Fundamental Aerodynamic Principles

2.1 Universal Law of Conservation of Energy and Mass

• This fundamental law states that energy and mass can neither be created nor destroyed; they
• can only be changed from one form to another. This principle underpins all aerodynamic
• theory.

2.2 The Principle of Continuity

• This principle describes how air behaves as it flows through a tube of varying cross-sectional
• area, like the flow around an aerofoil.
•  The Continuity Equation: The air mass flow (mass per unit time) is constant at all points
• along the tube. The equation is:
• Area (A) × Velocity (V) × Density (ρ) = Constant
•  Incompressible Flow: At low subsonic speeds (below approximately Mach 0.4 or
• knots), changes in air density are insignificant and can be ignored. In this case, the equation
• simplifies to:
• Area (A) × Velocity (V) = Constant
•  Core Concept: From this, we can see that a reduction in the cross-sectional area of the tube
• results in an increase in the velocity of the air flowing through it. This is key to
• understanding how an aerofoil generates lift.

2.3 Bernoulli's Theorem

• Bernoulli's theorem relates the pressure and velocity of a moving fluid.
•  The Principle: In the steady flow of an ideal fluid (one that is incompressible and has no
• viscosity), the sum of its static pressure and dynamic pressure remains constant.
•  The Equation:
• Static Pressure + Dynamic Pressure = Constant (Total Pressure)
•  Key Relationship: Since Total Pressure must remain constant, Bernoulli's theorem proves
• that an increase in velocity will cause a decrease in static pressure, and vice versa. This
• is the single most important principle in the generation of aerodynamic lift. Total pressure is
• also referred to as
• Pitot Pressure.