ROTARY-WING AIRCRAFT (HELICOPTERS)

FIXED-WING AIRCRAFT You have learned about the physical laws and forces that affect flight, the airfoil, and the rotational axes of an aircraft. Now, let's apply these principles to a fixed-wing aircraft in flight. First, motion must exist. Motion is provided by the thrust developed by the engine of the aircraft. This is accomplished by the force exerted by the exhaust gases of a jet aircraft or by the action of the propeller blades on a propeller-driven aircraft. The thrust overcomes the force of inertia and, as the fixed-wing aircraft accelerates, the air flows by the wings. The relative wind striking the leading edge of the wings is split and flows across the upper and lower surfaces. The camber of the upper surface acts as a constriction, which speeds up the airflow and reduces the pressure of the air. The lower surface, being relatively flat, doesn't affect the speed or pressure of the air. There is lower air pressure on the upper surface of the wing than on the lower surface. The fixed-wing aircraft is lifted into the air. Now that the aircraft is safely in the air, rotational axes come into play. If the nose of the aircraft is raised, the angle of attack changes. Changing the angle of attack causes the aircraft to pivot on its lateral or pitch axis. If you lower the right wing of the aircraft, the left wing rises. The aircraft moves about its longitudinal or roll axis. Assume that the aircraft is in a straight and level flight. There is a strong wind striking the aircraft's nose on the left side, pushing the nose to the right. This causes the tail of the aircraft to move to the left, and the aircraft is pivoting on its vertical or yaw axis. All of these forces are necessary for flight to begin or be sustained. ROTARY-WING AIRCRAFT (HELICOPTERS) The same basic aerodynamic principles you read about earlier in this chapter apply to rotary-wing aircraft. The main difference between fixed-wing and rotary-wing aircraft is the way lift is achieved. Lift The fixed-wing aircraft gets its lift from a fixed airfoil surface. The helicopter gets lift from rotating airfoils called rotor blades. The word helicopter comes from the Greek words meaning helical wing or rotating wing. A helicopter uses two or more engine-driven rotors from which it gets lift and propulsion. The helicopter's airfoils are the rotor blades. The airfoils of a helicopter are perfectly symmetrical. This means that the upper and lower surfaces are shaped the same. This fact is one of the major differences between the fixed-wing aircraft's airfoil and the helicopter's airfoil. A fixed-wing aircraft's airfoil has a greater camber on the upper surface than on the lower surface. The helicopter's airfoil camber is the same on both surfaces (fig. 3-9). The symmetrical airfoil is used on the helicopter because the center of pressure across its surface is fixed. On the fixed-wing airfoil, the center of pressure moves fore and aft, along the chordline, with changes in the angle of attack (fig. 3-9). If this type of airfoil were used on a rotary-wing aircraft, it would cause the rotor blades to jump around (dive and climb) uncontrollably. With the symmetrical airfoil, this undesirable effect is removed. The airfoil, when rotated, travels smoothly through the air. The main rotor of a helicopter consists of two or more rotor blades. Lift is accomplished by rotating the blades through the air at a high rate of speed. Lift may be changed by increasing the angle of attack or pitch of the rotor blades. When the rotor is turning and the blades are at zero angle (flat pitch), no lift is developed. This feature provides the pilot with complete control of the lift developed by the rotor blades. Directional Control A pilot controls the direction of flight of the helicopter by tilting the main rotor. If the rotor is tilted forward, the force developed by the rotor is directed downward and aft. Now, apply Newton's third law of motion (action and reaction). Lift will be developed in an upward and forward direction, and the helicopter will tend to rise and move forward. From this example, 3-6 Figure 3-9.—Center of pressure.