Wing fences?

[dannyweaver]

Most airliners are swept wing aircraft, so why don't they have/need wing fences? Do they not suffer from (is it called?) 'spanwise flow', or do they use some other technique? Probably an obvious answer to you lot, but I'm only a wannabe, so have pity on me! ;)

[wobblyprop]

You're right wing fences were added to wings to stop span wise flow. This is somewhat of a low speed problem.

Many modern airliners with swept wings have engines mounted on pylons protruding forward. This helps to generate a vortex over the top of the wing. The vortex acts like an aerodynamic wing fence.

It is also possible to have a notch somewhere along the leading edge. This also does the same as above.

I'm not sure what the likes of an MD11 have though.

The pylon protruding forward of the wing helps to balance the wing during high speed flight and prevent twisting.

[dannyweaver]

Mmm, I see.... But:

this is all very well for the likes of the A340 or B747, with two engines on either wing, but what about all the others with just one per wing? The engine is always located fairly near to the wing root, so surely this vortex you're on about won't have much effect? What about between this vortex and the root?
:confused:

[wobblyprop]

I knew you'd ask, I had to look up the name of these things.

If you look under the wing you will see vortilons.

They are box shape objects tapered at the forward and trailing edge.

See the example below (this is the best picture i can find at the moment.)

example (http://www.boeing.com/companyoffices/gallery/images/commercial/737700-07.html)

You will notice that outboard from the engine there are serveral of these vortilons. They have the same effect as the engine pylon.

The trailing edge of these usually breaks, moving back and downwards with the flaps.

I hope this helps.

[avialuver33]

I thought I remembered reading something about this, so I dug up one of my books. In the volume entitled "Modern Combat Aircraft Design", Klaus Huenecke says the following, and I quote:

The saw-tooth leading edge is a sure sign that the aircraft builder has not got the wing flow under control and therefore has to resort to this makeshift solution.

A fence is a very easy means of influencing the boundary layer. At one time this was frequently employed, but because of advances in wing design it has largely disappeared from combat aircraft.

By means of a boundary layer fence a virtual division of a sweptback wing into discrete areas occurs, so that undesirable spanwise boundary layer effects cannot spread, except to a limited extent, to neighboring areas. In particular the object is to prevent the drifting of the boundary layer into the outer areas which are endangered by separation anyway. A disadvantage of the boundary layer fence is its high friction and interference drag in high speed flight.

I'm not sure how much of this applies to commercial transport aircraft, but surely some of it has to be relevant. Hope this helps.

[mikef]

I remember reading somewhere that wing fences are incorporated when the aerodynamicist says "too hard, I want to go home" :D

[Smokie]

Wobblyprop,

Mmmmmmm, those "vortilons" look suspiciously like Flap track fairings to me. They actually house the rather large Screw Jacks that operate the Double Slotted Fowler Flaps on the 737.

At least they did when I worked as an Airframe fitter for Britannia many moons ago, hope this helps.:)

[wobblyprop]

Smokie,

You're probably more than correct.

Going from the principles of flight notes, that was the only explanation i could find.

Also, from the diagram in the notes, the vortilons are not as far forward as the leading edge but still produce an areodynamic fence.

I still think this flying thing is a black art ;)

[spekesoftly]

dannyweaver,

I suggest you might like to buy/beg/borrow/steal a copy of 'Flyer' magazine. The October 2002 edition has an excellent article written by John Farley (former Harrier test pilot, and current PPRuNer) entitled 'Going with the flow'. It is very readable, and mentions quite a lot about wing fences and spanwise airflow.

[Smokie]

Wobblyprop, so do I .:cool:

[PickyPerkins]

----- Start of Quote -------
wobblyprop I knew you'd ask, I had to look up the name of these things. If you look under the wing you will see vortilons. They are box shape objects tapered at the forward and trailing edge.
----- End of Quote -------

I had never heard of these vortilons. but now I see good pictures of of them on a Hawker 800XP in "Flying" magazine, November, 2002, p. 70 and 75. They seem to be pointy things projecting downwards and in front of the wing leading edge about 1/4 or 1/3 of a wing length out from the wing root to the wingtip. The caption says "The vortilon, like the stall fence, improves aileron effectiveness at low speeds".

This picture (airliners.net/open.file/231565/M/) of a Hawker 800 XP shows a vortilon as a small white feature under the leading edge of the wing immediately under the engine intake.

I understand from browsing the net that vortilons are popular on Long-ezes and are very effective in increasing stability at low speeds.

[A320_Murray]

Hey,
No expert here...but..I read in another forum that that wingtip fences are the same as adding an extra x amount of wingspan..useful when wingspan is an issue, i.e the MD-11!!!...hence the reason the 777 doesn't have them..could be wrong...:)

[PickyPerkins]

So far as I can make out, vortilons are not on the wingtips.

[A320_Murray]

Hey,
Yup sorry...I misread the post...:confused: aah well...

[low n' slow]

I see that some of you confuse vortilons with wingfences.

Wingfence: Big and ugly thing (look at the SAAB Lansen wing)

Vortilon: Little and aggressive looking and usually sits in a group.

However, I could also be confused now I realize.
Is vortilon = vortex generator?

I know however that the 737 has vortex generators placed just under the leading edge of the wing but still on the slat. I guess that these are the ones that wobbelyprop was looking for but couldn't find.

Now that we're into vortex generators and such: Does anyone know why the 737 has vortex generators right at the fuselage end. The little gap on the fuselage between the elevator and the rudder is covered in them. Seems odd to me to put them there... but they must be doing some good eh?

regards/lns

[Mad (Flt) Scientist]

To clarify, since there seems to be an element of confusion with the terminology.

A vortilon is precisely as described by PickyPerkins, and are fitted to the Hawker 800 as he notes, and also to the Bombardier Conti...oops Challenger 300. It is a flat plate surface mounted under the leading edge of the wing, and generally looks like it is intended to spear any birds or groundcrew near the L/E. It is a device for control of the wing flow (usually concentrating on stall) by means of a generated vortex.

A vortex generator is a more general term, but is normally meant to be a small device, typically triangular-ish, often placed in a group, which is used to either re-energise the local flow and prevent separation or used to "fix" a shock on a transonic aerofoil and prevent unwelcome effects due to the shock migrating.

A wing fence is a flat plate device, typically (but not always) wrapped around the leading edge and extending chordwise along some or all of the wing. It is used to either prevent (minimise) the effect of spanwise flow and/or to control the originating point and development of a stall.

And a winglet, which seems to have got caught up in this discussion, is a device on the wing tip which converts some of the "wasted" energy in the tip vortex into thrust, and gives the benefit usually asociated with a larger span. Some very advanced and specialised wing tip devices can almost eliminate the vortex, with attendent improvements to range, fuel burn, etc.

[Ref + 10]

From what I remember of my aerody classes, Vortex generators are used to promote "mixing" of air near the upper surface of the wing. This creates a turbulent area and moves the seperation point of the airflow rearward which makes the airflow over the wing more efficient.

Mad Scientist, I've never heard that winglets create thrust! I thought that they created more efficient lift characteristics over the wing area by preventing air "spilling" over the end of wing. Aren't lift and thrust different forces? Increasing lift results in a smaller demand for thrust which then translates into lower fuel flows but they are secondary effects of winglets not primary ones.

[eng1170]

Now seriously confused!

Low'n'slow - You mentioned the 737 having vortex generators UNDER the leading edge of the wing but still attached to the slat. I know that there is two groups of generators on the upper surface just o/b of the pylon, and then further out towards the tip, but have never seen them below the wing - is this on a 737NG?

I have also noticed the 4 on either side of the rear fuselage and just assumed that they are there to improve the airflow over the lower section of the rudder as ther is quite a gap at the bottom where it sits on its seal. But I could be wrong (wife tells me I usually am!!) and if anyone can enlighten me - great!

[Mad (Flt) Scientist]

Ref + 10
Mad Scientist, I've never heard that winglets create thrust! I thought that they created more efficient lift characteristics over the wing area by preventing air "spilling" over the end of wing. Aren't lift and thrust different forces? Increasing lift results in a smaller demand for thrust which then translates into lower fuel flows but they are secondary effects of winglets not primary ones.

Winglet Design (http://142.26.194.131/aerodynamics1/Drag/Page8.html)

specifically:
"Unlike the other methods mentioned above, the winglet does not strive to reduce induced drag so much as it uses it to create an offsetting thrust."

Essentially the winglet is designed such that as the wingtip vortex strikes the airfoil surface of the winglet, the "lift" created by the winglet is directed both inboards and SLIGHTLY forward. By this means some of the wasted energy in the wingtip vortex is extracted and used to provide thrust. This energy extraction has a secondary, but lesser effect, in that the less powerful tip vortex does create less downwash (and hence induced drag).

Because this is a three dimensional flow problem, the winglet must be carefully designed and is optimised for a design point. Winglets are therefore best suited for aircraft with well defined single (or limited) design points, such that the off-design point penalties are minimized.

(For that reason alone, I am not surprised to note that the Yak-140 soon lost its winglets; although I believe that the god of aircraft structures took a helping hand in that case:))

The effect that you are describing is more properly that of an "endplate"; see the following for a discussion:
Winglets and Endplates (http://aerodyn.org/Drag/tip_devices.html)
Essentially an endplate is a plate added to the entire end of the airfoil section; the design of the rear wings on an F-1 car, and the plates on the ends of the front wings too, are examples of endplates. As the article notes, endplates are a simpler, cruder approach to the same desire - a more efficient airfoil.

[Ref + 10]

Mad Scientist, I consider myself enlightened. :)

[Smokie]

Glider manufacturers, namely ASW and Schempp Hirth, discovered the benefits of winglets along time ago. (15-20 years ago)
They increased the max L/D of their gliders/sailplanes considerably.
Typically for the Ventus it increased from a glide angle of 1:42 to somewhere in the region of 1:48 and for the Nimbus 3 from something like 1:50 to 1:55, not quite the curvature of the earth but I think that they are still working on it ! :)

For you boys and girls who have never flown these superb machines...
Ah well... you have all missed out ! :) :) :)

[jungly]

At high sub-sonic speeds the local mach number is often close to 1.0. This often occurs mid way across the elevator or at the join of elevator and rudder.

Massive seperation will occur behind the shockwave and can mean that little airflow actually touches the control surface. It therefore becomes useless which is far from a desirable characteristic!

Many high subsonic acft have vortex generators near the rudders/elevators to re-energise the boundary layer thereby maintaining control effectiveness.

Obviously above Mach 1 this a moot point but thats where the all moving tailplane comes into play.

I suspect that is what you see on the 737.

*For interests sake have a look at the very inboard leading edge on the A340/330 wing. The slats dont sit flush. There is an 6" saw-tooth/notch.

[low n' slow]

Yes, they were placed on the NG aircraft. Three evenly spaced vortilons outside the enginepylons.

My conclusion is that these perform the same task as a wingfense by generating vortexes over the wing at high AoA.

Another interesting feature that I have noted is that when the slat is extended the geometric (or was it aerodynamic) washout is enhanced.
It never occured to me that this is an excellent way to make the wing more effective over a greater speedrange as this was never covered by the aerodynamics syllabus (at least not something that my class covered).

best regards/lns