Dirty wings?

Question

http://s203.photobucket.com/albums/aa220/avaitor2012/?action=view&current=0623070817a.jpg

You know when you fly and the wing appears burnt or dirty? Why is that? I assumed it was exhaust but I see it on outboard flaps as well as on the wing etc. What is this and where does it come from?

Answer 1

If you look real close you will see the vortex generators are attached to ribs running for and aft (structural points).  The generator disturb the airflow, air is dirty by its very nature.  The ribs are a fixed and at times a different temperature and attack the dirt particles, which stick to it.  Keep in mind the airflow on the leading edge splits the air steam and it flow up and over the wing carrying the fluids with it, which impact the vortex generators.

The real dark stains are grease and hydraulic oil coming for the leading edge slats and tracks nothing to worry about is normal.  Some of the stains behind the engines are carbon from the engines sticking to the upper surface of the wing.  All of this is normal unless you have a serious leaking line or a fuel cell fastener coming loose allowing fuel to weep into the slipstream.  This is why its important not to wipe the surface off until the mechanic have inspected it.

Since most of the difficulties of transonic flight are associated with shock-wave-induced flow separation, any means of delaying or lessening the shock-induced separation improves the aerodynamic characteristics. An aircraft configuration can make use of thin surfaces of low aspect ratio with sweepback to delay and reduce the magnitude of transonic force divergence. In addition, various methods of boundary layer control, high-lift devices, vortex generators, and so forth, may be applied to improve transonic characteristics. For example, the mounting of vortex generators on a surface can produce higher local surface velocities and increase the kinetic energy of the boundary layer. Thus, a more severe pressure gradient (stronger shock wave) would be necessary to produce the unwanted airflow separation.

A vortex generator is a complementary pair of small, low aspect ratio (short span in relation to chord) airfoils mounted at opposite angles of attack to each other and perpendicular to the aerodynamic surface they serve.  Like any airfoil, those of the generator develop lift.  In addition, like any airfoil of especially low aspect ratio, the airfoils of the generator also develop very strong tip vortices.  These tip vortices cause air to flow outward and inward in circular paths around the ends of the airfoils.  The vortices generated have the effect of drawing high-energy air from outside the boundary layer into the slower moving air close to the skin.  The strength of the vortices is proportional to the lift developed by the airfoils of the generator.

Vortex generators serve two distinctly different purposes, depending on the aerodynamic surface upon which they are mounted. Rows of vortex generators located on the upper surface of the wing just upstream of the ailerons delay the onset of drag divergence at high speeds and also aid in maintaining aileron effectiveness at high speeds. In contrast, rows of vortex generators mounted on both sides of the vertical fin just upstream of the rudder prevent flow separation over the rudder during extreme angles of yaw which are attained only when rudder application is delayed after an engine loss at very low speeds. In addition, rows of vortex generators placed on the underside (and occasionally on the upper surface) of the horizontal stabilizer just upstream of the elevators prevent flow separation over the elevators at very low speeds.

In summary, vortex generators on wing surfaces improve high-speed characteristics, while vortex generators on tail surfaces, in general, improve low-speed characteristics.

Answer 2

I'm going with "Dirt".

Answer 3

Yea its a combo of exhaust when the flaps are lowered and hydraulic fluids. It tends to happen a lot on the older 737 series.

I know that wasn't a 737 , I'm just saying the wing dirt collection trends to happen more on the older 737 series

Well I don't think thats dirt behind the VG's looks more like rust :) , possibly some moisture.

Maybe the pilot made a quick stop in his wifes house to pick up some lunch , hah sorry i had to much to drink tonight...kinda tore up the grass on the front lawn.

Answer 4

Most of what you are seeing is just extra grease that has collected dirt. Lots of moving parts in jet wings all the joints need to be lubed for easy movement. Nothing to worry about.

Answer 5

I would expect other pilots to know this, its lithium based lubricant. It sticks to metal and can stand much much higher temperatures. Its used liberally and gets all over the place.  If you get it on you doing a preflight it burns slightly.

I would expect it to get on the winglets from the ailerons.

Answer 6

The dirty stuff on the airplane is probably...brace yourself...DIRT.

Airplanes fly through air, and there is particulate matter in it.  Sometimes that dirt sticks to parts of the plane that get wet as the plane flies through clouds.  The vortex generators direct airflow over the top of the wings and that part tends to get wet first.  As the plane descends, the air gets a little dirtier and that dirt sticks to the plane a little bit.  The same exact thing happens to your car...as you drive it, it gets dirtier.

Washing airplanes is expensive and time-intensive.  Its not done as frequently as it once was, and so airplanes appear dirtier than they once did.

Answer 7

I disagree with everyone.  This is residue from water flowing over the vortex gens when you fly through rain.  Some of it is probably wearing off from the gen or the wing itself.

Answer 8

It looks like it might be oil or hydraulic fluid.Someone should say something to the pilot so they can take the plane to a garage to get it looked at.It looks dangerous.

Answer 9

Nope, engine exhaust. Hits the leading edge of the flap when they're extended flows over the top and bottom of the flap. If you could see the bottome it'd be obvious because it's dirty there to I guarantee.

Answer 10

Wow!  Really surprised at how many people thought it was exhaust on the upper surface of the wing.

Most likely it is dust, just the same as you get on your car.  It is forming in patterns like this because the vortice generators are breaking up the flow (as designed).  The air/dust gets trapped and adheres to the paint.

If you have localized patterns, it could be grease or even loose rivets (dark grey/black).

Answer 11

Zack, this is just exhaust that has collected on the aircraft. The exhaust has carbon particles in kerosene, and as the aircraft collects hours and landings, with  the engines using reverse thrust, it  collects the black on it, getting darker and darker with every landing. It gets washed off by major air carriers periodically. This can only be done at a few stations, and sometimes a particular aircraft for one reason or another will go longer than it is supposed to do so, making it very black and streaky.

Regards,

Dan

Answer 12

Dirt?  What dirt?  I don't see any dirt.