

Oct 18, 2004, 11:33 AM  

http://www.lcrcc.net/thrust_calc.htm
or read the the thread on "How do YOU measure thrust" http://www.rcgroups.com/forums/showthread.php?t=286867 
Apr 13, 2014, 09:53 PM  
Joined Dec 2010
228 Posts

Quote:
Here is the simplified version of my equation: F = 4.3924e8*RPM*d^3.5/sqrt(pitch)*(4.23333e4*RPM*pitch – Vac). F is thrust in Newtons, RPM is rotations per minute, d is prop. diameter in inches, pitch is prop. pitch in inches, Vac is aircraft airspeed in m/s. If you want thrust in other units: to convert newtons to grams, multiply newtons by 1000/9.81. To then convert grams to ounces, multiply grams by 0.035274. To convert ounces to pounds, divide ounces by 16. The generics of the equation can be found here, including an Excel spreadsheet to make your life easier, and the equation above is shown here in a more visually appealing form: http://electricrcaircraftguy.blogspo...equation.html The full derivation of the equation begins with Newton’s laws, and is shown here: http://electricrcaircraftguy.blogspo...ackground.html I hope this is useful to many people, as it is very useful to me. 

Sep 21, 2014, 08:24 PM  
Joined Dec 2010
228 Posts

Quote:
I appreciate this info. In regards to the following lines, I've known what you say to be correct for a long time now (probably a couple years): "In your equation, the last bracketed terms are (4.23333e4*RPM*pitch – Vac) which I interpret as being (Pitch Speed  Flying Speed). This implies that if Flying Speed is equal to Pitch Speed that the thrust is equal to Zero. This is not correct. At Pitch Speed (RPM X Pitch) the angle of attack along the entire blade is zero degrees which has the best LiftDrag ratio. Flatbottomed [more generally speaking: cambered] airfoils (propellers) still generate thrust at negative angles of attack up until Zero Thrust Speed." What I didn't know, however, are the lines after that (in red above), so that is very valuable info. I spent nearly all of my time on that equation focusing on getting the static thrust right for any given prop at any given RPM. I didn't, however, spend much time on the dynamic thrust part since I only had one single data point (wind tunnel data, see rightside of Fig. 1 here) to compare to, and I didn't have a book solution to know where zero thrust occurred. Nevertheless, I knew it was at a negative angle of attack, but I used pitch speed to start just so I'd have a rough initial guess. Where did you get those plots, by the way? I will def. need them to correct and improve my equation, and so I can include them on my site and properly cite where they came from. I'd like to read those pages surrounding the plots for sure too. 

Sep 21, 2014, 08:29 PM  
Toronto Canada
Joined Dec 2002
5,539 Posts

NACA Technical Report TR237
http://naca.central.cranfield.ac.uk/...report237.pdf In your equation you could multiply the Pitch Speed term by a factor to get Zero Thrust Speed. Zero Thrust Speed = Pitch Speed X (P/D+0.2)/(P/D) Just another comment: At Zero Thrust Speed the motor will be very lightly loaded. Because of this a reasonable estimate of its RPM is simply: RPM = Battery Voltage X Kv. This RPM could be significantly different than the static RPM depending on the efficiency of the motor. It should also be possible to include an estimate of the varying RPM information into your formula. 
Sep 21, 2014, 08:37 PM  
Joined Dec 2010
228 Posts

Quote:



Thread Tools  
Similar Threads  
Category  Thread  Thread Starter  Forum  Replies  Last Post 
Discussion  I know the RPM's and the prop size, how to calculate thrust?  Fiasco  Power Systems  16  Apr 11, 2006 06:49 PM 
Calculate Static thrust by RPM and prop diameter and pitch?  Xnaron  Power Systems  7  Mar 22, 2004 10:59 PM  
Can anybody with Ecalc help me? How much thrust will a 5x5 prop make, at 19,000 rpm?  I FO nut  Electric Plane Talk  25  Jul 01, 2003 10:26 AM 