- Wed Jul 09, 2008 7:28 am
I have the same issue on my U2.... but only with the VG on. The more VG, the worse it is.
I called WW about this a while back and they sent me this:
There are several factors that can cause flutter in the trailing edge at the tips.
1) A combination of loose leading edge sail tension (the tension on the sail anchor strap that attaches near the end of the rear leading edge) and overly tight tension carried by the tip wand will slacken the trailing edge, because the excess tension along the leading edge applied to the tip wand causes it to bend inwards and thus reduces the spanwise tension the wand puts on the sail. This can result from the sail anchor webbing strap stretching over time, from the adjustment on this strap having been loosened (on those gliders where the strap is adjustable), or from the wand adjustment being too tight. If the glider has an adjustable sail anchor strap, the strap should be tightened until, with the glider fully set up, the sail mount webbing is snug at VG loose and progressively tighter at higher VG settings. If the glider has a fixed length sail anchor, this adjustment can be obtained by shortening the length of the tip wand as necessary.
2) Tip battens adjusted too loose - if the four shortest battens at the tip are not tight enough there will be insufficient tension along the trailing edge. The shortest batten has a string on it and the next three have adjustable lever tips. With the glider completely set up, tighten the first three lever rib adjusters. Usually 3 to 5 turns (counter clockwise) on the shortest, 2 to 4 turns on the second shortest, and 1 to 3 turns on the third shortest will do the trick. On the shortest batten, adjust the string to where it requires a quite firm pull to install the second loop of string onto the batten tip.
3) Trailing edge string adjusted too loose or stretched (if the glider is so equipped – late model Sport 2’s, U2’s and T2’s no longer have the trailing edge string). To adjust this string, first de-tension all of the battens. Un-do the knot where the trailing edge string exits the jam cleat near the wing tip. After unwinding the string from the cleat, take a pen and mark where the string exits the cleat. With a pair of needle nose pliers, wind the string around the pliers several turns. (This is so you don't strip the coating off the string) Pull the string 7/8 inch and then re-cleat it. Re-wind the string around the cleat several turns as it was before and tie a new knot.
4) In severe cases, where the sail has been fluttering for some time, and the trailing edge hem has stretched or the sail resin has partially broken down, you may need to effect a takeup along the trailing edge hem itself. There are a few easy ways to do this without requiring that the sail be removed from the airframe for sail work.
a) You can put a twist in the first loop of the string on the shortest batten before engaging it on the batten tip, and then adjust the second loop to a very firm tension. This changes the angle with which the string pulls on the trailing edge in such a way as to apply more direct tension along the hem.
b) You can install a small zip tie through the two grommets to which this string attaches, and pull the grommets together, which will apply further tension directly on the trailing edge hem.
We have successfully removed some severe flutter (even at trim speed) from a number of Sports, U2s and T2s with these methods. The trick is catch the flutter early, because if you allow it to continue for a long time, the resin in the sail breaks down and the sail will require work to solve the problem. If none of the above adjustments are sufficient to remove the flutter, then the final fix is to have the trailing edge of the sail re-scalloped to correct the balance of tension between the sail body and trailing edge hem. This requires that the sail be removed from the airframe, and sent to us.