Subject: Number Fifty-One in the Series--North American P-51 Mustang (8 of 10)

From: [email-masked]
Date: 19 Feb 93 17:22:12 GMT

>From [email-masked]

The next chapter in the Mustang story involves the development of three 
experimental lightweight versions, the XP-51F, G, and J.  

In early 1943, joint discussions were held between British authorities 
and North American Aviation dealing with the subject of what the next 
generation of Mustangs should look like.  It was agreed that a 
thorough redesign would be carried out, mainly to reduce weight but 
also to simplify systems, improve maintenance, and enhance performance 
without changing the engine.  The new Mustang was to be designed to a 
combination of optimal British and American strength requirements, but 
mainly to those laid down in British Air Publication 970.  

Two prototypes were ordered under the designation XP-51F, the contract 
being amended in June of 1943 to cover the purchase of five XP-51Fs, 
all powered by Packard V-1650-3 engines.  Serial numbers were 
43-43332/43336.  The British Air Commission requested that two of 
these aircraft be given to then for evaluation.  A request by the 
Technical Command for the procurement of twenty-five service test 
P-51Fs was not authorized, since it was felt that prototype trials 
should be made before any quantity production was undertaken.  

The project was given the company designation NA-105.  Resemblance to 
the previous Mustang was only coincidental, since the structure of the 
aircraft was almost completely redesigned.  Most of the changes were 
made in an attempt to save weight.  The main landing gears were 
redesigned and the wheels and tires were greatly reduced in size.  New 
disc brakes were fitted to the wheels.  The wing was larger in area, 
having a straight-line leading edge, completely eliminating the 
familiar "kink" of the earlier Mustang versions.  The wing aerofoil 
was changed to an even newer low-drag "laminar flow" profile.  The 
inboard wing guns were deleted, the remaining four guns having 440 
rounds each.  The wing tanks were reduced in capacity to 102 US 
gallons and the fuselage tank was eliminated entirely.  The engine 
mounting was simplified, the "integral" engine cradle saving over 100 
pounds of weight and improving access to the engine.  The hydraulic 
system was simplified and increased in pressure.  The engine coolant 
and intercooler radiators were redesigned and installed in a 
completely new duct which had a vertical inlet which was placed even 
farther away from the underside of the wing.  The oil cooler was 
removed from the rear radiator group, enabling the latter to be made 
smaller and making it possible to eliminate the long and vulnerable 
oil pipes.  The oil was passed through a heat exchanger mounted on the 
front of the oil tank and next to the engine intercooler.  The flow of 
glycol carried away the heat from the oil.  The cockpit layout was 
improved, and the pilot's back armor was made integral with the seat.  
The canopy was made much larger in an effort to reduce the drag still 
further.  Control surfaces were improved, and the tail surfaces were 
made larger.  The ailerons were given a larger degree of movement, and 
the chord of the flaps and the ailerons were made equal.  Still more 
weight was saved by using a three-bladed Aeroproducts hollow-steel 
propeller.  Many minor metal parts were replaced with moulded plastic 
parts.  

Before construction began, it was agreed that the last two of the 
NA-105 airframes would be fitted with Rolls Royce Merlin 145M engines 
obtained from England under reverse Lend/Lease.  These aircraft were 
designated XP-51G and bore the serial numbers 43-43335/43336.  

Engineering inspections were held in February 1944.  The first XP-51F 
was flown by Bob Chilton on February 14, 1944.  The second and third 
XP-51F flew on May 20 and 22 of that year.  Equipped empty weight was 
about 2000 pounds less than that of the P-51D, and combat weight was 
1600 pounds less.  The engine was the Packard Merlin V-1650-7 engine 
of 1695 hp, same as the powerplant of the P-51D.  Maximum speed was 
466 mph at 29,000 feet, and an altitude of 19,500 feet could be 
reached in 4.9 minutes.  Service ceiling was 42,500 feet.  Normal 
range was 650 miles, and maximum range was 2100 miles.  Weights were 
5635 lbs. empty, 7610 lbs. normal loaded, and 9060 lbs. maximum.  
Wingspan was 37 feet 9 1/4 inches, length was 32 feet 2 3/4 inches, 
height was 8 feet 8 inches, and wing area was 233 square feet.  

Work on the conversion of the fourth and fifth NA-105 airframes as 
XP-51Gs began in January 1944, with the Merlin 145M engines arriving 
in February.  Five-bladed propellers were fitted, but the XP-51G was 
otherwise similar to the XP-51F.  The first XP-51G flew by Ed Virgin 
on August 9, 1944, the second machine following on November 14.  The 
engine was the Rolls-Royce Merlin 145M engine of 1910 hp., driving a 
Rotol propeller with five wooden blades (almost identical to the 
propellers of the Spitfire XIV).  It was readily apparent that this 
was the hottest Mustang yet-- maximum speed was 472 mph at 20,750 
feet, and an altitude of 20,000 feet could be reached in 3.4 minutes.  
Service ceiling was 45,700 feet.  Normal range was 485 miles, and 
maximum range was 1865 miles.  Weights were 5750 lbs. empty, 7265 
lbs. normal loaded, and 8885 lbs. maximum.  Wingspan was 37 feet 9 
1/4 inches, length was 32 feet 2 3/4 inches, height was 8 feet 8 
inches, and wing area was 233 square feet.  

The third XP-51F was shipped to the United Kingdom on June 20, 1944 
after preliminary flight checks.  It was painted in RAF camouflage and 
was named Mustang V.  The RAF serial number was FR409.  The A&AEE at 
Boscombe Down found the Mustang V to weigh only 7855 pounds in 
interceptor trim.  They rated it very highly except for a severe lack 
of directional stability which required frequent heavy application of 
rudder in certain flight conditions.  

The second XP-51G was shipped to the United Kingdom in February 1945.  
This plane was also named Mustang V, and bore the RAF serial number 
FR410.  It is widely reported to have achieved a speed of 495 mph 
during tests at the A&AEE at Boscombe Down in February 1945.  

Neither the XP-51F nor the G ever proceeded any further than the 
prototype stage.  The Merlin 100-series of engines had not quite 
reached the stage where they were fully ready for production.  In the 
meantime, the Packard Motor Car Company had set up its own development 
program and had come up with the V-1650-9 version of its license-built 
Merlin, capable of delivering a war emergency power of 1900 hp at 
20,000 feet with water/alcohol injection.  Packard said that they 
could deliver this new engine starting in late 1944.  Consequently, this 
engine was chosen to power the next production Mustang, which was 
designated P-51H.  Neither the P-51F nor the G were developed any 
further, although the work on these two airplanes was invaluable in 
the development of the P-51H.  

The last prototype in the lightweight NA-105 series was the XP-51J, 
which was similar to the F and G models except that it reintroduced 
the Allison V-1710 engine to bring the Mustang full circle.  The 
Allison engine was, however, the V-1710-119 version with a two stage, 
gear-driven supercharger, rated at 1500 hp for takeoff and 1720 hp 
with water injection at 20,700 feet.  Unlike earlier Allisons, this 
engine had an updraft carburetor.  The nose geometry was substantially 
modified, and all air inlets in the nose were completely eliminated.  
Instead, the carburetor air was taken in through a ram inlet at the 
front of the radiator duct and piped to the engine.  

Two XP-51J prototypes were ordered, with serial numbers being 44-76027 
and 44-76028.  44-76027 made its first flight on April 23, 1945, 
piloted by Joe Barton.  The XP-51J weighed 6030 pounds empty and 7550 
pounds normal loaded.  It was anticipated that a maximum speed of 491 
mph could be achieved at an altitude of 27,400 feet, but this was 
never achieved during tests because the new Allison had not yet been 
cleared for full power operations.  XP-51J Ser No 44-76027 was, in 
fact, loaned to Allison so that they could use it to iron out the bugs 
in their engine.  The other XP-51J prototype, Ser No 44-76028, was 
never actually flown, but was scavenged for spare parts to keep the 
other example flying.  The end of the war in the Pacific brought all 
further work on the XP-51J to an end.  

Sources:

  American Combat Planes, Ray Wagner, Third Enlarged Edition, Doubleday, 
  1982.  

  The American Fighter, Enzo Angelucci and Peter Bowers, Orion Books, 
  1987.  

  War Planes of the Second World War, Fighters, Volume Four, William 
  Green, Doubleday 1964.  

  United States Military Aircraft since 1909, Gordon Swanborough and 
  Peter M. Bowers, Smithsonian, 1989.  

  Fighting Mustang: The Chronicle of the P-51, William N.  Hess, 
  Doubleday, 1970.  

  Classic Warplanes: North American P-51 Mustang, Bill Gunston, Gallery 
  Books, 1990.  

  Famous Fighters of the Second World War, Volume I, William Green, 
  1967.  

  British Military Aircraft Serials, 1912-1969, Bruce Robertson, Ian 
  Allen, 1969.  

Joe Baugher       AT&T Bell Laboratories    2000 North Naperville Road 
		  Naperville, Illinois 60566-7033.   (708) 713-4548