I am attempting to build my DAW Dragonette with a V-tail and I am using your advise. The tail area will equal the original total area of 36.90 sq.in. However I am getting an unexpected result when I try to use your formula for calculating the tail dihedral.

The vertical area = 10.49 sq.in.

The horizontal area = 26.41 sq.in.

This is a ratio of 0.397

The TAN-1 of 0.397 is 21.7

Does this mean that I should build the V-tail with a dihedral of 22 degrees from horizontal. That's pretty flat!... Am I doing something wrong, or does the Dragonette just have a tiny rudder?

From : Don Stackhouse

No, you're doing it right. The calculated angle really is that flat.

I just measured the areas of my son's Dragonette, and got a vertical area of 10.94, and a horizontal of 26.09, essentially the same as your numbers. This calculates to an angle of 22.75, very close to your figure. The total area I get is 37.02, for an area per panel of 18.51 square inches. It looks to me like our numbers are in fairly close agreement, well within the bounds of manufacturing tolerances.

Yes, I agree, that sounds like a strangely flat angle, but it makes sense if you think about it. Remember my comment that lower aspect ratio wings need less tail dihedral, and higher aspect ratios need more tail dihedral? To understand why, look at the formulas for horizontal and vertical tail volume coefficients. The key difference between them is that where the Vht formula contains the MAC (Mean Aerodynamic Chord) of the wing, the Vvt formula contains the semi-span of the wing. For low aspect ratio wings, the MAC is proportionately larger in comparison to the semispan, so the stabilizer area is also proportionately larger relative to the fin. When you convert this to an "equivalent" V-tail, this causes the tail dihedral angle (which controls the ratio of stab-to-fin effects) to be flatter. If it makes you feel better, we've been doing a lot of tests lately with V-tailed MHLG's of our own (one of which, the all-composite version of the Nymph, is almost, FINALLY ready to start going "in the box and out the door"), and these tend to also have unusually "flat" tail dihedrals.

All that said, there's a couple of other factors that could cause some slight exceptions to the rule in this particular case. First of all, someone else asked about the fact that a conventional V-tail creates a rolling moment opposite to its yawing moment. In most sailplanes, the tail span is so small in comparison to the wing span that this effect is truly negligible. In the very-low-aspect-ratio world of MHLG's, this isn't quite as valid anymore, which may cause a need for a slight additional amount of tail dihedral to compensate. It also means that instead of keeping the aspect ratio of the tail surfaces about the same as the original conventional tail, you might want to make them a bit lower. This does two things: 1. It keeps the span of the V-tail panels down, which helps minimize the rolling effect, and... 2. It improves the Reynolds numbers of the tail panels, making them more efficient. In general, more span is usually better than more Reynolds number in terms of improving efficiency, but in the ultra-low Re world of MHLG's, the Reynolds numbers become a bigger issue.

In addition, I've noticed that my son's Dragonette has some slight dutch roll problems, suggesting that maybe the original fin is a bit too small. There are a lot of factors that influence this, so I won't hazard any guesses about whether all Dragonettes could benefit from a larger fin, but it's certainly possible. If so, this would also suggest a slightly higher V-tail dihedral, to improve the dutch roll/spiral stability balance.

If my son and I were doing this conversion to his Dragonette, I think I'd start at about 24 or 25 degrees per side, and just tack-glue the surfaces in place for the initial test flights, just in case we need to make some adjustments. Calculations and analysis aside, it always pays to keep your options open. Engineering is not an exact science, despite what many folks would like you to believe.

Any advise on what aspect ratio I should use for the V ?

On small models this isn't quite as critical, because of the increased importance of the Reynolds number issue. I think I'd favor keeping it low in this case, maybe around 2:1 for each panel, to minimize tail panel span and keep the adverse rolling effects low on this relatively low-wingspan aircraft. This gives you an average panel chord of about 3", and a panel span of about 6.17", which BTW is in the same general ballpark as some other MHLG V-tails I've tested successfully.

Don Stackhouse
DJ Aerotech