On Dec 25, 4:18 pm, Bertie the Bunyip wrote:
"Bravo Two Zero" wrote in news:EyZbj.1217$Gr4.669
@newsfe08.phx:

What is rudder flutter ?

A bit like the way a flag whips in a high wind. Any control surface can
flutter if the supporting surface is flexible enough to allow a
complimentary cyclic oscillation of the two surfaces (control and
stabilising surfaces).
Here's how it works. The control surface is displaced, loading up the
adjacent stabilising surface and bending it over to one side (the fin
in this case, but it can be the stab or the wing just as easily) The fin
has now suddenly been pushed to one side under tension and may now
spring back in the other direction carrying the hinged edge of the
rudder with it. the trailing edge of the rudder, however, will be left
behind in this move, deflecting and thereby making a control input that
contributes considerable force in the direction the rudder is now
moving. ( I know, I know, a pic here would help, but bear with me) the
fin is now stretched out in the other direction and ready to spring back
the other way repeating the cycle.
If the rudder is balanced on the hinge line, it will be far less likely
to support this cycle. The other way you can stem it is to make the stab
fin or wing very rigid and this is why biplanes can have as high a
redline as , say a Mooney might though the bipe might have no balance
area at all and the Mooney would. All the surfaces are too rigid to
allow the cycle to start.
You would think that a greater load on the surface, i.e, "G". would tend
to dampen the action, but the opposite is the case.
Flutter will appear at a lower speed if the surface has an aerodynamic
load on it.
Aerobatics are the most common arena for flutter, but poor maintenance
can also bring it on. A lot of high performance light planes have been
lost simply because they have been repainted without rebalancing the
surface, or the balance material has come loose or lost in a rebuild.
Mass balance is typically a bit of lead buried in a balance horn or some
other bit of the control surface ahead of the hinge line.
Aerodynamic balance can also be employed to help in this and that's
often where the mass balance is housed as well.
Large aircraft often use depleted uranium for the mass balance, BTW!

Bertie

To add to Bertie

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.

Cheers