Calculating Wind Speeds With RPM
04-13-2007, 09:27 AM
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Calculating Wind Speeds With RPM
I know that wind speeds can be determined by using a prop and tachometer. This in effect is called an anemometer.
Now here's my question:
What is the mathematical equation to determine wind speeds (in MPH) based on the RPM & Prop Pitch?
The one example that I was able to find on the internet is as follows:
RPM x Pitch x .00114 = wind speed in MPH
In this example, it is stated to use a 11x8 prop. But what if I wanted to use a different pitch on the prop?
Anyone give me a fairly basic equation here?
Now here's my question:
What is the mathematical equation to determine wind speeds (in MPH) based on the RPM & Prop Pitch?
The one example that I was able to find on the internet is as follows:
RPM x Pitch x .00114 = wind speed in MPH
In this example, it is stated to use a 11x8 prop. But what if I wanted to use a different pitch on the prop?
Anyone give me a fairly basic equation here?
04-13-2007, 10:33 AM
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RE: Calculating Wind Speeds With RPM
The easy way is DL a program that does it for you...
[link=http://www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm]http://www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm[/link]
That equation is fine, changing the pitch in the equation isnt going to mess anything up.
[link=http://www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm]http://www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm[/link]
That equation is fine, changing the pitch in the equation isnt going to mess anything up.
04-13-2007, 10:54 AM
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RE: Calculating Wind Speeds With RPM
Good enough for me! I'm actually using an 11x7 prop instead of the recommended 11x8 prop. I wasn't sure if the different prop pitch really changed anything in the equation or not. All I want is to get a close approximation of wind speed from the anemometer.
I made one from stuff I already had and it seems to work very well. I was just hoping to keep my equation as accurate as possible with the different pitch prop that I'm using. Thanks for your help!
I made one from stuff I already had and it seems to work very well. I was just hoping to keep my equation as accurate as possible with the different pitch prop that I'm using. Thanks for your help!
04-13-2007, 02:12 PM
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RE: Calculating Wind Speeds With RPM
Can some one explain where the .00114 factor comes from? When I do the math as follows:
Rpm (rev/min) x pitch (inches/rev) x [60 min/hr x 1ft/12inches x 1mile/5280ft] = miles/hr
The bracketed conversion factor should be .000947.
Is there a correction being applied for prop efficiency?
Brad
Rpm (rev/min) x pitch (inches/rev) x [60 min/hr x 1ft/12inches x 1mile/5280ft] = miles/hr
The bracketed conversion factor should be .000947.
Is there a correction being applied for prop efficiency?
Brad
04-13-2007, 02:38 PM
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RE: Calculating Wind Speeds With RPM
I think the first example was wind speed comming off the propeller, not mph of the airplane. You are correct, 0.000947 is the factor for theoretical mph of an airplane.
Ed S
Ed S
04-13-2007, 06:54 PM
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RE: Calculating Wind Speeds With RPM
There is no slip with a wind driven propellor. It "advances" at the pitch value.
Converting that to feet and reading rpm and converting the rpm to fps and then mph yields the correct number.
(88 fps=60 mph)
"Rpm (rev/min) x pitch (inches/rev) x [60 min/hr x 1ft/12inches x 1mile/5280ft] = miles/hr"
I built a double ended tester.. the props mounted on a Master Airscrew gear box for easy changing.
At my usual slope site on a windy day...
The Dwyer Wind Gauge served as the control..
Converting that to feet and reading rpm and converting the rpm to fps and then mph yields the correct number.
(88 fps=60 mph)
"Rpm (rev/min) x pitch (inches/rev) x [60 min/hr x 1ft/12inches x 1mile/5280ft] = miles/hr"
I built a double ended tester.. the props mounted on a Master Airscrew gear box for easy changing.
At my usual slope site on a windy day...
The Dwyer Wind Gauge served as the control..
