This Is Why RC Helicopters Have 2 Rotors


Why Do RC Helicopters Have Two Rotors?

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RC helicopters have what may seem to be a very bizarre design. The two rotor (or coaxial) design featured is not a common feature in modern helicopters. So, if you are expecting your RC helicopter to look exactly like a real helicopter, you may be surprised.

Why do RC helicopters have two rotors? The dual rotor design on an RC helicopter is to help the vehicle turn. These rotors spin in directions opposite to each other which ends up canceling out each other’s torque force. Once there is no torque force, the helicopter won’t spin around in a certain direction.

In this article, we will explain why RC helicopters have two rotors and what else about their design makes them different than life-size helicopters.

Why do RC Helicopters Have 2 Rotors?

RC helicopters are fixed pitch and they change their altitude by configuring the motor speed and the RPM of the main rotor. The amount of lift generated is correlated to the motor speed. The second rotor is always rotating so that smooth yaw is maintained (right or left rotation). This yaw is adjusted by the speed of the second blade relative to the primary blade.

Small remote-controlled helicopters are too small to effectively control with this method. Two counter-rotating blades are put onto the same axle of these helicopters. Sometimes, tail blades are put on more expensive models so that they have better control.

When the helicopter needs to change direction, it will change the speed of one of the rotors, so it is different than the other. The amount of torque generated will cause the helicopter to turn one way and it then can change direction.

The RC Helicopter’s Gyro

Yaw control is made easy in RC helicopters through the gyro component. The gyro is an electronic device that is connected between a tail rotor control and a receiver. The term yaw is used to describe the changing of helicopter direction by rotating the nose of the vehicle to different sides.

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The gyro can also be referred to as an accelerometer. It senses rotational movement in the helicopter that isn’t the result of signals sent to the receiver. Through fine-tuned adjustments to the blade pitch or tail rotor speed, all the obsolete yaw is removed.

Gyros can make these decisions at incredibly high speeds, so the pilot of the helicopter won’t even notice a yaw issue. They will only feel a very stable helicopter. Gyros also have settings that can be adjusted by the pilot to fit different scenarios.

Additionally, there is an upgraded version of the gyro known as a Heading Hold Gyro. Once it learns the orientation of the helicopter, it will keep that heading until the pilot puts in a manual yaw control that overrides.

After the signal is sent from the transmitter, the gyro will learn the new heading and keep the helicopter pointed in the correct direction until it gets another order.

The benefit of this is that the RC helicopters can avoid all the necessary changes in direction when they are forced around by wind gusts. The gyro knows all the effects from the weather are not changes in the yaw command by the pilot because it didn’t come through the receiver.

This feature was once rare and expensive, but now, it is implemented in almost all remote-controlled helicopter gyros.

What’s the Difference Between FP and CP Helicopter Control?

There are two forms of control in most rotor RC helicopters, collective and cyclic. Collective controls determine the altitude and cyclic controls the directions. The terms FP and CP refer to the collective pitch control of a helicopter. The cyclic pitch control technique is pretty much the same for CP and FP helicopters.

The pitch angle of the main rotor must be changed with the air for the air to flow efficiently over it. The amount of change is determined by the lift produced through the blades. The pitch angle of the blade as it moves through the air is known as the angle of attack.

The cyclic control changes the rotor disc’s pitch angle. This represents the imaginary circle that could be drawn in the air by the blades as they spin. Usually, CP and FP remote-controlled helicopters use a flybar to change the angle of the rotor disc.

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The flybar is the small rod that is located perpendicular to the main rotor blades. There is an airfoil paddle and at each end of the flybar so that it can move in response to the tilting of the swashplate. The swashplate is directly controlled by the servos and enables the translation of inputs into the motion of the main rotor blades.

Flybarless Systems

Flybars have been used on RC helicopters for many years, but as technology progresses, flybarless systems are rising in popularity. Flybar systems are being replaced by multi-axis electronic stabilization units. They may sound complicated, but they function similarly to gyros, and they can smooth out yaw control.

These flybarless helicopters look much more realistic than those with flybars because regular helicopters don’t have flybars. If you look at the rotor head of flybarless helicopters, they are much more streamlined because of the lower component count.

Hard to Control RC Helicopters

There are some very cheap RC helicopters such as the 2-channel helicopter. The downside of these is that they are very hard to control because they have no pitch control at all. They only have main and tail motor speed controls, and those controls are sometimes only on and off as opposed to actual speed changes.

The torque force of a 2-channel RC helicopter is compensated only slightly, which means the helicopter will be spinning around especially if the motor speeds have been changed.

With some practice, these helicopters can be flown in wide circles that look like real flights. The controls are also very basic and easy to use which makes them a great beginner RC helicopter. This design only works with remote-controlled helicopters.

Normal RC helicopter controls are a bit more complex to learn but they have a lot more functionality. These controls have:

  • Many different toggle switches for things like landing gears and throttle hold
  • Tail rotor left and right movement
  • Throttle and collective pitch up and down
  • Cyclic movement backward and forward as well as left and right.

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The Differences Between RC Helicopters and Life-Size Ones.

Remote controlled helicopters have curved flat surfaces instead of the solid aerofoil shape of real helicopters.

The square-cube law of physics states that, when an object grows in its size, the volume will grow faster than the surface area. This explains why bigger animals have a harder time cooling themselves and why it’s harder to build skyscrapers because they are so big.

When this law is used in the context of helicopters, it is apparent that at the scale of a remote-controlled helicopter creates tons of strength and power even if it was made sloppily and from low-quality materials.

On the other hand, a real helicopter is much larger and made to carry people and supplies. A craft of this size is really stretching the capabilities of the materials and engines that are currently available. The highly cambered airfoil of an RC helicopter would not be strong enough for a life-size helicopter blade.

Real helicopters also contain two sets of blades, they are just configured differently. Some big helicopters will have a tail blade or two sets of counter-rotating main blades at the rear or front of the helicopter.

Real helicopters could potentially be built like an RC helicopter, but they would be much less efficient and very hard to control. The main way a modern helicopter changes direction is to use the full pitch control of the primary rotor blades collectively or independently

The Takeaway

The two-rotor design of RC helicopters gives them more precision and smoother yaws, so they don’t end up spinning around in the wrong direction. When the two torque forces cancel each other out, the current directional spin is maintained. Essentially, having 2 rotors on your RC helicopter makes it able to fly properly for your enjoyment.

Matt Robbs

I love to spend time with my wife and 3 kids. There is no better way to get them off the couch and outside than for us to grab the RC cars or boat and enjoy the sunshine!

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