What can cause the shear webs on a Chrysalis to break?

I have a sort of continous minor problem with my Chrysalis in the shear-web area. Almost every time I go to the field, I come back with a couple of shear-webs to replace. Some of this has been due to the fingers hooking over the pegs (this only seems to happen when I get tired), but it sometimes happens under normal launching - sort of seems to be a weak spot. I glued the shear webs using medium CA (I think). Any ideas?

From : Don Stackhouse

Mike, when something breaks, it means that either it's not as strong as it needs to be, or that it's being asked to do more than it was designed and built for, or both. Unfortunately it's usually almost impossible to tell exactly which of these is the case in a specific situation, so it's usually best to attack this issue from both sides.

First we'll look at what you're expecting it to do:

If you're pulling down on the pegs, that's bad. Period. Don't do that.

Joe was under the impression that this particular Chrysalis is your first one, and that this particular wing was covered with Solarfilm, which led to some recurring flutter problems. Also, you're reporting shear web failures but not spar cap failures, which supports this conclusion. If this is the case, that is probably most of your problem. The recurring flutter puts high stresses on the joints between the shear webs and the spar caps, causing fatigue in the material and the glue joints. Now the bad news: to make it go away, you must eliminate the flutter, AND repair the existing fatigue damage. This means replacing any shear webs that have a lot of launch cycles on them or that show any signs of splitting or cracking, and re-gluing all the shear web joints in the inboard wing panels. Use thin c/a for this, and make sure it soaks through the thickness of the balsa shear web sheeting (more on that in a moment). Cover the inboard wing panels with Monokote or regular weight Ultracoat or Oracover. You can probably leave the Solarfilm on the tip panels IF you can get the new covering to stick to it.

The failures you're seeing during normal launches are quite likely to be latent damage that accumulated during the abusive launches. The fact that you're not actually folding wings regularly says that the Chrysalis is almost able to tolerate the present situation. This means that almost any improvement in launch stresses will probably have a very meaningful effect on this problem.

Since pulling down on the pegs mainly seems to be a problem when you're tired, practicing may or may not help, but adjustments to your grip and to the pegs might be very helpful. Check Joe's article in AJ&D on how to throw a Monarch. Note in particular how he wraps his fingers and thumb around the nose in front of the wing, forcing him to bend his wrist back to keep the model aligned with the throw. Besides additional speed that this gives the model at the point of release, it also makes it much more difficult to wrap the index and ring fingers over the tops of the pegs. By the way, pulling down on the pegs at the end of launch often provides the initial excitation that triggers flutter.

Have someone observe your throw from the side, and videotape it if possible. Check of you're moving the model in a straight line or in an arc. The model's path during the throw should be in as straight a line as possible, and the fuselage should be as parallel to that line as possible. If you're moving the model in an arc, it's probable that you're applying some negative loading to the wings during portions of the throw. This generates energy-robbing drag, and also applies very high (and totally unnecessary) loads to the wings. The model should be at as close to a zero-lift angle of attack as possible during the entire throw if you want the best launch and the least stress.

I also recommend that you laminate some extra balsa on top of your finger pegs to make them thicker, if you haven't already. This gives a much more comfortable rest for your fingers, and also helps prevent you from wrapping your fingers as far around the pegs.

Now let's look at what you can do to make your Chrysalis stronger:

First a warning: It's important to also do the stress-reducing actions discussed above. If you don't, then strengthening the shear webs will probably just move the problem to something else, like the spar caps or perhaps the tail.

Ok, back to shear webs. The original Chrysalis kits had some very light wood in this area in some kits, and shear webs only on the aft face of the spar. We have since specified shear webs on both sides of the spar out to rib F, and requested heavier wood for the shear webs from our wood supplier. Wood being what it is, no two pieces alike, and variations in density even within the same sheet, there are probably a few kits with lighter shear web stock in them than we would like. You might want to replace the shear webs with heavier wood if they seem to be from very soft stock. I'd recommend 8 to 10 lb/cu.ft. "C" grain balsa. If you're going to do something this drastic, you might also want to use 1/16" balsa (1.6mm) instead if the 1/32" stock that comes in the kit, depending on how much extra weight you're willing to add for an increase in strength.

In addition, if you built the kit with one of the thicker cyanoacrylates, the adhesive might not have soaked very far into the shear web balsa, leaving you with only a surface bond. This wastes much of the strength of the shear web. A water based glue such as WeldBond, Titebond or old-fashioned white glue does a much better job of this type of bonding. Unfortunately, since the original joints were made with cyanoacrylate, the surface of the bonding areas on your spar caps are now sealed, unless you do a lot of sanding (which might weaken the spar caps). For this reason I recommend thin cyanoacrylate for any repairs here, it's thin enough to soak thoroughly into the wood, but compatible with any existing adhesive residues.

If you're interested in serious "overkill" and lots of extra work for yourself, you could also add 3/32" or 1/8" shear webs between the spar caps (like an "I" beam) from rib B to rib F. I'd recommend using Weldbond or Titebond for those if you want to take that approach.

As I said above, now that you've substantially increased the strength of the shear webs, be careful that the point of failure doesn't simply move to something else, like perhaps the spar caps. Make sure you've eliminated any flutter, and that you aren't using any abusive launch techniques. Keep inspecting the entire structure regularly for any signs of new problems. These fixes should solve your problem, but in any case please keep in touch, we want to be sure you're happy with your Chrysalis.

Thanks and good luck,

Don Stackhouse @ DJ Aerotech

P.S.: Since receiving this, Mike sent the following additional information:

1) Material could very well be the problem. I did move from Solarfilm to a heavier material, but this may still be the problem. I did get some Monocote from yourselves, and will recover the wing with this when I have to do quite a bit of surgery.

This would explain any flutter present, and flutter's damaging effects are cumulative.

2) I have had a three spar failures. This has been on the top spar where it has been under compression due to the shear web failures. A little glass tape & CA seems to have fixed them.

As I discussed in the AJ&D article on Chrysalis spar failures on launch, the main cause of these is pulling down on the pegs at release, and the main failure scenario is a failure of the shear-web/spar-cap joint, which then allows the now unsupported spar cap to buckle.

3) I will work on my throw, since this is probably a major cause of the failures.

Sounds logical to me! The alternative would be to beef up the spars so that it wouldn't break no matter how hard you pulled, but who in their right mind wants a 20 oz. hand launch glider? (Note that at that point it probably doesn't qualify as a "sailplane" anymore!) The nice thing about developing a better throw is that you will have less fatigue in your arm, more height in your launch, more enjoyment in your flying, and if someday you opt for a high tech HLG like a Monarch 'D' or Wizard, you won't break it the same way!

4) The shear web balsa was very light and soft. I have been beefing up the shear webs when doing repair work, and so far none of these has given problems. I will check this carefully for future Chrysalis's.

This is the constant nightmare of anyone in the wooden airplane business! No two pieces of wood are ever alike, and no matter how carefully you purchase and inspect the wood for the kits, there will always be some that are at the extremes of the range. Even if we doubled the amount of wood inspection (which would probably almost double the price of the kit), there would still be a few pieces that would manage to sneak through. Balsa is particularly bad in this respect, with much wider variations in properties than other woods.

By the way, this is the main reason we decided to use smaller sizes of harder and less variable woods like spruce to carry most of the primary loads on the Chrysalis. That's also why the Monarchs and Wizards we build today have no balsa in them! We're looking for ways to make the Chrysalis design even less vulnerable to balsa's fickle personality, but as long as there is balsa in the kit, we will probably still have to rely on the builders to help us "keep an eye" on balsa's unruliness!

Don Stackhouse @ DJ Aerotech
djarotec@bright.net