If weight is constant and speed increases in level flight, what happens to the required lift coefficient CL?

• A. It decreases
• B. It increases
• C. It stays the same
• D. It becomes negative

Answer: (A)

In level flight, lift must balance weight. Lift is L = q S CL, where q = 0.5 ρ V^2 is dynamic pressure. Since weight is constant and L = W, you have W = q S CL, so CL = W / (q S) = W / (0.5 ρ V^2 S). This shows CL is inversely proportional to V^2. As speed increases, the required lift coefficient must decrease to keep the lift equal to weight. Physically, higher speed allows the wing to generate the same lift with a smaller angle of attack. Negative lift would imply downward force not needed in level flight, so it doesn’t apply here.