RC Helicopter Controls Understanding Cyclic
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. OK, 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 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
is increased (assuming the helicopter is not sitting on the side of a hill). If we match the amount of thrust from the propeller to the pull of gravity, 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. 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.
Now our helicopter controls don’t really tilt the rotor – they do something even better and it is called "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 tilt the helicopter in any direction. That is why it is called cyclic pitch control or just plain cyclic. Cyclic helicopter control is responsible for controlling pitch (elevator) and roll (aileron) of the helicopter.
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 its travel 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
this is not exactly where these pitch changes happen, but you get the 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
and the swash plate. The swash plate 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.
This is a picture of a typical Hiller rotor head with the swash plate below it. The swash plate will tilt forwards & backwards - left & right transferring the corresponding cyclic commands to the rotor head. The swash plate will also move up and down if you have collective pitch to change the pitch of the main rotor blades.Forward cyclic tilts or pitches the helicopter forward. Rear cyclic tilts or pitches the helicopter backwards. Right cyclic tilts or rolls the helicopter right.Left cyclic tilts or rolls the helicopter left.
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 is lost.
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 increase the amount of lift to the rotor blades so the helicopter doesn’t start loosing 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
too. That is why a steady breeze is better than a gusty breeze. We haven’t even started talking about
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 helicopter pitch forward, you will be fine. I wanted to go into more detail than that however so you know 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 rotor blades or fly bar move to initiate backward or side ways 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.
RC helicopters are all about taking your time and not rushing. You have heard me say that before and this won’t be the last time either. It is really simple – if you rush and cut corners you will have a hard go of it. If you take your time you will succeed and have lots of fun.
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