Forces acting on an airfoil[PDF]
Airfoils (or Aerofoil) are common for helicopters and airplanes. The difference is that for helicopters the airfoils are the rotor blades attached to the main rotor and the main rotor rotates about an axis. This is not the case for airplanes where the airfoils (the wings) are fixed.
See the scheme of basic aerodynamic forces acting on an airfoil below:
It is important to have some definitions about airfoil terminology to better understand the forces acting on it
- Leading edge: The front edge of an airfoil
- Trailing edge: The rearmost edge of an airfoil
- Chord: The length of the chord line from leading edge to trailing edge
- Chord line: A straight line intersecting leading and trailing edges of the airfoil
- Mean camber line: A line drawn halfway between the upper and lower surfaces of the airfoil
The relative Airflow has two components:
- Horizontal part caused by the blades turning and movement of the helicopter through the air
→ The Rotational airflow
- Vertical part caused by the air being forced down through the rotor blades
→ The Induced Flow
Induced Flow (or Downwash )
When blade pitch angle is increased, a downward flow of air is induced through the rotor blades creating a downward component of air that is added to the rotational relative wind
Rotor blade action changes the still air to a column of descending air. Therefore, each blade has a decreased Angle of Attack (see below) due to the downwash.
Angle of attack (AOA)
The angle of attack (AOA) is an aerodynamic angle, not easy to measure. Several factors may change the rotor blade AOA and the pilot has little direct control over the AOA, except indirectly through the flight control input. An other thing important to know:
- When the AOA increases, the Induced Flow decreases and more lift is produced
- When AOA decreases, the Induced Flow Increases and more power is required to lift
Blade Angle (or pitch angle)
It is a technical angle rather than an aerodynamic angle.
- The helicopter pilot's manual, Volume 1, 2nd Edition
- FAA helicopter flying handbook
- Principles of helicopter flight, 2nd Edition
- VID 514786 - Creation
DATE OF SUBMISSION
- 12:54, 12 November 2019
- This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.
- The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.