what do they really do? would they not just decraese the efectiveness of the control surface? please help

On full scale planes they do just what the name says, it is used to place weight opposite the control surface for balance. This isn't very practical for RC planes, because most of us don't want the extra lead weight in every surface. Also when you mass balance a surface you usually have to build a stronger surface to support the added weight. This ends up making planes heavy.

In one respect they do decrease effectivness of the control surface. However, for 3D flight this starts to work the other way. The bigger the counter balance, the smaller the stab which is good for tumbling, etc, because you don't have the larger flat portion of the tail plowing through the air.

That's my opinion.

Bill

They also help to prevent flutter!

The manual for my Ultra Sport 40+ says that the rudder and elevators are counterbalanced to prevent flutter. Wide open passes are freaking fast, and this is my first ever kit build, so my hinge gaps could pobably be better, and there's no sign of flutter!

How does counterbalancing help flutter? I can see how it would reduce servo load because the part of the moving surface ahead of the hinge line is actually generating forces complimentary to the servo movement, but I don't understand the flutter thing.

I forgot to point out that the reason for balancing is to help prevent flutter.
I'm no aeronautical genius, but my understanding is that when a surface starts fluttering it gets worse and worse due to the weight of the surface. IE: it starts moving and the weight of itself tries to keep it moving in that direction.
With it balanced, the weight is the same (or close) on each side of the hinge, which cancels the momentum of the surface.

I think the balances on the Ultra Sport are just for looks, and doesn't really make any difference.
However, I haven't looked at them or weighed them to see how much they are balanced.
If a surface is 100% balanced you can disconnect the linkage and the surface should "float" in a neutral position. You can't really check it like this unless you have it freely hinged.
Any % of balance helps and 100% is not necessary.
I know for a fact that on a full scale Cessna 172 the elevator isn't 100% balanced.

There are many different opinions on all this, and I'm sure that a search on this would bring up plenty of material.

Bill

I think that the term "counterbalance" in this instance means counter-balancing aerodynamic forces, not weight. After all, how would you counter balance a rudder?

You counter balance the rudder the same as the other surfaces. It doesn't have anything to do with the aircraft and it's surface being parallel to the ground. On a full scale aircraft you would remove the rudder from the plane, place it in a jig that uses the hinge bolts, positioned so that the surface of the rudder is parallel to the ground. Now with it in this position you use lead, brass, etc to balance the rudder to the recomended amount. Place the rudder back on the plane.
For RC planes it's a bit easier.

You are right though, in that the counter balance is more for aerodynamic effect than balance on an RC plane. I think the kit makers use these for looks more than anything else, or the fact that the full scale version has them so they use them on the model plane also.
I have seen very few kits that had weight balances or give instructions on balanced surfaces.


Bill

David_Moen,

Bill Mixon got it right on the control surface flutter, in another word the control surface vibrates/flutters at a frequency that is harmonic to it self or another component of the model.

"..........,but I don't understand the flutter thing."

Although it does not address counter balances per se, but this link http://www.airspacemag.com/ASM/Mag/I...FM/Hammer.html
would explain the flutter thing much better & there is a link at the end of the article that will take u to the movies of things that flutter in a real aircraft.

Hope this help
Hisham

Most of the counter balances on RC planes are of the aerodynamic type (and mostly for looks) the aerodynamic counter balances on full size are used to reduce the stick forces (or RC servo loads). Mass balances on the other hand are used to prevent flutter. Its done like its described in the previous posts. The only rc plane I've ever seen that recommends them is the US Air Core Colt, and they recommend using them on the ailerons. However they do benefit other rc planes. Me and a buddy of mine built a plane called the "Fun 15" that has massive control surfaces that are aerodynamically balanced, mine's glow and his is (was) electric. I make it a point to keep the speed down when the nose is down, he decided to try a power dive. The resulting elevator flutter turned it into a lawn dart. I added lead to the leading edge of my counterbalances to mass balance the sufaces and with the nose down and power on there's no sign of flutter. Have a similar small 3D electric that fluttered the elevator at high speed, a little lead on the leading edge of the counterbalances solved the problem.

Mike