Spring 2003 16.61AC3-5 Spiral mode - slow, often unstable From level fight. consider a disturbance that creates a small roll angle q>0 This results in a small side-slip v(vehicle slides downhill ow the tail fin hits on the oncoming air at an incidence angle B → extra tail lift→ yawing moment The positive yawing moment tends to increase the side-slip → makes things worse If unstable and left unchecked, the aircraft would fly a slowly diverging path in roll, yaw, and altitude = it would tend to spiral into the ground!! Sideslip Steadily increasing roll angle Yawing moment due to fin lift 7 Fin lift force Can get a restoring torque from the wing dihedral Want a small tail to reduce the impact of the spiral modeSpring 2003 16.61 AC 3–5 Spiral Mode - slow, often unstable. – From level flight, consider a disturbance that creates a small roll angle φ > 0 – This results in a small side-slip v (vehicle slides downhill) – Now the tail fin hits on the oncoming air at an incidence angle β → extra tail lift → yawing moment – The positive yawing moment tends to increase the side-slip → makes things worse. – If unstable and left unchecked, the aircraft would fly a slowly diverging path in roll, yaw, and altitude ⇒ it would tend to spiral into the ground!! • Can get a restoring torque from the wing dihedral • Want a small tail to reduce the impact of the spiral mode. 5