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If anybody could explain this in an intuitive way I'd really appreciate it. I haven't been able to find a clear answer on this. I understand that the increase in effective AoA/CL and the spanwise flow & resultant thickened boundary layer at the tip is responsible for the tip stall (if uncorrected for in the wing design), but I can't wrap my head around the reason why the effective AoA and CL increase in the first place. On a rectangular wing they decrease because of the upwash induced from the vorticies, so why doesn't it have the same effect on a swept wing? Most of the explanations I've found on here are geared for aerodynamicists and not the average joe pilot like me, and the explanations given in most pilot resources are overly simple if an explanation at all.

The book "Handling the Big Jets" by D.P Davies says "Too much sweep produces poor oscillatory stability and a tendency for the tip to stall, causing pitch up" and that's it..

ACE the Technical Pilot Interview by Gary V. Bistrow says "A simple swept and/or tapered wing will stall at the tip first [...] This is so because the outer wing section produces a higher wing loading due to the wing taper, which causes a greater angle of incidence to be experienced to a degree where the airflow stalls at the wingtips. The boundary layer layer spanwise airflow, also a result of sweep, further contributes to the airflow stalling at the wing tips."

This ATPL CBT video I watched says the airflow separation starts at the tips because the weak wingtip vortices allow separation earlier than the inboard part of the wing where the stronger vortices from the wider chord reduce the effective angle of attack... what?!

I have an interview coming up where this question could be asked, and I don't feel like I'd be able to deliver an answer with any sort of confidence at this point.

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You are reading the wrong sources.

I've never come across the claim that the lift curve slope of a swept wing increases towards the tip. That might happen in ground effect (when the tips are very close to the ground from the swept wing's incidence), but that effect is very weak, or when the wing tip has noticeably less sweep (crescent wing).

But not on "regular" swept wings in normal operation.

If, however, the intended meaning of your question is to ask why the tips of backward swept wings reach stall conditions earlier than the inner wing: That they do indeed. Read on for an explanation.

What your sources describe is foremost caused by the thickening of the outer boundary layer of backward swept wings. This is compounded by aggressive taper - but this taper effect is very similar in unswept wings, too. Next, when you plot aspect ratio over sweep angle, you will find a boundary above which swept wings develop undesirable stall behavior, so the wing's aspect ratio is another factor (which your sources stay mum about).

To prepare for the interview, I advise you to read some of the answers here, like:

And make sure you follow the links in the linked answers as well if something is unclear.

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  • $\begingroup$ Thanks Peter. So in simplistic terms, as AoA is increased, the spanswise flow starts to pile up towards the tips which increases the thickness of the boundary layer, making it more susceptible to separation. So when the AoA of the plane is increased further, the separation begins at the tips first because of its thicker BL. But it does not in fact have a higher effective AoA or CL since the airflow over the top of the wingtips doesn't have a larger pressure differential than the root (which makes sense - it has less lift anyways..) hence causing the tip stall. Is my understanding correct? $\endgroup$
    – Speedalive
    Jun 27, 2019 at 13:09
  • $\begingroup$ @Speedalive: Yes, your understanding is correct. $\endgroup$
    – Peter Kämpf
    Jun 27, 2019 at 17:11
  • $\begingroup$ Peter, I was initially confused as well, but it seems @Speedalive didin't mean the lift curve slope. He just wrote 'AoA/CL' because both are involved (and in the linear range can be used interchangeably). The standard idea for simplistic (just so as not to say bogus) explanations like quoted is that the [effective] AoA is somehow higher at the tips, but it doesn't imply that the slope is different. In my experience, pilots (and their sources) rarely understand the lift curve slope as a specific parameter at all. They know things are normally linear, but that's about it. $\endgroup$
    – Zeus
    Jun 28, 2019 at 1:48
  • $\begingroup$ @Zeus: Thank you, that makes sense. So the interpretation of "the effective alpha and/or c$_L$ increases towards the tips" should read "the tips get nearer to stall". That they do indeed. $\endgroup$
    – Peter Kämpf
    Jun 28, 2019 at 5:57
  • $\begingroup$ Thanks for the clarification! $\endgroup$
    – Speedalive
    Jun 30, 2019 at 16:08

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