The Wren pipe sizes were developed from testing that I carried out when the prototype 44 was first made. It was not an easy engine to start and I found it was not possible to start the engine if the pipe was too small. I did exactly what Henke did with his tests and mounted an engine on a test bench with its tail pipe and a thrust scale. I found that the pipe made with the divergent section leading to a straight section gave the best results which was a small gain. I tested with a 54 and a 66 size engine and all the results were published in RCJI. Before I did this work pipes would usually lead to a substantial loss of power. Very few people have been prepared to do the basic research work which has always surprised me. I did find the results were scalable and were applied to a twin P200 powered plane with good results.

I did play about a bit with a pipe that was divergent and convergent sections and was secured to the back of the engine. It had a very small outlet and was intended for a Me163. I also did work on bifurcated pipes which are often badly designed and loose power and make a fearful noise. All this was a long time ago now.

It is good to see that at least one person is prepared to do basic testing, well done Henke.

John