Microlight » Gyroplane Technical Knowledge » Aerodynamics of Autorotation

How Do the Blades Autorotate 

Airflow up through the disk during a vertical descent provides the energy to overcome blade drag and turn the rotor. 

For simplicity, the following aerodynamic explanation is based on a vertical auto-rotative descent (no forward airspeed) in still air. 

Under these conditions, the forces that cause the blades to turn are similar for both blades regardless of their position in the plane of rotation. 

Dissymmetry of lift resulting from airspeed is therefore not a factor. 

During vertical auto-rotation, the rotor disk is divided into 3 regions:

• Driven Region
• Driving Region
• Stalled Region

The Driven Region

• The Driven Region is nearest to the blade tips and normally consists of about 30% of the radius. 
• This area produces adequate lift for flight but also a lot of drag.
• The total aerodynamic force in this region is inclined slightly behind the axis of rotation. 
• Creating a dragging force which tends to slow the rotation of the blade. 
• The size of this region varies with blade pitch setting, rate of descent, and rotor RPM.