[quote]ORIGINAL: BruceTharpe

I made a tunnel from three slabs of sheet steel, joined at the corners with angle iron. Weight of the hot box is 15, maybe 20 pounds. I've felt it after starts, and it's definitely warm. I thought about using a stove pipe, but it would either need legs or somebody holding it during startup. In the end, I thought the heavier tunnel was the best way to go because it could just sit there with no concern about being blown away. If you use a pipe, I suggest using as large a diameter as practical, and make it heavy enough that it can just sit there during startup.
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Bruce, and all:
I have flown *another* designers airplane with the similar engine configuration using a 6" dryer duct....single walled...on a 2X4 base for over 100 flights (and a compadre's for 10 more) with out a problem of heating the bottom, or blowing it down the runway.
(Common sense does need apply re: wet starts). Weight is less than a pond. Does it get hot? You bet! so just don't touch the metal as you move it away from the airplane to taxi. My unsolicited 2 cents is *KISS* does apply.

Greg