Stall Without Flow
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Idealised, exaggerated representation of a beginning lift collapse (stall) |
What we call a "stall" can be described in German as a "lift collapse". Such a "collapse" does not happen purely by chance but is essentially dependent on the angle of attack in addition to the profile surface of the airfoil. This is very similar for most airfoils (with rounded nose approx. 18°, at flate plate 12-15°). In practice, it is then said that it was flown too slowly. From a physical point of view, however, it is primarily a question of the angle of attack.
A higher angle of attack (e.g. pulling on the control stick) leads to more lift and drag, but also to lower speed. The pilot can read the speed from his instrument (also audible from the wind noise), which is the reference for him. The decisive factor, however, is the angle in relation to the flight path. To reduce the risk of a "collapse", it is therefore advisable to approach a little faster (+ half the gust speed) when landing in gusty conditions.
Increasing the angle of attack causes a forward shift and intensification of these pressure areas. By continuing to pull, the "pressure cushion" becomes stronger and spreads further. The "pressure deficit" increases due to the even greater drop in the backshape of the profile. But what happens if you "overpull"?
Now, at the latest, we have to look at the pressure conditions at the end of the profile. The sums of pressure energy and kinetic energy (total pressure) of the trailing edge upper side and the trailing edge lower side are out of balance. While the pressure energy increases on the underside, the kinetic energy remains almost the same and on the upper side the kinetic energy (downward direction vector) increases but the pressure energy decreases significantly. the total pressure on the underside of the profile now predominates, e.g. at the end of the profile.
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Lift collapse (stall) in the flow tunnel - not in real flight |
Air now flows from behind around the trailing edge of the profile in the direction of flight, the pressure difference decreases - lift collapse (stall) occurs. This is not a sudden event, nor is it a complete collapse of lift. Depending on the profile, the event can happen abruptly, but it can also happen more gradually, so that the pilot notices the collapse of lift and can restore the glider to a flyable condition by immediately applying pressure (see lift diagram of corresponding profiles). Glued-on woollen threads in the stall flight test show this filling / flowing back.
The real movement of the pushed air particles could be shown in a sequence of images of the individual air particles. Alternatively, you can show a colored pressure distribution in which the direction vectors of the air particles (intensity by length of the arrows) are drawn. The total pressure (pressure energy + and kinetic energy) is shown on each directional arrow with a more or less large circle.
Controlled stalls (starvation - slow pull on the control stick with sufficient altitude) are generally not really dangerous if there is no spin tendency. In uncontrolled stalls, the "collapse" usually occurs on one side, triggered by an increase in the angle of attack. Caused, for example, by turbulence (with an upward component) in speed ranges close to the lower limit or in slow turns, e.g. when turning downwind. Uncontrolled stalls must always be avoided, as they are potentially life-threatening for us normal pilots with our airplaines, except we are aerobatic pilots.
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