Gordon rigg <<email>> writes:

Kathleen reports, and pictures on a Dutch blog confirm, that they are measuring the difference relative to vertical between the keel and the sprogs. The measurement should be done as Flip did it in Laragne where the keel is first propped exactly horizontal. Putting the keel 15 degrees nose up, or horizontal makes a difference of almost 1 degree on the outer sprog on a typical glider. This is about understanding 3D geometry and is not rocket science. If you don't understand it you need to just do it with a glider and you see it makes a difference. The sprogs on most gliders are not in parallel planes with the keel but they are angles outwards at the back. The more of horizontal the glider is the more you measure this "splay" angle rather than the twist.

Rig a glider and measure the sprogs with string or inclinometer. Then press down on the sail near the sprogs. That changes the string to keel measure 15 mm or so, which is a big adjustment. Close the glider and open it again, fly it and land it and it will go back to where it was more or less. This comes from all the little clearances in the holes that all the bolts go through. Normally they all on average sit fairly near the middle.

The main loading with the glider flying or on the ground is not in the same direction as when they are asked to hold the sprogs up. Press on the sprogs and all the bolts move in the holes to a different position. Now whack the glider in hard and holes in the nose plates and through the leading edges are no longer exactly round and perhaps nothing is exactly the same any more. However we know from experience that all these little things do not change the fundamental safety of the glider, or we really would have many accidents. If you want a safe glider get a design that has many hours flown and few reported accidents. This has always been the better measure of safety rather than what placard is stuck on the keel. The pilots know this, and the manufacturers (who are still in business) know this.

The DHV statement that these measurements are repeatable to 0.25 degree is totally flawed. Sure it might apply to the glider they measured and placed in their warehouse and then measured again - but not one that is rigged on uneven ground, jostled in the launch queue, flown, landed with average skill, carried quite a way across the field and then left in the sun before being measured again.

I can make every pilots glider fail with a bit of a sneaky press of the hand unless they set their trailing edge 15 mm higher than required. Maybe I could be world champion at Laragne (if I were allowed to go).

On a "lighter" note, Corinna cannot steer the glider on full VG? For sure it does not glide too well unless it is tight. So the glide difference last year with Kathleen was nothing to do with who had been "on the pies" after all!