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ALC-104: Helicopter - General and Flight Aerodynamics
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Flight Aerodynamics


Note: In this chapter, it is assumed that the helicopter has a counterclockwise main rotor blade rotation as viewed from above. If flying a helicopter with a clockwise rotation, you will need to reverse left and right references, particularly in the areas of rotor blade pitch change, anti-torque pedal movement, and tail rotor thrust.

Once a helicopter leaves the ground, it is acted upon by the four forces of lift, thrust, drag, and weight. This chapter examines these forces as they relate to flight maneuvers. In powered flight, the total lift and thrust forces of a rotor are perpendicular to the tip-path plane or plane of rotation of the rotor.

There are certain physical characteristics and effects that are unique to a helicopter and its rotor system while in flight.

 


Fig3-1.JPG
The fuselage of a helicopter with a single main rotor constitutes considerable mass being suspended from a single point.  It is therefore free to oscillate as a pendulum either longitudinally or laterally. See figure 3-1. As this pendulous action can be exaggerated by over controlling, control movements should be smooth and non-exaggerated.


 


Fig3-2.JPG
In order for a helicopter to generate lift, the rotor blades must be turning to create a relative wind that is opposite the direction of rotor system rotation. The rotation of the rotor system also creates centrifugal force, which tends to pull the blades straight outward from the main rotor hub. The faster the rotation, the greater the centrifugal force. This force gives the rotor blades their rigidity and, in turn, the strength to support the weight of the helicopter. See figure 3-2. The centrifugal force generated determines the maximum operating rotor RPM due to structural limitations on the main rotor system. As a vertical takeoff is made, two major forces are acting at the same time—centrifugal force acting outward and perpendicular to the rotor mast, and lift acting upward and parallel to the mast. The result of these two forces is that the blades assume a conical path instead of remaining in the plane perpendicular to the mast.


Fig3-3.JPG
Also referred to as conservation of angular momentum, it is readily demonstrated by viewing spinning skaters. When spinning skaters extend their arms, their rotation rate decreases because their center of mass moves farther from the axis of rotation. When their arms are retracted, their rotation rate increases because the center of mass moves closer to the axis of rotation. When a rotor blade flaps upward, the blades center of mass moves closer to the axis of rotation and blade acceleration takes place in order to conserve angular momentum. Conversely, when that blade flaps downward, its center of mass moves further from the axis of rotation and blade deceleration takes place. See figure 3-3.

 

 


Fig3-4.JPGKeep in mind that due to coning, a 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 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 articulated rotor systems since the blades are generally “under-slung” with respect to the rotor hub, and the change in the distance of the center of mass from the axis of rotation is small. See figure 3-4. The hunting action is absorbed by the blades through bending. If a two-bladed rotor system is not “under-slung,” it will be subject to Coriolis Effect comparable to that of a fully articulated system.



This effect is typically effective less than one rotor diameter above the surface. As the induced airflow through the rotor disc is reduced by the surface friction, the lift vector increases. This allows a lower rotor blade angle for the same amount of lift, which reduces induced drag. Ground effect also restricts the generation of blade tip vortices due to the downward and outward airflow making a larger portion of the blade produce lift. When the helicopter gains altitude vertically, with no forward airspeed, induced airflow is no longer restricted, and the blade tip vortices increase with the decrease in outward airflow. As a result, drag increases which means a higher pitch angle, and more power is needed to move the air down through the rotor. See figure 3-5. Ground effect is at its maximum in a no-wind condition over a firm, smooth surface. Tall grass, rough terrain, revetments, and water surfaces alter the airflow pattern, causing an increase in rotor tip vortices.

Fig3-5.JPG



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° later in the direction of rotation from the point where the force is applied. See figure 3-6.

Fig3-6.JPG


Analysis of a rotating two-bladed rotor system readily describes how gyroscopic precession affects the movement of the tip-path plane. Moving the cyclic pitch control increases the pitch angle and thus angle of attack of one rotor blade with the result that a greater lifting force is applied at that point in the plane of rotation. This same control movement simultaneously decreases the pitch angle and angle of attack of the other blade the same amount, thus decreasing the lifting force applied at that point in the plane of rotation. The blade with the increased angle of attack tends to flap up; the blade with the decreased angle of attack 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.


Fig3-7.JPGFigure 3-7 illustrates the effect of forward cyclic control movement; 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. In a rotor system using three or more blades, the movement of the cyclic pitch control changes the angle of attack of each blade an appropriate amount so that the end result is the same.