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 3,306 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: TBS Disco Gimbal Update
 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 20,946 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
27,200 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 29,703 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.
 Mar 04, 2009, 07:42 PM OFF TOPIC POSTER Des Moines IA Joined Dec 2005 4,810 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 20,946 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