At what airspeeds are single rotor helicopters most susceptible to UY?

• A. Airspeeds less than 30 knots
• B. Airspeeds greater than 60 knots
• C. Airspeeds between 30 and 60 knots
• D. All airspeeds equally

Answer: (A)

In a single-rotor helicopter, yaw control is provided primarily by the tail rotor, and the amount of directional stability you have depends a lot on how fast you’re moving forward. When you’re in hover or flying very slowly (low airspeeds), you have almost no translational flow over the fuselage and tail surfaces. That means less natural damping of yaw motions, and any upset or disturbance—changes in engine or tail rotor thrust, a gust, or a small tail rotor inefficiency—can produce a noticeable, uncommanded yaw that’s harder to counteract. As you pick up speed into slow-to-moderate forward flight, the aerodynamics begin to stabilize the aircraft: the added flow over the vertical fin and the tail area provides more damping, and the tail rotor operates in a more favorable airflow, making yaw disturbances easier to manage. So the susceptibility to uncommanded yaw is greatest at very low speeds, i.e., less than 30 knots.