Lift

What Is Lift?

Lift is one of the four fundamental aerodynamic forces acting on an aircraft, working opposite to weight. It is generated when air flows over and under the wings, creating pressure differences due to changes in airflow speed and direction.

Direction and Components

Lift always acts perpendicular to the relative airflow (also called the flight path). The resultant aerodynamic force acting on a wing can be resolved into two components: lift (perpendicular to the airflow) and drag (parallel and opposite to the airflow).

How Lift Is Produced

• The wing shape (aerofoil) and angle of attack (alpha) cause air to move faster over the top surface and slower underneath.
• According to Bernoulli's principle, faster airflow above reduces pressure, while slower airflow below increases pressure, generating lift.
• Some explanations also refer to Newton's third law: the wing deflects air downwards, and the reaction force pushes the wing upwards.

The Lift Formula

The amount of lift (L) generated can be calculated using:

L = CL × ½ × ρ × V² × S

Where:

• CL = coefficient of lift (depends on wing shape and angle of attack)
• ρ (rho) = air density
• V = true airspeed
• S = wing area

Angle of Attack and the CL-Alpha Graph

• As angle of attack (alpha) increases, CL (and thus lift) increases up to a point (CLMAX).
• Beyond CLMAX, further increases in alpha cause a rapid drop in lift (stall).
• The CL-alpha graph shows:
  • Zero-lift point: where the curve crosses the horizontal axis (no lift produced).
  • Alpha = 0: where the curve crosses the vertical axis (angle of attack is zero, but some wings may still produce lift).
  • CLMAX: the peak of the curve, indicating maximum lift before stall.

Operational Relevance

Pilots control lift by adjusting airspeed and angle of attack. At lower speeds, a higher angle of attack or use of flaps is required to maintain lift. Understanding these relationships is crucial for safe takeoff, climb, cruise, and landing.