Spanwise airflow

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YouTube - Air-Flow Characteristics on a 40 Degree Swept Back Wing

the above video indicates that as the angle of attack rises the span-wise airflow rises & more probably the leading wing edges will stall first. Also notice that no appreciable change in airflow occurs at the root(attachment with the fuselage) of the wings
YouTube - Flow Studies Over a Swept Wing Model
video indicates the use of leading-edge blowing techniques to arrest the span-wise airflow & make it flow towards the rear root of the respective wing (notice the surface tufts at leading edges flowing ~towards the fuselage). Same result too can be achieved using vortes generators such as wing fences on SU-22
250px-Wing_fence_sukhoi-22m-4.jpg

Now, I have two questions.
  1. Why in the very 1st place spanwise airflow occurs i.e why not the air just flow straight towards the rear of wing?
  2. Is this spanwise airflow problem more prominent in Delta wing or Swept wing configuration?
 

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  • #2
boneh3ad
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Same result too can be achieved using vortes generators such as wing fences on SU-22
250px-Wing_fence_sukhoi-22m-4.jpg
Those aren't vortex generators; they appear to besu-22 more closely related to something called a Gaster bump. The purpose of a Gaster bump is related to attachment line contamination rather than spanwise flow, as you will observe spanwise flow with or without such a device except very close to it.

Over the fuselage of a plane, the boundary layer is almost always turbulent before it even reaches the wing. On a wing, the turbulence continues from the wing root all the way down the attachment line, leading to early transition on the wing and a huge increase in drag. A Gaster bump prevents this from happening.

The fact that they extend so far back may be used to try and get rid of spanwise flow, but it is really only delaying the inevitable, as the pressure gradient alone on a swept wing will lead to spanwise flow.

Why in the very 1st place spanwise airflow occurs i.e why not the air just flow straight towards the rear of wing?
On a swept wing, you have two things that will help lead to spanwise flow. First, there is no leading edge stagnation point. Instead there is what is called an attachment line, which is essentially the linear analog. It shares everything in common with a stagnation point except there is a velocity along that leading edge of the wing away from the root. This means that as the air starts to flow over the wing, it starts out with some spanwise momentum.

The second reason is that on a swept wing, there is a pressure gradient that as a result of the sweep, tends to push the flow in the spanwise direction slightly.

Is this spanwise airflow problem more prominent in Delta wing or Swept wing configuration?
The phenomenon isn't really a problem, for starters. It also would be equally prevalent on a delta wing aircraft as it is on a swept wing aircraft provided they had the same sweep and airfoil. There isn't really an intrinsic preference for one design over the other.
 
  • #3
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regarding ambiguity b/w fences & GasterBumps(I can't find any picture. of the latter)
Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Not to be confused with wingtip fences, wing fences are flat plates fixed to the upper surfaces (and often wrapping around the leading edge) parallel to the airflow. They are often seen on swept-wing aircraft. They obstruct span-wise airflow along the wing, and prevent the entire wing from stalling at once. Wing fences are often used in addition to or instead of slats.
http://en.wikipedia.org/wiki/Wing_fence

Regarding 2nd question.
In my understanding the pure-delta wing is highly prone to stall at high AoA reason is the spanwise flow!! This problem thus can be obliterated by the use of canards as is seen in the modern fighters like Saab Gripen; J-10; EF-2000 & the stealthy J-20....
Some of the older Mirage III/5/50, Cheetah and Kfir found a partial remedy to their congenital woes through retrofit of small fixed canards, while the Viggen had fixed canards designed from the outset. These canards added to the overall lift in ways similar to the leading edge flaps/slats, except that they remained stuck out even when not needed in high speed flight!
MiG-21 was a different story relating to tailed-deltas
 
  • #4
boneh3ad
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Either way, a delta-wing is not going to be intrinsically more prone to spanwise flow. Now, the occurrence of the phenomenon may well be more catastrophic to a delta wing, but it is not more common just as a characteristic of the design. Spanwise flow occurs in any swept-wing configuration.
 
  • #5
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In my understanding the pure-delta wing is highly prone to stall at high AoA reason is the spanwise flow!!
Actually, you have only half of the answer correct. The reason why pure delta wings are more prone to stall at a lower [tex]\alpha[/tex] is because of the extremely low taper ratio. Lower taper ratios [tex]\lambda < 0.3[/tex](typically), allows for tip stalling to occur more rapidly since the lift distribution tapers off so rapidly (less material to divide the high low pressure regions). Of course this allows for larger effects due to tip vortices an hence problems due to spanwise flow.

But as boneh3ad said, every aircraft suffers from spanwise flow, due to vortices.
 
  • #6
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^^ Thnx both of you for your contributions
 
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