A helicopter is a complex machine and what makes it more complicated is the fact that it has lots of rotating parts. The parts which concern us about this topic are – Main Rotor and Tail Rotor.
Why Do We Need Tail Rotor?
A Helicopter produces lift by rotating its rotor blades. Rotor blades have an airfoil shape which produces lift when air passes over them. For rotation of main rotor blades, torque is needed which is provided by the engine through the transmission shaft.
Helicopters are designed to being able to hover over the ground while maintaining stability. But thanks to Newton’s Third Law of Motion, which states that every action must have an equal and opposite reaction, the torque provided by the engine tends to rotate the Helicopter in opposite direction of main rotor blades. This tendency of helicopter to rotate has to be counteracted.
To counteract this rotating tendency of helicopter Tail Rotor is installed on the helicopter. This tail rotor would produce thrust to oppose the spinning of the helicopter. The tail drive shaft would transmit power from the engine to the tail rotor. This design is very popular in small and medium-sized passenger helicopters but is not widely used in heavy lifting helicopters. This is because 30% of engine power is utilized by a tail rotor which is not used to lift the helicopter.
Apart from preventing the unwanted spinning of the helicopter, the tail rotor also provides directional control for the helicopter when it is hovering. Anti-Torque paddles are used to control the tail rotor thrust by changing the pitch of tail rotor blades, which then provides directional control.
Different Types of Tail Rotor Designs
- FENESTRON: – Tail Rotor is prone to damage from FODs and is often invisible while rotating which can be very dangerous for people around the helicopter. To mitigate these flows a new tail rotor design was introduced, which is called Fenestron. This design consists of small rotors inside a covering. This design limits the possibility of damage to people and also prevents rotors from FODs. Also, the aerodynamic losses are reduced in this design.
- NOTAR: – NOTAR stands for No Tail Rotor. This design does not include any rotor but uses aerodynamics and air pressure to produce the required thrust. This design consists of a fan that is located at the attachment of the tail boom to the fuselage. The fan sucks the air from the main rotor downwash and sands it at the end of the tail boom, from where it exits from small nozzles. The tail boom also consists of two slots from where a low-pressure air is thrown out, which creates a boundary layer around the tail boom through Coanda Effect. Due to this low pressure, the downwash from the main rotor also gets attached to the tail boom and produces sideways lift. So, NOTAR counters the spinning of a helicopter by the combination of Nozzle thrust at the end of the tail boom and sideways lift force at the tail boom.
Helicopters without Tail Rotor
Coaxial Rotor Helicopter:
This design consists of two rotors, mounted one above another on concentric shafts. Both rotors are rotating in the opposite direction to each other (contra-rotating). This design does not require a tail rotor because the rotors are rotating in opposite and the torque reaction of each rotor is balanced by other rotor.
The directional control in this is accomplished by changing the collective pitch of one rotor thus changing the required torque of that rotor. This creates an imbalance between the torque of both rotors and makes the helicopter yaw in the desired direction.
Inter meshing Rotor Helicopter:
This design consists of two rotors mounted at a slight angle to each other, rotate in sync with each other without colliding. Since the rotors are contra-rotating the torque required by each rotor is in the opposite direction which removes the need for a tail rotor. Direction control is achieved similarly to coaxial rotor helicopter.
Tandem Rotor Helicopter:
Tandem rotor design consists of two rotors mounted one in front of the other. Both rotors are rotating in opposite directions to each other. Since the torque of one is cancelled by the other rotor there is no net torque reaction on the helicopter fuselage.
Directional control on this type of helicopter is achieved by applying opposite cyclic pitch to each rotor. This type of helicopter design is used in military helicopters.
Transverse Rotor Helicopter:
Transverse rotor helicopter has two rotors mounted side by side to each other. Rotors are contra-rotating to cancel out the torque reaction. Transverse rotor helicopter uses the same mechanism for yaw as Tandem rotor helicopter.
There is one more design in the helicopter, which is called Tip Jet. It uses a jet nozzle at the tip of the helicopter to rotate its blade. This design eliminates the need for a transmission shaft and thus no torque is transferred to the rotor. Because of this there no torque reaction present on the helicopter fuselage.
The helicopter designs with no tail rotor are generally used in heavy lifting helicopters as all the energy of the engine goes to main rotors which lift the helicopter.