


Question
Fan Sweep Area question...
I have heard many different percentages of exhaust diameter to Fan Sweep Area . My question is this...
Do you take the FSA and take the percentage and work forward OR.... Do you take the the FSA, work forward and take the percentage of the final number....?? I am trying to come up with an exhaust tube diameter for the 65mm fan in my Sapac Eurofighter. My fan sweep area is 3002.62. If I then devide that by 3.14 it gives me 956.25. The square root of this gives me 30.92 , the radius, which then yields for all intents a 62mm diameter cone. Going %85 gives me a 53mm outlet. If I take the 3002.62 sweep area and take the %85 , this gives me 2552.23. Dividing that by 3.14 gives me 812.812, the square root which gives me 29mm the radius, or 58mm diameter. 2 different ways giving me 2 different answers. 




The second way is the correct way.




My vote is you want to take 85% of the FSA and then derive the radius, not 85% of the derived radius. If you take 85% of the radius and than calculate the area you are essentially squaring the 85% reduction and reducing the area by 72.25%.
Paul 



Thanks guys !!
58mm it is ! 



The first way you're taking 85% of some imaginary radius derived from the fan swept area. The second way you're getting a radius that will give 85% of the fan swept area.
As others have said, the second way is what you want. Dan 
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