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Aviation History
1951
1951 - 0504.PDF
V FLIGHT, 16 March 1951 The instrument layout is an attempt at providing the helicopter pilot's ideal. Key:— 1. Altimeter 2. Air-speed indicator 3. Artificial horizon 4. Air preuure (clutch) I. Boost (lute 6. Clock 7. Compass 8. Cross level 9. Cylinder temp. 10. Directional gyro 11. Fuel contents gauge 12. Fuel pressure gauge 13. Oil pressure (engine) 14. Oil temp, (engine) 15. Oil temp, (gearbox)' 16. Rate of climb 17. Suction gauge 18. Ammeter 19. Tachometers or decometers 20. Fuel warning lights 21. Clutch control 22. Bearing temperature warning 23. Boost or decometer BRISTOL 173 ... constant need for greater speed, helicopters will undoubtedly provide the most economic means of transport. So far as the Bristol 173 is concerned, it was some time ago that the company foresaw that a multi-engined multi-rotor machine would need to be developed if ever the helicopter was to be lifted out of the "interesting experi- ment" category. Mr. Raoul Hafner and his team therefore focused their attention on producing a design study which, whilst meeting the basic requirements, would at the same time make maximum use of the data established with the 171. Having decided on fundamentals and essential form, Bristols put up the proposal to the M.o.S. : a specification was writ- ten, a development contract issued and work on the 173 began. Today, the first flight tests with the prototype are expected to start shortly. Physically, as may be seen from the illustrations, the air- craft has a neat and plainly functional appearance. Essen- tially, the layout could not well be simpler : there is a two- seat cockpit up at the front, a forward engine-room, a long, roomy cabin and an aft engine-room. In planning the air- craft, the collaboration of the Ministry of Supply, all branches of the Services and British European Airways was engaged in order to fit the helicopter to meet the widest possible ambit of operational requirements. The first prototype will be used chiefly to prove the configuration. The second aircraft, however, should be fully representative of the production type, and it is at present intended that the first 173 to go into operation will be used by B.E.A. for operational investiga- tion of Orkney-Shetland and similar routes. So far as is now known, the aircraft will ably meet all the B.E.A. helicopter requirements although, naturally enough, a few minor changes may have to be made in the light of practice. It goes without saying that such an aircraft as this will be well-fitted to a wide variety of Service r61es. For example, 18 airborne troops could be carried; for ambulance work ten stretcher cases or ten sitting cases or combinations of each could be accommodated; for air/sea rescue work, the aircraft is a "natural," whilst other roles which spring immediately to mind are freight carriage, radar calibration, photographic, signal-line laying, mobile crane and, of course, air observa- tion post. It is also evident that for all the additional variety of less punitive specialized jobs which the helicopter does so well—crop-spraying, mountain rescue, short-haul air mail, freightage over difficult terrain and the like—the 173 offers unique capabilities. The fact that structurally the aircraft approximates to a large-diameter tube supported at each end both whilst in the air and on the ground means that the structure-weight case is eased, and this is reflected in the, at first sight, surprisingly light gauges of the structural elements. Weight has, in fact, been severely monitored throughout the whole design of the aircraft, and to this must largely be attributed the excellent disposable load figure of something over 51 per cent. The airframe is a semi-monocoque of basically ovoid section, made up by 22g, Z-section frames at 15 Jin pitch notched for 22g, Z-section stringers at 4$in pitch. On this skeleton is carried skin panelling of D.T.D. 710 duralumin, varying in gauge between 18 and 26, lap-jointed horizontally and butt- jointed vertically. In structural composition this is un- doubtedly the most simple all-metal multi-engined 13-seater aircraft ever built. In making an examination of the aircraft, the most straight- forward attack is certainly to work through the design from front to rear, separating out the power installation and rotor systems for subsequent individual analysis. On this basis, then, the cockpit takes precedence. Bristols took an unusual amount of care in planning the 173 cockpit; they consulted every source of informed opinion they could think of and, as a result, the aircraft is furnished with a control cabin which is, in effect, a synthesis of everything that a wide variety of helicopter pilots could desire. Dividing the cap- tain's seat (to port) from that of the second pilot is a console for engine ground-running, trim and bias controls. The primary flight controls comprise a collective-pitch lever, con- venient to the left hand of each pilot, a cyclic-pitch lever in the conventional control column position, and "rudder" pedals in which are vested yawing control. Control trans- mission and the distribution of control function between the rotors will be dealt with on a later page. The instrument layout, as may be seen from the detail drawing, is unusual; nevertheless, it is the arrangement chosen by the majority of pilots consulted. It will be noted that alternative provision is made on the panel for decometers, and in this connection, provision is also made for a Decca Flight Log display head to be installed at the forward extremity of the central control console. Although by comparison with many other (but smaller) helicopters, the cockpit glazing of the 173 does not appear lavish, in actual fact the void area is generous. With the exception of the front screen, which is of Triplex, the cockpit glazing is entirely Perspex. Both pilots' doors are flush- closing toggle-and-runner hung sliding units. Stowage is provided for radio and associated equipment in standard racks on the cockpit bulkhead behind the captain's seat. The central portion of the bulkhead takes the form of a vertical box conduit enclosing the flying control transmission, whilst on the starboard side, behind the second pilot's seat, a folding door gives access through the bulkhead to the forward engine room. This compartment is divided from the main cabin by a diaphragm bulkhead stabilized on its rear face by a grid of channel-section members : a small doorway in the centre of the bulkhead provides for emergency access between the forward part of the aircraft and the main cabin. The forward Leonides 23 HMV 550 h.p. helicopter power plant is mounted with its crankshaft axis tilted forward five Structuro/ly, the fuselage is extremely simple and is provided with escape hatch/windows of unusually generous area.
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