With reguard to the 2M Chrysalis: 1.) some of the ribs were cut too short to reach the the TE, primarily those closest to the center wing joint and 2.) the lack of a wing joiner rod both seem to compromise wing integrity.

Do you have any suggestions on how to add a wing joiner rod, or why one wouldn't be necessary? I already have one wing half built.

From : Don Stackhouse

All good questions, and I just happen to have good answers.

The ribs you noticed are the ones that come to a point, because they are relieved on their upper and lower edges for the balsa sheeting at the center section and the poly breaks. These include ribs A, B, J, K1, K2 and L. Since the sheeting is half the thickness of the trailing edge stock, the required thickness of the rib at that point is zero, and therefore the rib ends at the forward face of the trailing edge stock, instead of extending back into the notch like the other ribs. Because of some quirks of the way the notching machine is set up, as well as some requirements for other kits we make using the same machine, it's easier to leave a notch at those locations in the trailing edge, even though the rib doesn't use it. You can plug the unused notches with scrap balsa if you like, or just leave them there, it doesn't matter as far as the plane is concerned.

As far as a joiner at the center section, not only is it not necessary, it's also ineffective in thinner airfoils. Also, the sweep angle in the spars would make the design and manufacture of a joiner very difficult.

The loads are carried primarily by the material in the spar caps that is closest to the top surface of the upper spar cap and the bottom surface of the lower spar cap. The closer to the center of the wing thickness you get, the less effective the material is for helping with the bending loads. The only other thing that material would be good for in those inner areas would be shear, but there isn't much shear of a type they could help with in that spot on the wing center section. In a thin airfoil like what the Reynolds numbers of a plane like the Chrysalis requires for decent efficiency, the effectiveness of material beneath the outer surfaces drops off especially quickly.

To put it in perspective, a joiner between the spar caps of the 2-meter Chrysalis, assuming it was made from the same material as the spar caps (and assuming you could get around the grain flow problems caused by the dihedral angle) would have less than 26% of the bending strength of the spar caps on either side of it. Even if we deleted the doubler on the upper spar cap and increased the depth of the proposed joiner by an equal amount, that joiner would still have less than 48% of the bending strength of the spar caps it would be supposed to be joining. Obviously the conventional joiner approach in this case is a lot of work for both us and for our customers, with very little payback for all that trouble!

Instead, we put the "joiner" material where it will do the most good: on the outer surfaces of the upper and lower spar caps. See those swatches of unidirectional S-glass in the package of fiberglass materials? Those form "straps" across the outer surfaces of the left and right upper and lower main spar caps at the center section. Since they're even further from the middle of the airfoil thickness than the spar caps themselves, they have even more leverage to carry the load. Since they are a nice, conformable fiber material, they can bridge the gab without having any short or broken fibers, unlike a wooden joiner in that situation. And, since they are S-glass, they are a stronger material than the wooden spar caps they are joining. The net result is a far stronger connection than any "conventional" wooden joiner in that situation could achieve, and for a lot less trouble for both us as the kit manufacturer and for you, the builder.

Just make sure that if you sand on any of the center section fiberglass, you don't sand through the fibers in the middle of that lower spar cap S-glass strap! A good way to help that situation is to apply the fiberglass with thin epoxy (30 minute works well), then get some wide masking tape and neatly tape over the wet fiberglass, from dry wood on one side to dry wood on the other side. If applied neatly, the tape will hold down the edges of the fiberglass while the epoxy cures, resulting in a smoother surface. Once the epoxy has completely hardened, peel off the masking tape. You'll find a fairly smooth surface underneath, similar in texture to that of the masking tape. A little light sanding to smooth off this texture (without sanding into the glass fibers just below that texture!) will give you a surface smooth enough for covering, but without cutting the reinforcing fibers.

Sometimes the "obvious" or "conventional" method isn't the best. Often "the way we've always done it" isn't the best way to do it. In design, we need to question everything (just as you did in asking this question), and figure out what really makes the most sense for the particular mission we're trying to accomplish.

Good questions, and thanks for asking them.

Don Stackhouse
DJ Aerotech