Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 23. Air Speed Indicators

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 XXIII - AIR SPEED INDICATORS

106. Purpose and use.-a. The air speed indicator shows the rate of speed that the airplane is traveling through the air. Except in still air at normal sea level atmospheric pressure, the indicated air speed is different from ground speed. However, the pilot may calculate ground speed from the indicated air speed if he knows the altitude at which he is flying and the direction and speed of the wind.

b. Some specific uses of the air speed indicator are to aid in

(1) Estimating the actual ground speed of the airplane. This is necessary in cross-country flying when the time required to reach a landing field must be determined in bomb sighting, and gunnery exercise.

(2) Determining the best throttle setting for the most efficient flying speed.

(3) Determining the best climbing and gliding angles.

(4) Determining whether the speed attained in a dive is within the limits of safety for the structure of the airplane.

(5) Indicating to the pilot when the airplane has attained flying speed in the take-off, and when the stalling speed is being approached in landing.

c. The reliability of an air speed indicator is dependent upon the pressures delivered by a pitot static air speed tube and the response of the mechanism of the air speed indicator to these pressures. Because of greater reliability, as compared to pitot venturi tubes, electrically heated pitot static air speed tubes have been made the standard for the Air Corps. The pitot static air speed tubes are practically free from errors resulting from slight deposits of dust, oil, or water and electrical heating will prevent ice formation on the tube under the most severe icing conditions.

(1) Front view.   (2) Side view.
(
3) Rear view.
FIGURE 60.- Low range type of air speed indicator.

107. Description.-a. The two types of air speed indicators in general use are shown in figures 60 and 61. The internal mechanism and operating principle of all air speed indicators are the same, the only difference being in minor construction features and in the range of the instrument. The low range type indicates air speeds of 40 to 300 mph and the high range type indicates air speeds of 50 to 500 mph.

b. The indicator (fig. 62), consists primarily of an airtight diaphragm assembly D and a mechanism for multiplying its deflection. The mechanism is composed of a rocking shaft assembly R with diaphragm lever A1, and a long lever A2, a sector assembly S with a sector lever A3, and a hand staff G, and pinion P. A hairspring C, secured to the hand staff and anchored to the mechanism body, removes back-lash from the mechanism and holds the diaphragm lever against the diaphragm bridge. The entire mechanism is housed in an air-tight case. The pitot tube which is connected to the interior of the diaphragm and the static tube which is connected to the case are treated in detail as separate instruments in section XXIV.

FIGURE 61 .-High range type of speed indicator

c. In external appearance, the different air speed indicators are similar, except for the appearance of the dials and small variations in the length of the case. The cover glasses are held in the case by means of a snap ring. The back of each instrument contains a centrally located pitot pressure connection and an offset static pressure connection Each of these connections is made up of an internal pipe thread insert in the case and a nipple union.

108. Operation.-a. The mechanism of the air speed indicator responds to small differential pressure changes. The phosphor-bronze or berylium-copper diaphragm capsule is sufficiently sensitive to move under very slight changes of pressure. Since the impact pressure from the pitot tube is transmitted to the interior of the diaphragm capsule and the static pressure from the static tube is transmitted to the interior of the otherwise airtight indicator case and the outside of the diaphragm capsule, it thus responds to the difference between the pitot and static pressures. As the speed of the airplane increases, the pitot pressure inside the diaphragm causes the latter to expand. The rocking shaft picks up the motion by means of its diaphragm lever and in turn transmits this motion through the long lever to the sector and finally to the handstaff pinion. The pointer fastened to the handstaff indicates air speed in miles per hour.

Figure 62.-Diagram showing air speed indicator mechanism and connection to pilot static tubes.

b. The fact that air speed indicators may be in error due to installation and instrument maladjustment must be realized, and these errors must be taken into account when extremely accurate use of the indicator is required. Therefore, to determine true indicated air speed the corrections given below are to be determined and applied.

(1) Since the pitot static tube is seldom installed in an air stream free from disturbances caused by the aircraft structure, the differential pressure developed by the tube differs from the theoretical value. The error in the indication of speed due to this difference in differential pressure is called the "installation error."

(2) Indicators may also be in error due to the imperfect calibration to the standard air speed pressure relation or to the effect of temperature changes in the instrument. For a change in instrument temperature from +45° C. (113° F.) to -35° C. (—31° F.), the errors should not exceed 3 1/2. miles per hour at any point on the scale. There are various types of Air Corps standard computers which may be used to facilitate the correction of air speed indications.

109. Installation.-a. The air speed indicator is grouped with the other flight instruments. It is mounted so that the zero on the dial is on the top side. Installation is made in accordance with various airplane and instrument board drawings. Mounting screws as required are furnished with each instrument.

b. The P (pitot) and S (static) connections on the back of the instrument case are connected to the corresponding connections on the air speed tube by means of airtight lines of seamless copper or aluminum tubing. When seamless copper tubing is used, the fittings will be either three-piece solderless or cone unions and when seamless aluminum alloy tubing is used, the fittings should be aluminum alloy 17ST, anodically treated, solderless fittings. The connecting lines are carried inside the fuselage and wing structure to prevent damage. The altimeter and rate of climb indicator are also connected into the static line. Each pitot and static pressure line should incorporate a flexible connection located behind the instrument board in such a manner as to complete the insulation of the instrument board from the vibrations of the airplane structure.

c. The lighting system of each of the air speed indicators is designed for a 3-volt source. The elect rical connector socket in the back of the case is connected to a 3-volt supply by means of a standard electrical connector plug in accordance with airplane installation drawings or specific Air Corps Technical Orders affecting the various types of airplanes.

d. In the calibration of air speed indicator installations on various types of airplanes. it has been found in some cases that the maximum permissible diving speeds exceed the scale range of the instrument. Whenever this condition exists, the air speed indicator is replaced with one having the necessary higher scale range.

110. Maintenance.-The general points on service inspection and maintenance of instruments given in section II are applicable to air speed indicators. The specific points of inspection and the procedure for dependent operating units are given in section XXIV.