Rudder counterbalance ?

[Ron Van Putte]

On Apr 28, 2005, at 11:09 AM, Gray E Fowler wrote:

>
> "Why not mass balance for flutter elimination. When most full scale 
> aircraft
>  get painted they have to check balance of the surface ( ie Piper
>  Cherokee's).
>
> Because most spend hundreds of dollars removing ounces of weight.
>
> Flutter is natural frequency-speed driven. If you have flutter it is 
> usually an indication of a serious stiffness problem, either in the 
> set up or the structure itself. Instead of counter balances and otehr 
> tricks, just increase the stiffness. That will be the biggest bang for 
> the weight buck.

I agree with Gray that flutter is a natural frequency/speed-driven 
phenomenon.  However, I disagree that increased stiffness is the best 
way to go.  Most of what people call "flutter" is really "control 
surface buzz", because it is essentially a one-dimensional phenomenon.  
Flutter is reserved by experts in the area to refer to oscillatory 
phenomenon involving two or more degrees of freedom (like wing 
bending/torsion or wing bending/torsion/aileron deflection).

If the control surface is not statically mass balanced so that the CG 
is in front of the hinge line, control surface buzz will occur at some 
speed/altitude combination.  With increased stiffness, the buzz speed 
is increased.  If one puts in enough stiffness, the buzz speed can be 
increased higher than the airplane can reach.  This often requires more 
stiffness than it is possible to build in.

However, if the control surface is statically mass balanced so that the 
CG is in front of the hinge line, buzz involving the control surface is 
essentially eliminated.

Ron Van Putte
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