Handley Page, Lachmann, flow control and future civil aircraft John Green ABSTRACT Frederick Handley Page and Gustav Lachmann independently developed and patented the concept of the slotted wing as a means of increasing maximum lift. Subsequently they co-operated on the project and Lachmann joined Handley Page Ltd. The Handley Page slotted wing became used worldwide, generating substantial income for the company from use of the patent, and its descendents can be found on all modern transport aircraft. In the years following World War II, Lachmann led research at Handley Page to reduce drag by keeping the boundary layer laminar by surface suction. Handley Page led this field in the UK and developed a number of aircraft concepts, none of which came to fruition as full scale projects. However, looking to the future, the basic concept of laminar flow control holds out arguably the greatest potential of all technologies for reducing the fuel burn and environmental impact of future civil aircraft. 1. INTRODUCTION This is the story of two men of genius, Frederick Handley Page and Gustav Lachmann, Figs. 1 and 2. They were brought together by chance, as a result of having independently, and unknown to each other, invented and patented the same aerodynamic concept. During World War I they had been on opposite sides. Handley Page, who had been 28 at the outbreak of hostilities, established his company’s reputation as the designer of the large biplane bombers, the ‘bloody paralysers’ sought by the Royal Navy in 1914, that made a great contribution to the war effort in 1917 and 1918. Lachmann, 10 years his junior, transferred from the Hessian Cavalry Regiment to the German Flying Corps in 1917, crashed badly following a stall and spin, joined his squadron after six months in hospital only to be severely wounded in aerial combat and end the war in hospital again. In the early post war years both men patented the idea of a slotted wing as a means of increasing the maximum lift of a wing before it stalls, Lachmann in Germany and Handley Page in England. In 1921, having learned of each other’s work, they met, discussed at length and agreed to co-operate rather than compete. From this sprang a forty year friendship, for the last thirty three years of which Lachmann worked beside Handley Page as an employee of the company, in charge of aerodynamics, structures and slot development from 1929 to 1932, chief designer for the next four years and then in charge of the research department. Despite being chief designer of the Handley Page Hampden, one of the three medium twin-engined bombers in front line service with the RAF at the outbreak of World War II, Lachmann was interned in Canada as an enemy alien at the outbreak of the war. Pressure from Handley Page led to his transfer to the Isle of Man, where he was able to work for the company as a consultant. After the war he returned as head of the research department and continued in that post until his retirement in 1965, three years after the death of Handley Page. The men were alike in being trained engineers with an interest primarily in aerodynamics but with the ability to span the full range of disciplines needed to design and build aircraft. They were alike also in being well rounded men of wide cultural interests, noted for their humour and their gifts as raconteurs and conversationalists. In fact they were both brilliant, both polymaths and both men of their time. Handley Page stood out as the entrepreneur, the industrial giant, whilst Lachmann was the airman and scientist who, through his leadership of research, helped to establish Handley Page Limited as an innovative, pioneering company in many aspects of aircraft design. They both contributed to an ethos of designing aircraft by, in Handley Page’s own words(1), “unassailable aerodynamical logic”. The concept of the leading edge slot which brought the two men together is an expression of such logic. And when Lachmann set out in the 1950s in the quest to reduce aircraft drag by the use of surface suction to maintain laminar boundary flow, the aerodynamic logic behind the concept was clear and he did so with the full support of Handley Page. Both men were convinced of its potential and the company devoted a considerable amount of its own resources to the project. Unlike the leading edge slot, which became adapted worldwide and generated substantial licence revenue for Handley Page, laminar flow control, as demonstrated by the experiments of Handley Page and other organisations, has yet to be realised in a full-scale aircraft project. Nevertheless, the underlying logic remains unassailable, the pressure to reduce fuel burn is greater than ever because of the need to reduce climate impact and the time is now ripe to re-examine the work on laminar flow control done by Lachmann’s team in the 1950s and 60s. There is a connection between slotted flaps and boundary layer suction. Both aim, by manipulating the airflow, to achieve a substantial gain in performance, an increase in lift through the leading edge slot, a reduction in drag by boundary layer suction. The first part of the paper reviews the development of the slot, the second the progress at Handley Page on laminar flow control by suction and its potential role in reducing CO2 emissions from aviation in the 21st Century. 2. THE SLOTTED WING The aerofoil sections of the biplanes of World War 1 were relatively thin. As a result, lift increased steadily with angle of incidence until a critical angle was reached at which the airflow over the upper surface of the aerofoil broke away and lift dropped suddenly. The wing stalled. Fig 3 shows an illustration of the phenomenon as it was portrayed by Orville Wright in 1924. It was to delay the onset of this flow breakaway over the upper surface, so as to increase the maximum achievable lift coefficient, that Handley Page and Lachmann investigated slotted wings. 2.1 The Handley Page patent In a paper given to the Royal Aeronautical Society in 1911(2), Handley Page (he was 25 at the time, Lachmann just 15) discusses the experimental results then available on lifting surfaces. In his opening passage, headed “Some Theoretical Considerations”, he says, “To obtain a law giving the normal pressure on a plane as a continuous function of the angle of incidence of the impinging air from 0 to 90° is impossible owing to the two distinct forms of flow that occur on the back of the plane. From the horizontal position of the plane up to an angle varying in magnitude from 10° to 50° depending on the aspect ratio, shape and curvature of the plane, the air hugs the back of the plane, the suction due to the rushing air is felt directly on the back of the plane, and the pressure increases continuously as some function of the angle. At angles greater than this critical value the air leaves the back of the plane, a “dead” air region is formed there, and any reduced pressure or suction on the plane back tending to increase the total “lift” is then solely due to the drag of the “live air” stream at the edges of this dead air region.”.......and a few pages later, “....the critical angle at which the “live” air leaves the plane back is reached earlier in the case of planes of high aspect ratio, and the planes do not have such high maximum values as the planes of lower aspect ratio.......With planes of high aspect ratio there is not the same facility for the “feeding in” of fresh air at the plane sides to act as a link between the plane and “live stream” and therefore the live stream leaves the plane back at an earlier stage than in the case of the plane of lower aspect ratio.” The correct observation that it was the trailing vortices from the wing tips that delayed stall on low aspect ratio wings1, was clearly to the fore in Handley Page’s thinking when in 1917 he began experiments in a company wind tunnel to find ways of delaying the stall. His first idea was to create a wing of aspect ratio 6 divided into six sections by means of chordwise slits, with the idea that air flow through the slits would feed “live air” through to the plane back and delay the stall. The wing planform and the experimental results are shown in Fig. 4. The result was a slight increase in stalling angle but no increase in maximum lift coefficient. After a small amount of trial and error variation of the geometry, Handley Page abandoned this idea but, still with the thought of bringing “live air” through to the upper surface to invigorate the “dead air”, he then tested a wing with a spanwise slot. The first model and the results are shown in Fig. 5. Although not a striking improvement on the results with chordwise slots, it did not take much refinement of the geometry to demonstrate significant gains. There followed an extended period of experimental development, carried out in secret, with the first patent application being filed in October 1919 but with continued experimental development of the concept carried on through the following years. Extra impetus was given to the work by the announcement by the Air Ministry in 1919 that it proposed to hold a competition for a safe civil aircraft that was to have not only a reasonable cruising speed but also the ability to fly slowly.