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Aviation History
1956
1956 - 1723.PDF
7 December 1956 887 for orientation relative to the horizon. Missiles are, therefore,often roll-stabilized but not position-stabilized. That is to say, automatic corrective action is taken by the control system tomaintain the rolling velocity at or near zero, but the actual angular position of any wing pair relative to a datum is un-important. (It should be noted that roll-stabilization is required only to prevent rolling motion induced as a result of mechanicalmisalignments or aerodynamic cross-coupling. Rolling as a manoeuvre is never required, except where it is used to preventdeviation from the initial line-of-fire during the boosted phase of a ground-launched missile.) Plan shapes of missile wings vary widely, although thereappears to be some preference for forms with a moderately swept leading edge, a square tip and a straight trailing edge. Manymissiles have pure delta or rectangular plan-forms. Aspect ratios are normally low, and this "is a characteristic of all supersonicaerofoils: where, owing to the changed nature of the airflow, induced-drag reduction is no longer important. Moreover, low-aspect-ratio wings simplify handling, stowage and firing prob- lems. For air-to-air missiles the wing span determines how faraway the weapon has to be suspended beneath the wing of the parent aircraft, and consequently decides the size of the fixedlaunching structure that has to be carried. Problems of stowage are most acute in the case of missilesdesigned to be operated by naval forces. If large numbers are to be carried in a ship's magazine, it helps greatly if the wingsand control surfaces can be detached—though this hardly applies to defensive missiles, whose potential rate of fire would be muchtoo high to allow time for the assembly and precise alignment of eight surfaces on each round. Precise assembly is mandatory,because the disturbing airloads developed by even very small angular misalignment of any aerodynamic surface could be suffi-cient to absorb so much effort on the part of the servo control system that the margin of power left for manoeuvring would beseverely restricted. (As missiles cannot be test-flown prior to service it is impossible to trim out an incorrect flight attitudeinduced by airframe misalignments. This trouble can be avoided only by imposing stringent manufacturing limits on all structuralcomponents. Such requirements will raise problems in the pro- duction shops even more severe than those already experiencedwith the integral-structure items for high-speed aircraft.) The carriage of bombardment missiles aboard submarines hasbeen suggested. Here, although space restrictions will be even more severe, it should be possible to carry out accurate assemblyoperations, as rate of fire will be low and the initiative in this case rests with the missile operator. To some degree, the plan-form of missile wings must also bedictated by aeroelastic considerations. Wings having a thickness/ chord ratio of four per cent and less must be susceptible to failurelay flutter, owing to the low torsional and bending stiffnesses implied; although the wing-root attachment to the body can bemade more continuous and torsionally rigid than would be the case with an aircraft. The wings are usually attached to the bodyby a multiplicity of connections, often socket-headed screws, distributed along the chord above and below the wing root. Thisindicates that the centre-section structure is not continued through the body to interconnect each wing-pair, but that thecentral structure consists of a thick-walled tube extending the length of the wing chord and internally free from obstructions.The constraint of the root in bending is thus likely to be con- siderably lower than it would be for a continuous centre-section.Another aeroelastic defect to which thin wings are subject is divergence. The leading-edge section of a supersonic aerofoil isextremely thin and it is aerodynamically desirable to have as sharp an edge as possible. This is achieved so well in most missiles—and also in certain United States fighter and research aircraft— that the leading edges have to be protected by covers to preventthem from injuring personnel on the ground. This type of leading-edge structure consequently has little stiffness in thebending mode. If the stiffness is below a certain critical value, the incremental increases in air-loading experienced by theleading edge as it deflects will cause displacement to increase progressively until structural failure occurs. As the root sectionsof most missile wings are constrained along the entire chord, the leading edge tends to fracture along a line displaced at asmall angle to the transverse axis. Thus, the fact that many missile wings have swept leading edges may be dictated as muchfrom aeroelastic considerations as from aerodynamic ones. One especially intriguing paradox is the difference in plan-forms existing between the wings and control surfaces. The wings generally have swept-back leading edges, square tips andstraight trailing edges, whilst the controls frequently have a converse geometry of straight, or near-straight, leading edges,raked tips and swept-forward trailing edges. The controls are generally pivoted on an arm fitted at about 40 per cent chord.The control plan-form is dictated largely by the need to restrict the movement of the centre of pressure as the surface is moved,in order to keep the control torque within the capacity of the servo-power available. Since the control loads are brought to a TJ Fig. 4. Boost configuration: tandem (top); forward wrap-round, with canted Venturis; and rear wrap-round, with boost fins. shaft attachment, it follows from structural considerations thatthe thickness/chord ratio must be greater than that specified for the wings; in practice these ratios appear to lie in the region ofsix to eight per cent. Depending upon the plan-form, the cross- sections used for controls are of basic double-wedge or paralleldouble-wedge profile. Defensive (anti-aircraft) guided weapons can be divided intotwo main classes, the air-launched and the ground-launched. The operational range and the active flight-time of an air-launchedmissile is considerably less than that of its ground-launched counterpart. (Air-to-air weapons are, in most instances, merelyan interim solution to the air-defence problem.) A missile launched from an aircraft already possesses considerable kineticenergy. Its main motor or boost units accelerate it to a high supersonic speed. After burn-out, the missile coasts or glidesunder control for the remainder of its interception path. Its speed will eventually fall to a value at which the missile will beincapable of achieving its full design manoeuvres; but if the interception course does not call for these manoeuvres to beperformed the missile will remain operational. Finally the speed will fall to a lower limit at which the manoeuvre margin is sorestricted that the missile becomes non-operational. It will then be necessary to trigger some self-destruction device. Motors and Boost Units Fireflash is unusual among air-to-air designs in that it isaccelerated to its peak velocity by two external boost motors which separate at burn-out, leaving the engineless "dart" to con-tinue in free flight towards the target. Other air-to-air missiles, such as the Falcon, are equipped with an inbuilt motor whichis not jettisoned on burn-out. Missiles with external boosters have high drag during primary acceleration, but a small dart oflower drag is left to continue in free flight. The converse is the case for the missile with the internal motor. As designers of mostair-to-air missiles favour the latter layout it may be assumed that it is the more efficient. Another factor which probably influencesthe choice in favour of the integral motor is that, if external boosters fail to deliver the same thrust for the same time, yawingwill be induced; but design action can be taken to reduce this effect. Yawing cannot be serious with motors placed on themissile centre-line, even if the burning is erratic. Motors and boost units fitted to air-to-air missiles are of thesolid-charge type. Liquid-propellant motors are too complex to fit into such small structures and are also unnecessary in viewof the brief firing time. The power requirements of ground-launched missiles are morecomplex. In the first place, such a weapon has no initial kinetic energy, so that an additional launching phase (or boost phase) isrequired to accelerate the weapon from rest to above, or near, its operational speed. Acceleration is obtained by the traditionalmethod of providing very high thrust for a brief period by employ- ing external boost units, which fall away at burn-out. Operationalspeed is then maintained by an internal motor, which may be termed the "sustainer."In the boosting of ground-launched weapons there is a remark- able divergence of philosophy between Britain and the UnitedStates. Most American SAMs (surface-to-air missiles) are fitted with a boost unit in tandem with the weapon, whilst the knownBritish counterparts have wrap-round boosters. (Presumably this is the missile equivalent of the current aircraft-engine installationargument, "to pod or not to pod.") With a tandem boost several motors are assembled into a singlefaired container which fits on to the rear of the weapon. The • s
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