Drag is defined as?

• A. The net aerodynamic force perpendicular to the relative wind.
• B. The net aerodynamic force parallel to the relative wind, usually the sum of lift and weight.
• C. The net aerodynamic force parallel to the relative wind, usually the sum of side force components.
• D. The net aerodynamic force parallel to the relative wind, usually the sum of induced drag and parasite drag.

Answer: (D)

Drag is the aerodynamic force component that acts parallel to the relative wind and opposes the aircraft’s motion. The total drag you feel is the sum of parasite drag (skin friction, form drag, and interference) and induced drag (drag that comes from the wing producing lift). So, drag is the forward-to-backward force along the wind direction, not the lift or weight, and not a sideways force. The other ideas mix forces that aren’t aligned with the relative wind or include weight, which isn’t part of drag.