That annoying tip flutter

posted in: Steven Pearson | 0
Part of the test flight for all production gliders at Wills Wing is a pilot full-forward speed run at VD to check pitch pressure and sail quality. The sustained dive speed varies from the mid 30s on a Condor to as much as 80 mph on a T2. New gliders do not flutter at ‘Vd’ dive speed however older sails will invariably begin to flutter at much lower speeds. Fluttering will sometimes progress from a short-intermittent buzz at high speed to a debilitating and persistent beat at much slower gliding speeds with as little as 50 hours of airtime. Fortunately, you should be able to coax as much as 500 hundred flutter-free hours from a sail with just a little preventative maintenance. Curved tip gliders are more susceptible to flutter because there is more unsupported sail area aft of the roach line (defined by a line from the end of the leading edge to the trailing edge at the aft end of the root cord) so they require somewhat more attention and tuning.
There are 3 steps to correcting tip flutter.

1. Adjust your batten tension. The batten lever force should become firm about 15 degrees before the latch engages. As you sight up the batten pocket, you should see the lateral sail wrinkles and slack disappear just before the lever clicks in place. Do not tighten the batten so tight that the high point of the batten is visibly pronounced in the sail top surface. Battens from the root to mid-span can be somewhat looser since this area almost never develops flutter and high batten tension in the inboard section of the sail often hurts handling. Make your final adjustments after the glider is completely assembled.

   I check the batten tension every time I fly a glider and make changes as required. New gliders often require tuning on every flight for the first few hours until the sail is fully relaxed from stitch tension and residual creases in new fabric. Also, you may notice that the sail changes with temperature or if it has been set up continuously for a day or more.
2. Balance the leading edge and wand tension. This procedure varies somewhat depending on the model and configuration of your glider but the following principle applies to all Wills Wing curved tip gliders. The spanwise sail tension (preventing the sail from moving forward on the leading edge) should not be supported predominantly by the tip wand. Higher leading edge tension and correspondingly looser wand tension will reduce flutter. 
   You can check the tension on the leading edge sail mount through the outboard sprog zipper. At VG loose, the sail mount strap should be snug and without slack. On newer model T2s with an adjustable sail mount, the tension may be relatively tight at VG loose. The tension in the strap will become tighter at higher VG settings. 
Older model sails were mounted to the airframe with an adjustable webbing/tang assembly. If your sail has this configuration, tighten the webbing until it is snug when the glider fully assembled and VG loose. You’ll have to remove the clevis pin that secures the tang to the leading edge (with the wings folded) to make adjustments. A reference circle is drawn on the inside of the sail body showing the original tang position. Typically, you will need to tighten the webbing about 1/4 inch. Higher leading edge tension will be more effective at reducing flutter but may make lateral control stiffer and less responsive. 
   Later model sails have a fixed length sail anchor which is less likely to stretch and become out-of-tune but cannot be tightened. If your sail has this configuration and the sail strap is too loose then you must shorten the tip wands. The tip wand lever has 3 hole positions where it is bolted to the sail. Remove the lever from the sail and move it to the position closer to the cup which effectively shortens the wand by 3/16 inch. Alternately, trim your tip wand by 3/16 of an inch. Note that the stock tip wand length is 34-5/16 inches.
Later model T2 and T2Cs have an adjustable sail anchor to precisely control leading edge tension. If your sail has this configuration, tighten the leading edge approximately 3/16 of an inch. 
3. If your sail has already developed flutter then you must correct the problem without delay or the sail will be irreparably damaged. If the tuning changes described above do not solve the problem, install a small-gauge zip-tie between the grommets of the #2 (shortest) batten and tighten it enough to pinch the sail together about 3/8 of an inch. If the sail still flutters, the final remedy is to return it to Wills Wing for repair. In many cases, we can re-cut the trailing edge and extend the service life however it’s still important to adjust the sail as I described above.