Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 24. Air Speed Tubes

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

111. Purpose and use.-Air speed tubes are used on aircraft to furnish accurate values of impact (pitot) and static pressures caused by the motion of the aircraft through the surrounding air. Specifically, they are used to supply

a. Impact pressure to the sensitive element of the air speed indicator.

b. Static pressure to the cases of air speed indicators, altimeters, and rate of climb indicators.

112. Description.-a. The air speed tubes, of which there are two general types, are located in such a position that in flight they will be in undisturbed air. One type of tube (fig. 63(1)) is designed for installation on a tapered streamline mast extending below and forward of the nose of the fuselage. Another type of tube (fig. 63(2)) is designed for installation on a boom extending forward of the leading edge of the wing. This boom is tapered in such a way that a minimum of vibration will result under flight conditions. Although the construction of the two types differs slightly, their component parts and principle of operation are the same.

b. The air speed tube (fig. 64) is a two-section tube made of heavy gage brass seamless tubing. It is located on the airplane so that its axis is parallel with the longitudinal axis or thrust line of the aircraft out of the slip stream and free from disturbances caused by the aircraft structure. The forward section is open to the front in order that it may be subjected to the full force of the impact pressure. This impact pressure section is provided with a baffle plate  to prevent blow-back of moisture and dirt into the tube and a small drain hole at the bottom to dispose of moisture and condensate. Another drain hole is located in the shark fin part of the impact pressure section. While the leaks caused by these drain openings are so small that their effect on actual measurement of the itupact pressures is negligible, they must be closed or sealed with masking tape when the periodic leak test is made on the pitot pressure system. The rear of the static section of the tube is provided with three small openings on both the top and bottom surfaces which, because of their design and location, give accurate measurement of air in static condition and also provide a means of draining this section.

(1) Mounted under nose of fuselage.

(2) Mounted forward of leading edge of wing.
FIGURE 63.-Streamlined type air speed tubes.

c. Risers are provided in the shark fin projection at the rear end of the tube to which are connected the tube extensions leading to the instruments. This type of construction reduces to a minimum the possibility of fouling the system with ice and dirt. The air speed tube is provided with two heating elements as shown in figure 64 to prevent icing when such condition is prevalent during flight. These elements are of excellent construction and will not burn out easily. Most of them will give trouble-free service for the life of the tube.

FIGURE 64. -Cross section diagram of an air speed tube

d. As described in section XXIII, the impact (pitot) pressure is transmitted by airtight tubing connections to the pitot pressure connection of the air speed indicator and the static pressure is transmitted by airtight tubing connections to the static pressure connections of the air speed indicator, altimeter and rate of climb indicator.

113. Operation. Ordinarily the air speed tube requires no attention from the pilot. Pitot and static pressures are transmitted to the altimeter, air speed indicator, and rate of climb indicator instantaneously and without manual control. However, under ice-forming conditions in flight. the pilot should turn "on" the switch labeled "AIR SPEED TUBE." When icing conditions are no longer encountered. the switch should be turned "off'." The electrical circuit of the heating element of the tube is connected through the ignition switch for all engines in such a manner that when the ignition switch is "off." the electrical circuit of the tube is open. and when the ignition switch is "on" the electrical circuit of the tube will be controlled by the "on-off" switch labeled "AIR SPEED TUBE." Precautions must be taken by placing the switch in the "off" position while on the ground or when heat is not required so that discharging the battery will be prevented and the life of the heating elements in the tubes will be lengthened.

114. Installation.-In order to permit removal and replacement of the air speed tube, a union is placed in each connecting line at the point of attachment of the mounting boom or mast to the wing or fuselage. The union is accessible through an inspection door. In removing a tube first disconnect these unions, then remove the mounting screws and lockwashers. The tube may then be pulled out of its mount a few inches and the pitot and static tubing and the "snap-on" electrical connector disconnected. To replace the tube, the above procedure is reversed. For detailed installation instructions consult Air Corps Technical Orders on mounting and wiring for individual types of airplanes.

115. Maintenance.-a. Particular care should be exercised in keeping the air speed tube clean at all times. When airplanes are not flying, the air speed tube should be protected from dust, dirt, oil, and other foreign particles by an appropriate cloth, canvas, or leather sack with a streamer. The sack should cover all openings in the tube.

b. The electrical circuit of each installation is tested to determine whether the heating element of the tube is connected satisfactorily. With the heating element of the tube properly connected and operating from the combined generator and battery source of direct current at 14.25 volts, the voltage as measured at the tube will not be less than 12.0 volts; with a battery source of 12 volts, the voltage as measured at the tube will not be less than 10.5 volts.

e. With the instruments properly connected to the static pressure line, the static pressure slotted openings or holes of the air speed tube are suitably connected to a source of suction. The altimeter pointers are set to indicate zero. A suction is slowly applied in sufficient amount to cause the altimeter pointers to indicate 1,000 feet of alt itude (approxmately 1.05 inch Hg. or 14.24 inches water, suction) at which point the source of suction is "pinched off." During a period of 1 minute. the altimeter pointers should not change their position by more than 150 feet: that is. they should not indicate less than 850 feet of altitude. The altimeter or instrument board is tapped during the 1-minute period to remove the friction effect from the indication of the pointers. Do not apply pressure to static lines. Regulate the slow application and removal of suction by means of the indication of the rate of climb indicator which should not exceed 2,000 fpm. Individual altimeters, rate of climb indicators, and air speed indicators should be tested for leaks in accordance with the Air Corps Technical Orders governing their use.

d. In testing the pilot lines, the pitot pressure chamber drain holes of the air speed tube are sealed. With the air speed indicator properly connected to the pilot pressure lines, the pilot pressure opening of the air speed tube is suitably connected to a source of pressure. A pressure is slowly applied in sufficient amount to cause the air speed indicator pointer to indicate 150 mph (approximately 0.82 inch Hg. or 11.18 inches water pressure) at which point the source of pressure is pinched off. During a period of 1 minute, the air speed pointer should not change its position by more than 10 mph (it should not indicate less than 140 mph). The air speed indicator or instrument board is tapped during the 1-minute period to remove the friction effect from the indication of the pointer. Do not apply suction to pitot lines.