On some helicopters, running the length of the tail boom are “L” shape (or something similar) brackets that protrude about an inch. These are known as tail boom strakes and they act like spoilers.
Because the tail boom is underneath the rotor system, at a hover, very low airspeeds or sideways flight rotor down wash passes around the boom. Like an airfoil, this produces high and low air pressure areas that exert a force along the tail boom. This force decreases the tail rotor’s capability during hover and slow flight. At higher speeds the down wash moves to the rear and passes above the tail boom. Strakes control the airflow around the tail boom, thus increasing the tail rotor’s efficiency and decreasing the turbulent air, which improves yaw control.
During the late 1980s, NASA and the U.S. Army performed wind tunnel and flight tests to analyze the performance gain from adding a tail boom strake. The tests were performed using a Bell 204B helicopter. Published in 1993 the NASA Technical Report 3278 stated a 5-percent improvement in pedal control margin will provide an additional 2,000 feet of altitude capability or 500 lbs. of payload. The report concluded that the strakes improves handling qualities, reduces tail boom fatigue, improves climb and cruise performance, and increases yaw control safety margins for all single rotor helicopters with enclosed tail booms.