Army VTOL Surveillance Aircraft
The need for surveillance or observation of one military force by another has always been a key point of Army strategy since the earliest times. The first use of aircraft bythe U.S. Army can be traced to the requirement for observation during the Civil War when hydrogen ballons wereused. Surveillance was the military requirement behind theprocurement of the first heavier than air craft procured bythe U.S. military forces. It can be said that the Mohawk is truly a direct descendent of the Wright Model B.
Starting in the mid-1950s Army sponsored over a dozen flight research vehicles to explore the feasibility of combining a VTOL capability with fixed wing aircraft. In October 1959 the Army Aircraft Development Plan was initiated. This plan provided guidance for the future of Army Aviation and included ArmyStudy Requirements (ASRs). These ASRs described broad objectives to be incorporated in the development of aircraft forlight observation, tactical transportation, and manned surveil-lance.These ASRs were released to industry at Fort Monroe, VA, on 01 December 1959. In February 1960 the aviation industry submitted 119 design concepts as possible answers to the Army's future aviation requirements.
By the early 1960s part of this effort was a program of VTOL surveillance research aircraft. The Lockheed XV-4A (jet ejector) (VZ-10) and G. E./Ryan XV-5A (fan-in-wing)(VZ-11) configurations were selected because of their apparent suitability for the surveillance mission. Later, when tripartite development of the Hawker P-1127 was initiated, the Army evidenced strong interest in this configurationfor the same reasons. As a result, responsibility for the U.S. part of the project was assigned by the Department of Defense to the Army, and the aircraft was designated XV-6A.
A number of configurations are possible to satisfy the requirements of VTOL with high speed approaching Mach 1. While a number of configurations to achieve VTOL are possible, the technical problem of achieving a useful winged VTOL aircraft is that of reasonably matching the thrust or power required for vertical flight with that which is required for an efficient cruise. Thus the helicopter achieves a reasonable match since the rotor accelerates a large mass of air to a relatively low velocity. The jet engine also accelerates a mass of air, but to supersonic velocity, and a jet VTOL is limited in speed essentially only by the characteristics of the engine itself. Thus it can be said that these two configurations bracket the spectrum of VTOL possibilities, and match the power required for vertical flight with that of cruise with varying degrees of efficiency.
When either mechanical augmentation (Hawker P-1127) or aerodynamic augmentation (Lockheed Hummingbird) to a gas turbine is employed, the increased vertical lift obtained is at the expense of fuselage frontal area and fuselage capacity. If the augmentation system is relocated in the wing as has been done with the XV-5A fan-in-wing, the wing thickness must be increased with an associated penalty of premature wing wave drag increase as Mach 1 is approached. Configurations which use some scheme to augment basic engine thrust generally have higher drag than would an equivalent conventional take-off aircraft.
The U.S. Army began funding several theoretically promising V/STOL approaches in 1961. The XV-4A and XV-5A were not procured as competitors for any specific mission, but rather as flight research vehicles to produce essential technical data. Both the XV-4A and the XV-5A were procured as research aircraft and not as pre-production models.
A definite philosphy was employed in setting down the requirements for them. Both aircraft were to be designed to meet the IFR handling qualities requirements of helicopter specifications of Mil-H-8501A, and to incorporate stability augmentation to operate within desirable pitch and roll boundaries. In the previous test bed programs, the handling qualities were usually deficient about one or more axes. Some of the aircraft were flown through sheer skill and specific experience of the pilot. The stability augmentation system in each aircraft was designed to be very flexible and in effect makes a variable stability machine out of the airplane. This not only allowed optimizing the particular configuration, but also enabled investigation of parameters of interest to VTOL operations in general.
The very fact that jet engines are used for conventional flight allowed both these aircraft to attain high subsonic speeds. Since that particular end of the speed range was well documented, it was necessary only to reach such velocity as required to demonstrate the flexibility of the propulsion system. Since cost was always an extremely important factor in Army aeronautical research, it was decided that the aircraft should be designed for a maximum airspeed of approximately 450 knots, obviating the need for sophisticated structures, yet producing the desired results at the least program cost. The only requirement was that the aircraft should take off vertically on a sea level hot day, hover 5 minutes, and have a total endurance of one hour when carrying a pilot and copilot, or 300 lb of instrumentation.
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