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Aviation History
1957
1957 - 1680.PDF
DUPLICATED INPUT LINKAGE > SPRING BOX ON INPUT LINKAGE 768 FLIGHT SPLIT SECTION CONTROL SURFACE CONTROL SYSTEMS OUTLOOK . . . spurious signals—a damping rather than an exciting tendency.A "reversing lever" adjacent to the flying control valve is a simple way of ensuring the negative feedback effect. Hobson work on flying controls for military aircraft has ledto the development of electrical signalling between the stick and the powered control, since modern military aircraft are in any casemaking extensive use of electric and electronic control in the flight system and weapons control of the aircraft. There are also sub-stantial advantages to be gained in the problems of control at high induced temperatures. The work of Hobson and other firms inthis field has been discussed in the issues of Flight reviewing the Farnborough Show. The appearance of such a system in a pro-duction British aircraft has now probably been delayed by some years, but it is worth noting that an aircraft equipped with fullelectrical signalling of all its powered flying controls ("fly-by- wire" system) is shortly to fly in North America. A systemutilizing Boulton Paul flying controls and limited to the aileron channel has been flown in the U.K. in the Tay-Viscount. This does not mean, however, that the lead in this branch offlying control science has been surrendered. Contracts for the development of electrical signalling by flight experience have beencontinued, and Hobson now have a "half-way" electrically sig- nalled system flying in an Air Survey, Ltd., Dakota operating fromWhite Waltham. Although coupled to the stick, the output of the system does no work on the flying-control surfaces; the system ismerely operated in a representative way. A second and more ad- vanced system is shortly to be installed and flown for 1,000 hr. A word about the development of the system itself is appro-priate here, because it underlines Hobson's feelings about the provision of safeguards. The first system with Elliott amplifiersand A.C pick-offs (exhibited at the 1956 S.B.A.C. Show) had as a stand-by signalling circuit a mechanical linkage that was clutched-in if a failure occurred in the electrical line. Although it operated quite satisfactorily on the bench this conception was really inopposition to the general Hobson philosophy, since change-over from an electric main system to a manual stand-by is difficult toachieve and is complicated by the necessity for fault-sensing devices. There are also likely to be various difficulties in aligningthe electric and mechanical channels during installation. But the crux of the problem is that the mechanical channel must be madethe reference against which the electrics are checked, and this suf- fers from every one of the disadvantages which the electric signal-ling is put there to avoid; the worst of both worlds, in fact. The succeeding system was consequently all-electric, usingHobson thermionic-valve amplifiers and triplicated signalling channels, pick-offs, torque motors and valves. The need fortriplication is governed by the need to control the three conditions of an electrical signal: it can be off, it can be on, or it can providethe wrong information. Should a signalling error occur on one channel, the system will then be majority-ruled and continue towork satisfactorily. The American approach, incidentally, appears to be to use a multiplicity of pick-offs and signalling channels butto mix the signals in one amplifier and use one relay and one valve. And although for reliability multi-channel signalling isused, mechanical stand-by is retained. Shown schematically is a possible flying-control system for civil aircraft with two powered controls to each section of a divided control surface Return hydraulic lines and jack-position feedback linkage hcve omitted. Spring boxes protect the linkage from valve seizure In November 1956 Hobsons and the A. V. Roe weapons divi-sion stated liaison on the developed electrical signalling system which is soon to be installed in the test Dakota, and the twoorganizations are now working in close conjunction. Whether, in the light of changed emphasis, this particular con-trol-system development will proceed much further is difficult i foresee. It depends largely on whether British aircraft with avery high performance (in the terms of two or three years hence) and comprehensive electric and electronic systems are ever built.Although certain advantages for civil transports can be also shown it is most unlikely that such control systems would ever come intothe civil field without the usual military evolutionary process. But if this did occur, the widespread experience gained would eventu-ally result in natural civil acceptance. In time-scale, this might coincide with the advent of the supersonic airliner. Powered flying-control design for civil aircraft is developing, iHobson's view, along fairly well-defined lines. The requirements of A.R.B. and (in particular) C.A.A. demand coverage of a widerrange of failures than need be met in military applications, and the trend of design is governed by meeting each of these cases withthe simplest overall system. Among the conditions to be con- sidered are those governing a runaway servo, disconnection of theinput linkage, and fracture of any critical element in the system, The C.A.A. also demand that if the input circuit is not duplicated,the trim systemmust have sufficient authority to ensure that a : landing can be made in the event of disconnection of the normalcontrol system. Resolution of these stipulations tends to lead to the systemillustrated at top left. To cater for a disconnection or runaway failure (it is worth noting that, in Hobson's experience, no suchcase has ever occurred) the control surfaces are split, so that the failure of one section of a surface can be over-ruled by the remain- TORQUE STICK MOVEMENT q-FEEL V- FEEL Providing the pilot with feel proportional to equivalent air speed; as e.a.s. increases, the spring operating the force-controlled pressure regulating valve is compressed by the diaphragm, which reduces its effective area. This area reduction corresponds to the differ- ence in the v and q stick-rate curves shown on the left; the latter characteristic can be obtained by replacing the spring with a rod. SPEED Torque characteristics as a function of motor speed for D.C. (left) and A.C. (right) machines. The advantages of A.C. starting in spite of this apparent handicap are discussed in the text. ing, active sections. The number of divisions will depend upon thehandling of the aircraft with one section inoperative. In practice, the difficulties of providing such a trim system withpowered controls is such that it is usually simpler to duplicate the input circuit, and this is carried up to the input to the servovalves. The number of jacks used on each section is determined by the necessity for retaining stiffness of the surface should the outputof one jack become disconnected, or seal failure cause a complete loss of fluid. There are, consequently, separate hydraulic suppliesfrom engine-driven pumps to each jack of a pair. The less accept- able alternative is to use one system as a stand-by for the other;but the complications and potential unreliability of change-over mechanisms (which are brought into use only in emergency) arebetter avoided altogether. Weighed against this must be the reduc- tion to fifty per cent of the control-surface hinge moment if oneside of an active system should fail. Another consideration that figures in the design of a system isthe question of mass-balancing the control surfaces; it would be possible to dispense with one jack and use a suitable anti-flutterdamper if the surfaces were mass-balanced, but since in the case of a large aircraft the additional weight might be 700 or 800 lb,additional jacks and no balance are the obvious alternative. A further feature of the system shown is the pre-loaded spring b°ximmediately upstream of each control servo valve; should a valve seizure occur, the linkage can be moved against the associated PfTOT PRESSURE PRESSURE PROPORTIONAL-* TO SPEED E AS J STATIC PRESSURE -RETURN 3- SUPPLY PRESSURE
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