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Aviation History
1934
1934 - 1439.PDF
JULY 26, 1934 51 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT FIG. 2.—Component Parts of Compressor System, and Control Levers, a, Compressorand Windmill, b, Oil Reservoir, c, Oil Separator, d, Pressure Relief Valve, e, Test Cock, f, Main Control Cock, g, Course-change Cock, h, Air Expansion Chamber, j,Pitch Control Lever, k, "Pins " Lever. ing is a further argument against the use of an electric system, and the introduction of reduction gearing further increases the time lag with consequent loss of damping. The quantity of air required is of the order of four cubic feet per minute, compressed to a pressure of 30-35 lb. per sq. in. While such a supply is not large, it is beyond the capacity of compressed air bottles, and a compressor system is a necessity. Compressor System The compressor system has been designed to operate for long periods without attention, and possesses several unique features. A rotary type of compressor is used, which may be driven from the engine, but which is more usually mounted on a wing and driven by a small windmill by which it is driven at a speed of 1,200-2,400 r.p.m. The interior construction of the compressor is shown in Fig. 1. As may be seen, an eccentrically placed rotor has slots cut in it, in which hardened steel blades can slide under the action of centrifugal force. Air is drawn in through the inlet port and compressed by the rotation of the blades to the outlet port at the side. Unless the clearances between the ends of the blades and the case were very small, such a compressor would be very inefficient if some satisfactory seal were not pro- vided. It is found, however, that if a copious supply of oil is continually fed to the compressor a very satisfactory performance results, besides being accompanied by the minimum of wear. Referring to the diagram of Fig. 3, the oil is supplied to the compressor through the jet A, and is ejected from the compressor with the compressed air, in the form of an emulsion, to an oil reservoir. The emulsion is thrown against the walls of the reservoir through a jet, and this action causes the oil to flow to the bottom while leaving the compressed air nearly free of oil. An outlet is provided at the bottom of the reservoir, to which a filter is fitted through which the oil passes again to the inlet jet on the top of the compressor. The size of the inlet jet controls the quantity of oil supplied. The compressed air, as shown in the diagram, passes away from the top of the reservoir to an oil separator interposed between the compressor system and the auto- matic gear. The function of the separator is to extract the finer globules of oil, which it does by means of a jet through which the air impinges on a small plate. The oil separator is fitted with a drain cock at the bottom through which the oil collected may be periodically removed, while the compressed air is supplied to the gyroscopic apparatus and associated mechanisms from the outlet pipe at the top of the separator. Simple Qyroscopic Theory In order to convey a true understanding of the operation of the Automatic Pilot, it is necessary to describe very briefly the simple properties of a gyroscope, but for a more complete account of the principles underlying gyroscopic action, the reader must refer to any of the standard text- books on dynamics. Any rotating body has the property of tending to main- tain the direction of its axis fixed in space unless acted upon by some external force. The greater the angular momentum (or mass, radius, and angular velocity) of the spinning body, the greater is the tendency to maintain the fixed direction of its axis. The second property of a gyroscope which must be described is the apparently peculiar manner in which it responds to the application of an external force. In Fig. 4 a spinning gyroscope is shown mounted in gimbal rings
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