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1515Blue
7th Jul 2002, 03:53
the dc9 has vortlilons or votex generating pylons on the under surface of the wings. does anyone know exactly how they work? and for what purpose? is it anything like vortex generators on the upper surface of other airplanes to energize the boundary layer to delay separation?? the dc 9 32 which im familiar with doesn't have the regular vortex generators...

thanks

'%MAC'
7th Jul 2002, 04:17
Vortilons generate a vortex at high alpha, inhibiting spanwise thickening of the boundary layer and they help to stabilize the position of the leading edge vortex. On the DC-9 it is of interest that they were only added during flight test. The DC-8 has very behaved stall characteristics, and the Ė9 was a lose derivative of this four-engined jet. When it came to stall testing the TPs and FTEs found very different stall characteristics from the olí 8. Seeing an obvious difference with the wing, one having pylons and one not, they hung some pylons on the otherwise clean DC-9 wing. The stall became very docile with the pylons and they were eventually shaved to the form they are today. Many aircraft have upper-wing VGs, from military fighters to transports. The A-4 has a number of them on the leading edge. Many transports have VGs on the tail (fin) to suppress the boundary layer over the rudder.

1515Blue
7th Jul 2002, 04:24
thanks for the reply %mac,
could you please explain further what you mean by 'thickening of the boundary layer'? by this do you mean separation?--the change from laminar to turbulent flow? and also, could you talk a bit more about the leading edge vortex? thanks

'%MAC'
7th Jul 2002, 05:26
Yea 1515Blue, you got it, thickening of the boundary layer is the transition from laminar to turbulent flow, or for the DC-9 from semi-laminar, mostly turbulent flow to really turbulent flow with no cohesion.

At high alpha the boundary layer cannot negotiate the adverse pressure gradients formed at the wing leading edge, as a result the flow separates at the leading edge of the wing. The separated flow rolls into a set of vortices which reattach themselves to the wing. This reattachment line then encounters another pressure jump and again separates but is in opposite direction to the primary separation. The vortilon provides a mechanical blockage and creates a stable vortex that is at acute angles to the span-wise vortex, usually destroying it from that point on. The vortilon, though below the wing, creates a vortex above the wing, remember we are at high alpha.

1515Blue
7th Jul 2002, 05:47
hmmm...that's interesting. somehow though, i still cant picture the separated airflow 'reattaching' itself to the wing...

also, just a point of clarification... on a swept wing the tips stall first therefore the separation is inward????

how i wish we had a whiteboard...

'%MAC'
7th Jul 2002, 07:22
Yes it would be nice, but Iím afraid my drawing is worse than my writing (if can be believed).

Since I canít get to sleep, let me try to put you to sleep with more of this. The tips should stall last on a well designed wing. Raisbeckís Saberliner wing has the tips warped (twisted) down so that the wing root will stall first. Now on just a regular swept wing with no fancy clever stuff the tips will stall first because of 1. the leading edge vortex is thicker at the tips then the roots, (i.e. a span-wise diversion of the boundary layer) and 2. an increase in the local lift coefficient towards the tips, caused by a large downwash at the wings center and hence a relative upwash at the tips. That is why bending the tips down reduces the chance of tip stall, (alpha is lower).

Letís just simply and say that empirically it is shown that the rearmost sections of a wing tend to lose lift prematurely. This is a true statement and the theory behind it was shown mathematically by Pistolesi, Mutterperl, and Weissinger. Basically the simple lifting line fails for swept and curved wings.

As for visualizing the vortex, think of a sheet of paper rolled into a cone, overlapping itself. The pointy end of the cone is at the L.E. wing root, and the wider portion is at the leading edge of the wing tip, roughly parallel with the wing sweep. Now can you see from the vortex how the tips should stall first and then move its way in?

'%MAC'
7th Jul 2002, 07:31
Another thing to add, slats (from previous post) can be used (have been used) to depress the boundary layer of swept-wing tips, decreasing their tendency to stall.

1515Blue
7th Jul 2002, 07:34
...so basically the vortillons break or upset the span wise flow??? still trying to visualize it all... but thanks for explaining it.

'%MAC'
7th Jul 2002, 07:44
Yes, they are outboard of the wing root and disrupt the chaotic spanwise flow by creating a vortex at an acute angle to the span-wise flow. Span-wise flow acts somewhat parallel to wing sweep (oh, I'm opening myself up for lots of criticism) where vortilon induced vortices act in the direction of the flight path. That is their lateral axis are aligned in these directions. Oh, I hope I didnít lead you astray. I did edit the top post for clarity, I see how my clarity was opaque.

1515Blue
7th Jul 2002, 07:53
in that sense they aren't really like the vg's at the upper surface of other wings that ' energize' the boundary layer to keep it attached to the surface... the span wise flow flows around them???

'%MAC'
7th Jul 2002, 08:04
I would contend that the vgs on the upper surface energize the flow for the trailing edge, and have little effect on the span-wise flow. Vgs are generally in a line and create very small vorticies. A vortilon, sawtooth, or wing fence creates a healthy vortex to disrupt the span-wise flow. I'm sure they aid to some extent, but vgs are quite a burden drag wise.

1515Blue
7th Jul 2002, 08:10
i'm a bit more confused.. how can the vortilon located underneath the wing creat a vortex above it? still trying to visualize it

'%MAC'
7th Jul 2002, 08:21
We got a great volley going here.
The air strikes the underside of the wing at high alpha and negotiates the turn around the leading edge. The vortilon's position is very close to the stagnation line and it is designed to provide a vortex only at high alpha (alpha is angle of attack in American parlance, sometimes the British use the term angle of incidence). Remember that we really don't want to create vortices, because that is adding to our drag, but at high angle of attack that's okay. Is it getting any clearer? Sorry I can't write better.

1515Blue
7th Jul 2002, 08:30
it's a bit clearer...
so this is what i gather... the vortilons are there then basically to redirect the span wise flow... through the creation of vortices. but not really function in the same way as the upper wing VGs...


makes me wonder then why not all airplanes have them. could it have something to do with the 9 being a t tail?

'%MAC'
7th Jul 2002, 08:43
No, it has to do with the DC-9 wing, the flaps, the required performance speeds (remember they are based on stall speed), and quite possibly the joining of the wing to the fuselage and fairing. Remember it was an unanticipated modification after the stall tests were being flown. You can't just look at a wing and say, 'oh well that needs some vortilons.'

Clarification: The vortilon is there to destroy the span-wise flow and allow roll control at high pitch angles such as exist during take-off and landing. The vortilon achieves its objective by creating a strong vortex that is at an angle to the naturally occuring span-wise vortex (flow) destroying the span flow. Roll control is a funcion of the ailerons and the vortilon causes the tips to stall after the wing root, so that some roll is still available in the stall. (And that's if they work as designed.)

1515Blue
7th Jul 2002, 08:50
k...

thanks for all your replies %mac. let me sleep on this one and hopefully it'll sink in later on