To prevent the helicopter from spinning, a tail rotor is installed at the rear of the helicopter to provide horizontal thrust to push the tail of the helicopter in the opposite direction to which it wants to spin.
Think of when you push a paddle through the water in a canoe. You push the paddle towards the back of the canoe, but the canoe moves forward through the water. When the thrust produced by the tail rotor is equal to the torque produced by the main rotor, the helicopter will sit in a hover without sending the pilot dizzy! To get any aircraft off the ground an airfoil or group of airfoils have to produce more lift than the weight of the aircraft. The more weight, the more lift the main rotor needs to produce.
When a helicopter is in a balanced hover or in level flight the lift produced by the main rotor is equal to the weight of the helicopter. To descend, the main rotor produces less lift compared to weight, and gravity pulls the helicopter into a descent.
To adjust the amount of lift the main rotor produces, the pilot raises or lowers the Collective Control. This adjusts the pitch angle on all of the main rotor blades at the same time and creates more lift or less lift. As the Collective is raised, more pitch is applied the main rotor blades and more lift is produced.
As the Collective is lowered, less pitch is applied to the the main rotor blades and less lift is produced. When at the desired altitude the pilot does not move the Collective Control. The second task of the main rotor is to move the helicopter in any direction over the ground that the pilot wishes. This is the biggest benefit to a helicopter over a fixed-wing: Stable, accurate maneuvering over a spot.
To move the helicopter laterally in any direction, the pilot moves the Cyclic control. This mechanically tilts a device on the main rotor mast called a Swashplate. This swashplate transmits those control movements to each main rotor blade to adjust the pitch of each blade individually. When the disk tilts, it moves the upward acting force vector Lift to the side slightly and this makes the helicopter move in that direction until the Cyclic control is moved back to the center. As mentioned earlier, the main job of the tail rotor is to balance out the rotational torque created by the engine turning the main rotor.
To do this, the smaller tail rotor creates lift but in a horizontal direction — this is called Lateral Thrust. As the tail rotor turns, the pilot controls how much thrust it produces by adjusting the pitch of all the tail rotor blades together, just like the Collective on the main rotor. If you want to see our top RC Coaxial Helicopters that we like, you can view them by clicking here. If you want to know more about RC helicopters with two rotors, you will want to keep reading this article.
You can also check out this video to learn all about coaxial helicopters. RC helicopters with two sets of main rotors are coaxial RC helicopters, and these helicopters have their rotors one above the other, and no tail rotor.
Coaxial helicopters are often referred to as helicopters that are flying themselves. This means that even without having a lot of control, beginners can still fly this kind of helicopter. Advanced hobbyists would not typically prefer this kind of helicopter because it limits the amount of control that they can have over the flight and hence, frustrates them. The thing is that the less control you have over your helicopters, the less equipped you are to handle sudden situations, but this is quite normal with beginners.
Advanced hobbyists, on the other hand, would not appreciate this as they are already used to handling sudden breezes and obstacles like birds. This is why the coaxial helicopters are best suited for beginners. For advanced users, the coaxial helicopters can be quite frustrating because the advanced users would rather have more control than let the helicopter fly itself.
Tandem rotor helicopters are also two-rotor helicopters, but the layout of the rotors are different from that of coaxial helicopters. These tandem rotor helicopters come with two big horizontal rotor assemblies mounted one in front of the other. Single rotor helicopters, on the other hand, require a mechanism to neutralize the yawing movement that the single large rotor produces.
After all, the large rotor does most of the work lifting the craft into the air. It's this tail rotor that makes sure that all of that work doesn't go to waste. Tail rotors are how the helicopter counteracts the torque generated from the large central rotor. While the central lifting rotor spins incredibly fast to lift the craft, it creates a torque imbalance over the helicopter as a whole. Explained more simply, the helicopter wants to spin around to counteract the torque from the rotor.
The tail rotor balances the forces generated from the main rotor and also allows the pilot to adjust the direction the nose is pointing when the chopper is hovering.
Tail rotors are generally powered by the same driveshaft as the main rotor, allowing them to sync up. Tail rotors are either built onto the tail in a traditional design, or they are built into the tail in a fan-type configuration, called fan-tail or fenestron design. There is another design, however, that removes the second external rotor entirely and isn't a coaxial design. In this design, called the NOTAR system, a jet of air is sent through a vent on the tail of the craft to create a boundary layer of air flowing along the tail boom.
This low-pressure air changes the direction of airflow around the tail boom, creating thrust opposite to the motion created by the torque effect of the main rotor. A rotating vented drum at the end of the tail boom provides directional control.
While this is a little bit of a special case, it is technically still a two-rotor design, just a rather peculiar one. All this talk about different tail rotor designs brings us down to the root of the matter. At the end of the day, helicopters inherently have a torque imbalance , and each different tail rotor design is simply a way to manage that.
With that in mind, let's dive into more of the specifics behind why helicopters have tail rotors. When helicopters were first created, their designers faced the massive challenge of creating a craft that was able to hover while also being stable. Thanks to Newton's third law of motion, each and every action requires an equal and opposite reaction.
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. 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. The gyro can also be referred to as an accelerometer.
Through fine-tuned adjustments to the blade pitch or tail rotor speed, all the obsolete yaw is removed. 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. This feature was once rare and expensive, but now, it is implemented in almost all remote-controlled helicopter gyros.
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.
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