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A Wing Fence is a straight, vertical "fence" running from forwards to backwards down the wing.

enter image description here

This MiG-17 has three fences per wing. I've also seen them on delta-winged aircraft such as the MiG-25.

The purpose of the wing fence is to prevent the air from flowing sideways over the wing. Air instead becomes trapped between the fences and flows front to back as normal. This is the ideal condition for creating lift, because the air follows the camber of the wing.

So why doesn't the Concorde have them?

enter image description here

The even more extreme sweep (at first) of Concorde's wing seems to me like it would induce even more sideways-flowing air, meaning it should benefit even more from wing fences.

(I noticed the Tupolev Tu-144 doesn't have them either.)

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  • $\begingroup$ At least in some cases the modern replacement is a vortilon. But it seems the more modern and sophisticated wing-designs doesn't need fences. en.wikipedia.org/wiki/Vortilon $\endgroup$ – Peter Jun 6 '16 at 12:25
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    $\begingroup$ As I understand it, wing fences only really help when the aircraft is approaching stall speed. The slower airflow over the wing begins to move sideways causing an increase in stall speed and a nose-up tendency in a stall. I'm guessing that part of the solution used on the Concorde was to fly above stall speed high enough to not induce that issue. $\endgroup$ – Ron Beyer Jun 6 '16 at 13:27
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    $\begingroup$ Flying above stall speed is clever ^^ $\endgroup$ – Peter Jun 6 '16 at 13:34
  • $\begingroup$ I am not sure I would even call MiG-25 wings a delta. The leading edge is not swept that much. Less than that of MiG-15/17/19 I think. $\endgroup$ – Jan Hudec Jun 6 '16 at 14:34
  • $\begingroup$ @JanHudec I've heard it described as a "cropped delta". In any case, it's a Mach 2+ craft with relatively high wing sweep, so I thought that it and the Concorde would both push air to the side more than front-to-back. One has wing-fences while the other doesn't, which is why I mentioned it. $\endgroup$ – DrZ214 Jun 7 '16 at 6:25
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The main purpose of the wing fence is to prevent the boundary layer thickening in the wing due to the spanwise flow observed in swept wing, as can be seen below.

Spanwise flow

Image from fuckyeahfluiddynamics.tumblr.com

This is the reason for having wing fence in aircraft such as Mig-25- to avert stall and improve stability. In case of highly swept wing like the Concorde, this is not required as the mode of generation of lift is different- using vortex lift.

Concorde wing

Vortices over the Concorde wing, Separation in three-dimensional steady flow, ONERA

In fact, there is a significant cross flow, as can be seen in the following image:

Cross sectional Flow

Flow above the Concorde Wing, Separation in three-dimensional steady flow, ONERA

Due to this, the wing fences are not required in case of highly swept wings like Delta and Ogival wings (as in Concorde), where separation leads to vortex lift. Mig- 25 wings, though highly swept, was neither of these- the wing generated lift by conventional manner.

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  • $\begingroup$ Strange. Why are those "wingtip vorticies" over the middle of the wing instead of the tip? $\endgroup$ – DrZ214 Jun 6 '16 at 23:12
  • $\begingroup$ @DrZ214 They are not the conventional wingtip vortices. The mechanism of lift generation in this case is different- it is through the vortices themselves. See the question on vortex lift for details. $\endgroup$ – aeroalias Jun 7 '16 at 1:03
  • $\begingroup$ @DrZ214: The vortex in delta wings (and indeed a square or circle wing) are different from wingtip vortex. Think of it this way: hold a large piece of squarish cardboard or foam vertically (like a wall, not a wing). Then push it forward. Obviously the area behind it will be a partial vacuum. Also obviously the vacuum cannot last, air will rush in to fill it - thus forming the vortex. It is this partial vacuum that is a source of lift, not the vortex. That's one reason why this sort of lift is called "vortex shedding" because the vortex gets created and destroyed continuously. $\endgroup$ – slebetman Jun 7 '16 at 3:06
  • $\begingroup$ the boundary layer thickening in the wing due to the spanwise flow observed in swept wing What does this mean exactly? Are you talking about compressive heating against the leading edge? Also, in that first grayscale image, are the brighter spots representative of "thicker" flow? $\endgroup$ – DrZ214 Jun 7 '16 at 7:56
  • $\begingroup$ I'm pretty naive to this concept, but it would seem like the fences are not just unnecessary, but would be harmful to the vortex lift effect by possibly decreasing the cross flow above the wing at the bottom of the vortex. Is that accurate? $\endgroup$ – AaronLS Jun 7 '16 at 21:03
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The Concorde makes use of vortex lift (link).
While one of the reasons for applying wing fences is to prevent air from flowing sideways, they are also able to create vortices (R. Whitford, Design for Air Combat, 1987), though this only applies to wing fences that don't extend over the entire wing.

The need for vortex generators can be overcome if the wing itself is designed in such a way that a vortex is generated. Besides that, the vortex itself is very stable and also prevents air from flowing sideways.

Concorde vortex flow (source)

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