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aeajr
477 Posts |
 Posted - 10/25/2007 : 08:21:02 AM
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UNDERSTANDING AND FIXING POP-OFF PROBLEMS
by Ed Anderson
aeajr on the forums
If you launch your glider with a hi-start or a winch and are having pop-off problems, consider this a cry for help from your plane. It has a problem that needs to be fixed.
Hook location relative to CG -
A hook location that is in front of the CG will not cause pop-offs, but understanding hook location is the first step to fixing pop-offs. For hi-start or winch launching, your hook should be in front of the CG by up to a 1/2". You will want to move this back, closer to the CG point as you gain confidence with the plane and as you work toward more aggressive, higher launches. The closer to the CG, the steeper the angle of climb and the higher the launch. Some planes have adjustable hooks to make it easy to adjust this. My Spirit has 3 hook positions. I consider them Beginner, Sport and Competition, with the last one being right on the CG. My Thermal Dancer has an adjustable hook and my Legend has 2 spots. Note that as you move the hook back, the more unstable the plane becomes on launch and the more important it is to give it a good hard push/throw rather than just letting it go. Typically you will want to move the hook back over time, as you gain confidence with the plane in order to get the highest launch.
Note also that if you add or remove weight from the nose of the plane, as you adjust the CG, you are also changing the hook position in relation to the CG. Most glider pilots move the CG back over time as this usually leads to better handling and better thermaling. However as you move the CG back, the hook position is moving forward in relation to the CG. So, consider moving the hook to keep your launches high. Adjustable tow hooks make this easy.
Hook angle -
Your tow hook should be parallel with the bottom of the plane or bent slightly up toward the plane. There should be plenty of clearance so that the ring can come out when you want it to. Sometimes, a light weight hook can become bent and angled away from the plane. This will contribute to pop-offs. Push it back into shape. If this keeps happening, replace it with a stronger hook. Also, some people put a small notch in the hook right behind where the ring rides. This can help reduce pop-offs too.
H-Stab or elevator setting - The dive test
This is a test for trim and balance, called the dive test. This is best done on a calmer day to eliminate influence from the wind.
Take your plane up high and get into a smooth glide into the wind. Now put the plane into a dive of about 30-45 degrees and let go of the sticks. Does it tend to stay in the dive, coming out slowly over several seconds? Does it hold the dive, requiring you to pull out. If it does, that is good. However if it tends to pull out of the dive quickly, on its own, that is bad.
A properly balanced and trimmed plane should hold a dive of 30-45 degrees and come out of it very slowly. If it tends to pull out quickly, you have a problem. Your plane may be too nose heavy which will require you to trim in up elevator to hold the nose up. Or, your h-stab is up in the back. Your elevator may be trimmed neutral but the up angle of the stab is giving you the effect of up elevator. Either of these conditions will cause the plane to want to climb. In a normal glide you may not notice it but at the higher speeds of a dive, or on launch, the plane will want to go into a loop which, on launch, causes a pop-off. This also causes more drag which hurts your planes performance.
When I first got my Spirit I was having terrible pop-off problems. I thought is was my launch technique. I was new, what did I know. However one of the club experts performed the dive test described above. After we removed some nose weight, things got better but still the plane would pull out of a dive too quickly and it still tended to pop-off too easily. Temporarily I put in some down trim on launch to control this so I could fly that day, but we knew I had a problem.
Over time we determined that my h-stab was not parallel to the wing. I had to do some wood working to correct this, but once I got the h-stab right, the pop-off problem went away, the plane had a much better glide and thermaling became a lot more fun.
NOTE: If you have a fully flying stab, that is the entire stab moves on a pivot, then you do not need to go through this shim process. You automatically fix any wing to stab alignment problems when you adjust your trims. This is an advantage of a fully flying stab.
If the stab is glued on, one way to test for this condition is to shim the wing if you can. This works with top mount wings, but not with side mount wings. Do this after you feel you have made all of your balance adjustments or you will mask a nose heavy plane.
When you mount your wing, put a shim under the back of the wing. Start with something like 1/32 and keep adding till your dive test looks good. Chances are your pop-offs will stop too. Rather than aligning the stab with the wing, you are aligning the wing with the stab. This is not the best way to fix the problem but it does work and it is quick and easy. If correcting the stab is not an option, then adjust the wing to match it. Note that this will cause a gap under the wing. If you are going to use this as a permanent fix, then fill that gap with foam tape or some other filler or this will lead to drag, which is something to be avoided.
If you are putting in down elevator trim on launches to prevent pop-offs, chances are you have a trim problem. A properly balanced and trimmed plane, where the tow hook is in front of the CG, does not tend to pop-off on launch.
The Throw -
Until your plane reaches flying speed, it is in a stall state. In stall, there is little or no control on the surfaces and not enough lift from the wing. If you just let go, then your plane is a rock till the hi-start gets it flying. One of the side effects of this will be a tendency of the plane to drop left or right causing a wild launch or a crash. It can also cause over rotation, leading to a pop-off.
By throwing the plane, you get it up to speed quickly giving you more control and a smoother safer launch. Rotation will be smoother and there will be less tendency toward pop-offs. At first you can throw out flat or at about a 20 degree up angle. Over time you can bring that angle up to 45 degrees or even higher. The stronger the throw, the steeper you can launch resulting in higher launches. Competition pilots, using winches or strong hi-starts, will throw the plane with great force at angles over 70 degrees to get maximum height launches.
Summary
Pop-offs are a cry for help from your plane. It has a problem and it needs your help. It may have multiple problems. It may be nose heavy AND have a bent tow hook AND have an h-stab that is not properly set AND a pilot who is not giving it a good throw. Don't ignore it, don't fix it with down trim and don't add weight to the nose to prevent it. If you understand your plane, you will see that the problems are not hard to fix. Not only will addressing these issues fix your pop-off problems but you will get higher launches and have a better flying plane that handles better and thermals better and more easily.
Clear Skies and Safe Flying!
Best regards,
Ed Anderson
ESL Content Editor
aeajr on the forums |
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kzimmerm
4 Posts |
Posted - 03/14/2008 : 12:11:58 PM
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One additional small not to Ed's comments here is a slight modification to the tow-hook. To start with if the hook were not exactly parallel with the fuse but the bent slightly upwards, there is a less of a tendency to pop-off.
Another trick is to grind a slight ridge at the bend of the hook. This can be done with the use of a Dremel tool and narrow diameter grinding stone. The idea is to produce a slight ridge without compromising the strength of the hook. I've done this on a number of hooks and makes a world of difference. 
Kurt
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UNDERSTANDING AND FIXING POP-OFF PROBLEMS
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