Beginner’s Theory Part II
Now that we have an idea of how the helicopter basically functions we shall go into slightly more detail. The point of this exercise is to make sure the modeler has an ideal of how the helicopter control system should function. There are many different types of main rotor control systems available. What all helicopters have in common is the manner in which the rotor head is controlled. If the main rotor controls are checked for correct operation at the main rotor head, than all helicopters may verified in the same manner.
To tilt the rotor disk we cause the blades to change pitch cyclically in a synchronized manner through the swashplate. When maximum and minimum cyclic pitch are applied on opposite blades there will be a delayed action in that the disk will actually deflect 90 degrees later in the direction of rotation.
While gyroscopic laws may be used to explain this by analogy it is really the aerodynamic force that is delayed in reaction. In other words it takes time for the blade to react to cyclic pitch change.
What this may mean for the “green horn” when checking control displacement direction is that the cyclic action might appear to be happening early, when in fact it is functioning correctly. For an example, the forward tilt (forward cyclic) of the rotor disk will require minimum pitch applied over the right side and max cyclic pitch to be applied over left side of the heli.
This is for a counter clockwise rotating rotor when viewed from the top. Using this methodology for left and right cyclic (roll cyclic) one will see a maximum cyclic pitch change occur over the nose and tail. This paragraph needs to become a mind set since all main rotors behave in this manner.
The confusing part about helicopters is that different manufacturers achieve the cyclic control we described at the rotor head through various manners. For some the swashplate will tilt forward for a forward disk deflection, for others the swashplate will tilt at a 45 degree forward quadrant for the same forward disk tilt.
The reason this 45 degree system works is due to the fact that the rotating cyclic control system is adjusted by 45 degrees to compensate. This puts the cyclic pitch change back in the correct synchronization or “phasing”. The majority of helicopter designs use the forward tilt of the swashplate for a forward disk deflection.
Most common or garden variety models use two methods of operating the cyclic. These two methods are mixed together through the rotating controls. The mechanisms by which this mixing happens are called mixer levers. Each blade has its own mixer. The two parameters to be mixed are the direct cyclic control described earlier and that of the flybar.
The flybar is also called a “Hiller Servo Rotor”. This is because a man named Stanley Hiller invented it. Basically the same swashplate cyclic action controls and causes the paddles or small airfoils to fly high or low in a synchronized manner just like the main rotor disk. The neat thing about the flybar is when it tilts it is also connected to the blade pitch horns through the mixers.
This causes an additional cyclic factor to be input to the rotor blades. The flybar tilt through the mixers applies the correct cyclic control to the blades. Think of it like power steering, the swashplate flys the Hiller rotor which in turn flys the main rotor.
Here comes the hard part regarding a basic understanding of the Hiller portion. The Hiller paddles like the main blades take 90 degrees (1/4 revolution) after the maximum paddle pitch change to reach their highest and lowest deflection (flybar teeter). When this point is reached the maximum Hiller steering factor (output) is fed to the blades through the mixers. The blades then take another 90 degrees to react. From the maximum cyclic paddle pitch change it takes a total of 180 degrees for the main rotor disk to finally react.
Another thing worth noting about the Hiller portion of the control system is that collective is not applied since this servo rotor supplies no lift to the helicopter. The way we can prevent this from occurring is by means of a device called a “washout”. The washout is located between the swashplate and the flybar. The washout allows the swashplate to rise and fall for collective without binding at the paddle pitch control linkages. It also transfers cyclic from the swashplate to the paddle pitch control rods.
Most model helicopters have a washout. As a secondary function the washout drives the swashplate rotation as it connects to the mast. It is allowed to slide up and down on its driving pins. The pins are either located in the rotor hub or clamped to the mast by other means.
Now back to the mixers. The mixers also allow collective to be fed through to the rotor blades and still maintain any commanded cyclic factor. Mixers may be located on the flybar carrier or on the blade grip pitch horns. Since they are essentially levers, various adjustments are sometimes offered by way of other control rod mounting positions.
There are two common methods of controlling the heart of the helicopter. The rising and falling swashplate action used to control the collective may be handled by one servo or several. This depends upon the type of control system. You could have a conventional type control system with one roll, one fore/aft and one collective servo which all operate independently of each other.
The conventional system uses mechanical mixing through levers, mixer devices (moving servo trays or rocking servos) and associated belcranks prior to the swashplate control horns (swashplate input). This is sometimes called mCCPM
You might have the system called eCCPM which uses all three servos to raise and lower the swashplate and also share cyclic control duties. This mixing of both cyclic axis and collective is done electronically in the radio prior to the swashplate control horns.
There is a third control system which uses a separate servo for each control parameter but it has no mechanical mixing taking place prior to the swashplate control horns. The swashplate does not rise and fall for collective control. A separate rotating control rod runs up the mast to a rotating collective mechanism. The rod may run inside a hollow mast or inside a long slot machined along the mast length.
The next segment, Part III in the beginners section will deal with the flybar and swashplate in more detail. It shall also cover how forward airspeed affects the rotor.