Re: [Gibolin] G3-suit. VKB
In reply to:
Your questions bring me some more questions: what is the Reynolds number and in what is it interesting for wingsuit building? How can you determine the coefficient of lift. I tried to measure the angle of attack -snip-
disclaimer: I'm a pilot not an engineer
Reynold's number, in a sense, has to do with the drag of the fluid (air) over the surface of the object (the jumper). It is useful in scaled experiments such as small wind-tunnel models. Finding the critical reynolds number (somebody correct me if I'm wrong) on the model will help predict important airflow characteristics over a full-size version. So you can subject a small model to wind tunnel tests that you couldn't do on a real person, and get useful information.
Angle of attack is the angle between the chord of an airfoil (i.e. a straight line from a tracker's head to his toes) and the wind vector. If you wanted to estimate your AOA while you jumped 1) install an AOA vane on your body somewhere, or 2):
- use video to determine X while you track, where X is the angle between the horizontal plane and the chord of your body
- Determine your glideslope, Y, equal to the angle of your descent path as you track--get this from GPS data (90°=straight down, 0 = going perfectly horizontal)
- Subtract X from Y to estimate AOA (did I get that right?)
Note in the model picture, the AOA is approximately zero or slightly above. As others have noted, this is only possible in a very limited set of circumstances. It's not very useful, because unlike simple vector math, you can't just get a stream snapshot at 90° and also at 0° and expect any useful interpolated data.... the important stuff has to be determined experimentally. It would be interesting to see it at about 5-10 degrees higher AOA, and slightly less camber, I think.
Another thing you'll notice in the model picture is the distinct difference between the laminar flow over the top of the model and the highly turbulent separation underneath. This is basically what an inverted stall looks like... with excessive drag and poor controllability. If you adjusted the camber (i.e. the tracker lifted his forehead a little) I'd expect that to change. But of course, the AOA is probably unrealistic anyway so it is fairly moot.
Coefficient of lift... using C sub L at a given angle of attack, you can predict lift force based on certain factors. If you know the critical angle of attack and calculate C sub L for that configuration, you can find the max theoretical lift. Combine it with coefficient of drag, do some college math, and you can figure out a host of things such as L/D(max), best glide speed, best rate of descent, etc. Many things that might be useful to a nerdy tracker!
I'm sure I've botched the theory terribly...
maybe someone like 460 has some insightful comments.
I just want to echo how fascinating this is... I have never heard of anyone looking at tracking in a windtunnel before. That body-wing that flew whats-his-face over the english channel probably went through the wringer too. It's a start.
