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Aviation History
1962
1962 - 2140.PDF
FLIGHT International supplement, 20 September 1962 Air-Cushion Vehicles Structure Like the VA-3, the basis of the craft is a buoyancy tank. It measures 23ft 4in by 8ft 9in, and like all parts of the primary structure it is made of anodized material, assembled wet with an etch- primer and sealing compound and finished with DTD.5555 paint (one coat inside and two coats out). The tank has the form of a vee-nosed rectangle, and its buoyancy is sufficient to support the gross weight of the craft with an ade quate reserve. It is divided into 14 sections by transverse plate frames with extruded T booms extending right across the tank at a pitch of 18in. As shown in a sketch, it is divided into four rectangles by the stability jets, which, as in the VA-3, have the form of a St George's cross, which is easier to incorporate into the structure than the all-diagonal cross of St Andrew. As the main cutaway drawing shows, the top of the tank dips down along the centre on each side of the longitudinal stability duct, which is about 1 Jin wide. The transverse frames are joined across the duct by straps, which are bolted and riveted permanently. The transverse duct is made in left and right halves on opposite sides of a plate web which extends above the buoyancy tank and divides the whole plenum chamber into front and rear sections. The transverse duct is ahead of this web on one side of the craft and behind it on the other. This involves a very slight amount (not enough to be noticed) of pitch/roll coupling. Strength of the structure is adequate for an 8g end acceleration. Torsional loads are borne by the closed boxes which comprise the main buoyancy tank. This side elevation shows the layout of the engine and bevel-drive boxes in relation to the g.c; also illustrated is the lift airflow. The sketch of the underside, below, show the longitudinal and transverse nozzles, which are not normally seen. The stiffening members on the underside of the buoyancy tank have been omitted for clarity End loads in bending are reacted partly by a series of Welsh-hat stringers run ning along the underside of the tank the full length of the craft, jointed across the transverse stability ducts. Additionally, heavy longitudinal webs are inter- costalled between the transverse frames at a station about two-thirds of the way out from the centreline. Directly above these heavy longitu dinal webs is arranged a series of Warren- girder trusses built up from lin-diameter welded tubes in 14g light alloy, secured by machined fittings. These trusses are omitted over a 6ft length in way of the fans so that the latter may be withdrawn (they are arranged to slide on tracks to starboard, but could equally be removed to port). On top of the trusses are secured box-section full-length longerons, known as "stand-off booms," which are braced together and complete the primary structure. These booms provide the depth required to resist all longitu- Control layout and (inset) the roll-control mechanism on a larger scale dinal bending loads, and form the basis for the cabin door sills and floor support. Each of the transverse frames in the buoyancy tank terminates in a shear block and upper and lower lugs through which removable pins attach the outer duct sections. The latter provide a further 3ft width of buoyancy tank on each side, and are each divided into front, centre and rear portions. The latter are not designed to contribute to strength of the main hull, although they are located against each other by shear pegs outboard. The upper surface of each outer duct section is a paper- honeycomb sandwhich panel, to provide adequate stability against plenum- chamber pressure and permit it to be walked on. Depth of the plenum chamber, between the buoyancy tank and the main floor, is generally 6in. At its outer end the chamber curves down around the outer duct sections, the inner and outer walls of which are joined by num erous small fluted diaphragms. The final curtain jets issue from neoprene- fabric extensions, tied to maintain shape when under pressure, which project about 1ft below the bottom of the metal structure. These extensions increase the 58
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