My next project is a Sagitta 900. (An oldie but, I understand, a goodie.) I hope to build a v-tail for it.

Could you comment on:the advisability of an asymmetrical airfoil, like an upside-down wing with a flat "bottom" and any useful parameters like the point of maximum thickness and location of hinge line (in % of chord, say). I'm not looking for a design job, just some rough guidelines.

From : Don Stackhouse

I recently answered a question on the e-flight mailing list about cambered airfoils for tail surfaces. Although there are exceptions, in general they usually don't add any advantages of significance. The amount of continuous lift required from a tail in either the up or down sense is normally not enough to justify a measurable amount of camber. The tail is almost never as good at making lift as the wing (lower span + lower Reynolds number + lower aspect ratio adds up to relatively poor efficiency), so usually it's best to keep its lift close to zero if possible.

In addition, cambered airfoils can feed some interesting control forces back into the control linkages, resulting in some unexpected trim changes with airspeed. They can also complicate the requirements for differential ruddervator in V-tails, and introduce other non-linearities.

In cases of airfoil deadband problems, a little camber can move the dead spot out of the normal operating range. The slightly cambered Selig 8025 (an 8020 with a tiny bit of added downward camber) is one example of this. On all-flying tails it can sometimes help eliminate pitch control problems. On two piece, stabilizer-elevator type arrangements, elevator deflection introduces camber changes of it's own, so these types of tails tend to be less vulnerable to those kinds of problems. In two-piece (not all-flying) tails, the Selig 8020 has a fairly good track record. It's often thinned a bit, typically to around 6 to 7.5% thickness. In general it's a good idea to keep the tail airfoil thickness a bit less than the wing, for stability and Reynolds number purposes.

It's also important to have good stall resistance in the tail airfoil, and ideally you should also allow for the fact that it will be operating at different Reynolds numbers than the wing. This is part of the reason for the reduced thickness. Of course the reduced thickness and the lower Reynolds numbers both tend to decrease the maximum lift the tail can make, which therefore makes tail stall a bigger danger. The solution to that is to add tail area. It can be quite a juggling act, but in your case someone else has already worked out the bulk of it for you, in the Sagitta's original tail design. As long as you base your design on that, you should be reasonably close on your first try.

BTW, tail stall can occur with any type of tail design. One of its most common causes is a too-aft towhook position, which causes too much of the towline's tension to be loaded onto the tail.

As far as hinge line location, I think I'd start at around 60 to 65% of the way aft of the leading edge.

As far as tail size, find the area of the fin/rudder, and the area of the horizontal tail assembly. Add those two areas together. This is the total area of the V-tail. Divide that by two. This is the area of each v-tail panel.

Divide the area of the fin/rudder by the area of the horizontal tail. Find the arctangent of that number (any good scientific calculator should have that function). The answer is the dihedral from horizontal for each tail panel. Note, this method is an approximation, but it's a very good approximation, with a long and successful history.

Good luck, and please let me know how it turns out!

Don Stackhouse
DJ Aerotech