With the other thread raging, I got an email asking a far more fundamental question - how to you do square loops, anyway? That sounded like a pretty good question. I will make it anonymous, unless the author wants to identify himself. It's Fancheresque in scope, but gives you most of what you would need to know to troubleshoot on your own. Of course, it's much easier to have someone else with more experience to help, which would get to the problem almost immediately.
Could you describe how to do the squares?
I seem to get the first corner and maybe the second but I do not seem to be able to get the 2nd and 3 flats - after the second corner it seems all round
I am using a primary force and I have a hard point handle
Rounds are not bad, just big, but have not figured out the squares
Well, without having seen you attempt it, i am just speculating. The problems fall into several categories, any or all of which might be the problem:
Trim (roll/yaw): First and foremost, make sure that your airplane flies at the same roll angle when upright and inverted. If it doesn’t, that means the wing is warped or there is some misalignment. This is particularly problematic with “square” wings like the Primary Force, because theres a lot of lift at the tips and it’s not as stable against warps as a tapered wing, or one with LE sheeting. The fix is to find the warp (sight the wing from behind, put the spinner on your toe and look down at it. Visually center the TE in between the upper and lower surfaces of the wing at the root, then, without moving it, see if it also centered at the tips. If not, then twist and heat with a heat gun (carefully) until it is straight. If it’s just a little bit off , you can take piece of soft aluminum, cut out a piece about 1” x3”, glue or tape it do the lower surface of the outboard wing, and leave half of it hanging off the TE. Then bend it up or down to act as an aileron to compensate for the warp.
While you are checking for the *same* roll angle upright and inverted, note whether the outboard wing is high both ways, or low both ways. Low both ways suggest you have excess tip weight, high both ways suggests too little tip weight. The warp and the tipweight can both be present and add or subtract to each other. So, say the outboard wing is high upright and level inverted. That suggests that you have a warp AND not enough tipweight, and so forth.
Get that straightened out before trying again. If it appears to be level both ways, try adding small amounts of tipweight until you can *just barely detect* it with the outboard wing down upright and inverted.
Note that to first approximation, the more tip weight the more line tension there is, because the airplane is rolled away from you in steady-state flight, so some of the lift of the wing is vectored away from you, adding to the line tension.
However, when you are learning it is common to use a lot of tipweight (more than necessary) to increase the line tension. This doesn’t hurt anything too much in round maneuvers, and covers other trim issues by just overwhelming them. When starting to try square corners, the excess tipweight will cause what is known as “hinging”, which is a rapid roll motion that is proportional to how tight the corner is, the tighter you try to turn it, the harder it rolls.
Excess tipweight will cause it to roll away from you, so the first thing you see is the bottom of the airplane on insides (and top on outsides). This causes several issues- immediately, it will increase the line tension in a big “spike”, as the roll angle causes the lift of the wing to be aimed away from you which adds to the line tension. Typically, this will increase the control deflection, because you have more tension, the controls will be deflected further as the lines straighten out, the available torque to deflect the controls goes up, and a few other lesser effects. This will cause the corner to tighten up abruptly with no handle movement, exacerbating the roll, and feeding on itself to cause you to turn tighter than you wanted, and thus slowing the airplane more, making it turn further than you wanted, etc.
Unfortunately, the roll angle will only go “away” from you for a split-second. Since the roll motion pulled the lines up towards the inside of the turn, they will very shortly after swing back the other way, rolling the airplane in towards you (you now see the top of the airplane on an inside turn). This swing will cause the line tension to drop. It will continue to swing back and forth. This greatly interferes with both exiting the corner in the right direction, and also, will complicate the second corner if it is still signing back and forth, because you don’t know if you have any line tension, how much control you will get, etc.
These sorts of oscillations make the airplane very unpredictable to control, because you never know for sure what is going to happen.
If it is just a pure roll motion, you take out tip weight until it stops doing that, and doesn’t roll in the corners. Note that there can be an effect where it looks like it has the right amount of tipweight in level flight, a little too much in round maneuvers (starts to roll slightly away from you) and way too much in the squares. In that case, you want to add area to the outboard wing, without any “aileron effect”. You can also just compromise at the “best” setting, which might make it a touch light in rounds. We would add area to the outboard flap, usually call a “Wart”, because its’ an ugly clearly anomalous growth.
Another thing that can happen is if you have a lot of rudder offset, and the leadout position does not match. When you deflect the controls, you increase the line tension temporarily, because effectively you pull the lines toward you as you deflect them. This effect gets larger if you have a handle with lots of “overhang”, that is, the stiff parts connect to the flexible parts far in front of your hand. Your hard-point handle probably doesn’t have a lot more than the minimum possible, but the effect is always there. If you are flying around and the rudder it trying to point the nose away from you, and the leadouts are holding it from going further, as soon as you put in control deflection, it tries to pull the nose in at you, as the rudder effect is the same but the compensating leadout effect is greater because the line tension went up. This can do several things but at least it suddenly reduces the line tension, making your control input have less effect. Another thing that may/probably will happen is that since you are yawed with the outboard wing skewed forward to the previous position, the outboard wing will have more lift, causing the airplane to also *roll* in towards you, dropping the tension further. Also, the same sort of oscillation in the lines is set up, with the same effects as above.
This also tends to lead to oscillations in roll/yaw, with the same effect as above, unpredictable response.
A very common situation is what we named “Twister Disease”, as the stock Twister rudder offset leads to it. The rudder is very effective and has a large offset shown on the plans. This makes it want to yaw outboard, and the stock leadout position is far enough forward to keep it pretty close to “tangent” to the circle in level flight. So, if you adjust your tipweight as shown above, when you try to do a square corner , it will rapidly yaw nose-in, roll towards you, lose all the tension, and be in trouble. If you add tipweight to prevent that, you will have way too much for the rounds and level flight. The solution is in this case is to remove most/all of the rudder offset, set the leadouts further aft to make the “sag” of the lines, and then the tipwight much be good for all conditions.
Trim (pitch): The pitch trim/control response is controlled by the CG and the rate of the control motion (bellcrank, elevator horn, and handle spacing). For a no-flap airplane, regardless of the rest of the design, 15% of the chord works out surprisingly well in a wide variety of cases. It will at least be safe, if not optimal. So measure the “average” chord, multiply that length by .15, and that will tell you how many inches behind the leading edge you want the airplane to balance. Figure an 11” root chord, a 9” tip chord, the average is 10”, so the CG should be 1.5 inches behind the LE of the wing. If it is a too far aft, the response will be twitchy, and too far forward, and the response will be too sluggish, everything else being equal. There are some more subtle ways to deal with it, but I would suggest, if you aren’t currently flying full patterns, that you probably should just set it and leave it alone.
The control setup basically sets how fast the elevator moved with respect to the handle. I would suggest a rate of about 30 degree of elevator motion for about 20 degrees of handle motion. I would suggest you adjust it with the elevator horn setting, to as slow as you can get it, for the largest handle spacing, but you can just change the handle spacing.
I can’t guess whether you have it to fast or too slow. In past decades, almost everyone set the controls up too fast, meaning when they tried a square corner, they over controlled it drastically, causing the speed to die off, and making the rest of the maneuver impossible to complete. In recent years, however, I have found almost everyone setting up the controls too slow, meaning it is nearly impossible to get enough control deflection to turn sufficiently tight corners within the space available. A Primary Force has none of the problems we use to have with excessively fast controls (like on a Ringmaster, where you can tolerate +- 1/2” of elevator motion at most), and should have no issue doing square corners if otherwise in proper trim.
Flying technique: The most common problem people have with learning to fly square loops is trying to make the corners too tight, and the maneuver too small. If you just bury the controls in the corners you will upset the airplane, it will swing around wildly, or even stall the wing, and you have no hope of completing the maneuver. This is usually exacerbated by trying to make the maneuver too small. You have to allow the airplane to *fly* through the corner, make a detectable “straight line” section, then fly through the next corner. You never “snap” your wrist, you move it rapidly but smoothly to the desired deflection, hold it for a split second, then rapidly but smoothly return to neutral, hopefully with the airplane traveling in the right direction.
The specified maneuver dimensions are completely out of play for most people, and can only be met with extreme effort, perfect trim, and perfect power deliver on the best designs. And it’s still very difficult and prone to errors. People think of it, or have it in their internal clock to do it “snapsnapsnapsnap” when it’s really “turn-straight-turn-straight-turn-straight-turn-straight” Just as a timing mechanism, figure a each side a “slow second”, and actual count out loud slowly (slow second = 1 and a quarter seconds or something like that) or in your head, "one - two - three - four”. “snapsnapsnapsnap”, you will wind up completely out of control by the second “snap", and have to bail out or crash.
The other bugaboo is that things are going so quickly and are so unusual that you lose your orientation, and lose track of where horizontal is as you are exiting the second corner. This usual results in a semi-panic, because you know the ground is out there somewhere, but you have lost track. A solution, which is so simple that no one can believe it, is to force yourself to *stand up straight* and to “hold your head straight”.
When you are going to try the maneuver get your feet a little ahead of the airplane, and plant them so they are 90 degrees from where the center of the maneuver it going to be (downwind, presumably). Follow the airplane until your shoulders are also perpendicular to the maneuver, and parallel to your feet. Then stop rotating your shoulders and hold that position until you finish. Consciously and deliberately stand up straight, make sure your head it straight up and down, and then do the maneuver without moving your shoulders or feet. Follow the position of the airplane with your arm, so it’s a straight line to the center of your sternum, to your hand, to where the airplane is. The “straight up and down” will get you a reference you can count on for horizontal, without you having to see the ground. The shoulders and feet, will give you a reference frame for left and right, the maneuver should be generally centered on your shoulders.
This is much harder than you think to actually do, most people, after even a few flying sessions during training, develop all sorts of strange quirks with their body positioning. They make it feel comfortable and they can be very tricky to get rid of later.
Another important thing about piloting, and something that is diametrically opposed to what we tell raw trainees to keep the airplane alive on the first few flights, is to hold your arm straight and move your hand up and down for control, or hold your wrist rigid, and only move your elbow. For stunt, you want to make the majority of your control inputs with your fingers, wrist, and forearm rotation - if you have to start moving your elbow, that is an emergency. Hold the handle about a foot-18” in front of your sternum with your shoulder nominally perpendicular to the lines This will result in your elbow being bent at around 90 degrees and you can see that you can rotate your forearm for large handle motion. Control motions are either fine, medium, or large, so fingers, wrist, forearm. This also facilitates keeping the airplane in your “reference frame” as discussed above.
At any rate, this covers the big areas of problem that I have seen over the years. It would of course be very useful to get with someone experienced, who might diagnose your bigger problems immediately instead of you having to figure it out from an email.
Brett
Topic: How to do square maneuvers (Read 3655 times)