XP-51 Mustang

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North America XP-51 Mustang

Two P-51s that had been built be North American for their British order were diverted to the Army for evaluation as XP-51 aircraft serial numbers 41-38 and 41-39. Ship 41-38 came to Langley for evaluations by NACA in 1941, followed by 41-39 in 1943. The first airplane was finished with aluminum paint overall (not polished) and tail stripes. It is the airplane in the REAA Museum. The second XP-51 was finished in typical Army olive drab upper surfaces and neutral gray lower surfaces.


Contents

[top] Photo History

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12/29/1941

This is the North American XP-51 Mustang shortly after it arrived for NACA trials in December 1941. Built as the fourth Mustang, it was diverted from RAF production for U. S. Army use. The prewar polished metal finish would soon be covered in camouflage paint. Extensive flight tests were made on this aircraft at Langley. In the past few years, this particular aircraft has been restored to flight status by the Experimental Aircraft Association, and may be seen at their museum at Oshkosh, Wisconsin.
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09/04/1943

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01/22/1944

The North American XP-51 Mustang was the first aircraft to incorporate an NACA laminar-flow airfoil. This is the second XP-51, which arrived at Langley in March 1943.
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09/23/1943

This photograph of the P-51B is from Test 149 in the Full Scale Tunnel.
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07/24/1945

July 1945 photo is of the P-51B (lacks the bubble canopy of later models) outfitted for gloved wing airfoil research for profile drag.
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Semispan delta-wing model mounted on the upper surface of the wing of a Langley P-51D for transonic tests.
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Flight tests of the P-51H (Jack Reeder pilot). Note longer fuselage and taller vertical tail. A flow-survey rake is mounted on the forward fuselage behind the propeller. This version of the P-51 used a second-generation 66-series laminar-flow airfoil.
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XP-51 with wing-mounted cannon in flight tests at Langley.

[top] NACA Contributions

[top] Development of the Laminar Flow Airfoil

Laminar-flow airfoil model. Note that the thickest part of the airfoil is toward the rear of the airfoil (as far as 60% back from the leading edge) in contrast to conventional airfoils where the maximum thickness is closer to the leading edge.

By far the most important contribution made by Langley to the P-51 series was the airfoil used by North American for the wing. In 1939 the NACA researchers at the Variable Density Tunnel had developed a handbook of over 100 airfoils for industry’s use in designing wings. As that effort came to a very successful conclusion (many of the airfoils were still used as of 2011) two events occurred that would shape Langley’s most important contribution to the P-51.

The first event was the beginning of a revolutionary research program within that same Langley organization on the development of so-called laminar-flow airfoils which promised an unprecedented reduction in aerodynamic drag. By carefully shaping the airfoil in an unorthodox manner, researchers were able to demonstrate its remarkable benefits in the wind tunnel with highly-polished models.

Typical test setup for airfoil tests in tunnel. View is looking downstream in tunnel. Model is a wing section with a pusher propeller installed.

The second event was the introduction of the Langley Low Turbulence Tunnel which mitigated turbulence in its airstream to be more representative of the natural atmosphere. This capability was mandatory for testing the new airfoils. The new laminar flow airfoils were first tested there in 1939.

In 1939 the NACA performed many laboratory-type wind-tunnel experiments with smooth, polished airfoils of a new design called “laminar-flow airfoils.” Under these laboratory conditions it was found that very smooth laminar-flow airfoils exhibited markedly reduced drag because the boundary-layer flow (thin layer adjacent to the airfoil) over the wing remained smooth (laminar) without drag-producing turbulence. In addition it was found that the shape of the airfoil (not dependent on surface smoothness) delayed the onset of compressibility and shock waves that caused tremendous increases in drag.

When designers from North American were informed of the Langley results, they made the decision to include the airfoil on the P-51.

Although many writers have mistakenly referred to the laminar flow produced by the airfoil as the reason for the P-51’s performance in WWII that is not, in fact, the reason. It turns out that because the real-world conditions (camouflage paint, dirt and mud and structural “dings”) encountered in combat resulted in loss of the air-flow physics and laminar-flow effect exhibited by the polished models in the laboratory. However, the novel shape of the airfoil exhibited superior performance at high speeds compared to other U.S. or foreign airfoils and did indeed provide the airplane with a huge advantage in combat.

The airfoil was used on all operational models of the Mustang, including the ones flown by the Tuskegee airmen.

Late in the war a newer, more powerful version of the Mustang known as the P-51H was designed with a second-generation laminar-flow airfoil based on results from a new Langley tunnel known as the Low-Turbulence Pressure Tunnel. The airfoil was designated the NACA 66-(1.8) airfoil.

For further reading, the following reference is highly recommended for information on the NACA development of the laminar-flow airfoil and for the XP-51 experience:

Laurence K. Loftin, Quest for Performance: The Evolution of Modern Aircraft. NASA Special Publication SP-468, (1985) Pages 104-106.

[top] Flight tests of the XP-51

In 1941 the first Mustang was flown at Langley by Langley test pilots to evaluate its characteristics—especially the lack of sufficient ability to roll left and right during simulated combat. In a program involving wind-tunnel tests and flights by Langley test pilots, a new wing-mounted aileron (roll control) surface was conceived and implemented on Mustangs to improve its maneuverability.

Based on the flight investigation, a modified aileron system was used on all operational models of the Mustang, including the ones flown by the Tuskegee airmen. Without the modification, the Mustang would have been at a terrific disadvantage against German fighters during the war.

Many flight tests were conducted with other Mustang aircraft during the war years at Langley including specific and general research missions. These tests included flights with the P-51B, P-51D, and P-51H.

Please note that the P-51C version flown by the Tuskegee airmen was identical to the P-51B tested by Langley. It received the “C” designation because it was built in Dallas rather than California.

[top] Modifications to later models

Later in the war during 1944 Langley pilots were requested to evaluate a new model of the Mustang designated the P-51D. The new version had a “bubble” cockpit canopy which the pilots found significantly reduced the directional stability of the airplane. (Directional stability is the tendency of an airplane to “weathercock” its nose into the wind).

After the Langley tests, it was recommended that a new vertical tail shape (dorsal fin) be adopted for better handling qualities.

This modification was used on later P-51D models of the Mustang flown by the Tuskegee airmen.

[top] Full-Scale Tunnel Test

In 1943 the Army Air Forces requested that Langley conduct a “drag cleanup” test in the Full-Scale Tunnel to identify changes to reduce drag and increase the top speed of the airplane. This type of test was a specialty at the tunnel, consisting of removing protuberances, etc. and sealing the aircraft joints with tape, then removing the pieces of tape one at a time and noting the increment of drag contributed by that surface or gap. Over 30 aircraft were ultimately tested in this manner at Full-Scale and very significant increases in top speed had been obtained for most test subjects (typically, at least 20 mph).

However, the test results of the P-51 clean-up tests were disappointing. Its designers had learned a great deal from the previous NACA tests on earlier airplanes. The P-51B was found to be so clean in its as-received condition that the top speed could only be increased by about 3 miles per hour by the staff of the Full-Scale Tunnel. No significant modification to the P-51 came about from this test, which really turned out to be a non-event.

[top] Wing-Flow Flight Technique

In 1944 Langley was deeply involved in obtaining aerodynamic performance data at transonic speeds. Wind tunnels could not be used at that range of speeds because of shock-wave interactions with the tunnel walls, so other test techniques were required. The flight group at Langley conceived the idea of mounting small models on the wing of the high-speed P-51 and measuring the aerodynamic forces with instrumentation mounted in the airplane’s internal wing structure. The technique was enormously successful.

This use of the P-51 was strictly for research and did not impact the aircraft flown by the Tuskegee airmen.

[top] Further Reading

Laurence K. Loftin, Quest for Performance: The Evolution of Modern Aircraft. NASA Special Publication SP-468, (1985) Pages 104-106.

John Reeder, The Mustang Story: Recollections of the XP-51. NASA Facts NF171, April 1992.

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