11 Jan Helicopter Anti-Torque Systems
To understand torque and its effect on helicopters we need to reference Newton’s Third Law of Motion which states 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. In relation to helicopters, the engine turns the drive shaft, which turns the rotor. This rotation causes a torque (a rotational force). If the helicopter is on the ground the friction between the helicopter’s landing gear and the surface will prevent the torque force from spinning the fuselage. When the helicopter becomes airborne the force of friction is removed and if nothing is present to counteract the torque force the fuselage will rotate in a direction opposite the rotors. According to Newton’s Third Law, the action of the rotors creates an equal and opposite reaction (the fuselage spinning opposite the direction of the main rotors). For single rotor helicopters, counteracting the effects of torque can be accomplished in one of three ways.
The first way to counteract torque is with a tail rotor. A tail rotor is situated on the tail of a conventional helicopter. The purpose of the tail rotor is to reduce the effect of torque and the yaw motions inherit in helicopter flight. The tail rotor is comprised of two or four small airfoils that the pilot is able to control in the cockpit by manipulating the rudder (anti torque) pedals.
Helicopters that use dual-rotor systems do not require the use of a tail rotor as the two rotors are designed to spin in different directions canceling out the yaw created by one another 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 removes 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 wasn’t 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 transmission of the main rotor. This fan forces air at high speeds through the back of the craft, which creates lift and so adds to control.
As with any system, NOTAR has advantages and disadvantages. Its advantages include: Reduced noise levels – around 60% of the noise from conventional helicopters normally comes from the tail rotor. Safety – the tail rotor striking something causes many accidents. By removing the tail rotor you remove this possibility and there is a significant reduction in the helicopter’s vibration.
NOTAR’s disadvantages are it isn’t as efficient as a tail rotor and helicopters that use a NOTAR system will have a loss of maneuverability.
The last way to counteract 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 which is fitted within a housing. In application, a fenestron performs 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 having merely 2-4 blades and offers many advantages and a few disadvantages over a normal tail rotor. The advantages include: Safety – these are much safer for ground operations because the tail rotor is enclosed in a housing. More protected – less likely for foreign objects to get into the tail rotor and cause damage and reduced noise.
The disadvantages include: Higher weight and higher air resistance which leads to increased 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.