Correct tail pipe diameter for a P 120 engine
 

A friend of mine has a Pilot Dolphin airplane with stock tailpipe. I am installing a P 120 engine in it for him and got looking at the diameter of the tailpipe and was wondering if it is large enough to prevent unwanted thrust loss. It measures 2 7/8". Pilots advertising indicates that a 140 or 160 sized engine is said to be ideal and this is the only pipe choice so it should be fine with a 120.


Also wondering what is considered to be the ideal pipe to engine spacing. This tailpipe has a large bell mouth (approx 5") at the opening tapering down to the dimension mentioned above (2 7/8"). I currently have the engine tenativly mounted with aft end of engine tail cone dead even with the begging of the bell. I am guessing that some where around half way in to the bell would be about right or ideal.

From what I have seen from my Jet Guys who have models with pipes, there is typically about 3/4" between the tail cone and the beginning of the pipe's mouth. I have helped resolve starting problems when the tail pipe is too close to the end of the turbine's tail cone (i.e. turbine would not start w/o flames, popping and rumbling. Flames and popping/rumbling stopped after increasing distance between mouth of pipe and tail cone.) As I understand it, air needs to flow into the pipe thru the space between the cone and the mouth of the pipe. We are not just tunneling the turbine exhaust.

I'm not certain about the diameter of a pipe for a 120 but I do know that the distance between the end of a tail cone to the beginning of the pipe where the large bell mouth ends needs to be between 3/4" to 5/8". So the turbine tail cone needs to go into that large bell mouth until the said distance is achieved where the actual pipe begins.

Here's JetCat's catalog info...
http://www.rcuniverse.com/forum/atta...mentid=2187423