Forums: Skydiving Disciplines: Wing Suit Flying:
Hysteresis, or Why sometimes you barely move forward

 

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kallend  (D 23151)

Nov 17, 2006, 12:25 PM
Post #51 of 55 (2434 views)
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Re: [KrisFlyZ] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post

In reply to:
Where did the numbers for Cl and Cd come from?

Kris.

Aha!


pchapman  (D 1014)

Nov 17, 2006, 6:06 PM
Post #52 of 55 (2399 views)
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Re: [KrisFlyZ] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post

Re: Source of Cl and Cd.

Looks like Yuri started with curves like on the aerodynamics web page he linked to in his original post.

The numbers are not based directly on wingsuits, but just on a standard airfoil. Yet they are a starting point if one can't find better data.

Cd seems taken pretty much directly from the wind tunnel results for a NACA 0015 airfoil, a pretty basic symmetrical airfoil.

Cl is shaped like that for the 0015, but peak values are lower, perhaps simply adjusted to avoid being too optimistic, to reduce the max lift to drag ratio to some assumed best value, like the 2.8 in his graphs. Not sure exactly how the heights of the two peaks were chosen relative to each other.

I personally have not convinced myself just what the lift slope should look like. On the one hand there are airfoils with a clear pre-stall lift peak. On the other hand, a wingsuit is a very low aspect ratio device, despite the distinct wings outboard of the body, so one might use as a model some delta or blunt flat plate that has a more continuously increasing lift curve, without the "real airplane wing" pre-stall lift peak. One has to dig to find aerodynamic tests on anything even vaguely close to a wingsuit in shape.


yuri_base

Nov 17, 2006, 7:03 PM
Post #53 of 55 (2389 views)
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Re: [pchapman] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post

Yes, I started from standard airfoils and then "dumpened" the data a bit to take into account low aspect ratio and the fact that wingsuit consists of sections with very different characteristics. The absolute values are not important, they are just a vehicle to observe the trends and demonstrate the point.

I think you're absolutely right about the absense of the stall drop in lift curve for trackers. See the attached graph, it's the windtunnel data from VKB's post in BASE forum about their G3 tracking suit. There's just a tiny dent, if any, on lift and L/D curves at 15 degrees AoA. In this case, there's no "stall barrier" and the pitch angle is unambiguously related to the glide ratio.

We can only guess about real lift and drag curves for wingsuits. I still think that we do have the stall "drop", just because with the stall, the L/D curve is very sensitive to AoA near the best glide, and that's why it is so hard to find, and that's what we observe in practice.


(This post was edited by yuri_base on Nov 17, 2006, 7:04 PM)
Attachments: VKB-G3.gif (7.67 KB)


yuri_base

Nov 17, 2006, 9:27 PM
Post #54 of 55 (2375 views)
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The value of windtunnel data [In reply to] Can't Post

By the way, that little graph (L/D vs. AoA) has some valuable - and precise - result hidden in it. If we convert it to L/D vs. Pitch (using formula Pitch = AoA - GlideAngle = AoA - arctan(1/(L/D))), we get this graph.

L/D for the G3 flight (correctly extracted from planeout using acceleration data) was 1.6. Looking at the graph, the pitch angle corresponding to L/D=1.6 is either 5 or 23 degrees. To achieve the maximum L/D=1.8, the pitch should be ~10 degrees. Some balance tuning is needed to achieve the perfect pitch. The cleanest way to change balance is to use weights to shift the center of mass. The aerodynamic way to change the balance - changing the relative surface areas of different parts or changing body position - is more tricky, since it changes L/D curve itself, so the whole analysis needs to undergo several iterations.
Attachments: VKB-G3-pitch.gif (6.04 KB)


IslandGuy  (C License)

Nov 18, 2006, 5:28 AM
Post #55 of 55 (2354 views)
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Re: [yuri_base] The value of windtunnel data [In reply to] Can't Post

I'm missing something. I've always been under the impression that most conventional (non-slotted, etc.) wings stall at an AoA of about 18 deg. When I stall a wingsuit, I pretty much stop flying and start falling, just drop out of the sky. Looking at the charts, how is it that the best L/D is at AoA of 20 deg, with a wing that is presumably stalled?

So, with that said, what is it that I'm not getting. Is it the reference plane for measuring AoA or lift and drag? I know the math is all there, I'm just trying to understand so I can apply it to what I experience flying.


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