On only has to look up the thermal conductivity tables to see that if you
need to resist incoming radiation/convection, rubber/plastic etc. is a
non-starter compared with metals.
Take an area or 10cm x 1cm (typical of the area exposed to the exhaust pipe)
and a wall thickness of 1mm, a steel pipe would develop only 14 deg.C across
its wall for every incident kW. As 1kW is an overestimate of what really
arrives (we only have 60kw max. from engine and most of that is either
defeating drag or or going out of the radiatior) there is never going to be
any thermal problem (apart from some local rise in oil temperature before it
reaches the oil cooler and goes the way of all heat ).
By contrast rubbers are 500 times worse and will fry under such intensity.
Large wall thickness too. Copper is 5.5 times, Al. 3.3 times better than
steel.
There is no need for elaborate thermal calculations of the arrival energy at
the pipe.
A thermocouple (or simpler, blobs of Thermal paints) can give advance warning
of such problems.
Since reporting in-flight failure of the original system in April 99 (the
first ?) I have run with a metal pipe between rubber end pieces at the cool
ends, with no deterioration.
Graham C. G-EMIN
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