In a museum today I took a close look at one of those big General
Electric aircraft turbochargers, a type B-2-LY according to a sign at
the exhibit. The turbine portion is much different from any modern
turbo I've seen. Exhaust gas from the engine enters a toroidal plenum
through a port in its circumference. A second port 180 degrees away
goes to a butterfly valve in a short duct leading to the atmosphere.
This is the waste gate.

The side of the plenum that faces away from the compressor has a
circle of nozzles to direct the gas against the single stage axial
flow turbine wheel, which is on the exterior of the turbocharger! Of
course this requires the turbine to be flush with the aircraft
surface.

A small cooling air duct crosses the turbine's exposed face, like a
bridge. It connects to a stationary circular "cooling cap" which
covers the center of the turbine wheel. Air flows through the duct,
into the cooling cap, then exits through a narrow gap (about 1 or 2
mm) between the cooling cap and turbine disk, near the roots of the
blades.

The air duct continues across the turbine wheel and ends in an
attachment to the exhaust plenum. Apparently this is simply to give
support; the pipe comes to a dead end here.

In photos of a P-38 upper nacelle it's possible to see all this. The
cooling cap air duct is quite visible running fore and aft above the
turbine. Some photos show the turbine blades too. They appear as a
close-spaced pattern of radial lines, almost like a grille.

I've seen the same thing in pictures of B-24s. But later bombers used
a different turbine exhaust configuration. In the B-36 (B-29 & B-50 as
well, I think) the turbines were enclosed.

--

Paul Hirose
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