Don Stackhouse at DJ Aerotech tells how to reinforce the very thin wing ribs that tend to break easily.
Mike Whaley writes:
I have had a recurring problem with the wing ribs on my Chrysalis cracking and breaking during normal use.
For the other readers of AJ&D ["Ask Joe & Don", a feature on the DJA Website - MSW], I should point out that Mike's "normal use" includes some slope flying, which tends to be somewhat more rough-and-tumble than most thermal flying. Also individual flying styles, flying sites, and storage and transport conditions all have a bearing on the incidence of dings and crunches. Your mileage may vary.
That said, I must agree that the trailing edges of the Chrysalis are somewhat thin and in some cases fragile. This is a natural result of the need for low airfoil thickness in this region, plus the need for low structural weight to meet performance and handling requirements. Throw in the relatively huge variations in balsa properties (even with careful selection) plus some occasional rough treatment, and some recurring breakage is almost inevitable.
It's always where they attach to the trailing edge or anywhere up to an inch or so in front of it, and seems to affect the wing anywhere from the center to the tip.
Yup, that's a thin area. On the 2-meter Chrysalis we have a rear spar in that vicinity to help steady things a bit, plus the greater thickness that comes with being a larger model. Still, when you have a modern, thin-trailing-edge airfoil, this will be a trouble spot.
I have tried putting CA on the ribs in this area but it hasn't helped much. I'm wondering if the only way to effectively solve this may be to glue fiberglass cloth or 1/32 sheet doublers pieces to all the ribs up to 1-1/2 inches ahead of the trailing edge... it's just so thin there, I'm not sure that there is anything else that will help. Do you have any other suggestions?
There are some other things that will help.
First of all, I'd avoid using C/A for stiffening, it tends to make the wood brittle, and may actually make the problem worse.
The two main culprits here are bending and buckling failures.
The bending failures can happen from landing impacts, but a more likely cause is from handling. If you pick up the wing by the trailing edge, or set it down in such a way that the trailing edge has to support the weight of the model, chances are it's going to get bent. Although I'm sure you're already trying to avoid these situations, for the record I'd just like to say that the first step in minimizing damage is to handle the model more carefully.
The other big source of bending failures is covering damage. If you shrink the covering tight on one side before covering the other, or if you don't stick the covering down to the entire surface of the trailing edge stock and the edges of each rib, there's a good chance you will pull a bend into the ribs during covering. This is also one of the main sources of buckling failures; if you shrink the covering very tight all at once in one spot, especially if you haven't stuck it to the rib edges first, the covering tension can easily buckle a rib. If you got a little overenthusiastic with the sandpaper and inadvertently sanded down the ribs ahead of the trailing edge, this will of course weaken them and make matters worse.
Ok, fine then Mr. Sailplane Designer, I'm not doing any of those bad things, and I'm doing my very best to land gently and not abuse the structure, but even so I'm still getting failures. So now what do I do?
Fiberglass doublers on the faces of the ribs probably won't help the bending related failures, although they could help the buckling failures IF you put them on BOTH faces of each rib.
Some carbon or fiberglass (NOT Kevlar!) tow, smoothly and carefully glued to the top and bottom edges of each rib and extending about 1/2 inch (13 mm) onto the trailing edge stock on both sides will help with the bending failures, and maybe a little with the buckling failures. If you don't have any tow, just peel individual strands from the edge of some heavy fiberglass or graphite cloth.
Another option is to glue a piece of 1/8 inch square balsa or spruce alongside the rib, from the spar to the trailing edge, and inset into the trailing edge about 1/2 inch. If you're having trouble with cracks in the leading edges of the ribs from landing impacts, run the reinforcements from the leading edge, across the top of the lower spar cap, and back into the trailing edge. You'll need to taper the inset portion at the aft end to match the trailing edge, and do some excellent woodworking with tight-fitting joints to get the maximum benefit for the minimum weight. It might not be necessary to do every rib; every second or third rib might be sufficient.
Another probably easier option is to use bamboo skewers. Bamboo has the highest impact strength for its weight of any wood. You can probably find the skewers in the Chinese food section, meat department or the barbecue supplies (charcoal, etc.) section of your local grocery store. Get the ones that are about 1/16 inch (1.6 mm) in diameter. These are about a foot long (30 cm), and have a point on one end, much like the point on a toothpick. Insert them through the shear web from the front, alongside the rib and just above the lower spar cap (all of these fixes need to be connected to the spar, so that the loads from the trailing edge have somewhere to go). Slide the skewer back along the rib and stick the tapered point about 1/2 inch into the trailing edge stock. Trim the forward end of the skewer so that it snuggles up against the back face of the leading edge dowel. Now use some thin C/A, or better yet a thin aliphatic such as "Superphatic", to glue the entire length of the skewer to the rib, the spar and the leading and trailing edges. Once again, if your breakage problems aren't too severe, you might be able to get away with just every second or third rib.
The thing to bear in mind on all of this is that you're adding weight. Yes, we could have used a thicker airfoil and/or heavier materials in the kit, but the performance would have suffered. It's a tradeoff. The only airplane that is too strong to break is the one that's too heavy to fly. If you just want a fairly good slope ship for light-to-moderate winds, or if you live in one of those places where the average thermals regularly carry off Volkswagens and small children, then the extra weight may not be a problem. All airplanes are a compromise at best, and the compromise that best fits your needs will almost certainly be different than what suits someone else. Just remember that in airplanes, to an even greater extent than in most other engineered devices, whenever you try to improve performance in one area, you usually have to give up performance in other areas. You will have to decide for yourself what best fits your needs.
Don Stackhouse @ DJ Aerotech
I would like to hear from anyone who finds this information helpful. Please email me at email@example.com or catch me at the next IRKS meeting. Or of course you might see me at the flying field or the beach!! May all your launches be high, your thermals be big, and your landings on the spot!