TRIMMING AND THROWING HAND LAUNCHED GLIDERS Dan Garsonnin May 22, 2008

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1 TRIMMING AND THROWING HAND LAUNCHED GLIDERS Dan Garsonnin May 22, 2008 I am presuming that the glider has the normal trim set-up; i.e., zero-zero wing and stab, inner stab tip 1/8 higher than outer, slight left rudder, and a washed-in left wing I build mine in with a skewed dihedral joint -- but it can be warped in, just so it doesn t change. The challenge with these machines is that they must display stable and predictable flight patterns at glide speeds as well as launch speeds (between 10 mph and 70 mph), reach maximum launch heights, with no in-flight adjustment to compensate for the various speeds. In other words it must fly automatically... and it will, once it is properly trimmed. In fact, given sufficient height, these aircraft can be dropped in any attitude or position and they will right themselves and fall into their normal glide pattern. To do this requires quite delicate force compromises, all of which must be pre-set and unalterable once the plane is in the air. Sufficient height means 50 or 60 feet, which is about the most that many of us can muster on a good throw. If it requires all that altitude just to recover and achieve a normal glide situation, then there is no altitude remaining for duration; i.e., it would recover only just in time to land. So, for any duration at all, it will require a little effort on the launcher s part to achieve fast recovery at a reasonable height. The most important factor to the success of an airplane s flying is air speed. If too much air speed is lost, the wing stalls and the plane falls, which turns to a dive. Once flying speed has been achieved, the recovery process can be initiated, but time and altitude will have been lost. Hence, it is most desirable to arrive at a flight pattern that enables the aircraft to retain its flying speed at all times. If we were to throw the plane straight up, it would eventually arrive at the top of its climb with very little speed, and may even appear to stop altogether, before heading back down just as steeply and consuming all of the altitude. So, the idea is to never throw the plane in such a manner as to leave it hanging on its tail in mid-air. We can do this by throwing it in a banked attitude. In a banked attitude, the wings are not as supportive of the planes weight, and assisted by the presence of the fin, the plane tends to follow a more natural, curved trajectory. The aircraft is allowed to start nosing over at a higher speed and the fall begins at a speed higher than the stall speed. If the plane starts to fall while still in a bank, it will side-slip and the dihedral will roll the aircraft level. Recovery from a nose-up, climbing position is then almost instantaneous and no more height need be lost in a dive to acquire flying speed. So, launches are always made in a banked position to enable the plane to roll out of the climb, rather than stall and dive. These gliders are built to fly in left hand circles (for right hand throwers). For initial test, to ensure the plane s trim isn t such that it flies itself immediately into the ground, it can be gently thrown directly into its left glide with a slight left bank. Gentle throws are always made horizontally, and as more force is used, the nose is raised to steepen the climb. Each time, only that force which is needed to just carry the plane over the top of its trajectory is used. At the top, 1

2 the speed should be just slow enough that the plane just barely flops or settles onto its inside wing. Not only is the angle of the climb critical to the amount of force used in the throw, but just enough bank angle is used to allow, or precipitate, the drop onto its left wing. If the climb angle is too steep for the climb speed, the glider will be left hanging from its nose, prior to stalling and diving. Should the bank angle be too sharp for the speed used, or the climb angle, the plane will arc over at high speed into a cartwheel on the ground. This maneuver often resulting in splintered wing tips and tails is well worth avoiding and in all test patterns using a left climb, only very little left bank is ever used. These free flight gliders are made so as to always turn consistently similar sized circle at all speeds. This accomplishes several things: it keeps the plane flying in a smaller air locale hopefully one that s going up and at calm times, the plane stays in the same area and establishes a firm, predictable, and workable flight pattern. At low speeds, the factors that effect this left turn are: the stab is tilted toward one side introducing a sideways component to its lift, the left wing tip has a higher angle of attack dragging the left wing back, and the left wing is a little heavier than the right. This counter-intuitive notion of wash-in (down aileron) dragging a wing back (and down) deserves a bit of explanation. Just before a wing stalls, the drag increases steadily with every increase in angle of attack; however, the lift drops off a bit just prior to stall. As the plane settles into its minimum sink glide, most of the wing is at minimum sink angle (maximum lift), but the washed-in portion, not yet stalled either, is riding in that zone where lift has reduced a bit but the drag is even higher so it pulls the wing back and down. At higher speeds, as with a normally flying powered aircraft, the drag may skew the wing a bit, but the plane will still roll away from the wash you might need a touch of rudder to correct the skew or yaw to clean up the turn. At high speed, all the above factors are reduced and sometimes even reversed. With any increase in speed, the increase in aerodynamic forces (i.e., lift and drag) is proportional to the square of the increase in speed. This means that if you double the speed you will quadruple the lift and drag, and if you triple the speed, the increase in lift and drag will be nine-fold. Remember how hard it used to be to peddle your bike against the wind. Little wonder, when you figure that a 10 mph wind gave you four times the resistance than it did at 5 mph. It s not that hard to understand either, if you visualize a bunch of air molecules as you might find in the wind -- if we double the wind speed, not only does each molecule tug, pull, and drag at the surface twice as hard, but there are twice as many of them doing that for any given time. The slightly heavier left wing soon becomes insignificant in the face of the greatly increased aerodynamic forces that accompany the higher speed. The effect of the tilted stab may reverse slightly, but its overall effect will become insignificantly small, as a turning device. 2

3 In fact, the only turning device we use for left turn at high speed is the rudder. It has no effect (or very little) at low, optimum glide speeds, but it becomes very powerful with increases in speed. A measure of its power may be guessed at from its small size. Its power is so great that some counter measures are built in to control it at very high speeds or it would drive the plane into a steep, terminal spiral. The inside wing is washed-in, either warped or built-in. That angular difference, henceforth to be called differential, which used to drag the left wing tip back at low speed, now lifts that inside wing up, preventing the left turn from becoming too steep at high speed. Eventually, all of these effects, with their different speed ranges, are carefully blended and balanced for desirable flight qualities throughout the entire speed envelope. Their effects can be broken down into operating in three axes, one for each of the three dimensions of the plane s movement; i.e., pitch, roll, and yaw. If they can be viewed in this way, broken down into three, it becomes a little easier to determine whether, for instance the turn is too steep because the rudder is swinging the tail around or if the differential is insufficient to roll the plane against the force of the rudder as the speed builds up. The decision on how to correct a problem is based on rather fleeting observations of very subtle evidence and practice is required. But for sure, anything the aircraft does is with reason and is no mystery. Longitudinal stability is arrived at with much testing, as it depends on a blend of a static, nonchanging force and changing aerodynamic forces. Much of the situation has already been described on the first and second pages, but important points bear repeating here. The static margin, which is the distance between CG and neutral point (aerodynamic centre, complicated to calculate and we only need the concept here, not an actual figure), makes sure the plane goes nose first, and so as to not yank the nose straight down, is balanced by the stab, which compared to the wing is providing less lift (relative negative). Hand launched gliders have the added complication of looking for a certain reaction while it is falling, as well as flying, which no other aircraft or model need contend with. With the zero-zero set up, a long sweeping recovery from a dive is all we can expect, so it behooves us to avoid stalling and diving. If the plane, after stalling, could be made to fly soon, before it over-rotates into a step dive, much altitude could be saved. A large tail volume would encourage over-rotation, so we make the stab just large enough to stabilize the plane adequately and no bigger, to encourage a slower rotation into a dive, to allow flying earlier in the rotation. If the stab is too small, the plane won t hold the slowest glide without stalling. Faster flight won t be affected because any size stab would do then. All the efforts to know how to calculate the perfect tail volume are all for flying at maximum angle of attack at the slowest speed at minimum sink -- when stability is marginal. The Sweepettes and derivatives which I fly are carefully matched in stab size. I know that losing more than about sq inch from the stab will result in instability at minimum sink. And using a larger stab would result in excessive overrotation during stall recovery. And, no matter how carefully we play the CG location against the stab settings for the best stall recovery, it will never properly recover to normal flying state without the use of turn. 3

4 Attempts should always be made to catch the model before it lands on the ground, or some other even less forgiving obstruction leading edges suffer, both wings. Catch the model by its shoulder. It is the strongest, heaviest part of the model and is easily accessible. Get in the habit of catching it there and then there will be no hesitation or indecision in awkward situations. Under no circumstances should you try to catch the model by the tail, which may stay in your hand while the rest of the aircraft keeps going. As a matter of fact, try not to touch the tail at all, except for adjustment or cleaning. Pay particular attention to the tail when ever going in and out of the house or car. Precede it with your eyes. If you brush it against anything, even a coatsleeve, it will offer no resistance and will snap right away. Commencing a Routine Flying Day and Flying Technique Each new flying session requires a warm-up period for both you and your glider. Start gently and work-up gradually, and injury to both you and your plane can be avoided. Unless you are very familiar with the plane and you know it hasn t changed since the last time it flew, it will need checking-out. If the plane is new, you will need to establish the CG and stab incidence or elevators trim. A really well built plane will require only tiny changes. Commit your first flight to a landing on the ground. Push the glider straight out from your shoulder at its approximate glide speed and angle; i.e., slightly nose down, slightly banked to the left and only just fast enough to fly. The flight path should be almost straight at glide angle, perhaps a tiny rotation up but only tiny, certainly no down-tendency, hint of left turn. If no major adjustment is needed, then you can leave your vehicle (and glue, knife etc.), maybe grab a little plasticine, and head for the point in the field which offers the longest downwind run. I cannot over-emphasize the importance of going upwind to the furthest point of the field each and every time you throw your plane. It is nearly impossible to tell when your plane will stay in the air a little longer, or get caught in a particularly fast breeze and you ll then need every bit of space downwind that you can get. You may only need all that space once or twice in a flying session, but if it gets lost that once, it s game over. So, do return upwind each time. Get accustomed to launching from one spot, and move that spot to one side or the other if there is a wind-shift or if your launch pattern tends to deliver to one side. Test flights of low to moderate strength are made with a slight left bank and to the right of the incoming breeze so that the plane will turn into the wind. All the flights with a left bank can be made by just pinching the fuselage, under the wing, between thumb and the side of the index finger, much as you would hold a pencil. Start gently and then a little harder. Try for a halfcircle, and then three-quarters, until the plane will go out and boomerang back. Try for fifty foot circles (with a large glider). So far, none of this should have involved any height or downwind running, yet. Should all of this have been met with success, it s time to go for some small downwind flights with a little altitude to check for any recovery problems. Throw in the same direction, with a slight left bank, moderate force and steep enough to gain 25 or 30 feet in height. The pattern 4

5 we re looking for here is a slightly, or moderately steep climb with a constant left bank. At the top, the bank should remain the same but the tail should kick just a little after a bit of hesitation, as the plane settles into its glide. Should all the test flights have been reassuring, then it s time for some hard throws and real altitude. This requires a firm grip and firm control which is achieved by holding the model as before, but now the centre of the index finger-tip pad should contact the rear of the finger rest at the rear of the right-hand wing root. The thumb and middle finger are now pinching the fuselage and are stretched as far forward (hopefully to in front of the wing) as to be slightly uncomfortable with the strain on the index finger. That index finger must be stretched and strained in order to direct the plane consistently. This position will feel unnatural and uncomfortable, but I have tried others and I must admit, this one works the best. It is also the same grip the world s champions, and virtually all other flyers use. Do not forget to stretch your hand until you feel it straining. This locks the plane in your grip in one position, and it is essential that it is absolutely locked and in the same position for each throw. The hard throw seems to be particularly difficult for beginners to master and I m not sure why the concept seems logical and natural to me. It may just be difficult to train muscles that are operating in a twitch mode, as is required for throwing. The pattern this time is to climb to the right, transition... and glide left. With the grip firmly established, do your wind-up and throw I use a wind-up of 1 steps but Ron Wittman (indoor world champ) uses 25 yards(???). Throw harder each time, providing the pattern is good, until you are limited only by the speed and strength of your arm. Very steep climbs are used at this stage, with steep banks. In fact, throwing right is very safe because the left rudder, at these high speed launches, usually prevents the plane from arcing completely over on the right and will roll it left in time to avoid the ground. It is difficult to apply dangerous right bank with that left rudder. The plane should climb with constant or slightly increasing steepness as it goes up in a right spiral for between 100 to 150 degrees. When the speed decreases, the plane still hard over on the right wing, should appear to just hesitate. The tail might kick around another few degrees and the plane should begin to fall sideways at first but then quickly rolling onto its belly. If everything has been properly coordinated, it should fall into its normal glide pattern. Ideally, there should really be no hesitation at the top and the transition should be smooth; but a barely perceptible pause-and-kick can be exciting to a well-tuned observer. Failure to coordinate the launch will usually result in a massive stall and a long (altitude-chewing) dive to recovery. It is important to throw hard enough to gain sufficient altitude to allow for this stall recovery process. Knowing what the plane needs for a good launch is not quite enough one has to be able to deliver this condition... and very precisely, too. First-off, the plane is not omni-directional like a ball or a stone; it will only go where it is pointed. Hence, it must be pointed very accurately while it is being thrown. Remember how we are trying to lock it in your grip the same way each throw? If erratic launches persist, it is usually due to an inability to coordinate the wrist throughout the throw. The other probable fault is in the timing of the release during the arc of 5

6 the throw. Sometimes, intense concentration is needed to achieve this maneuver to which you ve not yet become accustomed. Sometimes, intense concentration ruins it more. Try to visualize some sort of stop-action in your mind to help analyze what is happening to your wrist during the throw. Focusing on a point in the sky (perhaps a cloud, or just an imaginary point) helps. Pick the point and aim for it. Of course the plane won t go straight for it and it will always veer off into its climb, but that s okay. The point is only there to stabilize your aim. Aim for it and follow through on the throw as if you were reaching for it with your fingers. Make that point the only thing in your mind. The problem is getting that plane away from you in the desired attitude. Often, beginners exhibit a strong reluctance to keep a sharp enough right bank at the release of the hard throw. To combat this problem, some kind of side-arm throw can be developed... which is a natural inclination. Watch for beginners who hold and begin their throw with sufficient bank, but the bank disappears during the throw and the release is flat. Do remember the different directions for light and hard throws. A left climb with a hard throw will drive the plane at high speed left, into the ground. Conversely, a right throw done too lightly will not leave enough height to recover from the sloppy right-left transition that invariably results. 6

7 ADJUSTING AND TROUBLESHOOTING It will probably help to refer back to that part earlier where the physical and aerodynamic forces are described. Should your plane need adjusting, as it undoubtedly will, the following methods have been used with success. Centre of gravity alterations are made with the addition of plasticine, or the removal of excess material, such as part of the nose weight. Aerodynamic trim settings are arrived at by forcing the trailing edge of the flying surface to another permanent setting. If the wood is thin, it might be induced to bend or warp permanently, but only very small changes can be achieved this way. The wood is bent in the fingers repeatedly and forcefully. Listen carefully and back-off when you hear slight cracking noises. Sometimes there is little or no warning and the piece breaks along the grain. This can sometimes be delayed a little if very firm pressure is maintained directly on the area of bending, but this is tricky. If a permanent set won t take, then the piece will have to be broken and re-glued. It s no big deal but it tends to leave a messier surface which drags in the air a little fine sanding will help smooth those edges. Care must be taken while gluing. Use only as much glue as is absolutely necessary. If the wood under repair is thin, be sure to apply equal amounts of glue to each of both sides match smear for smear if using anything other than epoxy, because there is always some shrinkage which will cause warps with more glue on one side. There is an essential trim phenomenon that must be understood, related to longitudinal (up and down) stability called stall frequency. Stall frequency is dictated by the relationship between the setting of the tail plane relative to the wing and the location of the centre of gravity (CG). The tail plane (or stab, or elevator) controls the angle of attack of the wing and the lift being developed. The lift also varies greatly with the speed. The CG, as discussed before, remains static. With one force varying and its counter-force not varying, there will be a change in flight quality with any change in speed. To a more or less degree, these settings will determine whether the plane has a tendency to dive or climb at various speeds. Diving is, of course, selfdestructive. We try to fly so that the tendency of the plane is to go up... but just a little. Let us briefly examine a stall. The plane is in an awkward situation that results in a loss of flying speed and the wing stalls then, the CG, being forward on the plane, brings the nose down and the plane dives. The violence of this maneuver depends on how much of the flying speed was lost, and this is often dependant on how steeply-up the plane was pointed at the stall. It can vary from a barely perceptible pause-and-nudge, to a complete mislaunch where the plane is left hanging on its tail and it flips180 degrees or more before plunging hopelessly ground-ward. Once the dive begins, the wing, held firmly in its angular position by the stab, begins to slowly lift the nose upward; but the speed continues to increase until the plane bottoms-out and begins to climb. During the climb, the speed begins to bleed-off. If the wing keeps lifting the nose up much beyond level before the speed bleeds-off, the plane will undergo another stall-and-recovery sequence. It is the quality of this repetition that is described by stall frequency. 7

8 If the stall frequency is high, the stalls and dives are short and quick. This high stall frequency characteristic is used by gliders with high drag such as the dime-store variety in the plastic bag, for a number of reasons. For instance, the speed build-up is low before the quicker bottomingout, and the quicker nose-up rotation during recovery is well matched with the faster bleed-off due to the higher drag. Any increase in speed, even for a short time, will cause these planes to pitch-up. Higher performing, sleeker planes do better with a low stall frequency. If we carry this to extremes, eventually the maneuver would take so long, and require so much height that it would dive to the ground before being able to recover. We are looking for a recovery time just short of that so the plane has time to just recover from a bad stall at altitude. Giving up-elevator will increase the stall frequency, and down elevator will decrease the stall frequency. To complicate things a little more, any change in the tail plane will necessitate a change in CG location to maintain the glide. If the CG is too far to the rear, the plane upon entering the glide, will slow-up, dropping its tail lower and lower, until it stalls abruptly, and repeats the whole performance all the way down. The number of times it does this is determined by the stall frequency and the initial altitude. In fact, any fiddling with the CG location or the tail plane will result in a change in stall frequency.. If the CG is too far forward, it will glide fast and groovy, and a little too steep. We would like our plane to float just fast enough to avoid a stall... and no more. Go back to earlier for the discussion on wing-tip differential and rudder effects for ideas on how to adjust the turn. Since 90% or more of the flight is spent in the glide, the reasons for keeping the plane traveling in a circle, especially in a breeze, are fairly easy to understand. But if only 10% of the flight will be spent at high speed, why bother to make it turn then? A good question, particularly since if it flew straight and fast, we could throw it directly up-wind and let it circle slowly back to us. The reason we have to make it turn at high speed is because that turn is the ultimate stall-damper. Remember that our plane has to fly well at high speeds as well as slowly, but there is no pilot to adjust for the changing lift with the changing speed. Trimming for a fast turn provides predictability, and hence... control. And, there are a number of aerodynamic features built into these planes, for instance to establish a good glide, which will change forcefully as the speed changes. We need one more counter-control to come in at high speed to combat these changing forces. We can in fact balance it so that it increases proportionally with other changing forces, and lock them all in place. The three forces we ll try to balance and tie together will come from the differential in the wings, or wing-tips, the angle of the tail plane, and the turn on the rudder. As speed increases, these forces being totally aerodynamic in nature, will all increase. It is up to us to balance them so that they all increase in desirable proportion. At speed, the wing tips are trying to roll right. The elevator is holding the wing so that lift increases, and the rudder is trying to turn left and slightly down. Any turn effects from the rudder will tend to dive the model so that the rudder counteracts the force from the differential as well as the stab. We try to make the model fly the same sized circle because it means an easier transition from high speed to slow speed. 8

9 If the model is trimmed to maintain a circle at any speed, then no matter what upset it encounters, it will ultimately and as quickly as possible, regain its normal flight pattern, even from a terminal velocity dive. These dives, more often than not, are the result from a mislaunch and a massive stall, perhaps from not using enough right bank in the launch. If the model were trimmed to fly straight while going fast, it would stall, drop its nose and dive, bottom-out, raise its nose and climb for another stall. The speeds would all be too fast for any of the low-speed trim effects to come into play, and the model would never achieve a glide circle or a glide at all. The ultimate stall damper is developed when the plane is made to turn consistently at any speed. As the plane picks-up speed and begins to rotate its nose upward, the rudder off-set turns the model left. This keeps the nose down and the plane banked until the speed can bleed-off and the low-speed glide-trim takes over. The transition from fast to slow should be almost imperceptible, as should the loss in altitude from the transition. I usually trim my plane so that it just has time to recover from a moderate mislaunch and go into its turn before touching down. On a total mislaunch like, hanging on its tail, which I don t expect to produce too often, it will not have time to recover and will dive to the ground. If too much rudder is used, any high speed will turn it into the ground, often with disastrous cartwheels. If too little rudder is used, the plane will not turn left sharply enough to smoothly switch into glide, and the plane will stall again. This may all seem like an extraordinarily complex blend of effects... and indeed it is. These hand launched gliders are difficult and intricate to fly effectively, but you can succeed in working out these problems and developing a very real knowledge... and that success will be gratifying. Following, is a troubleshooting chart. TROUBLESHOOTING Make all the adjustments in very small increments. Some of them have very profound, highspeed effects, such as with the rudder or elevator. There are only two, potentially really disastrous patterns to avoid: the stall and tail-slide close to the ground, and the left spiral, or high-speed, left turn into the ground. Almost anything else is pretty safe. When applying corrective alterations to the model, it is a good idea to restrict yourself to one adjustment at a time before testing -- to avoid overkill and so that you know which corrective action had effect. 9

10 CLIMB OR HIGH-SPEED APPARENT PROBLEM Dives with low bank Dives with steep bank or turns too sharply CORRECTIVE ACTION Bend trailing edge of stab up Examine wing for warps and if any, remove. Bend trailing edge of left wing down to reduce the bank. Reduce left rudder to open turn. If bank borders on being moderate or light, bend trailing edge of stab up. Climbs too steeply after diving, leading to further stalls. Bend trailing edge of stab down. Increase left rudder. Examine wing for warps could be excess of differential. Has tendency to roll right at high speed and/or while bottoming-out from diving. If wings are not warped, reduce wing differential, perhaps by bending left wind tip up. Increase left rudder 10

11 SLOW SPEED AND GLIDE APPARENT PROBLEM Shallow dives to the ground at steady angle of descent. Dives at ever increasing angle of descent. Slows down slower and slower until stall occurs (could be a gentle stall with quick recovery), usually recurring. Move CG rearward. CORRECTIVE ACTION Bend trailing edge of stab up and re-trim for high speed flight as well. Move CG forward. Bend trailing edge of stab down (tiny bit) and retrim for high-speed flight... watch for everincreasing dive. Reluctant to maintain, or fall into, consistent glide circle. Check wing for warps and ensure sufficient differential. Compare with high-speed tendencies and if they too suggest low differential, bend trailing edge of left wing down. Ensure sufficient stab-tilt; i.e., 1 to 1 degrees right bank. With a finished model, I split the top of the stab with a sharp blade against the fuselage on the right side, and bend/break that side down and re-glue -- this allows for stab tilt on one side and slight anhedral, but much easier than trying to reset the whole stab. Ensure that the inside wing is a little heavier than the outside wing this won t affect high-speed flight. Turns too sharp but not diving. Examine for excessive stab-tilt and reduce to 1 degree if needed. If inside wing may be too heavy, reduce wing-tip weight or add weight to right wing-tip until just lighter than left. Too tight a pattern but very stable. Typical of low performance, commercial planes, or rough-weather trim with emphasis on quick recovery. Probably, the stall frequency is undesirably high, the high-speed turn a little sharp, with a tendency to loop overhead on the 11

12 launch. Re-set combination of wing, stab, and rudder by doing bits of each: reduce rudder, bend trailing edge of stab down, and if any differential, bend trailing edge of left wing up to reduce, all to open the circle. This might necessitate a rearward shift of the CG to maintain a good glide pattern. 12