decreasing the lifting force applied at that point in the plane of  rotation. The blade with the increased angle of incidence tends  to flap up; the blade with the decreased angle of incidence  tends to flap down. Because the rotor disk acts like a gyro, the  blades reach maximum deflection at a point approximately  90° later in the plane of rotation.  Figure 2-31  illustrates the  result of a forward cyclic input. The retreating blade angle  of incidence is increased, and the advancing blade angle of  incidence is decreased resulting in a tipping forward of the  tip-path plane, since maximum deflection takes place 90°  later when the blades are at the rear and front, respectively. In a rotor disk using three or more blades, the movement of  the cyclic pitch control changes the angle of incidence of each  blade an appropriate amount so that the end result is the same. much smaller and the body is rotating almost faster than the  eye can follow. Because the angular momentum must, by  law of nature, remain the same (no external force applied),  the angular velocity must increase. The rotor blade rotating about the rotor hub possesses angular  momentum. As the rotor begins to cone due to G-loading  maneuvers, the diameter of the rotor disk shrinks. Due to  conservation of angular momentum, the blades increase  speed even though the blade tips have a shorter distance to  travel due to reduced disk diameter. The action results in an  increase in rotor rpm which causes a slight increase in lift.  Most pilots arrest this increase of rpm with an increase in  collective pitch. This increase in blade rpm lift is somewhat  negated by the slightly smaller disk area as the blades cone  upward. Hovering is actually an element of vertical flight. Increasing  the angle of incidence of the rotor blades (pitch) while  keeping their rotation speed constant generates additional  lift and the helicopter ascends. Decreasing the pitch causes  the helicopter to descend. In a no-wind condition in which  lift and thrust are less than weight and drag, the helicopter  descends vertically. If lift and thrust are greater than weight  and drag, the helicopter ascends vertically.  [Figure 2-32] The spinning main rotor of a helicopter acts like a gyroscope.  As such, it has the properties of gyroscopic action, one of  which is precession. Gyroscopic precession is the resultant  action or deflection of a spinning object when a force is  applied to this object. This action occurs approximately 90°  in the direction of rotation from the point where the force  is applied (or 90° later in the rotation cycle).  [Figure 2-30] Examine a two-bladed rotor disk to see how gyroscopic  precession affects the movement of the tip-path plane.  Moving the cyclic pitch control increases the angle of  incidence of one rotor blade with the result of a greater lifting  force being applied at that point in the plane of rotation.  This same control movement simultaneously decreases the  angle of incidence of the other blade the same amount, thus  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 2-33] In straight-and-level, unaccelerated forward flight (straight- and-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 a  helicopter initiates a move 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  disk 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 through the rotor  disk (up to a maximum determined by drag and the engine’s  limit of power), and more efficient flight. In order to maintain  unaccelerated flight, the pilot must understand that with  2-17 Figure 2-30.  Gyroscopic precession. Vertical Flight Gyroscopic Precession Forward Flight Forward A B C D Downward movement response here Upward movement Downward force applied here Upward force applied here