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Aviation History
1950
1950 - 1979.PDF
454 FLIGHT, 23 November 1950 SELACHIAN SELECTION The Douglas XA2D-1 Skyshark : PART II : Design and Performance This view of the Skyshark with wings folded accentuates the dimensions of the Aeroprod'jcts counter-rotating airscrews; they absorb 5,500 h.p. By STANLEY H EVANS, F.R.Ae.S., A.F.I.Ae.S. BECAUSE, in the nature of nautical things, the stabilityand control requirements for carrier-borne aircraftmust be unusually severe, considerable research and development have gone into this side of design, some of which can be diagnosed from a study of the Skyshark's tail ensemble. Thus, to obtain the desired elevator control forces an electrically operated adjustable incidence stabilizer is pro- vided for longitudinal trim. This permits the choice of a smaller elevator area, the combination of short elevator chord and shielded-horn-type aerodynamic balance at the tips giving lighter stick forces than would be possible with a fixed stabilizer and tab-trimmed elevators. We are returning, in fact, to the practice of 30 years ago—the days before the servo trim-tab came into vogue. Positive stability and effective control in the lateral-direc- tional sense are also virtues greatly to be desired in shipboard aircraft. However—as in the realm and relations of genus homo—such virtues are seldom obtainable without accom- panying vices in some degree or other, and so the final result is never quite the immaculate conception it promised to be on the designer's doodle-pad. This is plainly the case in low- wing aerodynamics applied to carrier aircraft, where the choice of dihedral angle is something of a compromise be- tween the requirements of low and high speed—from approach and wave-off conditions to the vertical dive case. The required dihedral to pick up a dropped wing with either ailerons or rudder close to the stall may be excessive for high-speed dives where application of the rudder gives excessive roll and yaw. Conversely, if the wing dihedral is too flat, instability in roll will probably show up when the wing flaps and landing gear are pulled down. Based on extensive wind-tunnel and flight research during the development of the Skyraider, the wing dihedral of the Skyshark remains the same at 6 deg, but the vertical loca- tion of the horizontal tail has been lifted, in effect, by the introduction of a 10-deg dihedral. Judging from the big "top-hamper" sail effect of the vertical tail, the rudder power looks adequate down to the stall, while the low taper- ratio of 0.5 and zero sweep of the wing should contribute towards effective aileron control throughout the speed range. Seeing that the Skyraider has positive stability and excel- lent control characteristics from the stall to a terminal velocity dive, and that the two types have similar wing-tail configurations, one may confidently expect the Skyshark to live up to its lineage. One noticeable external change resulting from the turbo- prop combination shows up in the fuselage lines. Although of characteristic Skyraider shape terminating in a deep ver- tical knife-edge, the effect has been accentuated in the Sky- shark by a big increase in depth and effective fin area aft of the wing. This increased vertical keel surface behind the e.g. is necessary, of course, in order to counteract the de- stabilizing influence of the lengthened nose, forward cockpit I AST week Stanley H. Evans described the general layout of the *- Douglas Skyshark (Allison T-40 dual turboprop) and its development from the Skyraider family. In this continuation he discusses certain aspects of stability and control suggested by the design, and makes some performance estimates. The word "selachian," incidentally, means "belonging to an order of fshes represented by the sharks and rays." blister and large airscrew-bhde "finnage." It may be just another dogmatic speculation on our part, but we suspect that this particular form of rear fuselage, with its smoothly integrated dorsal finning, induces a more effective weather- cock yawing action than the appendix type of dorsal fin sit- ting atop a bluff rounded body. Albeit dorsal finning remains an elusive art—a blend of inspiration and desperation. Hydraulically operated wing flaps of 40 per cent span by 25 per cent chord are of the Handley Page - N.A.C.A. single- slotted type having externally mounted plate-type hinges, which permit a large slot-gap when the flap is deflected. These hinges, while they openly violate the founding rules of the S.P.C.A., are clearly innocuous among the rest of the Christmas-tree decorations that seem fated to adorn the wings of most aircraft in the fighter-bomber category. It is astonishing, indeed, to note what can be hung on the lower surface of a wing in the way of bombs and rocketry without, apparently, impairing the performance too severely. (One is tempted to conclude that the airstream has developed the human trait of taking the line of least resistance; by crowding the flow over the upper surface, the lift may actu- ally be boosted more than the drag!) The air brake of the Skyshark is also operated off the hydraulic system, the brake flap being a quick-opening single panel mounted on the bottom of the fuselage just aft of the wing—similar in principle to the petaloid-type aft-fuselage air brakes first developed for the Skyraider. (The latter, how- ever, has additional panels on the fuselage sides.) Douglas studies in this field of design have also encompassed wing brakes or spoilers, reversible airscrews and parachutes; and, as a matter of historical fact, the first U.S. dive-brake wing installation was evolved at the Douglas, El Segundo, plant (then the Northrop Division of Douglas) in 1935, being in- corporated as double split perforated flap panels on both wing surfaces of the Northrop XBT-1 naval dive bomber— the primitive me nber of the long family line. But, weighing up the virtues and vices again, the fuselage- type air brake has certain advantages over the wing-type spoiler. Actually, wing spoilers mounted well forward on the upper surface have a very high drag effect—as much as twice that produced by the aft-fuselage type for the same deflection angle. Offsetting this virtue, however, are a couple of vices; namely, greater structural difficulty in mounting on
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