Building Scale Sailplanes 2
My original Mu 13 D3 model, described in the previous article, had been single channel ‘bang-bang rudder’ only. The transmitter was a hefty box that stood on the ground, heavy batteries inside and a ten or twelve foot long aerial standing vertically. One experience with this gear (not my mistake this time) was to have the aerial hit by someone else’s glider, a 10 foot span Sunspot, crashing into it and breaking it off. With the family I had moved to shelter on the lee side of the Ivinghoe ridge for a picnic. There were only two flyers at Ivinghoe that day, but Murphy’s law prevailed. The other pilot let his model drift too far back into the lee side downdraught and lost sight of it altogether as it came down fast. That was an event my wife did not let me forget, since the broken aerial fell on her. Fortunately it was only aluminum and no serious harm was done to us or the children. The other flyer had extensive repair work to do on his glider afterwards. I wonder if he remembers!
By the time I was able to build another scale sailplane, radio control for models had come on a great deal. It was possible now to operate four proportional controls with ‘sticks’ on a hand-held transmitter. What a great advance this was, but how big and clumsy those early four channel control sets look to us now! I learned to handle them with several ordinary slope soaring models.
The family moved from England to Australia which was another great change. After many delays, I set myself up again with a workshop. The house, built on a slope, had a large timber decking at the side with some limited headroom underneath. I dug myself a small workshop under there, laid a concrete floor and a couple of steps down, with walls of building blocks, and a window looking out over the garden.
I decided to attempt a 1/5th scale model of the original Mü 13, the 1936 prototype, lightweight Atalanta, built and flown by Kurt Schmidt. It was designed by Egon Scheibe of the Münich University Akaflieg. (Hence the Mü in the designation).
The Atalante was unusual for its time, having a built up, welded steel tube fuselage frame covered with fabric. This was very light. The wing was wooden with the usual strong main spar, plywood skinned D nose and fabric covering over the rear of the wing. Most unusually, there were flaps as well as ailerons. The tailplane was all moving. The tail moment arm was very short.
Enclosed, transparent cockpit canopies were now usual and the Atalante had one built up from light steel tubes, fitting neatly into the leading edge of the wing. As the photographs show, Kurt Schmidt’s head was virtually inside the wing and the outward view was extremely limited. He could look out over the top of the wing in banked turns, which was important when other sailplanes were sharing his thermals. His view downward was aided by large triangular transparent panels in the cockpit sides.
Building the model was straightforward although, as with all large models, there was quite a lot of it. I made the fuselage as a simple frame with main longerons of spruce, a good box at the front to take the radio gear, batteries and trim ballast and a few load-bearing cross frames from plywood. Since the model would be covered with translucent fabric, clear doped, shadows of the internal structure would be dimly visible so I did my best to imitate the full scale framework. For the wing joiner I used several strips of spring steel half an inch wide, set vertically in plywood boxes let into the main spars. These worked well. They were very strong and stiff in vertical bending but in bad landings they would flex easily in the fore and aft direction.
The wings were orthodox with spruce main spars with plywood webbing, and balsa skinning forward.
I decided I should be a little more adventurous with the ailerons and flaps. I had done some archery and knew that the best arrow shafts were made from very high quality, stiff alloy tubes. With a little care, a selection of these could (and still can) be found to fit perfectly inside one another telescopically. A tube of any required length can be made up of several shafts sliding into one another with a spot of glue to lock them firmly. (Glass-fibre shafts were also readily available, but carbon-fibre arrows had not been developed at this time. Archers often discard damaged arrows after use, and the discards can often be bought very cheaply for the tubing. Dealers may also give offcuts away because every archer has arrows cut down to a personal preferred length. I still use arrow shafts for many purposes.)
For the Atalante I made the control surface spars from arrow shafts. The ribs, all balsa, were shaped to fit onto the rear of the arrow shafts at the correct diagonal angles (see photos), and glued with epoxy to the tubes, which were sanded lightly to remove the anodising. A light trailing edge was added. The hinges were made by filing semi-circular holes in the tubes, inserting plastic wall plugs which had convenient central holes, and into these I placed right angled pieces of piano wire which, in due course, were glued into the rear spar of the wing (Figure 1). These made very good, almost frictionless hinges and gave no trouble in service.
The rest of the wing and tail were quite orthodox, except that the all-moving elevator was pivoted on an offcut arrow shaft mounted across the fuselage, fitting telescopically into offcuts built into the tail surfaces.
The Atalante had a very short nose and a lot of weight was needed right at the front to get the balance right. In fact, I needed a large, solid chunk of lead to get the c.g. forward and the radio receiver, battery and servos were as far to the front as possible. I should have used more lead.
All the controls were operated by pushrods. It was not feasible to mount those bulky and heavy old servos close to the control surfaces in the wing or tail. This way of doing things was virtually unknown in that era. A couple of extended arrow shafts ran from the cockpit area to the tail. The ailerons were driven by a single servo mounted between the wings, with pushrods. The flaps plugged into a simple cross-fuselage flap driver similar to, but larger than, those used for elevators on more ordinary models of the time.
Heat shrink fabric for models did not exist. I used light nylon fabric from a dress shop, glued on and doped, which I had often done successfully before with other models. Some of my friends still prefer this.
Winches and aerotows for models were unknown. People used hand held towlines for launches from flat land, but bungees were rare. The idea of thermal soaring with radio controlled gliders was only just beginning to occur to a few people and hardy anyone had actually done it. The Atalante model did not even have a launching hook. The normal thing with any large glider was to launch by hand from the crest of a slope directly into the lift. I had done this many times with small models and with my 10.5 foot span Mü 13 D3. But this had such a simple control system and the model was so stable that it was possible to use both hands for launching, and grab the press button (rudder left or right), when the model was already airborne.
The fat, slippery fuselage with its fabric skin now was impossible to hold firmly with one hand in a breeze, while managing the transmitter in the other. I confess, I had not thought of this problem until the day I went out for my first flight with the Atalante. A bad experience (read ‘mistake’) was waiting.
On my own with no-one to help, not even to steady a wing tip, I learned my lesson. With the hand-held transmitter and what turned out to be a highly sensitive elevator control, I could not grip the model firmly enough to get it into a safe position for launching. I should have recognised the situation and given up trying at once, but that would have meant a wasted journey to my flying site, over an hour’s drive from home to reach the hill side above the Barossa Valley. What I did is easily seen, in retrospect, to be quite stupid. I put the transmitter on the ground, used both hands to launch the model, and then tried to grab the transmitter. By the time I had it, the model was in a vertical climbing attitude about twenty yards downwind of me and ten feet high. I did not get it under control before it cartwheeled rather gracefully into the flourishing vineyard on the lee slope. At least there was no family picnic going on where it landed.
Surprisingly, the model was not badly damaged and it did fly again after some work. The ailerons and flaps never gave any trouble, but the elevator was extremely touchy until I added another substantial lump of lead to the front end. Even with the balance point at the traditional 33% of the mean wing chord, the Mü 13 was still very touchy. If building it again I would use even more ballast in the nose. It is worth noting that when Kurt Schmidt started to produce his factory version, Mü 13D, he added a slight sweepback to the mainplanes. I think this indicates that the Atalante, like my model, needed more weight up front.
Another experience awaited me. We entered the South Australian winter season which can, despite tourist brochures, be very wet and quite chilly. One morning after a stormy night, I entered my workshop under the decking and found it a couple of feet deep in water, with the Atalante looking very sorry for itself trying, unsuccessfully, to float. The lead in the nose, and the radio gear, didn’t help a bit in this situation. This time, the model really was not worth repairing and it was scrapped.
I also had to build a new, waterproof workshop, but that is another story.