- This article was published to accompany the STOL Mk.2 plan publication in October 1983. A new CNC kit and wood pack can also be purchased.
When the original STOL was designed in 1972 it wasn’t even called a STOL, just 242S – it was really little more than an aeronautical doodle. There was just the vague idea that it might be possible, with a really light, relatively high powered model, to operate in and out of normal sized back lawn with trees down either side.
The model that subsequently emerged had a faintly vintage look about it, particularly the structure, compared by the standards of the day. It flew very well, towed gliders, carried cameras and was mildly aerobatic, all on an O.S. 19 but it’s STOL (short take off and landing) qualities, while very good by normal standards, were not considered quite good enough to operate out of the aforementioned lawn in complete comfort.
So, after several months, split flaps were fitted. These bought about a big improvement in the slow flying capability, so much so in fact that dynamic stability, especially lateral stability became a problem at the ridiculously low speeds that were now possible. So the final modification was to fit slats to the outer half of the leading edges of the wings and this did the trick. ‘Garden’ flying now became common, first only in the near calm because the configuration of the runway meant that all take-offs had to be to the west and all landings to the east, regardless of wind direction. With a bit of practice however, first moderate cross wind components became acceptable and finally even down-wind components of a few knots were not too much of a problem. The model went on to do several hundred flights entirely without mishap from this somewhat unorthodox flying site.
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There were several snaps of course. First the wing, in a determined attempt to get minimum possible weight and maximum possible lift, was of apparently flimsy construction and with a concave under surface to boot. The original never actually broke but there was always that uneasy feeling that one day, a bit of unthinking aerobatting might break it. Second, the flaps and slats, being afterthoughts, left something to be desired from the engineering point of view. Finally the under cambered wing section helped to create a massive nose up trim change when the flaps were fully lowered. This latter feature was just about liveable, provided that you just about always got the speed well down before lowering the flaps and also arranged the trim so that it was full ‘nose up’ in normal level flight. This way the full trim range was available to counter the nose up pitch caused by the flaps.
So it came about that, after more than ten years of reliable service, it was decided to make an attempt to rectify the niggling snags by designing a completely new wing. The rest of the model had given no trouble at all so that remained the same. The undercarriage had been particularly reliable and capable of taking all those nasty plaster-it-on type landings that are occasionally necessary in STOL flying.
The basic thoughts behind the new wing were as follows; the ultra light weight of the old wing had proved to be unnecessary and a much more robust wing could be used with little detriment to the slow flight capability. A wing section a bit thicker with a blunter leading edge and less overall camber would give less sensitivity to small changes of angle of attack, have less centre of pressure travel and produce less pitching moment when the flaps were lowered. This latter was the only arguable part of the concept but, in the final result, the new wing was much stronger, nicer to fly and the nose up pitch-on flap deployment was only about half that of the original.
Slats again were not part of the original concept – it had been hoped that the new wing section wouldn’t need them. As with the Mk.1 however, the ultra-low airspeeds that were possible with the new plain flaps down still caused a certain amount of uncertainty in the yaw/roll area. So although the need is not desperate, slats of the type suggested on the drawing will be incorporated in due course.
The original managed quite well without ailerons – it would roll and fly inverted quite well – and so could the Mk.2 but with the luxury of aileron and rudder coupled on the same stick (using two separate servos but with a ‘Y’ lead joining them together at the aileron output of the radio) you get really crisp, accurate control at all speeds and it is particularly nice for inverted flying which the Mk.2 does better and easier than the Mk.1.
As far as normal flying is concerned, if you can fly at all, then you can fly the STOL. For normal take offs hold ‘up’ and slight right rudder till it gets moving then relax and it will take off without further interference. For a really short take off, apply about ten degrees of flap and hold full up until it takes off (10ft in no wind) but be ready to ease off the up as soon as it starts to climb. The controls might be a touch more sensitive than you might normally fly but you’ll soon get used to it.
For short landings, slow right down on the downwind leg, apply about half flap, then re trim. Make your base leg close and short then turn final and apply full flap. Control the rate of descent with the throttle and the speed with the elevator. Don’t make the approaches ‘ultra slow’ till you’ve had a bit of practice. If you’ve got the three point attitude as you cross the threshold, probably with the throttle at 1/3rd . As the runway comes up a final tweak on the up lever and closing the throttle can result in a very short, floatless landing. Tail up ‘wheelers’ are quite easy but take up a bit more room.
As far as construction of the new wing is concerned, the plan is self-explanatory. Just note that while the aileron bell cranks are symmetrically ‘handed’ the flap bell cranks are not. Seen from the top they both have the same angular orientation the left one is inboard of rib 2 and the right one is outboard of the other rib 2. It’s all pretty obvious when you think about it.
If you haven’t tried laminated tips before you will be very pleased with these, they are light, strong, and warp resistant and, unlike some ‘modern’ ideas, where half the balsa you bought finishes up as shavings and dust, all your balsa is stuck to the aeroplane.
There’s no way of knowing just how many STOLs have been built over the years but hundreds of plans have been sold and I’ve received letters from builders all over the world, so you won’t be alone.
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Click here to see David Smith's STOL build photos.
