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Faro
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Gloss Loss

Wed Jun 10, 2009 1:17 pm

When unpainted aircraft flight surfaces (wings, fuselage, fairings, etc) age and lose their gloss over 20 years of operation, what typical % drag increment do they generate? Does anyone know of any technical studies of this matter?

Faro
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tdscanuck
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RE: Gloss Loss

Thu Jun 11, 2009 4:41 am



Quoting Faro (Thread starter):
When unpainted aircraft flight surfaces (wings, fuselage, fairings, etc) age and lose their gloss over 20 years of operatio

Most of those surfaces are painted. The normal repaint schedule should take care of it. For the case of bare aluminum fuselages, they polish them on a periodic basis.

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RE: Gloss Loss

Thu Jun 11, 2009 1:45 pm



Quoting Faro (Thread starter):
When unpainted aircraft flight surfaces (wings, fuselage, fairings, etc) age and lose their gloss over 20 years of operation, what typical % drag increment do they generate? Does anyone know of any technical studies of this matter?

The reason they lose there gloss (shine) is because the operators don't take care of them. Look at AA MD-80's:


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Faro
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RE: Gloss Loss

Thu Jun 11, 2009 2:22 pm

Which is funny since shiny surfaces are slicker and therefore generate less drag, I would have thought it a priority to shine up the surfaces relatively frequently...

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klemmi85
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RE: Gloss Loss

Thu Jun 11, 2009 2:49 pm

Well, I might be wrong, BUT....

the AA livery doesn't look like someone goes out after every flight and makes em shiny. I rather suspect it to be some kind of chrome layer?!

Additional paint = additional weight = additional fuel burn. As for the wings for example, well... I don't believe they polish the wings, do they? That oughta be some ugly job. Imagine you had to polish a 744 / A380 wing, would be some work to do  Smile
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474218
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RE: Gloss Loss

Thu Jun 11, 2009 3:16 pm



Quoting Klemmi85 (Reply 4):
the AA livery doesn't look like someone goes out after every flight and makes em shiny. I rather suspect it to be some kind of chrome layer?!

The skins are polished aluminum. No chrome, no plastic coating, just 2024 alclad aluminum.
 
oly720man
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RE: Gloss Loss

Thu Jun 11, 2009 3:37 pm

If you can get hold of a book called Boundary Layer Theory, by Schlichting, there are graphs that relate the skin friction coefficient to the Reynolds number for different roughness heights. The effect on drag (skin friction coefficient) is highest when the roughness height is comparable to the height of the boundary layer (or laminar sublayer) which is on the wing leading edge and on the nose of the aircraft where the boundary layer is very thin. Further down the aircraft, the roughness is considerably smaller than the boundary layer and the surface (drag wise) behaves as if it is smooth.

Other work has been done, e.g. A.D. Young, The drag effects of roughness at high subcritical speeds, J Royal Aeronautical Society, V18, p534, 1950, that looked at the roughness of camouflage paint and the conclusions agreed with the graphs indicated above.

As for an absolute value for the change in drag, it would be necessary to integrate the change in skin friction coefficient over the whole surface to get an idea. I'd imagine it's in the 0-1% range, if that, i.e. in the noise of operational variation. I'm sure the bean counters watch each aircraft's performance and if the drag rise was that apparent, in terms of fuel use, for example, more planes would be polished more often.
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tdscanuck
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RE: Gloss Loss

Thu Jun 11, 2009 4:19 pm



Quoting Faro (Reply 3):
Which is funny since shiny surfaces are slicker and therefore generate less drag, I would have thought it a priority to shine up the surfaces relatively frequently...

Simply loosing the gloss is probably not a big enough impact on fuel burn to notice since, as Oly720man very nicely pointed out, the boundary layer is way thicker than that over most of the airplane. That's one of the reasons you can get away with button-head rivets at the back.

Duration-of-gloss is one of the specifications on aircraft paint, and aircraft do have a periodic paint/wash schedule (or polish schedule for bare fuselage).

Quoting Klemmi85 (Reply 4):
the AA livery doesn't look like someone goes out after every flight and makes em shiny. I rather suspect it to be some kind of chrome layer?!

Nope, just pure aluminum. 2024 clad is mostly 2024 with very thin layers of pure aluminum sandwiched to the surfaces.

Quoting Klemmi85 (Reply 4):
Additional paint = additional weight = additional fuel burn. As for the wings for example, well... I don't believe they polish the wings, do they? That oughta be some ugly job. Imagine you had to polish a 744 / A380 wing, would be some work to do

The wings are painted, not bare.

Tom.
 
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Faro
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RE: Gloss Loss

Thu Jun 11, 2009 4:24 pm



Quoting Oly720man (Reply 6):
If you can get hold of a book called Boundary Layer Theory, by Schlichting, there are graphs that relate the skin friction coefficient to the Reynolds number for different roughness heights. The effect on drag (skin friction coefficient) is highest when the roughness height is comparable to the height of the boundary layer (or laminar sublayer) which is on the wing leading edge and on the nose of the aircraft where the boundary layer is very thin. Further down the aircraft, the roughness is considerably smaller than the boundary layer and the surface (drag wise) behaves as if it is smooth.

Other work has been done, e.g. A.D. Young, The drag effects of roughness at high subcritical speeds, J Royal Aeronautical Society, V18, p534, 1950, that looked at the roughness of camouflage paint and the conclusions agreed with the graphs indicated above.

Many thanx, I'll try to get a hold of that. I was also guessing around 1% maximum drag increment, I would also guesstimate that most of that is "frontal" drag on the nose section and wing/empennage leading edges.

Quoting Oly720man (Reply 6):
I'm sure the bean counters watch each aircraft's performance and if the drag rise was that apparent, in terms of fuel use, for example, more planes would be polished more often.

I'm sure they do, being a bean counter myself albeit not of the aeronautical genus. I understand that their monitoring also takes into account those minute skin surface undulations which tend to develop over time and especially on the fuselage skin due to pressurisation cycles.

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474218
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RE: Gloss Loss

Thu Jun 11, 2009 4:35 pm



Quoting Tdscanuck (Reply 7):
The wings are painted, not bare.

Only the wing box and composite parts, the slats, flaps and aluminum (honeycomb) panels are polished.


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Faro
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RE: Gloss Loss

Thu Jun 11, 2009 4:45 pm



Quoting 474218 (Reply 9):
Quoting Tdscanuck (Reply 7):
The wings are painted, not bare.

Only the wing box and composite parts, the slats, flaps and aluminum (honeycomb) panels are polished.

Reminds me of an article I had read some 20 years back: European types were quoted as being highly resistant to corrosion because they were painted over almost everywhere whereas American airliners were deemed not as resistant because they showed more bare metal in the undersides. I guess by now corrosion resistance is pretty much the same on both sides of the pond.

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klemmi85
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RE: Gloss Loss

Thu Jun 11, 2009 7:09 pm



Quoting Tdscanuck (Reply 7):
Nope, just pure aluminum. 2024 clad is mostly 2024 with very thin layers of pure aluminum sandwiched to the surfaces.

Always learning something more, thanks  Wink
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