[Bob Kuczewski]

I just saw a topic posted on hanggliding.org asking about a "wind force product". Here's the URL:

    http://www.hanggliding.org/viewtopic.php?t=22840

Here's what I wanna know- and you should, too: how can our wind speed experience inform decision making at different altitudes? 15 mph winds at just-above sea level doesn't equate to 15 mph at 7000 ft. So, how can the we calculate wind pressure, to put speeds at varying altitudes into perspective? Ideally, I'd like an algorithm that takes barometric pressure and wind speed and results in a wind force product. (do most variometers include barometric data, or just "altitude" estimates?)

I'm not looking for a super-subjective estimate, as in weather reports where temperature is adjusted for humidity to provide a "feel" index; I wanna be able to say "at 3000 ft, 18 mph wind is equal to 11 mph at sea level".

The answer is a quantity called "Dynamic Pressre", and it's the factor that creates the forces that we feel on a glider.

In fact, both lift and drag (as well as other aerodynamic forces) are defined in a typically simple form:

    Lift = q x CL x Sw
    Drag = q x CD x Sw

The coefficients in these equations (CL and CD) are dimensionless quantities and they capture all of the shape-dependent characteristics of the wing. The "Sw" factor is the surface area of the wing. We can see that both lift and drag go up (and down) directly with changes in both the coefficients and the surface area of the wing.

So what about "q"? That's what we call "Dynamic Pressure" and it's defined as:

    q = 1/2 x rho x v^2

Where rho is the density of the air, and v is the velocity (so v^2 is the velocity squared - velocity times itself).

So the answer is that the force goes up with q. And q goes up with both the density and the velocity squared. So if you know the air density, and you know the wind speed, then you can calculate the dynamic pressure and the resulting force. You can also do that at sea level to compare the difference as Mavi requested. Now if he wants to do that based on altitude alone (as opposed to actual air density), then all he needs is a way to come up with density as a function of altitude. Of course the altitude / density relationship is not fixed, but it can be determined with varying degrees of accuracy using tables and/or standard instruments.

This was off the top of my head, so if anyone has any other thoughts, please chime in.

[TadEareckson]

Or you can just feel how your glider's responding at launch.