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Date: Wed, 16 Apr 2008 14:23:00 -0400
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Subject: Re: [LML] carbon-carbon composites
From: "bob mackey" <n103md@yahoo.com>
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Don asks several questions:
> Bob,
>    Do you have a solution to a problem where the exhaust gases
> exit the cowling area and have "charred" the epoxy airframe?
> I've seen this on a couple of LN-2's I've worked on, some others
> only just have discoloration of the paint in this area.
>
>   Obviously, a solution was to move the exhaust pipes down away
> from the fuselage, by either cutting and rewelding the tips, or
> shimming the system.
>   Would a Carbon/Carbon or other high end "patch" etc be effective??
> Would appreciate your comments.  Don Skeele   A&P

There are several ways to address the problem of charred skin
on the bottom of an LNC2. But first, let's take a look at exactly
what the problem is. The paint and epoxy become brittle, lose
adhesion, or bubble up because their temperature gets too high.
The temperature on that surface is determined by a combination of
the temperature and flow rate of the gases outside, and the thermal
conductivity in the materials. If the bottom skin were made of
aluminum, the heat load would be the same, but skin would conduct the
heat away rapidly and not get so hot. If the exhaust temperature
weren't so hot, it would not deliver so much heat to the skin.
If the materials were tougher, they could take they heat...

So there are at least three ways to beat that problem.
1) increase the thermal conductivity to carry the heat away from the
hot spot.
2) decrease the gas temperature blowing on the skin.
3) change the materials to something that can take the high temps.

Carbon-carbon composites could certainly take the high temps (3),
and also is much more thermally conductive than the foam-core
fiberglass sandwich panel (1).  Other resins such as bismaleimide
can take higher temps than laminating resins used by Lancair.

The foam core sandwich panel is quite a good insulator, so the thin
outer fiberglass skin might get almost as hot as the exhaust blowing
over it. Almost anything would conduct heat away from the surface better
than foam core.

Bob Belshe extended the tips of the headers slightly to deliver the
hot exhaust further from the surface (2).
http://mail.lancair.net:81/lists/lml/Message/46511.html

Jim MacKnight replaced the skin in that area with 0.090" stainless
steel (3). http://mail.lancair.net:81/lists/lml/Message/46527.html
Maybe Jim also included some insulation behind the stainless to slow
the heatflow into the fiberglass.

Another option would be to cool the exhaust gases where they
flow over the belly by using an augmenter and to accelerate
lower cowl cooling air. This may or may not produce any thrust,
but it should reduce the drag relative to pointing the exhaust
stacks away from the belly. High velocity exhaust leaving the aircraft
at right angles to the airflow is like a 3" pipe sticking out
and creating drag.

The conventional solution of a thin stainless steel skin with a bit
of insulation between it and the fiberglass skin is simple and effective.
The main reason to switch to c-c composite would be weight reduction,
or just maybe because you like high-tech black and would rather burn
money than paint.

-bob mackey