Swivel joint
The swivel joint ( English lead / lag hinge ) allows the rotor blade of a helicopter or rotary wing aircraft to lead or lag behind the rotation of the rotor head . It prevents overloading of the rotor blade root and the rotor head.
The swivel joint allows the rotor blade to move with the blade tip
- to move up to 5 ° further forward than the blade holder rigidly attached to the rotor head (to move forward) and
- lagging behind (lagging) up to 30 ° behind the rotation of the blade holder.
These movements are called panning .
Aerodynamic basics
The dynamic lift increases or decreases with the same angle of attack of the rotor blades as the square of the flow velocity . 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.
Effects on the main rotor
The fact that the leading rotor blade moves upwards due to the higher lift using the flapping hinge reduces its angle of attack and thus both lift and drag . Therefore, the leading rotor blade tries to be faster than the rotor head and to lead it. The retracting rotor blade moves downwards due to the lower lift using the flapping hinge. This increases its angle of attack and thus both the lift and the air resistance. The retracting rotor blade tries to be slower than the rotor head and to lag behind it.
Without swivel joints, the rotor blades and the rotor head or the rotor mast are subjected to strong alternating loads with each revolution, which would lead to a limited service life and premature failure of the components. Swivel joints are installed to relieve these components of the main rotor.
Since the pivoting of the rotor blades causes vibrations in the entire helicopter and damping due to the air forces acting only marginally in the pivoting direction is not sufficient, pivot dampers are installed to keep the vibrations as small as possible.
The swivel joint and flapping joint are often installed in a cardanic design in a common assembly .
Modern rotor blades
The modern fiberglass and carbon fiber - materials with their good elasticity - and fatigue strength values allow today the total absence of mechanical pan and flapping hinges with blades or rotors; the Bo 105 was the first helicopter to do this.
Instead, the rotor blades of modern helicopters 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
- Chapter 6. The pivoting movement of the rotor blades. In: Walter Bittner: Flight Mechanics of Helicopters. Technology, the flight dynamics system helicopters, flight stability, controllability. 4th edition. Springer Vieweg, Berlin et al. 2014, ISBN 978-3-642-54285-5 , pp. 85-88.
Individual evidence
- ^ Walter Bittner: Flight mechanics of the helicopter. Technology, the flight dynamics system helicopters, flight stability, controllability. 4th edition. Springer Vieweg, Berlin et al. 2014, p. 35.
- ↑ a b c Niels Klußmann, Arnim Malik: Lexikon der Luftfahrt , Springer Verlag, Berlin and Heidelberg, 3rd edition 2012, ISBN 978-3-642-22499-7
- ^ Walter Bittner: Flight mechanics of the helicopter. Technology, the flight dynamics system helicopters, flight stability, controllability. 3rd, updated edition. Springer, Berlin et al. 2009, ISBN 978-3-540-88971-7 , p. 85.
- ↑ Helmut Mauch: The helicopter flight school. With flight technology for RC-HELI pilots. GeraMond, Munich 2010, ISBN 978-3-7654-7349-4 , p. 70.
- ↑ Peter Siebenhofer: Aerodynamics using the example of the helicopter. Physics thesis. Bundesgymnasium / Bundesrealgymnasium Knittelfeld, p. 13.