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This is my first question here. I'm just a glider in education phase and not yet a pilot, but because I'm also physicist, I'm interested in some details more than other people.

My question is simple:

I know what a polar curve is. Why is this curve dependent on the wing load? For my knowledge in aerodynamics the polar curve is a property of the airplane/wings, but not dependent on flight state. Otherwise I know from my flight instructor, that gliders are often filled with water to increase weight, which gives a better gliding ratio.

What I mean you see on this curve: why is the curve changing with wing load? My only answer would be, that wings are flexed differently under different loads and that the different curves arise from (slightly) different geometries. Is there something else I have overseen?

enter image description here

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    $\begingroup$ I believe the answer to your question is that the speed polar changes with wing load, because you achieve the same „flow geometry“ (angle of attack) at a different speed. The dimensionless lift/drag polar in coefficient form (which is the one purely dependent on aircraft geometry, unless aeroelastic effects are considered) will not change appreciably with wing load. Not entirely sure though whether that answers your question, so posting as comment only. $\endgroup$ – Cpt Reynolds Aug 11 '18 at 18:44
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    $\begingroup$ Related $\endgroup$ – Pondlife Aug 11 '18 at 18:56
  • $\begingroup$ see my mistake. It is rather embarrassing... $\endgroup$ – michael Aug 12 '18 at 18:56
  • $\begingroup$ Hi @CptReynolds I like the Youtube channel! $\endgroup$ – Jihyun Aug 12 '18 at 19:17
  • $\begingroup$ @Jihyun not sure what you mean? I don’t have one... $\endgroup$ – Cpt Reynolds Aug 12 '18 at 19:51
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Oh god- I made a terrible mistake: While $C_{lift}$ and $C_{drag}$ are really independent on weight, this is (of course..) not the case for the velocity polar curve. The two curves scale perfectly with weight and are just subject to a simple transformation. No magic behind and completely transparent.

Sorry: Question Solved ;-)

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    $\begingroup$ You say: "Otherwise I know from my flight instructor, that gliders are often filled with water to increase weight, which gives a better gliding ratio." But then the heavier glider has a different polar curve, right? And it has: Read here why. $\endgroup$ – Peter Kämpf Aug 12 '18 at 21:15

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