Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 26. Rate of Climb 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 XXVI - RATE OF CLIMB INDICATORS

121. Purpose and use.-Rate of climb indicators, also called climb and dive indicators, are instruments designed to show gain or loss of altitude regardless of attitude of the aircraft. Specific uses for this instrument - are to

a. Show rate of ascent.

b. Show rate of descent.

c. Accomplish banked turns without gain or loss of altitude.

d. Establish constant and definite rates of descent when making an instrument landing.

122. Description.-a. Rate of climb indicators, which consist essentially of an indicator unit and a thermos chamber, are of several types. The older type (fig, 68) has the chamber as a separate unit and in the later type (fig, 69), it is built integral with the instrument case. These differences do not affect or change the principles involved or vary the method of operation,

b. Figure 70 shows the internal mechanism and parts of the indicator unit. The inside of the diaphragm A is subjected to the air pressure in the thermos chamber, connection being made by means of the diaphragm tube, case chamber connection B, and the con-

necting tube. The calibrated diffuser assembly V is also connected to the case chamber connection, The interior of the instrument case K is subjected to atmospheric pressure by means of the case static connection P. The deflection of the diaphragm is translated into pointer movement through a link, rocking shaft R, long lever arm L, sector assembly ,S, and pinion. to prevent injury to the mechanism from high pressure resulting from violent maneuvers, the diaphragm

movement in each direction is limited by a stop. A knob is provided for setting the pointer on zero. The pointer movement is accomplished by changing the position of the diaphragm which is mounted on the zero adjusting spring, This spring may be depressed or relieved by turning the knob.

c. The types having separate thermos chambers are housed in 2 3/4-inch instrument cases of molded bakelite construction provided with an airtight seal, while those in which the thermos chamber is built integral with the instrument case have a two-piece case. Both are provided with the standard :3-volt ring light, the receptacle being molded integral with the instrument case.

(1) Indicator

(2) Connecting tubing.                                    ( 3)Thermos chamber.
FIGURE: 68.- Rate of climb indicator with separate thermos chamber.

(1) Front view.                                                         (2) Side view.
FIGURE 69.-Rate of climb indicator with integral thermos chamber,

123. Operation.-a. The rate of climb indicator is designed to indicate a change in altitude of an airplane in flight. In level flight the pointer is horizontal; for any rate of climb a definite pointer indication in a clockwise direction occurs. In a similar way, for rates of descent, the pointer rotates downward or counterclockwise from the zero position to indicate the rate of descent. Each instrument has a sector stop to limit the travel of the pointer for ascent and descent to 1,900 feet per minute. Rates of ascent or descent in excess of 1,900 feet per minute will not impair the operation or accuracy of the instrument.

b. The rate of climb indicator is essentially a sensitive type of manometer. It operates from the differential between atmospheric pressure and the pressure of the thermos chamber which is vented to the atmosphere through a small calibrated leak. The restriction of the leak at the exit of the chamber causes a definite pressure differential to be established between the outside and the inside of the chamber when the atmospheric pressure changes, as in ascending and descending maneuvers. The measure of this rate of change of atmospheric pressure is indicated on the dial as a rate of change in altitude in feet per minute.

e. By use of a restriction to prevent oversensitiveness caused by "bumps," all rate of climb indicators have an inherent lag ranging from 6 to 9 seconds. Pointer oscillations which will be evident under turbulent atmospheric conditions should be interpreted as the response of a very sensitive instrument to actual conditions. These oscillations are inherent and do not interfere with the practical application of the information which the instrument furnishes. Further damping by use of restrictions would increase the lag in the instrument to an undesirable extent.

FIGURE 70.- Rate of climb indicator mechanism.

124. Installation.-a. The general points on installation of instruments given in section III are applicable to this instrument,

b. On instruments where the thermos tank and the indicator are separate units, the tank is mounted in a horizontal position at a distance not to exceed 3 feet from the indicator in a location where it will not be subjected to sudden or pronounced temperature changes.

There are two vents on the rear side of the instrument case, each provided with a threaded nipple. The vent marked "S" is connected with the static line and the vent marked "C" with the thermos chambe. If these connections are reversed, the indications on the instrument will also be reversed; that is, when ascending, the indicator will show descent and vice versa. The tubes used to connect the indicator and tank should not be altered in length, and if greater flexibility is desired, a standard flexible hose may be placed in the line provided a like length of the copper tubing is removed. When the copper tubing is used. it should have at least one coil with a radius of not less than 2 inches to absorb vibration and permit flexibility between the indicator and the tank. The tubing should he annealed after forming. These connections must be tight as a leak of even the slightest amount at this point will cause large scale errors in the instrument,

c. On late types of climb indicators where the thermos chamber is an integral part of the indicator, only one connection to the static line is necessary.

125. Maintenance.-a, The general points on maintenance of instruments given in section II are applicable to this instrument.

b. At periodic intervals, the static system and the indicator case must be checked for leaks. This is done by setting the pointer to zero with the adjuster, breaking the static connection as close to the indicator as possible, and attaching a 2- or 3-foot length of 1/4-inch rubber tubing. Apply a slow, continuous suction which will cause the pointer to indicate 500 feet per minute for 1 minute, then pinch the hose and watch the pointer; it should return to zero within the inherent lag period of the instrument. If it continues to rotate past zero in the counterclockwise direction. there is evidence of a leak in the instrument case and the instrument must be removed and replaced.