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Aviation History
1957
1957 - 0209.PDF
211 FOR NAVAL ATTACK An Analysis of the Douglas A3D Sky warrior of the U.S. Navy IN Flight for February 18, 1955, we published a full accountof the general principles which governed the design of theA3D Skywarrior, the standard heavy-attack machine of the U.S. Navy. That article was largely based on comments madeby Mr. Ed Heincmann, chief engineer of the El Segundo division of Douglas Aircraft, which was responsible for the design. It is now possible to present a more detailed appraisal of themachine, which is now in operational service with carriers at sea. Also included is a cut-away drawing, by one of our artists,which emphasizes the straightforward engineering of the airframe. As explained in the issue mentioned above, the design of theSkywarrior stems from U.S. Navy thinking of 1947-8, when it was determined that the most powerful striking aircraft capable ofoperation from the largest carriers should be purchased to fulfill a role of a more strategic nature than the operations to which theU.S. Navy—or any other navy—had been hitherto restricted. After much chopping and changing, the design finally settleddown in 1951. As is customary with El Segundo machines the weight was pruned by every possible means; and, although itwas decided to include a twin-20 mm turret in the tail (a Navy requirement), the final gross weight was held to 70,000 lb. Thefirst XA3D flew in October 1952 on the power of two Westing- house J40 engines, and later design-changes—some of whichwere major alterations—have since produced a compact, straight- forward and very effective operational aircraft. Power for theproduction A3D-1 is provided by Pratt and Whitney J57-P-6 units, each rated at some 11,000 1b thrust dry. A rather laterengine, the J57-P-10, is fitted to the current-production A3D-2 (one of which is shown in the heading picture). Although the airframe is almost entirely built from conven-tional materials used in classic manner, the structure is proving robust and well able to stand up to problems of corrosion andrough handling aboard ship. Much of it is made from 7075S high-strength aluminium alloy. The wing, which has a 36-degsweep angle and a thickness/chord ratio reduced from 10 per cent at the root to 8.25 per cent at the tip, has two plate sparswith relatively small extruded booms. A single 7075S sheet forms the whole skin between the spars from the centre-line ofthe aircraft out to the wing-fold axis. After investigation of the properties of tapered-sheet wings it was decided to employa constant skin-thickness of fin. This portion of wing is employed as an integral tank. An exceptionally large portion of the wing is arranged to fold,hinging upwards under the thrust of a hydraulic jack as a sketch (p. 214) illustrates. The hinge units are machined from steppedincreases in section in the extruded spar-booms, and the inter- spar skin is progressively reduced in thickness from |in at thefold to 0.032in at the tip. The sub-structure of the leading and trailing edges is stiffened by dished inner skins. Fuselage design was complicated by the need for a largebox-like weapons bay near the e.g., extending the whole depth of the hull and with the bomb-door openings roughly in linewith the large cut-out for the wing. Ahead of and behind the weapons bay are large self-sealing fuel cells; immediately beneaththe forward cell is a section devoted to power services, air-turbine motors, cabin conditioning and the like, while the flanks of therear fuel cell are recessed to accommodate the main undercarriage units. As shown in a sketch overleaf, the main legs retract aboutdiagonal axes so that the wheels can be housed in a vertical plane. The relatively narrow track of lift has proved no handi-cap to stability, even on a rolling deck; the tyres are of 43in diameter and the legs are 7|in-diameter assemblies machined The first production type of Skywarrior was the A3D-1, an example of which—with wings and vertical tail folded—is seen below. Now being built at El Segundo is the A3D-2, subject of the heading picture, which embodies minor changes in powetplants and equipment.
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