starline
Posts: 20
Joined: Fri Nov 25, 2005 8:43 am

B767 Vortex Gen. On Flaps

Fri Jun 11, 2004 1:19 pm

Hi,

saw for the first time on a Condor B767 vortex generators mounted on the bow of the flaps. Why are they mounted there, is the wing designed that bad that it's not useful putting them on top of the wing ?
 
QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

RE: B767 Vortex Gen. On Flaps

Fri Jun 11, 2004 6:52 pm

As you probably know, vortex generators add kinetic energy (KE) to the airflow passing over a wing; they're basically little airfoils which, for obvious reasons, produce vortices at their tips. Those vortices stir up the airstream and thus increase its KE. Now, without going into too much detail, air passing along a wing gets to a point where it doesn't continue along the wing surface, and separates - that certainly reduces lift, and can in extreme instances lead to a stall. So all vortex generators do is delay that separation by adding KE to the airstream, so that it continues along the wing surface for a longer period of time.

Okay, you probably know all that already. But why are those vortex generators on the flaps? After all, most vortex generators are mounted near the leading edge of the wing, at least on the wing itself. Well the bigger the flaps get - the more the wing's chord is increased - the greater the chance of early seperation. Even if the wing is fine by itself, deployed flaps can push it over the line. So, rather than creating turbulent flow earlier than necessary with the vortex generators placed on the wing itself, they're located on/just-in-front-of the flaps (where they're needed most). In addition, having the vortex generators on the flaps means they "retract" and disappear when the flaps do, so they're not around any longer than necessary. In addition to the 767, I know that the 777 for one is like that, as well as many/most other relatively large aircraft.

Cheers,
QantasA332
 
oly720man
Posts: 5761
Joined: Fri May 21, 2004 7:13 am

RE: B767 Vortex Gen. On Flaps

Fri Jun 11, 2004 8:57 pm

Not quite true about the flaps and the chord increasing.

The flow between the wing and the flap is accelerated and helps the flow over the upper surface of the wing (the high velocity past the flap reduces the local static pressure which makes it easier for the flow over the upper surface.) It could be argued that the chord gets shorter since there will be a gap between the wing and flap that the flow can go through. It is generally found that the effect of flap deflection has little effect on the stall incidence, just the amount of lift. Leading edge slats increase the stall incidence

Having never seen the vg's on the 767 flap (any photos anyone) I can only imagine that they are there for one of 2 reasons; either when the wing is at low incidence and the flap is shielded to some extend by the wing or when there is a large flap deflection so separation is delayed.


Andy
wheat and dairy can screw up your brain
 
Mender
Posts: 249
Joined: Wed Feb 04, 2004 9:36 am

RE: B767 Vortex Gen. On Flaps

Fri Jun 11, 2004 9:12 pm

The Vg's are fitted to prevent unintended roll oscillations during the final phase of landing. Airflow over the outboard flap can separate when the spoilers forward of the flap are deployed while the flaps are set at FLAPS 30.

Abrupt control wheel inputs will deploy the spoilers and can lead to roll oscillations with increasing magnitude. The installation of vortex generators on the leading edge of the of the outboard main flap will create vortices which help keep the airflow attached to the flap during spoiler deployment.

In other words. They are only needed to ensure the aircraft remains safe when landing in extreme weather.

They are also fitted to some 757's
 
QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

RE: B767 Vortex Gen. On Flaps

Sat Jun 12, 2004 6:50 am

Sorry Oly720man, my mistake. I think I was in a plain-flap frame of mind as I wrote...  Nuts

Cheers,
QantasA332

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