Angle of attack
In aerodynamics, the angle of attack or angle of attack is the angle between the direction of the flowing fluid and the chord of a profile . The profile can be part of a wing , a rotor blade , a sail or a turbine blade , for example . The size of the angle of attack, together with the flow velocity, determines the size of the dynamic lift . It is therefore an important parameter when operating aircraft, wind turbines, turbines or sailboats.
In the accompanying diagram shows the relationship between angle α and the coefficient of dynamic lift C L shown. It can be seen that as the angle of attack increases, the lift initially also increases, then reaches a maximum and then decreases again. The reason for this is that when the maximum lift is reached, the flow begins to detach from the profile. If the angle of attack is increased further, the flow will be completely interrupted . This greatly reduces the lift.
In stationary flight, the angle of attack is influenced by the center of gravity and the position of the elevator . At the same angle of attack, the lift force and the air resistance are proportional to the square of the speed of the aircraft (double speed gives four times the lift force) compared to the surrounding air. In straight flight, the lift is equal to the weight of the aircraft. Therefore, slow flight requires a particularly large angle of attack. Since the lift decreases again beyond an angle of attack that is characteristic for the respective wing profile, the angle of attack of the maximum lift determines the minimum speed at which an aircraft can fly (v min ).
In a few of the aircraft angle of attack is no attitude change on changing the setting angle of the wing relative to the aircraft fuselage controlled z. B. the Vought F-8 . The angle of attack also changes if the profile is changed by extending slats or flaps or if it moves due to updrafts or downdrafts. Beyond the critical angle of attack, the lift can be increased by strakes .
The stall and glide ratio of an aircraft are directly dependent on the angle of attack and only indirectly on the speed (the minimum speed and the speed of the best glide depend on the flight weight, load factor and other factors, while the associated angle of attack is fixed). It is therefore important to measure the angle of attack. This can be done with special instruments ( angle of attack indicator ). Side threads are common on gliders on the canopy and on hanggliders on the bracing. They show the direction of the air flowing past.
In the case of helicopters , the angle of incidence of the rotor blades of the main rotor is controlled uniformly or as a function of the angle via the swash plate , which changes their angle of attack. With controllable pitch propellers , the angle of attack and thus the thrust are changed in the same way with the change of the setting angle.
Propeller and fan
Propellers and fans have large angles at the hub and small at the tip, so that the angle of attack remains as homogeneous as possible.
Modern wind turbines use the change in the angle of attack of the rotor blades to regulate power. The aerodynamic efficiency of the rotor is adjusted by reducing the lift so that the nominal power of the generator is not exceeded.
In sailing, choosing the right angle of attack is an important part of sailing trim . The angle of attack of the sail must be matched to the current sail profile (curvature of the sail), since, in contrast to conventional wings, it is changed by the use of trim devices or passively by increasing or decreasing wind pressure.
- Ernst Götsch: Aircraft technology . Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8 .
- Joachim Schult: Sailing technology . 11th edition. Delius Klasing Verlag, Bielefeld 2004, ISBN 3-87412-140-2 .