In a level banked turn at a fixed bank angle, what is the effect on turn radius when airspeed decreases?

• A. Radius decreases
• B. Radius increases
• C. Radius remains the same
• D. Not enough data

Answer: (A)

In a level banked turn with a fixed bank angle, the lift that points perpendicular to the wings must support weight, so the lift magnitude is L = W / cos(phi). The horizontal component of lift, L sin(phi), provides the centripetal force needed to bend the path into a circle: m v^2 / r. Setting these equal gives m v^2 / r = (W / cos(phi)) sin(phi) = W tan(phi). Solving for the radius shows r = m v^2 / (W tan(phi)) = v^2 / (g tan(phi)). With the bank angle fixed, tan(phi) is constant, so the turn radius is proportional to the square of the airspeed. Therefore, as airspeed decreases, the radius decreases, making the turn tighter.