Issue 13 - F-14 Tomcat: Why move the wings?
By: David I Roberts
David I Roberts explains a brief history of swept wing and variable geometry aircraft up to the TFX.
Bird flight being the inspiration for the earliest dreams of men taking to the air, the concept of variable geometry has been around since the earliest attempts at designing (or imagining) a flying machine. From the mythical Daedalus and Icarus via the (probably fictional) dark age British King Bladud to the 11th century monk Eilmer of Malmesbury and many subsequent reckless tower jumpers, most early would-be aviators sought to fly by flapping wings strapped to their arms, without any realistic means of control or even a stabilising tail.
The exception was Leonardo da Vinci, who in 1487 sketched an ornithopter powered by the arms and legs of a prone pilot, and featuring a “rowing” motion. In other words, the wings could move fore and aft as well as up and down; this was almost certainly the first considered partial design for a “swing-wing” aircraft. Leonardo, despite his anatomical expertise, never quite grasped that it was torsional flexure of the feathers or membrane in the vertical stroke that provided forward thrust, and thus lift, but his research into aerodynamics remained the most comprehensive and detailed for centuries afterwards. In later life he concentrated on fixed wing gliders. One, a sail-wing hang glider with a remarkable resemblance to Sir George Cayley’s 19th century wing designs, was recently replicated and, with the addition of a fin, proved capable of piloted flight.
Designers of later ornithopters, some of which succeeded in flying in model form, and even at full size by the 20th century, preferred to concentrate on the mechanically simpler vertical motion. Fixed wings, or something approaching them, were more promising, as Leonardo had discovered four centuries earlier. Clément Ader, whose Éole reportedly made a brief hop-flight at Armainvilliers, France in 1890, equipped his fantastical steam-powered, bat-like creation with no less than six hand and two foot controls, none of which proved to exert much useful influence over the craft’s attitude and direction. Among the control systems was one by which the wings could be moved independently fore and aft to combine the functions performed in more conventional (and successful) aircraft by ailerons and elevators. Ader might have been better advised to equip his machine with a tail, as already developed by Sir George Cayley and Alphonse Pénaud, and with simpler, more manageable controls, but he was apparently the first person to leave the ground in a variable geometry aeroplane. His twin-engined Avion which followed embodied the same principles and, despite claims to the contrary, was no more successful. Coincidentally, the name Ader is close to the Welsh word for bird, which is aderyn.
Sweepback was ignored until researchers such as John W Dunne in the early1900s explored the stability it conferred on fixed-wing aeroplanes, about both pitching and yawing axes. Given the drag-producing profusion of struts and bracing wires typical of Edwardian aircraft, its potential for greater speed was not significant at the time. From 1907 Lt Dunne, initially employed as a kite designer at the Army Balloon Factory, Farnborough, designed a series of swept wing tailless gliders and powered aircraft which were tested at Blair Atholl in Perthshire under great secrecy, being the first aeroplanes to bear camouflage. In 1909, when the latest machine, the D.4 biplane, was finally beginning to get results, the British government, as so often, withdrew funding, and in 1910 Dunne set up the Blair Atholl Aeroplane Syndicate with the Marquis of Tullibardine, contracting with the Short brothers to build subsequent aircraft and testing them mainly at Eastchurch.
By 1912 the D.8 was proving so stable that a French pilot, Commandant Félix, was able to leave the controls and climb out onto the wing during a demonstration at Villacoublay. Ill health forced Dunne to retire from aircraft design and testing, but W Sterling Burgess in the USA acquired a licence to develop aircraft embodying his principles, selling various models, including seaplanes to the US and Canadian navies. One such seaplane, delivered in 1914, was Canada’s first military aircraft.
Swept wings saw little other use before the First World War; notable British examples were the Handley Page monoplanes and the G biplane, all having the swept wings with curved leading edges pioneered by José Weiss, who believed, correctly, that such wings, tapering in thickness from root to washed-out tip, would give automatic stability. During the war, however, the British services and manufacturers took no further interest in the principle. Albatros built a couple of experimental conventional biplanes with considerable sweep and another German company, DFW, produced its Mars Pfeil (Arrow) reconnaissance aircraft in some numbers. Austro-Hungarian companies Lohner and Oeffag also initially made two-seaters with rather acute sweep. Lohner fitted swept wings to their very successful biplane fighter flying-boats, which were copied almost exactly by the Italian firm of Macchi.
Excessive stability soon proved a liability as fighters evolved, and sweep angles were reduced to modest values on later two-seaters by builders such as Rumpler, Halberstadt, Hannover and AEG, who also applied it to twin-engined bombers. The main exponents of modest sweep in France during this period were Nieuport on their fighters and two-seaters and Morane-Saulnier, who applied it to late war monoplane fighters. The principal benefit in all these cases was an improved view from the cockpit. Sweepback can also be useful by allowing a greater range of centre of gravity travel.
Acute sweep enjoyed a revival in Germany soon after the war, when aviation was limited almost entirely to gliding. A photograph from about 1920 shows a small tailless biplane hang glider, clearly inspired by Dunne’s work, apparently with Alexander Lippisch aboard.
Lippisch collaborated with Gottlob Espenlaub to build a series of tailless gliders through the 1920s, and continued to design swept-wing aircraft, both gliders and powered aeroplanes, through the Second World War, the most famous product of his research being the Messerschmitt Me163 Komet. His research also doubtlessly influenced the adoption of swept wings, which delayed the onset of compressibility at high subsonic Mach numbers, for the Me 262 Schwalbe. In 1931 he had realised a swept wing could be made to carry more payload, with little loss of efficiency, by extending the centre-section aft to fill in the ‘V’ partially or completely. The delta wing was born. After the war, Lippisch moved to the US to help with, among other things, delta wing development. While he did not work on variable geometry, the planform of most swing wing aircraft, in which the wings at full sweep effectively combine with the tailplane to create a delta wing, owes much to Lippisch.
Two other important German pioneers, the brothers Reimar and Walter Horten, also developed tailless swept-wing sailplanes, which evolved into true flying wings of high aspect ratio and extreme aerodynamic cleanness. Some were converted to powered aeroplanes during the Second World War, and their twin-jet fighter-bomber, the Gotha 229, flew in 1944, with tragic results when test pilot Erwin Ziller was killed following an engine failure which showed up the aircraft’s Achilles heel, its marginal controllability under asymmetric thrust in the absence of a fin. The second, unflown, prototype was taken to the US, where its low radar signature impressed engineers. Combined with Jack Northrop’s contemporary wartime design efforts, Horten research contributed considerably to the American flying wing and stealth bomber programmes. Again, variable geometry was not involved, but the Horten brothers had put the high-speed potential of strongly swept wings firmly on the map.
GERMAN SWING WINGS
A Messerschmitt design, the P.1101, to meet a 1944 requirement for an emergency fighter, was partly completed by the end of the war. Having a single engine with a nose intake, it was planned to make its Me 262 wings adjustable before, but not during, flight to a sweep of 35, 40 or 45 degrees.
The prototype, about 60% complete, was eventually taken to the US, where the Bell Aircraft company, interested in the type since its discovery, took charge of it in 1948. It was never completed and was eventually scrapped in the 1950s, but it provided some data for construction of the X-5, of which more later.
In Britain, which between the wars had otherwise largely ignored acute swept-wing developments, the first practical variable-sweep aeroplane flew at Andover, Hampshire in 1931.
The Westland-Hill Pterodactyl IV was one of a series of tailless designs, with which John W Dunne had helped Geoffrey Hill at an early stage. The Pterodactyl types had all-moving wingtips to act as elevons at all angles of attack, and the Mk IV, a three-seat cabin monoplane, added a small amount of variable sweep about its roots and pivoted bracing struts to refine longitudinal trim in flight. A two-seat fighter, the Pterodactyl Mk V, albeit without the variable sweep, was built and compared well with contemporary fighters, but did not see production, its unreliable Rolls-Royce Goshawk engine being a liability. Hill proposed further developments, including a turret-equipped fighter, a four-engined flying-boat and a five-engined transatlantic transport, but none was adopted. The 1925 Pterodactyl IA is preserved at the Science Museum in London, and the family is perhaps best remembered as inspiration for the futuristic machines depicted in the classic science fiction film of H G Wells’s Things to Come.
A swept-wing light sportsplane, the Granger Archaeopteryx, inspired by the Pterodactyls and also built with input by Dunne, was the first tailless aircraft to have a tractor airscrew, all previous types being pushers. It first flew at Hucknall, near Nottingham, in 1930, and is preserved near there after years in the Shuttleworth Collection, where it flew again in 1971. It is hoped to restore it anew to flying condition.
In 1932, Waldo D Waterman built the first successful tailless aircraft of purely American design. Having swept wings with endplate fins, a pusher engine and a spatted tricycle undercarriage, it was intended for the private market. Its successor, the Arrowplane, won an award for a flight from Santa Monica, California to Washington, D.C. in 1935. That, however, was just the beginning; Waterman was working towards a flying car, a goal he achieved in 1937 with the Arrowbile, which had detachable wings and a Studebaker engine driving both airscrew and road wheels via a clutch. It complied fully with both aeronautical and automobile regulations. Its final development, the Aerobile with a Franklin engine, first flew in 1957 and is preserved in the National Air and Space Museum.
Other, more conventional, inter-war swept-wing aircraft are the De Havilland Tiger Moth, the Stampe SVA 4 and the Bücker Jungmann and Jungmeister, all having sweep to ease pilot entry and, particularly, exit in an emergency. Various two-seat reconnaissance and fighter-bomber aircraft of the period, notably the Curtiss Seagull, the Hawker Hart family and the Henschel 126, also incorporated the modest sweep favoured by Germany and Austria-Hungary in the First World War, for the same reasons of pilot view and ease of trimming.
In the UK postwar, just as Hawker, Avro, Handley Page, Supermarine and De Havilland were designing swept wing and delta aircraft, the visionary Vickers designer Barnes Wallis had, as usual, leapt far ahead of his contemporaries with the Wild Goose, a supersonic blended swing wing machine devoid of all control surfaces. Conceived in 1949 as a bomber armed with a stand-off missile and arising out of Wallis’s feelings of guilt over heavy aircrew losses in the Dams raid which had been his project, the Wild Goose was to be controlled by a combination of fuselage lift and differential wing sweep, in principle much like Ader’s system of 60 years before, but much more refined and practical. In the event, Vickers had to content itself with building the more prosaic Valiant. Somewhat overshadowed by Wallis’s projects was a patent for a variable-sweep supersonic fighter taken out by the British designer L E Baynes in 1949. It was never built.
Wallis’s design evolved into several proposals, including the beautiful Swallow supersonic transport which no doubt influenced Boeing’s planned Mach 3 733 of a decade later. The Swallow had swivelling pod-mounted engines on the wings and a cockpit contained in a cylindrical fairing which could be extended above the fuselage for better visibility in the landing approach. Funding was only obtained for a full-size version of one of these aircraft, a piloted version of Wild Goose to be built by Heston Aircraft as the JC 9. It was cancelled in 1952 before completion, but tests of trolley-launched radio controlled flying models, some rocket powered, proved the concept at speeds up to and above Mach 2. The remote control systems themselves represented a considerable innovation and much research. Tests continued with Swallow models until 1957, when the infamous Sandys White Paper put paid to nearly all state funded aircraft development in Britain.
A naval fighter, the Vickers 581 – using Wallis’s swing wings and twin engines above the rear fuselage with ramp intakes – was more conventional, having a fin and tiny triangular tailplanes mounted right aft. When this was turned down, Wallis and his team took the design to Langley Field in 1960 and tried to obtain US funding. He ruefully reported later: “We convinced the Americans too sincerely that this was a great idea so they decided to take it up for themselves instead of paying us a grant to do it in England.” The TFX resembled the 581 only in having a wing glove which blended into the fuselage above and behind the cockpit, but Wallis’s test data were reportedly applied wholesale to the American design. A final project, the Vickers 589, resembling a T.S.R. 2 with Lightning-shaped swing wings, was not proceeded with, but provided important data for the Tornado.
Meanwhile, back in the States, Bell completed the two X-5 prototypes, which first flew in 1951. A more complex wing sweep mechanism than that of the parent Messerschmitt P.1101 allowed the pivot points to move fore and aft for centre of lift compensation, using an electric screw jack to move them and disc brakes to lock the wings in position. Sweep, changed in flight, could be 20, 40 or 60 degrees, and sweeping through the full range could be accomplished in 30 seconds. The X-5 was tested up to Mach 0.9, but its poor tail position, high on a boom aft of the jet outlet and in the turbulence generated by the wings at some sweep settings, led to a vicious tendency to spin, which killed Captain Ray Popson at Edwards AFB in October, 1953 while the wings were swept at 60 degrees. Testing continued until 1955, and the X-5 ended its days as a chase plane at Edwards. The type was not adopted, as had been hoped, as an economically priced fighter for export, but it had proved the viability of variable-sweep wings.
In 1951-52, Grumman fitted swing wings and an all-flying tail to a Panther to produce the XF 10F Jaguar in response to Navy concern expressed in 1948 about operating swept-wing aircraft from existing carrier decks, already too small for the latest fast fighters, which had an unacceptably high landing speed. The wing moved between two fixed positions, 13.5 and 42.5 degrees with automatic unsweep in the event of failure, which mercifully worked even when the hydraulic fluid turned to ‘Jell-O’, as happened owing to poor maintenance during the test programme. The wings worked well, but the tailplane, which resembled a small canard delta aeroplane perched atop the fin, was a disaster. Freely pivoted and actuated by its small foreplane, it lacked ‘feel’ and tended to overcontrol, which in turn caused the wing slats to deploy as the aircraft pitched up.
The addition of four small fixed fins in a cruciform arrangement around the fuselage failed to alleviate the problem. In the end Grumman admitted defeat and substituted the conventional tail of a Cougar, which solved most of the stability problems. However, like the X-5, the Jaguar also spun suddenly and viciously. Another liability was the unreliable experimental Westinghouse XJ-40 engine, most of the problems with which were eventually traced to faulty assembly of a fuel control switchbox. An overlong cover fixing screw had been driven into the wiring. The XF 10F was abandoned in 1953 after 32 test flights, and a production order was cancelled. The Navy, by then building larger carriers with angled decks, no longer needed it. The flying prototype was relegated to barrier testing, while the uncompleted second aircraft was sent to a gunnery range as a target.
Notwithstanding the stability failures of the X-5 and Jaguar, the success of the swing wings prompted NASA to conduct further investigations into the system, using much of Wallis’s work as a basis. It was concluded that moving the pivot points outboard in a “glove“ fairing would provide better aerodynamics and more space in the fuselage. Wallis’s later Swallow designs show evidence that he had already come to a similar conclusion, but he had not yet moved the pivots as far out as NASA recommended in their 1958 report.
NASA also designed a simpler sweep mechanism than those of the X-5 and Jaguar. The way was clear for development of the Tactical Fighter Experimental, the versatile low-penetrating supersonic TFX required for the 1960s by both the USAF and the US Navy.