Patent GB-470492-A (GB)
Title: Improvements in or relating to control systems for the power units of aircraft
IPC: B64C11/30
Filed: 1935-11-12
Abstract: 470,492. Fluid - pressure servomotor - control systems. HALFORD, F. B., and MOULT, E. S. Nov. 12, 1935, No. 31236. [Class 135] [Also in Groups XXVII and XXXIII] In an aircraft power unit having a speed governor B controlling the pitch of the airscrew according to the engine speed and a fuel control device, such as the throttle G, automatically adjusted by a device responsive to the pressure on the induction side of the engine, a master control device controlled by the pilot, such as the hand lever D, simultaneously adjusts the speed and mean effective pressure of the engine, the master control device adjusting the setting of the speed governor and at the same time imparting a movement to the fuel control device additional to any movement thereof determined by the induction pressure. The hand lever D may also influence the strength of the mixture, the ignition timing, the change-speed gear of the supercharger, and the timing of a gun firing between the airscrew blades, and in certain positions overrides the automatic control of the engine speed and the throttle G. The invention is also applicable to compressionignition engines. As shown in Fig. 1, the blades A<1> of the airscrew are rotatably mounted on stub shafts projecting from a hollow shaft A<3> on which is slidably mounted a cap A<5> carrying studs A<6> which, through slotted lugs A<7>, rotate the blades from the position of coarsest. pitch into which they are turned by the centrifugal and aerodynamic forces acting on them towards the position of finest pitch by fluid pressure in the shaft A<3>. This pressure is derived from an oil pump B<1> driven by the engine and is controlled by a piston valve B<6> movable to connect a pipe A<8> leading to the shaft A<3> with an exhaust pipe A or with a pipe B<3> leading from the pump and fitted with a relief valve B<5>. The valve B<6> is controlled by a centrifugal governor B<8> driven by the engine, the governor being adjustably loaded by a spring B<9>, the upper abutment of which is provided by a sleeve C<3> geared to a rack C. The rack C is adjusted by a hand lever D through links D<1>, D<2>, D3 between two of which is connected a slidable cam F which through a bell-crank F<3> and links F<5> adjusts the contact breakers F<6> of the magnetos to adjust the ignition timing. The sleeve C<3> may move the valve B<6> positively by means of collars B", B<12> so that the blades A<1> may be put into the position of maximum or minimum pitch irrespective of engine speed. The charging pressure is controlled by a cam E which through a spindle E<3> operates on an aneroid chamber E<2>, which may comprise a stack of evacuated capsules, within a chamber E<1> exposed to the charging pressure by a pipe H<1> communicating with the induction pipe beyond a supercharger H. The cam E also is controlled by the pilot's lever D. A servopiston G<6> controlled by a valve E<4> connected to the chamber E<2> is coupled to a floating lever G<4> linked to the lever D and also through a bell-crank G<2> to the throttle G. A lost-motion connection G<3> permits the lever D to be moved to increase the engine speed further after the throttle G has moved against a stop G<7> to the fully open position. The carburetter J is arranged to enrich the mixture as the charging pressure increases and for this purpose a fuel valve J<4> is opened by the charging pressure acting through a pipe J<1> on an aneroid chamber in a casing J<3>. A further needle valve J<8> tends to cut down the fuel supply to compensate for changes in the air density and is operated by an evacuated container in a casing J<5> open to the atmosphere. With the lever D in the starting position indicated at 1, the valve B<6> is positively upheld and the blades A<1> are at the maximum pitch. As the lever D moves towards the position 2, the throttle G is opened appreciably before the rack C changes the airscrew pitch or the cams F, E effect the ignition timing or the charging pressure respectively. However, continued movement of the lever D changes the governor loading and the governor sets the airscrew pitch so as to tend to maintain a definite engine speed for each position of the lever. Somewhat below the minimum cruising speed of, say, 1600 r.p.m., the cam F begins to advance the ignition timing over and above the basic advance made by the usual timing governors and continues to do so up to a speed of 1700 r.p.m. Simultaneously, the cam E increases the charging pressure necessary to restore the valve E4 to its central position. Thus for each engine speed is a definite charging pressure and hence a definite power output. Above a speed of 1700 r.p.m., the ignition timing is gradually retarded and the power output increases steadily until the maximum cruising speed of, say, 2200 r.p.m. is reached. During a second stage in which the lever D is moved from 2 to 3, the charging pressure increases rapidly whilst the engine speed is increasing from, say, 2200 r.p.m. to 2400 r.p.m. and the power output required for climbing or emergency conditions in level flight is attained. Meanwhile, the cam F retards the ignition timing still further and the mixture is enriched by opening of the valve J<4>. In the third stage, the lever D is moved from 3 to 4. The engine speed increases up to, say, 2800 r.p.m. whilst the charging pressure increases little and the ignition timing is advanced little, if at all. In this stage, suitable "take-off" speeds are attained. The Specification shows by a graph the changes in power output, charging pressure, and ignition timing as the engine speed increases. Another graph shows the change in power output as compared with the sea level output with increasing altitude for various engine speeds. In the modified arrangement shown in Fig. 4, the engine speed, the charging pressure and the ignition timing are controlled by cams L, N, O respectively on a shaft K<4> connected to the pilot's lever D. The cam N operates a charging pressure control device E as shown in Fig. 1 through a bell-crank N<2> having an eccentric pivot N<3> adjustable by a lever N<5> to effect slight changes in the relation between the speed and the charging pressure. The cam L is coupled to the speed or pitch control device B. The shaft K<4> is coupled to the throttle G by a lever M, links M<1>, M<2>, a pin M<3> and a slotted arm M'. Lost motion to permit increased engine speed when the throttle is fully open, takes place when the pin moves into such a position that the slot in the arm M' and a slot in a fixed plate M<5> are aligned. The ignition timing cam O is linked to a floating lever O<3> operating the control valve of a servomotor O<5> which adjusts the differential driving gear O<10> of the magnetos F' and the distributers F<8> to alter the ignition timing. The latter is additionally advanced when the charging pressure corresponding to a given speed begins to fall above a critical altitude. For this purpose, the floating lever is urged by a spring P<2> against a pivoted cam P<1> controlled by an evacuated container in a vessel P<3> open to the atmosphere. The anchorage of this container is adjustable by a supplementary lever R connected to a cam R<8>. The lever R is also linked to a slotted lever R<2>. The needle valves Q', Q<8>, corresponding respectively to the valves J<8>, J<4>, Fig. 1, are operated directly by evacuated containers Q<2>, Q<1> exposed to the atmospheric and the charging pressure respectively. The container Q<1> is anchored to the lever R<2> so that the mixture strength may be altered in the cruising speed range, e.g. weakened when cruising over long distances, by operation of the lever R. A device R<10> connected to the lever M operates the lever R<2> if left in a position corresponding to a weak mixture when the lever D is at or near the starting position or in a high speed position when a rich mixture is desired. The cam R8 operates to advance the ignition when the mixture is weak. In the case of a multi-engined craft, the members R<2> are interconnected for simultaneous operation and the engine speeds are synchronized by means of adjusting nuts D<4>. In a modified arrangement, the mixture may be temporarily enriched for manoeuvring purposes by a solenoid arranged to adjust the pivot of a lever connecting the fuel valve and an aneroid chamber when energized by a push switch on the hand lever. The switch may be latched in the closed position or a separate switch may be provided if it is desired to maintain the enriched mixture for an appreciable period. Fig. 6 shows an arrangement in which the supercharger H has a change-speed gear U<12> controlled conjointly by the altitude and the position of the pilot's lever D. The latter is connected to a cam S<11> on a shaft S<12> which operates the speed or pitch control and the ignition timing control as already described. The cam S" acts on a floating lever S<14> to which the aneroid chamber of the charging pressure regulator E is connected. The changespeed gear U<12> is operated by a servopiston U' controlled by a piston valve T<8> having a lostmotion connection T<7> with an aneroid chamber T<4> in a vessel T<5> open to the atmosphere. The chamber T' is anchored to a lever T<2> movable by a cam T and supported by a bell-crank T<10> operable by a rod T<12> which may be connected to the lever R, Fig. 4. The rod T<12> gives the pilot overriding control of the change-speed gear. The effect of the lost motion device T<7> is to cause the step up in the gear ratio to occur at an altitude greater, say, by 500 ft. than that at which the change-down occurs. As the piston U<3> travels to the bottom of its cylinder to effect a step-up in the gear ration, a slotted lever U<5> connected to the piston U<3> actuates a link U connected to the floating lever S<14> so that the datum of the regulator E is moved to reduce the charging pressure. A cam U<10> acting through a lever on the link U<9> further modifies the charging press