Everybody loves the Kaman KMAX and its intermeshing rotor system is mesmerizing to watch.
But once people take a moment to think about it they become very concerned whether the rotors will collide with each other. Short answer is - they wont. They're mechanically synchronized via a gearbox and cannot get out of sync without a catastrophic mechanical failure. But thats not what this post is about. Its about the next thing that usually crosses people's minds when they watch the KMAX for long enough. How does it turn?
The answer is pretty interesting and it is actually three things going on that allows the KMAX to turn in place or "yaw". But before that I think its worth explaining how a traditional helicopter yaws.
The engine produces torque (a rotational force) to spin the main rotor. The torque force has a side effect of trying to spin the rest of the helicopter around the opposite direction. So a tail rotor is used to produce sideways thrust to counter that rotation. When the tail rotor thrust exactly equals the torque force of the engine, the helicopter remains facing forward. If the tail rotor thrust increases then the helicopter will yaw one direction (to the right with a clockwise main rotor and to the left in a counterclockwise main rotor). And if the tail rotor thrust decreases then the torque force naturally rotates the helicopter the other way.
This action is done via the left and right pedals in the cockpit which will yaw the helicopter accordingly.
The KMAX still has pedals yet no tail rotor. The intermeshing rotors spin opposite directions at the exact same rpm and so the torque force between them is neutralized. Without that variable tail rotor thrust to provide yaw, the KMAX must rely on three other methods to accomplish the same movement.
The first is it simply has a rudder.
There is a moveable rudder on the tail at the trailing edge of the vertical stabilizer. As either pedal is pushed, the rudder will swing to the left or right and deflect the airflow around it to produce a yaw reaction. This functions exactly like an airplane rudder. However its only effective at higher airspeeds. In a hover or at low speed there is little to no air flowing over the rudder which will have no effect on yaw.
For low speed flight the second and third methods are necessary. One of which is called differential cyclical pitch. As the rotor blades spin they create a continuous disc of lift. By tilting the rotor disc you can direct this lift to fly in in any direction.
Because the KMAX has two main rotors, each with their own cyclical pitch control, you can angle the discs in separate ways.
For example if the left main rotor tilted forward and the right main rotor tilted rearward the helicopter would yaw to the right around the center of mass between the two rotors. This is sort of like the motion of your hands as you steer the handlebars of a bicycle to pivot it.
The third and last method is probably the trickiest to understand and has to do with differential collective pitch. Collective pitch refers to the pitch angle of the rotor blades in unison. When the blade angle increases collectively the lift will also increase across the rotor disc.
Where there is lift, there is also drag and an increase in lift will also increase drag. This drag will try to slow the rotor rpm so the engine must increase power (and torque) to compensate. As mentioned above torque has an effect of trying to rotate the helicopter the opposite direction of the main rotor's spin.
And just like with differential cyclic pitch, the two main rotors have independent collective pitch control. One rotor can increase collective pitch while the other rotor decreases collective pitch. The torque on the higher pitch rotor will increase and the torque on the lower pitch rotor will decrease. This has an effect of yawing the helicopter opposite of the higher pitch rotor's direction of spin. Clear as mud?
So to recap with an example. A hovering KMAX wants to make a turn to the left. The pilot presses in the left pedal to begin the turn.
1) The rudder swings out to the left but because the helicopter is hovering there is no airflow over it to assist with the turn.
2) The left rotor tilts rearward and the right rotor tilts forward to rotate the helicopter around its center.
3) The clockwise spinning right rotor increases pitch and the counterclockwise spinning left rotor decreases pitch. This creates a torque imbalance that forces the nose to turn left.
All three of those things happen simultaneously with just the press of the left pedal and in vice versa with the right pedal. Pretty amazing that KMAN came up with a mechanical mixing device that can accomplish all of that with a single control input.
