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Old Oct 17, 2004, 07:51 PM
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Powell River BC Canada
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Plane speed /prop pitch/ RPM formula??

Sorry I'm rather mathmaticly challenged. Could someone tell me how to calculate theoretical plane speed from a give prop at a given RPM??

Oilman.
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Old Oct 17, 2004, 08:16 PM
Seattle, WA - USA
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Pitch speed = RPM x Pitch x (60/63360)
eg: 8000rpm x 6" x (60/63360) = 45.45mph
The 60/63360 converts from inches per minute to miles per hour.

Plane speed can be anywhere from ~80% to ~95% of pitch speed depending on a whole host of variables including p-factor (prop efficiency), airfoil shape, size, weight, construction materials, yada, yada, yada.

Good luck!
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Old Oct 17, 2004, 08:19 PM
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Powell River BC Canada
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thnks Mark.
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Old Mar 04, 2009, 12:06 PM
Night flier
United Kingdom, England, Bromyard
Joined Mar 2008
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well according to my calculations. my prop will b traveling at about 165!!!!! MPH,thats crazy lol. i will b using a 5x5 prop with a mega 16/15/4,and a 4s lipo lol. will b in a slipstream wing. so its goin to b fast......! Cant wait to get it built. going to b a handful lol
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Old Mar 04, 2009, 12:19 PM
Space Coast USA
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Space Coast
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A high reving high pitch prop on a piper cub will still make it fly like a piper cub. The plane has to be "fast" designed to take advantage of high pitch speed power plants.
Your slipstream would fit in that Fast category! Have a ball.
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Old Mar 04, 2009, 01:54 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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Quote:
Originally Posted by austin.laws
well according to my calculations. my prop will b traveling at about 165!!!!! MPH,thats crazy lol. i will b using a 5x5 prop with a mega 16/15/4,and a 4s lipo lol. will b in a slipstream wing. so its goin to b fast......! Cant wait to get it built. going to b a handful lol
Sorry, but you won't get pitch speed that high: 2200Kv x ~14v (under load) = 30800rpm. But you can't get Kv x V with a prop on, so reduce that to 85% of Kv x V...26180rpm

Now do the pitch speed calculation: 26180 x 5/1056 = 124mph.

One other suggestion - get a 5x5 EMP prop rather than the lousy 5x5 APC E or better yet use a Zagi Carbon 5.1x4.9.
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Old Mar 04, 2009, 04:13 PM
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East Anglia, UK
Joined Sep 2002
29,703 Posts
Pitch alone isn't enough. You need to calculate plane wetted area (wing area mostly) L/D ratio, and profile drag, to get the drag versus speed curve, and make a power versus speed curve.
That tells you how much power you need..

Then prop for a pitch speed about that speed.Most props work best at the 'geometrical pitch speed' - they actually still produce lift and thrust at almost zero angles of attack. And that is where they are most efficient too.

If you are building a slender racer, ultimately wing area dominates the drag equation.
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Old Mar 04, 2009, 07:42 PM
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Des Moines IA
Joined Dec 2005
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Thread started in (04) WOW but is it in a solid or gas state? Robert
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Old Mar 22, 2009, 07:15 PM
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USA, KY, Ashland
Joined Oct 2007
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where's the online calculator for this?

Thanks
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Old Mar 22, 2009, 10:43 PM
Space Coast USA
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Space Coast
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Try this one for kicks -
http://flbeagle.rchomepage.com/softw.../webocalc.html
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Old Aug 19, 2012, 11:42 AM
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Joined Aug 2012
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hi.. my ques it that.. if suppose due to the repulsive forces at the tip of the fan, a fan rotates.. how do i calculate the speed rpm of the fan with the given repulsive force.. could u help me out with formulas..
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Old Nov 28, 2012, 08:20 AM
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United States, NJ, Mt Holly
Joined Nov 2012
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http://brantuas.com/ezcalc/dma1.asp


try this
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Old Nov 28, 2012, 11:26 AM
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Mexico, BC, Mexicali
Joined Aug 2004
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You can use my Convertidora.
http://www.rcgroups.com/forums/showthread.php?t=1191623

Conversiones.

De Pulgadas a Centimetros.
De centimetros a Pulgadas
De gramos a Onzas
De Onzas a Gramos
Pies cuadrados a Decimetros cuadrados
Decimetros cuadrados a Pies Cuadrados
Pies cuadrados a pulgadas cuadradas
Pulgadas cuadradas a Pies cuadradas

De kilos a libras y onzas.

Calculo de KV Ratio

Duracion de bateria

Velocidad del modelo

Velocidad de perdida. Con datos en sistema Imperial y Metrico.(corregida)

Area para DF,

Para servos de oz a kilos.

Calculo de empuje para helices 2,3,4 y mas palas.(no muy confiable) en proceso de corregir.

de Centigrados a Fahrenheit.

Proyeccion de consumo de Ampers de 2s lipo a 3s, 4,5,6,7,-10S lipo.


Con formulas para el Kv y lectura del Drill press.
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Old Nov 28, 2012, 05:25 PM
Jack
USA, ME, Ellsworth
Joined May 2008
16,995 Posts
The Calculator page at flybrushless.com has a pitch speed calculator that works fine:

http://www.flybrushless.com/tools

For flying speed estimates I knock off 30% for draggy builds and 20% for the sleekest. If I want to know the real speed I send my eLogger for a ride with the GPS module attached.

The sell a pitot tube type air speed sensor for the eLogger too, I don't have one of those and they are much more complicated to install.

The drag is just too much of a variable for you to get anything but an approximation, if you really want to know you have to measure it.

They also have doppler effect speed interpreters you can put on a hand help cell phone too. Those listen to the inbound plane to estimate the speed. They don't work too bad I hear.

Jack
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Old Nov 28, 2012, 05:29 PM
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United States, NJ, Frenchtown
Joined Mar 2003
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Police radar guns work OK
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