

Aug 19, 2012, 06:25 PM  

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! 
Aug 20, 2012, 02:49 PM  

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. 
Aug 20, 2012, 03:12 PM  

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 
Aug 20, 2012, 04:28 PM  
Joined Aug 2009
1,398 Posts

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 
Thread Tools  
Similar Threads  
Category  Thread  Thread Starter  Forum  Replies  Last Post 
Discussion  Fan thrust vs prop thrust; why the difference?  PlaneFan  Electric Ducted Fan Jet Talk  3  Mar 04, 2012 11:55 AM 
Propeller thrust vs. RPM  sc_deimos  Modeling Science  14  Jul 06, 2004 04:57 AM  
MM Dandy RPM vs Thrust  ZRS4  Parkflyers  11  Mar 01, 2004 12:06 AM  
GWS RPM vs Static Thrust????  ZRS4  Power Systems  4  Feb 29, 2004 01:16 AM  
prop rpm vs. static thrust/pitch speed  opualuan  Power Systems  2  Feb 19, 2002 08:48 AM 