anti-torque systems for helicopters
To understand torque and its effect on helicopters, we must refer to Newton’s Third Law of Motion, which states that for every action there is an equal and opposite reaction. Torque is a measure of how much force acting on an object will cause that object to rotate. Relative to helicopters, the engine turns the driveshaft, which turns the rotor. This rotation causes a torque (a rotational force). If the helicopter is on the ground, the friction between the helicopter landing gear and the ground will prevent the twisting force from rotating the fuselage. When the helicopter takes off, the frictional force is removed and if nothing is present to counteract the twisting force, the fuselage will rotate in a direction opposite to the rotors. According to Newton’s Third Law, the action of the rotors creates an equal and opposite reaction (the fuselage rotates in the opposite direction to that of the main rotors). For single rotor helicopters, counteracting the effects of torque can be achieved in one of three ways.
The first way to counter torque is with a tail rotor. A tail rotor is located in the tail of a conventional helicopter. The purpose of the tail rotor is to reduce the effect of torque and yaw motions inherent in helicopter flight. The tail rotor is made up of two or four small airfoils that the pilot can control in the cockpit by manipulating the rudder pedals (anti-twist).
Helicopters using dual rotor systems do not require the use of a tail rotor as the two rotors are designed to rotate in different directions canceling the yaw created by the other without the need for a tail rotor.
The second way to counteract torque is with a NOTAR (No Tail Rotor) system. NOTAR is a fairly new form of anti-torque system developed by McDonnell Douglas. This system eliminates the tail rotor, which makes it much safer and also means less noise is generated.
In 1975 this concept was initially created by Hughes Helicopters; however, it was not until 1981 that a prototype helicopter made its first flight using the NOTAR system. Hughes Helicopters has since been acquired by McDonnell Douglas.
The NOTAR system uses a variable pitch fan driven by the main rotor drive. This fan forces air at high speeds across the rear of the craft, creating lift and therefore increasing control.
As with any system, NOTAR has advantages and disadvantages. Its advantages include: Reduced noise levels – around 60% of conventional helicopter noise typically comes from the tail rotor. Safety: The tail rotor hits something and causes many accidents. By removing the tail rotor you eliminate this possibility and there is a significant reduction in helicopter vibration.
The disadvantages of NOTAR are that it is not as efficient as a tail rotor and helicopters using a NOTAR system will have a loss of maneuverability.
The last way to counter torque is known as a fenestron. This is actually a form of tail rotor commonly known as a fantail. A fenestron is a tail rotor that is installed inside a housing. In application, a Fenestron functions the same as a conventional tail rotor, but there are design differences.
A Fenestron has between 8 and 18 blades, compared to a standard tail rotor which has only 2 to 4 blades and offers many advantages and some disadvantages over a normal tail rotor. Advantages include: Safety – They are much safer for ground operations because the tail rotor is enclosed in a casing. More protected: foreign objects are less likely to enter the tail rotor and cause damage and reduced noise.
The disadvantages include: Higher weight and increased air resistance, which leads to higher fuel consumption. Fenestrons are also more expensive to produce and less efficient.
Anti-torque systems are vital to the safe operation of a helicopter, regardless of the type of system used.