What exactly is "stall"

[chornedsnorkack]

What happens to an airfoil at "stall"? And how are "lift" and "drag" defined at high angles of attack?

Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust - no engines installed or engines shut down.

Absence of thrust means that in steady state, the plane is descending.

The plane might fly at "best glide" AoA. It would then have appreciable forward airspeed, modest rate of descent and good L/D ratio.

If the AoA is any lower, the total airspeed would be higher, L/D would be smaller and rate of descent would be higher.

Now, let´s increase the angle of attack. The forward speed would decrease, and so would L/D. But what happens to RoD?

On the other hand, imagine if the plane is not flying at all: imagine that it has AoA of 90 degrees!

A plane with zero forward airspeed still cannot drop out of the sky at any high speed. After all, as presumed above, it has low weight and large wing. Fast RoD at 90 degrees AoA would mean huge drag. The plane has to reach a steady state at a modest rate of sink and no forward speed. It would be "parachuting" vertically down.

How does the RoD of a parachuting airfoil compare with RoD of a stalling airfoil?

What happens if an airfoil is held at AoA of 80 or 70 or 60 degrees? It should still have a modest RoD - but it should also have a small but nonzero forward speed.

Can someone explain what really changes about the airfoil behaviour if you compare non-vertical parachuting (in "stalled" AoA) with flying "at the back of the power curve", at AoA slightly below "stall"?

[Andries]

Stall occurs when the airflow over the wing doesn't follow the shape of the wing anymore. in other words, when the airflow lets go of the wing.
The 25° angle of attack is a good example of complete stall.

From wikipedia :

Stalling an aeroplane

If attempting the stall for flight training purposes, be sure to carry out correct checks before hand such as the HASEL check. This ensures that the engine is in the right condition and the area around the aircraft is safe and acceptable.

An aeroplane can be made to stall in any pitch attitude or bank angle or at any airspeed but is commonly practised by reducing the speed to the unaccelerated stall speed, at a safe altitude. Unaccelerated (1g) stall speed varies on different aeroplanes and is represented by colour codes on the air speed indicator. As the plane flies at this speed the angle of attack must be increased to prevent any loss of altitude or gain in airspeed (which corresponds to the stall angle described above). The pilot will notice the flight controls have become less responsive and may also notice some buffeting, an aerodynamic vibration caused by the airflow starting to detach from the wing surface.

In most light aircraft, as the stall is reached the aircraft will start to descend (because the wing is no longer producing enough lift to support the aeroplane's weight) and the nose will pitch down. Recovery from this stalled state usually involves the pilot decreasing the angle of attack and increasing the air speed, until smooth air flow over the wing is resumed. Normal flight can be resumed once recovery from the stall is complete. The manoeuvre is normally quite safe and if correctly handled leads to only a small loss in altitude. It is taught and practised in order to help pilots recognize, avoid, and recover from stalling the aeroplane.

The most common stall-spin scenarios occur on takeoff (departure stall) and during landing (base to final turn) because of insufficient airspeed during these manoeuvres. Stalls also occur during a go-around manoeuvre if the pilot does not properly respond to the out-of-trim situation resulting from the transition from low power setting to high power setting at low speed. Stall speed is increased when the upper wing surfaces are contaminated with ice or frost creating a rougher surface.

A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a spin. A spin will occur if an aircraft is stalled and there is an asymmetric yawing moment applied to it. This yawing moment can be aerodynamic (sideslip angle, rudder, adverse yaw from the ailerons), thrust related (p-factor, one engine inoperative on a multi-engine non-centreline thrust aircraft), or from any number of possible sources of yaw.

Since most aircraft have an engine, some confusion exists between an aerodynamic versus engine stall. Many people seem to believe that an aircraft will drop out of the sky as soon as the engine stops in flight. In reality, the pilot can simply lower its nose to generate enough airspeed to maintain lift over the wings and so prevent a stall. The aircraft will then descend at a steady airspeed. The pilot then has time to find a suitable landing area or to restart the engine.

Put differently, all powered aircraft (even the biggest ones) become gliders when they lose all thrust. There have been cases of airliners running out of fuel at high altitude that landed successfully at airports a hundred kilometres away. However the distance which an aircraft can glide is directly related to the airspeed, but most of all the density altitude which the aircraft is at. The Gimli Glider is a celebrated example.

Stalls can occur at higher speeds if the wings already have a high angle of attack. Attempting to increase the angle of attack at 1g by moving the control column back simply causes the aircraft to rise. However the aircraft may experience higher g, for example when it is pulling out of a dive. In this case, the wings will already be generating more lift to provide the necessary upwards acceleration and so there will be higher angle of attack. Increasing the g still further, by pulling back on the control column, can cause the stalling angle to be exceeded even at a high speed. High speed stalls produce the same buffeting characteristics as 1g stalls and can also initiate a spin if there is also any yawing.

Symptoms of an approaching stall

One symptom of an approaching stall is slow and sloppy controls. As the speed of the aeroplane decreases approaching the stall, there is less air moving over the wing and therefore less will be deflected by the control surfaces (ailerons, rudder and elevator) at this slower speed. Some buffeting may also be felt from the turbulent flow above the wings as the stall is reached. However during a turn this buffeting will not be felt and immediate action must be taken to recover from the stall. The stall warning will sound, if fitted, in most aircraft 5 to 10 knots above the stall speed.

Stalling characteristics

Different aircraft types have different stalling characteristics. A benign stall is one where the nose drops gently and the wings remain level throughout. Slightly more demanding is a stall where one wing stalls slightly before the other, causing that wing to drop sharply, with the possibility of entering a spin. A dangerous stall is one where the nose rises, pushing the wing deeper into the stalled state and potentially leading to an unrecoverable deep stall. This can occur in some T-tailed aircraft where the turbulent airflow from the stalled wing can blanket the control surfaces at the tail.

http://en.wikipedia.org/wiki/Stall_%28flight%29

Greetz,

Andries

[SFo]

Stall is dependant of the AoA, not the speed. There is often reference to the 'stall speed' but only because in level flight the AoA depends only on the speed (if configuration stays the same, of course). A plane could stall at high speed if the AoA get too big (pulling the stick very hard).

After Cl Max, the L drops to almost 0 because there is no significant pressure difference bewteen the sides of the wing. Of course, a 'parachuting' aircraft would still be slowed down by its wings, but that just resistance of the air like for any other object, not lift. That's falling, not flying.

If you increase the AoA of a stalled wing, the forward speed might not increase (because you increase D) but L will drop a bit (it's already almost 0) and D will increase. RoD will increase too. Remember that best glide gives you the longest gliding distance, not the lowest RoD (which would correspond to Cl Max and give the longest gliding time). A plane will always have the same glide distance from a given altitude, regardless of its weight. But the glide time will be different.

[Bracebrace]

The lift of a stalled wing does not necessarily drop to 0. The lift drops faster with increase of AoA, that's true. Remember, the curve lift vs AoA continues above the stall AoA, although you notice the lift dropping. Ie in a spin, both wings create lift indeed, it's the difference in amount of lift on both sides that creates the autorotation.

If an aircraft flies down "nose down" 90°, it is very well possible to achieve a state of 0° AoA. If an aircraft flying forward is put in a 90° AoA state (should be nose up then), you achieve the same as you would throw a stone into the air. It doesn't stop, it's the combination of a forward moving decelerating object, and gravity force accelerating it downwards. All this, presuming the AoA stays 90°.

If the body angle stays 90° in this situation, you simply get different airfoil characteristics. The airflow attacks the profile at the trailing edge, leaving it at the leading edge. Different profile, different aerodynamics, different behaviour.