combination of the two. Typically, a composite component  consists of many layers of fiber-impregnated resins, bonded  to form a smooth panel. Tubular and sheet metal substructures  are usually made of aluminum, though stainless steel or  titanium are sometimes used in areas subject to higher  stress or heat. Airframe design encompasses engineering,  aerodynamics, materials technology, and manufacturing  methods to achieve favorable balances of performance,  reliability, and cost. As with fixed-wing aircraft, helicopter fuselages and tail  booms are often truss-type or semimonocoque structures  of stress-skin design. Steel and aluminum tubing, formed  aluminum, and aluminum skin are commonly used. Modern  helicopter fuselage design includes an increasing utilization  of advanced composites as well. Firewalls and engine  decks are usually stainless steel. Helicopter fuselages vary  widely from those with a truss frame, two seats, no doors,  and a monocoque shell flight compartment to those with  fully enclosed airplane-style cabins as found on larger  twin-engine helicopters. The multidirectional nature of  helicopter flight makes wide-range visibility from the  cockpit essential. Large, formed polycarbonate, glass, or  plexiglass windscreens are common. As mentioned, a helicopter’s landing gear can be simply a  set of tubular metal skids. Many helicopters do have landing  gear with wheels, some retractable. The two most common types of engine used in helicopters are  the reciprocating engine and the turbine engine. Reciprocating  engines, also called piston engines, are generally used  in smaller helicopters. Most training helicopters use  reciprocating engines because they are relatively simple  and inexpensive to operate. Refer to the Pilot’s Handbook  of Aeronautical Knowledge for a detailed explanation and  illustrations of the piston engine. Turbine engines are more powerful and are used in a wide  variety of helicopters. They produce a tremendous amount  of power for their size but are generally more expensive  to operate. The turbine engine used in helicopters operates  differently than those used in airplane applications. In most  applications, the exhaust outlets simply release expended  gases and do not contribute to the forward motion of the  helicopter. Because the airflow is not a straight line pass  through as in jet engines and is not used for propulsion, the Turbine Engines Powerplant and Transmission Landing Gear or Skids Fuselage Figure 1-93.  The major components of a helicopter are the airframe, fuselage, landing gear, powerplant/transmission, main rotor system,  and antitorque system.  1-42 Main rotor hub assembly Main rotor blades Fuselage Landing gear or skid Transmission Powerplant Airframe Tail skid Tail rotor Tail boom Stabilizer Pylon