Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 31. Miscellaneous Instruments

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXXI -MISCELLANEOUS INSTRUMENTS

146. Aircraft clocks.-a. Timepieces with various features are required for use on aircraft for general flight and navigation purposes. All standard instrument panels are equipped with one 8-day clock. These clocks have, in addition to the usual chronometer mechanisms, a sweep second hand. They are designed to operate on shock-proof panels and are mounted in the same manner as all other aircraft instruments. Some types have "elapsed time," "time out." and "stop" features incorporated in their design as shown in figure 94.

b. The ordinary maintenance of clocks consists of winding, setting, checking for accuracy, and regulation.

(1) In winding, no greater force should be used than is actually required. The clock is fully wound when the winding stem offers a slight resistance to further winding and any attempt to wind beyond this point will result in damage to the delicate mechanism.

(2) In checking for accuracy, the second hand is checked against the second hand of another clock or watch of known accuracy for a period of not less than 1 hour. When the rate of gain or loss is excessive the clock is replaced.

(1) Type A-7                                                     (2) Type A-8
FIGURE 94.-Aircraft clocks.

(3) Regulation of the clock is a base function. If regulation in conformance with the tolerance specified for the particular clock cannot be accomplished, the clock is sent to the depot for repair.

c. Pocket and wrist time and stop watches are also required for use in working various navigation problems. An example of each is given in figure 95.

(1) Type A-7 time and stop watch         /2) Type A-8 stop watch
FIGURE 95.-Navigation watches.

147. Driftmeters.-a. The purpose of driftmeters is to determine and measure the amount of drift when flying cross wind courses. Some additional uses of this instrument are to

(1) Determine relative bearings.

(2) Measure ground speed by timing.

(3) Aid in compass compensation and swinging in the air.

(4) Aid in air speed indicator calibration.

b. Several standard types have been developed for use on the different types of tactical airplanes.

(1) The driftmeter (fig. 96) is of the optical periscopic type and is installed on the side wall of the aircraft with the objective end protruding through the fuselage. The instrument has a field of view of 25° with the ground image one-third natural size and completely erect; that is, right side up and in correct right and left relationship. Accurately in the plane of the image is the reticle, rotated by means of the control dial adjacent to the eyelens. Angle of rotation of the reticle is read on the control dial or on the edge of the reticle itself. The eye distance is 500 mm (approximately 20 inches). With the eye at this distance from the eyelens, the full field is visible but the position of the eye is not critical. Due to the large eye distance, the instrument may be installed on or below the instrument board and may therefore be used by the pilot.

FIGURE 96.-Type D-1A driftmeter.

(2) The driftmeter (fig. 97) is a telescopic type of instrument of variable length with a trail sighting feature, variable magnification, illuminated recticle, and bearing scale. Eyepieces giving unit power and three-power magnification are furnished, the unused eyepiece being carried in the socket under the drift scale. The meters are supplied assembled in the minimum length of 3 feet over all and in some instances with extension tubes 2-, 3-, and 5-foot lengths.

(3) The driftmeter (fig. 98) is of the straight tube periscopic type incorporating a gyro stabilized reticle, variable magnification, a trail sighting feature, and provision for use as a bearing plate for terrestrial objects. The length overall may be any value from 3 feet 9 inches to 8 feet without effect on the optical characteristics. The instrument is mounted vertically in the airplane with the eyepiece convenient to the navigator and the objective end projecting below the airplane sufficiently to clear the fuselage. With his eye to the eyepiece, the observer sees images of the ground directly below projected on the reference lines of the reticle, or by operating a handle, shifts the line of sight up to 85° from the vertical. The entire assembly is rotated on a bearing in the mounting flange for drift or relative bearing measurements. Pitch or roll of the airplane has only slight influence on drift measurements due to the stabilization of the reticle. The field of view is 25° ; the magnifications are unity or 3-power. The reticle is illuminated for night use.

FIGURE  97.-Type B-2 drift meter.

c. In the maintenance of driftmeters, the external surfaces of the objective and eyepiece lens are checked to insure that. these surfaces are free of dust, dirt, oil, and finger marks. If dirty, first blow the surfaces with a light pressure blower or with the mouth. A light film of moisture will usually be noticeable. While the lens is damp, any soft tissue paper free of gritty materials is wiped gently across the surface. In order to avoid scratching of the lens, a fresh piece of the paper must be used for each wiping motion. If the lens is extremely oily, it will be permissible to moisten the tissue paper with a little carbon tetrachloride followed immediately by wiping with clean tissue. Never remove the lens or attempt to clean the internal parts of the driftmeter, as damage to the reference wires is certain to result. In order to check rapidly the adjustment of the vernier and the reference wires of driftmeters, the verniers are set to their zero indices and a light is held directly over the observer's lens (eyepiece). By looking through the driftmeter in the reverse direction, the reference wires are seen as rather large lines which should be parallel to the longitudinal axis of the airplane. Adjustment is made by loosening the set screw and shifting the vernier.

FIGURE 98. Type B-3 driftmeter.

148. Accelerometers.-a. Indicating accelerometers are used by pilots to check the forces imposed on the aircraft structure while flying. The use of the accelerometer enables the pilot to become familiar with the acceleration characteristics of different aircraft so that permissible values as given on the operating V-g diagrams will not be exceeded.

b. The indicating accelerometer (fig. 99) consists of a weight carried on an arm that swings through a limited angle parallel to the plane of the dial. Its motion is restrained by two springs which serve as a means of calibration. The angular motion of the axis is transmitted to the indicating hand which carries with it the maximum and minimum reading hands. The maximum and minimum hands remain at the highest positive and negative points reached in acceleration until reset to +1 g by a slight rotation of the setting knob. These hands will be reset after each maneuver. The dial is marked to indicate acceleration in gravitational units of a climb, dive, or turn.

FIGURE 99.-Indicating accelerometer.