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Old January 13th 07, 06:23 PM posted to rec.aviation.soaring
Dave Rolley
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Posts: 5
Default Grob Twin Astir Tailshaking

JJ is spot on! I've been through this myself in a Concept 70.

Early certification guidelines indicated that balancing the rudder was
not necessary on aircraft with a Vne below a certain speed. Of course
the guidelines were written for aircraft with steel tube or monocote
aluminum construction. GRP structures behave differently.

In the case of my Concept 70, the rudder was grossly out of balance. It
took almost 12 ounces of lead ahead of the hinge line to balance the
rudder. That was the short term fix. About two years later, while
refinishing the glider, I had a new rudder built out of glass, carbon,
and kevlar. Plus I had the fin spar inspected and, as it turned out, it
and the hinges needed repair. We stripped the old rudder and found at
least 4 layers of finish on it.

BTW, how serious is rudder balance? On a V-tail Bonanza the rudder
being out of balance by the weight of a silver dollar (1 ounce) on the
rudder trailing edge is enough to cause flutter and catastrophic
structural failure at speeds below the yellow arc. It is not something
to play with.

As to the rudder pedals moving, you would be surprised how much the
rudder cable can stretch. Since most glider rudder systems are cable
driven at the rudder regardless if push rods are used up front, unless
you are on the rudder pedal real hard, you might not notice the pedal
moving until the rudder deflections get rather large.

Since the control surface extracts energy from the airflow, the only way
to deal with flutter in flight is to slow down. Right Now! Don't wait
and don't think you can control it with control inputs from the cockpit.
You have to get rid of the active energy in the situation.

Dave

Andy wrote:
John Sinclair wrote:
Peter,
You may have experienced low frequency rudder flutter.


Can the rudder have low frequency flutter without the rudder pedals
following. I assume not, so were the pedals moving?

Andy