Main Rotors

The main rotors give the helicopter its lifting and control force. The lifting force is orthogonal to the rotordisc (upwards). By tilting the rotordisc, the direction of the lifting force is controlled. Each rotor individually adds to this force. When considering the rotors, a second important force comes into play immediately: rotordrag. This operates parallel to the orientation of the rotordisc, and in the opposite direction of rotor movement. This force must be overcome by the engine.

Rotors give the helicopter its lifting capacity in much the same way as wings do for an airplane. The rotors can be viewed as rotary wings (helicopters are, indeed, often classified as 'rotary wing aircraft'). In contrast to fixed wings, rotors are much thinner and more flexible and, due to the high rotational speeds they achieve, the structural forces upon them are significant. Rotors must be able to cope with these. Another important aspect of rotor design requires account to be taken of the fact that they move about a lot. This movement is not only in the plane of rotation, but the rotors also lead and lag in this plane and flap up and down.

Rotor designs use different profiles, and they can be constructed in many different ways. The same holds true for the materials used. A common design is the use of a symmetrical profile with a D-shaped extrusion, with the remainder of the chord made up of thin sheet metal comprised of an aluminium honeycomb.  Nowadays, composite materials are frequently used, again because of their combination of strength and low density, together with their good damping qualities

Rotorhead

The rotorhead is the result of a very complex engineering effort. Its purposes are to control the amount of lift generated by the rotors, and to provide direction control by tilting the rotordisc. Both of these tasks are accomplished by changing the rotorblade’s pitch. The amount of lift is controlled by changing the blade pitch of all of the blades simultaneously by the same amount. The control that the pilot uses for this purpose is called the collective. Direction control, by tilting the rotordisc, works by changing the blade pitch of each blade individually. The amount of pitch now depends upon its position in the circle of rotation. The control used by the pilot in these circumstances is called the cyclic.

The collective and cyclic pitch settings are transferred from a stationary mechanical system (the helicopter) to a rotating one (the rotating rotors). This is carried out by a device which is constituted of two discs, called swash-plates. One swash-plate does not rotate and is called the stationary swash-plate, whereas the other does (with the main rotors). The two plates are connected to each other with a bearing. The input controls are connected to the stationary plate. The control rods which connect to the blades (in order to set the desired pitch angle) are connected to the rotating swash-plate. In this way, mechanical control input is transformed from the stationary to the rotating rotorsystem, where the desired blade angles are realised.

As a result of various physical laws, the rotorblades tend to lead, lag and flap up and down. This results in very high moments and forces in the rotorhead construction at the position where the rotors are attached to the main shaft. There are several different designs of rotorheads. One of these uses hinges to prevent the moments from being transferred to the shaft, and in another, the resulting forces are absorbed by elastic materials. Furthermore, a rotorhead must be able to withstand the high centrifugal forces placed upon it by the high angular velocity of the rotorblades.

An important property is the total mass of the rotorblades since this is a measure of the amount of kinethic energy stored in the system. A significant amount thereof makes the rotorsystem less vulnerable to low rotor RPM because it takes longer to slow the blades down.

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