Document created: 1 June 04
Air University Review,
November-December 1972
Aircraft designers have long watched the flight of birds and the way they move their wings in flight. “If only an aircraft could be built to do this!” was the thought in the designers’ minds.
For many years, even before the Wright brothers’ epic flight, inventors have been working on moving wings for airplanes, wings that increased and decreased their length and width, wings that oscillated longitudinally, wings that flapped like birds’ wings. Some of these contraptions were actually built, and some of them even flew—sort of.
The practical idea of movable wings was introduced at a scientific convention in Rome in 1935. Dr. Adolf Busemann, a young German designer, read a paper on aircraft wings and high-speed flight. Dr. Busemann’s paper started aero engineers thinking about the advantages of movable wings. They found that one of the greatest advantages of sweptwings was the reduction of aerodynamic drag at high speeds. Research has since established that an airplane having zero sweep (wings at right angles to the centerline of the airplane) will produce the same drag at 540 miles per hour as an airplane having wings swept at 60 degrees flying at over a thousand miles per hour.
But some engineers realized that the movable wing concept had inherent disadvantages. When sweep angle increased, drag decreased but stalling speed increased. So the straight wing was ideal for low landing speed, and the highly swept wing was ideal for supersonic flight. From this simple statement of the problem comes the solution—variable sweep.
One of the scientists who was particularly impressed by the Busemann theory was Dr. Albert Betz of the Aerodynamics Research Institute at Göttingen, Germany. He set about to do further research on the idea. The work of Dr. Betz was noted by engineers at Messerschmitt, who felt that the concept might have an application to several high-speed aircraft that the company was then considering. Messerschmitt conducted extensive wind-tunnel testing to insure the validity of the theory.
In 1942 Messerschmitt began preliminary work on a design dubbed the P-1101. For over two years it was nothing more than a study program, but in September 1944 it was decided to produce one prototype aircraft. The German plans called for a single-place, mid-wing, single-engine aircraft with a 40-degree wing sweep. Attractiveness of other aircraft designs caused the P-1101 program to be considerably cut back, but it was decided that the prototype would be completed to serve as a flying test-bed for wing sweep and for new turbojet engines.
By this time, however, it was clear to the Germans that the war was almost over. In early 1945 a company of American infantry overran the P-1101 development facility. The Messerschmitt technical personnel had left every thing in perfect order, allowing the Americans to continue the swingwing research.
Although the basic engineering drawings and calculations for the P-1101 were never recovered, the aircraft was moved intact to Wright-Patterson Air Force Base, Ohio, where it was publicly displayed in 1945. Many observers considered the P-1101 a freak of engineering design and of little practical value. This, of course, would in future years prove to be very erroneous.
The P-1101 was truly an advanced aircraft for the time of its development. Its two-piece wing had steel spars, with wooden ribs, and a 40-degree sweep. The pressure cabin was located well forward in the upper part of the fuselage, followed by the fuel tanks, undercarriage retraction space, and a tail cone. The 1101’s wing span was some 27 feet, the wing area 170 square feet. The top speed was over 600 miles per hour at altitude.
Thus, the end of the war prevented the Germans from completing their first swingwing aircraft. There was one other aircraft using the movable wing concept, the P-1114. This novel design incorporated a provision for moving the entire wing assembly fore and aft along the fuselage to compensate for center of lift movement as flight speed increased.
Why the swingwing concept did not receive more attention cannot be definitely determined, but one main reason for the lack of interest was that engines were not powerful enough to propel the aircraft to the speeds at which variable sweep would make important contributions to performance. Also, the advanced German designers seemed to be more intrigued with the delta-wing design.
One hot morning in June 1951, a potbellied little white airplane streaked along the desert runway at Edwards Air Force Base, California. Then the skilled hands of Bell’s chief test pilot guided the tiny plane into the air, and America’s first swingwing aircraft had taken to the sky. It bore a marked resemblance to the P-1101.
Early in 1948 the Bell Aircraft Company, aided by the loan of the P-1101, began design studies on an aircraft that could change its wing sweep in flight. For a time it looked as though the Air Force might buy 24 of them, but an unfavorable evaluation by the Air Materiel Command reduced the program to a two-aircraft research endeavor. Designated the X-5, these planes were expected to demonstrate the best sweep angle for interceptor aircraft. It was made clear, however, that the X-5 was only a research tool, not intended for production, ever.
Not exactly the sleekest jet aircraft ever built, the X-5 looked something like a flying tadpole. The two-position adjustable wings were variable in sweep between 20 and 60 degrees. The X-5 had a takeoff weight of about 9500 pounds, the adjustable wing assembly weighing 1350 pounds. The engines of the plane were placed below the wing in the lower fuselage, to accommodate a variety of power plants and to have the engine out of the way of the sweep mechanism.
The mechanism for operating the sweep variation was truly an engineering masterpiece. The wings were mounted on hinges just outboard of each side of the fuselage. Inside each wing, near the leading edge, was attached one end of a ball-bearing screw jack. Shafts were then passed through the interior of the wings and into the fuselage, where they were driven by a gearbox. When the motors of the mechanism were operated, the screws rotated the wings on their pivots, changing the angle of the sweep. But the wings did more than just sweep when they were operated. In order to compensate for changes in pressure and center of gravity, it was necessary to slide the wings along rails mounted in the fuselage. At 20 degrees sweep, the entire wing assembly slid forward on the rails until, at 60 degrees sweep, they were about 27 inches forward of their starting positions. The sweeping and positioning actions took place simultaneously.
On the fifth test flight of the X-5, the sweep mechanism was operated for the first time. By the ninth flight, the sweep had been operated through its total limits. About that time, a strange characteristic of the X-5 was noted in the tests. At low speeds, almost all the available elevator action was required to level out the X-5 for landing. It had to be accelerated just before touchdown to keep from flying right into the ground.
Even though the X-5 had several deficiencies, a good deal of high-level interest was shown in it as a tactical fighter. Its advantages over bigger and heavier fighters of the day (e.g., the F-86 and F-89) were its much greater maneuverability and the fact that it could be carried in the C-119. But its complicated sweep mechanism and its limited fuel capacity and firepower led to its demise as a production fighter.
On 13 October 1953 one of the X-5s crashed when it failed to recover from a spin at 60 degrees sweep. The other now rests safely at the Air Force Institute of Technology, on loan from the Air Force Museum.
Even though the X-5 had been eliminated from consideration, it did not end the immediate history of the swingwing. Shortly thereafter, another strange airplane rolled out at the Flight Test Center. This was a great-grandson of the famous F4F Wildcat, and it featured two-position, inflight-variable swept wings.
The Grumman XF10F Jaguar was powered by a Westinghouse J40 engine generating 11,600 pounds of thrust. Its variable-sweep wings were mounted high on the fuselage. The wings were held straight for landings but could be swept back to 40 degrees for high-speed flight. The Jaguar featured a delta-shaped horizontal tail mounted atop the vertical fin. This replaced the conventional swept surface originally used.
The first of two XF10F prototypes flew in May 1953. For a time, it was thought that 30 of these might be ordered, but the XF10F proved to be a disappointment.
The Air Force’s newest fighter-bomber, the F-111, and its strategic counterpart, the FB-111, were the next of the swingwing aircraft. Several versions of this multimission aircraft have been built, including the bomber version FB-111 which has longer wings than the fighter version. The F-111 began life in 1959 when the Air Force defined an operational requirement for an advanced fighter (the TFX), which would later become the F-111.
Much of the technology involved with the F-111 design evolved from the X-5 and XF10F. However, with the F-111 sweep design, a slightly different approach was employed. In this aircraft the wing would sweep on its own pivot, well outboard of the fuselage. With this technique, the aerodynamic center remained relatively stationary throughout the wing’s full sweep.
Fully extended to 16 degrees sweep, the wing creates maximum lift, allowing short takeoffs and landings. As the speed increases and drag grows, the span and surface area are decreased by sweeping the wings to a maximum of 72.5 degrees. The wingtips come quite close to touching the leading edges of the tail. In the “folded position” the F-111 can move along at Mach 2.5 at altitude and supersonic on the deck. The wings can be placed in any intermediate position to perform any specific mission requirement.
Each wing pivots around an 8.5-inch-diameter steel pin, while the wing sweep is controlled by a hydraulic actuator. Working much like an automobile jack, large screws extend to determine the position of the wings. A pistol grip in the cockpit is the pilot’s control device. In the event damage occurs to the primary hydraulic system, a utility system will automatically cut out flow to nonessential subsystems in order to furnish power for the wing sweep and flight controls.
The F-111’s variable-sweep wing is going to play an important part in some aerodynamic testing. Shortly, an F-111 will be fitted with the so-called “supercritical” wing, which is specially designed to reduce high-speed drag. Equipping the F-111 with this wing is expected to greatly increase maneuverability and increase transonic performance without affecting the aircraft’s handling characteristics. The results of this testing should be very important to the F-111 and high-performance aircraft of the future.
For over a decade the Air Force has been looking for an aircraft to replace the B-52 bomber. The FB-111 is now partially fulfilling the manned bomber requirement caused by the phase-out of early-model B-52s and the B-58 fleet.
The so-called AMSA (Advanced Manned Strategic Aircraft) was the paper project for an advanced bomber (B-1). The AMSA also was to employ a swingwing. In November 1969 the Air Force released its request for proposals for the B-1 to interested bidders of the aerospace industry. The airframe contract was won by North American Rockwell, and General Electric won the engine contract.
Recently, North American displayed a full-scale mockup of the B-l. The “Big White Bird” is, indeed, a beautiful piece of airplane. The construction of the mockup was a big step toward possible future production of the B-l.
The B-1 will be able to fly at treetop level at almost 700 miles per hour and more than 1400 miles per hour at altitude. With its swingwings, it will be able to land on very short runways—quite amazing for an aircraft in the 350,000- to 400,000-pound weight class. The B-l’s swingwing mechanism is considered by engineers to be the most complicated system in the aircraft. As many as 35 different swingwing designs were examined before North American Rockwell decided on the present truss-type wing-pivot design.
So as to compensate for shifts in pressure and center of gravity, the B-1 uses a complex fuel system that transfers fuel within the fuselage to maintain aircraft stability. To maintain proper balance, fuel will be used from the mid-fuselage tanks first, from the wing tanks second, and from the forward and aft fuselage tanks last. Fuel can be pumped from the forward and aft fuselage tanks to the mid-fuselage tanks as the wings swing. An on-board computer will normally handle this intricate transfer, but it can be controlled manually from the cockpit. The sweep rates are geared to particular flight conditions and are slow enough to allow the fuel transfer to maintain the center of gravity.
The B-l’s wings can be swept or extended normally with only two of the four hydraulic systems. The wings can be swept from 15 degrees for takeoff and landing to sharply swept back at 67 degrees for high-speed flight. While engineers consider it highly unlikely that the wings might jam, the B-1 can be landed with wings fully swept—but, needless to say, it would be a much “hotter” landing.
The B-l’s first test flight is presently scheduled for the spring of 1974, with operational status in the late seventies. The B-1, engineers say, will last the rest of the twentieth century. It is, therefore, very possible that the entire Strategic Air Command fleet will be swingwing in the eighties, with the FB-111 and the B-l.
When the Navy canceled procurement of the F-111B, they found themselves in need of another aircraft to replace the F-4 Phantom. Once again the Navy went for another swingwing design in the F-14 Tomcat. The plane is being built by Grumman, long a manufacturer of Navy aircraft.
The F-14’s variable-sweep wing is the result of a tremendous amount of research work. One of the most advanced F-14 developments is “glove vanes,” which extend automatically from the leading edge near the fuselage at Mach 1, offsetting the shift in the F-14’s aerodynamic center. This leaves the horizontal stabilizer free for maneuvering, minimizing trim drag penalties and increasing combat agility. Also, flap activation is coordinated with the automatic wing sweep for maximum performance. The F-14’s maximum sweep is 68 degrees (from a minimum of 20 degrees), when the wing and tail surfaces are, for all practical purposes, one.
The F-14’s sophisticated Mach-sweep programmer provides for fully automatic wing sweep as a function of speed and altitude. Therefore, the pilot can obtain the maximum performance under any flight condition. As is true with the B-1, the F-14 pilot can manually control the wing sweep, but even then the programmer will maintain limited control on the pilot’s actions.
With its swingwing and powerful engines, the F-14 may make a formidable addition to the Navy’s striking power.
Our report on swingwing aircraft would not be complete without mentioning the swingwing aircraft of Europe.
The presently flying French Mirage G8 is the culmination of Mirage’s experience in swingwing aircraft, having built the G1 and G4 prior to the present G8 configuration. At full sweep the wings and tail have only a slight slit of space between them. It is powered by two engines and has a top speed of Mach 2.5.
Indications are that the G8 might well be the first variable-geometry aircraft to be ordered by the French Air Force. It could be a replacement for the Mirage III in the late seventies.
During the same period the British-German-Italian Panavia 200, the new multimission aircraft, is designed to enter service with the Royal Air Force, the German Luftwaffe, the German Navy, and the Italian Air Force.
The variable-sweep wing is the key feature that gives the 200 such a wide diversification of capabilities. Swept forward, it provides high lift capability, giving STOL performance from semiprepared fields and a very long loiter time. Swept fully back, it gives a low-drag, high speed capability with very good response at low levels.
Powered by two Rolls-Royce RB-199 engines of advanced technology design, the 200 is capable of Mach 2+ at altitude. Wing sweep range is from 20 to 70 degrees.
The first design of Boeing’s supersonic transport (SST) might have been the biggest swinger of them all.
When the sides were being formed for the battle to decide who would build the SST, it came down to two different SST concepts—the delta-wing design of Lockheed and Boeing’s swingwing. Boeing won and went about the job of building the largest swingwing ever. But that was not to be. The decision was made in 1969 to abandon the swingwing and go with a fixed, double-delta shape. Now the whole SST program has been scrapped.
One of the main reasons for the switch from the swingwing design was the tremendous weight penalty incurred by the swingwing mechanism. It was quoted that the weight penalty for the variable geometry was over 40,000 pounds, about 6 percent of the gross weight.
In the late 1960s, the Lockheed and the Air Force Flight Dynamics Laboratory conducted tests on a swingwing spacecraft. The spacecraft was designed to be a model for a reusable launch vehicle.
The triangular-shaped spacecraft has a small vertical tail with a movable rudder. The small delta wings swing into the airstream from the sides of the vehicle, about halfway down its length. The wings would be used after the spacecraft had re-entered the atmosphere and slowed down for a conventional aircraft-type landing. The future may see some application of this concept in returning space vehicles.
Those early experimenters who strapped contraptions on their backs and to their arms and jumped from precipices and bridges, frantically flapping their arms, knew the birds had something. While this review has shown that the swingwing has a firm hold on its domain of aeronautics, it is not as sophisticated as our bird imitators—it doesn’t flap; it merely swings. But it does fly!
Air Force Systems Command
Contributor
William G. Holder (B.S.A.E., Purdue University) is a space systems analyst with the Foreign Technology Division, Air Force Systems Command, Wright-Patterson AFB, Ohio. He has worked with the Boeing Company on the Bomarc B and the Saturn V. As a lieutenant in the U.S. Army, he served three years as an air defense guided missile instructor. Mr. Holder is the author of a number of technical articles and a book, Saturn V—The Moon Rocket (1969). . . Robert H. George (B.S.M.E., University of Arkansas) is a Supervisory Aerospace Engineer, Air Force Systems Command, Wright-Patterson, AFB, Ohio. He is a commissioned officer in the Air Force Reserve and has served on extended active duty with Headquarters Command and AFSC. He has thirteen years’ experience in aircraft systems engineering, aerodynamics and ballistic missiles, space launch vehicles, and spacecraft.
Disclaimer
The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.
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