I am sure everyone realizes to understand RC helicopter controls, we first must understand lift. Without lift, a RC helicopter or any aircraft for that matter won’t fly.
For anything to fly – the amount of lift produced has to be greater than the force of gravity pulling it back to the ground.
All things that fly, from birds to supersonic aircraft and yes helicopters too - rely on airfoils to create lift. An airfoil (the shape of all wings, or rotor blades if looked at edge on) creates lift by producing lower air pressure on the top of the wing than on the bottom.
This happens because of Bernoulli's principle as it applies to fluid dynamics which states faster moving air/liquid creates lower pressure due to the conservation of energy. To understand RC helicopter control, you don't really need to know the exact physics involved here - but you should at least know why & how lift is created when the air is moving faster across the top of a wing than the bottom.
Both the shape of the airfoil and its angle (called angle of attack - AoA) is responsible for this since the distance traveled over the top is longer than under it and the air has to speed up in order to travel over the longer top distance. In effect because of this increase in air speed and resultant lower pressure over the top of the wing, a vacuum is created above the wing sucking it upwards.
You can try a simple experiment right now to demonstrate how an air foil works and see Bernoulli's principle in action. Take a normal 8.5”x11” piece of paper and hold it between both hands length wise at the leading edge so the back edge of the paper flops down. You are now holding a very simple air foil shape (rounded leading edge followed by a long trailing edge).
Now blow down and across the piece of paper, at about a 45 degree angle. What happens? The back edge of the paper lifts up, even though you are blowing it downwards. You just created lift with your simple but effective paper wing. Ever wonder why a shower curtain gets sucked inwards when you turn the shower on? The flow of water from the shower head is creating a lower pressure area inside the shower than on the outside and the shower curtain moves towards the shower flow.
You may have noticed this as well when flying micro RC helis indoors. Have you ever wondered why the walls or windows in your house almost seem to be magnetic when your micro heli is getting fairly close to them; and no matter what you do, it's very hard or perhaps even impossible to overcome this strange magnetic wall or window attraction? It's a very real force the micro heli is experiencing, but it's got nothing to do with magnetism - it's all due to Bernoulli's principle.
When a micro heli (or any size helicopter for that matter) is near a wall, the fast moving downwash air flow from the rotor creates a low pressure area up against the wall which draws the helicotper into the wall or window. Has nothing to do with your piloting skills :-)
Helicopters don’t have wings of course – well actually they do... we call them rotor blades. You will see me interchange “wing” and “rotor” throughout this discussion. Just as long as we all realize that rotor blades behave just like wings, it will all make sense – I hope.
As long as the wing (or rotor) is moving through the air – low pressure is produced on the top of the wing and pulls the wing upwards. The faster the air flows over the wing the more lift. This is a very basic explanation – there are other forces involved such as drag and turbulence that limit the speed of air over a wing and ultimately limit the amount of lift produced.
The other component of lift is angle of attack or in helicopter control terminology “BLADE PITCH”. If you angle the rotor blade or wing up (leading edge higher than the trailing edge) you get even more lift. We call this positive or high angle of attack. What do you think we call it in the heli world? “Positive Pitch”.
This picture shows a typical symmetrical rotor blade with a special RC helicopter tool called a pitch gauge measuring the angle of attack or pitch of the rotor blade. As you can see, this blade is showing a positive pitch angle of about +13 degrees.
It might seem you could just use a flat board for a wing and just angle it upwards to get lift, but there would be so much drag and turbulence produced, the lift would be minimal for the amount of power that is being used.
In fact you could keep adding more and more power and the only result would be more and more turbulence and drag acting against the lift produced. Airfoils on the other hand are very efficient at creating lift with limited drag and turbulence - that's why they are used.
So now let’s take the air foil and pitch idea one step further. Let’s take our basic air foil example from above and create a mirror image of the air foil on the bottom of the wing. If we look at the wing edge on, the top and bottom both have the exact same air foil shape – they are symmetrical. Thus the name symmetrical wing or rotor blade – same on the top and bottom.
Having an air foil shape on both the top and bottom of a wing now means airplanes and helicopters can fly upside down. By changing the angle of attack or pitch to negative when upside down, the rotor now produces lift in the opposite direction – pretty neat.
Here is our RC helicopter pitch gauge now showing a negative pitch angle of about -9 degrees. Because this is a symmetrical rotor blade, if the helicopter was upside down, the rotor would actually be creating lift allowing the heli to fly or hover upside down.
Certainly not all rotors have symmetrical air foils – some have semi-symmetrical or flat bottomed air foils depending on how efficient the rotor has to be. Symmetrical air foils are not as efficient at producing lift as a flat bottomed air foil shape using equal amounts of power.
This is why birds don’t have symmetrical shaped air foil wings. The amount of energy to fly would be too great. I guess that is also why I have never seen a bird flying upside down for any sustained amount of time.
A quick note – when I am talking about flat bottomed air foils I am also referring to a hollowed out air foil. This is like our piece of paper wing experiment above, a kite wing, or a bird’s wing. There is no flat bottom on the wing, just the curved underside of the upper wing section.
This design is ideal for light weight micro fixed pitch applications and produces lots of lift - they are sometimes called high lift rotors.
I wanted to point this out because many micro and coaxial RC helicopters use this type of airfoil rotor design. The obvious reason is because it produces the most amount of lift for the amount of power, and has the least amount of weight (high lift to power & weight ratio).
In our helicopter hobby, most fixed pitched
helicopters will use flat bottom or hollow air foils to create the most
amount of lift for the amount of power available. As a result, smaller
engines or motors/batteries can be used for longer flight times and
lighter helicopters. This is a good compromise seeing that fixed
pitched helis can’t sustain inverted flight anyways.
Most of our single rotor collective pitch helicopters will use symmetrical shaped rotors coupled with powerful engines or motors. This is how we can produce lift when the helicopter is inverted. We simply use our helicopter controls to change the pitch angle from positive to negative.
There of course are RC helicopters with collective pitch that use flat bottomed or semi-symmetrical rotor blades to benefit specific areas of flight, such as auto rotations . However, most out of the box collective pitch, single rotor helicopter kits will come with symmetrical rotor blades because they are the best compromise for all areas of RC helicopter control.
To Sum Up:
Okay – we now understand all that, so how do we affect lift with our RC helicopter controls?
We already know that we can achieve lift by either speeding up the rotor blades or changing the pitch angle more positive or a combination of both. For our fixed pitched heli, we only have one option – increase the speed of the rotor.
On our collective pitch heli, we can speed up the rotor, increase the angle of attack, or do both. From the discussion in the fixed or collective pitch section – we know there are several advantages to using collective pitch and why it is preferred method of helicopter control over fixed pitch.
Now we can understand what is exactly going on with our helicopter controls in relation to lift.
These are very basic examples of lift theory and how it applies to helicopter controls. As we examine more areas of flight theory in the Direction And Cyclic Control section, you will see there are many other forces acting upon the helicopter that cause the amount of lift being produced by the rotors to be changing constantly.
This is why there is no such thing as a simple “up-hover-down” helicopter control button on your radio or in a real helicopter – the dynamics are always changing and you have to keep adjusting to stay in control.