P-51 variants

The North American Aviation P-51 Mustang was an American long-range single-seat fighter aircraft that entered service with Allied air forces in the middle years of World War II.

This article is a guide to some of the basic dimensional data, performance characteristics and armament of the P-51 series, using representative figures gathered from reliable sources.

P-51 Mustang Dimensions, performance and armament

The basic dimensions of the P-51/A-36 variants remained consistent from the XP-51 through to the experimental "lightweight" P-51s, although there were relatively minor variations in height and wing areas. The P-51H was the first production version of the Mustang to feature a lengthened fuselage and taller tailfin.

Power ratings

The power rating for the engines could vary according to the type of supercharger fitted, the size of the compressor impeller(s) and the gear speeds selected.

In the case of the Packard Merlin V-1650s, used from the P-51B on, two sets of power ratings can be quoted because these engines were fitted with two-stage, two-speed superchargers; the maximum power generated by the V-1650-3 was 1,490 hp (1,111 kW) at 13,750 ft ("critical altitude") (4,191 m) in "low blower" using convert|61|in|mm of Mercury "boost"; i.e. 61" Hg.Gruenhagen 1980, p. 183.] The "boost" is the pressure to which the air–fuel mixture is compressed before being fed through to the engine's cylinders (manifold pressure). Because air pressure and air density decrease with altitude the efficiency of a piston engine drops because of the reduction in the amount of air that can be drawn into the engine; for example the air density at convert|30000|ft|m|abbr=on is 1/3 of that at sea level, thus only 1/3 of the amount of air can be drawn into the cylinder and only 1/3 of the fuel can be burnt.

In the case of the Mustang the air being forced through the supercharger air intake was first compressed by the supercharger's first stage, or "low blower". A Bendix-Stromberg pressure carburettor injected fuel at 5 psi through a nozzle direct into the supercharger and the compressed air/fuel mixture was then directed to the cylinders. An intercooler was provided to stop the compressed mixture from becoming too hot and either igniting before reaching the cylinders, or creating a condition known as knocking.

Once the aircraft reached and climbed through the "critical altitude" the power would start to drop as the atmospheric pressure dropped; a second stage compressor was needed to allow the engine to continue to perform at higher altitudes. When the aircraft reached a certain altitude (20,000 ft (6,096 m) for the -3) the second stage, or "high blower" of the supercharger was started; the air/fuel mixture coming through the "low blower" then flowed through the second stage to be compressed again. [Note: The second stage starting was often accompanied by a noticable jolt, which inexperienced pilots often mistook for some type of engine malfunction.] This second stage blower required approximately 200 hp (149 kW) to drive it. As a result the maximum power generated by the V-1650-3 in "high blower" was 1,210 hp (902 kW) at 25,800 ft (7,863 m), using 61" Hg. [Smallwood 1995, pp. 132–136.]

The V-1650-7 used in later P-51B/Cs and in the P-51D/K produced slightly more power but, because of the use of slightly different gear ratios for the impellers, the critical altitude ratings of the supercharger stages were lower, 8,500 ft (2,590 m) and 21,400 ft (6,523 m) respectively. Experience showed that most air-to-air combat over Europe was taking place between these altitudes. [Gruenhagen 1980, p. 91.]

The power of the engine could also change according to the Octane rating of the aviation fuel being used. Higher octane fuels allowed boost pressures to be increased without the risks of pre-ignition or knocking.

The British and Australian air forces measured boost pressure using the formula of lbs/sq.inch (or psi). The normal atmospheric pressure at sea level is 14.5 psi, so a reading of +6 means that the air/fuel mix is being compressed by a supercharger blower to 20.5 psi before entering the engine; +25 means that the air/fuel mix is now being compressed to 39.5 psi. One pound of boost equals convert|2.04|in|mm of Mercury (Hg).

References

Notes

Bibliography

* Anderson, Peter N. "Mustangs of the RAAF and RNZAF". Sydney, Australia: A.H & A.W Reed PTY Ltd, 1975. ISBN 0-589-07130-0.
* Gruenhagen, Robert W. "Mustang: The Story of the P-51 Fighter (rev. ed.)". New York: Arco Publishing Company, Inc., 1980. ISBN 0-6680-4884-0.
* Knaack, Marcelle Size. "Encyclopedia of U.S. Air Force Aircraft and Missile Systems: Volume 1 Post-World War II Fighters 1945-1973". Washington, DC: Office of Air Force History, 1978. ISBN 0-912799-59-5.
* Smallwood, Hugh. "Spitfire in Blue". London: Osprey Aerospace, 1996. ISBN 1-85532-615-9.