Building solid controls

I mentioned this in another post, but thought it was worth starting a new thread and repeating again.

Many airplanes do not fly well simply because the controls are "soft". I had a Sterling Mustang profile with coupled flaps when I was a kid that was powered by K&B Greenhead 35. I still have the plane, by the way, although it hasn't been flown for a while. But, according to my memory, it was the tightest turning plane I have (had) ever seen in my life.

It was so good, I built a second one. Even copied the paint scheme. Put on another Greenhead 35. It stank! No matter what I did to it, I could not get it to turn tight.

I checked the CG, checked the weight, checked the elevator pushrod (to which I had attached a fairlead to keep it from flexing. I checked the amount of flap and elevator throw (all the same as the previous one.) I even tried increasing the size of the elevators. But it would not turn.

It was years later before I finally discovered it was the belcrank to flap horn that was flexing. I could hold the flap and move the leadouts, it was that bad. With no load, they moved fine. But under load (in the air) the pushrod was buckling inside the wing.

I cut the wing open, replaced the offensive (foul smelling!) pushrod with something substantial--and then, while I was at it, put new horns on the flap and elevator--and suddenly it turned as sharply as the original.

The moral: Use good hardware. Use solid horns or solid wire joiners between the flaps and elevators. Use good pushrods that don't flex and make sure there is no slop in the controls.

On some designs you may need to beef up the flaps or the elevators or the stabilizer. A lot of old designs have a thin stabilizer that will flex all over the place. Here is where you can cover it with carbon fiber. Do the same with the flaps if they are too "twisty" (sometimes a little heavier wood in the flaps is a good thing.) And then make sure you don't have gaps in the hingelines. If you do, seal them.

This will go a long way to solve a lot of problems that we fuss over because we can't figure out what is wrong. I have heard a lot of people say, "that plane won't turn". It is not always the design. It is quite often an unseen construction flaw.

Very well said Leonard. Unless you are using pull-pull controls, the linkage HAS to be rigid in all components, otherwise you are wasting your time. It is like having a precision hunting or target rifle or pistol, and you insert a sponge between the sear surfaces, and then expect to get a precise and crisp striker release.

Well said, Leonard.

However, there is an issue that still puzzles me. The total area of the flaps and elevators on a typical stunt ship are, within a range of (say) 10%, about the same. Some people advocate 1/8" dia hardware on the flaps and 3/32" on the elevator. Stiffness in the components is crucial but I think the loads on the horns are near enough the same. So why the difference in hardware sizes? Also, has anyone got any rules on when 3/32" wire becomes inadequate. A 500 sq in wing or smaller?

I have two rules of thum for control wire sizes, Geoff:

1) When in doubt, go heavier

2) When you can't bend it properly, go smaller.

There is a difference in stiffness between types of wire, and some horns are even made in "tweakable" and non-tweakable wire. The tweakable is softer and made to be able to be reset easier. But Matthew had a plane "tweak" itself on a trip to a contest. We attended church on Saturday night (A purposeful, bold plug) and drove up Sunday morning. He didn't have a chance to get in a practice flight before and discovered the shift in flap position only during the first official flight which he ended up aborting. Since then he has only used the "non tweakable" type of wire which still CAN be tweaked, but with great difficulty. This wire will, also, be stiffer and more resistant to twisting in the air.

We have used both sizes in the tail and I am not so sure it makes as much difference there. For one thing, the tail horn is generally shorter. The flap horn needs to be wider than the width of the fuselage at the wing, while the elevator horn needs to only we wider than the fuselage at the rear. We place the horns as close to the edge as possible when we are able to bend the wires ourselves, although bending the horn wires can be difficult when you need to do it yourself and the horn is already attached to the center. That factor alone may make your decision. The wider flap horn, even though made of 1/8 inch wire, is easier to bend because it is longer. The shorter elevator horn is easier to bend when made of 3/32 wire, but is less critical because it is shorter.

And, certainly, if you had a smaller, .020 size plane (or .049) you could size your horn wires appropriately smaller.

But the nose section and the controls, including the belcrank mount and leadouts, the pushrod and the horns, should all be made as "bullet proof" as possible. A "little" extra weight here might be a good precaution in ensuring a good flying and long lasting airplane.

Again Leonard, thanks for a clear and erudite explanation

It's 30 years since I engineered a CL stunt model but I've been engineering the tools or the steel components that build cars (first) and then buildings for 25 years and never thought about the inches more that we expect the flap horn to work in torsion.

As for the 'purposeful, bold plug' that's cool with me, my wife is the secretary for an Anglical parish in the north of Sydney. We understand where you come from.