed envelope.
But for anythng that goes very fast, you have to have 100% balancing,
which means it's statically balanced at the hinge line.


Bertie