An engineer's perspective on the Mustang

Flight Journal, Jun 1999 by Atwood, J Leland

North American Avistion'd (NAA) Mustang fighter is generally credited with a 20- to 30mph speed advantage over most of its WW II contemporaries . This speed advantage also permitted a considerable increase in range that required more fuel, but not enough to significantly reduce speed. Records show that some 275 U.S. aces were "made" in P-51s. The reasons for the Mustang's significant performance capability have never been clearly explained, and I hope to clarify why its aerodynamic features enabled this capability.

To begin: in 1940, the British Purchasing Commission, which I dealt with, had a member-H.C.B. Thomas from Farnboroughwhom I found to be familiar with the Meredith Report. This report outlined a feature that could enhance the performance of any internal-combustion engine at high speeds by using a radiator form of heat dissipation. A low-velocity airflow through the radiator was one element of this, and it was apparent to me that the larger the radiator, the lower the speed of the air flowing through it; this approached one of the Meredith Report's objectives.

I therefore offered Mr. Thomas sketches and other descriptions of a Mustang design that had the main radiator in the rear of the fuselage. The alternatives were wing radiators such as those used on the Spitfire and the Bf 109, and under-engine radiators such the P-40's; both positions limited radiator size and the length and size of the ducting that could be used to handle and control the cooling air.

In addition to the radiator's rearward position, after the design contract had been awarded and at the recommendation of NAA's aerodynamics group, it was decided to use a new airfoil of a class generally designated as "laminar flow." This was being developed at NACA (later NASA) at Langley Field, Virginia. A 1939 report by Eastman Jacobs and others at Langley contained the results of the tests of some small laminar-flow airfoils. The drag on these small models was quite low, and there was some hope that laminar flow could be achieved much farther back on an airfoil than had been predicted by previous investigators. The publishers of the report, however, warned that they had not been able to obtain laminar flow on wings of anywhere near the size of those required for actual aircraft and that their tests were to be taken only as the results from laminar-flow models of not more than six inches in width.

In spite of this warning, however, both Ed Horkey (leading aerodynamicist at North American) and Bell Aircraft's chief engineer, Robert Woods, decided to try laminar-flow profiles on the P-51 and the Bell P-63, respectively. Ihese airfoils were incorporated on the Mustang and the Bell airplane with the hope that laminar flow could be extended well back on their wings. Extensive efforts were made to polish and protect the P-63 wing's leadingedge profile, but the results were equivocal. Those who advocated the laminarflow wing felt that the Mustang's outstanding performance resulted from laminar flow over most of the wing. Kingcobra designers felt they were getting a similar effect, although that aircraft's performance did not justify this conclusion.

With respect to the Mustang, many tests-including some in recent years-have shown that extensive laminar flow was not developed on the Mustang wing and that the drag of the wing was probably no less than that of conventional wings of the same thickness and taper ratio. On the other hand, the Mustang's cooling drag was much lower. This was the result of using a ducted radiator with a large area and a slow-speed airflow through it (Pr and P2); closing up the exit and creating a backpressure restored the momentum of the cooling of air (momentum lost in radiator transit). This was possible because of the radiator's cooling capability, which, to be adequate in a full-power climb, was much more than that required at high speed and high dynamic pressure. According to calculations given in a supporting paper, the drag created by momentum loss in passing through the radiator can be reduced from some 400 pounds to close to 30 to 40 pounds because of the offsetting momentum of the jet thrust from the radiator exit (V2).

Since these two effects, i.e., the wing drag and the radiator momentum recovery, have never been disentangled in the literature, a technical reason for the Mustang's performance has never been clearly identified.

NACA had taken the lead in airfoil develop ment and had worked out a large series of airfoils that were used generally throughout the industry. For instance, the Spitfire wing was of the NACA 2200 series-13 percent thick at the root and 6 percent thick at the tip. This is the same airfoil series as is used on the DC-2 and the original North American BT-9 and AT-6 trainers. To improve the stall characteristics, I later changed the NACA 2200 series on the AT-6 trainer to the 4412 at the tip. It is quite probable that the Spitfire's wing, being only 6 percent thick at the tip, had a lower drag than the Mustang's wing as actually incorporated.