effects of counter-balanced rudder

[Wingin' Wayne]

I'm building a Formosa and am wondering if I should counter-balance the rudder? I scratch built a foamy with a counter-balanced rudder and it does a nice inverted flat spin. I'm hoping the Formosa will do the same! Does the counter-balanced rudder improve the ability to flat spin? Thanks for the advice.

[Sparky Paul]

Counterbalances are mainly used to aid weak servos in holding the surface out.
It could be useful for aiding flat spins, but opposite aileron will do the job.
Get the spin going and slowly push aileron to see what happens.

[hul]

Counterbalances are mainly used to aid weak servos in holding the surface out.
Aerodynamic balance (surface in front of the hinge line) reduces torque requirements, lets you use a weak servo like Sparky said.

Static balance (weight in front of the hinge line) reduces flutter. CG of the surface should be close to or on the hinge line for this. This also reduces shock load on the servo from hard arrivals for horizontal surfaces.

Hans

[Rodney]

Wingin' Wayne, I assume that you mean airodynamic counter balance, not mass counter balance. If so, yes it lets a weaker servo do the job. Mass balance minimizes the adverse effects of any shock loads on the servo machanics. Mass balance has little or no effect on whether or not you get flutter. By adding mass balance you may change the frequency at which flutter will occur but it does nothing to prevent flutter. The only way to prevent flutter is to have the resonant frequency of the structure fall outside the realm of any spurious or intentional frequency excitation. You do this by controlling the stiffness of the structure. Every thing will flutter given the proper stimulus.

[hul]

Mass balance has little or no effect on whether or not you get flutter. By adding mass balance you may change the frequency at which flutter will occur but it does nothing to prevent flutter. The only way to prevent flutter is to have the resonant frequency of the structure fall outside the realm of any spurious or intentional frequency excitation. You do this by controlling the stiffness of the structure. Every thing will flutter given the proper stimulus.
I was of that opinion too, but had to re-think. There was report in one of the german magazines about this. They tested this on molded gliders going straight down for several hundred meters, to a speed of 250mph (from memory). Mass balance on the rudder solved their flutter problem.
Moving a surface that has its CG not on the hinge line causes a reaction in the fuselage (combined CG will stay at the same location), fuselage moves to opposite way. A mass balanced surface won't cause that reaction in the fuselage and thereby removes the excitation.
I'll give you the details of the magazine tonight.

Hans

[peterangus]

Mass balance has little or no effect on whether or not you get flutter.

NO.....NO.....NO

A control surface which is not mass balanced can generate exitation forces which are exactly in-tune with the structure vibration, because they are a consequence of that vibration.

[globemaster3c17]

Mass balance has little or no effect on whether or not you get flutter. By adding mass balance you may change the frequency at which flutter will occur but it does nothing to prevent flutter.

Mass balance is extremely important when considering flutter. The closer you can get the balance point to the hinge line, the faster you can go before experiencing flutter. As an example, look at the rudder of a full-scale Cessna 310. The beacon is located on the leading edge of the rudder which is several inches in front of the hinge, and if that happens to get knocked off, you can't fly the plane because the rudder could flutter below Vne (never exceed speed) due to the shift in the balance point of the rudder. I can't tell you how many times I heard that from my instructor and the chief instructor when I was working on my multi-engine rating.

[Rodney]

The examples noted above that profess to show mass balancing eliminating flutter are somewhat flawed. What causes the change in where flutter occured is not because of the lack of ( or existance of) mass balance but because of the structural changes made in the stiffness or resononace point of the surfaces. You simple moved the range of susceptable frequencies into or out of the range of excitation. Flutter is a very complex problem and difficult to analyze as it contains so many factors that can make it vary; total mass, stiffness, mounting points and on and on. The only thing you can say for sure without many hours of exacting testing is that "Any change to the structure will change the point or frequency at which flutter will occur" Even the simple addition of a tape to seal the hinge gap will change stiffness and change the freqency at which flutter may occur. It is not the sealing of the gap, it's the change in structural stiffness that causes the change. The same goes for mass balancing, you are changing structural stiffness and total mass when you add the balance. We would spend months of testing on real aircraft to isoloate the cause and elimination of flutter without adding lots of additional costs and materials.

[peterangus]

The only way to prevent flutter is to have the resonant frequency of the structure fall outside the realm of any spurious or intentional frequency excitation.

Rodney

I can agree with that.

It is precisely that rule which is breached when you have an unballanced control surface.

When a wing vibrates, in bending, at its natural frequency, the aileron hinge line will experience vertical accellerations.

If the aileron CG is aft of the hinge, these vertical accellerations will result in small angular deflections of the aileron. [The aileron may have some free-play, and its control linkage will not be infinitely stiff]

These angular deflections will produce cyclic aerodynamic loads. These loads are exactly in-tune and in-phase with the wing vibration. This is your "spurious exitation" which may result in flutter.
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You are only seventy-five years behind the times. The "mystery" of flutter was solved circa 1930, after numerous fatal aircraft losses from this cause.