On Wednesday, May 9, 2018 at 5:07:21 AM UTC-7, Bruce Hoult wrote:

1) what is the function of "drag pins"

In typical sailplane practice, there are no "drag pins" per se; drag and thrust loads are accommodated by a torque couple comprised of spanwise tension outboard on the wing main spar and spanwise compression inboard applied between the flat faces at the root of the lift pin and at the end of the pin socket it fits into.

The loads reacted in this manner are primarily forward thrust loads, which in the normal flight envelope are much greater than the drag loads. Of course, abnormal overloads are most often drag loads applied when the wingtip encounters an obstacle on takeoff or landing.

2) how much load is put on things by pushing a glider around by the wing tips compared to opening the brakes at high speed, or by a Vne dive with brakes open? (and is either "necessary"?)

Just for arguement, let's consider a 15m glider with VNE-limiting airbrakes going straight down. Our hypothetical glider has a non-lifting mass of 500 lbs, so we know that the the brake on each wing is delivering 250 lbs of drag into the fuselage. Each airbrake is located 100" outboard of the root. The distance between the wing main spar and the aft lift pin is 12". Simple statics suggests that the compression force applied at each aft lift pin is on the order of 250lbf*100"/12" = ~2080 lbf. For comparison, this compression force would be developed by an aft pull on the wingtip of 2080lbf*12"/300" = ~83 lbf.

As I recall, US FAR Part 23 and the very similar JAR/CS 22 specify that a normal category glider be able to accommodate a 100 lbf chordwise force at the wingtip unless lower can be rationally justified. My impression is that most gliders are certificated to lower tip forces, the rational justification being "we handle them with care."

As I understand the issue at the root of this discussion, the problem is not so much overloads, but rather wear between the lift pins and sockets. Gliders being moderately limber, when you apply a chordwise force at the tip it separates the pin and socket from full engagement by some small but non-trivial amount; let's guess around 0.020" (0.5mm). And when the force is removed or reversed, the pin and socket go back together. Repeated cycles of this half-mm movement can, over time, result in wear and looseness. Remember that the wings typically weigh between 120 and 180 lbs each, so that's a vertical force of 60 to 90 lbs at each pin and socket connection while they're being worked in and out.

For the gliders I've designed, my advice is that an occasional chordwise force at the tip of up to 50 lbf while parking or moving the glider is not an issue. However, repeated cycles, for example when moving the glider from one end of the runway to the other, should be avoided when practical.

--Bob K.