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

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.

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.