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.

Originally Posted by SMorrisRC

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.

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

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.

Beat me to it Rich!

don't you have a plane you shoudl be building, or something? hahahahaha.

Ok, makes sense to me. I will go with the 85% since I want the most on top end.

good choice.

Originally Posted by LuvEvolution7

don't you have a plane you shoudl be building, or something? hahahahaha.

I'm on holiday!

pfffffft. excuses. LOL.