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pipe gap

Old 05-16-2006, 10:23 AM
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Default pipe gap

What gap should I leave between the outer tailcone of a J600R, and the bellmouth of a Wren jet pipe?
Old 05-17-2006, 03:16 AM
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Default RE: pipe gap

I don't know that combination myself, but its likely going t be somewhere in the 15-20mm range, based on the spacing people typically use in the panther with the J600R and PST pipe. Optimisation would require mounting the engine and pipe on a thrust measuring test bench with readings taken at intervals of a few millimeters.

Sorry I can't be more exact.

Old 05-17-2006, 09:24 PM
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Default RE: pipe gap

Hi Kelly,
John and I tested the J600R in the Starfire II yesterday and I used 1/4 to 3/8" according to Dean. It was apparently pretty good. I imagine there was some thrust loss in the 27" tube, but that is to be expected with a tube that long. However; there was no heat escaping with the thermal probe half way between the tail cone and the pipe mouth all the way around the mouth. In fact it was cooler a tad with the airflow being sucked down the pipe. The supreme test as far as I am concerned is when you put the hatch on. It should not suck down hard, which means there is not enough incoming air and you will suck some heat back, nor should it ever blow out as that means there is already heat escaping from the tail pipe into the fuselage. The thermal probe was cold even at the corrugations around the tail of the pipe showing that cool air is being sucked through with the rapid speed of the exhausting airflow from the inner tube of the pipe. Of course the fuselage was cool to the touch too.

Anyway Dean's range of 0.25 to 0.375" (6.5 to 10 mm) seems to be a pretty good starting position on this plane.

Old 05-18-2006, 03:00 AM
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Default RE: pipe gap

Thanks for the info folks. The instructions with the Wren thrust augmenting pipe give dimensions for two types of Wren exhaust cones, the original long cone with no inner cone at 10mm, and the newer short one with an inner cone at 30mm from the short outer cone. The J600R is a mix of both those types, a long outer cone and an inner cone. Thinking that it is the outer cone that matters, I have decided on a gap of 10mm. Len's experience suggests this will not cause problems. The Wren thrust augmenting pipes have a small intake and I guess a greater gap from the engine exhaust is more likely to allow hot expanding gas to spill over the outer edge of the bellmouth.

Old 05-27-2006, 11:47 AM
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Default RE: pipe gap

I have flown a PST 600R...the two piece tailcone on a gary Mueller made Wren pipe for two years with the gap at 10mm from the outer cone without a problem.
I did some bench testing with differing gaps, and any differences from 10-30mm was not measurable.


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