AVR Freaks

I recall that cup anemometers are drag wind turbines rather than airfoil wind turbines, and usually turn at windspeed/3. I guess this depends on the diam of the cup and the length of the arm, etc.

Yah know, I was thinking how to calibrate the darn things while reading down the thread. I think Bob hit it--mount the darned thing on a mast [to get it out of vehicle turbulence] on the top of your pickup truck on a calm day, and make a few trips back-and-forth on an open road at various speeds, and Bob's your uncle. No tape measure or algebra needed.

Lee

Hello Neil, Bob, Jim et al.

Thanks for posting the link to Dr. John Gorman's instrument Test Report. It saved me digging it out. I used to work at the Bureau where John still works. I used to be at various times the officer who bore those odd position titles of SRUM and SROO on the distribution list of John's report.

The 3 cup anemometer (with 120 degree angular separations) is indeed a strange beast. At the same time that the orthogonally exposed cup is being pushed by the wind, the other two cups are pushing against the same wind. And the three circular cross-sectioned arms are presenting less than ideal drag exposure to the wind. (aside: I am sure you will have heard how the Wright Bros would have flown further if they had not used drag producing circular cross-sectioned wing struts in their aircraft). All of the above is a simple explanation of why there is not a 1:1 relationship as implied in your (Neil) initial calculation. (another aside: John's exposure on the roof of his house fails WMO's test standards unless he is now 3 metres tall. Structure-induced vertical wind turbulence will contaminate all readings but not necessarily in an equal way. I will contact him tomorrow about that :lol: )

When working for the Saudi Met Service in the early 1980s, I was asked to do acceptance testing on a delivery of anemometers from their German manufacturer. Saudi did not have a wind tunnel, so I used my car with a boom holding the instruments ahead of the vehicle and had my 12 year old son log the readings while I drove back and forth along a test range on a still day at various speeds. The precise wind speed was not important because the spread of readings in the batch was enough to reject them all as unfit for service. I am confident that you could replicate such a precise "test range" :roll:

So you need a system visible from 50 yards away. I think you and Bob should get together and design a mini-AWOS, but perhaps have the speed being transmitted without need for the push to talk button.

I might even suggest a pitot tube (there you go Jim a fourth style) mounted inside a wind direction vane.

Ain't the weather a constant talking point !!

Cheers,

Ross

He might not know as much about it as you give him credit for.... I'm surprised he is even considering measuring the darn wind speed. I recommend something tamer like knitting.

... because of "surface roughness". There is an involved formula for calculating the wind speed at the standard 10m height based upon knowing the speed at a lesser height and the nature of the surface, eg mown grass, sand, etc. But that will not apply in your hillside situation.

So with the partially inflated wing and you still on the ground, you want to be able to look at an indicator of the wind speed at the height that the wing would be when you are being lifted off the ground. Correct?

The "Dines" anemometer invented by one of your fellow countrymen in the late 19th century uses a pitot tube. Australia still used them up until the 1970s at some sites. I have also seen a similar handheld instrument for measuring air flow in air conditioning ducts. There are also several cheap anemometers used by the "sailing crowd" (yachties). Either of these could be easily exposed appropriately. The indicator is the interesting bit as far as I am concerned.

Musing in the shower (inspiration-central), it occurred to me that a vertical totem pole painted in bands using eg the resistor colour code, to indicate decades of speed unit could be seen from all vantage points. The bands could be say one or 2 feet high. The "pointer" could be an outer black cylindrical sleeve that slides up and down the totem driven by your sensor/indicator interface. I would probably use a tensioned string drive (like the old radio tuning dials) with the motor battery etc at the ground level. The totem could be cardboard tubing to minimise crashing hazards to other flyers.

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Cheers,

Ross

Interesting indeed. The section on sonic anemometers claims the first such device was developed in the 1970s. This is wrong because in 1973 I was reading journals from the mid 1950s that described a room sized instrument produced in Japan. Unfortunately I did not keep my papers from that time but seem to recall that it operated at around 50KHz and used valves (hence the room size needs). I went on to design (circa 1978) an ultrasonic anemometer with air temperature measurement for use on drifting ocean buoys when I was at the Australian Bureau of Meteorology.

Cheers,

Ross