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Several pictures of the Concorde taking off, show a cloud of water droplets (fog) above the wings. Like this:

Concorde taking off from Heathrow

I assume the fog is created due to low pressure over the wings. But, doesn't the size location and visible turbulence of this pattern suggest that the wing is stalled? It certainly doesn't look like laminar flow.

I don't recall seeing this on other aircraft at takeoff.
What's going on? Is it unique to the Concorde? Is it exacerbated by ground effect?

This Wikia entry says:

...the undercarriage had to be unusually strong. This was due to the unusual loadings due to the high angle of attack that Concorde needed to take-off due to its delta-wing.

The "high angle of attack" would support my guess that the upper wing is stalled?
I could understand a stall if the aircraft was landing, but these seem to be near stall during takeoff?

Brock Adams
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  • This is by no means limited to the Concorde: https://c2.staticflickr.com/8/7170/6777607687_392329226c_b.jpg – Jon Story Jun 06 '16 at 01:32
  • @JonStory, Yes, but on other aircraft it only occurs when the wing is nearing full stall -- much more often during landings than in takeoffs. – Brock Adams Jun 06 '16 at 02:06
  • Not really - an airliner lands at or above Vref, which is ~1.3x stall speed. Takeoff (rotation) typically happens at a slower speed, below V2 - and V2 itself is only ~1.2x stall speed. That photo I posted is a 747 on takeoff, not landing. – Jon Story Jun 06 '16 at 02:16

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You're correct that the condensation is due to low pressure, but the wing is not stalled. The highly swept delta wing of the Concorde created lift be what is called vortex lift. From my answer in that question:

In the case of wings having sharp, highly swept leading edges like delta wings, the leading-edge separation vortex phenomenon occurs at subsonic speeds. However, the separation does not destroy the lift as in the case of low sweep wings; instead, it forms two vortices which are (nearly) parallel to the wing edges.

The centre of the vortex is a low pressure region, where condensation forms. In this figure, you can see the vortex formation in a model of Concorde.

vortex flow

Flow around Concorde in a landing Configuration by Henri Werlé, ONERA, image from efluids.com

A side view of the same thing:

Landing

  • Flow around Concorde in a landing Configuration* by Henri Werlé, ONERA, image from efluids.com

So, basically, the aircraft is still producing lift; only the mechanism is different.

This kind of lift production is quite common in case of highly swept wings. The vortex formation is also found in a number of other cases like chines, leading edge extensions (like this F-18), etc.

F -18

Flow visualization over a 1/48-scale model of an F-18 aircraft inside a water tunnel at the Flow Visualization Facility from NASA's Dryden Flight Research Center. Image from wikimedia commons

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aeroalias
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You're right to assume low pressure on top of the wing as cause of the condensation. As far as location and turbulence, this would probably have to do with something called "Span Wise Flow" in which air flowing over a wing doesn't just flow straight back, but tends to drive outward towards the wingtips and spill over the edge (a reason for 'winglets').

Mr. Tingle
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