Convair XF-92

The Convair XF-92 was an early American delta-wing aircraft. Originally conceived as a point-defense interceptor, the design was later used purely for experimental purposes. However, it led Convair to use the delta-wing on a number of designs, including the F-102 Delta Dagger, F-106 Delta Dart, B-58 Hustler, the US Navy's F2Y Sea Dart as well as the VTOL FY Pogo.

Design and development
Prior to August 1945, the Vultee Division of Consolidated-Vultee looked at the possibility of an swept-wing aircraft powered by a ducted rocket. Years earlier, the company had performed designs which involved liquid-cooled radiator engines. If properly designed, fuel would be added to the heat produced by small rocket engines, creating a "pseudo-ramjet".

In August 1945, the United States Army Air Forces (USAAF), soon to be renamed the United States Air Force, issued a proposal for a supersonic interceptor capable of 700 mi/h speeds and reaching an altitude of 50000 ft in four minutes. Several companies responded, among which was Consolidated-Vultee, which submitted its design on 13 October 1945. This design featured swept wings and V-tails, as well as a powerful propulsion system. Besides the ducted rocket, four 1,200 lbf rockets were positioned at the exhaust nozzle, along with the 1,560 lbf 19XB turbojet produced by Westinghouse.

A proposal by Consolidated Vultee (later Convair) was accepted in May 1946, with a proposal for a ramjet-powered aircraft, with a 45° swept wing under USAAF Air Materiel Command Secret Project MX-813. However, wind tunnel testing demonstrated a number of problems with this design.

In contrast to popular myth, this aircraft and subsequent Convair delta wings owed nothing to the research of Alexander Lippisch. Convair engineers designed the delta wing before becoming aware of Lippisch's work, using the 1944 work of Robert T. Jones on very thin delta wings. Lippisch's thick delta wing design was tested by the NACA at Langley Research Center, where it proved completely unsuitable for transonic and supersonic flight.

Using the 1560 lbf Westinghouse J30 assisted by a battery of six 2000 lbf liquid-fueled rockets, the layout placed the engine in a seemingly oversized cylindrical fuselage, with the pilot in a cockpit centered in the middle of the fuselage, serving double duty as a shock cone for the engine intake. The basic layout of the fuselage was very similar to the Miles M.52 design, although the M.52 did not use a delta wing. The design was presented to the U.S. Air Force in 1946, and was accepted for development as the XP-92.

In order to gain in-flight experience with the delta wing layout, Convair suggested building a smaller prototype, the Model 7002, which the USAAF accepted in November 1946. The design was similar in general layout as the original, but by placing the pilot in a conventional cockpit at the front, instead of centered in the fuselage, the resulting aircraft looked considerably less odd. In order to save development time and money, many components were taken from other aircraft; the main gear was taken from a North American FJ-1 Fury, the nosewheel from a Bell P-63 Kingcobra, the engine and hydraulics were taken from a Lockheed P-80 Shooting Star, the ejector seat and cockpit canopy were taken from the cancelled Convair XP-81, and the rudder pedals were taken from a BT-13 trainer.

Construction was well underway at Vultee Field in Downey, California when North American Aviation took over the Vultee plants in summer 1947. The airframe was moved to Convair's plant in San Diego, and completed in the autumn. In December it was shipped without an engine to NACA's Ames Aeronautical Laboratory for wind tunnel testing. After testing was completed, the airframe was returned to San Diego, where it was fitted with a 4250 lbf Allison J33-A-21 engine.

By the time the aircraft was ready for testing, the concept of the point-defense interceptor seemed outdated and the (now redesignated) F-92 project was cancelled. They also decided to rename the test aircraft as the XF-92A.

Operational history


In April 1948 the XF-92A was shipped to Muroc Dry Lake (later to become Edwards AFB). Early tests were limited to taxiing, although a short hop was made on 9 June 1948. The XF-92A's first flight was on 18 September 1948 with Convair test pilot Ellis D. "Sam" Shannon at the controls. After company testing, the aircraft was turned over to the USAAF on 26 August 1949, with the testing being assigned to Frank Everest and Chuck Yeager.

Yeager was the first Air Force pilot to fly the XF-92A. On his second flight he dived the aircraft in a 4 g split-S dive, reaching Mach 1.05 for a brief time. When approaching for landing on this flight he continued to pull the nose higher and higher in order to slow the forward speed to avoid the problems from his first attempt. Surprisingly, the aircraft simply wouldn't stall; he was able to continue raising the nose until he reached 45 degrees pitch, flying under control in that attitude to a landing at 67 mi/h, 100 mi/h slower than Convair had managed.

In 1951, the XF-92A was refitted with an Allison J33-A-29 engine with an afterburner, offering a thrust of 7500 lbf. The re-engined XF-92A was flown by Yeager for the first time on 20 July 1951. However, there was very little improvement in performance. In addition, there were maintenance problems with this engine and only 21 flights were made during the next 19 months. A final engine change was made to the 8400 lbf J33-A-16.

On 9 April 1953, Scott Crossfield began a series of flights on behalf of NACA. These tests revealed a violent pitch-up tendency during high-speed turns, often as much as 6 g, and on one occasion 8 g. The addition of wing fences alleviated this problem.

None of the pilots had much good to say about the design. Yeager commented "It was a tricky plane to fly, but ... I got it out to 1.05 Mach." Crossfield was more direct, saying "Nobody wanted to fly the XF-92. There was no lineup of pilots for that airplane. It was a miserable flying beast. Everyone complained it was underpowered."

Influence
The delta wing's thin airfoil cross section, low weight and structural strength made this a good combination for a supersonic airplane (although the fuselage was of a pre-area rule design). The large surface area of 425 ft² (39 m²), and the low wing loading it had as a result, led to excellent low-speed performance as well. Very slow landing speeds could be achieved, at the cost of extremely nose-high landing angles and the resulting poor visibility. The combination of good high-speed and low-speed characteristics was very difficult to achieve for other planforms. Although the XF-92 itself was not liked, the design concept clearly had promise, and the delta-wing became a "standard" layout on Convair designs through the 1950s and 1960s.

Of particular interest to aircraft designers was the unexpectedly good low-speed behavior Yeager had noticed on his second flight. The aircraft continued to remain controllable at very high angles of attack (alpha), where a conventional layout would have stalled. The reason for this turned out to be the unexpected creation of a large vortex over the top of the wing, generated by the airflow between the fuselage and leading edge of the wing at high alpha. The vortex became "attached" to the upper surface of the wing, supplying it with air moving at speeds much greater than the aircraft's forward speed. By controlling the flow in this critical area, the performance envelope of the delta could be greatly expanded, which led to the introduction of canards on most delta-wing designs in the 1960s and 1970s. More recently "mini-deltas", in the form of leading edge extensions, have become common on most fighter aircraft, creating the vortex over a more conventional wing planform.

Aircraft on display

 * 46-682 - Research & Development Gallery at the National Museum of the United States Air Force, Wright-Patterson Air Force Base, near Dayton, Ohio.

Operators

 * United States Air Force
 * United States Air Force

Popular culture
An unusual application of the XF-92A was as a movie model, stepping into the role of the "MiG-23" in the Howard Hughes film, Jet Pilot (1957 film), starring John Wayne and Janet Leigh. Due to the lengthy delay in releasing the film, by the time it appeared in 1957, the XF-92A's role had been left on the cutting room floor. It did appear in the film Toward the Unknown (1956) starring William Holden, again in the guise of another aircraft, this time, the F-102 Delta Dagger.