NACA-NASA and Thrust Vectored Approaches: from X-14 to YAV-8B

The advent of the gas turbine engine at the end of the 1930s, and its demonstration and incorporation on aircraft in the 1940s, set the stage for a revolution in flight propulsion that affected nearly all powered fly­ing vehicles by the mid-1950s. The pure-jet engine could power aircraft through the speed of sound and transport passengers across global dis­tances. The turbopropeller engine applied to tactical transports, and the turboshaft engine applied to helicopters and V/STOL designs, gave them the power to weight ratios and reliability that earlier piston engines had lacked, enabling generations of far more efficient aircraft typified by the ubiquitous Lockheed C-130 Hercules and the Bell UH-1 "Huey” helicopter.

As well, the jet engine enabled designers to envision STOL and VTOL aircraft taking advantage of its power. Initially, many designers thought that a VTOL aircraft would need to have many small jet engines for ver­tical lift, coupled with one or more major powerplants for conventional flight. For example, the delta wing Short SC.1, a British low-speed VTOL testbed design that first flew in 1957, had five small jet engines: four to produce a stabilizing "bedpost” of vertical thrust vectors and a fifth to propel it through the air. Other such aircraft, for example, the Dassault Balzac and Dassault Mirage IIIV, followed a generally similar approach (though none, however, entered service).[1427]

But other designers wisely rejected the complexity and inherent unreliability of such multiengine conglomerations. Instead, they envi­sioned a more efficient form of propulsion, vectoring the thrust of a jet engine so that the aircraft could lift vertically and then transition
into forward flight. This approach was pursued most successfully with the Hawker P. 1127, forerunner of the Harrier fighter family.[1428] Within the United States, Lockheed received an Army development contract for two research aircraft, the XV-4A, using a form of vectored thrust, whereby the exhaust of two jet engines buried in the wing roots would be deflected through a central fuselage chamber and mixed with air drawn through a fuselage intake, with this "augmented” exhaust enabling ver­tical flight. Optimistically named the Hummingbird and first flown in 1962, the XV-4A never enjoyed the success attendant to the P. 1127. Most seriously, anticipated augmented flow efficiencies were not achieved, limiting performance. The first aircraft crashed during a VTOL conver­sion in 1964, killing its pilot. The second was modified with a retrograde propulsion system reminiscent of the SC-1, using four lift jets and two thruster jets, and was redesignated the XV-4B Hummingbird II. It also crashed in 1969, though its pilot ejected safely. In contrast, the P. 1127 program went along relatively smoothly both in Britain and the United States. In the U. S., thanks to John Stack of Langley, it received strong technical endorsement, in part because the Agency was already follow­ing an important and evolving vectored-thrust study effort: the Bell X-14 program. America’s story of vectored-thrust research thus begins not with Langley and Ames’s exposure to the streamlined P.1127, but with quite another design: the X-14. Like the XV-15, the X-14 became one of the more successful research aircraft of all time, having flown almost a quarter century and contributing to the success of a variety of other programs.[1429]