FLIGHT INVESTIGATION OF THE EFFECT OF TAIL BOOM STRAKES ON HELICOPTER DIRECTIONAL CONTROL

A joint U.S. Army/NASA flight investigation was conducted utilizing an instrumented Bell 240B single-rotor helicopter to determine the effectiveness of horizontally-mounted tail boom strakes on directional controllability and tail rotor power required during low-speed, crosswind operating conditions. The purpose of the strakes was to separate airflow over the tail boom and change fuselage yawing moments in a direction to improve the yaw control margin and reduce tail rotor power required. Low-speed crosswind data were obtained in 5-knot increments at airspeeds of 0 knots to 35 knots and for 30° increments in wind azimuth from 0° to 330°. In right sideward flight at the most critical wind azimuth and airspeed (60° azimuth measured from the nose of the aircraft and 20 knots airspeed), the strakes improved the pedal margin by about 6% of total travel and reduced the tail rotor power required by 17%. The increase in yaw control and reduction in tail rotor power offered by the strakes can expand the operating envelope in terms of gross weight and altitude capability. No effects in forward flight were noted.