Helicopter control systems are quite diverse, but nowadays one can single out perhaps the three most popular and used ones – the tail rotor, Fenestron and Notar. All of them are aimed at compensating for the reactive moment from the main rotor of the helicopter, are designed to prevent the helicopter from spinning around its axis, and are the most important nodes in helicopter control. They have their own distinctive structural features, pros and cons. Let’s take a closer look at each of them.
1. Helicopter tail rotor
The tail rotor is designed to control the helicopter along the course, its purpose is to counteract the reactive moment from the main rotor and prevent the helicopter from spinning, the so-called yaw in aviation – the angular movement of the aircraft relative to the vertical axis.
The tail rotor is driven by the interaction of the propeller shafts through the main rotor gearbox, creating a thrust perpendicular to the helicopter axis vertically. The pitch, that is, the angle of inclination of the rotor blades is carried out using the control pedals in the helicopter cockpit. In practice, propellers with two blades are most often used, such as those installed, for example, on the Robinson 44, but depending on the technical features, the weight of the helicopter, there are control propellers with three, four and five blades.
The tail rotor partially takes away engine power, on average up to 10 percent. The single-rotor control scheme of the helicopter is quite simple to operate, does not require significant costs in terms of maintenance and repair, in comparison with other schemes. But in turn, the control screw creates more noise compared to Fenestron or Notar.
Fenestron is a closed-type tail rotor, the meaning of the word is translated from Latin as “window” or “small window”. Fenestron is part of the keel of the helicopter, it is installed in the annular channel, as a rule, it is part of the helicopter stabilizer.
In most cases, the diameter of Fenestron is two or even more times smaller in comparison with the diameter of the tail rotor we are used to. The number of blades can be in the range from 8 to 18. Visually, the very design of the tail unit with the use of Fenestron looks much larger than with a conventional bladed tail rotor.
Fenestron has a positive effect on aerodynamic characteristics, reduces the drag of the helicopter in the air, and is safer for people operating the helicopter due to its closed type of hull. Another important quality of Fenestrons is the minimum vibration indicators due to the higher rotational speed of the blades, thereby increasing the level of comfort while piloting the helicopter. The combination of Fenestron and a main rotor with 4-5 blades reduces the vibrations of the helicopter to a minimum, which makes these types of aircraft more acceptable for medical aviation.
In some cases, more power costs are required for hovering the helicopter, in comparison with the usual control rotor. But as a rule, cars with Fenestrons initially have a greater smell of engine power compared to lighter helicopters. Having a higher level of noise performance at high frequencies, Fenestron still works quieter than a conventional tail rotor for our perception.
Notar – no tail rotor. System assignment to counteract the reactive torque, as in the case of the tail rotor or Fenestron.
On a helicopter equipped with the Notar system, you will not find the propeller you are used to. The design feature is the rotary nozzle on the tail boom. A fan is installed inside the fuselage. The air flow, partly from the main rotor, enters through the air intakes with the help of a compressor, then the fan accelerates the air flow through the tail boom to the controlled nozzles. Thus, the pilot can control the helicopter, that is, make maneuvers relative to the helicopter axis to the left and right.
Helicopters with Notar are considered one of the safest, moreover, one of the “quietest” in the world. This unit is serially installed on the helicopters of the American company MD Helicopters.