24
$\begingroup$

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?

$\endgroup$
2
  • $\begingroup$ Guys, there's no need to add words before "lift to drag ratio". Not anymore than there is when asking about an actual aircraft $\endgroup$
    – Abdullah
    Oct 6 at 12:19
  • 2
    $\begingroup$ 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. $\endgroup$ Oct 6 at 13:02

1 Answer 1

30
$\begingroup$

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.

enter image description here

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.

$\endgroup$
11
  • 2
    $\begingroup$ Are you sure glide ratio (or descent gradient) is a direct expression of L/D? $\endgroup$
    – Max R
    Oct 5 at 13:05
  • 6
    $\begingroup$ @Max yes they're exactly equal, see aviation.stackexchange.com/q/80731/4108 $\endgroup$
    – Sanchises
    Oct 5 at 13:24
  • 3
    $\begingroup$ I wonder how much of that 120mph you could lose if you got the flare just right :) $\endgroup$ Oct 5 at 14:38
  • 5
    $\begingroup$ 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. $\endgroup$
    – John K
    Oct 5 at 14:49
  • 3
    $\begingroup$ @DaveGremlin, not enough. People have made safe landings with just a wingsuit; nobody has safely landed without one. $\endgroup$
    – Mark
    Oct 6 at 3:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.