Fly a P51 Mustang?

On Fri, 06 May 2005 18:11:12 -0400, gregg wrote:

Hi Corcky,

I have the paper here, somewhere, that describes the problem and the fix -
which is more or less as you describe.

What I wonder is:

Was this fix proagated through to the Packard Merlins, for the life of
production? Or were Mustang and later Spitfire Merlins fitted with some
other solution which also solved the problem?

thanks

Here ya go Gregg, looks like this answers your question. Google is
your friend :-D I just typed in "Spitfire" and "Orifice".

The British, with their extremely droll sense of humour, called the
original fix "Miss Shilling's Orifice" because the engineer who came
up with it was a woman by the name of Miss Tilly Shilling.

*** Begin Quote***
Carburettor design

One of the great problems as discerned by pilots was the tendency for
the carburetted engine to cut out under negative 'g'. Luftwaffe pilots
learned to escape by simply pushing the nose of their aircraft down
into a dive, as their fuel- injected engines did not cut out under
these circumstances. Many authors have criticised this aspect of the
Merlin design. In reality, like most engineering, it resulted from a
design compromise- the drop in temperature developed in a carburetor
results in an increase in the density of the fuel-air mixture when
compared to that of a fuel injection system. As a consequence the
Merlin produced a higher specific power output (horse power per pound)
that the equivalent German engine. It was felt that this gave a higher
power to weight ratio for the fighter and (rightly or wrongly) that
this outweighed the disadvantages. By 1941 Miss Tilly Shilling in
Farnborough had developed a partial cure for the problem. A diaphragm
across the float chambers with a calibrated hole (the infamous "Miss
Shilling's orifice"!) allowed negative 'g' manouvres, and was fitted
as standard from March 1941. Sustained zero 'g' manouvres were not
sorted out until somewhat later. In 1942 an anti-g version of the SU
carburetor was fitted to single and two-stage Merlins. 1943 saw the
introduction of the Bendix-Stromburg carburetor which injected fuel at
5psi through a nozzle direct into the supercharger and was fitted to
the Merlins 66, 70, 76, 77, and 85. The final development was the SU
injection carburetor which injected fuel into the supercharger using a
fuel pump driven as a fuction of crankshaft speed and engine
pressures, which was fitted to the 100 series Merlins.

*** End Quote***

Corky Scott

PS, in regards the higher horsepower per weight ratio claimed above,
I'm fairly certain that this had nothing to do with using a carburetor
versus fuel injection and the fuel density charge. I believe this had
much more to do with the fact that the British and all the allied
forces were using 100 octane fuel (100 octane fuel was Jimmy
Doolittles little known contribution to victory in WWII) for all their
combat flying machines where the Germans were limited for the most
part to much lower octane fuel. This allowed the Rolls Royce
engineers to raise the boost level of the supercharger in the Merlin
engine to the point where it essentially equalled the Daimler Benz
601. This all occured right at about the time the two fleets of
fighters clashed in the Battle of Britain. This, and the installation
of three bladed constant speed propellers brought the Spitfire's
performance to basic parity with the Me 109 and boosted the
Hurricane's performance to the point where it wasn't the slug the
Germans encountered over France anymore. Good thing too as there were
roughly three times the number of Hurricanes as Spitfires and
"Hurries" downed more German aircraft than the Spitfires.