Elevator Counter Balance

[Jamie]

I know that the reasons behind control surface flutter are akin to voodoo, but I have a few questions. Obviously on our RVs we are strongly cautioned about getting the elevators properly balanced, etc to reduce the possibility of flutter.

I share a hangar with an Extra 300 and guess what? The elevators have no counter balances on them. Next to me there is a homebuilder who builds his own designs (yes, they're very cool -- fully aerobatic, 12+ g's, etc). None of his designs have counterbalanced elevators. Obviously these aircraft are different beasts, but I would think the same principles still apply.

So what's the story? Why are some elevators counterbalanced and others aren't?

[pierre smith]

Hi Jamie,
Expect to hear from Scott Will, Philip Lerke and Kevin, engineers. As you probably read, I have an old acquaintance named Nick Jones near Beaufort, S.C. who had a Cassutt formula 1 disintegrate during new airplane testing at a race in Texas. He developed severe aileron flutter at around 260 MPH and shed the wings. He managed to bail at near 300 MPH and 100 feet. The opening shock gave him a hernia but he's glad to be able to tell the story.

His ailerons had the counterweight at the far end and he speculates that the rest of the aileron was not "damped" and was too limber and fluttered. He did say that Van has the best solution by having a counterweight all the way from tip to inboard end as the best counterweight possible for that reason. Our elevators and ailerons and wings are pretty stiff and he said that this mitigates flutter to a large degree, although we could still see flutter at higher than VNE.

As far as the airplanes you mention not having counterweights, the control surfaces must be either stiff or the guy is treadfing on thin ice if he's exceeding 150-160 MPH. Then again, the Pitts elevator and rudder don't have counterweights and neither did my Cassutt.

I'm looking forward to the engineers' replies.

See ya,

[Pirkka]

Originally Posted by Jamie

Why are some elevators counterbalanced and others aren't?

One guess here would be that those designs doesn't have this 'thing' (where the balance is located) going forward from the hinge line? This thing will affect stick forces as well... Would you like to build the Extra to avoid the extra weight?

[DGlaeser]

Don't think of flutter as control surfaces flapping in the breeze like a flag in strong wind. Think of the whole structure (wing, HS, even fuselage) bending and twisting as if it were made of rubber. When the structure is excited near it's natural frequency it becomes a tuning fork. The control surface is the obvious way that tuning fork gets 'twanged'. If the structure is stiff enough, it may be difficult to achieve the speed which excites it's natural frequency, so no special consideration is required for the control surface.
Balancing the control surface is meant to keep it from deflecting out of phase with the rest of the structure. When the wing/tail deflects up, a balanced control will tend to act as part of the whole wing/tail and 'stay in trail'. An unbalanced control will tend to sag down (due to inertia) when the wing/tail deflects up, which increases the angle of attack, which causes the wing/tail to deflect up more, and the control to sag more... Then when the structure 'springs back' and goes the other way, it happens in reverse (wing goes down, control goes up) - so you can see this out of phase action worsens tihs vicous cycle. When this is happening at the natural frequency of the structure large deflections result from relatively small 'twang' forces.
The 'voodoo' part of all of this is analytically predicting exactly when and how it will happen. It can be done with fair accuracy, but it is involved and expen$ive.

[Kevin Horton]

Originally Posted by DGlaeser

The 'voodoo' part of all of this is analytically predicting exactly when and how it will happen. It can be done with fair accuracy, but it is involved and expen$ive.

I used to believe that flutter could be reliably predicted too, then I learned about the flutter incidents during the E-6 TACAMO testing. The E-6 is a modified Boeing 707. Boeing experienced vertical tail flutter during the test program - the aircraft lost about half the vertical tail, but was landed safely. This occurred in the late 80s, or very early 90s. After that flutter event, you can be sure that Boeing pulled out all the stops to analyze the problem. Then they went back up on another test flight and lost half the vertical tail due to flutter again. If a company with the resources of Boeing can't predict the speed at which a surface will flutter, what hope does anyone else have?

A Historical Overview of Flight Flutter Testing

[DGlaeser]

Originally Posted by Kevin Horton

If a company with the resources of Boeing can't predict the speed at which a surface will flutter, what hope does anyone else have?

Excellent point. Obviously 'fair accuracy' leaves plenty of room for 'very inaccurate' I'm plumb out of snake oil, so my elevators will be balanced, and I have no plans to play beyond the VNE that other brave souls have established!