I have built a combat plane using a plastic flatbat, and coroplast wings with a V-tail. The V-Tail, (also made from coroplast) has 45 degrees of dihedral.

When I pull hard on the elevator the plane does a very hard barrel roll. I believe the problem is in the tail, since I have used the wing on another plane. What could be causing this.

From : Don Stackhouse

Something is causing an unequal response between the left and right sides of your plane.

Since this wing performed without this quirk on another model, I agree that it's probably not the culprit in this case. This means that your plane is probably getting the equivalent of a strong rudder input from some source, probably in the tail, whenever you apply lots of elevator.

So far I've told you stuff you already know. So what are the likely causes?

The first thing I'd check is your linkages to the tail. If you pull up elevator, do both ruddervators deflect upwards by exactly equal amounts? If not, then you don't have the linkage geometry equal for both sides.

If you push them back towards neutral while holding up with the transmitter, are they equally stiff? If not, then you probably have a buckling problem somewhere in the linkage for the less stiff of the two ruddervators. You need to make sure that both pushrods are well (and equally) supported over their entire length.

Is the tailboom and the tail's attachment to it sufficiently stiff in torsion? If not, then any inequalities between the two ruddervators could be causing the tail to twist when you apply elevator. This will make one half of the tail act like an elevator and the other half act like a rudder.

Are the torsional stiffnesses of the tail panels sufficiently great, and equal to each other? When they bend, does one of them tend to twist as well?

OK, so we've gone over the major mechanical/structural suspects, now what else?

Aerodynamics. You didn't say what the aerodynamic details of the rest of the plane were, but if the wing is putting major demands on the tail, it's possible that the tail is stalling. If it does, it's very possible that one side could be stalling before the other, resulting in the behavior you describe. Chances are that the two tail panels are not exactly equal in their construction and mounting, and the airplane is also not perfectly rigged and symmetrical, so the tail panels are therefore not working equally hard. This means that one will stall before the other, which on a V-tail acts like a huge rudder input. Excessive demands on the tail could be the result of very high pitching moments such as those generated by landing flaps, or they could result from a too-forward C/G, or from making the tail too small for the job in the first place.

One thing is for certain, that rectangular airfoil you get from a single piece of coroplast is prone to leading edge type stall behavior, with its very sudden and very large loss of lift. You'll need to make sure your tail is big enough that it never gets close to stalling.

Good luck, and please keep me posted on how this turns out for you!

Don Stackhouse
DJ Aerotech