Bernoulli’s Principle is a fundamental concept in fluid dynamics, explaining how a fluid’s speed relates to its pressure. In aviation, understanding this principle is important for grasping how aircraft achieve lift and maintain flight.
Understanding the Principle
Bernoulli’s Principle describes an inverse relationship between a fluid’s speed and its internal pressure. As a fluid’s speed increases, its pressure decreases; conversely, slower fluid movement results in increased pressure. This principle applies to both liquids and gases, including the air that surrounds an aircraft.
For example, if you place your thumb partially over a garden hose nozzle, the water speeds up as it exits the narrower opening. This increased speed is accompanied by a drop in pressure within the flowing water. Similarly, a shower curtain may be pulled inward when the shower is running because the fast-moving water and air inside create a lower pressure zone, drawing the curtain towards it.
How It Generates Aircraft Lift
The shape of an aircraft wing, known as an airfoil, is designed to harness Bernoulli’s Principle to generate lift. An airfoil typically has a curved upper surface and a flatter lower surface. As air flows over the wing, it splits into two streams, one moving over the top and one along the bottom.
The curved upper surface forces the air traveling over it to cover a greater distance than the air flowing beneath the wing. To cover this longer distance in the same amount of time, the air above the wing must accelerate and move faster. This increase in air speed above the wing results in a drop in pressure in that region.
Simultaneously, the air flowing along the flatter underside of the wing travels a shorter distance and moves at a slower speed. This slower-moving air maintains a higher pressure beneath the wing. The resulting pressure difference, with lower pressure above and higher pressure below, creates an upward force that pushes the aircraft into the air. This pressure differential is a primary component of the lift force.
Beyond Airfoils: Other Aviation Applications
Bernoulli’s Principle extends beyond wing lift, applying to various other aircraft components and systems. Pitot tubes, for instance, measure an aircraft’s airspeed based on this principle. These tubes measure the difference between the total pressure at the front of the aircraft and the static pressure of the surrounding air.
The difference between these two pressures is known as dynamic pressure, and it is directly related to the aircraft’s speed through the air. This pressure differential is converted into an airspeed reading displayed in the cockpit. Carburetors in older aircraft also utilized Bernoulli’s Principle for mixing fuel and air. Air speeds up as it passes through a constricted section, called a venturi, causing a pressure drop that draws fuel into the airstream to create a combustible mixture.
Complementary Principles in Flight
While Bernoulli’s Principle explains a significant portion of lift generation, it is not the sole factor enabling flight. The complete picture of how an aircraft stays airborne involves the interplay of multiple aerodynamic principles. Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction, also plays a substantial role.
As the wing moves through the air, its design and angle of attack cause it to deflect a mass of air downwards. This downward deflection of air, known as downwash, constitutes an “action” force. In response, an equal and opposite “reaction” force pushes the wing upwards, contributing to the overall lift. Therefore, lift is generated both by the pressure differential described by Bernoulli’s Principle and by the downward redirection of air mass as explained by Newton’s Third Law. Both principles are essential for flight, working together to overcome gravity and keep aircraft aloft.