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I've been reading material about the Perlan II experimental glider. They hope to take it up to 90,000 feet -- higher even than an SR-71 BlackBird.

But the Perlan II is just a glider. Can it really just climb thermals up to that level? Is there something special about its aerodynamic design that lets it do that -- or are all gliders capable of climbing as high as possible, with only the heat and oxygen needs of the human occupants being the limiting factor? Or is it just me that finds it odd that a glider can climb higher than a powered aircraft?

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Perlan II will climb to extreme altitudes on wave lift, not thermal lift. Wave lift is created when strong winds blow approximately perpendicular to a mountain range, and the wind speed increases steadily with altitude. This creates a standing wave in the atmosphere, kind of like the ripple behind a rock in a fast moving stream of water. On the leading edge of the wave, air is moving up, and that's where gliders fly. Standing waves can reach very high into the atomosphere, which is why Perlan II is expected to reach 90,000 feet.

Perlan II is different from normal gliders in that its wing is optimized for minimum sink, instead of best glide ratio. It's meant to float along while losing a minimum of altitude, which makes sense if it's going to fly in nothing but wave. Normal gliders are usually optimized more for gliding long distances, for which glide ratio is more important than minimum sink. Interestingly, Perlan II's wing looks a lot like RC model glider wings, which are also optimized for minimum sink, since RC gliders don't typically fly cross country.

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    $\begingroup$ Welcome to StackExchange and thanks for such a great first answer! Could you please explain for the truly ignorant (such as myself) what the difference between "minimum sink" and "best glide ratio" -- either with links or in-text? Thanks! $\endgroup$ – RoboKaren Feb 21 '16 at 16:46
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    $\begingroup$ @RoboKaren: min-sink and glide ratio are standard terms in aerodynamics. Sink rate is how much altitude you lose over time, for example, how many feet per second. So minimum sink is the lowest sink rate you can get. Glide ratio is how much distance you can travel per given loss of altitude, for example, how many feet per feet. So max glide ratio is the shallowest angle you can glide at. $\endgroup$ – slebetman Feb 22 '16 at 6:21
  • $\begingroup$ See my answer on minimum sink versus best glide aviation.stackexchange.com/a/25496/9195 $\endgroup$ – Ben H May 5 '16 at 22:27

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