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In this paper, it states:

The leading edge suction of the left backswept wing is larger than that of the right forward-swept wing at subsonic speed, and the lift vector of the left wing inclines forward (see Fig. 2(b)), which results in smaller drag of the left wing.

What causes the left back swept wing to have more leading edge suction? For clarity, this is an obliquely swept wing:

enter image description here

What is different about the right forward swept wing compared to the left back swept wing? Why does the left side have a higher amount of leading edge suction?

It also states that at subsonic speed this is true, which to me, hints that this is false at supersonic speed.

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    $\begingroup$ That's a very good question. If the leading edge suction is related to lift then, given the spanwise flow doesn't participate to lift, the RH wing should generate more lift (the leading edge is "more perpendicular" due to the sweep angle, so less spanwise flow). This is obviously wrong, I'm curious for the explanation. $\endgroup$
    – mins
    Commented Dec 2 at 13:51

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The paper shows an image for the chord-wise pressure distributions for an oblique wing (Fig 2a), which indicates how the pressure distribution is modified due to forward ('right wing') and rearward ('left wing') sweep:

enter image description here

In case of the forward swept wing there is a reduction in suction for the front section, and an increase in pressure for the rear section.

This effect is caused by local boundary layer thickness modification due to the spanwise flow. Important in this is to note that the flows are in opposite directions. In the case of rearward-sweep, the spanwise flow is from root to tip, whereas in forward-sweep the spanwise flow is from tip to root.

This answer shows the effect of rear-ward sweep on the boundary layer. It shows that the crossflow helps to 'thin' the boundary layer. As Vos and Faroukhi - Introduction to Transonic Aerodynamics explain on p.470:

We should recall that the boundary layer thickness at any point on the surface is a function of the distance that the flow inside the boundary layer has traveled from the leading edge (see the discussion on p. 305). With that notion, we can easily deduce that the boundary layer over the outboard wing is thicker than what would be expected based on two-dimensional boundary-layer properties. Conversely, the crossflow in the boundary layer causes the thickness of the boundary layer at the root to decrease compared to its two-dimensional counterpart.

  • In the case of the rearward-swept wing, we have spanwise flow from root to tip, this reduces the boundary layer thickness which accentuates curvatures thus leading to a higher peak.
  • In the case of the forward-swept wing, we have spanwise flow from the tip to the root, this increases the boundary layer which reduces the curvature thereby spreading out the lift.
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  • $\begingroup$ Sorry, forgot to paste the part of the paper that relates lift to leading edge suction $\endgroup$
    – ROIMaison
    Commented Dec 2 at 13:12
  • $\begingroup$ @mins I realized I misunderstood some text in the original paper. I rewrote the answer to focus on the chordwise distribution of the effects. $\endgroup$
    – ROIMaison
    Commented Dec 2 at 14:33
  • $\begingroup$ I’m sure I’m misinterpreting something, but why does the cross flow thin the boundary layer? I thought any flow that wasn’t exactly perpendicular to the chord would thicken the boundary layer? (I did read the linked answer) - Edit: I just re-read the quote in your answer, and that made sense, but why does the cross flow at the root thin the boundary layer? $\endgroup$
    – Wyatt
    Commented Dec 2 at 14:54
  • $\begingroup$ Thickening/thinning depends on the direction. In general, there is an obstruction in the center that prevents crossing from the other side (either a mirror plane or the fuselage). If inboard flow drives particles to the centerline, these particles cannot go anywhere, resulting in an accumulation of air particles; a thicker boundary layer. Conversely, in the case of outboard flow, the air particles are driven outboard. They cannot be easily replaced and the boundary layer becomes thinner. $\endgroup$
    – ROIMaison
    Commented Dec 2 at 15:05
  • $\begingroup$ Thanks for your answer. I've some difficulty to understand the paper: "The aerodynamic load of the right forward-swept wing is concentrated on the wing root; thus, the leading-edge suction of the right wing tip and the lift coefficient decrease". Let's assume the load is redistributed to the root, still the "thus" in the citation is not obvious. I can understand this changes the wing moments, but that doesn't explain why the lift coefficient and/or the LES coefficient decreases overall. Why would the lift at the tip "count" more than the lift at the root? $\endgroup$
    – mins
    Commented Dec 2 at 15:50

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