The Mississippi State University Parsons XV-11A Marvel was a boundary layer control research aircraft designed for the US Army. The feasibility of these unconventional features had previously been tested on a piston-powered testbed, the XAZ-1. This airplane was in essentially the same configuration as the "Marvel" and was referred to as the "Marvelette." With this background, the "Marvel" was successfully used for about 100 hours of testing and research flying. The program was then discontinued and the "Marvel" was placed in storage until 1982. Mississippi State University's Raspet Flight Research Laboratory [RFRL] is an integral part of the Department of Aerospace Engineering at Mississippi State University located at Bryan Field in Starkville, Mississippi. August Raspet, an aerophysicist, came to Mississippi State in 1948 armed with a $36,000 grant from the Office of Naval Research to conduct viscous flow studies using sailplanes.
In the 1960's the Aerophysics Department of Mississippi State University (now the Aerospace Engineering Department) undertook to produce a research STOL aircraft under sponsorship of the U. S. Army. This aircraft, the XAZ-1, was called "Mississippi Aerophysics research Vehicle, Extended Latitude" and was known under the acronym of "Marvel."
The target of research thrust was to test a configuration of features that would be of particular interest for an Army observation airplane. These features were: first, good pilot and observer visibility; second, a good utility cargo volume; third, STOL performance; fourth, unprepared field operation; fifth, a good cruising speed; and sixth, lower cost and simpler maintenance than a helicopter. To meet these targets, the airplane was laid out as a small pusher with an extended shaft from a turbine engine driving a ducted propeller. The prop duct was integrated into the tail surfaces.
Operations of single engine, boundary layer control, STOL aircraft since 1958 included the high lift super cub L-21, the modified Cessna L-19, the XAZ-1, and the XV-11A, with wing loadings ranging from 13 lb/sq h to 28 lb/sq ft. All of these aircraft had a distributed suction boundary layer control system for lift augmentation. The XAZ-1 and the XV-11A also had shrouded propellers for static thrust augmentation. The performance stability and control and handling qualities of these aircraft were evaluated and considerable experience gained in the operational aspects of such STOL aircraft with regard fo the handling qualities required for safe operation in the STOL mode.
The boundary layer is a thin film that forms on the surface of a solid body moving through a viscous fluid, like the wing of an airplane moving through the air. Within the film, velocity increases parabolic ally, from zero at the solid surface up to the free-stream velocity at the outer edge of the boundary layer. The depth of the layer varies with the smoothness of the surface, the viscosity of the fluid, and the speed of the flow, but it is never very large. The boundary layer was first identified and labeled by Ludwig Prandtl in 1904 in a classic paper that revolutionized this branch of fluid mechanics.
The flying qualities of wings can be enhanced in two ways, and boundary-layer control can help in both. The first is to decrease drag; the second is to increase lift. The most desirable way to decrease drag is to maintain laminar flow within the boundary layer and prevent a transition to turbulent flow. Laminar flow occurs when successive layers of air within the boundary layer slide smoothly over one another, from the stationary film at the surface up to the free-stream velocity of the outside air. Turbulent flow within the boundary layer occurs when these "streamlines break up and a fluid element moves in a random, irregular, and tortuous fashion," as when the smoke rising from a cigarette in a still room ceases to travel smoothly up but tumbles instead in eddies and curls. Over a normal wing, the boundary layer remains laminar over only a small portion of the wing chord before breaking up into turbulent flow. The area of turbulent flow experiences significantly greater skin-friction drag than the laminar flow.
The second way to improve the flying qualities of a wing through boundary-layer control is to increase the lift, especially the maximum lift, of the wing. Maximum lift can be increased by delaying the onset of separation of the boundary layer. As a wing's angle of incidence increases-as its leading edge is tipped up above the plane of flow of the free-stream air-its lift also increases, up to a point. Finally, however, the boundary layer on the upper surface breaks free of the wing altogether, reducing lift drastically. This is known as stalling. If the boundary layer can be kept from separating, the maximum lift of the aircraft can be increased, an important consideration in increasing takeoff-weight capacity and reducing landing speed.
The XV-11A the Mississippi Aerophysics Research Vehicle Extended Latitude (MARVEL) was a single-engined pusher monoplane fitted with a boundary layer control system. Parts were constructed by the Parsons Corp. The aircraft carried s/n 65-13070 and the first flight took place on December 1, 1965. It carried out its initial program of research on behalf of the US Army in the late 1960s. On completion of the flight program in April 1967, comprising 35 hours over 49 separate flights.
Although the aircraft is still in flying condition, failing interest in STOL aircraft of this type by the Department of Defense has curtailed research activities. The aircraft was put in storage to be revived in 1981 with civil registration N2768Q. It was rebuilt in the 1980s as a proof-of-concept for a utility aircraft.
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