On 6 Jun 2005 09:39:45 -0700, wrote:

Is there a simple (or complex) guideline for the relationships between
CG, pivot axis and aerodynamic center for a control surface to prevent
flutter?

Is it as simple as keeping the CG at or ahead of the aerodynamic
center, similar to designing for stability of the aircraft itself?

The Bearhawk and Christavia both have ailerons that have a pivot point
aft of the nose of the aileron. The Bearhawk requires that lead (or
some heavy substance) be placed in the nose of the aileron such that
the aileron is neutral while balancing on the hinge points. I
understand that the nose could be a bit heavy (such that the nose
pivots down when balanced on the hinges) and not compromise flutter
avoidence. But the Christavia does not require counterweighting,
although I've heard of one or two doing it anyway. The Christavia is
a lot slower than the Bearhawk, which makes a difference.

The other parts to flutter avoidence involve making sure there is no
slack in the control cables or rods, and that the controls themselves
are stiff enough such that their flexibility does not contribute to
the problem.

I'm not an aerodynamicist and I do not know or offer mathmatical
figures to support what I just said. When I built the ailerons and
other control surfaces for my Christavia, I did not change anything,
they are built according to the plans. No Christavia has reported a
problem with flutter and I'm not going to risk changing that.

Well, to be honest, I did drill lightening holes in the aileron spars,
but my rational there was that the spars were way over designed and
hang off three hinges. The control cable attaches right in the middle
of the aileron so there's no way for the aileron to twist when a
control input is applied. The holes aren't real close together and
not very big either.

It's a drop in the bucket anyway because the wings are massive,
overdesigned and very heavy (talking about a Mk 4 version of the
Christavia). Besides, max speed is 130 or so.

Corky Scott