Aerodynamics is the study of how air interacts with objects in motion, particularly aircraft. The principles of flight are governed by fundamental laws of physics that explain how forces are generated and how aircraft move through the atmosphere. Concepts such as motion, velocity, acceleration, relative wind, Newton’s Laws of Motion, and Bernoulli’s Principle provide the foundation for understanding lift, drag, thrust, and the overall behavior of an aircraft in flight.
The law of conservation of energy states that energy may neither be created nor destroyed, but only transformed from one form to another. This principle is fundamental to understanding how aircraft convert engine power into motion and aerodynamic forces.
Motion is the act or process of changing place or position. An object may be in motion with respect to one object and motionless with respect to another. For example, a person sitting quietly in an aircraft flying at 200 knots is at rest or motionless with respect to the aircraft; however, the person and the aircraft are in motion with respect to the air and to the earth.
Air exerts pressure whether it is moving or stationary. However, aerodynamic forces such as lift and drag become apparent when there is relative motion between the air and an object. A moving object in still air experiences aerodynamic forces as a result of its motion through the air. Likewise, the same aerodynamic effects occur if the air moves past a stationary object. Therefore, it makes no difference whether the object is moving through the air or the air is moving past the object; only the relative motion between the two is important. The airflow around an object resulting from the movement of either the air or the object, or both, is called the relative wind.
Velocity and Acceleration
The terms “speed” and “velocity” are often used interchangeably, but they do not have the same meaning. Speed is the rate of motion in relation to time, and velocity is the rate of motion in a particular direction in relation to time.
An aircraft starts from New York City and flies 10 hours at an average speed of 260 miles per hour (mph). At the end of this time, the aircraft may be over the Atlantic Ocean, Pacific Ocean, Gulf of Mexico, or, if its flight were in a circular path, it may even be back over New York City. If this same aircraft flew at a velocity of 260 mph in a southwestward direction, it would arrive in Los Angeles in about 10 hours. Only the rate of motion is indicated in the first example and denotes the speed of the aircraft. In the last example, the particular direction is included with the rate of motion, thus, denoting the velocity of the aircraft.
Acceleration is defined as the rate of change of velocity. An aircraft increasing in velocity is an example of positive acceleration, while another aircraft reducing its velocity is an example of negative acceleration, or deceleration.
Newton’s Laws of Motion
The fundamental laws governing motion and the forces acting on aircraft are known as Newton’s laws of motion.
Newton’s first law is normally referred to as the law of inertia. It simply means that a body at rest does not move unless force is applied to it. A body moving at a constant speed in a straight line will continue to do so unless acted upon by an external force. A force is required to change its speed or direction of motion.
Because air has mass, it behaves as a physical substance that is subject to Newton’s laws of motion. When an aircraft is on the ground with its engines off, inertia keeps the aircraft at rest. An aircraft is moved from its state of rest by the thrust force created by a propeller, or by the expanding exhaust, or both. When an aircraft is flying at uniform speed in a straight line, inertia tends to keep the aircraft moving. Some external force is required to change the aircraft from its path of flight.
Newton’s second Law states that the acceleration of a body is proportional to the net force acting upon it and inversely proportional to its mass. This relationship is expressed mathematically as:
Force = mass × acceleration (F = ma)
For example, increasing engine thrust causes an aircraft to accelerate, while increasing drag reduces acceleration.
Newton’s third law is the law of action and reaction. This law states that for every action (force) there is an equal and opposite reaction (force). In aviation, this principle is demonstrated by a propeller or jet engine accelerating air backward, producing an equal and opposite reaction force that propels the aircraft forward.
The three laws of motion that have been discussed apply to the theory of flight. In many cases, all three laws may be operating on an aircraft at the same time.
Bernoulli’s Principle and Subsonic Flow
Bernoulli’s principle states that when a fluid (air) flowing through a tube reaches a constriction, or narrowing, of the tube, the speed of the fluid flowing through that constriction is increased and its pressure is decreased. The cambered (curved) shape of an airfoil accelerates airflow over its upper surface, producing a decrease in pressure similar to the effect observed when a fluid passes through a constriction. [Figure]
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| Bernoulli’s Principle |
Diagram A of Figure illustrates the effect of air passing through a constriction in a tube. In Diagram B, air is flowing past a cambered surface, such as an airfoil, and the effect is similar to that of air passing through a restriction.
As the air flows over the upper surface of an airfoil, its velocity increases and its pressure decreases; an area of low pressure is formed. There is an area of greater pressure on the lower surface of the airfoil, and this greater pressure tends to move the wing upward. The pressure difference between the upper and lower surfaces of the wing creates a lifting force known as lift.