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I was told pulling the yoke quickly and all the way back stalls the plane immediately. I believe this is not possible as long as this occurs above the stall speed.

I think about a styrofoam plane: Increasing the pitch angle (same effect as pulling the joke) just makes the plane turn up with a small radius, but it doesn't stall.

Is this correct?

When I fly in straight line how can I provoke flow separation at wings above stall speed?

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    $\begingroup$ If you're talking about the cited stall speed in FCOM, turn off your anti-ice and go fly in the snow for a while, then you'll see how far wings can stall before hitting the stall speed. Not recommended though. $\endgroup$
    – JZYL
    Nov 27 '21 at 15:33
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    $\begingroup$ "I often hear "stall can happend at any airspeed"" -- "here plane is not stalled because of "hard yoke pull"" -- when flying at any speed, just yank that yoke all the way back, as far as it can go, however much force it takes. You'll probably stall. And all your questions will be answered. Can't guarantee a linear flight path though! $\endgroup$ Nov 27 '21 at 16:03
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    $\begingroup$ @JurgenM, I think your example of the stryofoam plane is hampering your understanding here. Yes, a very small, very light airplane will simply arc abruptly upwards with a rapid increase in pitch. But remember, the mass to surface area ratio increases dramatically as size goes up. Larger airplanes have a lot more load on the wings, and more inertia to overcome when pitching. They don't fly anything like your styrofoam model that just does a quick loop before striking you in the buttocks... $\endgroup$ Nov 27 '21 at 16:20
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    $\begingroup$ No, by definition. Stall speed is the speed at which the wing stalls. It's a variable that depends on loading, angle of attack, how the airfoil shape has been perturbed by icing, and many other factors. $\endgroup$
    – jamesqf
    Nov 27 '21 at 18:56
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    $\begingroup$ There are two linked questions top right, and I am certain that the subject of accelerated stalls has been covered in depth here, and on many searchable web sites. In fact, this is starting to read less like a question and answer, and more like a bunch of us trying to convince Jurgen to abandon a stubbornly held disbelief. I just don't know what else can be said on the subject... $\endgroup$ Nov 28 '21 at 3:32
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So when I fly in straight line how can I force flow separation at wings above stall speed? I think this is impossible.

Stall is a matter of angle, not speed

There is no stall speed, you can decrease speed or increase speed as long as you manage to remain below the stall angle.

Stall happens when the angle of attack exceeds the maximum angle of attack for the airfoil. So to stall you just need to increase pitch until the stall. This is how GA pilots train for stall prevention and recovery, see this video.

Let's take two examples:

  • In level flight, say at 60 kt, the stall angle of 15° is reached. Any attempt to climb by increasing the pitch will stall the wing and the aircraft will lose altitude (unless power is increased).

  • In steep descent, the speed is 200 kt, the pitch is quickly increased to level the aircraft, the angle of attack changes, say from -5° to 18°. The new angle of attack is larger than 15°, this immediately stalls the aircraft, even if the speed might be larger than 150 kt.

[My friend] thinks if he pulls the yoke all the way back as fast as possible, the plane will stall and immediately drop. I think this is impossible at speeds above stall-speed.

Your friend is right. This is the case of the last example. Again there is no stall speed, it's $\sf \color {orange} {\text {only}}$ a matter of angle of attack.

Reason for flow separation during stall

So flow separation is consequence of plane that "falling down" not initial reason why plane starts falling down.

No, flow separation is the consequence of air no able to follow the airfoil curve, because the angle between air direction and the curve is too high, so inertial effects are too strong compared to effects which make air following the surface. Inertial effects make air continue on its momentum instead of curving.

As air doesn't follow the airfoil curve, the pressure field is not able to create enough lift, and the aircraft loses altitude due to its weight. But losing altitude is not the cause of the flow detachment. To convince yourself, let's use a wind tunnel. The airfoil is fixed, so it cannot move down when lift decreases. From the linked video:

enter image description here

The airfoil is at the same location, but the flow has detached due to the angle of attack. The inertial forces won over the viscous forces, air now follows its momentum and eddies form:

enter image description here

So no vertical movement is required for stalling the airfoil.

Case of stall in level flight and apparent "stall speed"

Explain me why airflow stay attached at 61kts but separate at 60kts?

Again, this is not a matter of speed, but angle. You might be confused by the fact that if you want to fly level, and start decreasing the speed by decreasing the engine power, then the angle of attack must be increased. So:

  • You see an aircraft which speed is decreasing while the angle of attack is increasing.
  • At some speed value, the angle of attack will reach the stall angle, and the wing will stall.
  • We may call this speed the stall speed but this speed is valid only for flying level, while the angle is constant whatever the attitude. This is why we say there is no stall speed, but a stall angle.

If you try to stall the same wing in descent, the angle (the angle of attack, not the pitch angle) will be the same, but the speed will likely be much larger.

So instead of saying "the wing stalls at 60 kt, but at 61 kt is is still flying", you should say "the wing stalls at 15°, but at 14° it is still flying".

But actually the stall is not so abrupt.

Stall is not lift on/off, it's progressive

At some point the angle is too large, regardless of the speed, lift starts decreasing. Here is a typical lift coefficient vs. angle of attack curve:

enter image description here

Adapted from source

We see lift stops increasing with the angle of attack near 13°-15°. Then it decreases. This is the stall.

If the pilot just pitches the aircraft up, lift continues to decrease, and the aircraft loses altitude.

Can I climb while in the stall area?

Yes you can. That's not contradictory.

When the angle of attack is near the stall angle, the coefficient of lift is not 0, it is only decreased, if we continue to pitch up, it will continue to decrease, there is no possibility to increase it up (see the curve).

But the lift coefficient is not the only parameter when determining lift, velocity matters too, and even more than the lift coefficient Lift is proportional to $C_L$, but proportional to $V^2$:

$$L = \frac 1 2.C_L.A.\rho.V^2$$

($A$ is the wing area, $\rho$ is air density, two constant parameters).

So let say we're in a level flight at constant altitude (lift = weight). We reduce the power while increasing the angle of attack, so that our angle of attack is just at the beginning of the stall area. Obviously increasing the angle of attack will make the aircraft descend. But what happens if we can increase the power at the same time?

  • Let's say we pitch up a bit and the lift coefficient is reduced by 30% (e.g. it comes from 1.4 to 0.98).

  • Let's offset this reduction by an increase of velocity by 20% ($1.20^2\times 0.7 = 1$).

  • The result is L was multiplied by 0.7, then by 1.44, meaning it hasn't changed.

If we increase the velocity by 30% instead of 20%, then we starts climbing.

But let's not forget in the stall area of the curve,

  • lift coefficient decrease is large for a small additional angle of attack,
  • we won't be able to increase the velocity indefinitely,
  • maybe more crucial, we won't be able to increase velocity quickly enough, lift decrease will occur before the aircraft has accelerated, and if we try to maintain the altitude by a bit of additional pitch we might stall it for good.

Do I need lift at all?

I think about a styrofoam plane: Increasing the pitch angle (same effect as pulling the joke) just makes the plane turn up with a small radius, but it doesn't stall.

How do we gain altitude? By a force directed upward and larger than the weight. Lift is such a force, but it's not the only one.

For example a rocket has no wing, therefore doesn't produce lift, but the engines produce thrust. As long as thrust is larger than weight it's possible to ascend. Rockets have huge engines because they are heavy.

What about a styrofoam plane? It's very light, probably only a small force is required to make it gain altitude, therefore the propeller may be sufficient by itself to gain altitude if the airplane is pitched vertically. In that case it doesn't matter if the wings generate lift or are stalled. That's why you can exceed the stall angle, generate no lift and still can make the aircraft climb on its own momentum and the thrust provided by the propeller.

The wing in this case is used like the empennage, it just acts like a windvane and orient the fuselage. At this point, the aircraft doesn't fly anymore (flying means lift), it is propelled by the engine.

Stalling doesn't mean the aircraft must fall immediately. It falls only if there are no other forces but lift to oppose its weight.

Is it possible to actually reach the stall angle at high speed?

For many aircraft stalls are difficult to trigger at high speed because its actually difficult to reach the stall angle. This angle is about a 15° nose up relatively to the airflow (not to the horizon). At a large airspeed, the aircraft has enough momentum to climb when pitched up, and therefore it remains at a low angle of attack until the speed decreases and a stall at low speed is triggered.

A way to trigger a high-speed stall is to increase the load factor (g) e.g. by performing a steep turn.


For more details, see:

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Federico
    Nov 29 '21 at 14:33

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