Bernd Krag

The North American P-51 "Mustang" and the Laminar Flow Wing:
A Success Story or just an Illusion?

The North American P-51 "Mustang" is one of the best known fighter aircraft of the Second World War. Much has been written about the advantage in performance of the P-51 compared with other contemporary fighters by using a laminar flow airfoil. Its spectacular performance combined with excellent flying qualities and a very long range made it the best propeller driven fighter airplane of the war. The designer of the P-51 was Edgar Schmued, perfectionist and chief designer at North American Aviation. It was his intention to build an exceptionally clean aircraft showing the same aerodynamic characteristics like the smooth wind tunnel model. When in early 1940 the NACA first released information on the new laminar flow airfoil, Schmued decided to incorporate this new airfoil into the design of the P-51. The laminar flow airfoil promised about 20% less drag than conventional airfoils.

The introduction of the laminar flow airfoil into the P-51 design was observed with interest by the NACA. But NACA engineers expressed serious doubts concerning the performance of the laminar flow airfoil under real flight operations. Manufacturing irregulations, surface roughness and dirt would reduce the laminar flow to only a small region near the leading edge. The conditions of a highly polished windtunnel model could not be maintained on a real aircraft.

Nevertheless reports from England, where the NA-73 was flight tested, were very favourable with respect to the performance of the new fighter. RAF flight test engineers were convinced that the laminar flow had been achieved. But RAE scientists at Farnborough were very sceptical that the laminar flow airfoil was the key to its performance. They shared the doubts of their NACA colleagues. To find out what was that all about the US Material Command at Wright Field sent Dr. Edward P. Warner over to England. Warner's report was not conclusive with respect to the influence of the laminar flow airfoil on the performance of the NA-73. But he warned that the Germans could build aircraft of similar performance after a "Mustang" had got into German hands and the secrets of its laminar flow airfoil are disclosed.

But the fears of Dr. Warner were unjustified. At that time German scientists (Schlichting, Tollmien, Kawalki) had found a new way, to calculate the point of transition of the boundary layer from laminar to turbulent for compressible flow. This opened the way for the development of a new series of high-speed airfoils with superiour characteristics. First application was on the Messerschmitt Me 262 jet fighter.

In spite of that, the German aeronautical scientific community was interested in airfoils with laminar flow characteristics. Prof. Hermann Schlichting of the Technical Highschool Braunschweig did a lot of theoretical and experimental work to find laminar flow airfoils which would maintain its characteristics to very high Reynold Numbers. He investigated laminar flow airfoils of Russian and Japanese origin. It was the opinion of the scientific community that the laminar flow would disappear with increasing Reynold Numbers. The problem was that there existed no wind tunnel which could produce speeds high enough to come into the region of real flight Reynold Numbers (Re > 20 million) in order to prove the assumptions.

When the first P-51 Mustangs appeared over Germany in spring 1943, the good performance of this aircraft was well noticed. There were rumours that this aircraft had a laminar flow airfoil. Early in 1943 the German Air Ministry (RLM) provided an original P-51 wing in a good shape for further investigations. The AVA Göttingen measured pressure distributions at several wing sections and made drag measurements. The latter resulted in a surprising low profile drag. At this point Prof. Schlichting became interested. The same wing was put into the large 8 m wind tunnel A3 of the Luftfahrtforschungsanstalt Braunschweig-Völkenrode (LFA). It was Schlichting's intention to make boundary layer measurements and to estimate the point of transition on the airfoil where the air flow changes from laminar to turbulent. Measurements allowed Reynold Numbers up to 7,6 millions.The wing was carefully smoothed and a new coat of paint was applied. A problem was the position of the main spar where the wing surface had a small gap at ca. 25% depth.

The mesurements using a special probe which had to be moved very close to the surface were difficult. The slightest damage of the paint after the probe had touched the surface caused an immediate transition of the air flow from laminar to turbulent. The measurements were repeated for different Reynold Numbers and different lift coefficients. For the lowest Reynold Number (4 millions) the point of transition was measured at 50% depth on the upper surface. It moved to the leading edge with increasing Reynold Number, arriving at 20% for Re=7,5 millions. Measurements with different laminar flow airfoils including the Mustang airfoil were later continued in the large high-speed wind tunnel of the DVL, Berlin up to Reynold Numbers of 20 millions. These measurements clearly revealed the fact that the laminar flow effect completely disappeared at real flight Reynold Numbers. This was an expected but sobering result.

One important result of the wind tunnel investigations on the original Mustang wing was its low profile drag. This lead to a comparison of the Mustang wing with wings from different German aircraft. In 1944, rather late in a war, comparative wind tunnel measurements were made with original wings of the Fw 190, He 219, He 177, Ar 234, Me 109 F, Ju 288 and Martin B-26 "Marauder". Objective of these measurements was the estimation of the profile drag for different lift coefficients. These measurements revealed a clear advantage of the Mustang wing. Compared with the ideal smooth wing (wind tunnel model), the wings of the German aircraft had up to 60% higher drag than the ideal smooth wing. This reflects the poor manufacturing standards in Germany at that time. The Mustang wing had by far the lowest profile drag that could not be explained with the laminar effect alone, but to a large extent with the very good manufacturing standards at NAA.

Concluding you can say that the performance of the Mustang could not be attributed to its laminar flow airfoil. It was the overall low drag design of this aircraft with clean surfaces including the careful design of the radiator that was the key of its good performance. Edgar Schmued succeeded to build an aircraft as clean as the wind tunnel model - a remarkable aircraft and a remarkable designer.

References

  1. Ray Wagner: Mustang Designer Edgar Schmued and the P-51. Smithsonian Institution Press: Washington D.C. / London 1990.
  2. James R. Hansen: Engineer in Charge, A History of the Langley Aeronautical Laboratory 1917-1958. The NASA History Series. NASA: Washington D.C. 1987.
  3. Breford / Müller: Messungen am Originalflügel des Baumusters P-51 "Mustang". AVA FB. Nr. 1724/2, 1943.
  4. Kopfermann / Breford: Umschlagpunktmessungen am Originalflügel des Baumusters P-51 "Mustang". In: Deutsche Luftfahrtforschung, UM 2035, 1943.

Title Picture: Bernd Krag