Someone I know is building a big MB339. This particular jet tends to be tail heavy. The turbine is to be moved forward 6 inches thus increasing the tailpipe length 6 inches. I don't know how long the stock tail pipe is, but it looks to be pretty long already. The question is: In a fully ducted jet, will increasing the pipe length decrease thrust? Let the wisemen speak up.

I may not be considered wise on this, but maybe a Wren type of pipe could be the solution. It has a convergent section, and a straight section.
The straight section could per my instructions be made very long without much thrust loss.

Lars

Lars, we were thinking along the same lines in that making the pipe converge at the tail end would help in decreasing loss of thrust. I will relay your information. Making it converge to a certain point and then take it straight to the end sounds like a good idea. Thanks. Alex.

Guys, the Wren pipe doesn't converge, it diverges. From the bellmouth back a certain distance the pipe diameter increases. Then from that point back it's a constant diameter. Most other pipes converge slightly throughout their length. I have also seen some that are a constant diameter.

To answer your question whenever you flow a liquid or gas through a pipe the resistance increases with length. How much it increases is dependant on a number of factors. The most critical factor is pipe diameter. In this case as long as your pipe diameter is sized large enough for that engine and you are not making the exit diameter any smaller I doubt that you would detect any noticeable difference in thrust.

For example, as long as your pipe is of sufficient diameter that it isn't starting to choke off the flow and you replaced a 24" long pipe with one 30" long, keeping the same inlet and outlet diameters, then any change in resistance or thrust loss would be neglegible. If you have any doubt you could go with a slightly larger diameter for the longer pipe. I'm sure that if you contacted any of the custom pipe makers in the hobby that they could advise you accordingly.

Joe

Sorry, my screwup. Of course it diverges, as in tapers out from the exhaust nozzle.
I don't know the physics behind this, but I have a feeling that you trade off some of the excessive exhaust velocity for static thrust.

Lars