Patent GB-730966-A (GB)
Title: Improvements in or relating to mechanical and hydraulic remote control systems
Inventor: SYMON ALEXANDER
IPC: F15B7/00
Filed: 1952-04-07
Abstract: 730,966. Linkwork; toothed gearing. BRISTOL AEROPLANE CO., Ltd. April 1, 1953 [April 7, 1952], No. 8867/52. Classes 80 (1) and 80 (3). [Also in Groups XXIX and XXXIII] In a remote-control system, such as for a control surface of an aircraft, it is required to raise the natural frequency of the transmission sufficiently above the vibration frequency of the controlled device to avoid instability, and according to the invention the natural frequency is raised to a maximum by increasing the stiffness of the transmission-which may comprise linkwork, rotating shafts or hydraulic pistons and cylinders-in steps along the system from the controlled device (or " free end ") to a node in the system approximately in proportion to the fourth power of the distance from the free end, stiffness of a member being defined as the reciprocal of the strain produced in unit length thereof by unit load (i.e. the stiffness of a member is increased by decreasing the stress therein). A quartic curve Q, Fig. 1, shows the optimum distribution of stiffness along the length of a transmission from the free end O to the node N to give maximum natural frequency. For practical purposes the transmission is divided into three sections of equal length (FL1, L<SP>1</SP>L<SP>2</SP> and L<SP>2</SP>N) for which stiffnesses S<SP>1</SP>, S<SP>2</SP>, S<SP>3</SP> are chosen to give approximately equal areas above and below the curve. The three transmission sections, such as three rods 12, Fig. 3, connecting a hand-lever 10 (the " node ") with an aerofoil 11 (the " free end "), are of similar material and section but are interconnected by levers 13 so that the stress is greatest, and thus the stiffness least, in the section adjacent the free end. The velocity ratios between adjacent sections are #S<SP>2</SP>/S<SP>1</SP> and #S<SP>3</SP>/S<SP>2</SP>. The three sections may comprise three shafts, Fig. 4 (not shown), interconnected by spur reduction gearing, the hand-lever being geared to the first shaft so as to increase the stiffness of the transmission as a whole. The third shaft is connected by eccentric or elliptical gears to linkwork controlling the. aerofoil so that the transmission ratio varies continuously to give a fine control at certain parts of the movement. In a hydraulic transmission, Fig. 5 (not shown), a hand-lever is connected to a control surface by means of input and output cylinders connected by three ducts which are arranged in series and are interconnected by large- and small-diameter piston units so that the stress in successive ducts is increased. In a construction, Fig. 6 (not shown), where a single hand lever is connected through two shafts and two stages of reduction gearing to a transverse shaft whose two ends are separately geared, each through two further stages of reduction gearing, to two control surfaces, the transmission is regarded as a main system having a node at the hand lever, the stiffness of the two branch systems being taken as the sum of their separate values, and a secondary system having a node at the junction of the branch systems. It is stated that usually the natural frequency of the secondary system is so much higher than that of the primary system that no compromise in the distribution of stiffnesses is necessary. In a further construction wherein a control surface 79, Fig. 7, is moved by a hand-lever 80 through a train of shafts and gearing, a device 88 such as an inertia damper or a one-way coupling operates on the shaft 81 to provide a node at this point so that the transmission between the hand lever and the device 88 is not subject to vibration. Curves are given for a transmission of three equal-length sections showing the effect on the natural frequency of varying the ratios of the stiffnesses. The natural frequency may be increased by dividing the transmission into more than three sections but it is stated that in practice it is difficult to raise the frequency to more than 2.5 times the frequency of a transmission of uniform stiffness.