# Flow separation

coefficient of lift in dependence of the angle of incidence

as flow separation (English: stable) one designates the separation of the air flow of the surface of a flowed against article in the aerodynamics. In practice under it bearing areas, control surfaces, propeller , rotor blades are or in the engine area also meant compressor blades. It can concern equally the separation of a laminar, and a turbulent current.

## intended

flow separation [work on] for flow separation:

1. Enlargement of the angle of incidenceup to the value, at which no more lift increase takes place, at which the maximum coefficient of lift approx. max is thus reached, what at approx. 16° angle of incidence happens.
2. Increase of the speed at a subsonic profile into the transonic range. This is called in the technical language Highspeed stable.

## effects

the consequence is a decrease of the lift (with the propeller: the propulsion).

the pilot of a Starrflüglers can an flow separation purposefully cause, in order certain maneuvers to accomplish (z. B. Landing, bag flight, wobbling, aerial engagement maneuver).

ThoseEffect of an flow separation depends on the condition of the profile and the bearing area construction. With using and airliners with the construction to a good-natured „stable characteristic “one pays attention, so that when unintentional occurring the stable no abrupt lift decrements arise.

A stable condition, which mostly leads to the crash, is the so-called Deep stable with loss of the elevator effect.

## stable speed

although primarily on the angle of incidence depends, can the occurrence of an flow separation one to this angle in practice a speed assign. The smaller the speed of an airplane becomes, the more must the angle of incidenceare increased, so that the airplane without at height flies to lose straightforward. If one wants besides still without Höhenverlust a curve to fly, the angle of incidence must be increased additionally.

One calls the speed, at which it comes in the straight flight to flow separation, stable speed, or stable speed.If an airplane flies faster than slower with the stable speed, but than with the minimum speed specified by the manufacturer, then it comes into the bag flight. The current at the bearing areas is already turbulent, that lift strongly decreased, the airplane “sags through”. The current is however stillnot completely torn off and the airplane remains controllable - reduced -.

Take-off weight, position of the center of gravity and air temperature have effect on the “stable speed”. Ice beginning at the soil or during the flight at the Flügelvorderkante (this range is completely particularly responsible for the lift development) can lead to unexpected flow separation,since the profile of the carrying or control surfaces concerned changes.

## Highspeed stable

with the Highspeed stable comes it by the training of a shock wave as flow separation behind the shock wave, typical for the transonic speed range. If extremely much energy is not supplied (for example through) this condition is terminated a dive automatically, since the shock wave enormous resistance reduces produced and by it the speed again.

## one-sided flow separation

if it in the slow-speed flight by different speeds of the bearing areas (turning flight) at the slower bearing area to a one-sided flow separation comes,this leads to wobbling.

an unintentional flow separation at ground level can have dangers with an aircraft fatal consequences. By the drastically decreased lift and increased air resistance it falls “like a stone” from the sky.

With rotary-wing aircraft (helicopters) flow separation can to a sudden lift decrement at the main or tail rotor lead. Flow separation develops, because the angle of incidence of the rotor blades is too largely selected, for z. B. on overloading of the machine by an overheavy load at the load hook or by wrong flight tactics in the mountains. Only experienced pilots are in the conditions,to master such emergency situations.

## intended flow separation

the small lift with torn off current is purposefully used in some cases.

• With hanging gliders and birds a complete flow separation permits a standing landing briefly before the ground contact.
• In the paraglider haven flow separation is a means, over inTo eliminate emergency of linen unions and trailers.
• 3D-Kunstflugfiguren are flown under flow separation.
• The operating parameters of wind energy plants are in such a way laid out that with to high wind velocity the current tears off. Thus the load of the rotor can be limited without adjustment of the rotor blades (Pitchen).