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Aviation History
1955
1955 - 1114.PDF
226 FLIGHT Bristol Britannia . . . transmitted through high-speed torque tubes leading to the wingtips and tail. These tubes are very light and have the same expan- sion coefficient as the remainder of the airframc; their entirelength is carried in anti-friction bearings and the static loads on the pilot's controls are extremely low. At the surface-end of each torque tube the motion is re-convertedto a linear displacement applied to the tab which, in turn, moves the main surface. The entire system is now fully developed andis extremely popular with all pilots who have become familiar with it. Basic stick force is the sum of the circuit friction and the tabhinge-moment, neither of which is sufficient to give any reason- able feel. Some form of artificial feel, proportional to control dis-placement, was therefore considered desirable (the A.R.B. consider it mandatory) and this is provided for the ailerons and rudder bycoil springs, the neutral point of which can be varied by electric screw-jacks under the control of duplicated trim switches on thecockpit pedestal. For the elevators, however, some form of v-feel was essential(proportional to indicated airspeed) and this has been inserted by a special Hobson hydraulic feel-simulating circuit into which isfed stick-displacement and i.a.s. information. The characteristics of the system are such that the airframe cannot be over-loaded through the application of excessive elevator angle. Elevator trim is effected by an independent circuit driven byconventional hand-wheels. Torque tubes transmit the hand-wheel rotation to the innermost of the four tabs on each elevator, thesetabs being reserved for this purpose. All four of the rudder tabs are connected to the normal control system. Each aileron, how-ever, has five tabs, of which the innermost is actuated only by a separate circuit interconnected with the rudder torque-tube.The result is that application of rudder automatically increases the roll due to dihedral in yawed flight while retaining optimumdihedral for level flight. It is possible, therefore, to effect small corrections to heading—while approaching to land, for example—merely by applying pressure to the appropriate pedal. In a con- ventional aircraft this would cause yaw with sideslip. Such a system of free-surface controls requires careful provisionof control locking. In the Britannia, a lever alongside the engine power levers can be moved aft against a spring and retained bya spring-loaded catch. This energizes small hydraulic jacks which centralize and then lock all the main control surfaces. When inthe aft position, the locking lever restricts the movement of the engine power levers and so prevents the aircraft from becomingairborne with locked controls (the pilot's controls remaining free to move at all times). With the surfaces locked, the four enginestogether cannot be opened above flight-idling power, although one engine on each side can be taken to full power. Five con-spicuous warning lamps, one for each surface, remain lit on the shroud panel as long as the controls are locked. It will be observed that, since the operation of the controlsis dependent upon airspeed, the requirement of ground-testing presents unusual problems. The position of any cockpit con-trol is no criterion of the position of its associated surface, and accordingly a special panel on the starboard side of the cockpit roof carries desynn-type repeaters which continuously indicatethe position of all five surfaces. Opening up the two inner engines provides sufficient airflow for ground-testing the tail surfaces,but the ailerons are normally unlocked and tested while taxying or facing upwind. It is of passing interest to note that, until the l.a.s. builds upon take-off, there is nothing to prevent the Britannia's control surfaces from moving independently, once they are unlocked. Nodoubt it will be many years before the sight of a Britannia with the port elevator up and the starboard elevator down will cease tocause astonishment. In each gearbox unit, between the flight deck and the torquetubes, a common layshaft is employed for pilot input, autopilot servo and a hydraulic damper. The latter is inserted to preventthe transmission of shocks or any too-rapid movements. The autopilot is the Smiths S.E.P.2, one of the most comprehensiveand highly developed systems in the world. Flight Deck. Although no doubt the Boeing 377 will neverbe rivalled for spaciousness, it is our opinion that the Britannia has the best-engineered flight deck in the world at the presenttime. It is worth remembering, in this connection, that B.O.A.C. are not only at present the principal purchasers but they are alsothe sponsors of the aircraft, and it is with their most intimate help that the Britannia has reached its present excellence. Provision is made for two pilots, a navigator and a radio officer,with a supernumary seat-position available for a fifth crew mem- ber. A most important feature is that the Britannia is cleared foroperation under limited conditions (ferrying, for example) with a minimum crew of two. All essential instruments and controls canbe reached by one or other pilot without his having to leave his seat or even undo his harness. The pilots' instrumentation is fullyduplicated and of a highly developed character. The Britannia 100 has well-known Sperry instruments, including a C.L.2 gyro com-pass, an H.L.5 horizon and a Zero Reader, together with an I.L.S. indicator. In later Britannias these four instruments are replacedby the Smiths Dual-Flight system, which is an extremely advanced integrated installation employing no more than two panel instru-ments, named the director horizon and the beam compass. A description of this system will shortly be published. Both large central panels are occupied by engine instruments,four separate jet-pipe temperature gauges replacing the two dual indicators of the earlier machines. The centre portion of the roofcontains remote radio panels and on either side of this installation are transparent roof portions which are of particular assistanceduring circuit flying. In spite of their essentially simple character, special note shouldbe taken of the "systems" control panels in the cockpit. The fuel panel, for example, is virtually a diagrammatic picture of thefuel system. All fuel lines are indicated on the two-dimensional panel face, warning lights being inserted in their correct relativepositions and flowmeters taking the place of the engines. On the basic panel, space is left for the additional tankage of the long-range Britannia. It is doubtful if any aircraft has a more readily interpreted form of presenting system-information, and similarpanels are employed for the electric circuits. Owing to the free-turbine layout of the Proteus it is possible A general yiew of the first Britannia 300, now being built in the East Bay of the Bristol Aeroplane Com- pany's great assembly hall at Filton. This air- craft is scheduled to fly during the coming winter.
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