# Flapping joint

Schematic sketch of flapping hinge

The flapping hinge ( English flapping hinge ) allowing the rotor blade of a helicopter or rotorcraft to move with respect to the rotor circle plane up or down. It prevents overloading of the rotor blade root and the rotor head . Flapping hinges allow the rotor blade to move approx. 35 ° upwards and 5 ° downwards. These movements are called "hitting". The centrifugal forces generated by the rotation of the rotor prevent the rotor blades from moving too far upwards. In normal operation only deflections of ± 5 ° are achieved. This concept comes from Juan de la Cierva .

## Aerodynamic basics

In forward flight, the flow velocity and therefore the buoyancy is at the leading rotor blade by the wind increased, while they are reduced when returning rotor blade. Forward and backward moving rotor blades thus generate asymmetrical lift.

In addition, the rotor plane is tilted by the control input of the swash plate in forward flight . In addition to the asymmetrical lift, the rotor blades therefore also perform considerable aerodynamically guided movements up and down per revolution. In addition, there are disruptive influences from different wind currents and gusts, which superimpose further flapping of the rotor blades in the rotor.

## Effects on the main rotor

The rotor blades and the rotor head or the rotor mast are therefore subjected to strong alternating bending loads with each revolution, which without the free flapping movement would lead to a limited service life and premature failure of the components. In order to relieve these components of the main rotor, flapping joints are installed that allow a limited rotor blade movement up and down. As the leading rotor blade moves upwards due to the higher lift using the flapping hinge, the angle of attack and thus also the lift are reduced. The retracting rotor blade moves downwards due to the lower lift using the flapping hinge. This increases the angle of attack and thus also the lift.

The rotor blades beat perpendicular to the rotor plane. The air resistance that occurs is usually sufficient for damping. The installation of dampers for flapping joints is therefore generally not necessary.

Cardan joint

The swivel joint and flapping joint are often installed in one assembly with a cardanic design .

The modern GRP and CFRP materials with their good elasticity and fatigue strength values ​​mean that mechanical flapping hinges on rotor blades or rotors can now be completely dispensed with. The Bo 105 was the first helicopter which, thanks to its GRP rotor blades, could do without mechanically movable flapping joints. Modern helicopters therefore often have neither mechanical flapping nor swivel joints . The rotor blades perform the flapping and pivoting movements, depending on the design, via elastic areas on the rotor head or on the blade connection arm or on the rotor blade itself.

## literature

• Holger Duda, Jörg Seewald: Flight physics of the gyroplane. Understand and calculate. Springer Vieweg, Berlin 2016, ISBN 978-3-662-52833-4 , pp. 45-46.
• R. Randall Padfield: Learning to Fly Helicopters. 2nd edition. McGraw Hill, New York NY 2014, ISBN 978-0-07-180861-3 , pp. 27-28.