TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 1 - Hydraulic principles

SECTION I: HYDRAULIC PRINCIPLES

1. General.-In a study of hydraulics as applied to aircraft hydraulic systems, two of the basic principles of liquids must be considered.

a. Liquids are practically incompressible. A pressure of 100 pounds per square inch applied to a body of water causes it to lose only about 0.0003 of its original volume. Liquids, such as the oils and other fluids used in hydraulic systems, vary slightly from this figure but for all practical purposes their compressibility is negligible.

b. Pressure applied anywhere to a body of confined or enclosed fluid is transmitted with undiminished force in every direction (Pascal's law). This pressure acts at right angles to every portion of the surface of the containers with equal force on equal areas.

2. Application of hydraulic principles.-a. Because of the incompressibility of liquids they can be used to transmit a force. A simple demonstration of this may be illustrated by placing a stopper in one end of an iron pipe and completely filling the pipe with water. A second stopper is placed in the open end and a force is exerted on this second stopper. This force is transmitted through the liquid and forces the first stopper out of the pipe.

b. In his experiments Pascal found that a force of 1 pound pushing against a small piston will equal a force of 10 pounds pushing against a large piston, the area of which was ten times that of the small piston. This is illustrated in figure 1.

FIGURE 1 .-Multiplication of force (hydraulically).

c. It follows from Pascal's law that a force slightly in excess of 1 pound pushing against the small piston will move the large piston against which 10 pounds is pushing. This constitutes a mechanical advantage comparable to that shown in figure 2, and may be used for many practical purposes, e. g., hydraulic jack, hydraulic press, etc. One important fact, however, must not be overlooked.

FIGURE 2.-Multiplication of force (mechanically).

The amount (volume) of fluid forced out of the small cylinder (fig. 1) is exactly equal to the amount of fluid forced into the large cylinder. A definite volume of fluid has been transferred from one cylinder to another, therefore, the small piston must travel a greater distance than the large one in the same length of time. In other words, what is gained in force is lost in speed.

d. The foregoing principles of hydraulics may be applied to advantage in operating certain mechanisms on an airplane. Since a confined fluid will transmit a force, light oil may be conducted in tubing from one point to another on an airplane between which two points it is desirable to transmit a force. The force applied to the liquid at one end of the tube will be transmitted to the other end regardless of the length of the tube or the number of bends. Furthermore, by expanding the bore of the tube on one end and inserting a piston into this expanded section, the force applied at the small end of the tube will be increased at the large end. The increase in force will be in direct proportion to the expansion of the tube. Oil forced into the tube at the small end at a certain rate will move the piston at the large end at a proportionately slower rate but with an increased force applied against it. The pressure in pounds per square inch will be the same at both ends of the tube. By means of a piston rod, the motion of the piston, and the force exerted on it, may be transmitted to the mechanism to be operated. This application of hydraulic principles is the keynote of the operation of hydraulic systems installed in airplanes. For the hydraulic units commonly employed in airplane hydraulic systems see section II.