What causes Chrysalis wing failure on launch?

"We've had a good pilot break his Chrysalis on launch -- cracked the wing on launch, he believes, exerting force downward on the throwing pegs. He previously flew the Chuperosa -- a much heavier ship with a fingerhole, I believe. Any clues? I think the "feel" of launching with dual finger pegs is different from the fingerhole and I've also had a problem getting used to that feel."

From : Don Stackhouse

Bob,

Sorry to hear about your friend's misfortune with his Chrysalis. Yes, pulling down on the pegs as you throw will break almost any good handlaunch.

The Chuperosa he was used to flying is an old design, with a thick airfoil and a heavy structure. This gives it a stronger wing and a slower airspeed at release (more weight means your arm can't accelerate it to as high a speed), which helps reduce launch stresses. This makes it more tolerant of abusive launch techniques. This also gives it lackluster low speed performance better suited to a slope ship than a hand-launch thermal ship. It is possible to make a model tolerant of abusive handling in some parts of the country; the Chuperosa is a tolerably good HLG in southern California, but the thermals there are known to carry off children and small automobiles.

Pegs do have a different feel than the traditional finger hole, but I don't think that's really the answer. The Chrysalis can be built with a fingerhole (it's even shown as an option on the plans), but this won't necessarily keep you from pulling downward at release. One of the more effective measures is to modify your technique. Make sure that when you grip the pegs (or the edge of a fingerhole) the "ball" of your fingertip is resting on each peg, and that you are NOT hooking your fingertip around and over the top of the peg. Laminating some extra balsa on top of the pegs to make them thicker will help with this, and also make throwing much more comfortable. The small increase in peg drag in flight will not measureably effect performance.

Structurally speaking, we are now recommending that the builders put shear webs on both the forward and aft sides of the spar from rib B to rib F, then on the aft side only from F to the tip as shown on the original plans. In the cases of wing failures we've looked at, the failure seems to initiate as a failure in the glue joint between the shear web and the upper spar cap, causing a loss of support for the spar cap, thereby allowing the spar cap to buckle. The extra shear webs increase the area of the glue joints, add additional support to the spar cap, and make that support more symmetrical, all of which are good things!

We have had some reports of spar failures like you describe, not a lot, but when it occurs it sometimes happens multiple times to the same person. When something isn't a problem with the fleet but re-occurs with specific people, technique is a very likely culprit. We're looking at simple and practical ways to make the Chrysalis more tolerant of this (such as the shear web change), but virtually any HLG that is light enough to fly well in our midwestern conditions will also be vulnerable to overstressing on launch by bad technique. It looks like education will be a key part of any fix for this one.

Don Stackhouse @ DJ Aerotech