HEIGHT INDICATOR FOR AEROPLANES

Evening Star, Issue 20311, 21 October 1929, Page 11

HEIGHT INDICATOR FOR AEROPLANES

telling the distance from the ground Red, green, and yellow lights have now been adapted to aviation, as was demonstrated at the Ail-American Ancraft Exposition, Detroit, in the radio echo altimeter developed by Dr It. i. W. Alexandcrson, consulting engineer of the General Electric Company, New York. , , . ~ ' Tlie lights, however, do not indicate “stop,” “go," and “caution.” In the Alexandcrson altimeter, winch was shown as a laboratory model and ■which has been successfully employed in actual flights, the lights give a visual warning of depth. When the S re ® n licrht flashes on the cockpit panel the pilot knows ho is 250 ft above ground; when the yellow shows, is lOUit above ground; and the red lights give a positive warning that trie ground is only 50ft below the plane. Nearly two years ago army flyers from Wright Field consulted with engineers and scientists of the General Electric Company, outlining to them some of the most serious problems of aviation. Among these problems they mentioned the importance of a method for measuring the absolute _ height above ground. In rain and in fog, as well as in night Hying, the pilot has no means of really knowing Ins distance above ground, and consequently the perils of operation under these conditions have been very great. THE THEORY. Dr Alexandcrson sot to work on the theory that the time interval required for a radio impulse to travel from a plane to the ground and back again to a receiving set on the plane anight be practically adapted as a reliable altitude indicator. The apparatus shown at Detroit, while still in an experimental form, is the result ot many months of patient investigation by Dr Alexanderson and his assistants. The visual recording instrument is small and may bo mounted on the cockpit panel in full view of the pilot. It consists of a meter on which ground distance up to 3,000 ft may be recorded. It is in the lower regions, however, that danger lies, and it is in the recording of levels down to 50ft that he has been most interested. Three coloured lights—green, yellow,' and red—indicate elevations of 250, 100, and 50ft respectively, and the flashing of any one of these lights gives the pilot immediate warning of his position with relation to the ground. Because the time interval between the outgoing and the reflected radio impulse is so short, radio waves travelling with the speed of light, an indirect method of making such ments was adopted by Dr Alexanclersou. In his experiments he used an oscillating receiver, one of the typo which sends out a wave which may be picked up on other receivers as a squealing note or beat. The echo or reflected signal was picked up on the same receiver which sent out the wave. Dr Alexandcrson discovered that every time the aeroplane changed altitude by half a wave length ; a whistling note went through a complete tone cycle, from low_ pitched to a high pitch and back again to a low pitch. By counting the cycles of the tone it was possible to measure the altitude, the measuring stick being one-half the wave length of the antenna oscillator. By means of the. meter, graduated from 3,000 to 2Qoft ; the pilot may read his altitude within those limits at any time. The echoes indicating height are periodic, becoming stronger as the plane approaches ground. The periodic character of tho echo and the chance that the pilot would not see tho instrument at tho instant an echo was recorded presented a problem which Dr Alexandcrson met by developing a memory' meter. In this instrument. the echo is recorded as altitude when it occurs and the - meter continues to hold that reading until a. stronger echo, indicating a lower altitude, occurs. In approaching tho earth the memory meter gives a continuous indication of altitude. If depth sounding is desired when climbing, in which process tho echo is becoming weaker, a push button may 7 bo used to eliminate the memory features of the meter, and each succeeding reading is indication of the next echo. Thus a depth sounding may be taken at any time during the cruise whether the plane is ascending or descending.