From: Klusmanp@aol.com
Message-ID: <118.b8d302c.2984fbb3@aol.com>
Date: Sun, 27 Jan 2002 01:44:03 EST
Subject: Re: Rudder Balance, IV
To: lancair.list@olsusa.com
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In a message dated 1/26/02 12:08:32 PM EST, RWolf99@aol.com writes:

<< Thanks, Walter, but even with the fancy degree I know next to nothing 
about 
 flutter.  However, I can see how gravity affects flutter when it can exert a 
 hinge moment, as it can with a horizontal control surface (ailerons and 
 elevator).  What I don't understand is how it affects the rudder.  I don't 
 understand why the rudder needs to be counterbalanced at all.  But I could 
be 
 dead wrong about that, and "dead" is the operative word here. >>


I have a degree - but one that is not very fancy at all. I had no formal 
coursework in flutter but have read about it and had an opportunity to talk 
to someone who is an expert on the subject.

As I understand it, counterbalancing a control surface is required to 
"de-couple" the control surface (i.e., aileron) from the main surface (i.e., 
wing). This is a fancy way of saying you don't want the control surface be 
part of a mechanism that can contribute to flutter.

Imagine we are in steady, level flight. Imagine the wing experiences a gust 
that causes it to move up. Given that there is some flexibility in the 
control system or even the aileron itself, the aileron will tend to do one of 
two things depending upon its balance with respect to its hingeline.

If the aileron is "tail heavy" it will want to deflect "down". This downward 
deflection will increase the lift of the upward moving wing and will tend to 
add to the total amount of "up" deflection that the wing will see. As the 
wing gets to its max up-bending position and starts on its way "down" the 
deflection of the aileron will reverse and help contribute to driving the 
wing down. It is easy to see that the aileron is adding energy to this 
dynamic condition.

If the aileron is "nose heavy" it will want to deflect "up" as the wing 
starts its initial "upward" movement. In this case the aileron is reducing 
the lift on the wing and acting to oppose the upward motion. As the wing 
reaches its max upbending position and starts on its way down the aileron 
will want to reverse its deflection and again oppose the downward motion of 
the wing. Here the aileron is reducing the energy of the system or acting to 
dampen the motion.

A similar behavior can be shown for a wing that is twisting. If the wing 
pitches "nose-down" a counterbalanced aileron will deflect up and tend to 
oppose the twisting. 

Imagine the airplane is flying in knife edge flight. Gravity will have no 
influence on the aileron with respect to the motion about its hinge line. Yet 
the aileron will still add energy or remove energy from the wing during a 
gust or other disturbance depending upon its balance condition. The same is 
true of the rudder and elevators.

It is interesting to note that counterbalances are often found concentrated 
at the extreme outer portion of the control surface. This is where you see 
the greatest amount of movement in both twisting and up/down bending 
(flapping). This is where the concentrated mass of the counterbalance has the 
biggest effect of causing the control surface to oppose the motion of the 
main surface.

This is not to say that all control surfaces MUST be counterbalanced to avoid 
flutter. Certainly many aircraft have flown without counterbalanced surfaces. 
This is also not to say that a control surface is required for flutter. 
Recall a stop sign twisting and bouncing when the wind direction and velocity 
is just right. To avoid this engineers try to design the twisting frequency 
of (i.e.) the wing different (typically much higher) than the flapping 
frequency.

This is probably a gross simplification of all that is flutter. I've seen the 
formulas and they are pretty ugly. I do know that much of it depends upon the 
stiffness of the airframe/control system, the distribution of mass on the 
flying surfaces, and the speed of the aircraft. I have not had an opportunity 
to ride in a Lancair IV but I believe I've heard it is a fast plane, yes?

Paul Klusman
Cessna Aircraft

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