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After about 4 hrs on N121J,
it's clear that it needs exhaust modifications. The straight exhaust tubes
supplied by Lancair come out too close to the skin. The exhaust gas stream
appears to be flowing along the skin for about 3 or 4 feet before mixing with
enough of the airstream to cool it. If I keep flying with it as is, the
paint will start to blister and I'm afraid I might even see
delamination.
The factory suggested
simply opening up the rear of the cowl exhaust tunnels and deflecting the
exhaust pipes downward. Bad idea, since the ball joints won't accommodate
that much bend without leaking. For some reason, the ball joints were
pointed straight back, more or less level, instead of angling downward toward
the exhaust tunnels.
The short exhaust tube
sections can't be bent -- they don't fit into the usual benders for auto
exhaust pipes, and in any case, the thin-walled stainless they're made of would
collapse unless a mandrel bender is used. One solution would be to have an
aircraft exhaust company make new ones, which would undoubtedly take quite a
while and be expensive. What I've done instead is to order a U-bend piece
from Borla (www.borla.com) that consists
of a 6-inch radius, 180-degree section, with 8" legs, all in 2-1/2" diameter
T-304 stainless (Borla part number 19250). I'll bevel the ends of the
legs, then cut so that I have a 20-degree curve section on the other end.
I've cut the exhaust pipes off 10" from their ends, and found a gent locally who
can weld the thin-wall stainless.
This ought to bring the
open ends of the exhaust pipes several inches lower and farther from the
skin. Perhaps newer exhausts supplied by Lancair are different, but if
yours are like mine, think about getting the exit ends well away from the belly
of the airplane. With any luck, the noise level inside might even be
reduced a little.
Jim Cameron
Legacy N121J
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