HOW
AIRCRAFT FLY
An airplane flies because air moving over and under wings travels at different speeds, producing a difference in air pressure, low pressure above the wing and high pressure below it. The low pressure exerts a pulling force, and the high pressure a pushing force. These forces, usually called "lift", depends on the shape, area, and tilt of the wing, and on the speed of the aircraft. The greater distance over which the air must travel above the curved upper surface forces that air to move faster to keep pace with the air moving along the flat lower surface. According to Bernoulli's principle, it is this difference in air speed (velocity) that produces the difference in air pressure.
The aircraft speed has a great influence on lift. The faster the air moves over and under the surfaces of an airplane, the greater the pressure differential and, as a result, the greater the lift. As an airplane flies on a level course, the lift contributed by the wing and other parts of the structure counterbalance the weight of the plane. Within certain limits, if the angle of attack (the angle the plane is flying at) is increased while the speed remains constant, the plane will rise. If the angle of attack is decreased, that is, the wing is tilted downward, the plane will lose lift and start to descend. An airplane will also climb from level flight if its speed is increased, and it will dive if its speed is decreased. Lift varies directly with speed. In preparing to land, the pilot must ease the plane down and at the same time reduce its speed as much as possible. To compensate for the considerable loss of lift resulting from the decrease in speed, the pilot provides extra lift by altering the wing area, effective curvature, and angle of attack. This is done through the use of flaps, (large wing extensions located at each trailing edge). Most flaps are normally tucked into the wing during straight and level flight. If extra lift is wanted, the pilot extends the flaps out and down
The attitude (angle of attack) of an airplane (its orientation relative to the horizon and to the direction of motion) is determined by three control devices, each of which provides for movement about a different axis. The three devices include the movable sections of the tail, which are the elevators and rudders; and the movable sections of the trailing (aft) edge of the wing, known as ailerons. The control surfaces are operated from the cockpit by means of a control stick or wheel column and rudder pedals. Stick control is used in smaller, lighter airplanes, and the wheel, with its greater leverage, is generally used in larger airplanes.
Elevators provide for pitching movement around the lateral axis. A backward pull on the control stick or wheel column raises the elevators, thereby depressing the tail and lifting the nose of the plane for a climb. Forward movement of the stick or column produces the opposite effect, making the plane dive.
Ailerons, usually placed far out on the wing, control rolling movement around the longitudinal axis. Leftward movement of the stick or the wheel raises the left aileron and lowers the right, thereby banking the plane to the left. The reverse tilt occurs when the stick or wheel is moved to the right.
Rudders provide for turning movement around the vertical axis, changing the course of the plane to the left or the right. When the right rudder pedal is pressed, the rudder turns the plane to the right around the vertical axis. Pressing the left pedal produces a left turn.
HOW
HELICOPTERS FLY
A helicopter derives its lift not from fixed wings like those of conventional airplanes, but from a power-driven rotor (or rotors), revolving on a vertical axis above the fuselage. Helicopters can rise or descend vertically, hover, and move forward, backward, or sideways.
The rotor of a helicopter usually has two, three, or four blades. The rotor is driven by an engine, usually in the fuselage, through gears, which reduce the speed of rotation to less than the speed of the engine. An important feature of helicopter design is the development of devices to counteract the torque, or the reaction force developed when the rotation of the rotor in one direction tends to turn the fuselage in the opposite direction. The most common form of antitorque device is a small vertical propeller, similar to an airplane propeller, mounted at the tail of the helicopter in such a position as to push the tail to one side.
When a helicopter is rising vertically from the ground or descending vertically, the lift on all the rotor blades is the same, because they are all moving through the air at the same speed. When the craft is moving forward (or in any horizontal direction), however, the lift on some blades is greater than that on others. During each cycle the speed of the blades through the air varies, depending on whether the direction of the rotation is the same as or opposite to that of the motion of the helicopter. The airspeed of each blade is equal to its speed of rotation plus the forward speed of the helicopter during one half of the cycle and minus the forward speed during the other half. Hence, if the blades were fixed in a horizontal position, the amount of lift provided by each blade would vary during the cycle because lift increases with airspeed, and the helicopter would tilt to one side. To avoid this type of instability, most single-rotor helicopters have flapping blades. The blades are hinged close to the hub in such a manner that each blade rises when moving at greater airspeed to reduce the lift and drops when moving at lower airspeed to increase the lift. Thus, the effect of the varying airspeed as a result of blade rotation is nullified.
A helicopter can be flown in any direction, forward, backward, or sidewise, by tilting the rotor in the desired direction. Tilting the rotor changes its lift from purely vertical to a combination of vertical and horizontal. To turn a helicopter, the rotor is first tilted in the direction of the turn, and then the thrust of the tail propeller is altered to turn the fuselage in the desired direction. Ascent and descent in helicopters are controlled by increasing or decreasing the pitch of the rotor blades. In the event of a power failure, the rotor of a helicopter is disengaged and will autorotate like the rotor of an autogiro, maintaining enough lift to permit the craft to descend to a safe landing.