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Jun 25, 2009, 06:51 PM  

I always us 80% of the fan sweep area (FSA). The fan sweep area is the doughnut shaped ring that the fan blades push air through. You need to measure the diameters on the fan you are using. The calculate the exit diameter is easy the simplest way to do is like this:
De= sqrt(.80) * sqrt(D*D  d*d) De= diameter for 80% FSA D= rotor diameter d= diameter of rotor spinner/motor (depends on fan used) .80 = 80% FSA I will post a photo and do an example. Don 
Latest blog entry: My column in April 2014 Model Aviation


Jun 26, 2009, 12:03 AM  

Are you using the stock HP 3 blade fan/rotor?
Don 
Latest blog entry: My column in April 2014 Model Aviation


Jun 27, 2009, 08:59 PM  

OK here we go.
De= sqrt(.80) * sqrt(D*D  d*d) De= diameter for 80% FSA D= rotor diameter d= diameter of rotor spinner/motor (depends on fan used) .80 = 80% FSA Measuring the fan we get: D= 56 mm d= 24 mm De= sqrt(.80) * sqrt(56*56  24*24) De= .8944 * sqrt(3136576) De= .8944 * sqrt(2560) De= .8944 * 50.596 De= 45.25mm = 1.78 " 
Latest blog entry: My column in April 2014 Model Aviation


Jul 14, 2009, 05:12 PM  

I have used slightly oval at times but round I believe is the best as it distributes the airflow evenly. The EDF F117 I had had long rectangular slots and it worked great but they also had a nice gradual transition from round to rectangle to reduce losses as much as possible.
Don 
Latest blog entry: My column in April 2014 Model Aviation


Apr 13, 2010, 03:40 PM  

Here is a spreadsheet that I wrote back in December and modified today. It will calculate FSA when you input the fan shroud inner diameter and the fan hub diameter. It will output FSA from 100% down to 70% in 5% increments. It will also give you the exhaust diameter for 100% FSA down to 70% FSA in 5% increments. It will give you the diameter for both a single exhaust outlet or a bifurcated exhaust (two outlets from one fan). Input in the green cells only. The calculator works for both English and Metric units but you must input both diameters in the same units.
Example: if you put mm in for both diameters then area would be mm squared and exhaust diameters would be in mm. http://www.rcgroups.com/forums/showthread.php?t=1160865 Steve 
Apr 13, 2010, 11:35 PM  

Quote:
I use 1.8" on the 60mm fan. I try to keep the thrust tube no less than 56" long on the 60mm fan. 

Latest blog entry: My column in April 2014 Model Aviation


Apr 13, 2010, 11:38 PM  

Quote:
Thanks for posting that!!! 

Latest blog entry: My column in April 2014 Model Aviation


Jun 26, 2010, 10:24 PM  

Can somebody tell me if im right:)
Hey everybody I've been looking around for someways to make a thrust tube for my T45 to see if i could get any performance increase. After looking around on rcgroups and some videos i found a great idea make it out of a paper cup. So I did I got one from subway, first i was just going to cut it the right size fan not really worrying about the exit diameter. Then I did some research on thrust tubs and found a hole bunch of info on FSA and the percentages that it needs to be for making and effective thrust tube. On my research hunt for thrust tubes i did find most of the info needed but it wasnt all in one place so I decided to make a video. The video is over how to find your fsa and the right percentage it needs to be also on me installing it into my Sapac T45.
I'm about 90% sure that all the info and math in the video is right, but if im wrong can someone please tell me and I will fix it. Don't want to tell someone the wrong thing. well enjoy
j3tman, 


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