The basic classification of helicopter types is that of the number of main rotors and their disposition. According to the number of main rotors, it is possible to classify helicopters as single rotor, dual rotor and multirotor types.
Single rotor helicopters appear in many varieties. Helicopters of the single rotor scheme have a main rotor, mounted on the main fuselage and a tail rotor mounted on the tail structure (see Figure 3). This arrangement, which
was developed Ъу B. N. Yur’yev in 1911, provides a name for one classification.
The basic merit of single rotor helicopters is the simplicity of construction and the control system. The class of single rotor helicopters includes the very light helicopters (flight weight about 500 kgf), and very heavy helicopters (flight weight greater than 40 tons). Some of the deficiencies of the single rotor helicopter are:
Large fuselage length;
A significant loss of power due to the tail rotor drive train (7 – 10% of the full power of the engine);
A limited range of permissible centering;
A higher level of vibration (the long transmission shafts, extending into the tail structure, are additional sources of spring oscillations).
Dual rotor helicopters appear in several arrangements.
Rotors arranged in tandem; this is the most prevalent arrangement (Figure 5a)
Rotors in a transverse arrangement (Figure 5b);
A cross connected rotor scheme (Figure 5c);
A coaxial rotor arrangement (Figure 5d).
The basic merits of helicopters with a tandem rotor arrangement are:
Wider range of permissible centering;
Large fuselage volume; which allows it to contain large-sized loads;
Increased longitudinal stability;
Large weight coefficient.
Helicopters with a tandem arrangement of rotors can have one or two engines, which are located in the forward or aft parts of the fuselage. These helicopters have the following serious deficiencies:
Figure 5. Dual rotor helicopters.
A complicated system of transmission and control; /8
Adverse mutual interaction between the main rotors which causes, in addition, a loss of power;
Complicated landing techniques are required in the autorotation regime of main rotors.
The following advantages are attributed to helicopters with a transverse arrangement of rotors:
Convenient utilization of all parts of the fuselage for crew and passengers, since the engines are located outside the fuselage;
Absence of harmful interaction of one rotor with the other;
Higher lateral stability and controllability of the helicopter;
The presence of an auxiliary wing, where the engines and main rotors are located, allows the helicopter to develop a high speed.
Deficiencies of these helicopters are as follows:
A complicated system of control and transmission;
An increase in size and structure weight due to the presence of the auxiliary wing.
Dual rotor helicopters with cross connected rotors have a considerable advantage over helicopters with transverse rotors; they do not have an auxiliary wing, which reduces the size and structure weight. But, at the same time, with these advantages there is a deficiency, — a complicated transmission /9
and control system.
These helicopters are not produced in the Soviet Union. They are encountered, on occasion, abroad.
The basic advantage of dual rotor helicopters with coaxial rotors is their small size. Their disadvantages:
Deficient directional stability;
Danger of collision of the rotor blades;
In the Soviet Union, there are only light helicopters with this rotor arrangement.
Multi-rotor helicopters are not widely used in view of their complex construction.
In all dual-rotor helicopters, the main rotors rotate in opposite directions. In this way the mutual reactive moments are balanced, and the necessity of having a tail rotor is eliminated. Thus the power loss from the engine is reduced.