Kelley

I am planning to run a rear exhaust engine mounted upright such that the exhaust pipe will penetrate the firewall, run inside the fuselage for approximately 10 inches and then make a 45 degree bend and exit the bottom. The pipe will be 7/8 inch diameter. This is not a tuned pipe.

What is the best method of insulating the pipe? I have thought of automobile header tape but that seems heavy. What material is best, at the least cost, for this type of insulation and where do I get the material.

Mike James

If you can work it out, I think you'll find that air is the best insulator.

Just provide an cooling air exit that's approximately twice the surface area of the intake area, and allow air to flow over the whole assembly.

I did this with a YS .61 long stroke and tuned pipe, on a prop-powered sport jet I built about 10 years ago, and it worked acceptably. I think that almost anything you would actually wrap around the exhause tube would be inferior to just having the air flowing past it.

Ed Smith

I agree with Mike, Do not wrap anything around the pipe. Some years ago I used a full tuned pipe in my racing aircraft. The 40 size engine turned 27k and the pipe got hot! The pipe was totaly enclosed and exited the fuselage ot the side above the wing. I will attempt to describe the setup.

I used what is called a pressure cowl. The air entering the cowl was ducted to the engine and forced through the fins by a tube that was a tight fit around the engine. I made a fibreglass tube that enclosed the pipe for its full length 15". The tube had about 3/16" clearance from the pipe all round. I put three strips of silicone on the pipe to hold it in the middle of the tube. The only exit from the pressure cowl was down this tube. So, all of the coling air was forced through the engine fins and over the pipe. It worked perfectly.

Pressure cowls have been around for a long time. Control Line Team Racers have been using them for decades. Even the 1930s original VW bug had ducted air over the engine.

Most pylon racers using tuned pipes now use a totaly enclosed pipe with a full length pressure cowl.

Ed S

Kelley

A pressure cowl sounds like what I need. Thanks for the explanation. What is a pressure cowl made of that will withstand all of the exhaust heat inside the fuselage.

Mike James

Ed can jump in here with details on his method, but what we're both saying is...

If you get air flowing through this thing, you won't have "all that heat inside the fuselage" to worry about. The only time it will be an issue is if you let the plane run for long periods of time on the ground. So, take care of your engine, and minimize ground idle time. (like turbine flyers do)

Ed Smith

The latest version of the whole Pressure cowl. The front part of the cowl is double skinned. The inner skin is the tight fitting shroud around the engine at the front end of the cowl. The incoming air is ducted over the head fins and through the case fins. You will notice the vertical air inlet on the front of the cowl but not on the inner shroud. The inner shroud forces the air to the sides of the case. fuel going up the front transfer passage cools the front of the engine. The idea of this system is not just to cool things down but to maintain an even engine heat throughout the engine run.

The shroud in this cowl is a fibreglass molding. You expressed a concern about the heat inside the cowl. Mike is correct, excess heat is being carried away by the airflow. My first shrouds were made from the cardboard tubes from toilet rolls. The never burned out. They were also a perfect fit over a Nelson racing engine. Somebody new what they were doing when they designed them, (The toilet rolls that is)

Ed S

Kelley

Ed/Mike,

Thank you both very much for your explanations. I now understand and this is what I will do inside the fuselage of my prop jet. I do not have any experience with fiberglass but this seems like a good project to learn on.

Educol-RCU

My Feedback: (14)

Exhaust wrap will do the job..

SCALECRAFT

My Feedback: (13)

Hi

The same concept the micro tubines use to keep 700 degree centigrade hot gases from toasting thier fuses. If you can look at one, its designed as mentioned before, to pass cool air between the "pipe" and fuse.


Steve