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Aviation History
1956
1956 - 1722.PDF
886 FLIGHT Fig. 1. Aircraft need to bank in order to provide a lift-component for a turn. A cruciform-wing missile can generate a lift force directly in any required direction. MISSILE DESIGN . . .. . . -.;-.-r-.--.„;-,,-.^ ance by adversely affecting the inherent stability. Weight analysismust also be more detailed than in aircraft design, and ounces and even fractions of an ounce are important. One consequenceis that production items such as castings must be particularly closely controlled, since such components tend to become heavieras production proceeds, and the margin of permissible weight variation for individual parts is small.Another critical factor that must loom large in the initial design stage is die allocation of space. The total volume of themissile is determined by the amount of equipment it is required to carry. Too large a volume means higher drag, heavier struc-ture and greater pitching or rolling inertia, with a consequent reduction in performance. Too small a volume means, at worst,inability of the weapon to meet its specification; at best, some item has to be drastically pared, to provide room for what isabsolutely essential. Thus it might be possible to reduce the size of the warhead, although this inevitably reduces the operationalvalue of the weapon. It follows that the initial specification for a guided weapon—and the initial design concept to meet thisspecification—must be most carefully determined, because all aspects of the design are more closely integrated than is the casewith an aircraft. Any major miscalculation at the initial planning stage, requiring a design modification at a later stage, is almostbound to have a serious effect on the final performance, even if it does not cause the design to be abandoned. Guided weaponsare not, therefore, so amenable to "stretching" as are most con- temporary aircraft designs. Whereas additional "black boxes"can generally be incorporated in a developing aircraft, this is im- possible with a guided weapon unless envisaged in the initialconcept. Volume packing densities often exceed 50 lb/cu ft. Space and weight are thus two of the factors which confine g.w.design with extreme stringency. As aircraft conform to a general geometry, dependent upontheir role, so do guided weapons. There are two methods of manoeuvring the missile along the desired flight path.One mode of achieving a change in direction is to bank the weapon and apply control loads in a manner similar to that usedin conventional aircraft. This system is relatively slow in response, and so is suitable only for weapons flying a predeter-mined path where the most rapid manoeuvres are not required. The resulting "aircraft-type" configuration is usually specifiedonly for bombardment weapons, and even for these the configura- tion merely represents an interim stage until the true ballisticmissile has been evolved. Where high rate of manoeuvre is paramount, as it is for ••r~*: j defensive weapons which have to intercept agile and fast-movingtargets, a different missile geometry was very quickly (and almost universally) adopted, involving a cruciform layout with doublepairs of wings and control surfaces. The virtue of this arrange- ment is that, by movement of the appropriate controls, a forcecan be generated in the required direction as a component of the lifts induced on each mutually perpendicular wing pair. Thiscan be done in the minimum time, and without first having to twist or roll the missile about its longitudinal axis. Although canard layouts—in which the control surfaces precedethe wings—are employed in some widely used American missiles, the most frequent British configuration is more conventional,with the wings placed ahead of the controls. One reason for the reluctance to specify a canard layout is probably that, whenmanoeuvring, the main wings are operating in the downwash of the controls and the values of the lift reactions become difficultto predict; erratic and even reversed-sense motion can occur. Similar arguments also apply, in a less severe form, to thepositioning of the controls behind the wings; and in some designs —such as Fireflash—the cruciform has been indexed at 45 degto the wing cruciform to remove the controls from the severest downwash region. Control surfaces for guided weapons (other than those for thesubsonic bombardment types, which follow conventional aircraft practice, and certain American designs, such as Falcon) are ofthe all-moving pattern, no fixed surfaces being associated with them. These all-moving surfaces provide, by suitable differentialaction, the forces for rolling, pitching and yawing, and can simul- taneously generate the necessary forces for initiating any type ofmotion. Consequently, ailerons are not required and the wings are completely fixed surfaces. A variation in control layout within the general cruciform Fig. 3. Typical wings: rectangular parallel double wedge, trapezoidal biconvex and trapezoidal double-wedge. Fig. 2. Variation in control surfaces: moving tail (top), canard and moving wing. In these sketches the wing and control cruciforms are both in line. missile configuration can be achieved by transferring the functionof the moving control to the wing and arranging for the rear aerodynamic surfaces to be fixed. Lateral forces for manoeuvringthe missile and differential forces for rolling are then generated by pivoting the wing and altering its angle of incidence relativeto the centre-line of the missile body; this geometry is seen in Sparrow and Terrier. The rear fixed surfaces then perform thefunction of stabilizers, limiting movement of the combined centre of pressure of the weapon. One of the arguments in favour ofthis layout is that the response of the missile is faster than when the manoeuvre is initiated by rearward-positioned controlsurfaces. Another point on which guided weapons differ aerodynamicallyfrom aircraft is the proportion of the total lift contributed by the body. In supersonic missiles most of this body lift is gener-ated over the nose cone, which is usually of conical or ogive form; and, depending upon the relative proportions of body and wings,this component of the lift can contribute up to 15 per cent or more of the total. Presumably it would be possible, by makingthe body relatively short and fat, to generate from the nose cone alone the entire lift necessary to manoeuvre the weapon; butcontrol and stability problems would be considerable and test vehicles of this form have yet to appear.Drag is always one of the biggest factors; and, in parallel with aircraft designed for supersonic flight, ultra-thin wings of lowaspect-ratio are universally used. 'Thin" is a relative term, but it is hard to avoid using superlatives when describing missilewings. High-speed aircraft are fitted with wing sections having thickness/chord ratios ranging from four to ten per cent. Fromexamination of the wings of the missiles already shown to the public, it is clear that ratios ranging from five per cent down to aslow as two per cent are in existence. The two-per-cent wings are usually solid, but for four-per-cent wings and above, fabricationmight result in a lighter structure. Wing sections are generally biconvex, double-wedge or paralleldouble wedge. All these profiles are symmetrical about the median plane, so that they are the same either way up. Thelift-curve slope is the same for negative or positive angular dis- placements; and, as the missile body is also invariably sym-metrical about its centre-line, there is no aerodynamic reason
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