Forces balance in a vector triangle for steady descent

Steps:

• Constant speed means net force is zero, so L, R, and W form a closed vector triangle.
• W acts vertically downward.
• R acts opposite the flight path, upward at 15° to the horizontal.
• L acts perpendicular to the flight path, creating a 90° angle with R; the angle between W and R is 15°.

Why D is correct:

• D depicts a right triangle with 90° between L and R, and 15° between W and R, matching equilibrium where R = W sin 15° and L = W cos 15°.

Why the others are wrong:

• A: Shows acute angle between L and R, violating perpendicularity of lift to drag.
• B: Places W at wrong angle to R, not 15°.
• C: Incorrectly aligns L parallel to flight path, not perpendicular.

Final answer: D