Helicopter Flight Physics Lab: Understanding Dissymmetry of Lift and Blade Flapping

What Dissymmetry of Lift Is

Dissymmetry of lift is a phenomenon observed in helicopter rotors where the lift generated by one blade is not equal to that of the other, due to the varying angle of attack as the rotor blades move through their cyclic motion. This imbalance can lead to significant forces on the fuselage and tail boom, affecting the overall stability and performance of the aircraft.

Understanding dissymmetry of lift is crucial for designing efficient and stable helicopters, as it directly influences the control inputs required from the pilot.

Why Blade Flapping Occurs

Blade flapping refers to the up-and-down motion of helicopter rotor blades due to aerodynamic forces. As a blade moves through its cyclic path, the angle of attack changes, causing the blade to flap. This motion is governed by the lift generated at different points in the blade's rotation and can be described using the blade-element model.

The phase lag between flapping and cyclic pitch control is critical for maintaining stability and efficiency. The 90-degree phase difference ensures that the blade’s flapping does not interfere with its cyclic pitch movement, which is essential for controlling the helicopter’s direction.

Cyclic and Collective Trim

Cyclic trim involves adjusting the cyclic control inputs to counteract the effects of dissymmetry of lift. By tilting the rotor blades in a coordinated manner, pilots can balance the forces acting on the helicopter, ensuring stable flight. This is achieved through precise manipulation of the cyclic pitch controls.

Collective trim, on the other hand, involves changing the overall pitch angle of all blades simultaneously to adjust the helicopter’s altitude and speed. By doing so, the collective control helps maintain a consistent lift across the rotor disc, reducing the impact of dissymmetry.

Autorotation and Power Curve

Autorotation is a critical maneuver where the main rotor continues to generate lift without engine power. This occurs when the engine fails, allowing the helicopter to descend safely. The power curve, which shows the relationship between airspeed and available power, is essential for understanding how much speed can be maintained during autorotation.

The Glauert translational lift theory helps explain how the rotor system generates lift in autorotation by converting the kinetic energy of the descending rotor into potential energy, ensuring a smooth descent.

Frequently asked questions

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Everything above runs in your browser — open Helicopter Flight Physics Lab: Dissymmetry of Lift, Blade Flapping, Cyclic and Autorotation and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.