How does the Kmax control yaw?

[HDW]

K-Max Flight controls

DIRECTIONAL CONTROL SYSTEM: POWER-ON

Up to now, the intermesher rotors reacted in much the same manner as any helicopter. However, in the directional system the similarity ends. Due to the couter-rotating rotors, as explained in previous paragraphs, rotor torque is nullified. It is important to keep in mind that this is true only if both rotor receive equal pitch changes. So far in our discussion, we know how to fly vertically and in any given direction. To make turns, the rudder pedals are linked mechanically to the collective and fore-aft cyclic systems. Turning the intermesher is brought about by intentionally changing the torque relationship between the rotors with the application of the pedals.

When the pilot applies pedal, both differential collective and differential cyclic are applied to the rotors. For example,when right pedal is applied, the left rotor increases in pitch and the right rotor decreases in pitch (see fig. 5). This action is known as differential collective and causes the left rotor to produce more torque reaction and the right rotor less, thus turning the helicopter to the right.

Conversely, the pitch and torque reaction of the right rotor increases and that of the left decreases when left pedal is applied causing the helicopter to turn and roll to the left. It must be remembered that these effects occur while the helicopter is in powered flight.






As differential collective is induced in the rotors,another action as differential cyclic, takes place simultaneously. Application of right rudder pedal not only causes the left rotor to increase in pitch and torque reaction, but also tilt forward,(see fig.6). the right rotor decreases in pitch and tilts aft. This action tilts the left rotor forwards and the right rotor aft , causing them to „ push and pull „ in the turn. Thus an aerodynamic force is induced wich assists the helicopter in the turn to the right. The action of differential cyclic may be compared to the use of oars in turning a row boat. When in a right turn, the oarsman facing the stern uses the oar in his right hand to pull the boat around in the direction of the turn and either pushes with his left oar or allows it to drag in the water to assist in turning. Application of left rudder pedal in the helicopter causes the right rotor to „ push „ and the left rotor to „ pull or drag „ and the left rotor to „ pull or drag „ turning the aircraft to the left.







DIRECTIONAL CONTROL SYSTEM
„ POWER OFF „ ( AUTOROTATION )


As described in previous paraghraphs during power-on flights ( engine driving the rotors ), the differential torque reaction helps turn the helicopter. We have learned that in the intermesher configuration,in powered flight,the greatest torque reaction is produced by the HIGH lift rotor. During autorotation the rotors are driven by an external force, the flow of air up through the rotors produced by the helicopter`s rate of descent. Again, due to the characteristics of the intermesher configuration, the rotor having the HIGHER pitch now provides the greatest reaction due to rransmission friction and causes the fuselage to turn in the same direction as the fuselage. Consequently, application of right pedal would apply more pitch on the left rotor which would cause the helicopter to yaw to the left-an undesirable situation! The solution is in the incorporation into the controls a mechanism known as the „ reverser „ . The purpose of this devise is to maintain a consistent relationsship betwen the application of pedal and direction of turn. This is accomplished by reversing the differential collective in the rotors while in autorotation thus by reversing the differential collective in the rotors while in autorotation thus making the inside rotor in a turn the high pitch rotor and the rotor ( see fig. 7 ). The helicopter then turns in the desired direction.




REVERSER

To accomplish the above, the reversing mechanism is installed in the control module between the pedals and collective system. Ist only purpose is to reverse the differential collective to the rotors when in descending flight 0 – 10 percent collective position and autorotation. The reverser mechanism is a self – contained unit consisting of three control connections – input from the rudder pedals output to the collective systems,and control input from the collective lever.

The reverser never needs adjustment other than initial rigging.It is designed to mechanically and automatically revers the differential collective input to the rotor from the pedals as required between power – on flight and descending – autorotation. The reverser has two main control positions, „normal“ and „reverse“. During normal power-on flight, the reverser is in the normal position. With application of right pedal, the left rotor increases pitch and the right rotor decreases pitch.This causes the helicopter to turn to the right. On entering descents while maintaining the same amount of right pedal, the pilot lowers his collective lever to the full „ down „ position,and the signal rod from the collective lever automatically and mechanically causes an overcentering lever in the reverser to shift. This moves the output side of the reverser in the opposite direction from that applied by the right pedal. This action reverses the pitch between the rotors, decreasing pitch in the left rotor, and increasing pitch in the right rotor. The direction of turn is now in the same direction as pedal applied and,so far,the reverser has accomplished what it was designed to do. How ever, as in most reversing mechanisms, there is a transition area which, in this case, occurs in a neutral area, which the reverser must pass through in order to reverse the control direction.

The neutral area exists at approximately the 25 – 30 percent collective position. As the pilot lowers the collective lever from a normal powewr– on flight condition to the full „ down „ position for descents,the reverser shifts from normal to reverse. During this time, at some intermediate collective lever position, the reverser passes through this neutral area. There are times in flight when the pilot may fly at this intermediate collective stick position (partial power flight during a descent). If a pilot were to apply rudder pedal with the reverser in this area, there would be no differential collective output and all the pilot would be relving on for a turning force would be differential cyclic. Consequently, the directional control power would be less due to the lack of differential collective.

To augment this neutral area characteristic, a control linkage has been added to the differential cyclic system to augment the differential cyclic control. This is called the differential cyclic shifter.



DIFFERENTIAL CYCLIC SHIFTER ( D.C.S.

The primary purpose of the DCS linkage is to increase the output of the already present differential cyclic control ( more fore / aft tilting of the rotors ) so there will be adequate directional control whenever the reverser is in, or near, the neutral area.

Tthe result of all this mechanical mixing is that the pilot just pushes the pedal in desired direction to get normal aircraft response. Larger pedal inputs may be required, with the collective in the neutral area, to get the same aircraft response. More pedal response can be achieved by moving the collective control up or down out of the neutral area.

The pedals are also connected to the rudder which moves in direct proportion to pedal input. The rudder reduces pilot workload by increasing directional stability in forward flight.