Vectored V/STOL Comes of Age: The P. 1127, Kestrel, and YAV-8B VSRA

In 1957, Britain’s Hawker and Bristol firms began development of what would prove to be the most revolutionary V/STOL airplane developed to that point in aviation history, the P.1127. This aircraft program, begun

as a private development by two of Britain’s more respected companies, was the product of Sir Sidney Camm and Ralph Hooper of Hawker, and Stanley Hooker of Bristol. It eventually spawned a remarkable opera­tional aircraft that fought in multiple wars and served in the air forces and naval air services of many nations. Hawker had an enviable reputa­tion for designing high-performance aircraft, dating to the Sopwith fight­ers of the First World War, and Bristol had an equally impressive one in the field of aircraft propulsion. NATO’s Mutual Weapon Development Project (MWDP) supported the project as it evolved, and it drew heav­ily upon American support from John Stack of NASA and the Langley Research Center, and from the U. S. Marine Corps. (The P.1127 design was extensively tested in Langley’s 30-Foot by 60-Foot Full Scale Tunnel, and the 16-Foot Transonic Tunnel, helping identify and alleviate a poten­tially serious pitch-up problem exacerbated by power effects during tran­sition upon the original horizontal tail configuration).[1446] Powered by a

single Bristol Siddeley Pegasus 5 vectored-thrust turbofan of 15,000- pound thrust, the P.1127 completed its first tethered hover in October 1960, an untethered hover the next month, and, after extensive prepara­tion, its first transition from vertical to conventional in September 1961. As with the X-14 and other V/STOL testbeds, bleed air reaction nozzles were used for hover attitude control and, in the P.1127’s initial configu­ration, had no SAS. Low control power, aerodynamic suck-down, and marginal altitude control power made for a high pilot workload for this early Harrier predecessor. Even so, NACA researchers quickly realized that the P.1127 offered remarkable promise. NASA pilots Jack Reeder from Langley and Fred Drinkwater from Ames went to Europe to fly the P.1127 in June 1962, Reeder confiding afterward: "The British are ahead of us again.”[1447] His flight evaluation report noted:

The P. 1127 is not a testbed aircraft in the usual sense. It is advanced well beyond this stage and is actually an operational prototype, with which it is now possible to study the VSTOL concept in relation to military requirements by actual opera­tion in the field. The aircraft is easily controlled and has safe flight characteristics throughout the range from hover to air­plane flight. The performance range is very great; yet, conver­sions to or from low or vertical flight can be accomplished simply, quickly, and repeatedly.[1448]

Camm’s P.1127 led to the Hawker Kestrel F. G.A. Mk. 1, an interim "militarized” variant, nine of which undertook operational suitabil­ity trials with a NATO tripartite (U. K., U. S., and Federal Republic of Germany) evaluation squadron in 1965. The trials confirmed not only the basic performance of the aircraft, but also its military potential. So the Kestrel, in turn, led directly to a production military derivative, the Hawker Harrier G. R. Mk. 1—or, as known in U. S. Marine Corps service, the AV-8A. Eight of the Kestrel aircraft, designated XV-6A, remained in the United States for follow-on testing. NASA received two Kestrels, flying them in an extensive evaluation program at Langley with pilots

Jack Reeder, Lee H. Person, Jr., Robert Champine, and Perry L. Deal, under the supervision of project engineer Richard Culpepper.[1449]

Langley tunnel-testing and flight-testing revealed a number of defi­ciencies, though not of such magnitude as to detract from the impres­sion that the P.1127 was a remarkable accomplishment, and that it had tremendous potential for development. For example, a directional insta­bility was noticed in turning out of the wind, yaw control power was low but not considered unsafe, and pitch-trim changes occurred when leav­ing ground effect. The usual hot-gas ingestion problem could be circum­vented by maintaining a low forward speed in takeoff and landing. A static pitch instability was encountered at alphas greater than approxi­mately 15 degrees, and a large positive dihedral effect limited crosswind operations. Transition characteristics were outstanding, with only small trim changes required. Overall, low – and high-speed performance was excellent. Like any swept wing airplane, the Kestrel’s "Dutch roll” lateral – directional damping was low at altitude, requiring provision of a yaw damper. It had good STOL performance when the engine nozzles were deflected between purely vertical and purely horizontal settings. Indeed, this would later become one of the Harrier strike fighter’s strongest oper­ational qualities.[1450]

Like any operational aircraft, the Harrier went through progressive refinement. Its evolution coincided with the onset of advanced avionics, the emergence of composite structures, and NASA’s development of the supercritical wing. All were developments incorporated in the next gener­ation of Harrier, the AV-8B Harrier II, developed at the behest of the U. S

Marine Corps and adopted, in slightly different form, as the Harrier Mk. 5 by the Royal Air Force. As well, the AV-8B benefited from Langley research on optimum positioning of engine nozzles, trailing-edge flaps, and the wing, in order to obtain higher propulsive lift. (This jet age work mirrored much earlier work on optimum positioning of propellers, engines, and nacelles undertaken at Langley in the 1920s by the NACA).[1451]

Two AV-8A Harriers had been modified to serve as prototypes of the new Harrier II, these being designated YAV-8B. Though deceptively similar to the earlier AV-8A, the YAV-8B relied extensively on graphite epoxy composite structure and had a leading-edge extension at its wing – root and a bigger, supercritical wing. The first made its initial flight in November 1978, joined shortly afterward by the second. A year later, in November 1979, the second YAV-8B crashed after engine failure; its pilot ejected safely. However, flight-testing by contractor and service pilots confirmed that the AV-8B would constitute a significant advance over the earlier AV-8A for, during its evaluation program, "all performance requirements were met or exceeded.”[1452] Not surprisingly, the AV-8B entered production and squadron service with the U. S. Marine Corps, replacing the older Vietnam-legacy AV-8A.

In 1984, after the AV-8B entered operational service, the U. S. Marine Corps delivered the surviving YAV-8B to Ames so that Ames researchers could investigate advanced controls and flight displays, such as those that might be incorporated on future V/STOL combat systems called upon to conduct vertical envelopment assaults from small assault car­riers and other vessels in all-weather conditions. The study effort that followed built upon Ames’s legacy of V/STOL simulation studies, using both ground and flight simulators to evaluate a variety of guidance, con­trol, and display concepts, particularly the research of Vernon K. Merrick, Ernesto Moralez, III, Jeffrey A. Schroeder, and their associates.[1453] NASA designated the YAV-8B the V/STOL Systems Research Aircraft (VSRA). A team led by Del Watson and John D. Foster modifying it with digi­tal fly-by-wire controls for pitch, roll, yaw, thrust magnitude and thrust deflection, and programmable electronic head-up displays. Researchers subsequently flew the YAV-8B in an extensive evaluation of control

The NASA Ames YAV-8B V/STOL Systems Research Aircraft. NASA.

system concepts and behavior, from decelerations to hover, and then from hover to a vertical landing, assessing flying qualities tradeoffs for each of the various control concepts studied and evaluating advanced guidance and navigation displays as well.[1454] In addition to NASA pilots, a range of Marine, Royal Air Force, McDonnell-Douglas, and Rolls-Royce test pilots flew the aircraft. Their inputs, combined with data from Ames’s Vertical Motion Simulator, helped researcher Jack Franklin develop flying qualities criteria and control system and display concepts sup­porting the Joint Strike Fighter program.[1455] With the conclusion of the

VSRA aircraft program in 1997, NASA Ames’s role in V/STOL research came to an end.

In conclusion, in spite of the many challenges revealed in these summaries of V/STOL aircraft, the information accumulated from the design, development, and flight evaluations has provided a useful data­base for V/STOL designs. It is of interest to note that even though most of the aircraft were deficient, to some degree, in terms of aerodynamics, propulsion systems, or performance, it was always possible to develop special operating techniques to circumvent these problems. For the most part, this review would indicate that performance and handling – qualities limitations severely restricted operational evaluations for all types of V/STOL concepts. It has become quite obvious that V/STOL air­craft must be designed with good STOL performance capability to be cost-effective, a virtue not shared by many of the aircraft researched by NASA. Further, flight experience has shown that good handling quali­ties are needed, not only in the interest of safety, but also to permit the aircraft to carry out its mission in a cost-effective manner. It was appar­ent also that SAS was required to some degree for safely carrying out even simple operational tasks. The question of how much control sys­tem complexity is needed for various tasks and missions is still unan­swered. Another area deserving of increased attention derives from the fact that most of the V/STOL aircraft studied suffered to some degree from adverse ground effects. In this regard, better prediction techniques are needed to avoid costly aircraft modifications or restricted opera­tional use. Finally, there is an important continued need for good test­ing techniques and facilities to ensure satisfactory performance and control before and during flight-testing.

Today, NASA’s investment in V/STOL technology promises to be a key enabling technology in making the airspace system more environmen­tally friendly and efficient. Cruise Efficient Short Take-Off and Landing Aircraft (CESTOL) and Civil Tilt Rotor (CTR) promise to expand the number of takeoff and landing locations, operating in terminal areas in a simultaneous noninterfering manner (SNI) with conventional traffic, relieving overtaxed hub airports. CESTOL-CTR aircraft avoid the air­space and runways required by commercial aircraft using steeply curved approach and departure paths, thus enabling greater system capacity, reducing delays, and saving fuel. To fulfill this vision, performance penal­ties associated with STOL capability requires continued NASA research
to mitigate.[1456] While much still remains to be accomplished, much has already been achieved, and the vision of future V/STOL remains vibrant and exciting. That it is constitutes an accolade to those men and women of NASA, and the NACA before, whose contributions made V/STOL air­craft a practical reality.