Stunt outlook for '51

By Joe Wagner. Originally published in 'Model Airplane News', March 1951.

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Every model builder knows of at least one controline flier who can seemingly make his ship do everythlng except sit up and talk; who goes to contests and takes home the hardware while less accomplished modelers stand around in envy, asking each other, "How does he do it? What is his secret?"

The first and most important secret of the consistent contest winner is nothing more than constant practice. The second is complete familiarity with the contest rules, and obedience to them. And the third is a good airplane.

Most contest fliers use more-or-less modified kit models. Why? Because Ihey know that a lot of design and testing has already gone into the kit model by the time it goes on sale, and that it is capable of good performance. However, they also realize that all kit models represent a compromise; between high performance on the one hand, and cost and prefabrication on the other. Naturally, some are better fiers than others, but all can be improved by the builder who is willing to spend a little extra time in doing so. Now, the questions arise: How can you tell a good stunt ship from another not so good? How do you go about improving a kit model? What about original designs?

The most important fact that must be faced is that centrifugal force, that does so much for the controliner in keeping the lines taut, is the models greatest enemy when it attempts tight, smooth maneuvers. The same force thats blacks out the pilot of a reaI plane in a sharp pull-out, can play havoc with the flight path of a stunt model. In a small loop, for instance, the weight of a model may be increased as much as 20 times! But, although the welght of the model is greater, the wings still remain the same size; so that the surfaces that were quite ample to lift the model from the ground may not be able to maintain flight under the added load. When this happens, the model stalls, and a rough or mushy maneuver results.

The number of times a model's weight is increased (G's} in any maneuver depends on three things: the model's weight, its speed and the size of the maneuver. A model weighing 40 oz. will develop twice as many G's as one weighing only 20 oz. A loop 10' in diameter gives twlce as many G's as a 20' loop. But a model that flies at 80 mph develops four times as many G's as one that travels at 40 mph. For the benefit of those with a knowledge of elementary algebra, the formula for the number of G's developed by any model in any maneuver is:

Formula

G is the number ef times the model's weight is multiplied in the maneuver
W is the model's weight in pounds
S is the model's speed in mph, and
R is the radius of the maneuver in feet.

From this, certain conclusions are obvious: the weight of a stunt model should be kept at an absolute minimum; in competition, the largest maneuvers permitted by the rules should be flown at the slowest speed that allows easy completion of the stunt pattern. These are the essentials of prize-winning performance. Unfortunately, there are several other factors that must also be considered.

First of these is appearance. The possible number of points awarded for appearance is the same as a perfect series of horizontal and vertical eights combined and, although few stunt fliers would consider omitting these maneuvers from their patterns, equally few make the necessary effort to win top appearance points. The AMA rules give the requirements: the model must be well-built (a necessity anyway, if the model is to last very long), nicely-finished, and must look like an airplane.

Strength is essential too. There are few modelers indeed who do not have occasional crack-ups, but even discounting that dismal possibility, the model must still be strong enough to withstand the terrific stresses met with in contest stunt flying. This does not mean that the model must be heavy. Through stressed-skin construction (sheet balsa fuselages, planked wing Ieadng edges), unbelievable strength can be achieved with a minimum of weight. However, all joints must be double-cemented, to withstand vibration, and all points of doubtful strength should be reinforced.

Another feature a good stunt model must have is a good airfoil. This means an airfoil with high lift, low drag, and a high stalllng angle. The sections that best meet these requirements are quite thick, between 18% and 25% of the wing chord with rather sharp leading edges. These thick airfoils actually have less drag than the more conventional thin sections, strange as it may seem, while their lift and stalling angles are both much greater. Thick airfoils work wonders on tail surfaces too, rudders and fins as well as stabilizers and elevators. A horizontal tail with a 15% symmetrical airfoil is much more effective than the usual flat section; allowing smoother and tighter maneuvers since its stalling angle is so much greater (Such surfaces are most conveniently built by carving them from soft solid balsa, then cutting out the center portion and inserting pieces of sheet balsa at intervals, sanding these to a rib contour, and covering the assembly with Silkspan). This same idea can be equally well applied to the fin and rudder; however, instead of the customary straight fin and offset rudder, a better arrangement can be made by building the entire vertical tail as a unit, using a flat-bottomed lifting airfoil. The cambered side of the tail should be on the inside of the models circle, and the entire assembly offset 2 or 3 deg.

How about flaps? Well, let's put it this way: a perfectly good stunt model may be had without using flaps (witness the Senior and Open Stunt winners at the '50 Nationals), but performance can be bettered with them. The reason for this is that deflected flaps change the symmetrical airfoil to a lilting airfoil (the lift acting either upward or downward, depending on whether the flaps are down or up) and this lift, acting in conjunction with the elevators, permits tighter maneuvers without stalling. To be fully effective, flaps should be full-span, about 25% of the wing chord at their widest point, tapering to the tips to avoid tip-stalling the wing. Flaps should move opposite to the elevators, and their movement must never exceed 30% of the elevator movement.

A Stunt Model

Click on the drawing for a larger image.

The stabilizer and elevator of a stunt model must be kept out of the turbulence behind the wing, in order to be at all effective. This usually means placing the wing as low as possible, and raising the tail as far above the wing as is practical. This is particularly true of very close-coupled models, and those with flaps.

One of the most highly misunderstood factors in stunt flying is that of power. Most of the difficulties experienced by the novice contiol line flier can be blamed on simply too much engine for the airplane. Excess power means excess speed, and excess speed means trouble. The reaction time of the average person is much too slow for the rapid control movements necessary to control the flight of an 80 to 100 mph stunt ship, and the terrific line pull of such an airplane only results in numbed fingers - or broken lines. Also, even if the modeler is successful in flying such a model through a stunt pattern,- the maneuves are so rapid that a judge has the greatest difficulty in following them. Actually, the maximum power required by any stunt model is just enough to pull the model straight up. The largest stunt models on the market today will all fly the complete pattern quite nicely with a good 29 in the nose.

Of course, one of the prime requisites for successful stunt flying is a good, reliable fuel tank; but so much good advice on this subject has already been published that no more should be necessary here.

The landing gear makes a difference too, since 20 of the 335 possible flight points depend on its functioning properly. The wheels should be located slightly forward of the leading edge of the wing; the idea being to let the tail come up as quickly as possible, prolonging the take-off run so that the model doesn't make an unrealistic (and point-losing) leap into the air. And, contrary to general opinion, a rearward wheel location results in smoother landings. The landing struts themselves should be nearly rigid. A little fore-and-aft movement is all right, but any sideways twisting action is disastrous to smooth take-offs and landings. Large enough wheels should be used to allow easy take-offs from, and landings on, the usual rough contest site.

One ltem that no one seems to give much thought to is the lowly push-rod. In most stunt models, the push-rod is absolutely free inside the fuselage. In flight, the airstream exerts a considerable neutralizing force on the elevators, and when the up line is pulled, the bellcrank moves, but the elevators may not; the push-rod bending instead. When this occurs, the pilot has less than no control at all over the model's flight. To avoid this sad state of affairs, insert a small plywood bulkhead in the fuselage, half way between the bellcrank and the elevator horn. If the push-rod is passed thro a close-fitting hole in this bulkhead, no bending can possibly occur.

A small amount of sweep-back in the lead-out wires is essential for smooth control, since the control lines themselves sweep back from the model in flight. Lead-out sweepback does not help to hold the lines tight but it does keep the lead-out from being bent where they leave the wing guides, and eliminates the binding that this causes.

The balance point of any stunt model reflects a compromise: the more nose-heavy a model is, the harder it pulls out on the lines; and the further back the cg, the more maneuverable (and sensitive) the airplane. The best practice is to balance the model at the point where the front leadout leaves the wingtip. This should be about a third of the way back from the leading edge on a model with flaps, and about a quarter the chord back for a model without flaps. The center of the bell-crank should be even with this point, automatically giving the proper leadout sweepback.

The rudder should never be offset more than 20 deg.; 10 to 15 deg. being about right for most stunt models. If offset thrust is used it should be just enough to be barely noticeable from the top. Weight in the outboard wingtip should be just sufficient to make that wing hang a trifle low when the completed model is balanced on its center line. Don't go overboard in trying to keep the lines tight by doing things to the model, since the only result of excessive offsetting is to make the model "crab" outward inflight, ruining its maneuvering ability.

A stunt model should have enough pull on the lines so that the flier can rest easy in the knowledge that the ship won't come in at him, but not so much that it becomes a strain to hold on to the handle towards the end of the flight. Excess line pull is the result of either too much speed, or "crabbing"; both of these being serious evils in themselves. "Crabbing" can be easily detected by the flier; if both of the model's wheels are visible from the center of the circle during flight then the model is flying sideways, and the motor or rudder offset must be reduced. Excess speed can be corrected by running the engine a little rich, or by installing a smaller motor.

Any points which are not mentioned in the preceding discussion are either common practice (weighting the outer wing-tip, off-setting the thrust line slightly to the outside of the circle, etc.) or optional with the modeler. (For example, the vertical position of the thrust line seems to make no difference in the performance of a stunt model.) And now, having covered the requirements for a good stunt ship, some advice on flying technique might be in order.

The first and foremost requirement for a good stunt pilot is confidence. He must be confident of his airplane and of himself, and his abiilty. And the only way to breed confidence is through practice. This does not mean that four or five hours of flying a day is necessary. Several flights a week should be all that is needed, provided the flier keeps constantly trying to improve his pattern. It is a good idea to have an experienced stunt pilot watch occasional flights from outside the circle, as do the judges at a contest, to point out the bad and the good maneuvers in the pattern.

Be familiar with the rules - all the rules - and obey them. And don't be the character, a familiar sight at all contests, who is continually annoying the judges with trivial questions about the regulations. In fact, don't annoy the judges with trivial questions about anything, such as, "Hey, when is it my turn to fly?" and "How many points have I got?" Most judges have very good memories for such people - especially when their flights are being scored. After all, judges are human too.

When it is your turn to fly, be ready. Even if you're not sure when your turn is coming up, be ready anyway. And, once you get in the circle, make the most of your opportunity. Maybe your maneuvers aren't perfect. But, if there isn't too much wind, you can make them look a lot better (to the judges, anyway) than they actually are. (This method, described below, does not constitute cheating or poor sportsmanship. It is widely used by those contest fliers that know about it, and kept a deep secret from those who don't.) Let's go through a pattern to show you what I mean.

First, your model should have been all fueled up, primed, and ready, before you even took it into the circle. If you've done this. it should be a simple matter to start the engine and become airborne within the allotted minute. And there's five points. Take off directly down-wind, using a little down to keep the tail high and the model on the ground. If you can stretch the take-off to about a quarter-lap, you re a cinch to get full points for it. Before you give the judges your starting signal, fly around for a few laps to get the feel of the airplane, and to make sure that the engine is running properly. Now, any time you're ready, give the signal, and make it noticeable!

If you've got a good model, level flight is a snap. Just hold on to the handle and the airplane will do the rest. Do your climb and dive directly in front of the judges. It takes exceptional (and rarely encountered) ability for a judge to follow these maneuvers close up and still be able to pick out the flaws. Don't forget the level lap after each maneuver - you've got to give the judges time to mark down your (phenomenal) score.

Really good wing-overs are almost as rare as four-engine free-flights. It takes a real pilot to split the circle exactly in half, as the rules specify, but if you can't quite do it perfectly, don't despeir. Just do your wingover parallel with the judges' stand, and to them it will look spiffy. Don't worry too much about the maximum height on your loops, since you can do 40' loops on 50' lines and still be under the limit. Of course, the judges may not know that, so better not make them quite that big. If your loops tend to travel back and forth a little, do them facing at right angles to the judges. If your loops vary slightly in height, or aren't quite the same size, do them in front of the judges. Naturally, if you know that your loops (or any other maneuvers) are letter-perfect, do them where the judges can see them best: about 45 deg. to the judges' nght.

Inverted flight is just as easy as flying level right-side-up. Any nervousness here is only a state of mind, so forget it. And now let's go on to the horizontals. The horizontal eight is an extremely difficult maneuver to do properly - unless you know how. The rules say that both ends ol the eight must be round circles, and that the model must be in a vertical position at the center. It is a little unnerving to have to dive straight at the ground just before starting each half of the eights, but, if you do the outside loop first, so that the model points straight up instead of straight down in the middle, then there's nothing to it.

Vertical eights are next and, like the horizontals, they should consist of two circles rneetng at the center. If you're not absolutely sure of yourself on these, do them in front of the judges too. Overhead eights are a little tough, but if you face at right angles to the direction of the eights, you won't be so far off balance from leaning over backwards.

All there is to the square loop is four quick jerks of the handle, and now you're ready for the special maneuver. Don't pick something easy, like inverted square loops or rolling your wheels, because, even if you do it, perfectly, you won't get as many points as you will if you try something really hard; a square horizontal, for instance; whether you do it well or not.

Now, all you have to do is go around with the model until the engine quits, land, and it's all over. The landing is just another maneuver that you can leave to the airplane. Just keep the handle in neutral, and don't move it; not even the tiniest bit! and the model will rnake the prettiest landing you (or the judges) ever saw.

What will the trend in stunt models be in 1951? Well, the most noticeable change will be in the appearance of the models. More and more realism will become evident as the season progresses; profile jobs will become an unfamiliar sight at contests as the stunt modelers concentrate on building ships that look like real airplanes. Manufacturers, too, will take up this trend, some (as Sterling has already done) producing exact scale stunt models.

Models will become smaller, the "flying barn doors" rapidly disappearing and being replaced by smaller, neater, and better-proportioned ships - and capable of better performance, too. Most stunt jobs will be in the .29 to .35 class. Only a few die-bards will continue to use the 60's, and even an occasional class A model will end up in the winner's circle. There doesn't seem to be much chance of the AA achieving the same standard of performance as the larger ships during the coming year, although even that is not impossible, and some of the larger contests may include a special class for the under-.10 stunt models.

Trio of beauties at California contest. Front two ships Berkeley Zilch's, third Li'l Dupper Zilch. Designs by MAN contributing editor, Jim Saftig.

Three Zilch's

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