How is RC helicopter lift generated and controlled?
Great question! Without lift after all, an 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.
This is one of the most common questions I get asked by people who contact me for help. There are two primary reasons:
With those two points in mind, let's look at how lift is produced by today's RC helicopters. This should then help you understand and figure out why your heli is not overcoming the pull of gravity.
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 lift 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, the higher air pressure under the wing pushes the wing upward.
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 also creating air movement down the side of the curtain creating a lower pressure area inside the shower than on the outside. The result? The higher pressure on the outside pushes the shower curtain towards the shower flow.
You may have also noticed this as well when flying your 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? 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 has 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 lower pressure area up against the wall compared to the other side of the helicopter. This pressure differential then pushes your little heli into the wall or window. It 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.
We Need The Speed!
As long as the wing (or rotor) is moving through the air – low pressure is produced on the top of the wing and the higher pressure underneath pushes 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.
We Need The Pitch!
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 rotor blade edge on 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.
I demo how this "negative" pitch change occurs in the video below.
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 toy & micro and coaxial RC helicopters use this type of airfoil rotor design. The obvious reason is because it produces the most amount of lift with the least amount of power, and has the least amount of weight (high lift to power & weight ratio).
In our helicopter hobby, most fixed pitched helicopters (like our little micro coaxial above) 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 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.
The speed of these fixed pitched blades is altered by you the pilot, to adjust how much lift is produced. Faster head speed - more lift. Slower head speed - less lift.
Lift control by way of rotor speed adjustment can be somewhat vauge and imprecise, because motors and rotors can't speed up and slow down instantaneously.
So... If you have a fixed pitch RC helicopter that won't lift off, the problem is not enough rotor speed. Some common things to check:
With collective pitch, we change the angle of attack of the rotor blades while the speed of the rotor blade remains fairly constant to adjust how much lift is produced.
Less angle of attack - less lift. Greater angle of attack - more lift.
This method of controlling lift is both very precise and very fast.
Speed of the rotor on collective pitch RC helicopters is also adjusted, but more as a way to control power output for a specific type of collective pitch flying. The majority of lift control on all collective pitch helicopters comes from adjusting blade's angle of attack, not from the rotor speed (also referred to as head speed or rotor RPM).
Tame normal flying for example we will usually be running lower rotor RPMs, because efficiency, docile flight characteristics, and longer flight times are preferred. Opposed to maximum head speeds used for fast & agreesive aerobatic flying (3D), when maxmim power is needed over efficiency & long flights.
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 collective pitch control 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, or specific types of flying such as scale where inverted flight is not usually performed.
Once again, efficiency over performance!
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.
So... If you have a collective pitch RC helicopter that won't lift off, the problem could be not having enough blade pitch, not enough rotor speed, or a combination of both! Common things to check:
Summing Up RC Helicopter Lift
These are very basic examples of lift theory and how it applies to RC helicopter lift. 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” RC helicopter lift button on your radio or in a real helicopter – the dynamics are always changing and you have to keep adjusting the amount of lift produced to stay in control.