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Aviation History
1958
1958 - 0613.PDF
9 May 1958 gives practically no indication of the planform of the wing. How-ever, numerous unofficial photographs, taken during the NA.39's first flight, reveal a broad-chord wing with appreciable sweepand generous area. It is probable that its thickness/chord ratio is less than that of the Supermarine Scimitar since the design Machnumber ought to be somewhat higher. Moreover, the high manoeuvrability required for low-level attacks and toss-bombingimplies a design factor of at least 8 and perhaps 12, from which it can be concluded that the mainplanes are extremely stronglybuilt with multiple spars and thick skins, possibly incorporating integral stiffening. In his first annual report, written in July 1956,Blackburn's present chairman, Mr. Eric Turner, said, "We are also going forward widi the latest developments in integral con-struction, in which the structure is largely machined from the solid. . . ." Last August our sister journal Aircraft Productionpublished an extensive article on this work, which probably related to the wing of the NA.39. Although no details of this are apparent from the photographs,the wing is clearly fitted with devices giving the maximum possible lift coefficient. It is certain that the incidence of the wing itself isfixed, but, supplementing high-lift flaps, the leading edges may perhaps be hinged, or otherwise movable, to increase the effectivecamber at low speeds. The Scimitar has a simple hinged leading edge of the droop-snoot type. So also does the American Crusader,whereas such machines as the Skyhawk, Fury and Tiger have large leading-edge slats. A slat deals with a relatively large volumeof air at a relatively low velocity, and represents boundary-layer control in its longest-established form. Boundary-layer thickness increases with linear distance (acrossa chord or along the fuselage, for example) and also with velocity, so that the whole aircraft tends to be surrounded by a turbulentenvelope of "tired" air; moreover this boundary layer may break away from the skin, causing separated flow, high drag and reducedlift. It is highly doubtful that Blackburn have worried about the boundary layer over the fuselage, since attention to this can onlyreduce drag and is of economic interest only in the field of "global" laminar-flow transports of the type envisaged by Handley Page.The boundary layer over die wing, and perhaps over the tail also, is a different story, and it is worth while controlling it in order toreduce drag and increase lift, particularly to improve performance at the bottom end of the speed range. The aim is generally either tosuck the boundary layer away, through slots or perforations, into the interior of the wing and thence to a discharge outlet (asHandley Page have done) or to inject energy by some means to increase the momentum of the boundary-layer sheet itself. Thisenergy can be supplied by small vanes of the type frequently referred to as turbulators or vortex-generators, but these imposea measurable drag penalty. Alternatively, the tired air can be helped along to the trailing edge by injecting into it a high-velocity—possibly sonic—stream of air from spanwise slots. Such a slot can be placed close to the leading edge on the upper surface, whereit will have the same effect as a slat, but will deal with a smaller volume of air at higher velocity. In addition, or as an alternative, compressed air can be ejectedfrom a thin slot above the leading edge of each flap. This is doubly valuable in that it helps to preserve the flow over the flap whenthe latter is depressed, so that the lift coefficient, as well as the drag, is greatly increased (unlike a conventional flap which usuallystalls at full deflection) It is well known that the Scimitar has "blown" flaps of this type, and in our issue of April 4 an articleon the Scimitar commented "air is bled from a late compressor stage of each engine and ejected from a supersonic nozzle to giveboundary-layer control over the trailing-edge flaps. The supply pipes from each engine are interconnected so mat there is noadverse effect if an engine fails." It may be noted that the blown flaps of both the Scimitar and NA.39 are comparatively small, andif such a system is employed in the NA.39 it may well extend across the greater part of the span. An accompanying diagramshows the sort of basic layout which may have been adopted, and a graph indicates the gains in lift coefficient which are possible.It is perhaps appropriate at this point to turn to the question of propulsion, since in an aircraft like the NA.39 die lift, drag andthrust are intimately related by the joint thermodynamics of me powerplant and boundary-layer control system. From the out-set, Blackburn chose the de Havilland Gyron Junior turbojet, two of which are fitted at the,roots of die wings. The only thrustwhich may be officially published for a Junior is 7,000 lb (DGJ.l) although later versions, such as the DGJ.10, are known to havedifferent—and probably higher—ratings. It follows mat the pro- pulsion system of the NA.39 has characteristics which differappreciably from diose of the other naval twins; die total installed weight must be much less and die fuel required for a given rangeis probably somewhat lower. - Each Gyron Junior is apparently mounted right at die forwardend of its nacelle, being slung from two of die fuselage frames and fed widi air from a direct pitot intake (a further indication thatdie Mach number is not intended to reach much beyond unity). The cowling panels are clearly well forward of the strong torsionbox of the wing, which is probably attached to frames around the jet pipes. The latter are undoubtedly long, but their modestdiameter should minimize drag and weight. At their rear ends die pipes are toed outwards, pardy to reduce asymmetric yawingmoments but chiefly, no doubt, to prevent Coanda adhesion and damage to die rear fuselage due to ultrasonic buffeting. After-burning (reheat) is clearly not fitted, since in a machine like the NA.39 range is of far greater importance than speed. Each enginebay is equipped with a Graviner fire-protection system, which was fully proved during trials widi a full-scale rig in which everyflight condition could be simulated. Inboard, adjacent to the fuselage wall, is a small subsidiary intakewhich must swallow some of the boundary layer and may serve odier functions. Outboard of die powerplants are additionalintakes, of surprisingly large size, which probably entrain ram- air for ventilating die engine bays and cooling die jet-pipe tunnelsand accessories, and possibly for feeding fresh air to die cockpit- conditioning system.The following text has been issued by Plannair, Ltd.: "In the Blackburn NA.39, Plannair were required to provide cooling fora generator at an operational altitude where air is much thinner dian at ground level. The designers faced the problem of design-ing equipment capable of building up die very high pressure and large volume of air required for efficient cooling, whilst remainingwithin the strict size and weight limits specified by Blackburn and General Aircraft. "The outcome of an extensive development programme carriedout by Plannair, Ltd., was a multi-stage compressor capable of moving 270 cu ft/min of cooling air; an outstandingly high per-formance for equipment 12in long by 6in diameter, weighing only 15 1b. Interesting design features of the 14,000 r.p.m. compressorare its built-in gearbox (ratio 2:3) driven by a splined shaft from the main engine gearbox, and the four impellers and four statorsused to produce die high pressure rise."
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