


Discussion
Thrust vs Rpm
I have been cruching some numbers using the charts Rainer put up on the JetFan 90. In my simplistic way, I'm figuring that, for instance every 2500rpm would equal X amout of thrust increase. Right/Wrong?
What I need to know, as RPM increases is the thrust measured lineal or expotential? 






that would be applicable to a particular fan. if applied that way, yes.
you mean static or dynamic ( bench or in flight ). good to measure both as neither are equal how has that fan performed for you? 





stoneenforcer, havn't ran it yet, it's for my F86F. Hoping next summer it will be done.
But here's what I'm getting at. Chart below show 2 identical Scorpion motors 1210Kv. I took Kv of each: Using Kv x V=RPM 1210 x 38.88V=47,044rpm @ 4.1 Kg 1 Kg = 11,474 rpm 1210 x 37.75V=45,677rpm @ 4.3Kg 1 Kg = 10,622 rpm So, how can you get .2Kg more thrust with 1367rpm less? Based on RPM only for thrust I don't see any way to work out a graph to figure thrust. Granted there is a difference in pack size and C rating, shouldn't make any difference. 





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Last edited by stoneenforcer; Aug 21, 2012 at 02:46 PM.




Thanks, it seems there is know real way to do it. Just thought it would be interesting. Did try Rainer's JetFan 90 V2 calc, pretty close.






i think it is more on a curve ...when running full scale tubines i notice at 80% PLA ....not much noise ...when you get to 100% the dang plane is jumping and straining to move and jump the chalks ....im sure our EDF are the same it's the last 30%20% is where most of the thrust is made.






Cooler, would love to see a graph of the Jetfan 90 RPM/thrust.
Seems to me this would be pretty easy to do as we aren't really concerned with the motor or system efficiency at this point just thrust and RPM. Heck you could run the fan with a squirrel cage as long as you achieve RPM (as a friend of mine puts it....lol). Pack size and C rating only effects RPM so they really don't matter. Once the curve is established you should be able to extend it for higher RPM's of higher powered setups. So if you put your fan on a test stand and measure the thrust and rpm at 1/8th throttle, record, then 1/4 throttle, record, them 3/8 throttle, record up to full throttle. That will give you 8 points to plot. From this you could plot the points and find out if it linear or have a curve to them. From this data you can extract that if you make X RPM you should have Y thrust. I'm really interested in this but unfortunately painting now so I can't run my test stand. If no one has done this in about a week, I will. Setup will be inlet ring and open exhaust, 8S and 1500kv. I think I hit around 30K RPM with that setup. Terry 

Last edited by Chuchuf; Aug 19, 2012 at 02:02 PM.




Terry, It sounded like a good idea at the time, to use the numbers Rainer had put up in the charts. Kv x V=thrust. It just didn't work out very well. But, if you take a little time and use the Calc for the JetFan 90, you can see a definate pattern. Just for the nuts of it I ran a few numbers from the calc.
Using a base of 6s and 3.75V per cell  Kgrpm 2.532,047 2.632,657 2.733,255 2.833,824 2.934,418 3.034,984 Sorry, I wasn't inclined to take it all the way to 6.5Kg. I think anyway one would do this, it's still only guide. Now by plugging various thrust and V per cell figures you can get a real close estimate to what you want/will get out of it. From what I have seen this is the point where really good batteries shine. It's interesting how much the figures change by changing the cell V by even .05V. So for those who havn't DL'd the calc, I say get it and play around. 





Quote:
I think the problem with using a calc to determine the output is we have no idea how the calc determines it. If you plot the rpm horizontally and the thrust vertically, you will see that it is a straight line, ie linear from the number above. This tells me that the calc is coded that way. Now it may very well be that the output of an edf is in fact linear but we will have to run one, measure it and actually determine if that's the case. Once you know what the thrust vs rpm is, it's pretty easy to figure out what you want for a motor and batteries for the thrust you would like to obtain. ie I need to turn 40000 rpm to get 9 lbs thrust as an example. Back to painting........uggggg Terry 






Agreed. that would be the best. If someone who has a Het 700681200Kv on 12s wants to volunteer to run a bench test that would be great. Could start at 30,000rpm. On the high end it should go to at least 48,000rpm. By the calc the would be 5.8Kg That's about as far as you could go with 5000mah's.
Back to setting up the nose gear. Much more fun than painting! 





Thrust vs. RPM
On an un stalled fan, the flow velocity thru the fan is directly proportional to the rpm. Acceleration is proportional to the velocity thru the fan.
Likewise, the mass thru the fan must also be directly proportional to the rpm. Force equals mass times acceleration, F=MA. Example: if we doubled the rpm, since we doubled both the M and the A then the thrust would be 4 times as much. A 10% increase in rpm would be : 1.1 times 1.1 = 1.21 or a 21% increase in thrust. There are bound to be some fudge factors creeping in but that is the underlying physics. You might want to click on my avitar to see a discussion of a fan with all the numbers. It needs some work but the physics is all good. 



Originally Posted by CoolerByTheLake
Terry, It sounded like a good idea at the time, to use the numbers Rainer had put up in the charts. Kv x V=thrust. It just didn't work out very well. But, if you take a little time and use the Calc for the JetFan 90, you can see a definate pattern. Just for the nuts of it I ran a few numbers from the calc. Using a base of 6s and 3.75V per cell  Kgrpm 2.532,047 2.632,657 2.733,255 2.833,824 2.934,418 3.034,984 end Quote I calculated the thrust at 3kg using the 2.5 kg rpm in a google window as follows: 34984^2/32047^2*2.5 and the result was 2.97923098835 which is less than 1% off. The calc involved is apparently using the physics I discribed in my last post 



Thanks Tom I love numbers, but this is a little over my head! So if I were to extrapolate these thru 6.0Kg @.1 increments, we are good to go?






Tom,
I like what you have done to clerify what eCalc is doing. As I expected it just assumes that there is a linear relationship between RPM and thrust for the reasons you explained. Using the sample numbers that Cooler provided (which were linear) I went ahead and did the calculations out to 5.5Kg (12.1 lbs) of thrust showing the RPM required. Terry 

Last edited by Chuchuf; Aug 20, 2012 at 04:54 PM.




So did you come up with46,826rpm? Is that at 3.75V per cell?
Did you forget the attachment? 


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