In straight-and-level (constant heading and at a constant  altitude), unaccelerated forward flight, lift equals weight  and thrust equals drag. If lift exceeds weight, the helicopter  accelerates vertically until the forces are in balance; if thrust  is less than drag, the helicopter slows until the forces are in  balance. As the helicopter moves forward, it begins to lose  altitude because lift is lost as thrust is diverted forward.  However, as the helicopter begins to accelerate, 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 disk and more excess power. In  order to maintain unaccelerated flight, the pilot must not  make any changes in power or in cyclic movement. Any  such changes would cause the helicopter to climb or descend.  Once straight-and-level flight is obtained, the pilot should  make note of the power (torque setting) required and not  make major adjustments to the flight controls.  [Figure 2-36] 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 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.  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.  Figure 2-37  and  Figure 2-38  show airflow patterns at different speeds and  how airflow affects the efficiency of the tail rotor.  While transitioning to forward flight at about 16–24 knots,  the helicopter experiences effective translational lift (ETL).  As mentioned earlier in the discussion on translational lift,  Figure 2-37.  The airflow pattern for 1–5 knots of forward airspeed. Note how the downwind vortex is beginning to dissipate and induced  flow down through the rear of the rotor system is more horizontal.  Figure 2-36.  Changing force vectors results in aircraft movement.  Translational Lift Effective Translational Lift (ETL) P ower required (horsepower) Indicated airspeed (KIAS) 800 600 400 200 0  0  40  60  80  100  120 C Minimum pow er for level flight(V$_{Y}$ ) A B Maximum continuo us level (horizon tal) flight airspeed (V$_{H}$ ) Maximum continuous power available Increasing power for decreasing airspeed Increasing power for decreasing airspeed $_{ }$P o w e r e q u i r e d t o h o v e r O G E D o w n w a r d v e l o c i t y o f a i r m o l e c u l e s u s e d b y a f t s e c ti o n  o fr ot or 1–5 knots 2-24