Tail Rotors in Helicopters - How Do They Work, Why Are They Needed?
It's impossible for a helicopter to fly without two rotors, o actually, it's impossible for a helicopter to fly well without two rotors.
The configurations of those two rotors can change, however. Helicopters generally keep one of two different designs today, either single rotor or coaxial rotor.
The single rotor design is likely what you're most familiar with, but don't let the name fool you, this kind of helicopter actually does have two rotors, it's just that one of them is on the tail. The coaxial rotor helicopter, which we'll talk about near the end of this article, doesn't have a tail rotor, instead, it has two main rotors that lift the craft and steer as well.
Tail rotor designs
Focusing on the smaller tail rotor of single rotor helicopter designs, you might wonder why it's needed at all. 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.
Why do 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. When the rotor of a helicopter spins in one direction, there must be an equal and opposite force in the other direction. This is the root of the torque problem with helicopters. In order to get enough lift, a significant amount of torque must be applied to the main rotor, but engineers still have to solve for the equal and opposite reaction somehow.
Early helicopter designs utilized multiple rotors spinning in opposite directions. This is the same as the coaxial design we see today (also tandem, and intermeshing designs), which is an incredibly efficient, albeit complicated, method of achieving helicopter lift and solving the torque problem without the need for a tail rotor.
In these scenarios, 50 percent of the torque turns one rotor in one direction and 50 percent of the torque turns the other rotor in the other direction, making the overall torque vector of the helicopter 0.
However, while this design was initially popular, helicopter pioneer Igor Sikorsky settled on a different design, which would shape the future of helicopter designs. He developed a method of utilizing a single tail rotor mounted to the back of the helicopter to counteract the torque from the main rotor. This is now the most popular orientation in the world and has seen years of refinement and development.
While you may think that the traditional tail rotor design is the most optimum because it is the most popular, this isn't always the case. There are quite a few significant issues with tail rotor helicopters that aren't seen on coaxial or dual-main-rotor craft.
For one, tail rotors consume about 30 percent of the entire engine power of a craft. This is a major power drain, especially as helicopters' selling point is its ability to achieve lift and forward motion — something the tail rotor doesn't help with at all.
Tail rotors also tend to be very fragile and easily breakable in constrained flight, due to their placement on the back of the craft. Due to the need for helicopters to be light, the tail rotor is usually built to be just tough enough to achieve its job of countering torque. Since it isn't tasked with lifting the craft, the rotor doesn't have to be inherently strong. In its own right, this isn't a bad thing, but coupled with the fact that tail rotors are always out of sight of the pilot, many helicopter crashes are caused by the tail rotor striking something and breaking. When this happens, the helicopter loses the torque control, starts spinning, and it usually ends up crashing.
Tail rotors can also be a little bit finnicky when it comes to accurately controling the craft. Due to the various forces at play in the air around a helicopter, holding the craft steady on one bearing can be difficult in some circumstances. To counteract changes in wind velocity and direction, the tail rotor needs to speed up and slow down at a precise rate.
When it comes down to the aerodynamics that makes this happen though, tail rotors are basically just mini main rotor assemblies. With that said, there are plenty of different tail rotor designs. Below is a picture of a fenestron design, which is one of the most durable tail rotor designs.
Are there helicopters without tail rotors?
As we've mentioned before, coaxial rotor designs and dual-main rotor designs eliminate the need for a tail rotor, and they can create a safer and more stable machine.
In order to understand how a design with two coaxial rotors can far outperform other helicopters with tail rotors, we have to examine the physics at play.
By placing two rotors on a single axis and rotating them in opposite directions we get a net-zero torque around the main body of the helicopter. Through both mechanical means and electronic means, each rotor is perfectly timed and controlled to cancel out the net torque of the other rotor in real-time. This allows the coaxial craft to achieve rather significant hovering capabilities when compared to their single-rotor brethren.
And, as a side note, vertical takeoff isn't exclusive to rotorcraft. However, planes that harness that ability without rotors — such as the harrier jet — generally accomplish the task with much less efficiency and stability.
In coaxial designs, the improved ability to hover and maintain stable flight ultimately makes for better helicopters. Better helicopters mean that they are easier to control and much safer for the occupants. Theoretically, if one rotor broke in a coaxial system, the craft could still be landed safely.
Lastly, the application of coaxial rotors means that there is no inherent need for the craft to have a gyroscope to provide stability. The rotational effects of both rotors provide for a near-perfect gyroscope, improving the stability of the craft further.