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On another note I was just going back and doing some calculations to work out thrust tube exit diameter for a Wemo 90mm rotor. To get 85% it has to be 65mm and to get 80% it has to be 63mm exit diameter. Now to me that seems a little on the small side but using the below formula it's what I get, anyone care to comment???
ok I don't know how to do the math symbolys on my keyboard so I'll just write it instead: 2= squared symbol FSA= sqr root (fan radius x pi 2)  (spinner radius x pi 2) X x 80% / pi So for a Wemo 90mm rotor it looks something like this: FSA = sqr root (44.5mm x pi 2)  (20mm x pi 2) X x 0.8 / pi FAS = sqr root (139.819 2)  (62.84 2) X x 0.8 / pi FSA = sqr root (19549.352)  (3948.866) X x 0.8 / pi FSA = sqr root (15600.486 x 0.8) / pi FSA = sqr root (12480.388 / pi) FSA = sqr root of 3972.1158 63mm diameter exit for 80% OK maths class is over, but anyone care to see if I've done something wrong? just seems alot to choke it down to IMO from memory the thrust tubes you can buy from Wemotec are 75mm in diameter which according to the above formula would make them over 100% of the FSA. 



I think you only use the actual swept area of the blades so you have to minus the area of the spinner in the centre. Is that right?



Joined Jul 2007
9,089 Posts




Joined Jul 2007
9,089 Posts




Australia, VIC, Sale
Joined May 2007
3,593 Posts

jb, area of a circle is pi * radius squared, not radius times pi^2...
EDIT: My numbers... od 89 id 40 or 44.5 ir 20 oa 6221.138852 ia 1256.637061 fsa 4964.501791 80% fsa 3971.601433 r 35.55559028 d 71.11118056 72mm makes more sense; closer to the premade ones. Disturbingly, this also happens to be within a bulls roar of 80% diameter as S3 said. As my math teacher always said (and I hated)  correct answer, wrong working out. 1/10. Edit 2: Here's a quick and dirty excel spreadsheet. Just change the yellow cells. The rest are formulae. If you have q's about the meanings or working out, let me know. Feed this data in to Stumax's thrust/efflux calculator, and you'll look like an aeroeng in no time! 



Tomorrow morning is looking the goods.
@ Nitro can you please bring that Rx. I'll bring the little motor that got fried. 


Australia, SA, Evanston Park
Joined Mar 2010
5,080 Posts





Thanks Ody, so my working out was correct just formula was wrong cheers for the clarification SAWEEET!
Yeah Ozzi you gotta account for and remove the area of the spinner, so just blades only. Thanks for all the input guys...even you Zoot . Tomorrow does definitely look doable, reckon I might bring the bungee down in case the grass is long. 



Looks like I might have found a reason to buy a smartphone.

