Radio broadcast .. (1922-30)

"With the Night Mail" installed on a Curtiss R4L mail plane powered with a Liberty motor; so that the findings made (which had to be extensively altered to be satisfactorily applied to twin-engine planes), are applicable to the present postal planes which are practically all DL^B's, having single Liberty motors. This type was adapted first by the Post Office itself from the well-known DH4's — "coffin-boxes," they were called, because the gasolene tank was lodged between the seats. The principles of the " Robinson method," explained below, were utilized in the Post-Office-Navy experiments. Two definite problems were found: one, eliminating ignition disturbances of a severe nature, the other, providing a circuit reliable and simple enough to be used by a pilot unskilled in radio. The ignition disturbances were caused by the low and high-tension sides of the ignition circuit, which causes produced two separate and characteristic sounds. At first an attempt was made to shield the entire ignition circuit by means of metallic sheathing, but difficulty was experienced in securing adequate and frequent grounds. Next, an attempt was made to get results by sheathing only the direction-finding circuit, but here again, when the apparatus and batteries were sheathed with metal, although not of course the loops themselves, the weight was excessive and there was little improvement in the diminution of the disturbances. Finally a successful attempt was made by placing the direction-finding loops at the most distant point from the motor that the plane afforded. The Robinson method called for fixed "A" and " B " coils wound at right angles to each other, with the planes vertical. Both coils were constructed with rubber-covered aircraft wire and, at variance with accepted policy, were bunched together, since this materially aided in shortening the installation time and increased the wavelength of a given number of turns. The "A" plane of the coil, parallel to the longitudinal axis, was wound between the extreme entering and trailing edge struts. All loops were wound on a form placed on the side of a building, taped, then transferred to their permanent positions where they were again taped into place on the struts and wing surfaces with airplane linen, which was then "doped." The two leads from each coil were carried in a taping on the trailing edge of each lower wing to the fuselage. The " B" coil, at right angles to the "A" coil, was wound between the second and third rear struts THE AIR MAIL MAN Carries emergency rations in tin cans and bottles away from the fuselage and on the opposite side from the "A" coil. The second plane problem, that of providing a circuit reliable and simple enough for a pilot untrained in radio, was solved by mounting the instruments, including "B" batteries, sixstage amplifier, variable condenser and one double-pole double-throw switch, in one unit, on detachable brackets carrying elastic cord supports. These brackets were located beneath the pilot's seat along with the filament storage battery, which was also mounted on a bracket. With the plane equipment, including an improved helmet with close-fitting receivers, rubber ear cushions and an outer hood over all working satisfactorily, the problem of developing field localizers was than continued, with the cooperation of the radio section of the Bureau of Standards. The Army and Navy were much interested in all this, for they had used radio direction finders for planes during the war, but neither had developed satisfactory radio field localizers. Since a plane approaching a field while receiving direction-finding signals from a station on the field can come safely within half a mile, the problem was to obtain a means of supplying a localized signal, intelli