Patent GB-832766-A (GB)
Title: Improvements in or relating to force balances for instruments
IPC: G01L15/00
Filed: 1956-03-07
Abstract: 832,766. Indicating apparatus; dynamometers; pressure gauges. YOUNGMAN, R. T. March 4, 1957 [March 7, 1956; May 24, 1956], Nos. 7083/56 and 16058/56. Addition to 700,556. Classes 106 (2) and 106 (4). [Also in Group XXXIII] An angle of attack meter for aircraft comprises a case 24 maintained at static air pressure through a nozzle 23 and enclosing two capsules 1, 2 connected respectively by nozzles 21 and 22 to an opening to receive direct wind pressure q and to an opening located at 0.1 chord below the aircraft wing subjected to a pressure p. The movable diaphragms of the capsules carry links 3, 4 interconnected centrally of the case at 7, a master link 5 being pivoted at one end to connection 7 and at the other end to a pivot 6 on an arm 8 which is pivotally mounted in a bearing 9 coaxial with the normal position of pivot 7. A shaft 13 connected to arm 8 is driven through a gear train 10, 11 by an electric motor 12 and it is provided with a wiper 16 for a potentiometer 15 whereby a signal may be derived to operate an angle-of-attack indicator. A fixed contact 17 and a movable contact 18 carried by link 4 on a flexible arm are in circuit with a relay coil, which operates contacts for reversing the polarity of motor 12. In operation motor 12 continually reverses its direction of rotation about twice per second and when the link system 3, 4, 5 is in equilibrium and the direction of rotation of motor 12 is such as to move rotatable arm 8 (Fig. 2), in a clockwise direction it will increase the master link component force along link 4 and reduce it along link 3 thereby throwing the system out of balance. The master link 5 will then be caused to rotate in a clockwise direction about pivot 6 by the unbalanced forces in links 3 and 4 which in turn will combine a movement of rotation and translation, thus moving contact 18 out of connection with contact 17. Thereby the circuit to the relay coil will be broken allowing the relay contacts to spring back to reverse the polarity of motor 12, which will rotate arm 8 anti-clockwise until the system is thrown out of balance in the reverse direction. When contact is re-made by movement of link 4 re-closing of the relay circuit causes the motor again to reverse its direction of rotation and recommence the hunt cycle. Thus, when the pressures in the capsules are steady motor 12 will continually hunt back and forth, the backlash in the gears 10, 11 accommodating this without transmitting any substantial motion to shaft 13. Relative variation in pressures p and q will affect capsules 1 and 2 to cause contacts 17 18 to be held either open or closed thereby enabing motor 12 to run continuously in one direction or the other so that arm 8 and link 5 are rotated to a new position in which a state of equilibrium is achieved between links 3, 4 and 5 and the relationship between contacts 17 and 18 is such that a balanced hunting of motor 12 may re-commence. Such rotation of arm 8 to the new position of equilibrium may, as described, cause movement of wiper 16 or it may operate directly the pointer of a visual angle-of-attack meter mounted at B-B in Fig. 1. The instrument may alternatively be adapted as a mach meter, altimeter or airspeed indicator.