Oliver asks about asymmetrical V-tail design theory.

I would like to get your opinion of the allowable size of an asymmetrical V-tail in which the sides are staggered one ahead of the other. If the trailing edge of one side is ahead of the leading edge of the other, would the interference effects be reduced enough that the projected areas could be closer to the areas of a conventional crosstail rather than the sum of the areas? The asymmetry could be carried a step farther by making the tail volume coefficient of each side the same. That is, the forward side could be given a slightly larger area to compensate for its shorter moment arm.

From : Don Stackhouse

Interesting idea, but interference has nothing to do with it. In our experience, on typical R/C sailplanes the amount of "destructive interference" between the two panels is pretty much negligible. If, for a specific application, the interference was not negligible, then the total area of the V-tail would have to be GREATER than the total area of the equivalent conventional tail.

The reason why a V-tail (assuming interference is negligible) must have the same total area as an equivalent conventional tail (of the same control authority) is because we must be able to generate the same forces with both tail types in both pitch and yaw SIMULTANEOUSLY. A V-tail with the same projected areas in the top and side views could make the same pitch force, or the same yaw force, but not both at the same time. This could become a major issue in some flight conditions, such as when trying to recover from a spin.

As far as the interference issue, I think there are some fuselage interference issues involved as well, which are more significant on short-coupled airplanes with fat fuselages. There are other factors that enter into the question of equivalency between the two tail types. In the case of interference during rudder inputs, it would seem that a V-tail is at a disadvantage, although there are interference effects that cloud the issue for conventional tails when they're asked to make combined simultaneous pitch and yaw inputs. In addition, there are factors (such as the larger span / aspect ratio / chord of a same-total-area V-tail) that give the V-tail an advantage over the conventional tail. In actual practice, it's been our experience that the sum of all these various pro-and-con issues usually adds up to just about ZERO.

We've consistently been able to size tails for a variety of models, offered with the builder's option of either tail type, with negligible differences in handling and control authority between the two versions. In nearly all cases, by using the same total area, and V-tail dihedral based on the arctangent of the vertical tail area divided by the horizontal tail area (the constant control authority method, slightly different from the arctan((Av/Ah)^2) constant stability method advocated by Joe Wurts, Mark Drela and others), we've been able to make all the major equivalency issues, including stability, and also the dutch roll/spiral stability balance, come out nearly perfect on the first try. It's tough to argue with something that's consistently successful in actual practice.

As far as the merits of offsetting the tail panels one in front of the other as you suggest, I can see pros and cons. At first glance I'd expect to see minor effects overall, some good and some bad, but the interactions of tail surfaces with each other are quirky enough that I'd be hesitant to make a firm opinion either way without flying a few of them first.

Don Stackhouse
DJ Aerotech