I've finally managed (with some help) to build a model that I have wanted to try for a long time. Ever since I became infected with handlaunch fever,I have wanted to try a handlaunch flying
wing. A flying wing planform just seems right for hand-launch. It's light, couldn't be simpler, and it represents a very interesting design exercise.
My friend Tom Broski from Charlottesville, Virginia builds beautiful, fully composite hand launch wings that work very well on models like the DJ Aerotec Monarch. Tom is an artist and his wings are each small works of art. I have been impressed with his skill, craftsmanship,and artistry since I first met him at one of our Eastern Soaring League contests. I talked to Tom this summer about the idea of collaborating on the development of a handlaunch flying wing. He was quite interested, and I knew that he could make excellent surfaces for the plane. When he agreed to make wings for a prototype, I told him that it would probably take several of them to work out the details and get the plane flying well. For that reason, I suggested that he keep these development versions as basic as possible, not to waste his time making them beautiful. Later I was somewhat sorry that I had said that.
I had done several layouts for this plane over the last several years, but had never actually begun building one. I know from previous flying exercises that an effective flying wing with good performance could be developed. My own ideas had evolved to a swept wing with a cambered airfoil at the center and a symmetrical airfoil at the tip. With a linear blending of the airfoils in between, the inner part is where the lifting work is done, and the outer part, where it becomes symmetrical, does the job of the horizontal tail on a conventional design.
The late Dr. Walter Pannekin has developed equations for calculating the aerodynamic analysis needed to make this kind of arrangement fly properly. Alan Halleck from Oregon turned these equations into a computer program some years ago. Since Alan sent me a copy of the program, I have used it to check enough different flying wing arrangements that I was confident it would get me into the right ball-park for setting up our first prototype. I was not at all sure that it would get the plane right-on at the first cut. I believe this program is available from B2 Streamlines, PO Box 975, Olalla, WA 98359-0975. As always a SASE is appreciated when requesting info like this.
As I laid it out, the wing has a 60 inch span. The root chord is ten inches, the tip chord is six inches, and the leading edge sweeps back at a 22 degree angle. As I said, a very simple sailplane. I wanted the thin airfoils that I've come to appreciate lately; so for the root airfoil, I used the same SD-7037 thinned to 7.5% that I used on our conventional Skeeter-Hawk hand launch design. For the tip airfoil I chose the SD-8020 symmetrical airfoil thinned to 6%. Dr. Pannekin's equations showed that for proper trim with adequate stability, the wing tip should have eight degrees of washout and the balance point should be in the range of 7.5 to 8 inches behind the root airfoil leading edge.
Tom made a set of composite wings to my specifications and I put together the model and installed the radio. Battery, receiver, and servos are all located within the airfoil of the wing. The battery is four cells of 150 mAh capacity, just behind the leading edge of one wing. The 4-channel Airtronics receiver is in the same place in the other wing. HiTec HS-60 micro servos are set just far enough outboard that their leads will reach to the receiver with no extensions. These drive elevons that stretch along most of the span of the wing. I made a small keel that fits between the wing panels and extends about an inch below the wing. This provides a place to hold the model for launching. A cross dowel through the keel at the c.g. acts as a finger rest for hand-launching.
The plane is finished by putting on the fixed vertical fins at the wingtips. I flew it first without these, but I soon realized that while it did ok at high speed with extra nose weight, it would never be a good thermaller without some stabilizing fins. The ones I used are made of 1/16th balsa with a 4-inch bottom chord, 2.5 inch top chord, and they are 5 inches high. After actually throwing these off the plane a couple of times, I found that I had to fiberglass them on with a bit of s/4 ounce cloth and CyA adhesive.
The prototype has performed beyond my most optimistic expectations. I'm really surprised that the plane came out so perfectly on the first cut. The programmed size, washout, airfoils and c.g. seem to be close enough that I can't figure out any changes that would lead toward improvement. Even the little towhook, that I put on to allow launching with a mini hi-start, seems to be just right. I've probably used up all of the good luck I'll get for a year on this one. Or maybe I should give a lot of the credit for this success to Dr. Pannekin.
At this point about 12 people have flown the plane through hi-start launch to landing, and several of these are advanced beginners who are most comfortable still with polyhedral sailplanes. I thought it would take two or three prototypes to get it flying well, but I love it just as it is. I have had some great flights with the plane. It launches beautifully both by hand, and with the mini hi-start. I've even flown it in its first little local contest where I got a score about 90% of the winner's.
It is a flying wing, and everyone has a bit of a problem adjusting to the visual aspect
of this configuration. It's amazing though how quickly you can become oriented to its
strange appearance. When finished, trimmed and balanced the plane weighs 11.8 ounces. With almost 500 square inches of very thin wing it is both light and fast. The battery and receiver are as far forward as I can place them. I'm using a 150 mAh pack, but based on the weight I had to add
to the nose, a 250 mAh pack could probably be used. The balance point is 7.7 inches back from
the nose of the plane. The keel is made oflight 1.4 lb./cu. ft. balsa with x/16th ply doublers on each side. The keel could bebagged with fiberglass and the ply avoided. I'll probably do that on a future version.
There is a 5-inch length of carbon fiber arrow shaft mounted in the keel that fits into a 9-inch length of carbon fiber kite spar mounted in each wing. I also installed a 1/16th steel alignment pin one inch back from the nose. The elevons start 41/2 inches out from the root of the wing. At that point they are 2 inches wide. From there they extend along the trailing edge for 29 inches and have a width of 1.7 inches at their outer end. Elevon movement for elevator control is 5/16inch up and an equal amount down. For aileron control the elevon moves 1/4 inch up and 5/16 inch down. This reverse of normal aileron differential was necessary to stop a tendency of the wing to nose up in a turn. I found it was very necessary to good handling of the wing.
Well, I've wanted a handlaunch flying wing for a long time, and now I have one. I wish I hadn't suggested to Tom that he should avoid making it pretty. It would really look neat with one of his rainbow color schemes. However, all sailplanes are pretty in lift and I've had this one up there
quite a few times now. It does fit the concept of a very simple plane. Just a wing and an arm. A bit of lift and some tight circles and away it goes. No chance of mixing this one up with one of the other models at the field. It has all the right stuff to meet my specifications; light weight, thin airfoils, smooth curves, good looks and good flying.
HERK'S EMAIL ADDRESS IS herkstok@aol.com
