...I am working on a new V-tail HLG design and have finally gotten the prototype flying. Well really I just sort of cobbled together some stuff I had lying around.

The plane looks promising but it seems to drop its nose a little when circling. I think I need to tweak the differential but I was just reading some info saying that V-tails tend to pitch up when circling and so should have more down throw...this seems opposite of my situation...I wonder if I need to adjust the decalage?

From : Don Stackhouse

Michael, it doesn't sound to me like it has anything to do with decalage or differential.

First of all, regarding differential in V-tails: It's really very simple. There is a mutual end-plate effect between the two panels of the V-tail, but for a rudder input those effects are often not equal between the two sides. Although different designs see differing amounts of this, in general the up-going ruddervator sees more end-plate effect than the down-going ruddervator. If the deflections of the two ruddervators are the same (i.e.: zero differential), the up-going ruddervator will therefore generate more lift than the down-going one, which will tend to pull the nose up. By adding just enough differential to cancel out the unequal end-plate effects, we make the lift of the two rudders once again the same, so there is no change in pitch when you make a rudder input.

Get your model flying directly away from you in level flight, preferably at close to eye level if possible. Now give it a big rudder input and watch for any change in pitch as it begins to yaw. If you see a significant un-wanted pitch change, then you need to adjust your V-tail differential.

In addition, some folks feel more comfortable with a model that tends to drop its nose a little when entering a turn. They add just a little extra differential to get this effect. It's entirely a matter of personal preference. I, for example, prefer my models to be exactly neutral in pitch during a rudder input. You should set you model up in whatever way is most comfortable for you.

However, the behavior you describe (if I understand you correctly) isn't a differential problem, or even a V-tail problem. There are several other phenomena that could be involved in your turning problem:

1. A circling airplane is using some of the wing's lift to counteract the centrifugal force from the turn. This leaves less to hold the airplane up. The airplane naturally starts to descend, which increases airspeed and/or angle of attack enough to provide the necessary extra lift. A too far forward C/G will tend to increase the amount of nose-down pitch change that results from this. A touch of up elevator to increase the angle of attack (assuming you aren't too close to the stall angle of attack to begin with) can provide the extra lift, without the nose-drop. Just be careful, there are limits on how much angle of attack you can impose on the wing before it gets mad at you and goes out on strike. In a turn, your stall speed goes up.

2. In a steep bank, the yaw resulting from a rudder input into the turn will be directed partly toward the ground. This is a quirk of airplanes that don't have ailerons. A little more up elevator and a touch less rudder can help in this case.

3. You might not have enough wing dihedral, or your tail could have too much dihedral and/or area (the combination of tail dihedral and tail area is what determines how much vertical fin effect it has). The same behavior in a conventional or T-tail means that the fin/rudder is too big.

There is a balance between vertical fin effect and wing dihedral. If you have too much dihedral or not enough fin, you get "dutch roll" (this probably is not the problem you have).

If you have too much fin effect and/or not enough wing dihedral, then you get spiral instability. I suspect that this could be your problem. This is where the model wants to steepen up the bank angle plus yaw into the turn by itself, gradually sliding into a spiral dive. The yawing effect at higher bank angles that I discussed above is what causes the nose to drop. Increase your wing dihedral or flatten your V-tail angle and see if it gets better.

Don Stackhouse
DJ Aerotech