My wife bought me a Spectre HLG for Christmas, and when I untaped the wing panels, the tip panels both had a moderate twist to them - about 1/3" washout in one, and 1/2" washin in the other.

Is there a recommended way of eliminating this twist? I was thinking about laying them on a flat surface (bottom down) with some bags of lead shot on them, for a few weeks in a warm place. Any suggestions would be greatly appreciated

From : Don Stackhouse

Sorry about the problem. It's one of the nearly inevitable side effects of the need to keep weight to an absolute minimum. The extremely lightweight composite skins are sensitive to residual stresses in the foam, stresses from being packed in a box and shipped various places, or laying under a bed or in a corner for a month, differential thermal stresses from the temerature changes it sees during shipping, etc... The list is virtually endless. Also, although I'm pretty sure this wasn't a factor in your case, just for future reference one of the quickest ways to get some really bizarre warps is to leave a fairly new set of wings closed up inside a car on a hot and sunny summer day.

I have two bits of good news:

1. It's not hard to fix. 2. The longer the wing is kept in the right shape, the more the internal stresses dissipate, and the less tendency it has to warp.

To fix the problem, set your covering iron on low heat, typically about 150 to 170 deg. F (65 to 77 deg. C). Hold the root end of the panel between your knees and use one hand to twist the tip of the panel into the correct amount of washout. While holding in the correction to the twist, run the covering iron all over the panel on both top and bottom. Keep the iron moving; if you let it dwell in one spot for more than an instant, you might possibly overheat the foam core and cause the cell walls to melt (that's VERY, VERY bad!). Just run it all over the surface until the fiberglass skin softens and allows the wing to take on the new shape.

I generally heat one side of the panel completely, let it cool, check the washout to see if it changed significantly, then heat the other side of the panel and check again. If you're really paying very close attention, you'll feel the panel relax a little in your hand when the temperature of the fiberglass skin reaches the epoxy's "glass transition temperature", the point at which the epoxy starts to soften. Put the iron aside but continue holding the panel into the new twist for half a minute or so while it cools down. Let go of the panel and check the washout.

If you did it right, it should now have the desired twist. If not, try again, maybe twist the wing a little beyond the desired washout setting, but run the iron over the surface for a longer period of time. If that still doesn't work, turn up the temperature on the iron a TINY bit, maybe another 5 degrees F or so, and try again. Be careful, the biggest risk here is getting in too much of a hurry, trying to force it with too much heat, and ending up melting the foam core. Remember that old saying, "Haste makes waste!"

Once you've found the right temperature setting for your iron, the next time will be much easier. By the way, this same temperature setting is about right for ironing out the inevitable small dents from gravel and other objects you might slide across on landing, etc.. With just the right amount of heat, the foam will soften a little at the same time the gas in the cells expands from the heat, and the dent will push itself out. Be careful though, once again there's the risk of melting the foam if you get too agressive, and the only repair possible then involves surgery.

However, don't let all of the above scare you; it's not as tricky to do as it sounds. BTW, Joe and I repeatedly use this same technique on prototypes when we're experimenting with different washout settings during flight tests.

I'd set the inboard panels of your Spectre HLG to about zero to 1/32" washout, and the tip panels to about 1/16" to 3/32" washout. Like with the Chrysalis, a little more washout improves stability and handling in turns, while a little less improves high speed performance and launch height. These numbers are a good compromise, and are what I use in my own Spectre. In no case should you have any WASH-IN on any of the wing panels.

Check the washout of your model regularly, about every week or so, if you suspect it's been subjected to high temperatures, or if it seems to be performing or handling oddly. If you keep the washout at the correct setting for a few months or so, the wing will eventually get used to that shape and tend to hold it. It's a bit like a new set of violin or guitar strings, that have to be re-tuned almost constantly for the first 5 minutes or so after you install them, but after a month or so almost never need to be tuned.

Don Stackhouse
DJ Aerotech