Helicopters or rotary aircraft have always intrigued people. Did you know that every helicopter requires at least two rotors to fly? While there are many different types of rotors available, there is still a controversy over their placement.
Over the years, helicopter design has involved the use of several types of rotors including tandem rotors, single rotor, coaxial rotor or NOTOR rotors. The most typical design in helicopters is a single central rotor with an additional smaller rotor on the tail. To understand the different designs for helicopters, it is important to understand how helicopters fly. The tilt of the main rotors allows the helicopter to lift up, move forward, backwards or sideways. The engine is used to turn the rotors. Movement of the helicopter is based on the pitch or angle of the blades. The pitch or thrust that the rotation of the blades creates is what lifts the aircraft. Despite the different designs, all rotors are mounted on a shaft above the fuselage.
Two Rotors is a Must
Torque is defined as a force that causes twisting or turning. Newton's third law of motion, which defines that “for every action there is an equal and opposite action", comes into play with helicopter rotors. When torque is applied to the mast, an equal and opposite reaction occurs so that the helicopter is pulled in the opposite direction. The rotor will turn in one direction and the body of the helicopter will go in the other. Thus, there is a need to compensate – and that is usually achieved with a tail rotor. However, in a tandem rotor helicopter, two main rotors are installed. They operate in opposite directions and the problem of torque is eliminated. The rotors spin, one clockwise and one counter clockwise, the torque reactions balance and the engine can then spend 100% of the energy produced to lift the helicopter. Also in use are co-axial rotors which have two rotors that turn in opposite directions with one mounted on top of the other. Helicopters that utilize NOTOR technology do not have a tail rotor, instead they use an anti-torque system.
Tail Rotor Helicopters Problems
There are several reported problems with using a tail rotor to deal with the torque issue. Loss of power is one of those problems. Tail rotors can use up to 30% of engine power in a helicopter. Tail rotors are smaller than the main rotor and more likely to fail in the case of an accident. Pilots don't have a visual of the tail rotor and accidents happen when the tail rotor hits an obstacle the pilot can't see. If there is a loss of a tail rotor, the helicopter could lose all the anti-torque capabilities, causing the craft to rotate and crash. Tail rotors are fairly difficult to control accurately. Turbulence and crosswinds make it extremely difficult to hold a constant heading in a tail rotor.
Problems with the tail rotor dissymmetry of lift have also been reported. Dissymmetry of lift refers to a phenomenon brought about by the difference in lift that is present from advancing half of the rotor disk and the retreating the other half. Additional problems with a tail rotor include transitional lift, which develops when the helicopter advances into clean air causing extra thrust. This produces a yaw, moving the aircraft left to right. This transitional lift can make a tail rotor helicopter harder to fly. A loss of tail rotor effectiveness (LTE) has also been reported. This occurs because of the aerodynamic interaction between the main rotor and tail rotor.
Tandem Rotor Helicopters
In opposition to single rotor helicopters, tandem rotor helicopters do not suffer from uncommanded yaw. The two large horizontal rotor assemblies mounted one in front of each other are counter rotating so they cancel out the torque produced from the rotors. One of the major advantages of tandem rotors is that all the power produced by the helicopter engine can be used for lift, while in single rotor helicopters part of the power produced by the engine is used to counter the problem of torque. Tandem rotors also have a large center of gravity range and good longitudinal strength. The two sets of rotors allow for more weight with shorter blades. Disk loading, which is the comparison of the helicopter’s mass to the area of the main rotor disk, is lower in tandem rotor helicopters. Disk loading indicates how efficient a rotor system is, and the lower the loading the more efficient the helicopter. Helicopters with tandem rotors also use less power in hovering or low speed flight.
Cons of Tandem Rotor Helicopters
Noted problems with the tandem helicopters are the need for two large rotors, reauiring a complex transmission. This transmission must be linked between the rotors to ensure the rotors are coordinated so they do not hit each other. In addition, the efficiency of the rear rotor can be compromised by the front rotor. This can be lessened by boosting the distance linking the rotor hubs. Examples of tandem rotor helicopters include the Boeing CH-46 and 47. Though tandem rotor helicopters have been successful and safe, very few companies in the United States have built helicopters with the tandem design with the exception of Bell Helicopter.
References and Image Credit
Alpman, Emre and Long, Lyle N. "Understanding Ducted-Rotor Antitorque and Directional Control. Characteristics, Part II: Unsteady Simulations." Journal of Aircraft Vol. 41, No. 6, November–December 2004.
Anderson, J. D., Jr. 1997. A History of Aerodynamics And Its Impact on Flying Machines.
Image: USMC CH-46 by the US Marine Corps under public domain