Lift is the aerodynamic force that allows aircraft to overcome gravity and fly. Understanding how Newton’s laws of motion and Bernoulli’s principle contribute to lift generation is essential in the study of aerodynamics.
For an aircraft heavier than air to fly, it must generate enough lift to overcome the force of gravity. One of those obstacles, discussed previously, is the resistance to movement called drag. The most challenging obstacle to overcome in aviation, however, is the force of gravity.
A wing moving through air generates the force called lift, also discussed in the Lift and Basic Aerodynamics section. When wing lift exceeds the force of gravity and acts opposite to it, an aircraft is able to fly. The generation of lift is based on principles described by Newton’s laws of motion and Bernoulli’s principle.
Newton’s Basic Laws of Motion
The explanation of lift has evolved over the past few centuries through the application of basic physical laws. These laws help explain the forces involved in lift generation, but they do not fully explain all aerodynamic characteristics of lift. In fact, many symmetrical airfoils are also capable of producing lift.
The fundamental physical laws governing the forces acting upon an aircraft in flight were adopted from theories developed before powered human flight became possible. The use of these physical laws grew out of the Scientific Revolution, which began in Europe in the 1600s.
Driven by the belief that the universe operated according to predictable laws that could be understood by humans, many philosophers, mathematicians, natural scientists, and inventors spent their lives unlocking the secrets of the universe. One of the most well-known was Sir Isaac Newton, who not only formulated the law of universal gravitation, but also described the three basic laws of motion.
Newton’s First Law: “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.”
This means an object remains at rest or in motion unless acted upon by an external force. An aircraft at rest on the ramp remains at rest unless a force strong enough to overcome its inertia is applied. Once it is moving, its inertia keeps it moving, while affected by other aerodynamic forces. These forces may add to its motion, slow it down, or change its direction.
Newton’s Second Law: “Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration.”
When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force. This law explains how force changes the speed or direction of motion. It covers both changes in direction and speed, including starting up from rest (positive acceleration) and coming to a stop (negative acceleration or deceleration).
Newton’s Third Law: “For every action, there is an equal and opposite reaction.”
In an airplane, the propeller accelerates air backward; consequently, the air pushes the propeller (and thus the airplane) in the opposite direction—forward. In a jet airplane, the engine pushes a blast of hot gases backward; the force of equal and opposite reaction propels the airplane forward.
Bernoulli’s Principle of Differential Pressure
A half-century after Newton formulated his laws, Daniel Bernoulli, a Swiss mathematician, explained how the pressure of a moving fluid (liquid or gas) varies with its speed of motion. Bernoulli’s principle states that as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases. This principle helps explain pressure changes over an airfoil.
A practical application of Bernoulli’s Principle is the venturi tube. The venturi tube has an air inlet that narrows to a throat (constricted point) and an outlet section that increases in diameter toward the rear. The diameter of the outlet is the same as that of the inlet.
The mass of air entering the tube must equal the mass exiting the tube. At the constriction, the speed must increase to allow the same amount of air to pass in the same amount of time as in all other parts of the tube. When the air speeds up, the pressure also decreases. Past the constriction, the airflow slows and the pressure increases. [Figure 1]
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| Air pressure decreases in a venturi tube |
As a wing moves through the air, it changes airflow around the airfoil. Air pressure above the wing decreases while pressure below the wing remains higher, creating lift. At the same time, the wing deflects airflow downward, producing an equal and opposite upward reaction force in accordance with Newton’s Third Law.
Although Newton, Bernoulli, and hundreds of other early scientists who studied the laws of physics did not have the sophisticated laboratories available today, their work provided the foundation for modern aerodynamic theory.
How does a venturi tube demonstrate Bernoulli's Principle?
Why is Newton’s Third Law important to pilots?
Can a symmetrical wing produce lift?
Which law is the "true" explanation for lift?
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