Marske Aircraft XM-1
The XM-1 was designed and constructed as a research 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 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.
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 is the use of C.G. tow hooks. They have no adverse affect on the aircraft at any time on any type of tow. 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. The XM-1 used a Faubel airfoil section with a relative thickness of 14 %.
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 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.
Jim Marskes XM-1 glider made its first flight in 1957. Test flights began with a few soaring routine ground slides were made at speeds up to 30 mph to see how it would handle. 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 leveled off before trying a few gentle pull-ups to get the feel of the elevators. All controls responded smoothly and firmly. 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. When the towline was released it was flying only a few mph above the stalling speed. With the C.G. tow hooks located considerably below the center of gravity there was 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 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 it would not have far to go. Once again airborne it rose very quickly from the ground due to the kiting affect of the low tow hooks. On release, at about 12 feet it chased the tow car down the runway and was gaining on it. Leveling off, the airspeed dropped from 41 to 4·2 mph. This flight revealed a lack of directional control causing the craft, to rotate about its vertical axis as other plank designs have experienced.
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.
By September all modifications were completed and 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. 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. Each succeeding tow took me a little bit higher and became routine and uneventful. At 50 feet and dropping full flaps they behaved exactly as they were designed to. From this time on all take-offs and landings were made with full flaps, resulting in shorter take-offs and slower landing speeds. The flaps produce no buffeting whatsoever and are quite effective.
Using a 450 foot towline take-off was made with full flaps, requiring a 100 foot run: into the 5 mph head wind. Going up no lateral or directional control was required and the ASI indicated 55 mph all the way. Release was made at 350' and was very smooth, they’re being no tendency to pitch up.
On this prototype, numerous configurations were tested including wingtip fins, central fin, spoilers, flaperons. Based on these experiments, Jim Marske, aided by Bill Daniels, flew the Pioneer I in 1968.