Velocity=distance travelled/time
Since the path is circular,
     Velocity=pi*diameter/(time for 1 revolution)
This is essentially what Sean has in his formula:
     v = 2*pi*(RPM/60)*r
Only the way it is expressed is different.

Now, the above assumes perfect conditions. But
the system is FAR from perfect. Sources of error
include:

     Nonlinearity (of generator. Opto method is linear)
     Bearing friction
     Magnetic opposition (if driving a generator)
     Air friction due to the support for the cups
     Air friction due to the geometry of the cups

This last source of error can be quite large! When
the cup makes the round trip, it comes back INTO
the wind, and the contributed error has an error
that is somewhat "sine" in nature, due to the
source of the error being based on a circular function.
The error is not perfectly sinusoidal, and is also
very dependent on the number of vanes used.

Short radial rods work better than long radial rods.
Try to keep the radial rods as thin as possible while
still maintaining the necessary rigidity. The cups
should be as aerodynamically shaped as possible.
An egg-shape works better than a spherical shape.

Bottom line: you have to calibrate the thing. One
fairly easy method is to measure the output with the
device mounted to the top of your car. But you have
to eliminate any error that might be due to the wind
blowing. So the usual technique is to first take a reading
with the car aiming directly INTO the wind, and then
take another reading with the car going in the exact
opposite direction. Add the two readings and divide by
two. (car velocity+wind)+(car velocity-wind) = 2*(car velocity)

2*(car velocity)/2 = car velocity.

The two readings should be taken as close together in time
as possible, and only when the wind speed is fairly constant
both in velocity and in direction.

Large mall parking lots make good places to do the test runs
if you can find a time when they are somewhat deserted.
Always have one person driving and another supervising
the experiment to avoid an accident.

Fr. Tom McGahee

----- Original Message -----
From: Sean H. Breheny <shb7@CORNELL.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, February 10, 2001 8:03 PM
Subject: Re: [OT]: Wind Speed Formula??


> 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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