subject: Basics of Aircraft Flying [print this page] Basics of Aircraft Flying Basics of Aircraft Flying
Introduction
Man has always wanted to fly. Legends from the very earliest times bear witness to this wish. Perhaps the most famous of these legends is the Greek myth about a father and son who flew with wings made of wax and feathers. It was not, however, until the successful flight by the Wright bothers at Kitty Hawk, North Carolina, that the dream of flying became a reality. Since the flight at Kitty Hawk, aircraft designers have spent much time and effort in developing that first crude flying machine into the modern aircraft of today. To understand the principles of flight, you must first become familiar with the physical laws affecting aerodynamics.
PHYSICAL LAWS AFFECTING
AERODYNAMICS
LEARNING OBJECTIVE: Identify the physical laws of aerodynamics to include Newton's laws of motion and the Bernoulli principle. Aerodynamics is the study of the forces that let an aircraft fly. You should carefully study the principles covered here. Whether your job is to fly the aircraft and/or to maintain it, you should know why and how an aircraft flies. Knowing why and how lets you carry out your duties more effectively.
LAWS OF MOTION
Motion is the act or process of changing place or position. Simply put, motion is movement. An object may be in motion in relation to one object and motionless in relation to another. For example, a person sitting in an aircraft flying at 200 mph is at rest or motionless in relation to the aircraft. However, the person is in motion in relation to the air or the earth. Air has no force or power other than pressure when it's motionless. When air is moving, its force becomes apparent. A moving object in motionless air has a force exerted on it as a result of its own motion. It makes no difference in the effect whether an object is moving in relation to the air or the air is moving in relation to the object. The following information explains some basic laws of motion.
Newton's First Law of Motion
According to Newton's first law of motion (inertia), an object at rest will remain at rest, or an object in motion will continue in motion at the same speed and in the same direction, until an outside force acts on it. For an aircraft to taxi or fly, a force must be applied to it. It would remain at rest without an outside force. Once the aircraft is moving, another force must act on it to bring it to a stop. It would continue in motion without an outside force. This willingness of an object to remain at rest or to continue in motion is referred to as inertia.
Newton's Second Law of Motion
The second law of motion (force) states that if a object moving with uniform speed is acted upon by an external force, the change of motion (acceleration) will be directly proportional to the amount of force and inversely proportional to the mass of the object being moved. The motion will take place in the direction in which the force acts. Simply stated, this means that an object being pushed by 10 pounds of force will travel faster than it would if it were pushed by 5 pounds of force. A heavier object will accelerate more slowly than a lighter object when an equal force is applied.
Newton's Third Law of Motion
The third law of motion (action and reaction) states that for every action (force) there is an equal and opposite reaction (force). This law can be demonstrated with a balloon. If you inflate a balloon with air and release it without securing the neck, as the air is expelled the balloon moves in the opposite direction of the air rushing out of it.
BERNOULLI'S PRINCIPLE
Bernoulli's principle states that when a fluid flowing through a tube reaches a constriction or narrowing of the tube, the speed of the fluid passing through the constriction is increased and its pressure is decreased.
THE AIRFOIL
LEARNING OBJECTIVE: Recognize the terms used to describe the various parts of an airfoil section and the terms used in explaining the airflow lift generation. An airfoil is defined as that part of an aircraft that produces lift or any other desirable aerodynamic effect as it passes through the air. The wings and the propeller blades of a fixed-wing aircraft and the rotor blades of a helicopter are examples of airfoils.
AIRFOIL TERMINOLOGY
The shape of an airfoil and its relationship to the airstream are important. The following are common terms that you should understand before you learn about airfoils. Leading edge The front edge or surface of the airfoil. Trailing edge The rear edge or surface of the airfoil. Chord line An imaginary straight line from the leading edge to the trailing edge of an airfoil . Camber The curve or departure from a straight line (chord line) from the leading to the trailing edge of the Airfoil. Relative wind The direction of the airstream in relation to the airfoil. Angle of attack The angle between the chord line and the relative wind.
AIRFLOW AROUND AN AIRFOIL
The generation of lift by an airfoil depends on the airfoil's being able to create a special airflow in the airstream. This airflow develops the lifting pressure over the airfoil surface. As the relative wind strikes the leading edge of the airfoil, the flow of air is split. A portion of the relative wind is deflected upward and aft, and the rest is deflected downward and aft. Upper surface of the airfoil has camber to it, the flowover its surface is disrupted. This disruption causes awavelike effect to the airflow. The lower surface of theairfoil is relatively flat. The airflow across its surfaceisn't disrupted. Lift is accomplished by this differencein the airflow across the airfoil.This low-pressure area is caused by the air that is disrupted by the camber of the airfoil, and it is the key to lift. There is less pressure on the top surface of the airfoil than there is on the lower surface. The air pressure pushes upward on the lower surface. This difference in pressure causes the airfoil to rise. Now, you know that lift is developed by the difference between the air pressure on the upper and lower surfaces of the airfoil. As long as there is less pressure on the upper surface and more pressure on the lower surface of an airfoil, an aircraft has lift. Lift is one of the forces affecting flight.
FORCES AFFECTING FLIGHT
LEARNING OBJECTIVE: Recognize the four primary forces acting on an aircraft. An aircraft in flight is in the centre of a continuous battle of forces. The conflict of these forces is the key to all manoeuvres performed in the air. There is nothing mysterious about these forcesthey are definite and known. The direction in which each acts can be calculated. The aircraft is designed to take advantage of each force. These forces are lift, weight, thrust, and drag.
LIFT
Lift is the force that acts in an upward direction to support the aircraft in the air. It counteracts the effects of weight. Lift must be greater than or equal to weight if flight is to be sustained.
WEIGHT
Weight is the force of gravity acting downward on the aircraft and everything in the aircraft, such as crew, fuel, and cargo.
THRUST
Thrust is the force developed by the aircraft's engine. It acts in the forward direction. Thrust must be greater than or equal to the effects of drag for flight to begin or to be sustained.
DRAG
Drag is the force that tends to hold an aircraft back. Drag is caused by the disruption of the airflowabout the wings, fuselage (body), and all protruding objects on the aircraft. Drag resists motion as it acts parallel and in the opposite direction in relation to the relative wind. Up to this point, you have learned the physical laws of aerodynamics, airfoils, and the forces affecting flight. To fully understand flight, you must learn about the rotational axes of an aircraft.
ROTATIONAL AXES
LEARNING OBJECTIVE: Identify the three axes of rotation and the terms relative to the aircraft's rotation about these axes. Any vehicle, such as a ship, a car, or an aircraft, is capable of making three primary movements (roll, pitch, and yaw). The vehicle has three rotational axes that are perpendicular (90 degrees) to each other. These axes are referred to by their directionlongitudinal, lateral, and vertical. Perhaps the most descriptive reference is by what action takes place about a given axis or pivot pointroll, pitch, and yaw.
LONGITUDINAL AXIS
The longitudinal axis is the pivot point about which an aircraft rolls. The movement associated with roll is best described as the movement of the wing tips (one up and the other down). This axis runs fore and aft through the length (nose to tail) of the aircraft. This axis is parallel to the primary direction of the aircraft. The primary direction of a fixed-wing aircraft is always forward.
LATERAL AXIS
The lateral axis is the pivot point about which the aircraft pitches. Pitch can best be described as the up and down motion of the nose of the aircraft. The pitch axis runs from the left to the right of the aircraft (wing tip to wing tip). It is perpendicular to and intersects the roll axis.
VERTICAL AXIS
The vertical axis runs from the top to the bottom of an aircraft. It runs perpendicular to both the roll and pitch axes. The movement associated with this axis is yaw. Yaw is best described as the change in aircraft heading to the right or left of the primary direction of an aircraft. Assume you are walking from your work space to an aircraft located 100 feet away. You are trying to walk there in a straight line but are unable to do so because there is a strong wind blowing you off course to your right. This movement to the right is yaw.
FIXED-WING AND ROTARY-WING AIRCRAFT
LEARNING OBJECTIVE: Recognize thedifference in aerodynamic principles that applyto fixed- and rotary-wing aircraft.A fixed-wing aircraft depends on forward motionfor lift. A rotary-wing aircraft depends on rotatingairfoils for lift. The airfoil sections of a fixed-wingaircraft aren't symmetrical. The rotor blades of ahelicopter are symmetrical. These differences areimportant to you if you're to understand aerodynamicprinciples.
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.
