The Convair XF-92 was the first serious attempt to incorporate delta wings, typically angled at 60 degrees, into the layout of an airplane. The brainchild of a former Nazi scientist, this ingenious innovation helped humanity finally harness and control the power of supersonic flight.
Yet despite its massive implications, the delta wing was always troubled by persistent problems that have only been resolved recently with the rise of computer technology.
Contents
Background
By the 1930s, it was becoming increasingly clear that the high drag produced by traditional straight wings was hampering efforts at achieving the next breakthrough in aviation technology, that of supersonic flight.
As a result, new swept-back designs with angled wings began to be proposed by some of the top engineering minds of the day.
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One of these visionaries was Alexander Lippisch, a German scientist who helped build the infamous Me-163B-1 Komet operated by the Nazi Luftwaffe, he put forward the idea of a triangular-shaped delta wing, so called because it resembled the ‘Delta’ from the Greek alphabet.
Lippisch built on the knowledge he had acquired during his time on the design team of the Messerschmitt Komet, who themselves had been heavily influenced by Adolf Busemann and a paper he had presented in 1935 at the Volta Congress of High Speeds in Aviation on a brand new ‘arrow wing’ design.
Lippisch’s novel wing was different from conventional designs in that it was sharply angled to reduce drag as well as having a large surface area that would encourage more lift.
Surprisingly, Busemann and Lippisch were not the first to conceive the delta wing, with the first patent awarded to Englishmen J.W. Butler and E. Edwards all the way back in 1867, 36 years before the first ever manned flight by the Wright brothers.
Fortunately, Lippisch lived in an age where it was actually possible to test the delta wing in flight conditions.
As the Second World War commenced, Lippisch decided to put his theory to the test by constructing a delta wing glider made out of wood called the DM-1 that would be launched from a transport plane traveling at high altitudes.
By the end of the war in 1945, as Germany made its last stand against the Allied forces, Lippisch and the nearly finished DM-1 were seized by the Americans during their occupation of Prien.
The Americans took such a great interest in what Lippisch’s research had to show that they decided to enroll him in Operation Paperclip, a top-secret program spearheaded by the Joint Intelligence Objectives Agency which brought several talented German scientists to the US to help develop cutting-edge technology.
Development
Back in the US, the National Advisory Committee for Aeronautics (NACA), NASA’s forbearer, verified the advantages of the delta wing through a series of wind tunnel tests, confirming its potential to revolutionize supersonic flight.
They not only proved the merit of Lippisch’s work but also that of two American engineers, Michael Gluhareff and Robert T. Jones, who had independently come up with the same idea during the Second World War on the other side of the Atlantic.
The results piqued the interest of engineers working at Consolidated-Vultee, who decided to implement the delta wing into their latest project, the XF-92.
As part of the XF-92 program, Lippisch’s idea was to be incorporated into the Consolidated-Vultee Model 7002 aircraft, which was a test prototype intended to investigate how delta wings performed at low and high supersonic speeds.
In 1948 however, the XF-92 project was cancelled by USAF, but Convair staff, seeing its potential, were given special permission to continue work on the Model 7002.
After being delivered to Muroc Air Force Base in April 1948, the Consolidated-Vultee Model 7002 went through a series of tests for the rest of the year and throughout 1949, with test pilots Sam Shannon and William Martin at the helm.
They helped demonstrate that the delta-wing design was in fact flightworthy and that an enormous vertical tail, originally added in the belief that a delta wing might block airflow to the tail and make the plane impossible to control, was unnecessary.
Garnering renewed interest, It was next handed back to USAF for Phase II testing where Major Charles “Chuck” Yeager, the first man to break the sound barrier, would be heavily involved in charting out the performance metrics of this cutting-edge craft, now designated XF- 92A.
By February 1953, after mechanical problems and even the destruction of its tail cone during a fire, USAF finally finished its evaluation of the XF-92A and transferred it to NACA where pilot A. Scott Crossfield was selected to fly it for further assessment.
Crossfield’s first outing with the XF-92A was memorable, with the aviator unable to control it even during a taxi across a lakebed. Being a new design, the delta-winged prototype had become notorious in pilot circles for violently rolling up and down. As Crossfield recalls:
“Nobody wanted to fly the XF-92. There was no lineup of pilots for that airplane. It was a miserable flying beast”.
After rolling uncontrollably towards the edge of the lakebed and realizing his brakes were completely burned out, Crossfield spotted a country lane, maneuvering the craft so it travelled 100 yards before coming to a stop. The road itself was later renamed ‘Crossfield Pike’.
Between April 1953 and October 1953, Crossfield flew the XF-92A a total of 25 times. The first 13 flights were to find out more about its static longitudinal stability, dynamic stability, directional control, longitudinal and lateral stability and control, and low-speed stability and control attributes.
For the next 10 missions, the engineering team experimented with different wing fence configurations in an attempt to figure out how they could reduce the tendency of the swept delta wing to pitch up at low velocities.
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During this round of testing the XF-92A undertook 2 flights to see how its low-speed lateral and directional control fared without wing fences.
The first was completed without a hitch, but when Crossfield landed on the lakebed during the second flight, the nose gear fell apart. Although Crossfield emerged relatively unscathed from the incident, the XF-92A program was discontinued.
Legacy of the XF-92
The XF-92A demonstrated for the first time the feasibility of delta-winged aircraft while at the same time highlighting a lot of its practical problems, such as its propensity to aggressively pitch up.
After Consolidated-Vultee was renamed Convair, the company further tweaked the delta wing in its next model, the YF-102 Delta Dagger.
Wind tunnel tests indicated that the Dagger was able to accelerate through the speed of sound with relative ease, yet problems with drag, especially as it reached Mach 1, continued to present obstacles to supersonic flight.
In 1954, Richard T. Whitcomb found the solution whilst developing his theory of the ‘supersonic area rule,’ which argued that supersonic speeds could be attained by increasing the cross-sectional area from a pointed nose.
He recommended that anything disturbing the airstream, such as the canopy over the wings, tail, or cockpit, should be matched with a reduction in cross-section elsewhere.
The follow-up YF-102A, acting in accordance with the principles of supersonic area rule, thus assumed the appearance of a coke bottle, a shape that helped it to accelerate effortlessly through Mach 1 speeds.
With the delta-wing puzzle finally cracked, a whole new era of flight commenced. In the 1950s the revolutionary swept-wing would first be assimilated into the B-58 Hustler and even the experimental XB-70 Valkyrie bomber.
In the Soviet Union, the design would be consolidated into the aborted Tu-144 supersonic passenger jet and more successfully into the MiG-21, the most widely used fighter jet of the Cold War.
Additionally, the French Dassault Mirage III would adopt the delta-wing, as would the F-102/F-106 interceptor, XF2Y-1 Sea Dart jet-plane, and B-58 strategic bomber.
It would even be incorporated into the Space Shuttle that served NASA between 1981 to 2011. This reusable low Earth orbital spacecraft system employed a uniquely configured ‘cranked delta’ wing in which the leading edge of the wing featured a slight bend close to its midpoint.
This adjustment proved much better than the previous straight-winged layout, which had not provided sufficient lift at high speeds and altitudes when gliding back down to earth.
Safety Issues
On the other hand, despite its clear advantages, the delta wing still had many safety issues that had been known to pilots as far back as the Phase I testing stage of the XF-92 by NACA in the late 1940s.
Being unstable at low speeds they required high take-off velocities and landings, were unbalanced at high angles of attack, and produced huge drag when pilots attempted to keep their plane level.
In fact, high speed takeoff had been a determining factor in the first recorded crash of the delta-winged Concord in 2000.
Concerns about the safety of the delta wing had first emerged in the 1980s, and for a time the delta wing decreased in popularity as manufacturers sought out safer alternatives for their supersonic planes.
Luckily, the emergence of ‘fly-by-wire’ control systems saved the delta wing from the scrap heap. Pilots were now able to compensate for its unpredictable wing qualities with computerized programs that could control any jerky movements.
In addition, the invention of canards, which are small horizontal fins typically fitted on the fuselage in front of the wings, greatly improved stability, especially during high angles of attack.
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As a result, thanks to the advancements of the digital age, the delta wing remains a staple design feature of all modern jets looking to attain high levels of control past the sound barrier.
The original pioneer of this epoch-changing invention, the Convair XF-92 A model can still be viewed at the Air Force Museum in Dayton, Ohio, where it has remained on display since 1969.
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Specifications
- Crew: 1
- Length: 42 ft 6 in (12.95 m)
- Wingspan: 31 ft 4 in (9.55 m)
- Height: 17 ft 9 in (5.41 m)
- Empty weight: 9,078 lb (4,118 kg)
- Gross weight: 14,608 lb (6,626 kg)
- Powerplant: 1 × Allison J33-A-29 afterburning turbojet engine, 4,500 lbf (20 kN) thrust dry, 7,500 lbf (33 kN) with afterburner
- Maximum speed: 718 mph (1,156 km/h, 624 kn)
- Service ceiling: 50,750 ft (15,470 m)
- Rate of climb: 8,135 ft/min (41.33 m/s)
