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Stalls happen when the boundary layer on top of a wing slows so much that it essentially becomes part of the airfoil.

enter image description here

My question is, as the wings on larger aircraft present a longer flow path for the boundary layer, they slow the layer more. Right? So, would they not be more have a lower stall angle? Isn't this what is hinted at by the use of slots on airliner flaps?

Abdullah is not an Amalekite
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  • I think this answser contains relevant elements (link between chord length, Reynolds number and stall angle of attack). Good answer should refer to these 3 concepts. – Manu H May 07 '20 at 13:25
  • I think there are concepts to clarify. boundary layer does not move (it is a layer in which air molecules are affect by friction over the wing surface), and separation point is where flow is no longer laminar. For stall, several definitions exist depending if you speak of the critical AoA or the stalled regime. – Manu H May 07 '20 at 13:37
  • @ManuH I think both the definitions you mentioned can be used to answer my question - a more stallable wing will get trailing-edge separation at lower AoA. But that's just what I'm guessing. I may be wrong. – Abdullah is not an Amalekite May 07 '20 at 15:07
  • @ManuH In the answer you linked, Peter said "generally a higher Reynolds number means your friction effects go down, translating into less friction drag and a higher stall angle of attack" which I don't get, because this implies that a slower, boundary layer airflow is less likely to reverse (what?) – Abdullah is not an Amalekite May 07 '20 at 15:12
  • @Bianfable I've never once claimed to be a "knowledgeable" person at this site!! I've asked a ton of questions and, occasionally, I've managed to come up with a reasonable answer. – FreeMan May 07 '20 at 15:40
  • @ABJX I linked this answer in order to point out that some elements already exists on aviation.SE and answers to your question should bring new elements or bring them differently. If there is something you don't understand, don't hesitate to ask a new follow up questions about specific concepts you've just discover. – Manu H May 07 '20 at 16:13

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Even assuming the same airspeed, a larger chord would increase the free-stream Reynolds number. As a general trend, higher free-stream Reynolds number will delay the stall boundary due to earlier transition to a turbulent boundary layer, as can be seen below for a 2D section data from wind tunnel:

Re_Clmax

Image ref: NASA Contractor Report 4745

This answer has a different and equally good illustration of the effect in terms of lift curves.

Of course, things get more nuanced in 3D, whether the airfoil is optimized for natural laminar flow, and in transonic flow, where the maximum lift trend with Reynolds number can reverse at an intermediate Reynolds number.

JZYL
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  • So the transition to turbulent is the only reason why a higher chord/Reynolds gives a higher stall angle. – Abdullah is not an Amalekite May 07 '20 at 15:14
  • @ABJX For normal airfoil/wing, I believe so. But I can't be certain whether that's the only physical process. However, the correlation is observed time and time again in wind tunnel and in literature. – JZYL May 07 '20 at 15:20
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    @ABJX: No, there is more. The boundary layer may be absolutely thicker than on a small wing, but is relatively thinner, so viscosity has less influence on the flow conditions. Also, larger aircraft tend to have higher wing loadings, so they fly faster which increases Reynolds number again and reduces the relative effect of gusts. – Peter Kämpf May 07 '20 at 20:11
  • so there is flow separation at a lower AoA but it is less severe due to the relative thinness of the boundary layer? – Abdullah is not an Amalekite May 08 '20 at 10:29
  • @ABJX No, boundary layer separation will be delayed to a higher AOA. Furthermore, as Reynolds number increases, the laminar separation bubble will also get smaller for the same AOA. – JZYL May 08 '20 at 11:06
  • wait then why use slotted flaps and slats? they cut up the wing into several smaller wings. – Abdullah is not an Amalekite May 08 '20 at 15:32
  • @ABJX If it's just slotted flap, the stall AOA actually decreases compared to plain airfoil, even though the CLmax has massively increased. As for slats, it definitely delays the stall AOA. My understanding is that a slat reduces the suction peak, thereby reducing the adverse gradient on recovery. – JZYL May 08 '20 at 16:04
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Any wing is 100% likely to stall when its critical angle of attack is exceeded, so no, larger wings are not more likely to stall than smaller wings. Both are certain to stall when the necessary condition is reached.

To address the edit in the question, are large aircraft more likely to stall... any aircraft will stall if it's flown to the point of exceeding the critical angle of attack. Put it there, and just like a wing in isolation, the probability is 100%. If the question is an empirical one, then all sorts of other factors come into play:

Small training aircraft are stalled routinely and intentionally as part of the training syllabus. Large jets are almost never intentionally stalled (an FCF flight would be the rare exception, since training is done in the simulator), and there are multiple warning systems in them to help the pilots avoid such a condition. Plus, with a crew of two pilots who are typically highly experienced, the likelihood of getting there inadvertently tends to go down considerably, when compared to a small aircraft that's often flown single-pilot by somebody who may not have extensive experience and may not fly as often as those who are paid to fly the larger jets.

To compare the AOA where the clean (no high-lift devices deployed) wing on a transport category aircraft stalls to the AOA where the wing on a light aircraft stalls is an academic exercise, but it doesn't relate to "is this aircraft more likely to stall than that aircraft." Both will fly fine when operated in the normal envelope, and both will stall when flown to that point.

Ralph J
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    Excellent point. Perhaps the OP should refine the question: i.e. "would the same airfoil shape, scaled up, stall at a lower critical AoA?" – Michael Hall May 07 '20 at 14:58
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    I guess I should rephrase the question. – Abdullah is not an Amalekite May 07 '20 at 15:03
  • -1 This isn't an answer, it's a put-down. Is the critical angle of attack more likely to be exceeded in a large aircraft? – Dave Gremlin May 07 '20 at 19:34
  • @DaveGremlin That's a whole different question. I've exceeded the critical angle of attack dozens, probably hundreds of times in small aircraft, because it's common to practice stalls in training aircraft. Never done it in thousands of hours in large aircraft, because that maneuver is reserved for the simulator in such aircraft. Does that mean that one is "more likely to" stall the small aircraft? Any aircraft can get there if you put it there, intentionally or not. Your version of the question has a behavioral aspect to it that the O.P.'s didn't. – Ralph J May 07 '20 at 20:54
  • @DaveGremlin Also, the answer is NOT a put-down, but pointing out that the question as phrased doesn't really have the sort of answer that the OP may be searching for. "Likely" is the wrong criterion. Ask the question differently, and it gets a different answer. But "same airfoil scaled up" or "big jet with its wing vs C-152 with its wing" or whatever other rephrasing of the question you have, is NOT what was asked. – Ralph J May 07 '20 at 21:08
  • @RalphJ - Your original answer was: "Any wing is 100% likely to stall when its critical angle of attack is exceeded, so no, larger wings are not more likely to stall than smaller wings. Both are certain to stall when the necessary condition is reached." This doesn't give the questioner anywhere to go and comes over as very dismissive. People who ask questions on this site are not always experts in the subject, hence the question. If there's a problem with the question it would be preferable to provide guidance rather than two sentences that are desinged to illustrate the OPs ignorance – Dave Gremlin May 08 '20 at 09:01
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    @DaveGremlin I have no problem downvoting questions that I think are poor, but this question has zero DV. The problem with the "is likely" phrasing is that it's a behavioral, not an aerodynamic, issue. Aerodynamics is pretty deterministic. And, reference the OP's comment above, he understood the need to refine his question in order to get at what he was interested in. – Ralph J May 08 '20 at 09:51