ORIGINAL: Miniflyer
First as a rule: effective thrust is created at the exit point of the exhaust gases. If a turbine pushes 50lbs, and looses 10lbs through a pipe (exagerated), the turbine in front will most likely still be producing around 50lbs. The 10lbs can be found along the path of the pipe, pulling the other way.... (trying to paint a very very rough picture: the turbine pulls forward with 50lbs and the pipe pulls aft 10lbs, thus pushing the airframe 40lbs in total).
In practice, the tail pipes actually change the design of the engines !!!
Say an engine is running at 30 lbs at 650C.. Install this engine in a plane and it puts out 27 lbs and runs at 600C.
The reason it only has 27 lbs is not because of 3 lbs of skin friction of the pipe. Its because the pipe has caused a 50deg
temp drop. Has very littel to do with skin friction or tail pipe velocity. I have mounds of test data that show this.
Eddie Weeks