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  1. 1. 3. Forces acting on a helicopter The three main forces acting on a rotor blade are the lift force, the drag force, and the centrifugal force. Lift is the upward force caused by the interaction between the air flow and the airfoil. Drag is the force of the air resisting the movement of the airfoil. The centrifugal force represents the tendency of the rotor blade to fly away from the center. Because of the circular motion, the air velocity is a lot higher at the tip of the blade than at the base. Because of the quadratic relation between lift and speed, and drag and speed, the lift/drag increases quadratically with respect to the distance to the center. Because of the forces on the rotor blades, the blades have a tendency to cone. This means they tend to tilt upward during flight. This is caused by the combination of lift and the centrifugal force. The centrifugal force tries to make the blade as horizontal as possible, while the lift tries to move the blade up. The combination of these forces mean that the helicopter blade is rotated slightly upwards. When the pitch of the blade is changed, the lift generated by that blade changes too, this means that there's a relation between the angle of attack and the coning angle. A tilted swashplate thus results in an asymmetric cone.
  2. 2. The motor of the helicopter makes the blades spin relative to the helicopter fuselage. However, because of action and reaction, this means that the fuselage has a tendency to spin in the opposite direction: this is called anti-torque. To alleviate this, another force needs to be generated outside of the central axis of the helicopter. In most helicopters, this force is the thrust from the tail rotor. While in forward flight, the thrust generated by the blades depends on their position. Some areas will generate more lift than others, which means that the net result isn't a vertical force anymore. This net result can be split up in lift and forward thrust. The sum of the lift and the weight determine if the helicopter goes up, down, or hovers. The sum of the forward thrust and the total drag determine at what speed the helicopter moves forward. Gyroscopic precession 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
  3. 3. approximately 90° in the direction of rotation from the point where the force is applied (fig. 13). Through the use of this principle, the tip-path plane of the main rotor may be tilted from the horizontal. The movement of the cyclic pitch control in a two-bladed rotor system increases the angle of attack of one rotor blade with the result that a greater lifting force is applied at this point in the plane of rotation. This same control movement simultaneously decreases the angle of attack of the other blade a like amount, thus decreasing the lifting force applied at this point in the plane of rotation. The blade with the increased angle of attack tends to rise; the blade with the decreased angle of attack tends to lower. However, because of the gyroscopic precession property, the blades do not rise or lower to maximum deflection until a point approximately 90° later in the plane of rotation. In the illustration (fig. 14), the retreating blade angle of attack is increased and the advancing blade angle of attack 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. Figure 13 - Gyroscopic Precession Principle: When a force is applied to a spinning gyro, the maximum reaction occurs 90° later in the direction of rotation.
  4. 4. Coriolis effect When a rotor blade of a three-bladed rotor system flaps upward, the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place. Conversely, when that blade flaps downward, its center of mass moves further from the axis of rotation and blade deceleration takes place (fig. 19). (Keep in mind, that due to coning, the rotor blade will not flap below a plane passing through the rotor hub and perpendicular to the axis of rotation.) The acceleration and deceleration actions (often referred to as leading, lagging, or hunting) of the rotor blades are absorbed by either dampers or the blade structure itself, depending upon the design of the rotor system. Two-bladed rotor systems are normally subject to CORIOLIS EFFECT to a much lesser degree than are three-bladed systems since the blades are generally "underslung" with respect to the rotor hub, and the change in the distance of the center of mass from the axis of rotation is small. The hunting action is absorbed by the blades through bending. If a two-bladed rotor system is not "underslung," it will be subject to CORIOLIS EFFECT comparable to that of a fully articulated system. Figure 18 - The axis of rotation is the imaginary line about which the rotor rotates and is perpendicular to the tip-path plane. Figure 19 - Coriolis effect is the change in blade velocity to compensate for the change in distance of the center of mass from the axis of rotation as the blades flap. CORIOLIS EFFECT might be compared to spinning skaters. When they extend their arms, their rotation slows down because their center of mass moves farther from their axis of rotation. When their arms are retracted, their rotation speeds up because their center of mass moves closer to their axis of rotation.
  5. 5. The tendency of a rotor blade to increase or decrease its velocity in its plane of rotation due to mass movement is known as CORIOLIS EFFECT, named for the mathematician who made studies of forces generated by radial movements of mass on a rotating disc.