Probably everybody has had airplanes which turn better one way than the other. Many articles on trimming stunt ships have been published which cover this topic. In most cases authors recommended bending of flaps and elevators to get satisfying flight characteristics. There’s nothing wrong with this. If there’s no access to the control system of our stunter this is the only cure for the problem. Most stunt ships are one piece aircraft so there is no other choice. This cure may not be the correct solution for the problem, but it may be sufficient to make the model flyable. In most cases only minor corrections should be necessary, so there’s no reason to solve one problem by raising another one. Nevertheless, if we haven’t found out what caused this unwanted behaviour we might build our next airplane the same way as we did the last one and come up with the same results. Now we’re not talking here about building errors like crooked fuselages, incorrect angles of flying surfaces, warps etc. Let’s have a look at the control system.
Slowly but steadily models with detachable wings and tails are getting more popular. On these ships one of several advantages is that we have access to the control horns. This allows us to change and trim the neutral setting, flap deflection and elevator deflection, and the ratio of flap to elevator deflection.
Some years ago a new model of mine needed so much flap bending that I got curious about the reason. Since it was a “detachable-everything” stunter I took a close look at the control system. Still not understanding the problem, I made a drawing of the whole system, imitated the controls with some pieces of wire and plywood, made a nice toy - and suddenly the light dawned. ( Besides, that’s a practical way to simulate mechanical connections and movements which are not quite clear at first glance ).

Using conventionally designed wings and bellcrank locations on a take apart airplane the pushrod runs from the bellcrank - usually mounted at about the ( vertical ) centreline of the wing - to the flap horn. The vertical distance between the two attachment points is roughly 25 mm. Depending on bellcrank design and dimensions we have a certain pushrod travel ( X ) . Half of this dimension is used for “up”, half is used for “down” ( X/2 ). Fig 1 shows this arrangement. We don’t need to delve deeply into mathematics, the drawing shows it all. With the “wire and ply” system we can easily see what happens when we move the pushrod. Despite the same travel in both directions it’s quite obvious that we have different horn deflections - thus flap deflections - on up and down ( see Fig 2 ). I’m well aware that the difference in deflection is not that much as shown in the drawing, since the angle of the pushrod is smaller in reality. I have slightly exaggerated the drawing so it’s much more obvious and it’ much easier to see, even without measuring.

The solution to the problem is simple. The horn has to be installed so that it is at right angle to the pushrod, i.e. at 90 degrees to the direction of movement. This can easily be found if we draw the bellcrank and horn position in our plan. The bellcrank hole is connected with the horn hole by a line, and we’ve got the exact pushrod angle ( see Fig 3 ). Now we simply bend the flap horn as required , or make our own horn according to the drawing. Basically the same goes for the elevator horn. Again the drawing will show the slant pushrod with the required shape of the horn.
There’s quite a number of models with a sheet tailplane. Some kit manufacturers don’t care about such subtle details, or the plan doesn’t give such detailed information. At the first look the problem might appear of a different kind, but it’s basically the same phenomenon. I’m talking about poorly designed or installed elevator horns. Often these are made as shown in Fig 4. This installation has the same effect as an angled pushrod ( non 90 degrees installation ). The hole of the horn is not in the same vertical plain as the hinge axis of the elevator, it’s in a more rearward position. In Fig 5 we can see the pushrod travel as X/2 on both sides of neutral, and the arc which is described by the horn hole. There’s a big discrepancy between both ways, and it can easily be seen that elevator deflection will not be symmetrical. There are places where this is intentionally done; RC flyers call this “differential deflection” and mount their control horns accordingly. However we fly control line aerobatics, and we dearly need symmetrical deflections.

This arrangement will also add two other problems. As can be seen in the drawing the horn hole will not only move horizontally but vertically, too. On it’s arc it will finally get to a point where it will not be able to move horizontally any more, even though there is still more pushrod travel available. No matter how strong the poor pushrod may “push” ( if there’s any force left, at all !) , here is “end of movement”. Also please note distance “D”. This is the vertical distance (= moment arm ) from the hinge axis to the horn hole where the pushrod is working. As you know a shorter moment arm requires higher force to do the same work. I’m not a doctor in mathematics and I cannot work out the numbers. But it’s quite obvious that at large deflections we need high forces. Probably with this horn installation we’ll soon run out of force - and of air ! The solutions to these problems are simple, see Fig 6. There are also RC plastic horns which are shaped to overcome this problem. They are quite suitable for small models.
While we’re on the subject of elevator horns here’s another thought. Tailplanes of modern big airplanes often have increased in thickness, sometimes to one inch. When building the stabilizer it’s necessary to provide enough clearance for the fully deflected horn. This requires to file a deep groove into the trailing edge of the stab; exactly at a point of high stress. In order not to weaken this area many builders are using horns with a somewhat crooked shape. The shape of the “crook” depends on the stab thickness. As shown in Fig 7 less material has to be removed from the stab for the same horn deflection as compared to a straight horn. Fig 8 shows two variations of horns with variable attachment points. The horn on the right has a slider running in a slot for continuous adjustment.
These “angled” problems can also occur at the bellcrank side. Fig 9 shows an example where the bellcrank has been mounted somewhat “tilted” when in the neutral position. It can be seen now that the pushrod has less travel when the bellcrank is rotated “right” ( in the drawing ) and vice versa. Bellcrank location can be another problem. Most plans show the bellcrank mounted in the middle of the wing ( spanwise ). The flap horn is installed in the middle, too. Inserting the pushrod will bring it into a slant position now which will create a very similar situation as seen in Fig 9. To avoid this arrangement we can construct a special flap horn with unequal length wire arms and the horn sitting off centre ( more outboard ). This is not always possible. In this case I prefer to move the bellcrank more inboard, mount it close to the left centre rib, and have the pushrod running at or very close to 90 degrees to the bellcrank drive arm ( Fig 10 ).
Fig 11 was included only to show the difference in play which results from different bellcrank arms. Even if play is the same the pushrod and hence the flap/elevator will have more play when attached to a shorter drive arm. This is one of the benefits of longer moment arms and one of the general reasons of the increased dimensions of a modern control system.
We’ve come a long way since the invention of our control system. Who’d have expected that it would ever be the object of scientific research and development. This article hardly claims to have supported this. It’s a collection of some basic knowledge, intended to help the advanced beginner understand the geometry. No effort was made to explain the process of developing a suitable system for your own flying toy. That might be stuff for another story.