Should I be concerned about the span-wise balance of the Vee-tail?

In the process of building the Chrysalis I received as a Christmas gift (what a wife!), and just finished initial construction of the V-tail. I've noticed that one side is heavier than the other. Obviously spanwise balance is important, especially in the wings, but should I also be concerned about the tail, since the moment is so much shorter?

From Don Stackhouse :

Dave, interesting question but it's not a significant problem. Even lateral balance of the wing isn't that significant unless it's WAY off. The extra weight you would add to the tail trying to laterally balance it unnecessarily would cause much bigger problems than the lateral balance itself. What really matters is lateral balance of the finished aircraft, and even that is only an issue if it's off enough to cause one wing to stall significantly before the other. Unequal washout settings between the two sides of the wing are far more important, and even minor variations in that from normal construction tolerances will totally overshadow any reasonably normal lateral balance discrepancies.

Now I can hear the purists out there shouting "Oh, yeah, Don, prove it!" OK, lets look at a hypothetical case using a typical 59" span, 400 sq. in. hlg as the "guinea pig". Lets assume that somehow you built the model so far off that it needed 1/4 ounce (more than 2.5% of the total aircraft weight for some Chrysali, and more than 6% of the wing weight, a relatively huge discrepency) in one wingtip to balance properly. Let's also say that the Mean Aerodynamic Chord (or MAC) is 14" out from the fuselage. This depends on planform, but that's a reasonably typical number. 1/4 oz. at the tip equals a difference in lift between the left and right wings (measured at their MAC's) of 1/4 oz. x 29.5"/14", which is about .53 oz. We decide to make a slight washout increase on one side and a slight decrease of the same amount on the other side to generate the correction of .26 oz. of lift for each wing.

How much change is needed? A typical airfoil for a hlg changes its lift coefficient by about .1 for each degree change in its angle of attack, and at 15 mph (a fairly typical airspeed for a hlg), the change in lift coefficient required is about .02, so the angle change required is about 0.20 degrees, or about .024" at the MAC (less than 1/32"!), or about .051" at the tip.

Measurable, yes, but within the range of washout errors we see. And remember, this is for a relatively huge balance error in the wing. To get the same effects from an un-balanced tail, the discrepency would have to be about 4 times bigger than that!

What you would see in this particular case is a tendency to roll slowly to one side at high speeds and the other way at low speeds. You would also see this phenomenon in a model with washout discrepencies that has been trimmed out with rudder, or tail alignment problems trimmed out with aileron. In fact, this change in lateral trim with airspeed occurs just about any time you fix a problem in one parameter by adjusting something else. No great surprise here.

Tail lateral balance just isn't a problem in any but the most extreme cases. Instead of worrying about the balance issue too much, concentrate that extra effort on building the model as straight and warp free as possible. This is far more sensitive and important than lateral balance.

Just build it straight and go have some fun. Good luck with your Chrysalis!

Don Stackhouse @ DJ Aerotech
djarotec@bright.net