Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 6. Manifold Pressure Gages

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 VI - MANIFOLD PRESSURE GAGES

21. Purpose and use.-a. The purpose of this gage is to show manifold pressures under various engine operating conditions when the engine is equipped with a supercharger.

b. Specific uses of the manifold gage are to

(1) Prevent oversupercharging when operating engines at low altitudes.

(2) Indicate loss of engine power when flying at high altitudes,

(3) Indicate safe power output of engines,

(4) Serve as a guide when adjusting automatic controls for external type superchargers.

(5) Eliminate use of throttle stops.

22. Description.-a. The manifold pressure gage (fig. 9) consists of an airtight bakelite case containing an evacuated pressure sensitive capsule or aneroid, and a multiplying mechanism for amplifying the movement of the pressure sensitive element and transferring it to the pointer. The manifold pressure to be measured is admitted to the case through a connection in the back of the case. The corrugated faces of the pressure capsule are therefore acted upon by two pressures; one of very low value which is constant acting on the inside of the capsule, and the other a variable pressure acting on the outside of the capsule. The deflection of the capsule walls is therefore proportional to the difference in the two pressures and the instrument is thus completely self-compensated for changes in altitude or barometric pressure. The pressure inlet is fitted with a restrictor which effectively excludes foreign matter and also serves to dampen out rapid fluctuations of pressure.

FIGURE 9.-Manifold pressure gage.

b. There are two types of manifold pressure gages in service. One indicates absolute pressures within the range of 10 to 50 inches Hg. The range of the other is from 25 to 65 inches Hg. The dials of both are graduated in increments of 1 inch Hg. and numerals are placed at each 5-inch interval on the scale. The pointer, numerals, and main graduations are painted with a luminous material and under average conditions of night flying will give sufficient luminosity to be legible. The gages are provided with the standard 3-volt light with the receptacle molded integral with the instrument case or with reflected fluorescent light.

23. Operation.-a. The manifold pressure gage is designed to measure absolute pressure and its operation is continuous. When the engine is inoperative the gage reading will depend upon the local barometric pressure, and under these conditions functions the same as an ordinary barometer.

b. Figure 10 is an inside view in simple form of the manifold pressure gage. When the pressure transmitted through the tube connection on the rear of the case is raised or lowered, the aneroid, which is evacuated, contracts or expands. This movement is transmitted to the pointer H through the link L, rocking shaft R, sector S, pinion P, and staff G. The force of the hairspring C, which is supported on disk D, serves to keep all parts tight against one another.

FIGURE 10.-Manifold pressure gage mechanism.

c. Manifold pressure gage indications are governed by engine speeds and the type of supercharger employed. One type of supercharger, the internal or geared type, is located in the induction system between the carburetor and the cylinder intake ports, and the manifold pressure gage indicates the pressure in the manifold between the supercharger outlet and the cylinder intake ports. When the external or exhaust driven type is used, the carburetor is located between the supercharger and the cylinder intake ports, and the manifold pressure gage indicates the pressure in the manifold between the supercharger and the carburetor. In engines equipped with fuel injectors, the manifold pressure gage indicates the manifold pressure immediately before the cylinder intake ports.

d. Although the manifold pressure gage is very sensitive, it is ruggedly built and will withstand the vibrations to which it is normally subjected under service conditions. However, it should not be submitted to rough handling.

e. When operating any of the modern aircraft engines, it is necessary to make a careful study of the handbooks and operations instructions for each engine to determine the maximum and desired manifold pressure readings for take-off, climb, dive, and cruising.

24. Installation.-a. The instrument is attached to the instrument panel in the space provided as shown on instrument panel drawing in such a position that the pointer is vertical when registering 30 inches Hg. When making the connection to the instrument, the threads on the nipple are cleaned and a drop of oil added, care being taken to remove any solder deposits on the union seat. The connecting union is tightened until it seals properly to give a tight joint without subjecting the instrument to excessive stress.

b. Whenever the instrument is installed on a vibrationproof instrument board, a length of flexible metal tubing approximately 10 inches long is connected between the instrument and the copper tubing extending to the engine manifold point of connection. A drain cock is located in the line, as close to the instrument connection as possible, for clean-out purposes. The control handle for opening and closing this valve is located so that it can be operated conveniently from the pilot's seat.

c. In addition to the above, the general installation instructions given in section III should he consulted.

25. Maintenance.-a. When the engine is inoperative, the manifold pressure gage reading should correspond to the local barometric pressure. Since the airplane altimeter is a very accurate barometric pressure measuring instrument and its accuracy is very dependable, it may be used as a standard of comparison for checking the manifold pressure gage.

b. A manifold pressure gage may be checked with the airplane altimeter by first setting the pointers or hands of the altimeter to zero and then applying vibration by tapping the panel lightly four or five times to remove any friction effects that may exist in the mechanism of either instrument. With the pointers of the altimeter set in this manner, the barometric scale on the altimeter will show the local pressure in inches of mercury. The reading of the manifold pressure gage should agree with this pressure within 0.4 inch Hg.

c. After the engine is started, the drain cock in the manifold pressure gage line should be opened for about 30 seconds while the engine is idling. This will clear the line and gage of any condensate that may have collected there. When the drain cock is closed and the engine is idling, the pointer should move to the left since the absolute pressure in the manifold will be low, that is, 10 to 15 inches Hg. As the throttle is advanced and the engine rpm increases. the pointer on the gage should move to the right or in a clockwise direction.

d. The pointer should always have a rather slow and steady movement and be free of any rapid oscillations regardless of how quickly the engine speed is accelerated or decelerated. Any variation in operation other than this is an indication of either of two defects : leak in the line or case, or improper damping adjustment.

e. The instrument may be checked and tested for case leaks and proper setting of damping adjustment by first disconnecting the line to the gage at the engine end and then applying pressure until the gage indicates 50 inches Hg. If the line is then closed, a leak will be indicated by a return of the pointer to atmospheric pressure. The proper adjustment for the restrictor is checked by suddenly releasing the pressure when the gage indicates 50 inches Hg. The indicator should reach 32 inches Hg. in not less than 1 second or more than 2 seconds.

f. The general maintenance operations for this instrument are covered in instructions given in section II.