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Aviation History
1961
1961 - 0012.PDF
10 FLIGHT, 6 January 1961 Part of the test equip- ment at the safety bar- rier and arrester gear proving base at Bedford Arresting RN Aircraft . . . danger of the main ram bottoming in the main cylinder, as evenat the end of the spline barrel stroke some clearance orifice remains. If this happens and the aircraft still has way on, the aircraft/energyabsorber link may part with disastrous consequences. The Link For all hydraulic gears the link between the aircraft and the gearis a steel wire rope stretched across the deck with an initial tension and raised above the deck by steel springs in the shape of a bow.In the ship case the bowsprings are retracted by power. The wire rope from the deck is led via a system of guide pulleys to one ofthe pulley blocks at either end of the main ram; it is then wrapped around the two crossheads missing every alternate pulley. Asecond wire rope from the deck is led to the other pulley block and wrapped around the two crossheads on the remaining pulleysin the opposite sense from the first rope. The reeving system is indicated in Fig 1. One unit therefore serves two centre-spanson the deck and each wire when rigged is a continuous length of 150 fathoms which serves opposite ends of two centre-spans.As the length across the deck is subject to heavy wear, means are provided for removal so that the centre span may be changed.The wire used in the system is of high quality and special con- struction determined by tests at NAD and at the manufacturers,British Ropes Ltd, Doncaster. Basic Problems The design requirement is the dissipation of a given amountof energy in a certain distance and within the strength of the aircraft and the limits of the human frame. The energy absorbedin a distance of just over 200ft is in excess of 15 x 106ft-lb and this gives rise to a retardation approaching 3g. The limits for theman are uncertain but they are probably of the order of 6g for a sustained load and appreciably higher for peak loads. The dis-tance in which the aircraft has to be stopped is determined by the ship layout and is unlikely to alter appreciably. If uniformretardation were achieved throughout the stroke then higher entry speeds than those currently obtaining could be accepted. Inpractice the first part of the stroke is used in accelerating the moving parts of the system, of which the rope is a big proportion,up to the speed of the aircraft. These impact loads occasioned by inertia put high forces on the aircraft. Uniform decelerationis therefore not achieved, the ratio of maximum to mean retarda- tion being about 1.5 for modern gears. The main problems are therefore to use as little of the strokeas possible in accelerating and to secure as closely uniform an acceleration as possible. As far as the hydraulic system isconcerned these two points are dependent upon the profile of the grooves in the spline barrel, i.e. at the beginning of the strokethe discharge orifice must be wide to enable speed to be built UP rapidly and must then gradually reduce in an attempt to achieveuniform retardation. The hydraulic efficiency is high, for if the mean pressure for the stroke is calculated from the amount ofenergy absorbed by the unit and related to the maximum pressure produced the ratio is approximately 1.25. The difference betweenthis figure and the overall maximum to mean retardations of 1.5 previously quoted is occasioned by tensions built up in the wirerope system by wave phenomena. This is at present the principal problem and the main obstacle to increased entry speed. The major parameters which bear upon the problem of attempt-ing to achieve uniform retardation within the strength limits of the aircraft, the wire rope, the main cylinder and the pulley bearingsare listed in the next section. The variation of these parameters throughout the arrest is not amenable to calculation and they aretherefore recorded from the unit under test conditions at Bedford. From these results, performance is established and improvementsrecommended. In the course of establishing the performance, mechanical faults occur and remedies are proposed. Theseimprovements in performance and operation result from close co-operation between McTaggart Scott, DGS and NAD. Main Parameters The variables measured are rope tensions, main cylinder pres-sure, ram stroke and aircraft or deadload retardation. Rope Tension. This is measured using a five sheave rope rider.The rope is deflected around a sheave mounted on a spindle which is supported through strain-gauged links. Under ten-sion the rope attempts to straighten and the pulley sup- port is loaded, thus causing an alteration in the gaugeswhich is recorded and related to rope tension. This rope rider was developed in NAD and has proved extremely successful. Ropetensions are measured at the deck edge sheaves and just before and just after the gear. The difference in tension recorded at thesepositions is of the order of 10 tons for high energy shots and is caused by reflections and pulley and rope friction as the tensionwave passes through the gear. A strain-gauged coupling can also be used to measure rope tension by recording the change causedby the elongation of a connection in the rope. This method suffers from the disadvantage that the connecting lead to the strain gaugeis subjected to the same motion as the rope with an increased danger of failure. The measurement of the tensions in the centrespan has not so far proved feasible. Attempts are now being made to determine the loads induced in the supporting structure, eitherof the hook in the aircraft case, or of the bollard in the deadload case, by strain-gauging the supporting members. Indications arethat successful records will be achieved but the estimation of rope tensions at the engaging point from the loads in the supportsinvolves a knowledge of the wire geometry at the hook throughout the arrest which is a fundamental property very difficult to assess,as the following brief explanation will indicate. The behaviour of tension waves in a complicated wire systemis imperfectly understood. The only detailed UK study of the problem* applies accepted theory to the arrester gear case overa limited range during the early stages just after impact. This application has not been systematically proved by controlledexperiment although a test vehicle to do this has been part of NAD's programme for some time. It is the old story of day-to-dayproblems taking priority over the furtherance of fundamental knowledge. However, measured tensions accord reasonably wellwith theory for high entry speeds. The facts fundamental to the study of tension wave phenomenaare that the speed of propagation of a longitudinal stress wave is given by V P i.e., the speed of sound in the rope. The speed of propagation of atransverse disturbance is given by V m where E = elastic modulus, p—density, g=acceleration of gravity,T = tension, m = line density. From these two fundamental formulae the following relation-ship can be deduced for transverse impact of the kind experienced in the arrester gear application This expression is a first approximation and assumes no initialtension in the wire. It can be expanded to give a more rigorous treatment. When the rope is struck a longitudinal tension wave is en-gendered which stretches the rope between the point of impact and the front of the stress wave and this stretch gives the slackfor the formation of the transverse wave. The transverse wave is partially reflected and partly transmitted by impact with thecentre span couplings and the deck-edge sheaves, and subsidiary longitudinal and transverse waves are set up, some of which returnto the impact point until the whole system vibrates violently. It can be shown that for the case of impact with the deck edge * Behaviour of Ropes under Longitudinal and Transverse Impact. by J. Thomlinson. RAE Report No NA227.
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