lift and thrust are greater than weight and drag, the hel- icopter ascends vertically. [Figure 3-10] In straight-and-level, unaccelerated forward flight, lift equals weight and thrust equals drag (straight-and-level flight is flight with a constant heading and at a constant altitude). If lift exceeds weight, the helicopter climbs; if lift is less than weight, the helicopter descends. If thrust exceeds drag, the helicopter speeds up; if thrust is less than drag, it slows down. As the helicopter moves forward, it begins to lose alti- tude because of the lift that is lost as thrust is diverted forward. However, as the helicopter begins to acceler- ate, the rotor system becomes more efficient due to the increased airflow. The result is excess power over that which is required to hover. Continued acceleration causes an even larger increase in airflow through the rotor disc and more excess power. Translational lift is present with any horizontal flow of air across the rotor. This increased flow is most notice- able when the airspeed reaches approximately 16 to 24 knots. As the helicopter accelerates through this speed, the rotor moves out of its vortices and is in relatively undisturbed air. The airflow is also now more horizontal, which reduces induced flow and drag with a correspon- ding increase in angle of attack and lift. The additional lift available at this speed is referred to as “effective translational lift” (ETL). [Figure 3-12] In or during forward flight, the tip-path plane is tilted for- ward, thus tilting the total lift-thrust force forward from the vertical. This resultant lift-thrust force can be resolved into two components—lift acting vertically upward and thrust acting horizontally in the direction of flight. In addition to lift and thrust, there is weight (the downward acting force) and drag (the rearward acting or retarding force of inertia and wind resistance). [Figure 3-11] When a single-rotor helicopter flies through translational lift, the air flowing through the main rotor and over the tail rotor becomes less turbulent and more aerodynami- cally efficient. As the tail rotor efficiency improves, more thrust is produced causing the aircraft to yaw left in a counterclockwise rotor system. It will be necessary to use right torque pedal to correct for this tendency on takeoff. Also, if no corrections are made, the nose rises or pitches up, and rolls to the right. This is caused by combined effects of dissymmetry of lift and transverse flow effect, and is corrected with cyclic control. 3-5 Figure 3-11. To transition into forward flight, some of the ver- tical thrust must be vectored horizontally. You initiate this by forward movement of the cyclic control. Figure 3-12. Effective translational lift is easily recognized in actual flight by a transient induced aerodynamic vibration and increased performance of the helicopter. Figure 3-10. To ascend vertically, more lift and thrust must be generated to overcome the forces of weight and the drag. TRANSLATIONAL LIFT FORWARD FLIGHT Resultant Resultant Lift Thrust HelicopterGLYPH<.notdef> Movement Weight Drag No RecirculationGLYPH<.notdef> of Air More HorizontalGLYPH<.notdef> Flow of Air ReducedGLYPH<.notdef> Induced FlowGLYPH<.notdef> IncreasesGLYPH<.notdef> Angle of Attack Tail Rotor Operates inGLYPH<.notdef> Relatively Clean Air 16 to 24GLYPH<.notdef> Knots Thrust Lift Weight Drag Vertical Ascent