"Stephen Harding" wrote in message
...
Time to do my share to improve the S/N ratio here at r.a.m,
or at least make a try at it!

It's easy to overlook. No exhaust pipes visible protruding
from the sheet metal around the Allison engines on the P-38.

Given that Lookheed, and Kelly Johnson in particular, paid
a lot of detailed attention in the design and execution of
the aircraft, I'm wondering why they never made an effort
to use exhaust thrust to squeeze a few extra hp/mph out of
the engines for this aircraft.

Much of the energy of the exhaust would already have been extracted by
the turbo-supercharger but I expect there was still some residual
thrust in it.

The thrust of a Merlin was I believe around 300lbs. (about 140kg or
1400N). At 440 mph (200m/s) that would be equivalent to 280kw or
400hp more at the shaft if prop efficiency of around 80% was allowed
for.

Turbochargers probably make more sense for slower moving aircraft.

Kelly Johnson was not all that happy with the GE trubosuperchargers on
the Alison because they had been specified by the airforce rathern
than properly integrated by Lockheed.

Aircraft like the P39 and Allison P51 did not perform well at altitude
because they did not have the turbosupercharged Allison only a single
stage super charger. Apparently the USAAF had neglected super-charger
development in favour of turbo-charger development.

The Merlin with a two stage intercooled supercharger and jet exhaust
nozzles could do the job almost as good without a turbo.

The P39 was tested with a turbo, I don't know if the P51 ever was.
There was a turbo-compounded Allison as well in which the turbo drove
the main engine shaft via a hydraulic trogue converter instead of a
supercharger.

It seems that for raw material reasons the special alloys needed were
reserved mainly for 4 engined bombers with the exception being P47s
and P38s.

The Germans because they had lower octane fuels had to use larger
lower boosted engines and instead relied on variable speed single
stage superchargers. Their raw material problemes were quite severe
even though they had by 1938 BMW turbo chargers that were reliable and
could cope with 850C temperatures.
The only turbo-supercharged engine to enter service appears to have
been the BMW801T which made it into a small number of high altitude
reconaiseance Ju 388's (there was a night fighter version as well
designed to interecpet high altitude B29s should they begin night time
bombing)

The Ta 152 did have a two stage intercooled supercharger on its
version of the Jumo 213 engine.






The Spitfire in particular, but I believe the P-51 as well,
all made use of exhaust thrust to gain a bit more speed. Given
the P-38 was designed in 1937-ish and didn't appear in person
until about 1939, perhaps the advantage of using thrust to
boost speed a little wasn't really thought of? Perhaps the
Allison at the time of design just didn't have the oomph for it?

Also, exactly where does the P-38 engine exhaust exit the
engine? Seems to be a big, upward facing pipe in the vicinity
of the turbocharger, but never really knew if that was turbo
intake or an exhaust of some sort.

Upward directed exhaust doesn't seem a good choice.

It probably kept exhaust duck back pressure low and got rid of a
draggy exhaust nozzle.




SMH