## Saturday, January 7, 2023

In this lesson I go over how Cyclic Feathering and Blade Flapping compensate for dissymmetry of lift.  As a quick summary, dissymmetry of lift is the difference in lift that exists between the advancing and retreating side of the rotor disk.  If this were allowed to continue without compensation, there would be no way to maintain forward flight. The advancing side would continue to produce more lift than the retreating side and this would produce a constant rolling motion to the left.  But luckily helicopters are designed to compensate for this using blade flapping and cyclic feathering. These allow the pilot to reduce some of the lift of the advancing blade while simultaneously increasing the lift of the retreating blade.

Blade Flapping occurs when the advancing blade flaps up in its plane of rotation while the retreating blade flaps down.  This up flap velocity results in a downward flow of air (also known as induced flow) over the airfoil.  This shrinks the angle between the resultant relative wind and the chord line thereby reducing the angle of attack in the blade. Simply put, this reduces lift generated from the advancing blade.  The reverse is true for the retreating blade. The retreating blade has a down flap velocity resulting in an upwards flow of air on the airfoil. This increases the angle between the chord line and the resultant relative wind thereby increasing the angle of attack in the blade. This increase in angle of attack results in increased lift being produced by the retreating blade.

Cyclic feathering occurs when the pilot introduces a control input using the cyclic.  In the case of forward flight, the pilot would want more lift generated on the aft portion of the rotor disc than on the forward portion of the rotor disk. Due to gyroscopic precession this input must be made 90° prior in the axis of rotation. This results as an increase in pitch at the 9 o'clock position and decrease in pitch at the 3 o'clock position. This results in a greater angle of attack in the retreating blade and a lesser angle of attack and the advancing blade. Thus, this is offsetting dissymmetry of lift.

In this lesson I go over how Cyclic Feathering and Blade Flapping compensate for dissymmetry of lift.  As a quick summary, dissymmetry of lift is the difference in lift that exists between the advancing and retreating side of the rotor disk.  If this were allowed to continue without compensation, there would be no way to maintain forward flight. The advancing side would continue to produce more lift than the retreating side and this would produce a constant rolling motion to the left.  But luckily helicopters are designed to compensate for this using blade flapping and cyclic feathering. These allow the pilot to reduce some of the lift of the advancing blade while simultaneously increasing the lift of the retreating blade.

Blade Flapping occurs when the advancing blade flaps up in its plane of rotation while the retreating blade flaps down.  This up flap velocity results in a downward flow of air (also known as induced flow) over the airfoil.  This shrinks the angle between the resultant relative wind and the chord line thereby reducing the angle of attack in the blade. Simply put, this reduces lift generated from the advancing blade.  The reverse is true for the retreating blade. The retreating blade has a down flap velocity resulting in an upwards flow of air on the airfoil. This increases the angle between the chord line and the resultant relative wind thereby increasing the angle of attack in the blade. This increase in angle of attack results in increased lift being produced by the retreating blade.

Cyclic feathering occurs when the pilot introduces a control input using the cyclic.  In the case of forward flight, the pilot would want more lift generated on the aft portion of the rotor disc than on the forward portion of the rotor disk. Due to gyroscopic precession this input must be made 90° prior in the axis of rotation. This results as an increase in pitch at the 9 o'clock position and decrease in pitch at the 3 o'clock position. This results in a greater angle of attack in the retreating blade and a lesser angle of attack and the advancing blade. Thus, this is offsetting dissymmetry of lift.