Stall (aviation)
When coating an aircraft which is angle of attack of the wing so far increased, the air flow that the profile resolves what a so-called stall ( Engl. Stable ) and thus an almost complete loss of dynamic lift a result. With conventional wing profiles, a stall occurs at an angle of attack of around 15 °, but this value is strongly dependent on the profile of the wing.
The German designation indicates the cause of this flight condition: The pilot has pulled the elevator too hard . In English usage ( stall ), on the other hand, the emphasis is on the resulting flow condition on the wing - the verb to stall generally denotes an interruption of a continuous process.
Dangers of stalling
Apart from aerobatic maneuvers and special situations (e.g. instruction flights), a stall is always unintentional. It is particularly dangerous near the ground, since the loss of lift immediately leads to a loss of altitude and the remaining altitude may no longer be sufficient to bring the aircraft back into a normal flight position. However, occurrence at a higher altitude can also harbor dangers, e.g. B. by a one-sided stall , which then usually leads to a spin , or by entering a so-called deep stall .
There are two main situations where the risk of overtaking exists: in slow flight and when flying in tight turns with particularly high G-forces. In both cases, the flight is at a high angle of attack, so even small changes in the flight attitude can lead to the critical angle of attack being exceeded.
Another cause of stall can be a sudden change in the direction of flow as a result of wind shear .
Avoidance
In many aircraft models, the designer tries to design the properties of the wings in such a way that a stall does not occur over the entire surface at the same time, but gradually from the inside out. This can be achieved, for example, by twisting the wing slightly so that the angle of attack decreases further and further towards the outside, a so-called twist . Due to this shape, the flow breaks off near the fuselage first and causes the stall warning system to respond before the entire wing is affected. In this way, a sudden loss of lift is prevented, and the ailerons also remain effective in this partially excessive flight condition.
Another constructive measure to avoid a stall or at least to limit its effects is to influence the longitudinal stability of the aircraft. Shifting the aerodynamic pressure point behind the center of gravity gives the aircraft a certain top-heaviness, which is compensated for by a downward force exerted by the elevator. When the speed drops, this force on the elevator decreases and thus leads to the lowering of the nose of the aircraft, which thus gains speed again. The same mechanism also leads to a lowering of the nose when the stall occurs, which leads to an increase in speed and thus counteracts the stall.
Attitudes
Stall speed is also possible at speeds that are significantly higher than the stall speed shown in the pilot's manual , which applies to unaccelerated straight flight. When turning, with accelerations greater than 1G downwards, the stall speed increases accordingly. In a curve with a 60 ° bank angle, for example, the downward acceleration is 2G and the stall speed increases by a factor of about 1.41 compared to straight flight. A stall at a vertical acceleration of> 1G is also referred to as an accelerated stall . The danger of an accelerated stall does not only exist in turns, but also in a go-around maneuver, in which the pilot goes too quickly into the climb. This sometimes happens unintentionally when the aircraft is still trimmed for landing and then suddenly given full thrust. The nose of the aircraft rises surprisingly quickly for the pilot, which then leads to the so-called elevator trim stall .
When catching after an overturning, it sometimes happens that the pilot wants to transition back to level flight too quickly, which can then lead to another overturning ( secondary stall ).
If the pilot overshoots in uncoordinated flight (aileron and rudder pressure are not coordinated), one speaks of a cross control stall . This type of stall occurs most frequently in the traffic pattern on a badly planned final turn for the final approach. If the pilot tries to save a curve that has been planned too tightly by excessive rudder , he can cause this very dangerous type of stall, which can often no longer be intercepted near the ground.
Interception (English: recovery )
In almost all cases it is possible to return to normal flight from the stalled state - provided there is sufficient remaining altitude. The most important action is to reduce the angle of attack again by pressing the aircraft nose down. Only then should the wings be rolled horizontally and the thrust regulated accordingly. After a normal flow over the wings has been restored, the vertical movement can be intercepted and the planned flight path resumed.
Pulling over in flight training
As part of flight training, pilots are prepared to recognize the signs of an incipient stall and to react accordingly. This also includes exercises in which the aircraft is deliberately pulled over at a safe height and then intercepted. The stall is practiced both with full thrust ( power-on stall ) and without thrust ( power-off stall ) in order to familiarize pilots with all potential situations in which there is a risk of stalling. The execution of such maneuvers is even part of the examination scope in many countries
Stall in aerobatics
In some aerobatic maneuvers, a stall is deliberately caused, i.e. deliberately 'overdrawn'. These include the spin , the snapped roll , the pushed roll , the cobra maneuver , the lomcovák and the male (aerobatics) . In some of these maneuvers the stall is deliberately used to bring about particularly drastic changes in flight attitude, for example in the case of the broken roll.
swell
- Niels Klußmann, Arnim Malik: Aviation Lexicon . 3. Edition. Springer, Heidelberg Dordrecht London New York 2011, ISBN 978-3-642-22499-7 . Page 17 ( Angle of attack ), Page 264 ( Pull over )
Individual evidence
- ^ Bill Cox: The Reality of Deep Stalls. In: Plane and Pilot Magazine. July 9, 2013, accessed March 14, 2018 .
- ↑ a b "Airplane Flying Handbook". FAA -Federal Aviation Administration, 2016, pp. 4–5 ff. , Accessed on March 15, 2018 (English).
- ↑ "Private Pilot Practical Test Standards". (No longer available online.) FAA -Federal Aviation Administration, June 1, 2017, pp. 40 ff. , Archived from the original on June 15, 2017 ; accessed on March 18, 2018 (English). Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
