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Aviation History
1958
1958 - 0612.PDF
628 FLIGHT NA.39 . . . deck reaction is probably the most important. In addition, thedynamic and mechanical properties of hooks meeting wires com- bine with the permissible run-out along the deck to limit thelanding speed, and the two factors together give the designer a fairly clear indication of the wing area which he will needfor a given section profile or maximum lift coefficient. In practice, as will presently be shown, lift coefficient and wing section areby no means as inter-related as they once were, for boundary-layer control, which is undoubtedly used in the Blackburn aeroplane,can increase the available lift by as much as 100 per cent under certain conditions. It has many times been suggested that designing an aircraft tofly supersonically (at any altitude) either doubles its weight or halves its range. This is a sweeping approximation, but it servesto show that the penalties of going beyond—especially just beyond —the speed bracket which has become known as "high subsonic"incurs substantial penalties. The three big variables confronting the team at Brough were therefore speed, range and structuralstrength. It is extremely interesting to compare the Blackburn machine with its counterparts in the U.S. Navy. The new tactical strike machine for American carriers is theGrumman A2F, which is relatively small and conventional. For "big league" operations, however, the North American A3J-1, thefirst example of which is almost complete at Columbus, shows the way American thought has progressed. Powered by a pair of rear-mounted J79s, with afterburners, the A3J is intended for a high- altitude speed of approximately Mach 2, and for a low-level speedof something over Mach 1. One assumes that North American designed the A3J to a closely defined specification requiring suchperformance. To the writer it seems curious that the U.S. Navy should have accepted the heavy penalties of such performance,since Mach 2 at height seems much less valuable than Mach 1 + on the deck; it is certainly not fast enough to embarrass a reallycapable surface-to-air missile. Perhaps the Navy just wanted a bomber that could fly as fast as the Air Force's Hustler. As a result of its high speed the A3J is a machine of no meanproportions. Had its sponsors concentrated upon the primary mission requirement—supersonic speed at low level—they wouldcertainly have been able to cut the A3J's size and weight and to have reduced its cost by about 50 per cent. Moreover they wouldnot have been presented with a flying machine which, so far as the writer can calculate, can be operated only from a Forrestai-class carrier or from a shore base. In contrast, the Fleet Air Arm —possibly because they had no choice—have elected to buy anaeroplane tailored to the dictates of the low-level mission and not intended for the Mach 2 high-level type of sortie. One has only to glance at the NA.39 to appreciate that it is notbuilt for Mach numbers much beyond unity. Yet it is designed specifically to do the low-level mission, and this it can prob-ably achieve as well as any other machine, not even excepting the fearsome A3J. Moreover, it may well represent a new class ofrelatively cheap and versatile strike aeroplane capable of fulfilling many mission requirements of both navies and land-based airforces. This photograph of the first NA.39 was taken some weeks ago at the Blackburn airfield at Brough. Production machines would probably not have the dorsal aerials, nor the nose probe. In fact it is fairly evidently Blackburn's bid to produce theoptimum tactical strike aeroplane. That it is area ruled has already been stated in positive terms (in fact its coarse applicationleads one to wonder whether or not the area ruling came after the basic design was laid down). The area rule is of the transonicvariety—of which more anon—and the design cruising speed is likely to lie in the "high subsonic" Mach range between 0.95and 1. Without area rule the cruising speed would almost certainly be well below 0.9. In addition, the manoeuvrability would bereduced and in all probability the crew would be subjected to a rougher ride. The latter is a factor of great importance whenoperating at low level, and one which can prevent a strike machine from making use of its full performance envelope. The fact thatthe NA.39 can fly faster than sound would seem to be an inci- dental by-product of its other characteristics, and not a designrequirement. Yet the obvious requirement of designing for full e.a.s. robs theaircraft of the advantages in lightness which subsonic cruise would otherwise confer. In fact, the development of a producible air-frame which can operate at a speed of the order of Mach 1 at sea level is one of the most difficult tasks with which a designercan be confronted. In either the subsonic or the supersonic regimes it is possible to know the position of the centre of pressure andto obtain a clear idea of the airflow around the whole aircraft, but in-between conditions change drastically and rapidly. Divergentflutter must be avoided at all costs, and this alone demands a structure of immense stiffness and a completely irreversible con-trol system; for example, the aerodynamic loads can give rise to severe torque stress in the main wing box and differential loadingcan occur between one side of the aircraft and the other. As is usually the case, the photograph of the aircraft whichhas been released for publication by the Ministry of Supply so 40 BOUNDARY LAYER CONTROL AT FLAP HINGE LINE I WING LEADING EDGE... 4 B 12 INCIDENCE- DEGREES This hypothetical diagram, prepared by one of "Flight's" artists, may show in general terms the sort of air-blowing (super-circulation) system employed in the NA.39. Air is tapped from a manifold surrounding each of the de Havilland Gyron Junior engines and is then passed through suitable ducting to nozzles above the wing near the leading edge and trailing edge. The nozzles may be thin spanwise slots, or multiple fishtails as shown. The system must be made to operate with either engine out of action. The small graph indicates the gain in lift coefficient which simple flap-blowing can achieve.
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