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Comment by ajuc | Hacker News Reader

There's thermals (a spot on the land that is warmer than the surroundings so the air rises there), and there's topography that causes rising air (for example when the wind blows at a face of a hill - the air has no choice but to go up).

RC gliders can stay in air basically forever flying back and forth in rising air along a hillside. It's called slope-soaring and it's great fun.

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Here's an RC glider doing 545 mph with that technique: https://youtu.be/MoaWlKC3wIM

I don't know how they manage to control it or measure the speed, but it looks legit.

Wow. That's not how it usually looks like :)

Usually it's very slow and relaxing. Looks like this:

https://youtu.be/Ev5fSVi1PrA?t=152

That is dynamic soaring [1] which is I guess a subset of slope soaring, and is pretty rare and nuts. It generally requires flying in the lee of a slope rather than in front of it.

Regular slope soaring is relatively more sedate, fun and accessible if you have the right slope.

[1] https://en.wikipedia.org/wiki/Dynamic_soaring

> Dynamic soaring is sometimes confused with slope soaring which is a technique for achieving elevation.

> RC gliders can stay in air basically forever flying back and forth in rising air along a hillside.

And there's been some effort to have autopilots do it automatically https://ardupilot.org/plane/docs/soaring.html

Somewhat more impressively, albatrosses do that on the forward faces of waves, soaring for days far away from thermals and hills.

There's research that they use dynamic soaring more, getting energy from the different wind speeds at sea level and a bit above https://www.youtube.com/watch?v=uMX2wCJga8g

Gotta love the end of the video aha! But the whole video is very interesting. As a kayaker myself I can definitely “feel” the phenomenon they are talking about of extracting energy by alternating on two sides of a flow gradient.

Next up in kayagliding...

That is very interesting. I always assumed sea birds used combination of wing in ground effect [1] and the lift of wind hitting the front of waves to cover long distance, but this explanation makes much more sense for birds crossing oceans.

[1] https://www.youtube.com/watch?v=vkVVnx70FgI

Edit: It just occurred to me that the albatross is in some way surfing the wind in much the same way a surfer pumps along a wave to generate speed.

> RC gliders can stay in air basically forever flying back and forth in rising air along a hillside. It's called slope-soaring and it's great fun.

This was my thought also. The Andes Mountain Range are rather mountain range right next to the ocean. I would think that is a pretty ideal place for a bird to slope soar for a long distance.

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