Patent GB-623587-A (GB)
Title: Improvements in or relating to instruments for determining speed and/or distance travelled relative to the air
IPC: G01P5/16
Filed: 1946-11-29
Abstract: 623,587. Fluid-pressure speed indicators. REILLY, H. L., SCRIMSHAW, F. H., and BONE, A. G. Nov. 29, 1946, No. 35456. [Class 106 (ii)] [Also in Groups XXVI and XXXVIII] Aircraft &c. speed or distance travelled relative to the air is measured by an instrument of the kind in which pitot impact pressure and pressure from the outlet of a motor-driven fluid-pressure fan &c. are applied in opposition to pressure-responsive means for controlling the speed of the fan motor, characterised in that the pressure-responsive means afford a smaller mechanical advantage for the fan pressure than for the pitot pressure applied thereto, and in that the fan intake is connected with a source of pitot pressure so that the fan pressure is pitot pressure plus the pressure produced by the fan, and the fan is constructed such that its effective radius decreases with the increase of its rotary speed. It is an improvement on the instruments described in Specifications 565,214 and 579,216 to allow an extended speed range up to about 550 knots. As shown,' a composite diaphragm has a domed central portion 11 secured to an annular portion 12 clamped in a casing 15, 16 at its inner and outer edges 13, 14. The three fluid-tight compartments 18, 19 and 20 thus formed are connected respectively with a source of pitot pressure through a pipe 21, fan pressure through a pipe 22, and a source of static pressure through a pipe 23. A fan 33 (see Group XXVI), constructed so that its effective radius decreases with increase of its rotary speed, is mounted in a casing 38 having a domed intake portion 39, and driven by a motor 31. The pitot pressure pipe 21 is connected also to the intake 39 of the fan, and the pipe 22 is connected to the casing 38. The motor 31 is connected by gearing 58 to an indicator 59 showing miles travelled, and may also drive an electric transmitter 40 which drives one or more repeater motors for remote distance or speed indicating units. A battery &c. 48 drives the motor 31 when a contact 41 on the dome 11 engages a fixed contact 47 to energize a relay 49, 50 and close the battery circuit. In use, with the contacts 41, 47 open, the fan will decelerate until the pressure in compartment 19 due to the fan pressure and pitot pressure plus the static pressure in compartment 20 is overcome by the pitot pressure in compartment 18, when the composite diaphragm 11, 12 will move upwardly to close contacts 41, 47. The motor 31 will accelerate and the fan pressure in compartment 19 will increase in compartment 19 to depress the diaphragm and open the contacts 41, 47 to cause the motor to once more decelerate. The motor will hunt about a mean speed dependent upon the speed of travel, relative to air, of the craft in which the instrument is installed. With this arrangement, the pressure in compartment 19 increases more than linearly with the square of fan speed, and consequently the fan would run at too low a speed were it not for the automatic decrease in the effective radius of the fan, which has the effect of increasing the speed at which the fan must rotate for the pressure in compartment 19, 20 to balance or overcome that in compartment 18. To maintain the temperature of the fan casing 38 substantially the same as that of the air outside an aircraft in which the instrument is installed, the fan casing has a jacket in communication with the outside air. In one form, the fan casing 38 comprises a dished member within a second dished member forming a jacket, both secured to a base-plate fitted in an opening in the wing or fuselage, and openings or air ducts allow air to circulate within the jacket.