Lift Weight Thrust Drag Figure 2-26.  To maintain a hover at a constant altitude, the lift  must equal the weight of the helicopter. Thrust must equal any  wind and tail rotor thrust to maintain position. The power must be  sufficient to turn the rotors and overcome the various drags and  frictions involved.  Powered Flight Hovering Flight Cyclic feathering changes the blade’s AOA differentially  around the rotor disk and creates a differential lift. Aviators  use cyclic feathering to control attitude of the rotor disk. It  is the means to control rearward tilt of the rotor (blowback)  caused by flapping action and (along with blade flapping)  counteract dissymmetry of lift (discussed in chapter 3). Cyclic  feathering causes attitude of the rotor disk to change but does  not change the amount of net lift the rotor disk is producing. helicopter to hover comes from the both the lift component,  which is the force developed by the main rotor(s) to overcome  gravity and aircraft weight, and the thrust component, which  acts horizontally to accelerate or decelerate the helicopter in  the desired direction. Pilots direct the thrust of the rotor disk  by using the cyclic to rotate the rotor disk plane relative to the  horizon. They do this in order to induce travel or compensate  for the wind and hold a position. At a hover in a no-wind  condition, all opposing forces (lift, thrust, drag, and weight)  are in balance; they are equal and opposite. Therefore, lift  and weight are equal, resulting in the helicopter remaining at  a stationary hover.  [Figure 2-26] Most of the changes in AOA come from change in airspeed  and rate of climb or descent; others such as flapping occur  automatically due to the rotor system design. Flapping is the  up and down movement of rotor blades about a hinge on a  fully articulated rotor system. A semi-rigid system does not  have a hinge but flap as a unit. A rigid rotor system has no  vertical or horizontal hinges, so the blades cannot flap or  drag, but they can flex. By flexing, the blades themselves  compensate for the forces which previously required rugged  hinges. It occurs in response to changes in lift due to changing  velocity or cyclic feathering. No flapping occurs when the tip- path plane is perpendicular to the mast. The flapping action  alone, or along with cyclic feathering, controls dissymmetry  of lift. Flapping is the primary means of compensating for  dissymmetry of lift.  While hovering, the amount of main rotor thrust can be  adjusted to maintain the desired hovering height. This is done  by changing the angle of incidence (by moving the collective)  of the rotor blades, and hence their AOA. Changing the AOA  changes the drag on the rotor blades, and the power delivered  by the engine must change as well to keep the rotor speed  constant.  The weight that must be supported is the total weight of the  helicopter and its occupants. If the amount of lift is greater  than the actual weight, the helicopter accelerates upwards  until the lift force equals the weight of the helicopter; if lift  is less than weight, the helicopter accelerates downward. Pilots adjust AOA through normal control manipulation of  the pitch angle of the blades. If the pitch angle is increased,  the AOA increases; if the pitch angle is reduced, the AOA  is reduced.  The drag of a hovering helicopter is mainly induced drag  incurred while the blades are producing lift. There is, however,  some profile drag on the blades as they rotate through the air  and a small amount of parasite drag from the non-lift-producing  surfaces of the helicopter, such as the rotor hub, cowlings, and  In powered flight (hovering, vertical, forward, sideward,  or rearward), the total lift and thrust forces of a rotor are  perpendicular to the rotor disk. Hovering is the most challenging part of flying a helicopter.  This is because a helicopter generates its own gusty air  while in a hover, which acts against the fuselage and flight  control surfaces. The end result is constant control inputs  and corrections by the pilot to keep the helicopter where it is  required to be. Despite the complexity of the task, the control  inputs in a hover are simple. The cyclic is used to eliminate  drift in the horizontal plane, controlling forward, backward,  right and left movement or travel. The throttle, if not governor  controlled, is used to control revolutions per minute (rpm). The  collective is used to maintain altitude. The pedals are used to  control nose direction or heading. It is the interaction of these  controls that makes hovering difficult, since an adjustment  in any one control requires an adjustment of the other two,  creating a cycle of constant correction. During hovering flight,  a helicopter maintains a constant position over a selected  point, usually a few feet above the ground. The ability of the  2-14