Generally speaking, does twist increase spanwise flow, because high pressure at wing root stream toward lower pressure at the tip?

I will write example with sailing jib:

When sail upwind, jib windward telltales can stream up or at 45 degress up(often middle and top one), leewward stream all horizontal.

I cant figure out what change wind direction in upward direction at windward side of sail?

Here is possible solution:

  1. Too much sail twist

  2. Jib sweep angle

  3. Jig angled to leeward maybe cause this spanwise flow

  4. Boat hull redirect wind in upward position

enter image description here

telltales: green windward, red leeward


3 Answers 3


This is a sailing question, but we must look predominately at the sail design, which comes to a very fine point, and also the speed of the wind and the boat, both much lower than typical aircraft.

So, there will be "spillage" of higher pressure to windward towards the finer end of the sail, while the leeward telltales are straight from the forward movement of the sailboat.

However, Reynolds number of this system is too low (Velocity) to generate an airplane-like circulation.

A square or rectangular jib would reduce spillage, but would capsize the boat easier (this is actually analogous to increased torque stress on the wing by not having an "elliptical lift distribution").

Wind tunnel studies would help here, but the spillage of pressure windward seems normal.


EDIT: when I answered, the question consisted only in the very first paragraph so I didn't know yet it was actually a nautical question.

On fixed-wing aircraft twist is mainly used, together with chord variation, to obtain a lift distribution as elliptical as possible, in order to minimise induced drag.

Moving along the wingspan, everywhere lift changes a so called trailing-vortex is shed backward and contributes to the formation of the wake. This vortex is correlated to the bent of the flow on the upper and lower wing’s surface as visible in this picture:

A more correct representation of the "tip" vortices

Spanwise flow distortion. Source: https://cdn-images-1.medium.com/max/1600/0\*2gL-QOXqx51qNdAE.jpg

To simply summarise: lift changes spanwise $\Rightarrow$ trailing-vortex is shed in the wake $\Leftrightarrow$ flow on the wing bends.

So, does twist increase spanwise flow? It depends on if and how much this twist makes the lift change.

For example, on a rectangular wing with no twist, the lift changing is concentrated toward the tip and therefore the biggest spanwise flow has to be expected there. On a jetliner’s wing, the spanwise lift is more uniformly distributed and so is the spanwise flow.

The following picture shows a sample of different lift distributions:

Spanwise lift distributions

Several spanwise lift distribution. Source: https://1.bp.blogspot.com/-e8479jPK0N0/U28fSOl8nrI/AAAAAAAASkM/35wCfWrvFq4/s1600/lift_distribution.jpg

The red line is for a rectangular wing; the target lift distribution is the elliptical one; and $\lambda=0$ denotes a delta wing.

From the picture it can also be understood how twist works: twisting the wing so that its tip has less incidence makes lift diminish there and a more ideal distribution is achieved. As pointed out by @RobertDiGiovanni this has also a relieving structural effect.

Bonus material

Sweep angle generates spanwise flow too: with a swept wing, the approaching speed can be decomposed in a component perpendicular to the leading edge plus a component parallel to it. This latter (called $V_n$ in the following picture) is completely spanwise flow:

Freestream flow decomposition

Speed decomposition. Source: this answer

Twist is used also on rotary-wing aircrafts but whit the double effect of both optimising the bladewise lift distribution to minimise induced drag and to compensate for the rotating speed becoming smaller and smaller (until null) toward the root. So, also in this case everywhere the lift changes spanwise a trailing-vortex is shed in the wake and the flow on the blades's surface bends, just like in the first picture.

When the helicopter is flying forward and the blade, during its rotation, is passing over the nose or over the tailboom, the freestream flow is also investing the blade completely spanwise.

  • 2
    $\begingroup$ Downvoter: "Please consider adding a comment if you think this post can be improved." $\endgroup$
    – sophit
    Sep 6 at 8:23
  • $\begingroup$ This answer contains a lot of effort, but interestingly enough, twist lowers wing tip AoA, which may reduce the wing tip vortex (as well as giving the wing more desirable stall characteristics). Elliptical lift distribution helps reduce wing tip (torque) stresses, but reduces overall lift (which is why some folks drop slats when heavy). Spanwise flow is much more an issue with swept wings, hence the development of winglets. $\endgroup$ Sep 6 at 10:32
  • $\begingroup$ @RobertDiGiovanni: thanks for your comment. Yes, the twist is used to reduce the wing tip vortex, I wrote it but maybe it is not clear, I'll adjust the text. Anyway I didn't consider any structural related issues here since it was not in the (original) question $\endgroup$
    – sophit
    Sep 6 at 10:44

Yes, at least across the wing's lower surface. For twists gentle enough to still have positive AoA at the tip, more twist means more AoA at the root, which means more pressure at the root's lower surface. Air flows from high pressure regions to low pressure ones however it can. The greater the pressure difference, the greater the flow. QED.

To see for yourself, tape pieces of yarn to a cardboard "flat plate airfoil," grasp it by its root and tip, hold it in front of a fan, twist it, and watch how much the yarn deflects spanwise.


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