Bernoulli’s Principle in Flight

Bernoulli’s Equation for Incompressible Flow

Bernoulli’s equation is fundamental to understanding lift: Total Pressure = Static Pressure + Dynamic Pressure. In low subsonic, incompressible flow (below roughly 300 knots at sea level), as air accelerates over a surface, its static pressure drops while dynamic pressure rises, keeping total pressure constant.

Application to Wings and the Venturi Effect

A wing acts much like a venturi tube. As airflow encounters the curved upper surface, streamlines converge and velocity increases, causing static pressure to decrease above the wing. Below the wing, airflow is slower and static pressure is higher. This pressure difference generates lift. The same principle applies in a venturi: where the duct narrows, airflow speeds up and static pressure falls.

Dynamic Pressure, IAS, and Air Density

Dynamic pressure reflects the kinetic energy of moving air and is calculated as q = ½ρV², where ρ is air density and V is velocity. Indicated Airspeed (IAS) is directly related to dynamic pressure, so changes in airspeed or altitude (affecting density) change the lift produced for a given wing area and angle of attack.

Streamlines and Pressure Distribution

Converging streamlines indicate increasing velocity and decreasing static pressure; diverging streamlines mean the opposite. Around an aerofoil, the upper surface typically sees higher velocity and lower pressure, while the lower surface experiences lower velocity and higher pressure. The wing’s camber and angle of attack (alpha) shape this distribution, directly influencing lift.

Compressibility Effects

At higher speeds, air becomes compressible, which disrupts the pressure gradient and reduces the maximum coefficient of lift (CL) achievable. For most general aviation operations, Bernoulli’s Principle applies accurately, but pilots must be aware of compressibility effects at higher speeds.

Bernoulli and Lift: Operational Relevance

While Bernoulli’s Principle is a classical explanation for lift, Newton’s third law (action/reaction) also plays a role. Both are valid perspectives, but exam questions typically focus on the pressure-difference model described by Bernoulli.