Exhaust Tube? To taper or not.Opinions please - RC Groups
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Jun 14, 2012, 09:29 AM
it wasn't me flying that plane
Hover or die's Avatar

Exhaust Tube? To taper or not.Opinions please

I have seen arguments for both routes..I used a straight tube on my scratch scorpion that I affixed a cone (built from a sippy cup)to at the end of the straight tube to drop the exhaust diameter..(over the last 4 to 5 inches)

I have also read that you should taper the exhaust through it's entire lenth..But have read that this increases friction along the entire lenth of the exhaust..

I need to build 2 tubes for my scratch Raptor and I'm wondering which route to take..

Any comments would be appreciated along with a little info on why you feel the way you do on this subject...
Thanks fellas for all the great info I have read and been given here at RCG..
Take care...........chris
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Jun 14, 2012, 09:53 AM
Registered User
Doug Bateman's Avatar
Just build two tapered tubes. Make them the desired diameter at the exhaust end or 85% of fan swept area. I don't agree with the taper adding more friction than a straight run.
Jun 14, 2012, 10:11 AM
5kW Club
racer1's Avatar
I am sure there is a magic formula, but I have always used a tapered cone.

Then I used for the exit, If I wanted thrust about 80% diameter of the fan, speed I went about 70% of the diameter.

Then I adjusted to suit my needs.

Use this program it sure makes thurst tubes super easy.

Jun 14, 2012, 10:28 AM
it wasn't me flying that plane
Hover or die's Avatar
You see fellas,,2 diff opinions in 2 post...both respected!!
Thanks for posting,yes I have read that it does and that it doesn't cause extra friction...

thank you for posting the link and what you choose for thrust versuses speed with fine tuning..All this info is very useful to many..

The Raptor is 62" X 44" and will probably push 10lbs(all up) so as much as I like speed I will lean towards the higher thrust exit diameter..Using 2 80mm JetFans on 6s (HET 700-68-2250)

Thanks fellas,plese keep the comments coming.........................................Chr is
Jun 14, 2012, 10:43 AM
Do it Right, the first time!
CoolerByTheLake's Avatar
Hover or die You could PM falcon 5, he went with straight tubes with taper on the ends for his T-38. He may have some opinion on this.

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