Question

Explain the difference between lift and drag.

Hint:

Lift is the useful force that keeps the aircraft up in the air (perpendicular to flight). Drag is the inevitable resistance force that tries to pull the aircraft back (opposite to flight).

Answer 1

Step 1: Introduction to Aerodynamic Forces:
Every aircraft in flight is acted upon by four primary forces: Lift, Weight, Thrust, and Drag. Lift and Drag are the two primary aerodynamic forces generated by the physical interaction of the aircraft's wings and body with the surrounding air.

Step 2: Defining and Analyzing Lift ($L$):

• Direction: Lift acts perpendicular to the relative airflow (direction of flight), pointing upwards.
• Origin: It is generated primarily by the wing's aerofoil shape. According to Bernoulli's Principle and Newton's Third Law, air moves faster over the curved upper surface of the wing, creating a low-pressure zone on top and a high-pressure zone underneath, which pushes the wing upward.
• Mathematical Equation: $$L = \frac{1}{2} \rho v^2 S C_L$$ where $\rho$ is air density, $v$ is velocity, $S$ is wing area, and $C_L$ is the coefficient of lift.
• Action: Lift overcomes the downward force of Weight ($W$) to keep the aircraft flying.

Step 3: Defining and Analyzing Drag ($D$):

• Direction: Drag acts parallel and opposite to the relative airflow, resisting forward motion.
• Origin: It is generated by friction between the air and the aircraft skin, pressure differences (form drag), and the lift-generation process itself (induced drag).
• Mathematical Equation: $$D = \frac{1}{2} \rho v^2 S C_D$$ where $C_D$ is the coefficient of drag.
• Action: Drag acts as aerodynamic resistance. It opposes the forward pulling force of Thrust ($T$) generated by the engines.