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Aviation History
1961
1961 - 0009.PDF
FLIGHT, 6 January 1961 9 Arresting RN Aircraft DEVELOPMENT WORK AT RAE BEDFORD By R. H. TOWNSEND, RCNC THIS article, from the "Journal of the Royal Naval Scientific Service,"is reproduced by permission of the Director-General Ships, Admiralty. Responsibility for expression of facts and opinions is that of the author,who was until recently with the Naval Air Department, RAE Bedford. NAVAL arrester gears are designed and built exclusively byMcTaggart Scott & Co Ltd, Loanhead, near Edinburgh,to requirements specified by the Director-General Ships. A prototype gear or gears is supplied to the Naval Air Department,RAE Bedford, for proving and developing. This work is the main concern of the Arresting Section of the Ships InstallationsDivision of Naval Air Department, which is staffed by scientific and experimental officers drawn from the MoA and the RoyalNaval Scientific Service. There are two facilities which exist at RAE Bedford for theproving of arrester gears, and the first of these is the safety barrier and arrester gear proving base. This site is powered by a corditecatapult which can propel a deadload of up to 40,0001b at speeds up to 150kt. The advantage of the base is that a representativeimpact can be engendered without the use of piloted aircraft and hence more economically and with greater control and reliability. The second site is the arrested-landing deck, which is a 6,000ftlong runway with a pit one-third of the way along its length. In this vault, arrester gears are installed and coupled to centre spanswhich are stretched across the runway, the whole installation being representative of a ship unit. Aircraft are taxied into thesewires; they cannot be flown in, as in the ship case, because of the difficulty of driving a 6,000ft concrete runway along at 28kt. Thetwo main functions of this installation are die confirmation with live aircraft of the gear performance figures established by trialsat the proving base, and strength proofing of aircraft, i.e., loading the arrester hook with a force greater than the working load itwill receive in service. Aircraft proofing is also necessary for stores and modifications which are added to the aircraft throughout itslife. These, then, are the main tools at the disposal of the NavalAir Department and this article will indicate their use in the evaluation of an arrester gear currently being fitted to HM ships toenable diem to operate the new generation of Naval aircraft. The gear comprises fundamentally an energy absorber and anaircraft/energy absorber link. Fig 1 shows the primary hydraulic system for an arrester gear.The main ram and cylinder both carry at their extremities a bank of pulleys assembled on a shaft. The main cylinder is anchoredand hence the set of pulleys attached to it is referred to as the fixed crosshead. The ram is free to move by compressing the fluidand the set of pulleys at the free end of die ram is known as the moving crosshead. Initially, when die main ram is fully extended fig 1. Schematic diagram of typical arrester gear Arrest of a Vickers-Supermarine Scimitar during RAE trials the whole system is full of fluid at a pressure of approximately6501b per sq in. When the aircraft engages, load is transferred to the moving and fixed crossheads via the linking system and diemoving crosshead begins to move. This motion forces the fluid in the main cylinder through a control valve which is locatedclose to the fixed crosshead. The control valve is known as a spline valve and is a variable orifice with a wide gap at thebeginning of the motion reducing to almost nil at the end of the arrest. In this way energy is converted into pressure and dis-sipated in heat. The fluid, after being forced through the spline valve, passes to the liquid side of the piston accumulator througha non-return valve. When the aircraft has come to rest the non- return valve is closed by the pressure on the air side of the pistonaccumulator, which is maintained by air bottles at approximately 6501b/sq in. The aircraft is disengaged; then, to restore the unitto the initial conditions, the resetting valve is opened and the 6501b/sq in from the air bottles acts on the main ram througha line by-passing the non-return valve. The moving crosshead moves back to the reset position, the resetting valve is closedand the unit is ready for another engagement. The piston accumu- lator is fitted to ensure separation of die air and liquid surfacesso that aeration troubles may be reduced to a minimum. The spline valve is die decisive factor in the behaviour of theram and hence the characteristics of the arrest. The spline barrel, diat part of the valve which dictates the variation of the dischargeorifice, consists of a series of grooves of varying depth cut on the outer surface of a cylinder. Three grooves at either end of adiameter correspond and these serve for one particular range of aircraft weights. By rotating the barrel another pair of threegrooves is brought opposite die ports which are connected to die main cylinder. During the arrest the barrel moves axially and dievariation of discharge orifice is thus decided by the varying depth of the splines in the spline barrel. This barrel motion is controlledby die telemotor ram which is fixed to the main ram and forces fluid through the transmitter cylinder to the minimum orifice endof the spline barrel. A sectional view of the principal pans of the spline valve is shown in Fig 2. The spline profiles are so cut that for a given aircraft weightthe energy involved in an engagement at the maximum speed anticipated is absorbed in the full stroke of the main ram and splinebarrel. At the higher energies the spline barrel movement is asymptotic to the working stroke and to all intents and purposesis fixed over the speed range in which a ship normally operates. If the wrong weight setting is used or the aircraft's relative speedinto the gear is greater than gear performance allows there is a Fig 2. Sectional view of spline valve FLUID FLUID FROM MAIN CYLINDERS SPLIT COJPLING FLOW OF FLUID DURING ARRESTSPLINE BARREL',TURNING GEAR FITTED HERE FIXED CROSSHEA MAIN i*/LINGER ,7=T COMPRESSED AIR FLUID FROM TRANSMITTER CYLINDERHYDRAULIC FLUID SHOWN SHADED GUIDE1 SPLINESSPLIT COUPLING EXHAUST TOACCUMULATORS MOTIONOF BARREL DURING ARREST VAS/E STROKE INDICATOR
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