There is yet another periodic force with a phase-displacement angular separation of 90 degrees. The total result of this action is a rotor tilt to the rear which is completely independant of any additional cyclic stick action and which causes an angular separation between the control axis and the thrust axis of the rotor. The following graphic illustrates this relationship: It then follows that if the maximum upward and flapping velocity is directly over the right side of the helicopter, the maximum displacement or actual flapping will take place over the nose of the aircraft.Ĭonversely, if the maximum downward flapping velocity is directly over the left side of the helicopter, the maximum displacement or actual flapping will take place over the tail of the aircraft. The force-displacement phase is 90 degrees, and is not affected by blade mass or any kind of air dampening. In the study of cyclic pitch, in a dynamic system such as a main rotor system with intertia, there is a phase angle between the maximum applied force and the maximum displacement. The flapping velocities are at maximum values directly over the right and left sides of the helicopter, because at those locations the airspeed differential is at its maximum. (This discussion assumes counterclockwise blade rotation, for clockwise rotation, they are reversed) It is understandable that the maximum upward flapping velocity will take place directly over the right side of the helicopter, and the maximum downward flapping velocity takes place directly over the left side of the helicopter. The resulting larger angle of attack retains the lift that would have been lost because of the reduced airspeed.įlapping Velocity, both upward and downward, must be of such a value as to increase or decrease the angle of attack so that the lift will remain constant. In the case of the retreating blade, the opposite is true:Īs it loses airspeed, reducing lift causes it to flap down (or settle), thus changing its relative wind and angle of attack. The blade flaps (or climbs) upward, and the change in relative wind and angle of attack reduces the amount that would have been generated. The advancing blade, upon meeting the progressively highter airspeeds brought about by the addition of forward flight velocity to the rotational airspeed (of the rotor), responds to the increase of speed by producing more lift. Blade Flapping is the up and down movement of a rotor blade, which, in conjunction with cyclic feathering, causes Dissymmetry of Lift to be eliminated.
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