Swept Wings spanwise flow and tip stall

[matcv88]

Hello everyone.

I have found one thread about this with a couple of answers but I am still confused about few things about swept wings and I hope someone can help.

In a swept wing there is a spanwise flow from the root towards the tip on the upper surface, and from the tip towards the root at the bottom is this correct?

Can someone explain me why does this happen?What causes the span wise flow on a swept wing?

Also the span flow plies up air at the wing tip, and this makes the tip stalling first as this thicker layer get easily separated. Why is this? Why if the boundary layer at the tip is thicker this would make the wing prone to a tip stall?

Thanks a lot in advance

[AdamFrisch]

Well, spanwise flow happens because air behaves like a liquid and will take the path of least resistance. In this case it's along the swept leading edge it tends to move as that offers less resistance than going over the wing to some degree (very simply explained - there's more to it). This however mixes with the air that does go over it (all the air can't "go around" the wing, most of it has to go over it), which creates a blend of the two working itself outwards along the span both on top and below the wing. The thicker the wing (and the more it needs to displace), the more spanwise flow in a swept wing. Same thing goes for tip vortices - they're the end result of the spanwise flow meeting.

There have been a few planes with forward swept wings (a glider comes to mind) where the spanwise flow actually goes inwards, towards the fuselage.

[keith williams]

In a swept wing there is a spanwise flow from the root towards the tip on the upper surface, and from the tip towards the root at the bottom is this correct?


The above statement is not true. I suspect that you are confusing the effects of wingtip vortices and the effects of sweepback.

With a straight wing, the high pressure below the wing leaks around the wingtips, to the upper surface. This produces wingtip vortices, a span-wise flow from root to tip under the wing and a span-wise flow from tip to root above the wing. None of this has anything to do with sweepback.

With sweep back the air meets the wing at an angle and this cause the flow to be deflected outwards both above and below the wing.


Also the span flow plies up air at the wing tip, and this makes the tip stalling first as this thicker layer get easily separated. Why is this? Why if the boundary layer at the tip is thicker this would make the wing prone to a tip stall?


The outward deflection of the airflow increases the length of the path that the air must follow when moving over the surfaces of the wing. As the air flows over the wing surfaces friction causes it to slow down. This reduces the kinetic energy and dynamic pressure of the boundary layer, making it more difficulty for the air to follow the contours of the wings. This in turn makes it easier for the boundary layer to separate from the wing, thereby causing stall. The air is slowest, and the boundary layer is thickest at the wingtips because the air at this point has taken the longest path. So this is where stall is most likely to start.

To understand why a low energy boundary layer is more prone to stall we need to look at the pressure distribution and what the airflow is attempting to do. As air flows aft from the leading edge of a wing it is initially flowing from an area of comparatively high static pressure to an area of lower static pressure. Gasses naturally from from high pressure areas to lower pressure areas, so this first part of the motion poses no problems.

But after the airflow has passed the point of lowest pressure it must move from an area of comparatively low static pressure into an area of higher static pressure. To maintain this flow the air must have sufficient kinetic energy and dynamic pressure to push its way into the higher pressure area. The kinetic energy and dynamic pressure are lowest within the boundary layer, so this is where the flow is most likely to break down first. When the boundary layer energy reduces to a point at which it cannot continue to flow over the surfaces it will separate from the surface, and the overall flow in that area will stall.

[matcv88]

keith Williams I don't know who you are or what you do but THANK YOU!
last couple of times I wrote here asking questions you always answered with detailed replies and REALLY REALLY well explained.


I really appreciate everyone that spends 5 minutes trying to make a rookie like me understand something, but you keith and your replies are incredibly helpful thanks a lot again

[keith williams]

Glad to be of help.

I firmly believe that student pilots should learn and understand as much as possible about the technicalities of flight and I do my best to assist them in doing so.

[Christophe Dupret]

I do Confirm Keith williams, your post is amazingly clear. The point is... Why is this topic not explained like this on any ATPL cursus like jepp or oxford...

[Funny Username]

Yes! A brilliant explanation, Keith. So true what Chris D said too. Why cant it be explained how you did in books and courses. Cheers.

[paco]

Maybe this will help as well....


http://www.captonline.com/swept.png

The dangerous bit is that, as there is less lift at the tips, the Centre of Pressure moves inwards, to the root, and forwards, following the line of the wing. When this gets in front of the C of G of the aircraft, it will cause a pitch up.

http://www.captonline.com/sweptcp.png

[Halfwayback]

By Moderator
Keith actually does teach ATPL students and we here at PPRuNe are very grateful to him for the time he spends here helping aspiring pilots climb the ladder. The same for Paco.

HWB

[paco]

Aw shucks! <blush> :)

[Halfwayback]

That'll be a first!

HWB

[PDR1]

There is a further wrinkle in that the spanwise flow component from the root (heading outboard) is opposed by the spanwise flow component from the outer wing panels (heading inboard) to produce a "stagnation" at some point along the span, and this will generally be where the stall initiates. The problem is that this point is not very well "defined" (ie it is a function of a wide range of parameters, some of which are not very controllable) and that's why swept wings are prone to unpredictably asymmertic stalls and wing-drops.

There are a number of solutionms to this problem including wing LE fences, LE Notches, dog/sawtooth leading edges and even leading-edge camber changes. All of these except the last attempt to nail the stagnation point into a known place by creating small vortices that impose the desired pressure conditions at high alpha. If interested you can find a more detailed discussion of this in Daryl Stinton's Green Book.

[paco]

More pieces to the puzzle - thanks for that! :)

[paco]

Adverse Pressure Gradient...... :)

[Rotorhead123]

I do Confirm Keith williams, your post is amazingly clear. The point is... Why is this topic not explained like this on any ATPL cursus like jepp or oxford...
Mr Williams,
Thanks for the best explanation on spanwise flow. I think I was always taught that the wing tip high pressure could not effect the upper surface of the wing because of the forward motion of the aircraft, thus spinning wake behind the aircraft. If this is inaccurate,and I'm sure it is,please explain.

If you ever have time and energy could you explain, Bound Vortex. I have watch endless Youtube video"s on the subject but don"t understand it.

Thank you,

[paco]

It's called a bound vortex because it stays with the wing. It is a theoretical model that uses vortices instead of a physical wing. For engineers only!

When it starts to move, there is a stagnation point on the rear upper surface of an aerofoil, due to the time lag, which makes the air leave the surface and produce a vortex just above the trailing edge - this would be the starting vortex which contributes to the circulation effect and allegedly contributes to the upwash at the front.

According to the circulation theory, the starting vortex creates a counter flow that reduces the speed of the flow underneath the wing to increase the static pressure there, while that above is increased, to reduce the pressure further. The resulting pressure field helps to create the upwash at the front of the wing, through a process called circulation, which is the motion around curved paths of the particles of air affected by the passage of an aerofoil surface. It is generated by any body moving through the air at subsonic speeds where pressure gradients arise and flows are induced as the fluid concerned tries to regain the status quo. The air doesn’t rotate about the wing, but we get an equivalent effect if you imagine that the airflow over the upper surface is the same as the average speed plus a bit, and that through the lower surface is similar to the average speed minus a bit - rather like the difference between a headwind or a tailwind when it comes to groundspeed. It can be increased with local increases of camber (flaps, etc.)

So, at subsonic speeds, there is a circulation that travels with the aerofoil that forms a bound vortex across its span, so called because it stays with it.
When it imparts a circulation to the air, an aerofoil will experience an equal and opposite reaction. This effective torque (denoted M) is called the pitching moment, and is nose down when lift acts normally.

[Rotorhead123]

Pack,
Thank you for this great explanation!

[Rotorhead123]

a stagnation point on the rear upper surface of an aerofoil, due to the time lag,

Could you please expand on time delay.
Thank you

[paco]

The air is moving over the front of the wing but not the back, immediately. That creates a wee vortex