Stall (fluid mechanics)

As a stall (English: stall ) is called in aerodynamics the replacement of the air flow from the surface of an object is streamed. In practice, including wings, control surfaces, propellers, rotor blades or in the engine area are also meant compressor blades. This may equally be the replacement of a laminar and a turbulent flow.

Causes

Two causes are responsible for the stall:

Effects

The result is a reduction in the buoyancy ( the propeller: propulsion ).

The pilot of a fixed-wing aircraft can specifically induce a stall to perform certain maneuvers (eg landing, stall, spin, air combat maneuvers ).

The normal response of an aircraft to a (complete) stall is a tipping or dive forward. Aerodynamically stable aircraft catch it ideally after a short time again.

The effect of a stall is dependent on the nature of the profile and the airfoil structure. For commercial and commercial aircraft in the construction of a " good-natured stall characteristics " is paid (less abrupt tipping ) so that in case of accidental occurrence no abrupt buoyancy losses in the stable.

A stable state, which usually leads to a crash, the so-called Deep stall with loss of elevator effect.

Stall Speed

The occurrence of a stall solely depends on the angle of attack. Nevertheless, it can be assigned to a speed in practice this angle. The lower the speed of an aircraft, the more the angle of attack must be increased so that the aircraft without losing altitude level flight. Will you also fly a curve yet without loss of height, the angle of attack must also be increased.

The speed at which it comes in straight flight to stall is called stall speed, stall speed, half English stall speed, or even as an English foreign word stall speed. When an aircraft flies faster than stall speed, but slower than the minimum speed specified by the manufacturer, then it gets into the stall. The flow on the wings is already turbulent, greatly reduced the lift, the airplane " bagged by". The flow is not yet completely demolished and the aircraft remains - restricted - controllable.

Weight, center of gravity and air temperature have effect on the stall speed. Ice accumulation on the wing affects the profile, thereby increasing the stall speed in unpredictable ways.

High Speed ​​Stall

When high-speed stall occurs by the typical for the transonic speed range formation of a shock wave to stall behind the shock wave. Unless it is extremely fed a lot of energy ( for example, by a nosedive ), this state is inactive by itself, as the shock wave generated enormous resistance and thereby reduces the speed.

Compressor stall

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

Stall occurs at a plurality of blades, there is a compressor pump, called surge. This results in a back flow of the air downstream from the compressor stages, until sufficient back pressure is present. This repeats itself periodically ( frequency 5 to 30 Hz ) while the counter-pressure of the compressor stage remains at the level that triggered this process.

The damage causes a compressor stall or Compressor Surge, can make an engine change required.

One-sided stall

When it comes in slow flight by different speeds of the wings ( turning flight) at the slower wing to a one-sided stall, this may result in a tailspin. For this reason, it is especially in slow flight at high angles of attack important " coordinated " to fly.

Nothing to do with the spins, the so-called spiral dive. While the flow is torn down only one side at a tailspin, it is applied to both surfaces of the spiral dive.

Hazards

When an aircraft a stall near the ground can have fatal consequences. The buoyancy loss and the increased air resistance let it fall like the proverbial stone to the ground; is no longer sufficient amount of for discharging the stall, there is a hard impact on the ground.

Therefore, many aircraft are equipped with a stall warning system that alerts the pilot to impending demolition.

For rotorcraft ( helicopters ) of the stall can lead to a sudden loss of lift on the main or tail rotor. Stall occurs if the angle of attack of the rotor blades is selected to be large. This can happen if the machine is overloaded by too heavy a load on the crane hook or wrong flight tactics in the mountains. Only experienced pilots are able to cope with such emergencies and to bring the helicopter back to a regular flight attitude.

Intended stall

The low lift at torn flow is used specifically in some cases.

• For hang-gliders and birds allowed a complete stall shortly before touching a standing landing.
• In the sport of paragliding stall is a means to eliminate linen throws and cravat in an emergency.
• 3D aerobatic figures to be flown under stall.
• The operating parameters especially from smaller wind turbines are designed so that at too high a wind speed flow stalls. This can be limited without adjusting the rotor blades, the load of the rotor.