Stall

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Flow field at a stall on a profile in the wind tunnel
The lift coefficient drops sharply when a certain angle of attack is exceeded from the maximum value.

The stall ( engl. Stall is) in the fluid dynamics , the separation of the flow from the surface of an object. It plays a role in aerofoils , propellers , rotor blades , turbine / guide vanes, etc. It can be the replacement of a laminar or a turbulent flow .

causes

Two causes can cause the stall:

  • Overturning by an angle of attack that is above a value characteristic of the respective profile .
  • Increasing the speed on a subsonic profile in the area close to the sound. This is known as a high speed stall or high speed buffet .

Effects

A glider in a controlled deep stall. The high angle of attack can be recognized by the side threads on the wing tips

The result is a reduction in lift (with propellers: the propulsion). The pilot of a fixed-wing aircraft can deliberately cause a stall for certain maneuvers (e.g. deep stall, spin , air combat maneuvers ).

The normal reaction of an aircraft to a stall is to pitch (dive) forward. Aerodynamically stable aircraft take off again after a short time (→ longitudinal stability ).

The impact of a stall depends on the profile and wing construction. In the case of commercial and commercial aircraft, attention is paid to good-natured stall characteristics (less abrupt demolition) in the design so that no sudden loss of lift occurs if the stall is accidentally entered.

In the deep stall , the stall affects not only the wings, but also the elevator . This usually leads to a crash due to inability to maneuver.

Stall speed

The speed at which the flow breaks down (" stall ") in straight flight is called the break -off speed (occasionally break - off speed ), stall speed , half - English stall speed or, as a foreign word, stall speed . In most aircraft, the profile shape can be varied (e.g. by means of buoyancy aids ) in order to reduce the stall speed in slow flight.

In general, the following applies: The lower the speed of an aircraft, the more the angle of attack has to be increased so that the aircraft flies straight ahead without loss of altitude. Will you also still without loss of altitude flying a curve must be the angle of attack also increased because in turning flight also the centrifugal force must be absorbed by the lift. For each wing profile, a maximum angle of attack can be specified, above which the flow breaks off and the lift suddenly collapses.

If the speed of an aircraft in the air is above the stall speed, but also below the minimum speed specified by the manufacturer, the aircraft goes into deep stall . The flow on the wings is already turbulent, the lift is greatly reduced, and the aircraft sags. However, the current has not yet completely broken off and the aircraft remains controllable to a limited extent.

The air density , the weight of the aircraft and its center of mass influence the stall speed. Ice accumulation on the wing changes the profile and thereby increases the tear-off speed in an unpredictable way.

High speed stall

When high-speed stall occurs through the typical for the transonic speed range formation of a shock wave to stall behind the shock wave. Unless an extremely large amount of energy is supplied (for example by a dive), this state ends by itself, as the shock wave generates enormous resistance and the speed is reduced again as a result.

Compressor stall

A compressor stall is a stall on a single compressor blade inside a jet engine . The basic cause for this is an instability of the air flow within the compressor or a change in the air speed within the compressor without a compensating change in speed (which ultimately leads to a change in the angle of attack of the compressor blades).

If the flow stall occurs on several blades, it comes to a compressor pumping. Called surge . Air flows back from downstream compressor stages until there is enough pressure again. This is repeated periodically with a frequency of 5 to 30 Hertz, as long as the back pressure of the compressor stage remains at the value that triggered this process.

The damage caused by a compressor stall or a compressor surge can make it necessary to change the engine.

One-sided stall

When turning, the air flows past the inner wing more slowly than the outer one. Slow flight can lead to a one-sided stall. Then the inner wing produces significantly less lift than the outer one; the aircraft tips 'inward' without the pilot's intervention and can spin . The slower you fly and the tighter the curve radius, the greater the likelihood of a one-sided stall.

Stall stall on steep incline

The so-called spiral fall has nothing to do with the spin . While the current is interrupted on one side during a spin, it is applied to both surfaces during a spiral fall.

hazards

A sagging close to the ground can result in a crash landing or a crash. Many aircraft have stall warning systems that signal the pilot of an impending demolition. In the event of a stall at great heights, there is enough time to stabilize the flight condition again as soon as the stall has been reliably recognized. On the crashed Air France flight 447 there was a stall in storms that the crew did not recognize as such, which led to a loss of control.

In rotary wing aircraft ( helicopters ) the stall can lead to a sudden loss of lift on the main or tail rotor. The stall occurs when the angle of attack of the rotor blades is too large. This can happen if the machine is overloaded by an excessive load on the transport hook or incorrect flight tactics in the mountains.

Intentional stall

In some cases, the low buoyancy of a broken flow is used specifically:

See also

Sources and literature

Individual evidence

  1. ^ Niels Klußmann, Arnim Malik Lexicon of Aviation , Springer Verlag, Berlin Heidelberg. ISBN 978-3-540-49095-1
  2. https://aopa.de/entwicklung/upload/PDF/Publikationen/AOPA_Safety_Letter/12_ASL_Ueberzüge.pdf
  3. http://www.austrianaviation.net/detail/auch-in-der-luft-nicht-ins-schleudern-kommen/