




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 pfactor (prop efficiency), airfoil shape, size, weight, construction materials, yada, yada, yada. Good luck! 
Latest blog entry: Pusher Style Folding Prop SetUp




United Kingdom, England, Bromyard
Joined Mar 2008
849 Posts

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




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. 



Quote:
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. 




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. 



where's the online calculator for this?
Thanks 



Try this one for kicks 
http://flbeagle.rchomepage.com/softw.../webocalc.html 







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. 


USA, ME, Ellsworth
Joined May 2008
18,224 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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