Cyclic and Collective Pitch. Aviator inputs to the collective and cyclic pitch controls are transmitted to the rotor blades through a complex system. This system consists of levers, mixing units, input servos, and stationary and rotating swashplates and pitch-change arms. In its
simplest form, the movement of the collective pitch control causes the stationary and rotating swashplates mounted centrally on the rotor shaft to translate vertically (move up and down). The movement of the cyclic pitch control causes the swashplates to rotate (tilt) as if on a gimbal; the direction of tilt is controlled by the direction in which the aviator moves the cyclic. Their unique design allows the swashplates to both elevate and tilt together, effectively changing a
non-rotating input to the stationary swashplate into a non-rotating output from the non-rotating swashplate.
Cyclic Pitch Change. A change in cyclic stick position causes the rotor blades to change pitch independently about the rotor disk (cyclic feathering). By aerodynamic reaction the blades will climb or descend as they rotate around (Figure 4-3). In this way, the virtual axis is tilted in the direction of desired flight. To pass through points A and B, the blades must flap up and down on a hinge or teeter on a trunnion. Although the virtual axis is tilted in the direction of desired flight as previously discussed, control inputs to the rotor system are actually made such that the point of lowest blade pitch in the plane of rotation occurs 90° prior to the direction the virtual axis is tilted.
Figure 4-3 Rotor Flapping Due to Cyclic Input
A cyclic movement in one direction decreases blade pitch at the point in the rotor disk 90° earlier while increasing blade pitch by the same amount 90° later. The decrease in lift resulting from a decrease in blade pitch angle and AOA causes the blade to flap down with the blade reaching its maximum down flapping displacement 90° after lowest blade pitch in the direction of rotation.
An increase in lift resulting from an increase in blade pitch angle and AOA causes the blade to flap up; the blade reaches its maximum up flapping displacement 90° later in the direction of rotation. Figure 4-4 shows the resulting change to the attitude of the rotor disk. The cyclic pitch change causing blade flap must be applied to the blades 90° of rotation before the lowest flap and highest flap are desired. To tilt the rotor disk forward, the lowest cyclic pitch on the blade needs to be over the right side of the helicopter and the highest cyclic pitch over the left side. Phase lag is accounted for when control systems/mixing units are designed and it is ensured that when the cyclic is pushed forward, the action tilts the swashplate assembly to place the cyclic pitch accordingly,. The rotor always tilts in the direction in which the aviator moves the cyclic.
Figure 4-4 Phase Lag
The relationship between cyclic control inputs, flapping and rotor disk response is summarized in Figure 4-5.
Cyclic displacement
Greatest blade pitch, Force, and flapping velocity upward
Highest flapping displacement
Least blade pitch, force, and greatest flapping velocity downward
Direction of disk tilt, lowest flapping displacement
Forward 9 O’clock 6 O’clock 3 O’clock 12 O’clock
Right 12 O’clock 9 O’clock 6 O’clock 3 O’clock
Aft 3 O’clock 12 O’clock 9 O’clock 6 O’clock
Left 6 O’clock 3 O’clock 12 O’clock 9 O’clock
Figure 4-5 Relationship Between Cyclic Inputs and Rotor Response
Cyclic Pitch Variation. Figure 4-6 illustrates the typical cyclic pitch variation for a blade through one revolution with the cyclic pitch control full forward. The degrees shown are for a typical aircraft rotor system; the figures would vary with the type of helicopter. With the cyclic pitch control in the full-forward position, the blade pitch angle is highest at the 9-o’clock position and lowest at the 3-o’clock position. The pitch angle begins decreasing as it passes the 9-o’clock position and continues to decrease until it reaches the 3-o’clock position; the pitch begins to increase and reaches the maximum pitch angle at the 9-o’clock position. Blade pitch angles over the nose and tail are about equal.
Figure 4-6 Cyclic Pitch Variation-Full Forward, Low Pitch
Figure 4-6 shows that the blades reach a point of lowest flapping over the nose 90° in the direction of rotation after the point of lowest pitch angle. Highest flapping occurs over the tail 90° in the direction of rotation after the point of the highest pitch angle. Simply stated, the force (pitch angle) that causes blade flap must be applied to the blade 90° of rotation before the point where the aviator desires maximum blade flap displacement.
A pattern similar to that in Figure 4-6 could be constructed for other cyclic positions in the circle of cyclic travel. In each case, the same principles apply. Points of highest and lowest flapping will be located 90° in the direction of rotation from the points of highest and lowest blade pitch.
408. CONING
Coning is the upward displacement of the rotating rotor blades due to a combination of
aerodynamic (lift) and centrifugal forces. The rotating blades of a helicopter produce very high centrifugal loads on the hub and blade attachment assemblies. In fact, the centrifugal force on the rotor system can be many times the weight of the load actually lifted (Figure 4-7).
In the example, the centrifugal force on a 48,000 pound helicopter is on the order of 80,000 pounds. Centrifugal Force plays an important role in, flapping, coning blade strength, and blade shape during operation.
Figure 4-7 Centrifugal Force and Coning
In rotary-wing aircraft, this is the dominant force affecting the rotor system. All other forces act to modify it. As a rotor system begins to turn, the blades begin to rise from the static position because of centrifugal force. At operating speed, the blades extend effectively straight out when the rotor system is at flat pitch (collective full down) and not producing lift. As the aircraft develops lift during takeoff and flight, the blades rise above the straight-out position and assume a coned position. The balance of forces establishes the blade at an angle from the flat plane that is referred to as a coning angle. Alteration of lift or centrifugal force establishes a different coning angle. A horizontal force of 40 tons balanced by a lifting force of 4 tons on a given blade would yield a coning angle of tan -1 (4/40) = 6° . The amount of coning depends on RPM, gross weight, and G-forces experienced during flight. Excessive coning can occur if RPM is too low, gross weight is too high, an aircraft is flying in turbulent air, or the G-forces experienced are too high. This excessive coning can cause undesirable stresses on the components and a decrease in lift because of a decrease in effective disk area (Figure 4-8).