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If a person falls at terminal velocity, and tries to steer himself with his limbs, what is the maximum lift to drag ratio he can achieve, and what position would that be in?

Abdullah is not an Amalekite
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  • Guys, there's no need to add words before "lift to drag ratio". Not anymore than there is when asking about an actual aircraft – Abdullah is not an Amalekite Oct 06 '22 at 12:19
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    Even in case of an actual aircraft, say a glider, it is arguably more correct to say that the best L/D ratio or maximum L/D ratio is (say) 40/1, rather than simply to say that the "L/D ratio" is 40/1. That's the whole point of the "polar curve" graph, to show how L/D continually varies with airspeed (angle-of-attack). I didn't edit your question (title), and maybe it's a bit overstepping the norm to make an edit along these lines, but technically speaking the edit did make the (title) more correct. – quiet flyer Oct 06 '22 at 13:02

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I'm an ex skydiver. It's a position known in skydiving as a "Flat Track", and is more or less the same configuration as a ski jumper takes. Legs together (ideally), bent forward at the hips slightly or flat, arms at the sides, palms flat. You're trying to turn your body into a "lifting body" type aircraft.

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In skydiving videos, it's the position you see jumpers taking to get away from each other when it's time to deploy their parachutes and they need to cover a lot of ground. There is another position called a "Delta Track" where you keep your legs apart and relaxed. Horizontal speed is about half of a flat track.

The horizontal velocity achievable while flat tracking is about 60 mph (making it quite a dangerous maneuver near others if you don't know what you're doing).

With a vertical terminal velocity of about 120 mph, which drops to around 80-90 mph while tracking, this makes the unenhanced human body's L/D while tracking, at 50-60 mph horizontally, at about 0.6/1 to 0.7/1. Adding a Wingsuit improves the L/D to better than 1/1, maybe approaching 2/1.

Everything you wanted to know about tracking here.

John K
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    Are you sure glide ratio (or descent gradient) is a direct expression of L/D? – Max R Oct 05 '22 at 13:05
  • Presumably the vertical terminal velocity in "flat track" is significantly less than maximum possible t.v. -- (which would be in what posture, head straight down?). Many readers may wonder about this, so maybe consider adding that info (ballpark number for that t.v. value) as an additional slight enhancement to this great answer? – quiet flyer Oct 05 '22 at 13:14
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    @Max yes they're exactly equal, see https://aviation.stackexchange.com/q/80731/4108 – Sanchises Oct 05 '22 at 13:24
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    I wonder how much of that 120mph you could lose if you got the flare just right :) – Dave Gremlin Oct 05 '22 at 14:38
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    Actually I had overlooked that point. The vertical velocity will be less than the normal 120. A wiki article says 90 vertical. I was trained that the horizontal speed approaches 60 to get the idea across how dangerous it can be. So it gives an L/D of .66 not .5. Thanks for the comment. – John K Oct 05 '22 at 14:49
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    @DaveGremlin, not enough. People have made safe landings with just a wingsuit; nobody has safely landed without one. – Mark Oct 06 '22 at 03:38
  • @Mark 9 is the guy landing in the boxes the real deal? I always thought that was a hoax. – John K Oct 06 '22 at 12:50
  • What is all the bulk at his ankles? Is that the "between the legs" part of a wing suit? – FreeMan Oct 06 '22 at 13:55
  • It's not a wing suit. It may be material added at the bottom of the legs to provide some stability, like fins sort of. Back when I was jumping, super baggy jumpsuits were all the rage, because they were supposed to reduce terminal velocity to about 100 mph. – John K Oct 06 '22 at 15:23
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    @Mark People have come pretty close. Just a sprain, for instance. But that relies heavily on a good landing area and luck. – Spitemaster Oct 06 '22 at 17:08
  • @JohnK No, it was a real thing. But it was set up very carefully, and was still astonishingly dangerous! – Graham Oct 06 '22 at 17:39