Could you tell me how many mm (or in.) movement are needed on each ruddervator for going up, down, left and right?

I am puzzled with the movements (up/down) I have to put on the ruddervators.

From : Don Stackhouse

The amount of deflection required for ruddervators is usually about the same as the corresponding deflections for a conventional tail. The specific amount depends on the individual model and on your personal preferences, but most good kits will give you some good initial values in the instructions. The Monarch and Wizard series models we produce all include recommended deflections in the instructions that come with the kit.

What direction to move them is fairly simple once you understand how a V-tail works. Of course for elevator, both ruddervators move up for "up" and down for "down", about the same amount you would move a conventional elevator.

Rudder inputs are a little more involved, but still not too hard to understand. When you move the ruddervators in opposite directions, the "up" and "down" portions of their lift forces cancel each other, but their sideways lift force components combine to push the tail to one side or the other.

For example, for a "right" rudder command, move the left ruddervator up and the right ruddervator down. The left ruddervator will try to push the tail down and left (which pushes the nose up and right). The right ruddervator will try to push the tail up and left (pushing the nose down and right). If the controls are set up properly, the "ups" and "downs" will be equal and will therefore cancel out each other. Meanwhile, both ruddervators are trying to push the tail to the left (and therefore push the nose to the right), which results in a "right" rudder command.

For left rudder, we use the same pattern in the opposite direction. The left ruddervator goes down, the right ruddervator goes up, the "ups" and "downs" cancel, the tail goes right and the nose goes left. Voila! Left rudder!

Now, one other little quirk still to deal with. We want the "ups" and "downs" to EXACTLY cancel, so that a rudder (yaw) command does not also cause an unwanted elevator (pitch) response. However, in many cases the two halves of the V-tail do not generate the same amount of lift force from the same amount of ruddervator movement. The ruddervator that moves "up" often gets an end-plate effect from the other half of the tail, which helps it generate a little more lift. Meanwhile, the "down" moving ruddervator doesn't get this same end-plate effect, causing it to generate not quite as much lift. If we keep the deflection of both ruddervators equal, we will get a slight nose-up effect every time we apply a rudder input. To fix this problem, we use a little differential. We adjust the controls, either with the control linkages or with the transmitter programming, so that the "down" moving ruddervator moves just a little father than the "up" moving ruddervator, just enough to make their lift forces exactly equal.

The amount of differential required depends on the specific model; some require a moderate amount (perhaps a 25% difference between the two sides), while others require none at all. In any case, it's not terribly critical, the model should be fully controllable with no differential at all. The instructions may give you an initial setting, but you'll probably want to fine-tune that a bit once you've done some flight testing. Once again, it's to some extent a matter of personal preference; some folks prefer having a little pitch response, either up or down, with their rudder inputs. The important thing here is to adjust your model so that it's comfortable for YOU.

Don Stackhouse
DJ Aerotech