Rotor (Aeronautics)
Overview
A rotor in aeronautics refers to the rotating part of a helicopter that generates lift. It consists of a mast, hub and rotor blades. The mast is a cylindrical metal shaft that extends upwards from the helicopter's transmission. At the top of the mast is the attachment point for the rotor blades called the hub. The rotor blades are then attached to the hub, and as the hub spins, so do the blades. The spinning blades generate lift which allows the helicopter to fly.
Rotor Types
There are two main types of rotors used in helicopters: the semi-rigid rotor and the fully articulated rotor.
Semi-Rigid Rotor
A semi-rigid rotor system has two blades which are rigidly attached to the rotor hub. The hub is then able to tilt and move as a single unit. This type of rotor system is simple and lightweight, making it ideal for small, light helicopters.
Fully Articulated Rotor
A fully articulated rotor system has three or more blades which are attached to the rotor hub with multiple hinges. These hinges allow the blades to move independently of each other, which provides greater flexibility and control. This type of rotor system is more complex and heavier, but it is also more stable and capable of handling larger, heavier helicopters.
Rotor Dynamics
The dynamics of a rotor system are complex and involve several key principles of aerodynamics and physics. These principles include lift, drag, centrifugal force, and gyroscopic precession.
Lift
Lift is the force that opposes the weight of the helicopter and allows it to rise into the air. It is generated by the rotor blades as they spin and move through the air. The shape of the rotor blades - flat on the bottom and curved on the top - causes the air pressure on top of the blades to be lower than the air pressure underneath, which creates lift.
Drag
Drag is the force that opposes the forward motion of the helicopter. It is caused by the resistance of the air against the rotor blades as they spin. There are two types of drag: parasite drag and induced drag. Parasite drag is caused by the friction of the air against the surface of the rotor blades, while induced drag is caused by the generation of lift.
Centrifugal Force
Centrifugal force is the force that pulls the rotor blades outward as they spin. It is this force that keeps the rotor blades extended and counteracts the bending forces that are exerted on the blades as they lift the helicopter.
Gyroscopic Precession
Gyroscopic precession is the principle that a force applied to a spinning object will result in a movement 90 degrees in the direction of rotation. In a rotor system, this means that a change in pitch applied to a rotor blade will result in a change in lift 90 degrees later in the rotation.
Rotor Control
Controlling a rotor system involves changing the pitch of the rotor blades to alter the amount of lift they generate. This is done using a combination of the collective and cyclic controls.
Collective Control
The collective control changes the pitch of all the rotor blades simultaneously. Increasing the collective increases the pitch of the blades, which increases lift and causes the helicopter to rise. Decreasing the collective decreases the pitch of the blades, which decreases lift and causes the helicopter to descend.
Cyclic Control
The cyclic control changes the pitch of the rotor blades individually as they rotate. This allows the pilot to control the direction of the helicopter by tilting the rotor disc in the desired direction.