piclist 1999\05\07\030442a >
www.piclist.com/techref/index.htm?key=airspeed
BY : Mark Willis email (remove spam text)

Herbert Graf wrote:
>
> > I didn't see the Electronics Now project but I've seen this done using a
> > small pilot bulb with the glass broken off as the sensing element. This
> > wouldn't use too much current from a Ni-Cad pack.
>
>     That sounds like a good idea. AFAIK the EN article used a solid state
> device self heating characteristic to determine airspeed. I think it used
> two transisters, one as a control device and one as a sensor. I remember
> reading it, was interested in making it until I found out that calibration
> wasn't that easy, it involved traveling in a car on a windless day, just
> couldn't be bothered with that. I can look it up if someone is interested.
> TTYL

I've been thinking about calibration;  Borrow time at a campus wind
tunnel, for higher speeds?  (Borrow a friend with an airplane?  <G>)

For slower speeds, you should be able to swing the unit on the end of
a rod, inside, at certain rotational speeds & get some good slow speed
calibration information.  (Want faster?  Attach a 1/2 Hp big power drill
to the center point <G>)

You need to measure the RADIUS, and rotational speed, from that figure
out the tip speed.

For example, 8' (wood 1"x2"?), 2Hz rotation rate, you get a rough
speed at the tip of the rod of:

Pi = 3.14159 etc <G>
r  = 4' here (I assume you pivot the rod at it's center)
T = 0.5 sec (= 1/(2 Hz))
V = Velocity = 2*Pi * r / T = 50.26 Ft/S, * 5280/3600 = 34.27 MPH

(Don't forget that you're talking some radial accelerations here -
don't tangle with that rod - Don't overdo the G forces here, and so on.
Shorter rod at lots faster RPMs gives higher speed capability with less
G forces <G>)  Good idea to balance that rod, too.  I'd sure put my PIC
& electronics near the center.  Could swing 6" and counterweight the
short end, too.

Mark