Helicopters are feats of human engineering. Their ability to take off and land vertically, and thereby access areas inaccessible by plane, makes them indispensable for many types of operations. And as most people already know, they owe this ability to the large main rotor at the top which produces the lift that makes them take off to the skies.
But why do helicopters have a smaller tail rotor, mounted vertically to the main rotor?
Helicopters have tail rotors to counteract the torque caused by the main rotor. In simpler terms, without a tail rotor, the body of the helicopter would start to spin in the opposite direction of the main rotor. The tail rotor simply counteracts this force, so that stable flight can be achieved.
In essence, it’s all about Newton’s third law, which states that “For every action, there is an equal and opposite reaction.”
In other words, as the main rotor starts to spin, the body will start to spin the opposite direction. And in order to keep the body from spinning, a small rotor is added to keep push against this spinning force, to keep the
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How Does A Tail Rotor Work?
Even though it might look like the tail rotor is powered by its own engine, that’s not the case. The tail rotor is geared to the main engine of the helicopter, which also powers the main rotor.
As it’s linked to the main engine, the tail rotor usually spins at a speed that is proportional to the speed of the main rotor.
In most helicopters, the ratio between the main rotor and tail rotor resides somewhere between 3:1 to 6:1.
This means that if the ratio is, for instance, 6:1, the tail rotor will spin six times as fast as the main rotor.
How Helicopter Pilots Control the Tail Rotor
Now, in order to adjust the thrust provided by the tail rotor, the pilot will push his set of pedals that control the pitch of the rotor blades. This, in turn, decides how much air is moved by the rotor and lets the pilot make fine adjustments to keep the helicopter stable in, for instance, windy conditions.
By adjusting the thrust provided by the tail rotor, it’s also possible to make the helicopter rotate around its own axis.
When the pilot pushes the left pedal, the pitch increases, and with it, thrust as well. In other words, the helicopter starts turning to the left around its own axis, away from the torque produced by the main rotor.
Similarly, the pilot decreases the pitch by pushing the right pedal, thereby making the helicopter turn to the right as it follows the torque produced by the main rotor.
What Happens if the Tail Rotor Fails?
As the rail rotor of a helicopter is what keeps it from spinning uncontrollably, a tail rotor failure could be devastating if not handled correctly.
As soon as the tail rotor is lost, the helicopter will immediately begin to spin. The severity of the spin is largely impacted by the airspeed and power setting. With higher airspeeds, the flow of air rushing past the helicopter’s body tends to prevent it from spinning. In case a high power setting is used, the body will be subjected to more torque, which will cause it to spin more.
What does the pilot need to do In case of a tail rotor failure?
It the tail rotor fails, it’s essential that the pilot immediately reduces power to the main rotor, to stop the helicopter from spinning.
Depending on the speed and the exact phase of flight, this could mean everything from completely rolling off the power, to maintaining the current power setting.
The next step will be to enter autorotation.
Autorotation is when the rotors are powered by the upward directed flow of air as the helicopter glides towards the ground.
During autorotation, the helicopter is no longer subjected to torque from the main rotor. This means that stable flight is attainable, even without the tail rotor.
The pilot then makes sure to safely glide to a suitable landing site.
If you want to read more about how helicopter pilots enter and maintain autorotation, we recommend our article answering the question ” can helicopter glide?“.
The Three Types of Anti-Torque Systems
As we have established so far, the tail rotor is a form of anti-torque system, as its purpose is to counteract the torque produced by the main rotor.
However, while normal tail rotors are the most common solution, there are other ones as well.
Let’s look at the two most common types, together with their advantages and disadvantages!
Ducted Fan/Fenestron
The Ducted fan or Fenestron could be said to be a conventional tail rotor, but with two major differences:
More and smaller blades: A Fenestron usually has somewhere between 8 to 13 blades, whereas conventional tail rotors rarely have more than 4. They are also smaller and generally spin at higher speeds than conventional tail rotors.
Protection: The blades are mounted inside a shroud. This means that they are protected from much of the turbulence experienced during flight. This makes them more efficient, as well as safer, as the shroud helps prevent tail rotor strikes against, for instance, the ground.
Advantages
- Quieter: A large share of the noise emitted by helicopters is produced as the tail rotor interferes with the airstream caused by the main rotor. As a fenestron is enclosed in a shroud, this becomes a much smaller issue.
- Safer
Disadvantages
- Heavier
- More expensive
- Less effective in strong crosswinds
NOTAR
NOTAR stands for “No Tail Rotor” and is a system that makes use of the air that’s pushed down by the main rotor blades, together with an airflow that is generated by a propeller inside the tail. The latter airflow is directed through an exit slot along the bottom right side of the tail, where it mixes with the air from the main rotors. This keeps the helicopter from spinning.
If you want to learn more about the NOTAR system we recommend you read the relevant Wikipedia article.
Advantages
- Safer: There is no rotor that can be damaged.
- Quieter
Disadvantages
- Expensive
- Heavy
Why Some Helicopters Don’t Have Tail Rotors (Dual Rotor Helicopters)
As you probably know by now, the tail rotor exists to counteract the torque from the main rotor. However, there are two helicopter types that have completely left out the tail rotor (or similar system) in favor of a second main rotor.
So how does this prevent a helicopter from spinning? Well, it’s very simple! To cancel out the torque from the first rotor, the second rotor is made to spin in the opposite direction.
This means that all of the engine power is used to generate lift, whereas some of the engine power in a single rotor helicopter goes to the tail rotor. In that regard, helicopters with two main rotors are somewhat more efficient.
Another benefit of dual-rotor helicopters is that they generally are less noisy. A large share of the noise comes from the tail rotor, as it interferes with the airflow coming from the main rotors.
With this said, let’s just quickly take a look at the two major types of dual-rotor helicopters.
Tandem Rotors
Tandem rotor helicopters are equipped with two main rotors that are mounted at either end of the helicopter’s body. By placing the rotors wide apart, a wide center of gravity is achieved. This makes them ideal for transporting heavy loads.
Even though the rotors are wide apart, they are powered by the same engine(s). This requires a complex transmission system, which increases costs.
Coaxial Rotors
In a coaxial rotor helicopter, rotors is placed on top of the first one. Just as with the tandem-rotor models, the two rotors are set to rotate in opposite directions, which ensures that the torque is canceled out and that the helicopter doesn’t spin.
Coaxial Rotor Helicopters usually are able to lift more while maintaining the same engine power, when compared to single rotor helicopters Again, this has to do with the lack of a tail rotor that takes some of the engine power to operate.
Conclusion
- Helicopters have tail rotors to counteract the torque from the main rotor.
- Using a tail rotor is just one of several ways to cancel out the torque. Some common approaches include NOTAR, Fenestrons, and dual rotor designs.
- In case of a tail rotor failure, the pilot must quickly enter autorotation to ensure that the helicopter doesn’t start to spin uncontrollably.
- Dual rotor designs come with the benefit of not having to provide power for a tail rotor, which increases the lift load while the engine power remains the same.
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