AIR LOG or AIR DIS TANCE RECORDER, an in strwnent which measures the dis tance flown by an aeroplane rela tive to the air. It consists of two units : a transmitter which is in stalled on a wing strut or in some other position on the aircraft where it receives undisturbed air, and an indicator which is mounted on the instrument board. The transmission may be either pneumatic or electric.
The transmitter comprises an impellor or wind-mill, which re volves as the aircraft moves through the air, the number of revolutions being proportional to the air distance travelled. The motion of the impellor is reduced by gearing to actuate a valve any unit of distance, such as geographic miles, nautical miles or kilometres. The instrument is rendered inoperative below a predetermined minimum speed, in the pneumatic type by relating the air pressure to the speed in such a way that operating pressure is not available below the minimum speed, and in the electric type by using a centrifugally controlled switch. This protection is necessary to prevent the operation of the air log by the wind, while the aircraft is on the ground. (C. H. C.) AIR PORT: see AERODROME.
This appliance was introduced by James Watt in 1765, when he invented the separate condenser for steam engines, in order to extract a mixture of air, water and vapour from the condenser, so providing the maximum practical vacuum. The air-pump may be built with the engine and its piston moved to and fro by a rocking lever, or sometimes by an exten sion of the engine piston-rod through the back head of the cylinder. A separate drive by engine or motor makes the air pump an independent unit. An arrangement of valves allows the inrush of air and vapour on the suction stroke, and expulsion through other valves on the return stroke.
A simplified pump has a conical-shaped piston or bucket (fig. r) which dashes at high speed down on to the water collected below which it drives upwards into the pump barrel above the bucket, which has just uncovered the ring of ports. The water, vapour and air shoot up the barrel and out through the top valve.
Modern pumps move the air independently of the water. The ejector type of air-pump is the most popular. A steam jet ex tracts the air and vapour from the condenser, which it discharges to the second jet, which finally ejects the air against atmospheric pressure. The ejector may incorporate a small condenser, an arrangement seen in the drawing (fig. 2), consisting of a ser4es or contact at a predetermined unit of distance, the resulting im pulse of air or electric current operating a ratchet in the indicator. The latter unit is simply a counter. By properly relating the pitch of the impellor blades and the gearing, the instrument will record of small tubes through which water from the condenser (termed the "condensate") is pumped. The primary steam jet sucks in air in the direction of the arrow at A, and discharges it along with the operating steam into the first condensing compartment. The secondary jet withdraws the aerated vapour from the first com partment and sends it with its operating steam into the second condensing compartment. The heat remaining in the steam after it has done its work is thus transferred to the condensate, so acting as a feed-water heater to the latter on its way to the boiler. The tire pump is a familiar example of simple reciprocating class, though strictly it should be termed an air-compressor. (See also VAcuum-Pump.)