Initial forward flight Calm wind hover Before takeoff During takeoff Lift Centrifugal force Resultant blade angle Figure 2-29.  During takeoff, the combination of centrifugal force  and lift cause the rotor disk to cone upward. Figure 2-28.  Because the helicopter’s body has mass and is  suspended from a single point (the rotor mast head), it tends to act  much like a pendulum. Coriolis Effect (Law of Conservation of Angular  Momentum)  Coning As lift on the blades is increased (in a takeoff, for example),  two major forces are acting at the same time—centrifugal  force acting outward, and lift acting upward. The result of  these two forces is that the blades assume a conical path  instead of remaining in the plane perpendicular to the mast.  This can be seen in any helicopter when it takes off; the rotor  disk changes from flat to a slight cone shape.  [Figure 2-29] If the rotor rpm is allowed to go too low (below the minimum  power-on rotor rpm, for example), the centrifugal force  becomes smaller and the coning angle becomes much  larger. In other words, should the rpm decrease too much,  at some point the rotor blades fold up with no chance  of recovery. The Coriolis Effect is also referred to as the law of  conservation of angular momentum. It states that the value  of angular momentum of a rotating body does not change  unless an external force is applied. In other words, a rotating  body continues to rotate with the same rotational velocity  until some external force is applied to change the speed of  rotation. Angular momentum is the moment of inertia (mass  times distance from the center of rotation squared) multiplied  by the speed of rotation. rearward flight groundspeed equals the windspeed, then  the helicopter is merely hovering in a no-wind condition.  However, rearward hovering into the wind requires  considerable care and caution to prevent tail strikes. Changes in angular velocity, known as angular acceleration  and deceleration, take place as the mass of a rotating body  is moved closer to or farther away from the axis of rotation.  The speed of the rotating mass varies proportionately with  the square of the radius. It is important to note that there is a difference in the amount  of pendular action between a semirigid system and a fully  articulated system. Because of the hard connection (offset)  of the latter, the centrifugal force pulling out on the blades is  transferred to the fuselage, and the fuselage tends to follow  the rotor attitude. The semirigid system is a true pendulum,  with thrust required to create a moment around the fuselage  CG to allow for control of the fuselage. This comes into play  later when mast bumping is discussed. An excellent example of this principle in action is a figure  skater performing a spin on ice skates. The skater begins  rotation on one foot, with the other leg and both arms  extended. The rotation of the skater’s body is relatively  slow. When a skater draws both arms and one leg inward,  the moment of inertia (mass times radius squared) becomes  In order for a helicopter to generate lift, the rotor blades  must be turning. Rotor disk rotation drives the blades into  the air, creating a relative wind component without having  to move the airframe through the air as with an airplane or  glider. Depending on the motion of the blades and helicopter  airframe, many factors cause the relative wind direction to  vary. The rotation of the rotor disk creates centrifugal force  (inertia), which tends to pull the blades straight outward  from the main rotor hub: the faster the rotation, the greater  the centrifugal force, the slower the rotation, the smaller  the centrifugal force. This force gives the rotor blades their  rigidity and, in turn, the strength to support the weight of  the helicopter. The maximum centrifugal force generated  is determined by the maximum operating rotor revolutions  per minute (rpm).  2-16$^{Initial rearward flight}$