Home Table of Contents

6.4 General Helicopter Aerodynamics

Quiz

Back Next
Rotor Thrust

Rotor Thrust is the component of the total reaction force, which acts along the axis of rotation. In flight, the weight of the helicopter which is hanging beneath the rotor is supported by rotor thrust, not lift directly. Since by definition lift is the component of the total reaction force which acts perpendicular to the relative airflow. Total Rotor Thrust (TRT) is the combined thrust of all the blades acting along the axis of rotation, (at right angles to the plane of rotation, or tip path plane).

Thrust, like lift, is generated by the rotation of the main rotor system. In a helicopter, thrust can be forward, rearward, sideward, or vertical. The resultant of lift and thrust determines the direction of movement of the helicopter.

The solidity ratio is the ratio of the total rotor blade area, which is the combined area of all the main rotor blades, to the total rotor disc area. This ratio provides a means to measure the potential for a rotor system to provide thrust.

The tail rotor also produces thrust. The amount of thrust is variable through the use of the antitorque pedals and is used to control the helicopter’s yaw.

Rotor Drag

Rotor Drag is the component of total reaction, which acts in the plane of rotation, parallel to the rotational flow. The helicopter's engine must overcome rotor drag to keep the blades turning. This force is called torque. Remember drag, as opposed to rotor drag, is the component of the total reaction force acting parallel to the relative airflow, and is not as directly responsible for retarding the rotor system as rotor drag.

Total Reaction

The Total Reaction is the single resultant force of the pressure acting on an aerofoil and, as a vector; it can be broken down into various component vectors.

Relative Airflow

The Relative Airflow (RAF) is the single airflow presented to an aerofoil after adding up all smaller airflow components.

Rotational Airflow

Rotational airflow is the component of relative airflow parallel to the plane of rotation, caused by the rotation of the blades.

Induced Airflow

Induced flow is the component of relative airflow at right angles to the plane of rotation. It occurs when air is being accelerated down through the helicopter disc.

Centrifugal Reaction
Thrust Centrifugal Force

Centrifugal force always acts parallel to the plane of rotation and is perpendicular (90 degrees) to thrust.

Rotor Disc
Surface Area

The Rotor Disc is the area contained within the tip path plane, swept by the rotor blades.

Disc Area
Coning Angle
Coning Angle

The Coning Angle is the smaller angle between the Tip Path Plane and the longitudinal (spanwise) axis of the blade. Coning angle depends upon centrifugal force and rotor thrust. Centrifugal force always acts parallel to the plane of rotation and acts to reduce the coning angle. Rotor thrust acts to increase the coning angle. As the coning angle increases, the area of the disc decreases. This causes a higher disc loading.