Swept Wing

Mig-15 versus F-86.

 

With the introduction of jets in the latter half of World War II applying sweep became relevant. The German jet powered Messerschmitt Me 262 and rocket powered Messerschmitt Me 163 suffered from compressibility effects that made them very difficult to control at high speeds. In addition the speeds put them into the wave drag regime, and anything that could reduce this drag would increase the performance of their aircraft, notably the notoriously short flight times measured in minutes. This resulted was a crash program to introduce new swept wing designs, both for fighters as well as bombers. The Focke-Wulf Ta 183 was a swept wing fighter design with a layout very similar to that later used on the MiG-15 that was not produced before war’s end.

A prototype test aircraft, the Messerschmitt Me P.1101, was built to research the tradeoffs of the design and develop general rules about what angle of sweep to use. None of the fighter or bomber designs were ready for use by the time the war ended, but the P.1101 was captured by US forces and returned to the United States, where two additional copies with US built engines carried on the research as the Bell X-5. The last jet fighter designed by Willy Messerschmitt the HA-300 had swept wings, Delta Wing in this case.

The Soviet Union was intrigued about the idea of swept wings on aircraft at the end of World War II in Europe, when their “captured aviation technology” counterparts to the western Allies spread out across the defeated Third Reich. Artem Mikoyan was asked by the Soviet government, principally by the government’s TsAGI aviation research department, to develop a test-bed aircraft to research the swept wing idea-the result was the late 1945-flown, unusual MiG-8 Utka pusher canard layout aircraft, with its rearwards-located wings being swept back for this type of research. When applied to the jet powered Mig-15, its maximum speed of 1,075 km/h (668 mph) outclassed the straight-winged American jets and piston-engined fighters first deployed to Korea.

von Kármán travelled to Germany near the end of the war as part of Operation Paperclip, and reached Braunschweig on May 7, discovering a number of swept wing models and a mass of technical data from the wind tunnels. One member of the US team was George S. Schairer, who was at that time working at the Boeing company. He immediately forwarded a letter to Ben Cohn at Boeing stating that they needed to investigate the concept. He also told Cohn to distribute the letter to other companies as well, although only Boeing and North American made immediate use of it.

In February 1945 NACA engineer Robert T. Jones started looking at highly-swept delta wings and V shapes, and discovered the same effects as Busemann. He finished a detailed report on the concept in April, but found his work was heavily criticised by other members of NACA Langley, notably Theodore Theodorsen, who referred to it as “hocus-pocus” and demanded some “real mathematics”. However, Jones had already secured some time for free-flight models under the direction of Robert Gilruth, whose reports were presented at the end of May and showed a fourfold decrease in drag at high speeds. All of this was compiled into a report published on 21 June 1945, which was sent out to the industry three weeks later. Ironically, by this point Busemann’s work had already been passed around.

Boeing was in the midst of designing the B-47 Stratojet, and the initial Model 424 was a straight-wing design similar to the B-45, B-46 and B-48 it competed with. A recent design overhaul completed in June produced the Model 432, another four-engine design with the engines buried in the fuselage to reduce drag, and long-span wings that gave it an almost glider-like appearance. By September the Braunschweig data had been worked into the design, which re-emerged as the Model 448, a larger six-engine design with more robust wings swept at about 35 degrees. Another re-work in November moved the engines to pods under the wings, as the Army was concerned about engine fires potentially destroying the aircraft. The resulting design would have performance rivaling the fastest fighters, and trounced the straight-winged competition. The basic layout of engines on pylons under swept wings is still used on most airliners today.

In fighters, North American Aviation was in the midst of working on a straight-wing jet powered naval fighter then known as the FJ-1. It was submitted it to the Air Force as the F-86. Larry Green, who could read German, studied the Busemann reports and convinced management to allow a redesign starting in August 1945. A battery of wind tunnel tests followed, and although little else of the design was changed, including the wing profile (NACA 0009), the performance of the aircraft was dramatically improved over straight-winged jets. With the appearance of the Mig-15, the F-86 was rushed into combat and straight-wing jets like the P-80 and F-84 were soon relegated to ground attack. Some such as the F-84 and F-9 Cougar were later redesigned with swept wings from straight-winged aircraft. Later planes such as the F-100 would be designed with swept wings from the start, though additional innovations such as the afterburner, area-rule and new control surfaces would be necessary to master supersonic flight.

The British also received the German data, and decided that future high-speed designs would have to use it. A particularly interesting victim of this process was the cancellation of the Miles M-52, a straight-wing design for an attempt on the speed of sound. When the swept wing design came to light the project was cancelled, as it was thought it would have too much drag to break the sound barrier, but soon after the US nevertheless did just that with the Bell X-1. The Air Ministry introduced a program of experimental aircraft to examine the effects of swept wings (as well as delta wings) and introduced their first combat designs as the Hawker Hunter and Supermarine Swift.

The German research was also “leaked” to SAAB from a source in Switzerland in late 1945. They were in the process of developing the jet fighter SAAB Tunnan, and quickly adapted the existing straight-wing layout to incorporate a 25 degree sweep. Although not well known outside Sweden, the Tunnan was a very competitive design, remaining in service until 1972 in some roles.

The introduction of the German swept wing research to aeronautics caused a minor revolution, especially after the dramatic successes of the B-47 and F-86. Eventually almost all design efforts immediately underwent modifications in order to incorporate a swept wing. The classic Boeing B-52, designed in the 1950s, would remain in service until into the 21st century as a high subsonic long-range heavy bomber despite the development of the triple-sonic B-70, supersonic swing-wing B-1, and flying wing designs. While the Soviet never matched the performance of the B-52 with a jet design, the intercontinental range Tu-95 turboprop bomber also remains in service today. With a near-jet class top speed of 920 km/h, it is an unusual in combining swept wings with propeller propulsion and remains the fastest propeller powered production plane. By the 1960s, most civilian jets such as the Boeing 707 adopted swept wings as well.

By the early 1950s nearly every new fighter was either rebuilt or designed from scratch with a swept wing. The A-4 Skyhawk and F4D Skyray were examples of delta wings which also have swept leading edges with or without a tail. Most early transonic and supersonic designs such as the MiG-19 and F-100 used long, highly swept wings. Swept wings would reach Mach 2 in the arrow-winged BAC Lightning, and stubby winged F-105 Thunderchief, which was found to be wanting in turning ability in Vietnam combat. By the late 1960s, the F-4 Phantom and MiG-21 which both used variants on tailed delta wings came to dominate front line air forces. Variable geometry wings were employed on the American F-111, F-14 and Soviet Mig-27, though the idea would be abandoned for the American SST design. After the 1970s, most newer generation fighters optimized for maneuvering air combat since the USAF F-15 and Soviet MiG-29 have employed relatively short-span fixed wings with relatively large wing area.

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