Many sailplanes with bagged wings use the upper surface skin as the hinge for flaps and ailerons. Are these models using some sort of flexible material in addition to glass along the hinge line?

I noticed that if you just use the glass it starts to fatigue and crack after a while.

From : Don Stackhouse

Scott, since our 2-meter Monarch and our Wizard HLG were some of the first to use this technique, I think I can answer your question.

The hinge is designed using the same principles as a leaf spring. A leaf spring gets more flexible, and can also handle more deflection, if it is thinner, longer, and/or made from a more flexible material. In this case, the "length" of the spring corresponds to the width of the hinge gap, and the "thickness" corresponds to the thickness of the skin. In the case of composite skins, the stiffness depends not only on the type of material ('E' glass works fine for this, Kevlar and 'S' glass not as good), but also on the weave of the cloth, the layup schedule and the fiber orientation (+/-45 degrees is good, 0/90 not as good).

My guess is that the cracking and debonding you saw was the result of not enough hinge gap for the skin design and the amount of deflection required. If you bend a leaf spring too far, the material gets damaged. Nothing mysterious about that, just good old-fashioned machine design techniques.

As you make the hinge gap wider, the hinge gets more flexible, and can also handle more deflection without being overstressed. However, it's possible to go too far with this. An overly wide gap can result in a control surface that can move up and down at its leading edge by bending the hinge into an "S" shape. This invites flutter, skin debonding, aerodynamic anomalies, and generally undesireable behavior. The cure is to thicken the skin, but this of course increases hinge stiffness, which requires a gap increase, which requires a skin thickness increase, which requires a gap increase, which requires... Fortunately the relationships between the parameters involved are not exactly linear, which means it's usually possible to find a combination that works. The exact gap required is very dependent on the particular design and the amount of deflection desired.

One other point, skin hinges work better for small deflections. Developing a skin hinge that can successfully bend 90 degrees to one side of neutral is a LOT more trouble than one that only has to bend 30 or 40 degrees. This was one of the original reasons for the very wide flaps on the Monarch 2-meter; wider flaps don't need to go down as far to make the same amount of drag for landing (although it also improves their efficiency for thermalling and cruise).

Stress analysis techniques are difficult to apply to this situation, but they can help you get a good initial design. Probably the easiest method, and one that generally gives better quality answers than number crunching, is to find an old wing panel with the same skin layup as the one you're planning to use, then simply cut some control surfaces in it, with a variety of hinge gaps. Start with a gap you think will be too narrow, then widen it up until you get something close. Computers and numbers are fine, but a few minutes with an old wing panel and an X-acto knife will give you a much better answer for a lot less trouble. Sometimes the old-fashioned low-tech method is the best!

Don Stackhouse
DJ Aerotech