A discussion of the .46 size Ultimate in another thread caused me to investigate an idea that I'd thrown out about the aileron hookup in my World Models Ultimate.

The WM model used one servo to drive the lower ailerons. The lower ailerons were connected to the uppers with regular horns that stuck up/down and a solid connecting rod between the two horns. It looked obvious that the configuration would introduce a differential throw to the upper ailerons compared to the lower ailerons. The uppers ought to move less than the lowers and they would move even less going down than they'd move going up.

Well, I didn't really know what the amounts were because I didn't rig my model with the supplied hardware and certainly didn't even use anything like that layout. But now, the discussion in the other thread has convinced me that I ought to work out the details to see just how "bad" the ARF would be for others who followed the directions. And the situation warrants it's own thread.

I used the plywood template that came with the ARF that's used to align the two wings when you're installing the top wing. It's a perfect place to start for where all the pertinent points will wind up. And I used the discarded hardware to give me accurate measurements to use.

It turns out that the setup will give a fair ("fair"?? maybe "lousey" would be a better word) amount of differential to the upper ailerons. And it's going to be skewed both ways: up:down and left:right since we're talking about a biplane here.

Since the servo drives the lower ailerons, they're going to move "full lock" as the transmitter stick is moved. But since the hinge point of the connecting rod is relatively far above the lower aileron center line it might push that rod differentially to/from the upper aileron. Truth is, the angle the rod makes relative to a line drawn from the connect point to the surface hinge line is fairly close to a right angle FOR THE LOWER AILERON and the movement imparted to the rod by the lower aileron movement isn't badly different when pushing up compared to pulling down. It's the upper aileron connect point location that messes up the whole deal.

When the lower aileron moves the suggested 10mm, it moves the lower aileron about 22degrees. And that deflection moves the connecting rod just a small amount less going up than going down, maybe 10% less. But the connection for the upper aileron is a real problem. When pulled down, it actually only moves the connection hole a very little amount down. And while the lower aileron moves 22degrees, the upper one is only going to move about 10degrees. It's not so bad going the other way. When the lower moves 22degrees, the upper will go up about 20degrees.

Now, think about it. This is happening one direction on one side of that little biplane but in the other direction on the other side.

When you move your tx stick to get a roll to the right.....
The right wings are going to see ailerons that're about 10% different in deflection, with a change in lift and drag coming from the top wing giving less than the bottom. But the left wings are going to have down ailerons that are about 50% different in deflection between top and bottom, with the top wing's aileron moving less than half what the lower wing's aileron moves.
Not only are you going to get a significant difference in lift and induced drag between the top and bottom but a lot different left side to right side, AND what you're getting is opposite what you often want from simple aileron differential on a monoplane. We often rig aileron differential into an a/c to have more drag on the wing that's inside the turns. This ARF deal is going to give more drag and less lift to the outside wing!

I got a feeling that my model has a lot less twitchiness than the ones put together by the book, and I bet it is a lot less apt to snap unexpectedly. But I'll never know. And truth is, these little planes sometimes don't do what the books say they should. I bet some of them fly decent. (Of course, it they fly decent with this screw up built in them, wonder how they'd fly if........)