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Aviation History
1950
1950 - 1988.PDF
0 ' \ 461 Calibre comparison : (left to right) Q.22in ; QASSin revolver; 0.303'm ; O.Sin ; 20 mm. The two last-named rounds have "rimless" cases. fired with a perfect aim. This "technical" dispersion— peculiar to the gun, its installation, and its ammunition— must be multiplied by a " personal" factor peculiar to the man concerned. For an average marksman, this factor approaches a value of 0.5. In the Luftwaffe, an evaluation of films taken during actual air combats proved that average shots achieved aim- ing dispersions of 10 to 12 per mine of the firing distance, whilst good marksmen had 8 per mille aiming dispersion only. A four-engined medium bomber may present, from the position of attack, a target area of, say, 300 sq ft. The lethal area calculated on the basis of technical burst- dispersion and the personal factor may be, for the range at which fire is opened, 7,000 sq ft. Then, at best, no more than (300/7,000) x 100=4.5 per cent hits can be expected of all the shells fired during this burst. For a 20-sec burst from four 20 mm guns, about four hits could thus be reasonably expected. Actual air-combat experience has yielded dispeision values of 2 per cent of the range with 20 mm guns, i.e., the diameter of the lethal circle at 600 yards' range was not less than 72ft, or about three- quarters of the span. "Fire density" means the number of shells which pene- trate the lethal circle within a given unit of time. High fire-density and small dispersion are, for a given projectile, guarantees of success in air combat. Self-propelling missiles, such as rockets, have energy of motion imparted to them because of the thrust force; when the force ceases, they behave like gun-fired projectiles. The thrust force prevents neither drop nor dispersion; but the trajectory becomes flatter because of the retention or increase of the speed. Salvo-firing experiments with the original model of R.4/M proved a technical dispersion of approximately 10 per mille over ranges of 200 yards. Improved types exhibited substantially less. The "time of flight," i.e., the interval between ignition of the propellant charge and impact on the target, is another factor important for probability of destruction. The flying target moves over considerable distances whilst the projectile is under way. The greater the range over which the fire is opened, and the slower the projectile, the less the probability of a hit. As to manoeuvrability, the faster the target the less capable will it be of changing its course quickly, so the use of automatically computing sights is a quite practical method of coping with the situation. The ideal case is, of course, the pilotless aircraft, which is unable to take evasive action. The preceding reference to dispersion concerned firing from an aircraft in the direction of flight. Queer things happen when firing at an angle to this direction: aero- dynamic forces then deflect the projectile, because it enters the air stream obliquely. With a fast spinning shell, a Magnus cross-force results from the circulation. When leaving the muzzle, the shell's trajectory is deter- mined by three imposed motions: translation along the barrel; velocity of the muzzle in respect to the air at rest; and rotation due to spin. Deflections of the shell due to aerodynamic forces produce, in turn, gyroscopic effects. When these deflections can no longer be neglected (i.e., in view of high aircraft speeds and long ranges), due allow- ance must be made in the technical dispersion if the barrel does not point in the direction of the intercepter's motion. Semi-flexible gun installations—which, because of reduced manoeuvrability, might eventually become necessary for supersonic intercepters—present particular problems in the adaptation of automatic computing and triggering sights. The trajectory of a rocket, too, greatly depends upon the motion of the " platform" from which it is launched. Supersonic missiles, like the R.4/M, preferably have initial guidance, such as launching tubes or rails; the optimum length of these is critical. Also, the attitude of the rail in respect to the path of the launching aircraft has an effect upon the subsequent trajectory. Spinning rocket-missiles produce higher dispersion than fin-stabilized, unrotated ones. A major disadvantage in intercepters flying near their critical Mach number is their lack of steadiness in yaw. Present-day high-speed intercepters suffer from " snaking," i.e., from undamped yawing oscillations. This presents a great difficulty in air combat. Somewhat belated steps are now taken to provide gyroscopic " auto-stabilizer " devices to suppress snaking and give a steady gun platform. These devices (an experimental model by the R.A.E. was recently shown at Farnborough) allow the pilot free use of the rudder for taking aim. (To be continued) " ' FACILITATING FLIGHT-RESEARCH A USEFUL flight-test research tool has been developed byBlackburn and General Aircraft, Ltd., to supplement the scope of the automatic observer. Known as the Pressure Scanner, the device is basically a selector valve, into which run pipes from each point at which pressure is to be measured, and from which are connected a number of pipes to the pressure- recording instruments. The input ends of the pipes connected to the instruments are carried on a moving member in the scanner, which is driven in controlled steps across the ends of the pipes from the vents. By this means, each instrument is connected successively to a number of vents, so allowing the pressures at a large number of separate poults to be individually recorded by a small number of instruments. The instrument panel is photographed by a camera auto- matically timed so that the exposure is made just before the moving member transfers from one row of pipes to another. Instrument lighting is synchronized with the camera to reduce the electrical load, and die time allowed for the instrument reading to settle (three seconds) is more than enough to avoid lag in any normal length of pipe connecting the scanner to the vent. Such a scanning speed allows individual pressure measure- ments to be made at the astonishing rate of 300/min, while using only 30 instruments. FLEXIBLE BEARINGS TN the orthodox flexible bearing, inner and outer metal -«- elements are usually bonded by an intervening rubber sleeve, the elasticity of which permits partial movement of one element in relation to the ouier. In an unusual modification of this method, seen in the patented Clayflex bearing, there is die important difference that the rubber sleeve is not adhe- sively bonded to the metal; instead, it is stretched to a greater axial length before assembly, with the result that it assumes a permanently "active" state in which it tries to return to its former, larger diameter. This tendency, however, is pre- vented by the metal sleeves, so that a constant radial pressure is exerted which effectively prevents any creep between metal and rubber, provided the approved angular movement (up to 70 deg, according to bearing type) is not exceeded. If it is exceeded, the rubber will not tear; the bond is, in effect indestructible. For bearings intended to work in the presence of certain oils and other liquids, and in extremes of temperature, special synthetic elastic elements are used instead of natural rubbers. Some of the hundreds of standard Clayflex bushes, together with various special applications, are illustrated in a catalogue issued by the makers, Howard Clayton-Wright, Ltd., Welles- bourne, Warwickshire.
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