FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1953
1953 - 0557.PDF
FLIGHT, i May 1953 553 COMET ENGINEERING . . . to about 12 man-hours (three men for four hours) and the civil Ghost 50 is a compact unit requiring no further disassembly once it is removed from the aircraft. The installation of the Ghost is shown in the Flight drawing on p. 552, which shows the port inner engine. The engine is carried by two simple trunnion bearings, incorporating Metalastik bushes to prevent the transmission of much high-frequency noise to the airframe. At the top is a single steadying tie-rod; all three pick-up points are secured by single bolts, and all the pipelines, cables and control runs are fitted with quick-release joints. The main air-intake duct is attached to the engine by a single toggle fastener; the jet-pipe—a lengthy unit owing to the considerable wing-chord—is secured by four simple clamps. When the latter are released, the pipe may be slid out to the rear on runners. At present, B.O.A.C. use a mobile crane for removing and replacing the Ghosts, which weigh slightly less than a ton apiece. The engine can be removed downwards once the trunnion bolts have been undone and the disconnections made, there being no need for particularly accurate positioning. A tensiometer sling has, however, been found necessary, to guard against overloading the lifting gear or damaging the engine. Apart from scheduled inspections, the Ghosts are given daily and pre-flight inspections; but these are quite minor and take one man about twenty and ten minutes respectively. A ground run is only carried out after 35 hours flying or if the aircraft has been out of use for 72 hours, and it should be noted that a Ghost—any turbojet, for that matter—maybe run-up to full power directly from cold. The entire engine bay can be reached from the ground, and B.O.A.C, have not purchased any special trestling or staging for Ghost overhauls. One of the most heartening features of the Ghost's behaviour has been the manner in which, time afte; iiiie, a snag has given no evidence of its presence until final detection at London Airport. This should not be taken as implying that the Comets are cruised happily with a sea of troubles on board; rather, that the troubles have been of a minor nature of a kind that could be rectified when the Comet has returned to the maintenance base. As a result, that scourge of efficient airline operation, the non-scheduled over haul, has very rarely been necessary. The actual figures are noteworthy, and may be taken as indica tive of the sort of performance to be expected as the Ghost and its axial successors are further developed. The failure-rate per thousand flying hours has been 0.8, counting non-productive flying, crew-training and route-proving. The current rate in normal service is 0.49 unscheduled engine changes per thousand hours which, as A. Cdre. F. R. Banks recently pointed out, may be compared with the figure of 0.6 established in service by two of the most highly developed radial piston-engines. The Ghost is, of course, singularly robust and so simple that there is not very much that can go wrong. By far the major proportion of failures have occurred in the ancillaries. The alter nator cooling-fan for example (necessary on the ground only), runs at up to 20,000 r.p.m. and the ball bearings originally fined to its shaft did not stand up to the operating conditions. The solution was a switch to Vandervell thin-wall bearings, lubricated by an oil film, which has proved quite satisfactory. Relighting is not a feature of normal operations, and even low- altitude stacking is at present done on four engines. But incorrect pilot drill with the pump-isolation switches can put an engine out and the easy relight of the Ghost is considered an asset. With the high-energy ignition system, now used in place of the original torch igniters, relighting is quite straightforward even above 25,000ft. And there is no doubt that later engines will better this performance. Following on from this it may be said that the Ghost turbine/ compressor unit runs in ball, roller and safety bearings, the latter bringing the engine to rest if all the others fail. If complete engine failure were to occur in flight the engine would, unless structural damage prevented it, continue to windmill at about 1,000 r.p.m. under the influence of ram air. Neither B.O.A.C. nor de Havilland consider there is any case for a means of stopping the engine dead in the air, although the C.A.A. are reported to be considering making this requirement mandatory. Time-expired Ghosts are sent to the D.H. test-beds and, in every case, they have been run up to full thrust. The only deterior ation in performance has been a very slight rise in jet-pipe temperature and specific fuel consumption. As regards perform ance deterioration in flight B.O.A.C. do not employ any form of installed thrustmeter—as favoured by the C.A.A.—nor do they consider there is any need for such a device. Any loss of power, or malfunctioning, of a Ghost can be detected at once. Before leaving consideration of the Comet power plant, one obvious question to be answered is "do de Havilland and B.O.A.C. think that their choice of buried (as opposed to podded) engines was correct?" Both partners answer an unqualified "yes"; later Comets will continue to employ installations of this type. Landing Gear.—The D.H. designed undercarriage has given outstanding service and has required no modification whatever. Maintenance of the gear has posed no problems, in spite of the fact that it is the first time an airline has employed a bogie under carriage, or units built around large, light-alloy forgings. Inherent in bogie-equipped aircraft is a limit on the turning radius, but this has not led to damage or tyre-scrubbing, neither has it impaired the Comet's ground mobility. The main bogies have stood up well to severe treatment, including landings with considerable drift on to indifferent runways. Certainly, the bogies ride better than equivalent single-wheel units (as were fitted to two Comet 1 prototypes) and they have also permitted operation from much poorer surfaces. Another advantage has been the ease with which the small tyres and wheels can be manhandled. The mainwheel tyres are Dunlop 35.00x17 size and they operate at ii5lb/sqin at 105,000 lb aircraft all-up weight. Tyre behaviour has been fully up to expectations, and no par ticular trouble has been experienced from slamming of the front bogie-wheels on to the runway. The present tyre-life works out to roughly 100 landings for the mainwheels and between 100 and 200 for the nosewheels. The latter wheels are unbraked and are splined to a common shaft to eliminate shimmy. This performance is underlined by the airfields through which the Comets fly regularly. Many Comet ports of call—including London—are still partly under construction and contractors' lorries have a habit of strewing runways with small bits and pieces. The Comets, with their eight ribbed mainwheel tyres, suffered by picking up such paraphernalia in the tyre ribs. This, and the severe tension across the tread, has now led to the intro duction of a new Dunlop "dimpled" tyre which, having no ribs, collects no damaging material; also, it is broader, and stronger under side-load. The power-steered nose unit has also given fine service. A steer- able nosewheel is essential on such an aircraft for it is not the practice to use differential engine power for ground manoeuvring, neither would this have much effect. The difficulty of bringing jet transports to a halt after landing is one of the current "anti-jet" views being voiced in the United States. It is true that the Comet has not had to contend with icy runways—except at London, where there is plenty of room. But the landing has never been a critical factor in any of B.O.A.C.'s operations with the aircraft, and this includes services through airfields with runways of normal length, sometimes in conjunc tion with very adverse combinations of altitude, ground tempera ture and gradient. The brakes are duplicated Dunlop disc units, and B.O.A.C. praise them highly. Last year, B.O.A.C. decided to fit discs of increased thickness in order to comply with the requirements of the worst accelerate-stop case without overheating. An unexpected windfall is the fact that these discs can, after an undetermined but considerable number of landings, be machined smooth and re- plated ; this can also be done a second time, trebling the effective life of the units. When it is recalled that there are 16 brake discs to each Comet, this is seen as an appreciable saving. Dunlop's Maxaret anti-skid equipment is to be fitted to the B.O.A.C. Comet 2s, and the device may be fitted as a retrospec tive modification to the Series is. Hydraulics.—A study of the Comet manuals reveals the unusual extent of the hydraulic system, and it also leaves one slightly dazzled, for there are five separate circuits and each is allotted a distinctive colour. The systems which are of de Havilland design are: (1) Green system, serving landing-gear retraction, nosewheel steering, wheel- and air-brakes, flaps, flying-control secondary servos and windscreen wipers. The green circuit is fed by a hydraulic pump on each outer engine. (2) Blue system, charged with the vital task of actuating the flying-control primary servo units. Blue fluid cannot be intro duced into any other system, and vice versa. The circuit is connected with a pump on each inner engine. (3) Yellow (emergency) system, which takes over the flying- control secondary servo units in the event of failure of both blue and green circuits. Power is supplied by an electrically driven pump, and the yellow service can also be used for ground-testing the flying controls when no other power is available. (4) Red (stand-by) system, which can be used for emergency lowering of the undercarriage, and operation of flaps, wheel- brakes and steering. Again, power is supplied from an individual electrically driven pump and the red supply can be passed through all normal systems, other than the flying-control primary servos, for ground testing. (5) Broken-red (manual) system, the hand pump of which can be used to energize the emergency down-lines of the undercarriage only. Each system is labelled in its own distinctive colour, and this has made servicing much more rapid and straightforward than would otherwise have been the case. The principal supplier of
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
