Oct 17, 2004, 07:51 PM Registered User Powell River BC Canada Joined Sep 2003 888 Posts 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.
 Oct 17, 2004, 08:16 PM Seattle, WA - USA Joined Sep 2003 3,237 Posts 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! Latest blog entry: Super Simple Gimbal (SSG) - My Version
 Oct 17, 2004, 08:19 PM Registered User Powell River BC Canada Joined Sep 2003 888 Posts thnks Mark.
 Mar 04, 2009, 12:06 PM Night flier United Kingdom, England, Bromyard Joined Mar 2008 831 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
 Mar 04, 2009, 12:19 PM Space Coast USA Space Coast Joined Oct 2000 20,722 Posts 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.
Mar 04, 2009, 01:54 PM
Registered User
Chattanooga, Tennessee, United States
Joined May 2003
27,197 Posts
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.
 Mar 04, 2009, 04:13 PM Registered User East Anglia, UK Joined Sep 2002 29,688 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.
 Mar 04, 2009, 07:42 PM OFF TOPIC POSTER Des Moines IA Joined Dec 2005 4,784 Posts Thread started in (04) WOW but is it in a solid or gas state? Robert
 Mar 22, 2009, 07:15 PM Registered User USA, KY, Ashland Joined Oct 2007 3,340 Posts where's the online calculator for this? Thanks
 Mar 22, 2009, 10:43 PM Space Coast USA Space Coast Joined Oct 2000 20,722 Posts Try this one for kicks - http://flbeagle.rchomepage.com/softw.../webocalc.html
 Aug 19, 2012, 11:42 AM Registered User Joined Aug 2012 1 Posts 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..
 Nov 28, 2012, 08:20 AM Registered User United States, NJ, Mt Holly Joined Nov 2012 5 Posts
 Nov 28, 2012, 05:25 PM Jack USA, ME, Ellsworth Joined May 2008 16,699 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
 Nov 28, 2012, 05:29 PM Registered User United States, NJ, Frenchtown Joined Mar 2003 9,056 Posts Police radar guns work OK