!    Aluminum trailing edge cap !    Aluminum or polyurethane and nickel abrasion leading edge strip Additionally, rotary rudder blades may have deicing provisions, such as electrothermal blankets that are bonded into the blade's leading edge. or a neoprene anti-icing guard embedded with electrical heating elements. Q4-17. What is the main advantage of rotary-wing aircraft over fixed-wing aircraft? Q4-18. What are the three types of landing gear used on helicopters? Q4-19. The directional control and antitorque action of the helicopter is provided by what group? AIRCRAFT HYDRAULIC SYSTEMS LEARNING OBJECTIVE: Identify the components of aircraft hydraulic systems and recognize their functions. The aircraft hydraulic systems found on most naval aircraft  perform  many  functions.  Some  systems operated by hydraulics are flight controls, landing gear, speed brakes, fixed-wing and rotary-wing folding mechanisms, auxiliary systems, and wheel brakes. Hydraulics has many advantages as a power source for operating these units on aircraft. !    Hydraulics combine the advantages of lightweight, ease of installation, simplification of inspection, and minimum maintenance requirements. !    Hydraulics operation is almost 100-percent efficient, with only a negligible loss due to fluid friction. However, there are some disadvantages to using hydraulics. !    The possibility of leakage, both internal and external, may cause the complete system to become inoperative. !    Contamination by foreign matter in the system can cause malfunction of any unit. Cleanliness in hydraulics cannot be overemphasized. COMPONENTS OF A BASIC SYSTEM Basically, any hydraulic system contains the following units: !    A reservoir to hold a supply of hydraulic fluid !    A pump to provide a flow of fluid !    Tubing to transmit the fluid !    A selector valve to direct the flow of fluid !    An actuating unit to convert the fluid pressure into useful work A simple system using these essential units is shown in figure 4-21. You can trace the flow of fluid from the reservoir through the pump to the selector valve. In figure 4-21, the flow of fluid created by the pump flows through the valve to the right end of the actuating cylinder. Fluid pressure forces the piston to the left. At the same time, the fluid that is on the left of the piston is forced out. It goes up through the selector valve and back to the reservoir through the return line. When the selector valve is moved to the position indicated by the dotted lines, the fluid from the pump flows to the left side of the actuating cylinder. Movement of the piston can be stopped at any time simply by moving the selector valve to neutral. When the selector valve is in this position, all four ports are closed, and pressure is trapped in both working lines. 4-19 ANF0421 PRESSURE LINE SELECTOR VALVE IN "DOWN" POSITION RETURN LINE SELECTOR VALVE IN "UP" POSITION WORKING LINES HAND PUMP ACTUATING UNIT RESERVOIR Figure 4-21.—Basic hydraulic system, hand pump operated.