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Old Jul 22, 2011, 07:10 AM
You win again gravity!
SMorrisRC's Avatar
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It will make a difference, you want a gradual reduction to the exit. I think you'll be more limited by cg requirements, so just have the tube as long as will fit into the plane. I think any differences between that and optimum will be quite small.
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Old Jul 22, 2011, 08:38 AM
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Quote:
Originally Posted by SMorrisRC View Post
It will make a difference, you want a gradual reduction to the exit. I think you'll be more limited by cg requirements, so just have the tube as long as will fit into the plane. I think any differences between that and optimum will be quite small.
Ok, I'm putting it on a 70mm HET setup and it will have about an 8 inch long thrust tube. Do these thrust tubes improve top end only or overall performance? Thanks for your help, greatly appreciated.
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Old Jul 22, 2011, 08:55 AM
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well, that's a good question. a thrust tube can do both for you. it deffinitely increases top end if you use a restricted opening of anythig less than 100% FSA. 85-90% is a good compromise. it also increases performance by focusing the efflux column. take for example a profile F-22 that a buddy of mine has. it's basically a profile jet, with a 64mm fan right in the center of the airframe. the performance was marginal at best. he asked me why and I told him to put a thrust tube on there. he tried one at 100% FSA and the performance was much better. then he gradually decreased the orrifice to about 88% and it flew even better. so the answer to your question is "both".

Rich
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Old Jul 22, 2011, 08:57 AM
You win again gravity!
SMorrisRC's Avatar
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In theory, 100% FSA exit gives maximum static thrust. As you reduce the exit diameter from there, static thrust drops off, but efflux velocity (hence, top end speed) increases. The trick is to find the balance that works for you. The typical exit diameter as a good balance is around 80%-85% FSA.
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Old Jul 22, 2011, 08:57 AM
You win again gravity!
SMorrisRC's Avatar
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Beat me to it Rich!
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Old Jul 22, 2011, 09:02 AM
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don't you have a plane you shoudl be building, or something? hahahahaha.
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Old Jul 22, 2011, 09:03 AM
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Ok, makes sense to me. I will go with the 85% since I want the most on top end.
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Old Jul 22, 2011, 09:16 AM
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good choice.
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Old Jul 22, 2011, 09:26 AM
You win again gravity!
SMorrisRC's Avatar
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Originally Posted by LuvEvolution7 View Post
don't you have a plane you shoudl be building, or something? hahahahaha.

I'm on holiday!
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Old Jul 22, 2011, 09:35 AM
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pfffffft. excuses. LOL.
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Old Jun 15, 2014, 11:55 AM
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Quote:
Originally Posted by SMorrisRC View Post
FSA is as easy as PI*r^2 of the fan (so, for a 70mm fan, r would be 35) minus PI*r^2 of the motor (70mm fan is usually around 28mm motor, so r would be 14).
The exhaust diameter is then usually around 85% FSA as a rule of thumb for general performance. (so take FSA *0.85, divide by PI, square root it to get r).
Simples!
First off, thanks for making it simple

I was wondering, is it okay if the intake area is more than 100%? Im building a twin edf Avro Arrow and i chose to make the airplane bigger it should be. The intake came out to be 245% :O

Will this affect the performance in anyway?
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Old Jun 15, 2014, 12:26 PM
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Build thread link- http://www.rcgroups.com/forums/showthread.php?t=2184842 , and yes the intake size that large would be a problem.. for any fan. These are only 50mm fans.
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