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Aviation History
1923
1923 - 0255.PDF
MAY 10, 1923 AN OPTICAL ALTITUDE INDICATOR FOR NIGHT LANDING _* By JOHN A. C. WARNER, Bureau of Standards THAT practical commercial aviation has come to stay must be admitted by even the most sceptical. The rapid advance which the past few years have witnessed is unmistakably the forerunner of greater activity in this comparatively new field of communication and transportation. It is at once evident, however, that the greatest benefit cannot be derived from the use of aircraft as commercial carriers unless their operation can be extended over the full 24-hr, day ; for their inactivity during the hours of darkness robs them to a great extent of their advantage over the systems of ground transportation. . • For this reason the problem of flying at night and under other conditions of low visibility is now demanding the attention of many aeronautical experts. We must equip ouTjairways and aircraft with suitable means for surmounting which provides a characteristic image on the landing surface ; as shown in Figs. 2 and 3, a rectangular bar is projected by B and two blunt arrowheads by C. The ground image from A is the altitude figure representing the particular altitude for which the projector is set. In determining the altitude of the aircraft the pilot simply turns the wheel attached to the pinion mating with rack F until the light beam from A intersects that from C at the landing surface. As A rotates, a toothed metal disc, G, attached to A, and extending through the walls of the pro- jector tube into the light-beam, is caused to rotate in a definite manner by virtue of the action between the teeth of G and those of the fixed rack with which they engage. The rotating disc acts as a rotatable object screen, for it is pierced by openings in the form of the altitude numerals corresponding THE JENKINS NIGHT ALTITUDE INDICATOR : Fig. 1, the complete instrument. the obstacles offered by these adverse conditions. This, of course, involves the installation of markers and beacons to clearly define the routes and fields, and also the equipping of aircraft with suitable instruments for navigation and landing. One of the most ingenious of the devices intended for use in night landing, especially emergency landing, is a very simple optical instrument known as the Jenkins night altitude indicator. Referring to Fig. 1, we note three projectors : A, B and C, each of which is equipped with an incandescent lamp properly mounted at the upper extremity to project a beam of light downward through the tube to the ground or water upon which a landing is to be made. Two of these projectors, B and C, are attached rigidly and parallel to each other to the side of the aircraft, while the third, A, is made rotatable (upon rails D and E) through a certain angle in a plane parallel to the fore- and-aft axis of the ship. Motion of A is brought about by the aviator, who manipulates a hand-wheel operating through a pinion mating with rack F. Projectors B and C are each equipped with an object screen • National Advisory Committee for Aeronautics (U.S.A.). TechnicalNoteKo. 123. , to the particular setting of the projector. Thus it is that the image seen upon the landing surface between the arrow- heads projected by C is that of the altitude numerals cut in G, through which the light passes. The altitude may also be observed on the transparent scale H, for an opening in the case containing the illuminating element of A allows a beam of light to fall upon the scale graduation which corresponds to the particular setting of the projector at which the ground images are seen to meet. The intersecting beams (from A and C) form two sides of a triangle whose altitude, determined with the instrument, is also that of the aircraft above the landing surface. Inasmuch as 50 ft. is the lowest direct indication of altitude for the instrument described, the illuminated bar image projected by the fixed source B is employed in estimating alti- tudes of less than 50 ft.' It will be seen that as the aircraft approaches the ground with all three projectors stationary, the numeral " 50 " will move from the arrowheads toward the bar image. The prevailing altitude is then estimated by observing the position of the altitude image with respect to the bar and arrowheads. For example, an altitude of 25 ft. would be indicated when 255
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