Hi all, My qestion is:
What is "FLATTER" (Not sure about the spelling!)

How can it be avoided and has balancing the elevators and ailerons have anything to do with it?


Regards
Erick

Erick,

Flutter is the uncontrolled oscillation of the control surface. You may hear it as a "buzzing" sound at full throttle or in a dive. It can lead to a catasrophic failure of the surface or the plane. It's frequently caused by excessive airspeed and sloppy linkages and made worse by heavy surfaces.
Balancing your surfaces would certainly help. Just lightening them would make a difference. I would seal the gap between the stab and elevator, etc. Replace the pushrods with new stiffer units and eliminate the slop. use ball-links, carbon rods, stiffer horns, whatever you need to make the entire linkage bullet proof.

Chris

A bit of weight (mass balancing), like a small brass strip on the leading edge of a control surface will help as well. Depending on the chord and surface area of the control surface weights will differ.

How do you calulate the balance point of a control surface? Is it done essentially the same way a wing is balanced? With the hinging done from the front as it is, does that make a difference in determining the balance point?

Blue Skyy

Mass Balancing is used as a last resort after checking all the things that Chris300s mentioned. Inch long strips of balsa is double sided taped to the leading edge of the control surface a bit at a time till the flutter stops. I have only used that method once on an ARF.

correction, I mean strips of inch long brass, not balsa........!!

Erick,

Fly777 makes a good suggestion for mass balancing. The idea is to move the balance point forward to the hinge point.
You don't mention what type of plane or hinging material you're using but here's the steps that I'd take ranging from the cheap and greasy to the most complex.
1. Firm up the control rods, if you're using EZ clevises screw a nut on the rod before the clevis and tighten it on the clevis to reduce slop.
2. Seal the gap with clear tape or monokote, this will prevent the migration of high pressure air through the gap to the low pressure side.
3. Replace long wood or wire pushrods with arrow shafts or carbon tubes. Terminate in ball links.
3b. If you're using nyrod or similar product, make sure that the sleeve is well supported inside of the fuse so that it cannot flex under load.
4. Replace cheap-o horns with more robust units from Sullivan or Robart. If the plane's giant scale use Rocket City 8/32 links.
5. Open up the aerodynamic counterbalances and add lead shot and epoxy to balance the surface.
5b. Without aerodynamic counterbalances you can epoxy a plate or wire to the tip extending forward of the hinge line and add lead to the end.
6. Move the hinge point back into the surface (nothch the surface and use Robart hinge points to move the hinge point behind the leading edge). Look at full scale rudders to get the idea.
7. If #6 is already in use, add brass strip on the leading edge to mass balance the surface.
8. Rebuild the surfaces out of lighter material. Carefully select the lightest wood that you can get away with. If the surface is solid wood you can cut lightening holes. While you're at it, add an aerodynamic counterbalance to the surface, stay below 40% of the surface size and you should be ok.

Chris

A little more about flutter.

Air going over and under the wing is never quite smooth. As with a lot of things in nature it oscillates and this shows up in the magnitude of the pressures and velocities. This is still existing at the trailing edge of the wing where is is probably the largest.

The pressures and the resulting forces go up as the square of the velocity. So the faster you go the more force these oscillations have.

Each control surface/control system (and also the wing) has a natural frequency. It is the frequency that when it is vibrated at will cause the oscillations to increase. If you hook up a variable frequency sound maker to a taunt length of pianio string and slowly increase the frequency of the sound you will find a frequency at which the string will really vibrate.

As an example of this I have seen an aileron vibrate as the engine is throttled up and down.

With the airplane, if the frequency of the little pressure oscillations, and the magnitude of the forces that come from velocity, match the natural frequency of the control surface then you get flutter.

The loose linkages (and the other things noted by the other writers) allow the flutter don't change the frequency of the flutter but do decrease the amount of force that is needed to start it.

The stiffer the control surface is the higher frequency the natural frequency is. If you have the control surface stiff enough it will be outside of the ability of the airplane to produce it. The big slow flying hovering airplanes with almost no structure in really big surfaces are just waiting for flutter to occur.