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If I recall correctly, the best competition gliders have a L/D ratio as high as 60:1. What imposes this limit? Is there a maximum theoretical L/D ratio, or could sufficiently advanced materials allow a glider with a L/D ratio of, say, 200:1?

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    $\begingroup$ For lighter-than-air craft, there is no such maximum. Buoyancy goes like the third power of the linear dimensions, but weight and drag go only like the second power. $\endgroup$ – Ben Crowell Apr 1 '15 at 21:08
  • $\begingroup$ @BenCrowell that may well be, but I don't think lighter-than-air craft count as gliders $\endgroup$ – raptortech97 Apr 1 '15 at 21:10
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    $\begingroup$ @BenCrowell: Weight also goes up like third power. $\endgroup$ – Jan Hudec Apr 2 '15 at 7:12
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    $\begingroup$ @JanHudec: The weight of a balloon is proportional to its surface area, not its volume. $\endgroup$ – Ben Crowell Apr 2 '15 at 12:42
  • $\begingroup$ @Jan Hudec nice to see some of your vintage commentary. Surprised that it was not surmised that "weight" = (density x volume of aircraft) - (density x volume atmosphere) - lifting force generated from motion. Drag is friction from motion. Buoyancy is insignificant for most aircraft, yet a pressurized cabin weighs a bit more at altitude. Was trying to calculate ton miles per gallon for Graf Zeppelin. Of course a motionless balloon has infinite lift to drag, but how much would it's volume effect drag once underway? $\endgroup$ – Robert DiGiovanni Feb 7 at 10:42
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With current technology the L/D might go up to 70 or 75, and going higher would require an almost impractically large wing span. Gliders need to fly in tight circles to use updrafts, and the larger the wingspan becomes, the bigger the speed difference between inner and outer wing will be. Also, landing such a wide wing without dropping a wingtip will be very hard. Smaller wings with a high aspect ratio will have a low chord length, leading to smaller Reynolds numbers, which translates into a steep increase of friction drag if the aspect ratio is increased without increasing wing area. Therefore, only adding wing span will help, and this runs into a soft wall beyond the 30 m of designs like the Eta. In addition, the current mass limit of 850 kg will make bigger aircraft unattractive for competition pilots.

The Concordia (pdf!) is claimed to have an L/D approaching 75, but I have learned over the years to see theoretical predictions as invariably optimistic, and realistic performance, with bugs on the wing and all, will never quite measure up to the hoped-for ideal.

Werner Pfenniger proposed to use boundary layer laminarisation by suction (pdf!) to reduce friction drag and proposed glider designs with L/D ratios in excess of 100. Turbines at the wingtips would drive the suction pumps, so these would still be unpowered aircraft. But so far nobody has dared to turn his visions into reality.

While laminarisation avoids the increase in friction drag of a turbulent boundary layer, a moving wall would eliminate viscous losses altogether. Now the friction generation is between the moving wall (this could be a taut foil running between two cylinders at the forward and aft end of the wing) and the fixed structure. With current materials there is no hope of reducing drag this way, but who knows what tricks will be possible in the future.

A glide ratio of 100 or more looks rather unlikely in the next 50 years.

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  • $\begingroup$ Ah, I forgot that gliders have to circle updrafts. $\endgroup$ – raptortech97 Apr 1 '15 at 20:39
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    $\begingroup$ Is there a theoretical limit though? Kind of like a speed-of-light for L/D? $\endgroup$ – slebetman Apr 2 '15 at 2:31
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    $\begingroup$ No, there is no thoretical limit: anything which produces some lift whilst drag tends to zero will tend towards an infinite L/D ratio. $\endgroup$ – thepowerofnone Apr 2 '15 at 11:20
  • $\begingroup$ @thepowerofnone So what's the plan to eliminate skin friction? I want to get rich. :-) $\endgroup$ – Calphool Apr 2 '15 at 15:38
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    $\begingroup$ @raptortech97: Yes, correct on both counts. Ideally, the forward location of both cylinders should move with the angle of attack, such that the stagnation point is exactly between the two. Quite a challenge! $\endgroup$ – Peter Kämpf Apr 2 '15 at 21:54
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Loosely speaking, L/D ratio is limited by the aspect ratio of the wing (its length to its chord width) and the friction of the wing surface (which is why frost/ice on the wing is a bad thing -- it dramatically increases wing friction).

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Yes, you can have a L/D as high as you'd like. But I don't think you'd enjoy flying in milliKelvin liquid helium superfluid which has zero viscosity...

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The lift and drag coefficient are defined with a fixed and uniformed pressure. Therefore, gravity and Archimedes' principle should not be involved in this topic. The maximum lift-drag-ratio is obtained with infinite wings. 2D airfoil optimization with genetic algorithm give the following optimization for the lift/drag ratio: 1/0.00166/0.002=300 000

https://optimization.mccormick.northwestern.edu/index.php/Wing_Shape_Optimization

This is a theoretical limit of course....which do not have some much sense in the present real world with finite wings, clear-air turbulence, surface roughness, water on wings,etc...

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