SAILPLANE

 

The XM-1, as its name implies, was designed and constructed as a re-search and development sailplane. This sailplane is intended to be the first of a series to discover the most practical soarer for the weekend flyer. The plank layout was chosen for several reasons. It has simplicity of design, very good performance for its size, forgiving flight characteristics, being stall and spin proof and light in weight making the ship easy to handle on the ground. Of course there are a few features I myself would like to see in a sailplane so naturally they have been incorporated into the design. A few of these are smooth and flowing lines which not only aids performance but also is very pleasing to the eye.

 

Flaps have been installed instead of the usual spoilers to allow a quick take-off on auto or winch tow and they provide a low landing speed. Another advantage of flaps is that the top of the wing is kept clean and eliminates-the possibility of water running into the wing if caught out in the rain. Another feature, which I am very pleased with, is the use of C.G. tow hooks. They have no adverse affect on the aircraft at any time on any type of tow. The correct word for it is smooth.In order that the sailplane may be trailered and stored easily, disassembly of the ship is necessary. Only the wings are removed from the pod and are joined in the usual manner. The wing has a span of 38 feet, a constant chord of 51 inches and an aspect ratio of 9. Since the aircraft was to have a moderately low wing loading a 14% reflex airfoil was chosen for good high-speed performance.

 

In order to keep all lines as clean as possible for efficiency, molded fiberglass is used wherever possible in the structure. The main wing spars are of laminated spruce tapered in thickness from root to tip to distribute flight loads evenly. The main wing ribs are of standard truss type construction while the nose ribs were sawn from 1/4-inch marine plywood. A rear spar is used mainly for fastening fittings, eleven hinges and to transmit drag loads to the fiberglass skin. The elevons are controlled by pushrods throughout while the rudders are cable actuated. Molded fiberglass covers the leading edge D tube and the first five root rib panels aft of the main spar to carry the drag loads. This consists of two laminations of glass cloth secured to the wing with epoxy resin. The basic structure of the XM-I as Et was being constructed. The fiberglass fuselage shell is just as it came from the mold. Construction of the fins are quite simple being basically a spruce and plywood frame covered with fabric.

 

During flight tests in 1957, smaller and heavier fiberglass covered fins were used with drag flaps but were discarded clue to their ineffectiveness. All fiberglass sheets were formed on a smooth flat metal sheet that was given a heavy coat of paste wax. One side of the fiberglass sheet thus formed is rough while the surface which was against the metal surface is smooth as glass. The smooth surface is used as the outside finish. Then securing fiberglass to fiberglass, or fiberglass to wood, the surfaces that will come in contact with one another must be made rough in order to give the resin something to cling to. So far, I have found epoxy resin to be the best gluing agent for fiberglass but you must work fast before the resin hardens. The leading edge fiberglass sheets were formed in a jig because a flat sheet of two laminations cannot be drawn around the leading edge. The fiberglass shell of the fuselage was formed over a built up plaster mold. This shell consists of three to four layers of glass cloth, four layers being used where higher strength is required. Sanding fiberglass by hand can be an exhausting job so a belt sander was rented. A tubular steel frame was welded together to fit into the fiberglass shell. This frame carries all flight, towing and landing loads imposed upon the aircraft.

 

To secure the frame to the shell, all tubing that was to come in contact with the shell was wrapped with strips of glass cloth and given a coat of resin. After it hardened it was sanded down and fitted to the shell. Additional strips of glass cloth were then given a coat of resin and stretched between the shell and the wrapped tubing. When this had hardened it was sanded down and given another coat of resin to give it a smooth finish. The remainder of the shell was strengthened by the addition of two fiberglass bulkheads and reinforcements in the nose. In the cockpit a control wheel is used rather than the conventional stick. The rudder pedals now operate in a conventional manner made possible by the highly differential bellcranks, which allow only one rudder to swing out at a time.

 

 

FLIGHT TESTS OF 1957

Test flights were carried out during September and October whenever Weather conditions permitted. A few soaring routine ground slides were made at speeds up to 30 mph to see how it would handle. I had a bit of trouble getting used to the independently operating rudders. However, aileron control was very good and it was also possible to balance the glider on its wheel by the use of the elevons. The next tow was to be a slow acceleration up to 40 mph to see if it would fly. The tow started off well, lateral control was good but the rudders were, a bit sluggish. At 30 mph the control wheel was brought back and the ship responded with the nose coming up to take-off position. At 40 she was airborne and I leveled off and held her on a straight course for a few seconds before trying a few gentle pull-ups to get the feel of the elevators. All controls responded smoothly and firmly. Looking ahead I could see the tow car was approaching the end of the run. Upon release of the towline the sailplane suddenly dropped as if stalled out. Applying full backpressure did not remedy the situation and the resulting drop to the ground shook both glider and pilot. I was thankful that I had a thick cushion under me and a strong glider to withstand the shock of a hard landing. That was enough flying for that day, at least till I found out what had happened.

 

Checking with Paul Wilhelm who was driving the tow car, I learned that when the towline was released I was flying only a few mph above the stalling speed. What had happened was that with the C.G. tow hooks located considerably below the center of gravity there is a tendency for the tension of the towline to hold or bring the nose up. Also, by applying full backpressure suddenly is much the same as raising flaps on an airplane just before touchdown.

 

The following weekend we decided to have another crack at it. I climbed into the cockpit and gave last minute instructions for the flight. This flight called for 50 mph and a free flight glide. Release was to be made only a few feet off the ground so if it would drop I would not have far to go. Once again airborne I rose very quickly from the ground due to the kiting affect of the low tow hooks. At about 12 feet I pulled the panic button, I mean the release button and gritted my teeth for a possible 15 foot drop. But instead I found myself chasing the tow car down the runway and was gaining on it. Leveling off, the airspeed dropped from 41 to 4·2 mph. My glide seemed exceptionally flat and I just floated down the runway. This flight revealed a lack of directional control causing the craft, to rotate about its vertical axis as other plank designs have experienced. Since it would be unwise to try any further tests it was decided to send the ship back to the workshop for modifications.

 

MODIFICATIONS OF 1959

Because college studies kept me busy throughout 1958, modifications were not completed until September of 1959. The first change to be made was the relocation of the tow hooks to eliminate the kiting affect. This was accomplished by raising them approximately 5" to bring them closer to the center of gravity. The previous lack of directional stability had required the pilot to be on his toes all the time. To rectify this situation the fuselage length was increased by 9 inches and the hood given a dorsal fin look. A new and larger set of fins were constructed which replaced the drag flaps with differential rudders of double the previous area. Also, the independently operating rudder pedals gave way in favor of the conventional setup. During previous take-offs I often found myself sitting there with both rudder pedals depressed, which added drag besides being disconcerting.

 

 

TEST FLIGHTS OF 1959

By September all modifications were completed and I was anxiously awaiting a chance to fly her again. After assembly of the craft at the airfield and several ground slides later it became apparent that there was a marked improvement in response to the controls. Satisfied so far we decided to see how the new CG hooks worked. On this next tow control again was firm and responsive. The take-off was very smooth with no corrective force of any kind necessary. The kiting and yawing affect had completely disappeared and the ship had become as stable as any sailplane I had ever flown. Each succeeding tow took me a little bit higher and became routine and uneventful. As yet I had not tried the flaps for fear of how the stability of the craft would be affected. However I soon: found myself at 50 feet and was forced to use them in order that I would not overshoot the runway. Dropping full flaps they behaved exactly as they were designed to. From this time on all take-offs and landings were made with fullflaps, resulting in shorter take-offs and slower landing speeds. I might add that the flaps produce no buffeting whatsoever and are quite effective. The MX-1 without a doubt was now ready for its first high flight around the field. Now using a 450 foot towline the take-off was made with fullflaps, requiring a 100 foot run: into the 5 mph head wind. I climbed cautiously at first but near the top of the climb the wheel was ail the way back. Going up no lateral or directional control was required and the a.s.i. indicated 55 mph all the way. Release was made at 350' and was very smooth, they’re being no tendency to pitch up. While going around the field at 70 mph: indicated air speed, the longitudinal feel of the aircraft reminded me very much of the Piper Tri·Pacer. Wheeling around on final I lowered the flaps and set her down feeling very satisfied and pleased. It is a feeling, which makes every second of the thousands of hours of work designing and building seem well worth while. Unfortunately winter crept in early with its wet weather and put an end to the test flights. I am looking forward to many pleasant hours soaring in the MX-1.