Building Scale Sailplanes 3
Foka 4 and the Long Man of Wilmington
I should not be writing this because the experiences I describe here are not those encountered in building from scratch. My Foka model came from a kit. My excuse was that I was living for a while in rented accommodation in Golders Green, suburban north London, while working in England for a year. I had little free time, few facilities or tools and only a kitchen table to work on.
I have no photographs of my model Foka but some of the full scale aircraft are shown here. It was a fifteen metre span sailplane intended for the so-called Standard Class World Championships. The class was intended to produce safe sailplanes of good performance for ordinary club pilots to fly in competitions. The best example was the Ka6 CR, which probably needs no description. The Foka 4 stretched the specification to its limits. No concessions were made for the inexperienced or clumsy pilot; it was the sensitive thoroughbred racehorse to the Ka 6’s trustworthy hack. It never came near the original intention of the class.
The pilot did not sit but lay almost supine, so the fuselage could be reduced to the minimum cross section. The wheel, required by the class rules, was almost completely buried in the fuselage belly so to land on any kind of rough surface invited trouble. It was also very ‘slippery’, picking up speed rapidly when trimmed forward, and it was easy to overshoot on landing despite the airbrakes.
I flew a Foka 4 once and found it delightful. The controls were brisk and light to the touch. It required a lot of care especially on aero tow. The view straight ahead through the long, low canopy was barely adequate and seriously distorted. If the glider got just a little high, the towplane disappeared altogether behind the pilot’s feet, which could be fatal for the tug pilot who might find his tail pulled violently up. Off tow, the handling was excellent and the performance outstanding for its period. In 1965, a Foka won the Open Class World Championships, competing against some very much bigger and more expensive sailplanes, including the famous D – 36 Circe glass ‘gummiflügel’ of three metres greater span. (When the Foka 5 was developed some time later, it had a much more sensible fuselage and cockpit, was much more pedestrian and the performance was not so good.)
So to the model.
I do not enter competitions and am usually content with a model that looks right when it is flying. I put in only a token effort for things that are not readily visible. My models are covered by the expression, stand off scale. Much depends on how far off the viewer stands!
Even by these criteria my Foka 4 was not a scale model. The kit came from Graupner and as a kit very good, but the resemblance to the full-scale aircraft was rather slight. It was in that strange category often described as ‘semi scale’. That is like semi-pregnant or semi-symmetrical, a logical impossibility. Either one is, or is not, pregnant. A wing profile either is, or is not, symmetrical. A model either is, or is not, to scale, but what precisely does that mean?
There have to be compromises. We do not, and probably cannot, build a scale model glider exact in every detail inside and out, but we are not trying to make museum exhibition pieces. We want to fly the model and we want it to fly more than once. Flying models get much rougher treatment and, in proportion to their size, are expected to survive much worse shocks than their full-scale counterparts. Even if we could magically reduce, to our chosen scale, a full sized aircraft with all its parts down to the last rivet, screw, fibre of glass, carbon or aramid it would be too delicate to handle, let alone survive a less-than perfect landing on bad ground.
We compromise especially with structures. We do not, as a rule, build up ribs or fuselage frames of wooden gliders in the way the full size parts were built. Such model parts will normally be cut from sheet balsa or plywood. A quarter scale model wing might be skinned in 1.5 mm (≈1/16th inch) thick balsa sheet. Model fuselages may be planked with 3 mm (≈1/8th inch) balsa. No full-sized sailplane had skins four times these thicknesses, and certainly never in balsa.
We can get closer with the thinnest available plywood, 0.4 mm (≈0.016 inches) which, at quarter scale, represents 1.6 mm. This is indeed about the thickness found in highly stressed skins on full-scale wooden gliders. I was present in 1974 at a scale contest in Germany where the judges did investigate the structure. An outstanding model of a Rhönbussard, which, if judged from the outside only, could have won, was marked down because the fuselage had been moulded in glass-fibre-plastic. The judging panel said it should have been built with a 0.4 mm plywood skin. It is true that the old time plywood skinning was never as smooth and wave free as moulded plastic, but to see the difference on a model requires very close scrutiny. If a model is to be entered in a competition at this level, it is wise for the entrant to find out long in advance what the judges will be looking for.
Allowing that for most of us, the internal structure and skin thickness is not normally to be considered, the minimum qualification for a scale sailplane must surely be that the general outline is correct. All the measurable dimensions should be as close as possible to the nominal scale figures and in serious competitions, the judges do use tape measures. The kit model Foka 4 would have failed such a check, but it looked nice anyway.
A word about wing profiles needs to be said. It is very fashionable for scale model gliders to be given airfoils quite different from the original. There is usually little or no justification for this. With the fairly large scales used for radio controlled sailplanes, the profiles of the full-sized aircraft work well on the models too. It is disappointing to see the external appearance of a model, and also its flying characteristics, spoiled by the use of a wing profile taken from well known catalogues of specialized competition model sections. Such things as wing thickness and camber, if wrong, show up immediately to the knowledgeable observer. The arguments used in defence of this practice are usually based on superstition rather than practical experience.
It is inescapably true that the airfoils of a model will not have the same lift and drag performance as the full sized wings. This is because of the lower Reynolds numbers, the so-called scale effect. But scale profiles on model gliders do not suffer the drastic deterioration that applies to ‘peanut scale’ and lightweight, indoor free-flight models. The scale sailplane with the same airfoil as its full-scale counterpart, will fly well and soar magnificently in thermals, or above the slope, and will perform aerobatics. In flight, a large scale sailplane with accurate dimensions including the correct wing profile and with the balance point in the right place, will respond to the controls very much like the full scale prototype. It will not win, and should not be entered, in pylon races or multi-task competitions. Use a specialised contest model for that kind of fun. Be content with the scale model that flies well, soars, and above all, looks right on the ground and especially when it is in the air.
This does not mean accurate scale models will always be easy to fly. Some of the aircraft we like to copy were, and are, not easy to manage in the air and models of them show similar characteristics. The Atalante I described in a previous article, was a good example of this. But this is part of the interest of the game. We must learn to fly our scale models just as the pilots had to learn to fly the originals.
Three of the profiles shown here have been used with success on various of the Author’s models. Others of similar shape, thicknwess and camber, would be quite satisfactory on a scale model sailplane of moderate size.
There are a few wing profiles I admit I would not like to try. The fourth profile here, the rather weird Göttingen 652, was used on a few famous sailplanes, like the Fafnir, Scud 2 and Hjordis in the early nineteen thirties, was very thick, very strongly cambered, and would not probably not behave very well even on a large model. It was actually not very successful even on the Fafnir. Yet this airfoil has been used, long ago, on some lightweight free-flight models which did fly, so it is not impossible. But I would say, as a general rule, if the wing section of the full-sized aircraft really does seem totally unsuitable for a scale model, it would be best to choose a different prototype to copy. Fortunately there are not many examples where this applies.
When it comes to matters like cockpit detail, covering materials and paint schemes, the modeller can go as far as he or she is inclined. But there is need for more compromises. If visible from outside we should put appropriate control sticks and levers in the cockpit, and a realistic instrument panel. It isn’t necessary to make the control levers and the instruments work. There surely should always be a simulated pilot, but there is not much point in a full length dressed doll if, as in many cases, only the head and shoulders, or the torso, will be visible. If the cockpit canopy exposes everything, a complete pilot will be needed but not many people will attempt to make the clothing perfect to the last button. (Has anyone ever made a scale, working model of a zip fastener for a flying suit?)
The wing was straightforward, built up with pine spars, balsa ribs and skin. There were no ailerons, spoilers or airbrakes, so the dihedral angle was increased to ensure good control with only the rudder for lateral control. This alone took it out of the scale description.
The most important lesson I learned from assembling this wing was that ordinary contact adhesive is very good for attaching balsa skins to open frame structures. At the time this was quite new to me. I had previously skinned models with balsa cement and a myriad of pins. (The now commonplace white wood glue was almost unknown and treated with suspicion. Cyanoacrylates were not available at all.) Balsa cement was the traditional glue, it worked quite well and gave just sufficient working time for things to be pinned together while the glue set. The Graupner kit instructions recommended contact adhesive for the wing skinning, I used it, and found it extremely easy and quick. I have often used it since for skins over open frames. (Alongside the Foka kits in the model shops, were plenty of plastic-foam cored wings with balsa skins. It was usual then to stick the skins on with carpet glue which did not attack the foam.)
The Foka fuselage was another lesson. It was moulded entirely in plastic, but a plastic with characteristics quite strange to me. I do not remember the correct name or acronym for this material but I do not think it has been used very much for model aircraft since the Graupner Foka. It had an almost soapy, or slightly greasy, feel, was somewhat flexible and entirely white. It also seemed quite heavy and one of the suggestions in the kit instructions was that the rear part of this fuselage should be carefully scraped down in thickness to reduce the weight aft of the wing. I confess I did not do this, but was ready to add sufficient lead ballast in the nose to compensate. Another, more irritating characteristic of the stuff, was that it would not take glue, paint or enamel. The builder of this kit was virtually forced to accept a totally white finish and it was not even easy to stick decals onto it. If it crushed or cracked badly, it could not be repaired.
I finished the Foka and it flew very well. I took it regularly slope soaring at Ivinghoe Beacon, especially when the weather was not very good for flying full-sized gliders at Dunstable. On many occasions I was the only model flyer there, which probably could not happen now.
When flying near Eastbourne where my parents lived, flying I learned again two more lessons that I knew already in principle and actually had directly experienced before, on the receiving end, as it were. (See my previous article about the Mü 13D3).
The first lesson was, not to fly in such a way that the model might hit someone on the ground. My wife and mother had, most unusually, come out to the flying site with me that day. It was on the South Downs very close in fact to Itford Hill, the place where the first ever (full-scale) glider competition was held in England, in 1922. I was demonstrating how I could bring the glider down to land close to my feet. Instead it hit mother. She was, fortunately, not bruised and was very forgiving but I felt a proper fool.
At another site in those same hills nearby, there is a huge, primitive and slightly indecent carving of a nude human male figure, cut into the turf on a north facing, steep two hundred foot slope. He seems to be holding two long, upright poles, one in each hand. He is an enormous piece of iron- or stone-age graffiti, but not sprayed from a can. He must have been dug with flint axes or crude spades to reveal the underlying white chalk rock beneath the turf. This mysterious, full-frontal spreadeagled outline, has been boldly exhibited to the entire countryside for thousands of years. (It is carefully tended now by worthy and respectable preservationists.) It is known as The Long Man of Wilmington.
There are legends surrounding him and superstitions abound. The new fearful warning I can issue is, don’t fly your model glider above the Long Man in a southerly wind. The slope he is carved on faces more north, so in a southerly it is on the lee side of the Downs. On such days, with his long poles he stirs the air up very thoroughly, sufficiently to throw glider totally out of control. I carelessly let the Foka drift that way, it dropped out of sight and was wrecked beyond repair at a spot somewhat lower than the Long Man’s important parts. I do not to believe in superstitions, but I do know there are powerful clutching hands on the downwind side of hills.