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We are often told that wing sweep causes the airflow to be deflected outwards. I can understand this for the underside, but for the upper surface it seems inconsistent with other effects.

For example on a sharply-swept delta, sideways flow out from below, up around the leading edge and back in again over the upper surface becomes dominant. It feeds the vortices employed by Concorde to enhance lift at low speeds. The sharper the sweep, the more the air over the upper wing flows inwards. The same effect is seen in miniature with tip vortices on a straight wing. This makes sense to me because the pressure above the wing is low, so that would naturally attract air in towards that zone.

So, does sweeping a straight wing really reverse such effects, or do say wing fences on the upper surface actually stop the air flowing inwards?

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    $\begingroup$ Related: Where is the spanwise flow? How does the span wise flow point the air towards the wingtip? $\endgroup$
    – user14897
    Mar 11, 2020 at 9:57
  • $\begingroup$ Thanks, !this image certainly helps: it shows that both can happen in different places. $\endgroup$
    – Guy Inchbald
    Mar 11, 2020 at 19:45
  • $\begingroup$ Note that first you are asking about swept wings then you change subject to deltas. The filled-in area of delta wings may seem insignificant but they cause the wings to behave very differently from swept wings. Some of the drastically different characteristics of deltas over swept wings: deltas can employ the creation and detachment of vortices to create lift (vortex shedding) an effect not seen on swept wings, deltas generate high drag when turning causing it to slow down but also allow it to achieve tighter turning circle, deltas surprisingly retain aileron control very deep into stall etc. $\endgroup$
    – slebetman
    Mar 12, 2020 at 15:46
  • $\begingroup$ @slebetman Yes one has to be careful. But in other ways the delta is often treated as a class of swept wing and much theory applies to both: certainly, both feature wing sweep in some form. So I think that one helpful strand of these answers might be to bring out any relevant aspects where the delta acts as a typical swept wing and where it differs. $\endgroup$
    – Guy Inchbald
    Mar 12, 2020 at 15:51
  • $\begingroup$ The primary difference is the filled-in area creates a low pressure zone (because air could not flow into it from below) that doesn't exist in swept wings. You can easily imagine such a low pressure zone may suck airflow inwards instead of outwards. I'm not sure if it does. What I do know is that it encourages the formation of vortices along the entire leading edge (instead of the usual tip vortex) and also encourage those vortices to detach and join the airflow (leading edge vortex shedding). As such the difference feels quite relevant to me $\endgroup$
    – slebetman
    Mar 12, 2020 at 23:09

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As a partial answer following up on those to date, here is an image from Where is the spanwise flow? How does the span wise flow point the air towards the wingtip?, showing a snaking in-then-out flow over the wing. Note the inward flow over the leading edge and wing section, which is just the point where wing fences are located.

Airflow over swept wing

On sharply-swept deltas which sit inside the bow shock cone at supersonic speeds, 1980s studies of supercritical attached flows show the air flowing upwards round the leading edge and forming a crossflow to meet an inboard "crossflow shock". See for example W.H. Mason, “A Wing Concept for Supersonic Maneuvering,” NASA CR 3763, December 1983. But it is less clear to me whether this inbound crossflow is physical, or a theoretical notion along the wing profile which, due to the sharply tapering chord with span, physically might still flow outwards:

Supercritical crossflow on sharp delta

This contrasts with the low-speed vortex flow over such wings (e.g. Concorde) at high angles of attack, where the vortex inflow is detached and the boundary layer flows outwards.

So it seems the full the answer must be complicated, with the pressure distribution along the chord, and hence the in-out flow changes, depending on the angle of sweep, the speed of flight, the angle of attack and how far above the wing you are considering.

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  • $\begingroup$ Yes, it is indeed complicated. You will even see a change in flow direction over the thickness of the boundary layer. Near the wall and past the suction peak the flow is outwards while the outer flow of the boundary layer still flows inwards, as does the outer flow. $\endgroup$
    – Peter Kämpf
    Mar 12, 2020 at 19:19
  • $\begingroup$ And regarding fences: They sit at an angle to the local flow which produces a vortex. This vortex, not the fence itself, helps to prevent the outward movement of the boundary layer. Complicated indeed. $\endgroup$
    – Peter Kämpf
    Mar 12, 2020 at 19:20
  • $\begingroup$ @PeterKämpf So even a fence locally experiencing inward flow would create a vortex impeding the outward flow further back? $\endgroup$
    – Guy Inchbald
    Mar 12, 2020 at 19:25
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It is deflected outwards by swept back wings and inward by front sweept wings. That is the main job for wing fence (to prevent the outside flowing of the airflow). enter image description here

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    $\begingroup$ That is the received wisdom which I am specifically questioning here. $\endgroup$
    – Guy Inchbald
    Mar 12, 2020 at 10:13
  • $\begingroup$ First portion of contact the airflow go inward is true but then it travels outward. All depends on the AOA and speed at the moment. Some spelling along with the Leading Edge all the way from the root of the wing to the tip if the Leading Edge it has big curvature and not sharp enough $\endgroup$
    – George Geo
    Mar 12, 2020 at 10:55
  • $\begingroup$ But most of the air travelling on the top side of the that type of wings is to the tips $\endgroup$
    – George Geo
    Mar 12, 2020 at 10:56

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