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Aviation History
1958
1958 - 0146.PDF
150 FLIGHT, 31 January 1958 Above is a schematic diagram of an elevator or rudder artificial-feel unit. The inset (aileron unit) replaces the double-acting rate-spring by mechanical stops. On the right are elevator and rudder feel-curves. THE VULCAN STORY . . . original 707B had simple upward-hinging plates, but the 707Apioneered the type finally chosen, in which rotating surfaces are mounted on perpendicular arms which move vertically out fromthe wing. Incorporation of this design in the Vulcan was com- plicated by the increased number of moving parts and the reducedstiffness of the surrounding aircraft structure. There are three brake positions: shut; medium drag (open and slightly rotated);and high drag (fully rotated), the last position being obtainable only with the undercarriage down. Although relatively small,these brakes have an extraordinary effect, adding two and a half times the profile drag of the clean aircraft. All the foregoing factors had been fully resolved by the timethat the first prototype was received by the A. and A.E.E. at Boscombe Down late in 1955. Nevertheless, a year previously acharacteristic had been discovered which eventually led to a valuable modification to the wing. It was found during tests on the 707A (WD 280) and theVulcan prototypes that application of g at high altitudes could generate minor buffeting. Although in no way serious, it wassufficient to pose a fatigue problem in the outer wings. Continued investigation in flight showed that the buffet regime becameuncomfortably near the performance boundary predicted for later versions of the Vulcan with more powerful engines. Accordinglythe 707A and Vulcan prototypes were subjected to g loadings to produce high angles of attack, which were then extrapolatedto simulate the behaviour of the developed Vulcan at its much greater operational altitudes. It should be emphasized that the buffet threshold is the point atwhich disturbance first becomes perceptible; the vibration experi- enced is, for example, much less than that caused by starting thepiston engines of an airliner. Avro, the R.A.E. and the N.P.L. attacked the problem along a broad front and investigated thebasic causes. A diagram on the previous page illustrates the three different factors causing the flow to separate from the uppersurface of the outer portion of the original Vulcan wing. At low to medium Mach numbers, the region A (in the diagram)experienced what approximates to a conventional stall. As the Mach number was increased it was found that the presence ofshock waves close behind the leading edge were a further cause of separation, notwithstanding the existence of supersonic expan-sion around the leading edge itself. At a still greater Mach num- ber the sonic region progressed back across the wing and theseparation followed the pattern illustrated at C. In Avro's words: "One basic factor affecting the incidence at which separationoccurs is the ratio of the peak local lift coefficient near the tip to that of the wing as a whole. On the basic Vulcan the hightaper ratio and the elliptic load distribution combine to give a peak value of this ratio at 1.56. It has been found, however, thatsmall variations in planform do not affect the basic elliptic loading of the delta wing, and therefore any extension to the chord nearthe tip is directly effective in reducing the local lift coefficient. The leading-edge modification consists of a 20 per cent chordextension over 'the outer 20 per cent of the wing, this extension reducing to zero at about 50 per cent semi-span. The effect is toreduce the peak lift-coefficient ratio to about 1.3, and this can be regarded as a direct increase in the incidence at the buffetthreshold of about 20 per cent. "However if full benefits are to be gained in all three regionsthen a combination of the planform change with other modifica- tions is required. The subsonic separation of region A can bedelayed by a greater radius of curvature on the upper surface near the leading edge, and this has been done by drooping the extendedleading edge. Unfortunately a drooped extension of this type also delays the formation of the supersonic expansion which is bene-ficial in region B, and this has been promoted at an earlier incidence by sharpening the leading edge. Neither of these modi- fications really affects the onset of rear separation and here thefundamental cure would be a reduction in sectional thickness ratio. It has, however, been found that vortex generators willre-energize the boundary layer sufficiently to give a substantial gain in the lift coefficient at separation in region C." Flight trials of the modified wing were conducted with a707A and the modification was thereupon transferred to the Vulcan itself early in 1955. The modification was confined to thefirst 12^ per cent of original wing chord. A number of production B. Mk 1 aircraft had been completedwhen the revised wing was introduced, but all were speedily brought up to the new configuration (which was designatedPhase 2). The first Vulcan B.I, XA 889, was delivered to Boscombe Down for preliminary acceptance trials shortly beforeEaster 1956, and it was checked out within a month. Initial acceptance thus occurred nine years five months from the receiptof the specification, and three years seven months from the first flight. Three months later, in August 1956, the first delivery wasmade—to No. 230 Operational Conversion Unit at Waddington, Lincolnshire. Operational Vulcans with Bomber Command are assigned toNo. 1 Group, and squadrons already named as recipients include Nos. 83, 101 and 617. The performance of the aircraft in servicehas been truly outstanding—except for a singularly unfortunate electronic malfunction during the S.A.C. bombing trials at Pine-castle A.F.B., Florida—and in our first appraisal of the aircraft through the eyes of the R.A.F- (Flight, September 27, 1957) itwas remarked that its utilization rate was better than that of any other machine in Bomber Command, large or small. Production Vulcans have been successively fitted with Olympusengines rated at 11,000, 12,000 and 13,000 lb thrust. From the earliest days, however, the overall powerplant installation has beensuitable for units with a thrust of up to 16,000 1b, and this is the initial type-tested rating of the Olympus 200, or BO1.6, whichis in production for the Vulcan B. Mk 2. The Olympus 200 is a quite outstanding engine, and was discussed in our issue ofFebruary 15, 1957. In order to utilize the substantial increase in power from this engine, Avro had to conduct a major airframeredesign, the full ramifications of which will become slightly more apparent when the first B.2 flies later this year. It is possible to comment on the wing of the new Vulcan, sincethis has been fitted to the second prototype, VX 777. The modi- fication was completed late in August last year and 777 has sinceserved as the aerodynamic test vehicle for the B.2. It is evident that the new wing takes the kinking process, which produced thePhase 2 wing of die Mk 1 aircraft, a stage further. As far as can be determined the inner wing of the B.2 is the same as that of theMk 1, but the sweep angle outboard of the first kink has been reduced. As a result the thickness/chord ratio over the outer partof the wing has been greatly reduced, while both area and span have been increased. The inescapable conclusion is that—evenhad the power remained the same as before—the B.2 will be able to fly much higher than its predecessors and be able to pull appre-ciably higher g at all operational altitudes. The flying of VX 777 at Farnborough indicated that all the movable surfaces on the trail-ing edge are elevons, serving as either ailerons or elevators. No details may yet be given of the number of Vulcans builtor on order, or of the proportion of the total which will be of the new Mk 2 version. Suffice to say that the Vulcan is one of themost potent bombing aeroplanes flying anywhere in the world at this time; and the only jarring note in an otherwise magnificentpicture is that the R.A.F.'s establishment for such aircraft is rela- tively so small that Avro cannot build the Vulcan anything likeas rapidly as their capacity permits.
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