The Aerodynamics of the V-bombers - Part 4

28 October 2022

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Dr Steve Liddle CEng FRAeS, is a Vulcan to the Sky Trustee and Principal Aerodynamicist at Aston Martin Formula One Team. The articles here are republished from Steve’s occasional series on ‘the aerodynamics of the V-bombers’, that he writes on the Vulcan to the Sky Trust LinkedIn page.

Four Digits and a Revolution

In 1940, as the Spitfires clashed with their Bf109 foes over the Kent countryside, it might have been a surprise to their pilots to discover that a key piece of the aerodynamic technology of both opposing aircraft had come directly from the firmly neutral USA. The aerofoil sections used in their wings were the result of a systematic investigation conducted at the expense of the US federal government, by the National Advisory Committee for Aeronautics (NACA) at its Langley Memorial laboratory at Hampton, Virginia. In the panic following the launch of Sputnik in 1957, NACA itself was reorganised as part of the newly created NASA organisation…

The flow of German aeronautical engineering talent and information to both East and West in 1945 is well known, but the achievements in aerodynamics during early part of the twentieth century particularly by Ludwig Prandtl and his team also made them prime targets. In 1920, Prandtl’s protégé Max Munk began work as an advisor to NACA, following a special order issued by President Wilson that allowed him, as a very recent enemy, to come to the United States and work. Munk developed aerofoil theory but crucially conceived and recommended the construction of a wind tunnel facility that could replicate the conditions

found in flight at speed and altitude. NACA’s Variable Density Tunnel was commissioned in 1922 and began to contribute to Munk’s project: a parametric understanding of aerofoils, providing experimental data that would allow engineers to select appropriate designs for their aircraft.

The physical shape of an aerofoil could be described in terms that included how thickness related to length (chord), where the maximum thickness was positioned along the chord and the curvature of the mean line (camber). NACA’s programme resulted in a major report in 1933, which described the results of 78 variations on a basic theme (Jacobs et al, 1933), with the geometry changes easily decodable from a name made up of four digits which gave camber in % of chord, location in 10ths of chord and finally thickness in % chord. Because the tests illustrated the aerodynamic effect of varying these parameters independently, for example the effect of reducing thickness at constant camber between a 2412 and 2410 shape, engineers could understand how these trends impacted their designs.

Figure from NACA TN460, showing the aerofoil sections studied. (Jacobs et al, 1933)

Supermarine’s experience with the Schneider trophy seaplanes had convinced them of the merits of a thin wing for low drag at high speed, which was supported by the NACA data but not British advice, which due to then unknown problems with wind tunnel testing did not see much advantage in thinning the wing below about 15%. They chose NACA 22xx sections, thinning from 13% root to a remarkable 6% at the tip (Ackroyd, 2016). Back in the dimensional world, threading the ammunition belts over the top of the eight Browning machine guns was now unlikely; they would have to be mounted at the side, requiring a widely spaced battery along the wingspan and hence the chord outboard to be large. Although certainly not the only drivers, the key input of reliable aerofoil data giving confidence to pursue an aerodynamically thin wing with internal volume for the guns was certainly a push towards the elliptical planform with which we are so familiar. The layout of the Spitfire ‘A’ wing (the original design, armed with 8x machine guns) is shown below, illustrating that the four guns and their ammunition belts are arranged as closely as possible, but still require a large chord outboard to accommodate both their length and the aileron behind.

Spitfire ‘A’ wing planform diagram, illustrating the layout of the 4x Browning machine guns 

In Germany, Messerschmitt adopted a different strategy of a relatively small wing planform augmented by leading edge slats for high lift; a modification to the basic NACA sections was incorporated to reduce the pitching moment in a wing that was slightly thicker than the Spitfire’s, but ultimately of similar section.

Example data from NACA TN4600, showing the 2R1 12 Aerofoil, as used (with scaled thickness) on the Messerschmitt Bf109. (Jacobs et al, 1933)

NACA’s investigation continued and looked to alter the camber line to meet a design lift coefficient, in the 5-digit series of the later 1930s. These were available by the time that AVRO came to design their twin-engined medium bomber that became the Manchester and ultimately the Lancaster, which used the 230xx series ranging from an 18% thick root to 12% at the tip. At some point, the shape would have passed through 15% and hence have been a 23015, which was the root section adopted by Focke Wulf AG for its FW190 fighter, used in the second half of the war to defend against attacks by RAF Bomber Command’s Lancasters.

Advice from the high-speed wind tunnel research at the Royal Aircraft Establishment during and immediately after the war (Mair, 1950) suggested that thin, symmetrical sections were likely to be useful at high-subsonic speeds, as they minimised both profile drag and the change in pitching moment resulting from shockwave formation. This also reflected the results of tests in Germany during the way years. For their high-speed jet bomber proposal that would become the Vulcan, AVRO settled on the NACA 0010 (10% thick, zero camber) section, from the same family as that used by the Spitfire, and the result of NACA’s research back in the early 1930s but which could ultimately be traced back to Max Munk and WW1-era Germany.

References

Jacobs, Eastman N.; Ward, Kenneth E.; and Pinkerton, Robert, M; The characteristics of 78 related airfoil sections from tests in the NACA Variable-Density wind tunnel; NACA TN460, November 1933

Ackroyd, J. A. D.; The Aerodynamics of the Spitfire; Journal of Aeronautical History, paper 2016/03, 2016.

Mair, W. A. (Ed). Research on high speed aerodynamics at the Royal Aircraft Establishment from 1942 to 1945. ARC, R & M No. 2222, 1950.

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