Overkill was a 100" span swept flying wing built by myself and my friend, Keith Findling, about six years ago. We calculated the sweep angle using a formula published by the Kuhlmanís in their RCSD column, "On the Ďwing." Overkill used the E222-E230 series of airfoils designed by Dr. Eppler specifically for use by swept flying wings. For those who arenít familiar with them, the Eppler 222 is an undercambered section, while the E 230 is a reflexed section. The root chord was 350mm, while the tip chord was 325mm. The wing featured no washout. According to the formula we employed, the E222-E230 series does not require washout when a sufficient sweep ratio is used.
However, for reasons Iím still not certain of, the wing turned out to be extremely unstable in pitch. I donít know if we screwed up on the calculations, or whether there was something wrong with the formula we used. (This was not the Panknin Twist formula.) Still another complicating factor was the fact that it was later discovered that the published positive pitching moment for the E230 was only half that measured by actual wind tunnel tests. In any case itís very likely that this was at least a contributing factor.
Construction: The wing was glass-skinned grey foam. We were worried about the foam core soaking up too much epoxy and tried to save weight by laying up the skins on a sheet of mylar and then epoxying them to the core. This turned out to be a bad idea, as you gain back more weight than you save. The glass was laid up at 45 degrees for torsional stiffness. The full-span spar was truly overbuilt. Spar caps were 3/16 aircraft spruce with an end grain balsa shear web and 1/64 ply on either side. The spar tapered from 3/4" wide at the root to 1/2" at the tip. (Yes, you could sit on it.) The wing was built in one piece to save time and weight, and because this was an experimental plane. The angled wing joiner was a million or so laminations of 1/64th ply expoxied into the root ends of the spar. (By now you can probably guess why we named it Overkill.) Winglets were made from sheet balsa. At least in terms of construction we seemed to have made the right decision in overbuilding everything. Even when Overkill was flown at very high speeds, it never exhibited any flutter problems.
The flying characteristics of Overkill could best be described as "divergent." It was very stable in yaw and roll, but required constant input in pitch to keep it flying straight and level. Once the nose started to come up, it would continue to do so until the wing stalled. Conversely, if the nose was allowed to drop, it would progress until it developed into a vertical dive.
Despite its pitch stability problems, performance was excellent. Overkill displayed an extremely good L/D and a good sink rate. The design used elevons for aileron and elevator function, and also had fairly large flaps for landing that were about 1/3 span of the wing. The flaps created almost no discernible negative pitching moment and were surprisingly ineffective at reducing speed. Even when fully deployed at 45 degrees, they did almost nothing to slow Overkill. And speaking of speed, flying this beast was an adrenaline-generating experience. When given the slightest amount of down elevon, it would accelerate like a banshee. On one occasion, I flew straight off launch into a huge thermal. Even though Overkill was pitching first nose up and then nose down in the turbulent thermal, she gained height very rapidly and soon orientation was getting hard as I was looking at the bottom of the wing. At this point, I dropped the flaps fully and gave a touch of down elevon to fly her clear of the thermal. Within seconds, Overkill streaked over our heads at a height of about seven hundred feet with a roar that we could plainly hear on the ground. This wing would certainly move!
In addition to the ball-bearing pitch behavior, stall characteristics were also extremely nasty. Any kind of stall, no matter how gently entered, would result in the dropping of one wing followed by a nose-down, unrecoverable spin all the way to the ground. This propensity for violent tip stalling is what eventually did Overkill in. During a bungee launch Overkill popped off at about sixty feet. I flew her around and set up for a landing. Once I had it pointed toward me, however, it quickly became obvious that it was going to overshoot by miles--the L/D was so good it always took me by surprise. As Overkill zoomed past overhead, I panicked and dropped full flaps. We had just increased flap throw from 45 degrees to sixty-five degrees. Overkill was moving pretty smartly and ballooned into a stall when I dropped the flaps. This was followed by the usual tip stall and nose-down spin into the ground from thirty feet. There was a splintering impact as the wing broke into two halves, and that was that. The damage was repairable, but as the wing was so unstable, we decided it wasnít worth the bother.
Conclusions: For those considering building a wing today, I would not recommend the E222-E230 series. In the five years since we built Overkill, a whole variety of modern, and much more efficient flying wing airfoils have been designed (nearly all come from Germany). At the time, a number of flying wings had been built in Germany that used the E222-E230 series, but they all featured higher sweep rates. We designed Overkill with a fairly low aspect ratio (about 7) as I was concerned about having high enough Reynolds numbers. In hind sight, having seen the speeds that swept wings cruise at, I now realize this was unnecessary. An aspect ratio of 10 to 11 would probably be a good conservative design parameter.
Still, even though it wasnít a successful project, it certainly gave us a taste of the kind of performance that is possible with a swept flying wing. Since that time, much more information about designing flying wings has become available. One of the chief sources is Bill and Bunny Kuhlman at B2 Streamlines. Be sure to check out their site on the web. They also sell a variety of books on flying wings and general RC sailplanes for those interested in designing their own.