Why?

Because there doesn't seem to be much information on the internet explaining basic Theory of Flight for Newbies.

There's plenty of "what to do" but not alot of "why" and I suppose there are plenty of books out there but they cost money and are probably a bit too in-depth for the new RC helicopter Pilot. Especialy for those that have gone out and bought an RTF model. The manuals seem to miss out how it all works. So I'd like to try and rectify that to the best of my ability.

About me:

 I was trained by the Royal Air Force as an Airframe Technician in 1983. I left in 1996 but in-between  spent many years working on both Seaking and Wessex Helicopters including a couple of years teaching. My first job was carring out major servicing on the Wessex HC 2 at Shawbuty in 1984. I left the RAF whilst doing the same, but on the Sea King.

       A Wessex HC 2 of No. 2 FTS RAF Shawbury.                RAF Search and Rescue Sea King

  I don’t intend to spend much time talking about specific RC models. I own a Walkera Dragonfly 22E which I am trying to fly and have found that knowing what it's supposed to be doing an advantage. Forgive me if most of this is compared to real aircraft but I know more about them.

  Watch this space as I attempt to explain a few mysteries. Why do you need  a tail rotor? Why does the model want to slide to the left? Why bother setting it up?

  Where to start? Well at the beginning. Helicopter flying is a matter of balance. What do I mean  by balance? Well there are 4 forces involved in flying. Lift, weight, drag and speed or velocity. To fly lift has to overcome weight, speed has to overcome drag.

LIFT & WEIGHT.

 We all know about weight. It makes things heavy yes? So what is lift? It's what we have to create to overcome weight and get off the ground and its done using an aerofoil or wing.

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  A wing has to have air flowing over it to produce lift and there are a couple of ways of doing that. First you can push it through the air just like a fixed wing aircraft or spin it around and around like a helicopter. Move it faster and it will create more lift, hence the fixed pitch model helicopters that use rotor speed to the control lift. Wind flowing over a blade it will also create lift which is why rotor blades "sail" up and down whilst a helicopter is sat on the ground in a breeze or are tied down in the wind. Rotor blades being more flexible than wings this sailing can cause the blades to hit the fuselage.

  An aerofoil will also produce more or less lift depending on its angle relative to the air flow (angle of attack) But if this angle gets too great it will stop producing lift and "stall". This will also happen when the airspeed becomes to low. This angle on a helicopter is known as the blade pitch angle and is changed using the collective lever (the throttle stick on your model if its a collective model or on the left hand side of the pilots seat on a real one). This increases the pitch "collectively" to all the blades at the same time and not only increases the lift but causes drag to raise its ugly head.

DRAG

  The enemy! As we either increase the speed of the rotor or increase the pitch of the rotor blades we also increase drag. Its not proportional either. As we increase lift by one unit drag will increase by greater than one.

 To overcome this drag we have to add more speed or rather power. In other words push the blades around harder.

TORQUE.
  Another force which causes us budding helicopter pilots problems. Pushing the blades around is easy if the helicopter is on the ground, it can push back against that ground. Once it the air it’s a different matter. Without something to push against the energy is distributed between turning the rotor head and turning the helicopter itself. It will spin like a thing possessed! Hence the tail rotor or second set of rotor blades rotating in the opposite direction (contra-rotating). These give something to push against and can also be used to control the machine about it’s vertical axis. Ah new word maybe?

Axis

  Three axis around which a flying machine moves. The vertical axis is an imaginary line which runs on a helicopter straight down through the main drive shaft. On a fixe wing aircraft it is about 1/3 of the way back from the leading edge of the wing and straight down through the fuselage.

 The lateral axis runs at right angles to this across the aircraft left to right. The longitudinal axis runs from the front to the back.

  So when a helicopter moves it moves around these axis. What does that mean? Imagine holding a pencil and rolling it in you fingers. Hold it vertical and you can see the movement around the verticle axis. Then do the same for the others. Movement around the vetical is yaw. movement around the lateral is pitch and around the longitudinal is roll.

THE ROTOR DISC & SWASH PLATE

This is the disc that the rotors create as they spin around. You can see it quite clearly. Why is this important? Because its position effects the movement of your helicopter. Tip it in any direction and your machine will travel in that direction. This is why the center of gravity  (CofG) is so important. If the helicopter is slightly heavy towards, for instance its nose then when it lifts off the nose will drop. This in turn will pull the disc down at the front and off you go staight into foward flight. Not good if you are trying to hover. To overcome it you might add trim on your transmitter but if you do this it will always have to be there and you are reducing the available movement on the stick. The same applies for any imbalance. If the left side is heavy, you're off to the left and so on.

Just below the rotor head ( the bit that joins the blades) is a device know as the swashplate. This is the clever bit of engineering that allows you to make adjustments to the moving rotors. Normally divided into two parts. The rotating and the non rotating stars. They are both attached to a ball with a hole through the middle with the main shaft running throught it. Movement of a servo is translated through the fix star to the rotating star and then to the blades. It's position dictates the position of the rotor disc. Hence making sure it is level before flighing. If it is tilted the model will fly in the direction of the tilt. This is the cyclic control.

HOW DOES IT ALL WORK TOGETHER.

So your model is sat on the ground facing away from you. As you raise the throttle the rotor spins up. Up to about 50% that is almost all that happens. Although you are now inducing torque. As long as the giro on your model is working it should stay straight. Then from 50% up collective pitch will increase. This will increase torque and increase drag. The solution is to increase the force from the tail rotor and increase the power to overcome the drag. Then as the tail rotor force increases the model will tend to drift to the left as the tail rotor works like a propellor pushing the model that way. It's over come on the real thing by  angling the main gearbox to the right, but I've considered this on my model and it would mean major modifications to do it, so I'm afraid we're stuck with it.

  Now its down to balancing the thing in the air ! Any slight movement of the model may well tilt that rotor disc and off you go again. Thats why its so hard to hover. Then wind or forward flight brings a whole new set of problems.

LEADING AND LAGGING BLADES, BLADE DAMPERS AND FLAPPING HINGES.

So whilst hovering you are trying to hold that rotor disc level. To fly forward you are going to have to tip that disc forward. You are sharing some of  the power that is lifting the model off the ground and directing it forward. So some of the lift is now going to be used to pull the thing forward there is less to hold you up, so you may have to add a little throttle/collective to make for it. 

Now as we discussed before as the air speed over the aerofoil increases so the amount of lift it produces increases. Youre going to have to imagine this as I can't find a good picture. The blade that is moving forward towards the nose of the aircraft is also effected by the airflow generated by the forwward movement.

So a little sum. If the advancing blade is travelling at 10mph around the disc we then add the speed of the foreward movement, lets say 10 mph. The airflow over this balde it is now 20mph. Loads of lift.

But the blade that is travelling away from the nose, known as the reatreating blade is travelling with the airflow created by the forward movement.

Same sum again for the retreating blade. Travelling around the disc at 10mph. But this time we take away the forward speed. Total airflow is now 0! So we have loads of lift on one side and none on the other. Your model is going to roll over towards the reatreating blade at a great rate of knotts and probably crash! But in reality it does'nt. Why? This is where the tightness of your blades matters.