

Bangalore, India
Joined Nov 2003
151 Posts

prop rpm vs thrust chart
I am looking for prop rpm vs thrust chart for GWS props. Couldn't find this anywhere on the net. Please direct me if you know some place where this is available.
 Rajesh 




Thrust is not a constant, static value (unless you're building a helicopter, or a model design pretty much just to hover)..
It varies significantly with airspeed.. www.motocalc.com ..a 


Bangalore, India
Joined Nov 2003
151 Posts

Andy,
I know it changes with airspeed, I am looking for static thrust figures. Need it for my experiments, static thrust gives me an idea how well I am doing. The problem is I have built a tach, but don't have a thrust measurement setup yet. I have checked the motocalc site, didn't see this information there. Is it possible to get this table from motocalc ?  Rajesh 


United States, NC, Belmont
Joined Mar 2004
684 Posts

Static thrust calculator
Here's a great staic thrust calculator for electric props with links to calculators for many but not all APC and GWS props:
http://members.jcom.home.ne.jp/42232...aticthrust.htm It's from a German Club and appears to be fairly accurate. Don't worry about the electric part of the calculationdoesn't seem to make much difference for just thrust calculations. 


Bangalore, India
Joined Nov 2003
151 Posts

Thanks for the link, Don. I had seen this link earlier, but its prop choices stops just above the sizes I am interested in. I am looking for props from 2.5x0.8 to 6x3. I tried the generic calculator, but the figures for the same dia and pitch differs significantly from that given by the GWS calculator. So I believe the results for smaller sizes would not be accurate either.



Toronto Canada
Joined Dec 2002
5,605 Posts

I don't know whether this will help you, but it's a simple spreadsheet which I stripped off a much larger version which I use for motor and propeller analysis.
It would be relatively simple to expand it to create graphs of Thrust vs RPM for most props. You will need to know the motor constants Io, Ra and kV. If you are interested only in power, RPM or thrust output, the spreadsheet may still be used by putting in any motor constants and adjusting the voltage until you achieve the RPM or whatever variable that you require. A couple of caveats. The sheet was not written in Excel, but is Excel compatible and for this reason I haven't protected the fields. The fields with formulas should be protected. There are 2 hidden rows which store the results of an intermediate quadratic equation. They should not be removed. The spreadsheet was originally written for nicads. It may be used for lipoly cells by either modifying the sheet or by noting that the nominal voltage for lipolys is 3.6 Volts or 3 times that of a nicad. The cell resistance for lipolys is higher as well. All propellers are not created equal. Propellers of the same dimensions from different manufacturers can have different areas and therefore different thrust and drags. This doesn't mean that one propeller is more efficient than the other but these differences should be compensated for. The easiest way is by changing the pitch slightly. The spreadsheet is based on power and thrust formulas published years ago. These equations were known as the Abbott equations. Power (WATTS)=P(in.) X D(in.)^4 X RPM^3 X 5.33 X 10^15 Thrust (oz.)=P(in.) X D(in.)^3 X RPM^2 X 10^10 The constants in these equations may be modified to suit the brand of propeller that you are using. Although I have found them to be reasonably accurate, the thrust equation does not take into account the thrust from a stalled propeller. Props with pitch/diameter ratios greater than 0.6 are statically stalled and the thrust measurement given by this program would be slightly higher than reality for these props (as are many other programs), if not compensated for. The power measurement, however, is reasonably accurate. The measured static thrust in these instances would approximate that of a propeller of the same diameter with a max pitch of 0.6 the diameter. This could be implemented in the spreadsheet. That being said, I have found the spreadsheet extremely useful for comparison purposes or for determining if you are going to burn out your motor from excessive current. The term Eg used in the spreadsheet is the voltage generated by the back EMF of the armature. It gives an indication as to how much of the applied voltage is doing useful work. All other fields are self explanatory. Perhaps someone proficient at Excel could clean it up a bit. The file is not actually a .zip file but an Excel .xls file. After downloading, rename the file propeller.xls. EDIT: I have uploaded a cleaned up version in which all the input variables are placed on one line. All formulas remain the same except for the thrust field which compensates for propellers with pitch/diameter ratios greater than 0.6. 



Included in the attached zip archive is an Excel spreadsheet with all my measurements for GWS props, and a thrust calculator. To calculate thrust at another rpm, enter your rpm along with the prop's diameter, pitch, and tconst from the measurements section.
If you have a prop that is not listed, you can try guessing its tconst by comparing to a similar prop that I have tested. In general, slowfly props have higher tconst than hyperdrive, and a higher pitch/diameter ratio also increases pconst (up to a point, then it decreases as the prop stalls). Note that my tconst is NOT the same as Motocalc's tconst (which is calculated from power, not rpm!). 


Bangalore, India
Joined Nov 2003
151 Posts

Thank you, Bruce, that was the data I was looking for. The spreadsheet is quite useful to get approximate results once one measurement data is available to determine the constant. The problem is constant varies a lot from prop to prop, so atleast one measurement has to be made before the equation can be used.
Martyn, Could you explain the statement "Props with pitch/diameter ratios greater than 0.6 are statically stalled" ?  Rajesh 


Toronto Canada
Joined Dec 2002
5,605 Posts

The pitch angle of a propeller blade (in degrees), varies along the length of the blade. The angle is greatest at the root and decreases linearly towards the tip.
It is for this reason that its pitch is specified in inches and not degrees. Most airfoils with these aspect ratios stall at pitch angles from 12 to 17 degrees (depending how it is measured). Therefore a propeller with a majority of its pitch greater than 17 degrees is fully stalled and creates little thrust, but lots of drag. It can be demonstrated mathematically that the pitch of a propeller blade with a pitch/diameter ratio=0.6 has a pitch angle at its tips of approximately 11 degrees. This means that the angle of the blade as you approach its root is greater than 11 degrees and is proportional to the distance from the tip. Half way down the blade the pitch would be 22 degrees and stalled. This also means that you can trim the tips from a propeller to reduce its diameter but stilll maintain its pitch (in inches). The majority of thrust from a propeller comes from the outer portion of the blade because thrust is proportional to the diameter to the third power. If the pitch angle at the tip is 11 degrees, the majority of the blade is at a pitch angle greater than 11 degrees and is becoming stalled. As the aircraft increases its speed through the air from rest, the angle of attack of the propeller will decrease and the blade will become unstalled. This is why real aircraft have variable pitch propellers, so they can have efficient thrust at low speeds and still achieve high airspeeds by increasing the pitch. For pitch/diameter ratios less than 0.6, the portions of the blade which generate most of the thrust are unstalled. Most formulas for thrust do not take into account this nonlinear characteristic of propellers and any thrust measurements made statically do not accurately represent the true performance of a propeller if its pitch/diameter ratio is greater than 0.6. 



Can someone do me a favor? I do not have Excel at home and therefore cannot check out my RPM vs. Thrust numbers. But using the site given by MGDON, I am getting some pretty wild numbers for the following RPM figures using GWS props:
5x3 at 15000RPM is calculated as 7+ oz thrust 6x3 at 12400RPM is calculated as 10+ oz thrust 8x4.3 at 6600RPM is calculated as 9+ oz thrust The thrust numbers look just too good to be true. But if the actual thrust is within 7580% of calculated figures, I am about to be a very happy person. Jun Nolasco Quote:





Hi Jun  I can't give you exact data for those rpm numbers, but they are not too far away from reality.
I have measured on my test stand: 5x3 GWS HD: 14280 rpm = 5.85oz, 18990 rpm = 10.00oz 6x3 GWS HD: 12720 rpm = 7.92oz 6x3 Cox: 11980 rpm = 9.01oz 8x4.3 GWS RS: 6750rpm = 9.72oz Cheers, Phil 


United States, NC, Belmont
Joined Mar 2004
684 Posts

Nolasko,
If you go to Bruce Abbots reply (no.8 above), he has Text prop thrust included along with the excel one. You don't need excel to open it. Some examples from his chart: If you open it remember the thrust figures are in grams so you have to divide them by 28.3 to get ounces. 5/310,200= 97.9 grams of thrust= + 3.46 oz.thrust (he doesn't go higher for 5/3's 6/39500= 155 grams= + 5.48 oz. 8/6 ( he doesn't include 8/43) @ 6100= 11.3 oz. I agree with you that the website I mentioned is a bit optimistic or we're not reading it correctly. I believe Bruce Abbotts chart is much more accurate. 



Just visited this forum and I wanted to say thank you for all the info. I am building small foamies now of 28 ounces down to 1.5 ounces, and I think your work will be helpful.
This should be a sticky thread, too valuable to not be! 
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