Will the rudder action on a vtail be affected by adverse yaw as one surface goes up the other down in rudder mode? Does one need differtial in a vtail or should surfaces move equally up and down?

First of all, although this isn’t part of Walter’s question (but you’d be surprised how many folks seem to be confused about this!), a V-tail does NOT work like ailerons, it does NOT directly roll the airplane, and the direction of travel of the ruddervators for rudder commands is the OPPOSITE of the direction that ailerons would move for the same command.

For example, for a yaw to the LEFT, the LEFT ruddervator goes DOWN and the RIGHT ruddervator goes UP. The “downs” and “ups” should cancel each other (so there is no effect on pitch), but both ruddervators are pushing the tail to the right, which swings the nose to the left.

As far as adverse roll from the tail, yes an upright V-tail does make a rolling force that is opposite the direction of the desired yaw, but the span of a typical V-tail is so small compared to the wing that this adverse roll is truly negligible. An inverted V-tail results in a proverse yaw, but it has so many other drawbacks that overall it’s usually better to use an upright V-tail.

Both types of V-tail normally operate at relatively low (and usually negative) lift coefficients. That, plus their relatively small span, means that they contribute nothing of significance in the way of adverse yaw, unlike ailerons. For that matter, a really well-designed set of ailerons doesn’t make any significant adverse yaw either. I’ve built flaperon HLG’s (such as our old Monarch ‘CX’) that could roll quite well with no rudder help at all, even though they had the same polyhedral as the poly 2-channel Monarch ‘C’.

Now the catch: there is no effect on pitch ONLY if the forces developed by both ruddervators is exactly equal. Unfortunately this isn’t always the case. For may V-tails, the ruddervator on one side (usually the up-going side) gets a bit of an end plate effect from the other stab, while the other ruddervator doesn’t get as much benefit. As a result, for equal angular deflections, one ruddervator (usually the up-going one) makes a little more force than the other ruddervator, which results in a slight pitch change (usually nose-up) whenever you apply rudder. By adding a little differential we can give the ruddervator that doesn’t get the end-plate help (usually the down-going one) just a little more travel, so that the forces developed by the two ruddervators are equal. Presto! No more un-wanted pitch change during rudder inputs.

Set up your V-tail model with no differential, i.e.: both ruddervators travel equal amounts for a rudder input. Now go out and fly it. Watch the pitch attitude closely when you apply rudder. Does the nose tend to rise? Then program in a little differential so that the down-going ruddervator moves a little bit farther than the one going up. If the nose tends to drop when you apply rudder (this is unlikely, unless you have an inverted V-tail), then do the opposite. When you have just the right amount of differential, the pitch attitude will stay the same when you apply rudder.

BTW, it is possible for a conventional tail to have pitch effects with rudder application. Fortunately it tends to be less noticeable, but unfortunately it’s usually a lot more difficult to program out, unless you have a really sophisticated computer transmitter.