I found your articles on Kevlar vs. glass fuselages very instructive. Now, I'm trying to decide whether to bag my wings with 1 oz Kevlar ($45/sq yd) or 1.4 oz glass ($4/sq yd), and am wondering if you could help me decide.

From : Don Stackhouse

Kevlar/epoxy is about 2/3 the density of fiberglass/epoxy, so the thickness of 1 oz Kevlar fabric will be about the same as 1.4 oz glass. When you figure in the weight of the epoxy, the weight of the two layups will not be that much different between the two, although there will still be some advantage in favor of the Kevlar. However, in our experience, 1.4 oz glass is way more than you need for an HLG. So is 1 oz Kevlar, but unfortunately that's about the lightest Kevlar available. For this reason, in our experience a Kevlar layup tends to be considerably heavier than a glass layup for an HLG if the glass layup is not grossly overdesigned.

The Kevlar will be much stronger in tension, but its compressive strength is so much worse than fiberglass that its overall strength in an impact is not likely to be that much different. Since under compressive buckling loads the stiffness of the material and the thickness of the layup for the same weight are the dominant factors, it does show some advantages there, and I believe this is the real reason for its success in composite fuselages.

The other big factor is that in a crash or collision, the Kevlar tends to fail in compression by micro-buckling of the fibers, but without actual breakage of the fibers, just crushing and crumbling of the epoxy matrix. The laminate has failed, but it still looks intact, even though it has lost most of its stiffness and structural integrity. It can be field repaired by wicking in some thin cyanoacrylate (do NOT try that on a foam-cored part, it will eat the foam), but it will still have only a fraction of its original properties. In this regard its legendary durability is to at least some extent a misconception and a myth. What real advantage it does have is actually due to its lower density, and therefore greater thickness for the same weight, in addition to the higher elastic modulus. This does help in the case of a fuselage where the typical unsupported thin shells usually fail in buckling rather than true compressive crushing of the material. It would probably be less of a factor in failures of the foam-supported skins of a wing. In any case, the major loads of a wing are generally carried by the spars, not the skins.

Yes, a Kevlar-skinned HLG wing will be more durable than a properly designed glass skin, but mainly because it is much thicker and heavier than it needs to be, assuming the spars are doing their job. Since you only have 1.5 meters of span as specified in the rules, any extra weight will show up as an increase in induced drag, which will hurt at all speeds, especially thermalling and cruising. If you live in an area where the thermals typically can carry off small children and an occasional Volkwagen, then this might not be that much of a handicap. However, if you ever face conditions where you have to work hard to stay up, you might find yourself wishing you'd gone for the lighter weight instead of the higher durability, as your expensive and overweight model loses the flyoff round against its lighter and higher performing competitor.

Many of the contest-level DLGs on the market have Kevlar bagged wings, and I have heard several experienced flyers say that these Kevlar wings last much longer. I am a bit skeptical that it can make such a big difference, especially since most DLGs have pretty substantial uni carbon spar caps to carry much of the load during launch. I've never seen a DLG wing fail before (glass or Kevlar), and am not sure where the main structural challenge lies.

The skin helps the torsional stiffness (and therefore the flutter characteristics), although the spars can have a great deal to do with that as well. However, the biggest factor in the skin is ding and dent resistance. The question here is how much extra weight you're willing to tolerate, and how much handling and performance you're willing to sacrifice, in order to get that extra durability. If you're not in the habit of treating your model like an EPP sloper, or going out and playing Kamikaze against your competitors, that unnecessary extra durability could come at a high price. Yes, midairs do happen, but if you fly with even a little common sense it shouldn't be the overriding concern that drives the entire design. Built-up film covered wooden airplanes are far less durable in this regard than either glass or Kevlar skinned models (although they do tend to be more easily repairable), yet you don't hear folks complaining that they are inadequate!

What say you? If you were to build a DLG, what material would you use?

For a composite HLG model, I'd use well designed glass skins with well designed carbon spars, in an airplane that was aerodynamically designed to fly well at the lightest weight that this well optimized structure could achieve. This will give you both the best high speed and launch, and the best low speed performance possible.

The key is to optimize the entire aerodynamic design of the model for the weight that the structure can achieve. Virtually all of the analyses that claim that "heavier is better" have one way or the other managed to botch this crucial step.

If it was a larger model such as a 2-meter or open class ship, where the heavy 1 oz Kevlar was not a gross overkill, the Kevlar might make more sense in at least some cases. Even then, an outer layer of glass (because of its higher compressive strength) with an inner layer of Kevlar might be both lighter and stronger than a solid Kevlar layup designed for an equal amount of dent resistance (in a denting impact, the outer surface sees compression while the inner surface sees a tensile load). In my previous occupation I developed some successful prop spinners for large aircraft that used a carbon outer shell with an inside layer of Kevlar, for similar reasons.

Kevlar is fantastic (even better than carbon) in tension, less than hlaf the strength of hardware store E-glass in compression, and only available in a limited number of weights and thicknesses, which in most cases are thicker than necessary for HLG's. If you consider all of these issues and it still makes the most sense sense in your particular application, then use it.

Don Stackhouse
DJ Aerotech