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As AOA is the angle between relative airflow and Chord Line. Then after you increase the angle of attack and fly at that angle of attack for some amount of time and the relative airflow changes accordingly. Does the AOA then become ZERO. If it does does it virtually mean that the we have more cushion to reach the critical angle of attack and stall the aircraft?

I have attached an image that shows the relative airflow changes as the attitude changes. It is from the Transport Canada Flight Training Manual.

enter image description here

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  • $\begingroup$ No, the AOA does not become 0. The relative airflow does not change over time. Where did you get this idea from? Did you read this somewhere? $\endgroup$ Oct 8, 2023 at 16:16
  • $\begingroup$ I have attached an image from Transport Canada FTM. Please have a look. Maybe this picture is misleading and I have misunderstood the explanation. $\endgroup$
    – Nish
    Oct 8, 2023 at 17:19
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    $\begingroup$ Those Transport Canada images are misleading because they have led you to believe something that is not true. Aircraft RARELY climb or descend with 0 AOA, and even in cruise flight there is usually some AOA. $\endgroup$ Oct 8, 2023 at 18:13
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    $\begingroup$ As you have correctly written, the "AOA is the angle between relative airflow and Chord Line". In all the three cases of your picture the AoA seems to be just the same. What's you doubt exactly? $\endgroup$
    – sophit
    Oct 8, 2023 at 20:11
  • $\begingroup$ The image shows that the direction of the relative airflow changing with the attitude of the aircraft or atleast it looks like that. That led me to believe that Relative Airflow changes as you pitch up or down. $\endgroup$
    – Nish
    Oct 8, 2023 at 21:51

3 Answers 3

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To clarify. The angle of attack is the angle between two vectors: the flight direction vector and the vector on the plane of the wing, which is also directed in the direction of motion (in some calculations, the direction of this vector is considered differently, and correction formulas are used). The higher the angle of attack, the higher the lift force. The higher the speed, the smaller the angle of attack must be to support the weight of the airplane.

The picture you attached only considers the case where the airplane speed is so high that a very small angle of attack is needed to support the weight of the airplane. It will be near zero, but it will only become equal to zero when the airplane is vertically off the ground and there is no wind. As for the change in angle of attack during or after the maneuver... You change the direction of the wing vector, but due to inertia it takes some time for the body to change direction, at that point the angle of attack will increase and then decrease only when both vectors are close to each other. If this coincides with the direction of your question, feel free to ask follow-up related questions!

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    $\begingroup$ RE "It will be near zero, but it will only become equal to zero when the airplane is vertically off the ground and there is no wind"-- I find this sentence hard to follow. Are trying to describe the situation where a/c is parked stationary on level ground and there is no wind? Some rewording may help. $\endgroup$ Oct 10, 2023 at 0:03
  • $\begingroup$ @quietflyer I took it to mean a dive straight at the ground: "Pointed vertically at the ground". At which point all "lift" is created by drag, not an angle between airflow and the wing. $\endgroup$
    – Azendale
    Oct 12, 2023 at 17:44
  • $\begingroup$ @Azendale -- ok, but the wind would be irrelevant. If you are talking about the external meteorological wind. Assuming we're not complicating things by including shears / gradients, the wind doesn't matter, could be 100 mph and would make no difference. $\endgroup$ Oct 12, 2023 at 18:38
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The diagrams reproduced in your question appear to be intended to illustrate the same, small (but non-zero) angle-of-attack in all 3 cases.

As for your quote--

Then after you increase the angle of attack and fly at that angle of attack for some amount of time and the relative airflow changes accordingly.

I think what you may be missing is that "flying at that angle of attack" doesn't imply flying at a constant pitch attitude. For simplicity imagine some sort of weird loop1 where we make a special point of holding the AoA constant starting from immediately after we've first "pulled G's" and began making the flight path curve. Since the direction of the velocity vector will constantly be changing throughout the loop, so will the direction of the relative wind, by the same principle that is illustrated in the diagrams in your question. So your quote above is arguably true. But AoA is constant, not decreasing.

But transitioning from horizontal flight to a constant nose-higher pitch attitude (one that eventually results in a stabilized climb) is a different situation. If we constrain the problem that way, one could argue that once the pitch attitude is established, we'll see the angle-of-attack decrease due to the changing direction of the flight path during the time interval that the flight path is still curving upwards.2 We could further constrain the problem in such a way that the angle-of-attack could indeed end up at zero, if we are flying with non-symmetrical airfoil that still generates lift at zero degrees AoA.

Exactly what we'd be doing with the elevator during this time is a different question that we can't really answer without knowing a few more constraints. (To a first approximation we'd expect to need to be moving the stick or yoke forward during the time interval that the flight path is still curving upwards, but there are many complicating effects that cause exceptions.) From a practical piloting perspective, it generally wouldn't be fair or useful to say that the mere fact that we are transitioning into a steady-state climb is the fundamental reason that the angle-of-attack is decreasing during this maneuver, or that climbing flight intrinsically tends to offer stall protection, or that climbing flight is generally conducted at a lower angle-of-attack than cruising flight. The decrease in AoA during the period that the flight path is still curving upwards can be viewed as an artifact of the fact that we've arbitrarily chosen to hold the pitch attitude exactly constant at this moment, rather than the AoA. Yet flying by reference to pitch attitude rather than purely by reference to AoA is indeed something that we actually do in many real-world instances.

For an extreme case of this scenario, forget about starting from "cruise flight" and forget about the initial increase in angle-of-attack in your quote above, and instead simply imagine that we are established in horizontal slow flight deep on the "back side" of the thrust curve or power curve, with the nose very high3, and then we add still more power (maybe we have an afterburner or two?) to accelerate without allowing the pitch attitude to change. Given sufficient power, and a non-symmetrical airfoil, we certainly could eventually end up in very high-airspeed climb with zero degrees AoA. The diagrams included in your question are clearly not intended to illustrate this sort of scenario!

And to frame the problem in yet another way, in a sustained, truly vertical climb the wing actually must indeed be at the zero-lift AoA4,5. Which would indeed protect us from stalling, as long as we constrain ourselves to manipulate the controls in such a way as to stay in that sustained vertical climb. But looking at extreme cases like that widens the discussion beyond what you seem to be asking about in your question.

So, it's complicated. We have to be very specific about which variables we are constraining when we ask questions like this. The fundamental reason an airplane stalls is that the pilot moves the control stick too far aft, thus placing the wing at the stall AoA.6

Footnotes--

  1. It's interesting to consider exactly what such a loop would look like and how different the required control inputs would be from a "normal" well-flown loop. The answer is far from obvious, and in most aerobatic light planes we can't accurately monitor AoA directly. A good potential new ASE question?

  2. Take care to note the importance of the word choice here-- "curving"-- a "curve" is not a linear climb at constant rate or even a linear climb with a positive rate of increase in airspeed and vertical speed. In the thought experiment here, the time that the flight path will actually be "curving" upwards as the pitch attitude remains constant (and the AoA is therefore decreasing) will tend to be brief. And don't overlook the fact that the upward curve of the flight path will typically, but not necessarily, have been preceded by an initial increase in AoA.

  3. A delta-winged aircraft might be would be a good choice for this demonstration, due to their good flight characteristics at "high alpha".

  4. So long as the aircraft is not tailsliding! Also, ignoring the need to offset any tail force, thrust line angle, etc.

  5. See for example the first set of diagrams in this related ASE answer: Does lift equal weight in a climb?

  6. This simplified argument is not intended to imply that the stick or yoke position that commands the stall angle-of-attack truly remains constant during turning flight or loops-- see for example this related ASE answer: What has happened to make me experience negative G with the control stick FULL AFT near the top of a loop?

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  • $\begingroup$ Sorry, just got kind of addicted to this one, will lay off on making any more updates. $\endgroup$ Oct 10, 2023 at 2:47
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I'm finally starting to understand where the OP is coming from-- I believe he is noticing that if the a/c started in the situation in top figure, and then instantaneously pitched up to pitch attitude of middle figure, then presumably the change to the direction of Rel Wind shown in middle figure would not be instantaneous. Which is true, the flight path must curve to a new direction for that to happen, and that is not instantaneous. (Maybe just a second or two, but not inst.)

But as to whether we eventually end up back at the original AoA, or higher, or lower (e.g. zero), depends on what control inputs we make and what part of the flight envelope we are in. For example consider the case where the instant pitch up (which obviously temporarily increases AoA) puts the plane so far on the back side of the power curve that rather than climbing, the sink rate goes way up, so the flight path curves downward. (Maybe we've chopped the throttle too at the same time.)

Well, now if we keep that pitch attitude constant, we'll see the AoA increase further as the flight path curves earthward till reaching a steady-state again. (Not a very pleasant one!) So no you can't make general statements that the AoA will always tend to decrease, maybe all the way to zero, if you pitch the nose up abruptly and then hold that pitch attitude. But my other answer covered a case where that could indeed happen.

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