STOL II (oz13963)

About this Plan

STOL Mk II. Radio control sport model. Wingspan 55 in.

Quote: "STOL Revisited. Peter Russell presents the MkII wing version of his famous 'Short take off and landing' aircraft.

When the original STOL (oz9889) was designed in 1972 - it wasn't even called a STOL then, 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 a 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 OS 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 brought about a big improvement in the slow flying capability, so much so 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 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 downwind 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.

There were snags, 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 undersurface to boot. The original never actually broke, but there was always that uneasy feeling that one day a bit of unthinking abandoned 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 livable with, provided that you 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. All 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, was 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 Mark I, 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 Mark II but with the luxury of aileron and rudder coupled on the same stick (using two separate servos but with a 'V' 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 Mark II does better and easier than the Mkl.

As far as flying is concerned, if you can fly at all, you can fly a 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 (ten feet in no wind!) but be ready to ease off the up as soon as it starts to climb. The controls mght be a touch more sensitive than you normally fly but you 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 rest of it right, you will be almost in the three point attitude as you cross the threshhold, probably with the throttle at about 1/2. 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 drawings are self-explanatory. Just note that while the aileron bellcranks are symmetrically 'handed', the flap bellcranks 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, 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.

The original Mk I STOL (oz9889) was published both in this magazine and the American 'Model Aiplane News' and, if you are thinking of joining the STOL club it might be an idea to take a look at the original article.

There is no way of knowing just how many STOLs have been built over the years, but hundred of plans have been sold and there have been letters from satisfied customers from such diverse locations as North and South America, Finland, Sweden, Norway, West Germany, Saudi Arabia, Iran, Malta, Malaysia, Australia and New Zealand, so you won't be on your own!"

Direct submission to Outerzone.

Supplementary file notes

Article, thanks to RFJ. Previously listed with the STOL (oz9889) plan.

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