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XV-8A Fleep - Technical Description

The U.S. Army Flexible Wing Aerial Utility Aircraft was designed to carry out the functions of a light utility aircraft. The vehicle is essentially a self-propelled flying cargo platform supported from a Rogallo type flexible wing. A pilot's seat and the necessary flight controls are provided at the forward end of the plat-form. An engine, pusher propeller, and a V-tail are mounted at the rear of the platform. Provision is made for manually folding the wingand tail surfaces.

The basic body structure was in the form of a flat deck. A raised platform at the forward end supports the pilot's seat, nose wheel, control mechanism, instrument panel, and nose fairing. Fittings on the pilot's seat back and at the sides of the platform attach the wing supportstruts. Other fittings at the aft end of the platform provide attachment for the engine mount truss and tail surfaces. The useable cargo area, 64 inches wide and 80 inches long, is fitted with twelve standard flush-type cargo tie-down rings. Because of the open-deck design, long slender cargo items may extend both forward and aft of the normal cargo area. Riveted aluminum alloy sheet and extruded sections are utilized in fabricating the platform structure. Jacking pads are provided on the lower surface of the platform atthe main landing gear and at the forward end of the cargo area. The forward end of the pilot's cockpit is a removable fiberglas fairing. Thefairing support framework also supports the instrument panel and the transparent plastic windshield. The pilot's seat, an integral part of the vehicle structure, is equipped with a standard seat belt and shoulder harness. Space has been provided to accommodate a back-pack typeparachute.

The landing gear is of the tricycle type. The nose andmain tires and wheels are the same size and type to minimize spare part requirements. The main landing gear tread is 9.0 feet, and the wheel-base is 10.63 feet. Large, low-pressure type III tires aid operation from soft ground or rough fields. Landing loads at the main wheels are absorbed by cantilever Fiberglas springs extending from both sides of the platform structure. Heat treated steel axles which mount the aluminum alloy wheels are bolted and clamped to the outboard ends of the springs. Single disc type hydraulic brakes incorporated in the main wheels are hydraulically actuated by a master cylinder in the pilot's cockpit. Pressurized hydraulic fluid is supplied to the brakes through flexible hoses encasedin wire braid. An oleo strut type shock absorber is incorporated in the nose landing gear. The nose landing gear assembly attaches to the forward end of the sheet metal platform by a tubular tripod type structure. The nose wheel which can be steered through an angle of 25 degrees either side of center by operating foot pedals in the pilot'scockpit produces a turning radius of 27.83 feet. The foot pedals are connected to arms extending from the sides of the shock absorber pistontube by a simple cable and pulley system. The nose wheel is aligned ina fore and aft position in flight by a centering cam.

The wing is composed of throe main structural members: a rigid center keel,and rigid right and left leading edges. The two leading edges join the keel atthe apex and form a near-triangular wing planform. The keel runs longitudinally aft from the apex along the center line of the wing. The flexible membrane, made of Dacron with a polyester coating, is continuously attached to the leading edges and keel. The leading edges have a 50-degree sweep angle. The totalwing area in flat planform is 450 square feet.

The wing is of the foldable flexible type made up of a rigid keel, two rigid leading edges, a rigid spreader bar, flexible membrane, fittings, and attaching hardware. The forward ends of the leading edges attach to the forward end of the keel to form an apex which sweepsback at a 50 degree angle. The spreader bar which attaches to the keel at about midway, supports the leading edges to produce the proper sweepangle, and transmits the leading edge lift loads to the keel. The wing keel is a tapered sheet aluminum alloy boxtype structure. A fitting at its forward end supports the leading edge members. The keel attaches to the spreader bar by a hinge fitting at thekeel 46 percent station. Fittings are provided forward and aft of the mainhinge to attach the pitch trim control cables. The leading edges are hollow aluminum alloy spars which have a symmetrical streamlined cross section, and taper from a maximum cross section near the spreader bar attachment toward both ends. An aluminum alloy channel at the maximum cross section serves as a shear web. The attachment at the spreader bar is a swivel fitting with one axis lying along a chord-line and the other axis forward of and parallel to the leading edge. The attachment at the keel is aspherical rod end type fitting. Since the spar is free to align itself to the load, and the wing membrane is attached along the trailing edge, membrane tension is always applied to the plane of maximum spar stiffness. The aft 13-1/2 percent of the leading edge is hinged to permit a 5 degree motion in a chordwise direction to provide additional lateral control. The hinge mechanism incorporates linkage to multiply the mechanical advantage of the actuating cable used to control the position of thehinged leading edge portion in flight.

The wing membrane fabric is square weave Dacron cloth coated on both sides with olive drab polyester resin. The coated material is flexible and extremely weather resistant. Total weight of the coated fabric is 8 ounces per square yard. The coated fabric has a tensile strength of not less than 200 pounds per inch in the warp direction, and not less than 120 pounds per inch in the fill direction. The membrane is attached along the keel and leading edges with machine screws. Metal reinforcing strips are bonded into the reinforced, bonded,and sewn edges of the membrane. To prevent trailing edge flutter, the aft edge of the membrane is scalloped, and thin wooden battens (3 perlobe) are retained in pockets sewn in the trailing edge membrane. A reinforcing cable, the length of which is adjustable on the ground for rolltrim, is sewn into a hem along the aft edge of the membrane.

The propulsion system consisted of a six cylinder, aircraft reciprocating engine equipped with a fixed-pitch propeller employed as a pusher, and an exhaust-driven ejector cooling system. A steel tube truss supported the engine near the aft end of the platform structure. Four flexible rubber mounts are used to attach the engine tothe truss. The propeller thrust line is inclined 3 degrees up at the rear with respect to the platform surface. The exhaust driven ejector cooling system is self-regulating, and requires no action on the part of the pilot. Sheet aluminum baffles direct the cooling air through cooling fins on the engine cylinders and heads.

The performance capabilities of the airplane were all within predicted values. The cruise capability was such that a 100-mile mission can be flown at maximum gross weight. Take-off and landing performance proved the STOL capability of the airplane. At maximum gross weight, the take-off distance over a 50-foot obstacle is 1,000 feet. Landing distance to clear a 50-foot obstacle is 400 feet. During the course of the test program, the airplane proved to be a reliable and easy aircraft to maintain and service. Some test operations were conducted from unprepared desert surfaces, establishing the capability for operation from areas other than regular airfields. The operational and flying techniques are basically similar to those of lightweight conventional aircraft. The two-control system lends itself to simplicity and provides adequate control power to permit a fixed wing incidence trim setting for the entire flight including take-off, climb, cruise, descent, and landing.

The handling characteristics of the aircraft were good. Control harmony between the longitudinal and lateral control systems was excellent, enabling the aircraft to be flown with one hand. Stability in all cases was positive with only light forces required. The flight characteristics of this airplane were similar in most respects to those found in a conventional airplane with a comparable light wing loading

The aircraft was safe and pleasant to fly for an Army pilot of average skill. Data available indicated that, with improvements, the concept can be developed into a flying truck with reduced experience and skill requirements for tho operator. Helicopter and light plane experience aids intransition to this aircraft, although such experience was by no means necessary. The aircraft is capable of rough field operation with certain advantagesover fixed-wing aircraft or helicopters.

Safe landing characteristics with engine power at idle were demonstrated. The system was highly sensitive to turbulence and rough air which is uncomfortable, but is self-damping to a high degree. The wing appeared to lose lift in some conditions of turbulence, causing some degradation of climb and descent performance. Crosswind operation investigations were continuously conducted. The results suggested that limitations will eventually be established that were quite compatible with light aircraft of about the same weight.

The idea of a primitive, low-cost, low-maintenance, limited-performance but useful acrial device was clearly demonstrated. For example, only one operation out of 47 was delayed due to aircraft maintenance. This program did not represent an operational evaluation environment; however, the low maintenance and support required was very unusual for an experimental aircraft. The aircraft met or exceeded all predicted performance goals and demonstrated its ability to haul bulky cargo shapes and a useful load almost equal to its empty weight. The ability of the aircraft to operate as a light STOL utility vehicle with a 100-mile range was established.

Length26 feet
Wing Span (spread)33.4 feet
Width (wing folded)10 feet
Height (wing at zero incidence)14. 5 feet
Wing Area (flat plan form)450 square feet
Wing Sweep (leading edge)50 degrees
Wing Keel Length26. 0 feet
Length of Platform (cargo area) 80 inches
Width of Platform64 inchesWheel Base10. 6 feet
Wheel Tread9 feet
Propeller Diameter7 feet
Weight Empty 1,115 pounds
Engine Oil15 pounds
Fuel150 pounds
Pilot170 pounds
Flying Weight Without Cargo1,450 pounds
Cargo Payload 850 pounds
Design Gross Weight 2,300 pounds



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