Helicopter Forward Flight

In steady forward flight, with no change in airspeed or vertical speed, the four forces of lift, thrust, drag, and weight must be in balance. Once the tip-path plane is tilted forward, the total lift-thrust force is also tilted forward. 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 force opposing the motion of an airfoil through the air). [Figure 2-31]

Figure 2-31. To transition to forward flight, more lift and thrust must be generated to overcome the forces of weight and drag.

Figure 2-31. To transition to forward flight, more lift and thrust must be generated to overcome the forces of weight and drag.

In straight-and-level, unaccelerated forward flight (straightand- level flight is flight with a constant heading and at a constant altitude), 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 down 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 from a hover, the rotor system becomes more efficient due to translational lift (see translational lift on page 2-19). The result is excess power over that which is required to hover. Continued acceleration causes an even larger increase in airflow, to a point, through the rotor disk and more excess power. In order to maintain unaccelerated flight, the pilot must understand that with any changes in power or in cyclic movement, the helicopter begins either to climb or to 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 2-32]

Figure 2-32. Power versus airspeed chart.

Figure 2-32. Power versus airspeed chart.