Helicopters are one of the most versatile modes of transportation in existence and give the pilot complete access to a fully 3-dimensional space. Because of this, helicopters, whether RC or full size, are one of the most difficult vehicles to learn to control.
A train can travel in 2 directions: forwards and backwards.
A car can travel in 4 directions: forwards, backwards, left and right.
An airplane can travel in 5 directions: forwards, left, right, up and down.
Whereas a helicopter can travel in 6 directions: forwards, backwards, left, right, up and down. Plus it can also rotate 360 degrees in either direction and do all of that while inverted.
In a traditional full size helicopter, the pilot is facing forwards controlling the cyclic with one hand and the collective with the other.
However, in an RC helicopter, you’re not always looking the same way the helicopter is flying which makes it quite difficult to keep oriented and stay in control. I’ve even heard full size helicopter pilots say that an RC helicopter is harder to fly – though in reality it’s probably not – but keeping orientated when you’re not facing the same way the helicopter is can be extremely difficult to master.
How Do RC Helicopters Fly?
So, how does an RC helicopter fly? Someone once said that it beats the air into submission – and in fact, it kind of does.
RC helicopter flight can be broken into 5 basic components and their functions. The main rotor, tail rotor, swash plate assembly, collective control and cyclic control.
The Main Rotor
A conventional RC helicopter has its main rotor above the fuselage (its body) which consists of 2 or more rotor blades extending out from a central rotor head, or hub assembly.
The main rotor of an RC helicopter is what produces the lifting force that allows the RC helicopter to fly. The rotors on a collective pitch RC helicopter are shaped just like the airfoils of an airplane wing, only they are thinner, narrower and both sides are symmetrical.
As the rotor blades rotate through the air, they generate lift. The amount of lift generated is determined by the pitch angle (and speed) of each rotor blade as it moves through the air. Pitch angle is referred to as the angle of attack when the rotors are in motion.
Some cheaper RC helicopter models which use fixed pitch rotors, generate lift by speeding up or slowing down the motor and therefore the speed of the main rotors. This results in a much slower responsiveness of the heli, since it takes some for the motor speed to speed up or slow down. As far as I know, there aren’t any real RC helicopters which use fixed pitch available on the market anymore an don’t go picking one up at a garage sale – there’s a reason they’re no longer made.
Depending on how the RC helicopter is set up, the main rotors typically spin at a constant headspeed ranging anywhere from 1,500 RPM to 3,000RPM. Larger size .60 or .90 RC helicopters will usually have a headspeed of 1,500RPM to 2,000RPM where smaller .30 or .50 size heli’s might have a headspeed in the 2,000RPM to 3,000RPM range.
To increase thrust, or lifting power, you simply need to increase the pitch of the main rotor. On an RC helicopter, the angle of attach (pitch) can be anywhere from +15 degrees to -15 degrees, though most RC heli’s are somewhere in the +11 to -11 range.
The pitch angles of the blades are controlled by the collective and the cyclic control which are transferred to the main rotors through the swash plate.
The Tail Rotor
Since Newton’s law states that “For every force, there is an equal and opposite reaction force,” as soon as the RC helicopter leaves the ground, there is there is nothing to keep the helicopter from spinning in an opposite direction to the torque force generated main rotors.
To stop the spinning of the body, a force which counteracts the force of the main rotors needs to be applied to stop it. In a single rotor RC helicopter, this is usually done by a smaller set of rotors attached to a long tail boom called the tail rotor, which is used to control the yaw, or rotation, of the helicopter.
The amount of thrust the tail rotor produces is determined by their angle of attack.
Increasing the angle of attach (pitch) of the tail rotor blade will increase the thrust, which will push the helicopter in the same direction as the main rotor blades, while decreasing the pitch decreases the amount of thrust, allowing the natural torque force of the main rotors to take over letting the helicopter rotate in the opposite direction to the main rotors.
A gyro, either mechanical or piezo-electric, measures the difference in rotational force between the helicopter and tail rotor and adjusts the pitch of the tail rotor accordingly to hold the RC heli steady.
The tail rotor is typically mounted at a 90 degree angle from the main rotor and provides a sideways thrust which counteracts the rotational force applied by the main rotors to hold it straight.
Depending on the gearing ratios, the tail rotor typically turns 3 – 6 times faster than the main rotor.
Tail rotors in RC helicopters are typically driven by a belt or a torque tube which is powered off the main gear which also powers the main rotor. In some cheaper models, a separate motor is used to power the tail.
The Swash Plate Assembly
The swash place on an RC (or full size) helicopter is used to translate the pilots commands into the motion of the main rotor blades and / or flybar.
The swash plate assembly fits on to the main rotor shaft beneath the head of the heli and consists of one rotating and one non-rotating disc.
The lower, non-rotating disc is linked directly to the cyclic and collective controls which are controlled by servos under the command of the pilot’s transmitter inputs.
This non-rotating disc is attached by a bearing to the second rotating disc, which turns with rotor and is linked to the main rotor blade pitch horns.
The swash plate can be made to tilt in any direction according to the cyclic controls, or move up and down to change the pitch of the rotors under the collective control, which allows the pilot to control the RC helicopter in a 3-dimensional space.
The collective control raises the entire swash plate assembly as a unit. As the swash plate rises or falls, it changes the pitch (angle of attack) of all rotor blades simultaneously and to the same degree. This is known as collective control.
Therefore, when the collective control is increased, it will raise the entire swash plate assembly increasing the angle of attack. Increasing the angle of attack increases the lift of the main rotor, causing the heli to gain altitude, while decreasing the angle of attack decreases the lift.
Since all blades are changing pitch together, the change in lift remains constant throughout every full turn of the blades.
The cyclic control works by tilting the swash plate up or down and increasing the pitch angle of a rotor blade individually as they revolve, so the angle of attack on one side of the helicopter is greater than it is on the other.
As the pitch angle changes, the lift generated by each blade changes and this unbalanced lift causes the helicopter to tip towards whichever side is experiencing the least amount of lift.
This allows the helicopter to move in any direction around a 360-degree circle, including forwards, backwards, left and right or any combination of the 4.
For example, when the cyclic control is pushed forwards on your radio transmitter, the swash plate tilts forwards increasing the angle of attack (and lift) in the rear of your helicopter which causes it to move forwards.
Because of the cyclic and collective pitch control of the main rotor blades and the pitch control of the tail rotor, your engine RPM and therefore the speed of the main rotor blades, can be kept at a constant rate increasing maneuverability response time.
Your radio transmitter handles all the mixes and translations between cyclic and collective movement, so all you need to think about is which way you want your helicopter to fly, not angles of attack or swash plate tilting.Have a question about this article or anything else RC Heli related? Ask it in our brand new RC Helicopter Forum and you'll get expert answers quick.
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