Improved rotor efficiency resulting from directional flight is called translational lift. The efficiency of the hovering rotor system is greatly improved with each knot of incoming wind gained by horizontal movement of the aircraft or surface wind. As the incoming wind produced by aircraft movement or surface wind enters the rotor system, turbulence and vortices are left behind and the flow of air becomes more horizontal. In addition, the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight. Figures 2-37 and 2-38 show the different airflow patterns at different speeds and how airflow affects the efficiency of the tail rotor.
Effective Translational Lift (ETL)
While transitioning to forward flight at about 16 to 24 knots, the helicopter goes through effective translational lift (ETL). As mentioned earlier in the discussion on translational lift, the rotor blades become more efficient as forward airspeed increases. Between 16 and 24 knots, the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air. The flow of air through the rotor system is more horizontal; therefore, induced flow and induced drag are reduced. The AOA is effectively increased, which makes the rotor system operate more efficiently. This increased efficiency continues with increased airspeed until the best climb airspeed is reached, and total drag is at its lowest point.
As speed increases, translational lift becomes more effective, nose rises or pitches up, and aircraft rolls to the right. The combined effects of dissymmetry of lift, gyroscopic precession, and transverse flow effect cause this tendency. It is important to understand these effects and anticipate correcting for them. Once the helicopter is transitioning through ETL, the pilot needs to apply forward and left lateral cyclic input to maintain a constant rotor-disk attitude. [Figure 2-39]
Translational thrust occurs when the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight. As the tail rotor works in progressively less turbulent air, this improved efficiency produces more antitorque thrust, causing the nose of the aircraft to yaw left (with a main rotor turning counterclockwise) and forces the pilot to apply right pedal (decreasing the AOA in the tail rotor blades) in response. In addition, during this period, the airflow affects the horizontal components of the stabilizer found on most helicopters which tends to bring the nose of the helicopter to a more level attitude.
When a helicopter is hovering, the tail rotor is operating in very disturbed airflow. As the helicopter achieves ETL, the tail rotor begins to generate much more thrust because of the less disturbed airflow. The helicopter reacts to the increased thrust by yawing. Therefore, as the helicopter achieves ETL, you must reduce tail rotor thrust by pedal input at about the same time that you need to make cyclic adjustments for lateral tracking, acceleration, and climb.Are you ready to start your journey learning to fly helicopters? Learning to Fly Helicopters, Second Edition, provides details on the technical and practical aspects of rotarywing flight. Written in a conversational style, the book demystifies the art and science of helicopter flying.