What is meant by drag?

• A. The aerodynamic force acting perpendicular to relative wind striking the airfoil.
• B. The aerodynamic force acting in the same plane as the relative wind striking the airfoil.
• C. The thrust generated by engines.
• D. The lift component perpendicular to relative wind.

Answer: (B)

Drag is the aerodynamic resistance the air presents to the airplane moving through it. It acts along the direction of the relative wind, opposite to the motion through the air, so it lies in the same plane as the flow hitting the airfoil. This distinguishes it from lift, which acts perpendicular to the relative wind, and from thrust, which is produced by the engines. Understanding drag as the along-flow resistance helps explain why it increases with speed and why propulsion must overcome it to maintain flight.