In a vee tail, if I have a small percentage excess over the amount required for stab area, can changing the vee angle be used to fine tune spiral stability?

IE. Within Reason, Is slightly excessive horizonrtal stab area much of a concern?

From : Don Stackhouse

Generally that can work, within limits. If spiral stability is already something positive, then changing tail moment arm is the most effective way to change it. However, if you have negative spiral stability, reducing the tail area can improve it.

The dihedral angle of a V-tail is what determines how much of the V-tail's authority is devoted to pitch, and how much to yaw. Increasing the tail dihedral is therefore the same as reducing the area of a conventional tail's stabilizer and correspondingly increasing the area of its fin. You can trade off some of one to get more of the other. For most sport models, the amount of pitch stability is not a huge issue; you can move the C/G back to reduce the static pitch stability, and having an excess of dynamic pitch stability from too much horizontal tail area is usually not a big problem, within reason. Therefore, starting with a bit too much tail area so you can get more fin effect (if required) by increasing the tail dihedral is a perfectly valid approach.

In fact, it's even been used on some full-scale airplanes with conventional tails, when they discovered that they needed more fin area. It was often easier to just add some dihedral to the stabilizer to give the fin some extra help, than to build a whole new set of tooling for the fin. However, there are other reasons for adding dihedral to the stabilizer (such as curing interactive vibration problems between the stab and the props), so don't assume when you see a plane with dihedral in the stab that it was done for fin area concerns.

Don Stackhouse
DJ Aerotech