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Aviation History
1955
1955 - 0206.PDF
206 FLIGHT, 18 February 1955 SKYWARRIOR . . . installation; at least one projected form of the design incorporatedunits buried in the roots of the wings. It was, however, con- sidered that the engines should be either completely buried orm6unted on pylons as long as ground clearance would permit, for any compromise was found to lose many of the advantagesof either method. The final configuration was a pair of single pods carried well below, and ahead of, the wing; this made pos-sible the attainment of optimum accessibility, and did not inter- fere with wing tankage or with the large and continuous flaps.Incidental advantages were that engines could be changed with the use of standard torpedo-lifting trucks without any specialhoisting gear, and that—if it became necessary—a change of engine type could be accepted with minimum change to the airframe.The latter point became very important later. In the late 1940s the El Segundo engineers began a majorprogramme of investigation on the flutter characteristics of the A3D wing. The load factors to which the aircraft was beingdesigned were considerably lower than those employed in other types of aircraft intended for comparable performance, and it wastherefore expected that aeroelastic considerations would be of paramount importance. Two main investigations were conducted: a preliminary flutteranalysis with the analogue computor owned by the California Institute of Technology and a test programme with a flutter modelto supplement and check the analogue analysis. One of the advantages of the analogue computor is that it lendsitself readily to the investigation of a large number of distinct cases; in the A3D programme, approximately 1,000 flutter caseswere examined. The dynamics of the wing were represented by an equivalent mechanical system which was in turn translated intoits equivalent electrical analogue, the force equations being repre- sented electronically. The mechanical system for the wing wasa beam in five sections, with coupled bending and torsion degrees of freedom; fuselage and tail masses were included in the innerbeam section, rotational inertia and spring terms were used to represent aileron rotation and the nacelle was represented by asuspended mass with a rolling inertia and spring. Early in the analogue programme it was found that the flutterspsed of the design was below the maximum design-speed. This was the type of blow which many design teams have had to faceduring the past few years, and it probably threw the A3D wing team into turmoil.The flutter mode was of the bending-torsion type on the basic wing, with or without nacelles (although, of course, the nacelleshad a marked influence on flutter speed); the frequency was rela- tively high. A survey of the effects of various modifications wasundertaken and the behaviour of many configurations was balanced against the corresponding increase in weight (diagram, page 208).As a result it was recommended that the nacelles should be relocated rather farther forward and outboard and that both thewing torsional rigidity and pylon rolling rigidity should be increased. Work on the flutter model was not completed until after me pro-totype design had been fixed (incorporating the changes resulting from the preliminary analogue analysis). The balsa/lead modelwas tested first in the El Segundo 40in x 30in tunnel, and then in the GALCIT 10ft tunnel and the variable-density tunnel ownedby the Southern California Co-operative organization. Testing in the latter tunnel enabled corrections to be made for compressi-bility effects at high Mach numbers. For ease of production, transport, repair and overhaul, the structure of the A3D is built in the sections shown. Much of it is sub-contracted. FIRST PILOT. SECOND PILOT TO STARBOARD. DITCHING HATCH SELF-SEALINC FUEL TANK BOMS FALSE i BATTERIES AND OXYCEN EQUIPMENT AUXILIARY POWER UNITS AND ELECTRONICS BOHB-BAY ANTI BUFFET RAI The model tests confirmed the analogue analysis for the barewing, but also showed—disturbingly—that flutter occurred with the J40 nacelles attached, at a speed well within the normal speedrange. The critical mode consisted of wing first-bending coupled with inner-panel torsion, nacelle motion also being considerable.Even with the nacelle pylons made as rigid as practicable, flutter became unstable at less man the design limiting speed. But otherdevelopments were more favourable: the critical mode developed slowly and was not wildly divergent; the addition of wing-fuelweight either stopped the inner-panel torsion-flutter or raised it beyond the limiting speed; no flutter was met at top altitudewithin the speed limit; and, fortuitously, a different engine was chosen for the production machine, which permitted stable flightat all speeds and altitudes. The new engine was the Pratt and Whitney J57 two-spoolhigh-compression engine, giving 10,000 lb thrust without resorting to an afterburner (this engine also took over from the J40 inanother El Segundo machine, the F4D Skyray). The increased weight of the J57 and the different nacelle geometry were instru-mental in finally overcoming the flutter problem; it was also found, by ground-resonance testing, that the critical inner-paneltorsion frequency of the full-size aircraft was rather higher than the model had predicted. The design of the fuselage was, by comparison, quite straight-forward. A very large fuselage was chosen, accommodating all the items normally provided for as well as the main undercarriageand a considerable proportion of the fuel. The chief invariables were the crew compartment, weapons bay and—once it had beendecided to incorporate it—the tail gun barbette. The weapons bay occupied the whole volume of the centre fuselage : eneaththe wing. Ahead of the bay was placed a large fuel tank and a tank of similar capacity was positioned immediately behind thewing. The main undercarriage units were attached to strong frames at the rear of the weapons bay, and were arranged to retractbackwards to lie on either side of the rear tank. Before outlining the design of the internal equipment somenotes may be given on the structure itself. The whole airframe was designed to be made in light-alloy stressed skin, there beinga high percentage of 75ST material. The fuselage structure was planned around longitudinal and transverse members chosenaccording to the location of the numerous miscellaneous cut-outs; long, heavy keel members were run from nose to tail, picking upsome of the weapons-bay and wing loads as well as the landing gear, catapult spools and arrester-hook attachments. The wing was designed as a two-spar structure, and the flutterrequirements already outlined dictated a skin thickness of no less than fin between the spars all the way from the aircraft centre-line to one-quarter of the span of the outer panels. The outer wings were arranged to fold upwards hydraulically and, in orderto save weight, the large lug-fittings accommodating the hinges and locking pins were designed integral with the upper and lowerspar booms. These booms were machined from stepped extru- sions. The vertical tail also was arranged to fold hydraulically. On the subject of flying controls, it is El Segundo philosophythat the aircraft should always be capable of being flown manually. Accordingly, the A3D system, although fully powered hydraulic-ally, was given elevator and rudder boost ratios sufficiently low
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