Hi Herbert,

Warning: I'm not an aerodynamics expert, this is only a speculation and I
invite answers from people who know.

I think that it should just be the simple formula:

v = 2*pi*(RPM/60)*r

where r is the radius of the vane (from pivot point to some point in the
cup), and v is the speed in whatever units r is in, per second.

The idea is that some point on the vane must be going at the same speed as
the air, in the case where the air has been going at a constant speed for
long enough for the vane to settle down to a constant speed. I think this
is true because, otherwise, you would have a vane moving faster or slower
than the air going over it, and it would be equivalent to a vane spinning
in a calm wind.

The only troubling thing is how to measure r. If you had a very small cups
on the vane and extremely thin rods connecting them to the hub, then it
would be easy. Otherwise, there will be some point on the vane (almost
certainly in the cup) which acts like a centroid (similar to the center of
pressure in the rocket discussion today). This point may change slightly
with wind speed, too.

So, my guess is that if you go with a very lightweight vane(so it doesn't
have much inertia) with small cups and very thin rods connecting them, you
can use the formula above.

Sean

At 07:49 PM 2/10/01 -0500, Herbert Graf wrote:
>      I'm thinking of adding a wind speed measurement device to my house
>monitoring system and I was wondering if anybody can point me to a formula
>that relates the wind speed to the RPM of the "vane". I am using the device
>with several "cups" connected to a bar that rotates in response to wind.
>Thanks for any pointers. TTYL
>
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