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I have a question about Stall, which I have difficulty to understand.

According to the theory, stall happens when :

1- Speed is slow then a certain limit.

2- Angle of attack is greater than a certain limit.

In the case of angle of attack, there is being spoken about relative wind along the wings. But here is my confusion. Relative wind is always shown parallel to the horizon, which strikes the wings. While in my understanding, in the sky there is equal air and wind everywhere. We create our own hard wind for our wings, by moving fast through the air. Right?

So it should not make any difference in which direction we move, along the horizon, or at a steep angle upwards or downwards, relative to the horizon. The relative hard wind will be created straight in opposite direction where we move fast. Because we are also moving in the same direction as our wings angle of attack. As I have shown with blue line in my illustration above.

If so, then there is no question about angle of attack at all. Because we always create our own wind by moving fast in any direction in the sky.

It would be different if we are moving parallel to the horizon, but only our wings have a greater angle of attack relative to the fuselage and the horizon. (As it is often explained in the example of, hand out of a car. While car is moving horizontally, but only the angle of our hand changes relative to the car). Then this stall theory is understandable.

If we look at the fighter jets and acrobatic airplanes, then we see that they can climb vertically up against the horizon, at 90 degree bank angle and fly upside down too. And they do no stall, because they create their own relative straight opposite wind by moving fast in any direction in the sky.

So why in normal planes we do have to think about straight only horizontally coming relative wind and accordingly the angle of attack against it?

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    $\begingroup$ You can drop the number 1 point on your list. Regardless if speed, stall always happens when a certain angle of attack is exceed. Also, the question is very confusing, maybe check out this: aviation.stackexchange.com/questions/2903/… $\endgroup$ – Jpe61 Aug 22 at 16:57
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    $\begingroup$ This question may be badly phrased, but its meaning is clear. It deserves an upvote. $\endgroup$ – Guy Inchbald Aug 22 at 17:16
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    $\begingroup$ Unfortunalely, alpha dot, that is the rate of AoA increase, also has an influence on stall. And Mach number. And Reynolds number. And aspect ratio. And wing sweep. It is rather complicated. $\endgroup$ – Peter Kämpf Aug 22 at 18:59
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    $\begingroup$ @S.M. Nawaz, Your blue lines would be correct if the airplane were keeping a constant AOA and pitching up to climb. However, the whole purpose of the pictures is to show increasing AOA in level flight up to the point of a stall. $\endgroup$ – Michael Hall Aug 23 at 1:49
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    $\begingroup$ @PeterKämpf looking at the question I don't think opening that can of worms is justified here. $\endgroup$ – Jpe61 Aug 23 at 7:27
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I think this has been at least hinted at in other answers, but, just to put it succinctly, the diagrams aren't actually intended to show the airflow as always being parallel to the horizon. The airstream is just shown flowing along the horizontal axis of the graph because it's convenient to illustrate it that way. There is no implication intended in the diagram that the horizontal axis of the diagram is actually parallel to the Earth's surface.

The behavior of the airfoil is exactly the same regardless of the actual orientation of the wing and airflow relative to the horizon. All that matters is how the airfoil is oriented relative to the airflow. You can rotate those diagrams any way you want relative to the horizon and what they depict will remain true.

The blue airflow lines you've drawn on the diagrams seem to be assuming that the airplane is always climbing at an angle roughly equal to the angle between the horizon and the wing's chord line, but this is not true. You can fly with a high pitch-up angle while maintaining level flight if you fly slowly enough, for example. You will practice this (called "slow flight") when training for a private pilot certificate. You can (and often do) even descend with a nose-up attitude. And, if you put the nose up too far, you can descend very rapidly with a nose-up attitude after your wings stall. For example, Air France 447 was falling 10,000 feet per minute with a nose-up attitude as one of the pilots held the stick back.

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  • $\begingroup$ This expands nicely on my answer. $\endgroup$ – Guy Inchbald Aug 23 at 10:30
  • $\begingroup$ @reirab - Now I began to understand that the angle of attack is not always the flight path. I thought the plane should go wherever its nose is pointed. But apparently it does not happen, if the thrust power is not sufficient to climb up, then the plane goes forward in an angel relative to the upcoming wind. So again a crazy question in my mind. What would be the direction of the relative wind when a plane pitch up its nose and give more thrust to climb up? Would be the relative wind then parallel to the airfoil? $\endgroup$ – S.M. Nawaz Aug 23 at 12:26
  • $\begingroup$ we can add that basic aerodynamics courses may include 2D wind tunnels, where the flow is in fact horizontal, and the wing can be held by the student at different AOA to get a feeling of the forces, similar to the concept that a hand outside the car window. $\endgroup$ – Gürkan Çetin Aug 23 at 15:23
  • $\begingroup$ @S.M. Nawaz: Re "I thought the plane should go wherever its nose is pointed.", have you ever landed a plane? The nose is definitely pitched up as you descend to touch down on the runway. $\endgroup$ – jamesqf Aug 23 at 16:37
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    $\begingroup$ @reirab: Indeed, the AoA is almost always positive, even in a normal descent. (That is, excluding diving, wingovers, and the like.) If you observe e.g. airliners on a long final, they'll almost always have a nose up attitude. $\endgroup$ – jamesqf Aug 24 at 3:19
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First of all, the relative wind is relative to the airfoil, it has nothing to do with any other direction.

We almost always show the relative wind as horizontal in aerodynamic diagrams, because it is the reference flow for the diagram. Which way up the plane is, or which direction it is moving, does not affect the aerodynamics. All that matters is the relative wind.

The angle of attack is measured relative to this wind.

By the way, slow airspeed is not a condition for stalling, it is a consequence of the stall conditions at low altitude. The stalling speed, at any altitude, is a consequence of the high angle of attack required to maintain lift. The maximum altitude is where the stalling speed and the maximum speed coincide.

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  • $\begingroup$ I think, relative wind is not coming from any particular direction. But we get relative wind created in any direction we with certain speed. The matter of "angle of attack" would be if wind direction is fixed only horizontally and we are moving against it. But in fact there is almost no wind if we do not move. So why do we see relative wind always parallel to the horizon? $\endgroup$ – S.M. Nawaz Aug 22 at 18:00
  • $\begingroup$ Yes. Which direction the relative wind is physically coming from (the direction of travel), and which direction we draw it in an aerodynamic diagram, are different things. $\endgroup$ – Guy Inchbald Aug 22 at 18:03
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    $\begingroup$ @S.M.Nawaz If you were making a diagram of angle of attack, which direction would you put the relative wind from? $\endgroup$ – HiddenWindshield Aug 22 at 22:44
  • $\begingroup$ @HiddenWindshield - I have modified my question above and added a photo, as I think what the relative wind is. $\endgroup$ – S.M. Nawaz Aug 22 at 23:44
  • $\begingroup$ @S.M.Nawaz So, when you draw an AoA diagram, you always draw the wind angle as coming from 25 degrees above horizontal? Ok, that's fine, just a little weird. $\endgroup$ – HiddenWindshield Aug 22 at 23:49
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Look at a typical diagram illustrating angle of attack. Where on the diagram do you see the horizon? How do you know the horizon is parallel to the bottom of the picture frame?

It's easy to fall into the trap of thinking that the relative wind is parallel to the horizon, because so many aircraft spend so much time flying so that the relative wind is parallel to the horizon. But in an aircraft that is climbing or descending in calm air, or one that is flying level in a downdraft or updraft, the relative wind is not parallel to the horizon.

If you like, you can rotate the entire diagram on the page. No matter which way you point it, as long as you have the same angle between the relative wind arrow and the chord of the wing, it is the same angle of attack, as illustrated on this page.

The whole point of angle of attack is that when figuring out the lift of an aircraft's wings, it does not matter where the horizon is. What matters is the interaction between the surrounding air and the wing, which usually depends on the relative motion of these two things, including the direction of that motion.

It is true that in most cases the relative wind around an aircraft has much more to do with the propulsion and control of the aircraft than with anything else. But you are mistaken if you think that an aircraft, even an extremely high-performance jet fighter, will always travel in exactly the direction in which the centerline of the fuselage is pointing.

Consider an air show in which one jet fighter is flying "straight up" and another is flying level. Can you tell, just by looking at it, that the fuselage of one aircraft is at exactly a 90 degree angle to the other fuselage? Or that the flight path is exactly 90 degrees from the other flight path? Are you 100 percent sure the "straight up" fighter is not actually flying a path 2 degrees away from vertical, while its fuselage is angled 1 degree on the other side of a vertical line? Small angle differences like that are significant when we're talking about angle of attack.

And since you bring up aerobatic aircraft, they often move in directions that are not "straight forward" in the direction the fuselage is pointing. Many aerobatic maneuvers involve stalls (high positive angle of attack) and flying upside down usually involves a negative angle of attack.

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There is no difference between "normal" planes and "acrobatic" planes as far as stall behavior. If the angle of attack exceeds the critical angle, the wing will stall, period. This is true for any airplane.

I think your confusion lies in this idea that the chord line of the wing is always parallel with the wind flow. That is wrong. As the airplane slows, lift is reduced due to the lack of airflow over the wing. Thus, the pilot has to increase the angle of attack to increase lift to maintain level flight. The opposite is true of increasing airspeed: in fact, some airplanes actually have to maintain a negative angle of attack in cruise to avoid climbing through too much lift. The weight of the plane and the amount of G force it's experiencing are also factors.

In your question, you wrote, "It would be different if we are moving parallel to the horizon, but our wings have a greater angle of attack relative to the horizon." You say that like it's not true, but it is in fact pretty much the definition of level flight.

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  • $\begingroup$ "There is no difference between "normal" planes and "acrobatic" planes as far as stall behavior." This is really not true. Stall behavior varies quite a lot between even different aircraft within ones that would be considered "normal," let alone between normal, stable aircraft and ones designed for high maneuverability like aerobatics aircraft or fighters. On most airplanes, flow doesn't just separate from the entire foil all at once. Different parts of the airfoil stall sooner or later than others and how this happens has large impacts on stall behavior. $\endgroup$ – reirab Aug 23 at 2:25
  • $\begingroup$ @reirab Yes, but as far as what the OP was asking, which is about the very basics of stall, there's no difference. $\endgroup$ – HiddenWindshield Aug 23 at 4:01
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Thank you for your question. Your most recent edit points to where your misunderstanding stems. Relative Wind is a function of flight path relative to the airfoil chord line. It is how the airflow interacts with the wing surface. Your frame of reference is the airfoil, not the ground. Align the Relative Wind with the flight path of the airfoil/wing. For big picture understandings, we can consider this roughly the same as the flight path of the aircraft with a little consideration given to how the wing is angled on the fuselage (Angle of Incidence).

It is the same concept whether the Relative Wind/Flight Path were parallel, perpendicular, or at an angle to the ground. It is true whether the aircraft is flying horizontally, vertically, inverted, straight up, or straight down. This is even true in a loop. The flight path (therefore the Relative Wind) would be roughly the tangent of the loop. Or, even if the airfoil/wing is spinning in circles in front of, above, or to either side the aircraft. Do some research on P-factor, auto-rotation, and vortex ring state. After all, propellers and rotors are airfoil with chord lines and angles of attack.

You are assuming that the aircraft’s flight path changes with attitude. Your blue arrows all represent the Relative Wind if the aircraft airfoil chord lines were relatively parallel to its flight path. That is only true in the first diagram. In the second and third diagrams, the chord line of the airfoil is not parallel to the flight path. The flight path in all three diagrams is from left to right, parallel to the top and bottom of the page.

In the first diagram, the airfoil is flying level into the Relative Wind created by the movement of the aircraft through the airmass. In the second and third diagram, the aircraft’s flight path has not changed. The airfoil’s position in the Relative Wind has changed. This could be caused by a sudden change in attitude. The aircraft’s flight path would not change until aerodynamics and power plant thrust overcomes the aircraft’s momentum. It could also be caused by the reduction of power necessitating an increase in nose up pitch in order to maintain level flight. For instance, pulling back on the control yoke abruptly and violently would change your attitude before it would change your flight path. Also, doing straight and level slow-flight would cause you to fly with your nose in an abnormally nose high pitch attitude.

To use your example of fighter planes and acrobatic planes, let’s look at some real world examples. If you ever watch a fighter plane rapidly change pitch, you will notice fog or clouds form just aft of the leading edge of the wing. This visible moisture is not visible when the aircraft is flying straight and level in unaccelerated flight. It only happens when the aircraft abruptly changes attitude. No matter how it looks, the aircraft’s change in flight path is not as abrupt as its change in attitude. In the case of an abrupt pitch nose up, the angle of attack will abruptly change until the aircraft’s flight path realigns itself to the new attitude.

Don’t make the ground or the horizon your frame of reference for Relative Wind. Don’t even use the direction of flight based on the aircraft’s longitudinal axis. Make your frame of reference the path of flight of the chord line of the airfoil through the airmass. The way the airmass hits or the air molecules interact with the airfoil/wings determines the Angle of Attack.

Relative wind is opposite of the aircrafts 3-dimensionally path or track of flight regardless of its attitude. If the aircrafts path of flight is straight up, straight down is the direction of its Relative Wind. You can extrapolate the same point to a banking aircraft. Relative wind is depicted as horizontal to simplify the imagery of a complex subject. It is similar to the fact that most maps and sectionals are printed with North at the top regardless of which direction you are actually facing. If you are confused, just turn the paper to orient it correctly.

Try this with your diagrams with your blue lines removed. If you are flying straight and level in unaccelerated flight, the first diagram would represent your Angle of Attack. If you were to reduce power to slow your airspeed while maintaining the same attitude, you would start to descend in altitude. The second diagram would represent this if you turn it to keep the airfoil chord line parallel to your frame of reference (the actual ground) in your visual field. If you were to bring the power to idle while maintaining the same attitude, you would descend at a faster vertical speed. The third diagram would represent this if you were to turn it so that the airfoil chord line remains parallel to your frame of reference (the actual ground) in your visual field.

To understand this from the frame of reference of the airmass, think of yourself as a human shaped air molecule either stationary or moving at a different speed or direction than the airfoil. Your feet are pointing toward the earth and your head toward the sky. If the airfoil from the previous paragraph strikes you while in level flight like in diagram one, the leading edge would hit you right in the gut. If the airfoil were to strike you while in a descent or in nose-up attitude level flight at a slower airspeed like in diagram two, the underside of the airfoil would hit you in the forehead. If you increase the descent rate while maintaining a level attitude or increase nose up pitch in straight and level slow flight like in diagram three, more of the underside of the airfoil would hit you on the top of your head.

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  • $\begingroup$ Now I began to understand that the angle of attack is not always the flight path. I thought the plane should go wherever its nose is pointed. But apparently it does not happen, if the thrust power is not sufficient to climb up, then the plane goes forward in an angel relative to the upcoming wind. So again a crazy question in my mind. What would be the direction of the relative wind when a plane pitch up its nose and give more thrust to climb up? Would be the relative wind then parallel to the airfoil? $\endgroup$ – S.M. Nawaz Aug 23 at 12:30
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    $\begingroup$ @S.M.Nawaz - Close. The Relative Wind is always roughly the flight path of the airfoil which is almost the same as the flight path of the aircraft. The flight path relative to the airmass is not always the direction in which the nose of the aircraft is pointing. The aircraft’s flight path will depend on attitude control as well as power measured in engine thrust and/or airspeed (you need at least one if not both of these). Remember, Lift makes an aircraft climb, descend, or remain at constant altitude. Lift overcomes gravity. Thrust overcomes drag regardless of attitude. $\endgroup$ – Dean F. Aug 23 at 13:29
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    $\begingroup$ Only at an extreme nose up pitch attitude does engine thrust contribute enough to overcoming gravity for it to make lift not a factor. As airspeed slows (which will be the case in nose up pitch), lift decreases. No airspeed, no lift. And, lift is always perpendicular to the airfoil chord line. But, with sufficient airspeed, you can continue to climb even without thrust as power. The Angle of Attack is the difference between the relative flight path through the airmass of the airfoil and the chord line of the airfoil. It does not necessarily rely on thrust. Think of a glider. $\endgroup$ – Dean F. Aug 23 at 13:41
  • $\begingroup$ During Slow Flight, your pitch is nose up. You can ascend, descend, remain at constant altitude, or turn. Below a certain speed threshold and under a certain power setting, altitude is controlled by power. Airspeed is controlled by pitch. “Pitch for airspeed. Power for altitude.” is a pilot mantra when flying on the backside of the power curve. On the front side, changing pitch to a nose up attitude will make the aircraft climb. The relative wind will even eventually align itself with the airfoil chord line, given enough airspeed and/or power. But, you will rapidly begin to lose airspeed. $\endgroup$ – Dean F. Aug 23 at 14:30
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S.M. Nawab, your question demonstrates an intuitive understanding of “relative wind” which is something that many others don’t always get right away. So, your root question is:

“why in normal planes do we have to think about straight coming relative wind and accordingly the angle of attack against it?”

The short answer is that we DON’T have to think about it that way. If you understand relative wind you already have a leg up on understanding that a stall can happen at any airspeed and attitude relative to the horizon.

The reason that relative wind is depicted as horizontal in most instructional diagrams is because “normal planes” spend most of their time in straight and level flight. This makes it easier to teach the concept of Angle of Attack to beginning students.

By the time you get to the point where you might need to worry about stalling at 3Gs while inverted at the top of a loop you should have this basic concept mastered!

ADDENDUM:

Don’t take this the wrong way, but I think you have allowed your intermediate level comprehension of relative wind in high performance aircraft and unusual attitudes get in the way of understanding a beginner level illustration. There is more than enough material here and in the other answers to put you on the right path to understanding, but I just have a couple pointers that my help you out in the future:

  1. Always make sure you understand the learning objective of whatever picture, graph or chart you are looking at. Read all accompanying text related to the picture! Context is very important. This very standard illustration is showing how increasing AOA leads to airflow separation and eventually a stall. It would not accomplish that point if you made an unfounded assumption that the aircraft was adding power and pitching up to climb.

  2. Make sure you are clear on what is being held constant, and what is a variable. In this illustration altitude is assumed to be constant, (therefore relative wind) and AOA the variable, and it is increasing to create additional lift as the aircraft slows down. If the relative wind were to change as you assumed, AOA would be shown as a constant.

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    $\begingroup$ So I have to consider that relative wind is always coming horizontally parallel to the ground. And we are changing our angle of attack into that wind? While I think that there is (almost) no wind. The relative wind we create ourselves for our wings, by moving fast in any direction. So our relative wind should always come exactly from opposite direction of our heading. Because only our movement cause the wind. $\endgroup$ – S.M. Nawaz Aug 22 at 18:43
  • $\begingroup$ @S.M. Nawaz. I'm not sure what your question is... (or whether you understand the point in my answer) You are correct that the aircraft creates relative wind by its motion through the air. Therefore it comes opposite to the direction of travel. It is depicted as horizontal for simplicity. $\endgroup$ – Michael Hall Aug 22 at 18:54
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    $\begingroup$ @S.M. Nawaz, everything you are saying about relative wind is 100% correct. Except that when you say that “we still assume that relative wind is coming horizontally and parallel to the earth surface” you are incorrect. I don’t assume that, and apparently you, (correctly) don’t assume it either. So who is the “we” you describe? I presumed you were referring to a simplified diagram in a flight manual, but maybe you need to explain where you are getting this incorrect assumption from. $\endgroup$ – Michael Hall Aug 22 at 19:58
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    $\begingroup$ @S.M.Nawaz - Actually, sticking your hand out of a car is the given example because changing your hands angle relative to the movement of the car does not change the hand’s direction nor speed. It only changes the Angle of Attack. I also gave the example of the skydiver changing the angle of their hands. Regardless of the change, their direction of travel is roughly straight down. Just like an airplane can drop roughly straight down, in a flat attitude, if it stalls. Even at full power. This is why Angle of Attack is considered as the important factor in a stall instead of airspeed. $\endgroup$ – Dean F. Aug 23 at 13:52
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    $\begingroup$ @S.M.Nawaz - Do some research on Power-on Stalls and Accelerated Stalls (stalls caused by increased load factor in a turn or bank attitude). Remember, the direction in which you point the nose of the aircraft is not always the direction in which it is going to fly. Do some research on Slow Flight and Uncoordinated Turns. That is why the car’s direction of travel is irrelevant. Only the Relative Wind created by the hand’s movement through the airmass relative to the hand itself is relevant to Angle of Attack. Even when the car is climbing a hill and not level with the horizon. $\endgroup$ – Dean F. Aug 23 at 14:01
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It's quite a bit of a simplification, but I can't see anyone pointing this in the other answers: If your plane is not climbing or descending (level flight) then your motion will be parallel with the horizon and so will be the relative wind (albeit in oposite direction).

If the wing (well, plane) was in a climb then that blue arrow should indeed be pointing a little bit down (depending on your speed and rate of climb, google Flight Path Angle).

If the wing is in a descent, relative wind would indeed have a slight upward component.

This would only happen if you choose the horizon to be your frame of reference (ie. Horizon is always horizontal and on the X axis of our graph. Vertical is always on Y axis). In practice you can choose to have the direction of motion as your X axis, and Y axis just something perpendicular to that motion. In this second case the relative wind will always be "horizontal" because that is how you decided to draw the axes.

Note that in first case (horizon is X axis) the wing will have to be rotated at it's corect pitch angle, while in second case (flight path is X axis) the wing will be rotated only by the aoa. Second is a bit simpler. And second is what most graphics will show

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