another pet peeve - Printable Version

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another pet peeve - .041 Safety Wire - 06.08.2010

I've always questioned every FM tuners tendency to give the RR powered British fighters the negative g cutout. I will agree that those from 39 to 41 had it but after that ?. Here is a great little write-up on the introduction of the SU and Stromberg carbuerators to the Merlins in 42 and the use of a float chamber diaphram to reduce this problem in 41. Both the SU and Bendix carbs were pressure carbs that were not affected by 0 g manuvers.


One of the great problems as discerned by pilots was the tendency for the carbureted 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 criticized 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' maneuvers, and was fitted as standard from March 1941. Sustained zero 'g' maneuvers 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 function of crankshaft speed and engine pressures, which was fitted to the 100 series Merlins.

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