krassihg wrote:In general, being heavy on a glider will "stretch out" the far right side of the polar and give better glide performance at high airspeed, unless the glider is being so overloaded that the wing is being seriously distorted.
I think the word "stretch out" is not correct. The polar shape is the same, it is just "shifted" down and to the right, when the pilot is heavy ( within the range). This mean you get more speed, but also more sink ( the L/D is the same) , and also the min sink gets bigger, and your stall speed also gets bigger. This also means that when you fly with a heavier weight, you have to watch your stall speed . I have seen pilots changing from a bigger glider to a smaller and because they are accustomed to certain position of the speed bar, get stalled and surprised.
See my post immediately above about each point on the polar corresponding to a given angle-of-attack. That also means a given bar position. Let's say for simplicity that we prefer to trim for min sink. Min sink occurs at the top of the polar and when we shift that data point down and right to represent a heavier pilot, that shifted point still represents the min sink angle-of-attack and the trim bar position. To a first approximation there is NO reason for a heavier pilot to not find the bar still to be at trim at min sink, if the bar was at trim at min sink for a lighter pilot. Just as a sailplane loaded with hundred pounds of water ballast still trims to min sink at the same trim lever setting that it had when it was empty. If the water is centered at the glider CG.
Changes in the relationship between trim bar position and trim angle-of-attack (and remember that stall is a function of angle-of-attack) have nothing to do with the shifting of the polar. Rather, they are a result of the following.
1) The wing washes out (distorts) at heavy loadings which unloads the tips and pitches the nose up. A heavier pilot therefore finds the wing tends to trim to a higher angle-of-attack than a lighter pilot, for the same hang point.
2) The hang point is rarely precisely at (or vertically in line with) the CG of the glider, at the trim angle-of-attack. If the hang point is slightly aft of the glider CG, this will make tend to make the wing trim to a higher angle-of-attack for a heavy pilot than a light pilot. This will exacerbate (make worse) the effect described in 1. If the hang point is slightly ahead of the glider CG, this will tend to make the wing trim to a lower angle-of-attack for a heavy pilot than a light pilot. This will lessen, and in extreme cases could even reverse, the effect described in 1.
Note that for a given glider, the hang point may be ahead of or behind the glider CG at trim, depending on where the pilot has put the hang point in the adjustable range, and even depending upon the glider pitch attitude and angle-of-attack that he is trimming for. (If the glider CG is higher or lower than the hang point, a change in the pitch attitude will move the hang point slightly forward or aft relative to the glider CG-- this effect is probably trivial, but does exist.)
Near the middle of the range of adjustable hang points I would expect effect #1 to dominate over effect #2, because I would expect the hang point to be quite near the hang glider C.G..
Also, the fact that heavy pilots typically need to hook in further forward than light pilots to trim for the same angle-of-attack, suggests EITHER that effect #1 usually dominates over effect #2, OR that the whole range of normal hook-in points is typically a little behind the hang glider C.G.. The latter possibility seems a little unlikely to me.
(Food for thought- why don't see more dramatic changes in trim with more dramatic changes in loading? If the glider trims at min sink for a 150-pound pilot, and trims to a significantly higher angle-of-attack for a 200-pound pilot at the same hang point, why doesn't it trim to a MUCH lower angle-of-attack when kiting during ground-handling with only 10 pounds of load on the hang strap? Or... maybe it does? Maybe the glider is kiting at the min sink angle-of-attack with the pilot exerting ZERO force on the bar or down tubes, whereas in flight the glider needs the weight of the pilot's hands and forearms resting on the bar-- not entirely trivial-- to keep the angle-of-attack down to min sink? Or is the change-in-washout effect negligible until we start to get really heavily loaded? Or does being in ground effect induce a trim change, so that we need to be cautious about comparing in-flight trim to ground-skimming trim or kiting-with-feet-on-the-ground trim? Perhaps more to the point, if ground speed is zero and the wind is strong and smooth enough to comfortably "kite" the wing with the hang strap somewhat loaded but the pilot's feet still bearing some weight, there is probably a large wind gradient, tending to unload the (close-to-the-ground) tips and pitch the nose up to a high angle-of-attack despite the light load on the hang strap... )
Note also that to the extent that effect #1 is taking place, our model of shifting the polar right and down as described in the post above is flawed. We really need to introduce more degradation of the sink rates at high airspeeds for the heavy pilot to show the unfortunate effect of the increased washout. But hopefully his performance at high airspeeds will still remain better than if we simply shift the whole polar down and right with no "stretching".