Cyclic Helicopter Controls are somewhat difficult to understand. If we look at the rotor blade one step at a time during a rotational cycle, helicopter-control will make much more sense.
Besides lift, our RC Helicopter rotor blades also produce thrust. This is what allows our helicopter to fly forwards, backwards, sideways, or any combination of those.
This is where an airplane wing and helicopter rotor blade take on different characteristics. The rotor is spinning just like a propeller and therefore not only produces lift it also produces thrust.
If you think of a rotor as a propeller, it is really easy to understand how a helicopter can fly in all directions. Okay, let’s take that complicated rotor system off our RC helicopter and replace it with a simple propeller. This propeller is on the top of our helicopter exactly horizontal & level to the ground so the thrust is pulling the heli straight up.
In this very simple example, the helicopter will lift off the ground and go straight up as the propeller speeds up or the pitch is increased (assuming the helicopter is not sitting at an angle such as on the side of a hill). If we match the amount of thrust generated by the propeller to the pull of gravity force, the helicopter now hovers.
Now let’s tilt the propeller on the helicopter a little bit with our cyclic helicopter controls; how about tilting it forward a little off perfectly level horizontal. What happens? There is still thrust pulling the heli up but there is also a small component (or vector) of thrust pulling the heli forward now. Same idea if we tilt the propeller backward – the helicopter will move backwards. These two movements are referred to as pitching. More specifically pitching forward and pitching backward.
What about side ways, exactly the same - tilting the propeller to the right will produce thrust to pull the helicopter to the right – opposite if you want to move to the left. These movements are referred to as rolling.
On the RC helicopter, we often call the spinning rotor the "Rotor Disc" because in flight, you can see this opaque disc above the helicopter. When on the ground you can also see this opaque rotor disc tilting forward, backward, left & right when giving cyclic stick commands on your radio just as if our propeller example was tilting in all those same directions - so how does that happen?
Now our helicopter controls don’t actually tilt the
rotor on the rotor mast, but they do allow the rotor disc to tilt because of "CYCLIC PITCH
CONTROL". How does it work? By changing the angle of pitch of each rotor
blade as it makes one revolution or cycle, we can vary the amount of lift (or thrust if you prefer to think of it that way) anywhere withing the rotor disc and thereby move the helicopter in any direction. That is why it is called cyclic pitch control or just
plain cyclic because the rotor blade pitch change occurs during each revolution or "cycle" of rotation.
You will often hear of aileron and elevator being used in the RC heli world since RC radios (fixed wing and heli) refer to those two channels as aileron and elevator - never cyclic roll & cyclic pitch. Helicopters of course don't have ailerons and elevators so to be correct, I always try to use the terms cyclic roll and cyclic pitch. I just wanted to mention why you will also hear aileron and elevator thrown around from time to time.
Let’s look at our forward flight example of tilting the propeller again. This time instead of tilting the propeller forward we will change the pitch angles of the rotor blade. For forward flight we will want the rotor to lift on the rear half of the rotor disc more than the front half; in effect, tilting or pitching the helicopter forward. What do helicopter pilots (RC and real) call this – forward cyclic.
You might think to achieve more lift on the back than the front; the rotor blade will have more pitch when it is at the back half of the rotor disc than the front half. However, because of the principle of gyroscopic precession this is not where these pitch changes occure, but hopefully you get the basic idea of how it works.
By increasing pitch on one half of the rotor disc and decreasing it on the opposite half, the rotor will want to tilt. Seeing that the rotor is firmly attached to the helicopter – the heli will follow along and also tilt in the same direction. Once the helicopter is tilted in that direction, the thrust coming off the rotors pulls it in that direction, just like tilting the propeller.
Notice I have only been talking about one rotor blade as it makes one revolution or cycle. This is to keep things simple in this explanation. Of course for balance reasons we need at least two rotor blades. The same pitch changes will occur to each rotor blade as it makes one revolution or cycle. It makes no difference if there are two, three, four, or five rotor blades – the same cyclic pitch changes occur to each rotor blade as it makes one revolution.
All these cyclic pitch changes are possible because of the rotor head and the swashplate. The swashplate is what makes much of helicopter control possible. It is basically a mechanical linkage system which mechanically joins the non-rotating helicopter control systems to the rotating control portions of the main rotor head.
The above photo shows a typical swashplate and rotor head. The swashplate is the heart of cyclic control. The swashplate converts the non rotating inputs from the servos going up to the bottom half of the swashplate into rotating command outputs that are transferred up to the rotor head & blade grips by the control rods. Basically, the rotor disc follows what the swashplate does. If the swash tilts forwards, so will the rotor disc causing the heli to start moving forward. On a collective pitch RC Helicopter (such as in this example), the swashplate will also move up and down to "COLLECTIVELY" change the pitch of the main rotor blades at the same time. That is why it's called "Collective Pitch".
With all these cyclic changes going on to keep the helicopter under control, the resultant lift is always changing. As the helicopter pitches or rolls in a certain direction, some of the upward lift/thrust is lost to directional thrust.
If you just imagined the helicopter started to loose its fight against gravity you are 100% correct. Unless the cyclic helicopter controls are small, you will have to also increase the amount of lift to the rotor blades so the helicopter doesn’t start losing altitude. The opposite is of course true when you level out and all the lift is now directed against the pull of gravity again. Unless you decrease rotor lift, your helicopter will start gaining altitude.
I think you are starting to see some of the changing dynamics going on that make flying a helicopter a challenge. Wind – even light breezes will push you around and change the translational lift too. That is why no wind or a steady breeze is better than a gusty breeze. We haven’t even started talking about torque yet – but the good old gyro has helped us out a lot with that one.
If this all sounds too complicated or difficult – don’t worry. You don’t have to understand it all right away. As long as you understand what the different cyclic commands do – i.e. forward cyclic makes the swashplate tilt forward and results in the helicopter pitching & flying forward, you'll be fine. I wanted to go into more detail than that however so you know exactly why it moves forward.
You will have to understand this by the time you set up your helicopter. If you push your cyclic stick forward on your radio and see the swashplate tilt backwards to initiate backward flight instead of forward flight – this information will have just saved your helicopter from a crash.
Helicopter Controls sometimes don't make much sense at first. However once you realize how it all works and how each control is so dependent on the other - it makes you realize how incredible these aircraft are. Cyclic is easy if you just remember whatever direction the swashplate tilts in, that is the direction the heli will move in.