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How Important Is A Plane's Skin For Aerodynamics?  
User currently offlinea380900 From France, joined Dec 2003, 1118 posts, RR: 1
Posted (1 year 10 months 1 week 4 days 20 hours ago) and read 4305 times:

I tend to assume that what matters is shape. I remember hearing over the years that paint (or material or even the way paint is applied) could matter too. And usually where there is friction, the surface does matter at least a bit. For instance, I think I've read in here that dirty planes were less efficient.

To what extent is that true or not? Does it matter for all domains of flight? What about Concorde, did "skin" matter particularly for this plane?

Any insight would be appreciated.

[Edited 2013-02-09 17:47:46]

15 replies: All unread, jump to last
 
User currently onlineBMI727 From United States of America, joined Feb 2009, 15831 posts, RR: 27
Reply 1, posted (1 year 10 months 1 week 4 days 19 hours ago) and read 4265 times:

Quoting a380900 (Thread starter):
Does it matter for all domains of flight?

Yes. It changes drag so it's more important anywhere drag is particularly important, so basically cruising and anytime you want to go especially fast.

Quoting a380900 (Thread starter):
I remember hearing over the years that paint (or material or even the way paint is applied) could matter too.

It does. A skin friction coefficient varies with the surface, whether it's polished or painted in various ways. Things like flush riveting and panel gaps can make a significant difference too. All of this is usually balanced against weight as well. Paint is heavier than a polished surface, although polished surfaces take more maintenance. Also, the 737 takes a drag penalty for not having main gear doors, but the additional weight of having them makes leaving them off a better option overall.

When you talk about going especially fast, you also have to consider thermal performance. Some surface finishes are better in this respect than others. The blue Pepsi Concorde could not fly as fast for as long as the normal white Air France and BA ones since it would heat up too much.



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17171 posts, RR: 66
Reply 2, posted (1 year 10 months 1 week 4 days 15 hours ago) and read 4189 times:

Quoting a380900 (Thread starter):

I tend to assume that what matters is shape.

Shape matters. The shape results in form drag and interference drag. Along with skin friction drag, these three make up parasitic drag. All three forms of parasitic drag increase with the square of speed. So twice the speed gives twice the parasitic drag.

Given this exponential increase in drag with increased speed, a low drag surface becomes more important the faster the aircraft. Laminar flow is a way to decrease skin friction drag.

Skin friction drag is not only dependent on the surface texture itself, but also the surface area. Bigger plane means more surface area, and thus more surface for the fluid (air in this case) to have friction against.


I believe it was Singapore Airlines that was experimenting with a non-smooth "sharkskin" textured paint some years ago. Apparently it decreased drag significantly but the paint was more expensive. Don't know what became of that.

Quoting a380900 (Thread starter):
Does it matter for all domains of flight?

There is always skin friction drag as long as the aircraft is moving, but given the shape of the drag curve the higher the speed the more significant a factor parasitic drag becomes.

Quoting a380900 (Thread starter):
I think I've read in here that dirty planes were less efficient.

Yes they are. But you have to counter that against the cost of washing the plane.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlineflyingturtle From Switzerland, joined Oct 2011, 2529 posts, RR: 14
Reply 3, posted (1 year 10 months 1 week 4 days 12 hours ago) and read 4135 times:

Quoting Starlionblue (Reply 2):
I believe it was Singapore Airlines that was experimenting with a non-smooth "sharkskin" textured paint some years ago. Apparently it decreased drag significantly but the paint was more expensive. Don't know what became of that.

I wondered about that too. AFAIK these experiments were done about ten or twelve years ago, as I read about them in a pop sci magazine...

That the paint affects aerodynamics was a pain during WW2, when attempts at "precision" bombing were done.


David



Keeping calm is terrorism against those who want to live in fear.
User currently offlineglen From Switzerland, joined Jun 2005, 225 posts, RR: 2
Reply 4, posted (1 year 10 months 1 week 4 days 11 hours ago) and read 4120 times:

Quoting Starlionblue (Reply 2):
All three forms of parasitic drag increase with the square of speed. So twice the speed gives twice the parasitic drag.

It is correct that parasitic drag increases with the square of speed.Therefore twice the speed gives four times the parasitic drag. Twice the drag with twice the speed would mean only linear increase.



"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
User currently onlineBMI727 From United States of America, joined Feb 2009, 15831 posts, RR: 27
Reply 5, posted (1 year 10 months 1 week 4 days 10 hours ago) and read 4108 times:

Quoting Starlionblue (Reply 2):
I believe it was Singapore Airlines that was experimenting with a non-smooth "sharkskin" textured paint some years ago. Apparently it decreased drag significantly but the paint was more expensive

I would imagine that such paint would be used only towards the rear of rather bluff portions of the plane in order to trip the boundary layer and delay flow separation. For the rest of it, you'd want the smoothest paint possible for the lowest friction coefficient.

I've seen some indications here that Boeing is playing around with active boundary layer suction, which would be a logical next step. It's just a matter of making such a system practical and reliable.



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17171 posts, RR: 66
Reply 6, posted (1 year 10 months 1 week 4 days 9 hours ago) and read 4062 times:

Quoting glen (Reply 4):
Quoting Starlionblue (Reply 2):
All three forms of parasitic drag increase with the square of speed. So twice the speed gives twice the parasitic drag.

It is correct that parasitic drag increases with the square of speed.Therefore twice the speed gives four times the parasitic drag. Twice the drag with twice the speed would mean only linear increase.

Correct. I totally miswrote there.  



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDarkSnowyNight From United States of America, joined Jan 2012, 1412 posts, RR: 3
Reply 7, posted (1 year 10 months 1 week 4 days 8 hours ago) and read 4037 times:

Quoting Starlionblue (Reply 2):

Quoting a380900 (Thread starter):
I think I've read in here that dirty planes were less efficient.

Yes they are. But you have to counter that against the cost of washing the plane.

On a 120 day cycle (which is about typical for short to medium haul a/c; you can go to about 280 on LH aircraft), that wash would have to cost about $2500 before it becomes an issue. Can't speak for the rest of the world, but in the US, the going rate is a little less than 20% of that.

As with everything, there will be some variation based on type & usage, but for the most part, it would be fiscally irresponsible for an airline not to wash their planes.

Quoting BMI727 (Reply 1):

It does. A skin friction coefficient varies with the surface, whether it's polished or painted in various ways. Things like flush riveting and panel gaps can make a significant difference too. All of this is usually balanced against weight as well.

But don't forget that really only matters on the fwd 2/3rds of most planes. By the time you get to the APU box (assuming we're talking about a C-5), the boundary separation is almost two inches at cruise. This is why we see button heads all over the place on an empennage. Even though they actually add weight, and protrude about 1/2 cm beyond the skin, they're a lot cheaper, so, on they go.



Posting without Knowledge is simply Tolerated Vandalism... We are the Vandals.
User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 8, posted (1 year 10 months 1 week 4 days 5 hours ago) and read 3963 times:

Aircraft are the only vehicles where I never seen faired out paint lines. They are typically hard tape lines and do create resistance to airflow as witnessed by close observation of high time airframes...you can actually see where moisture has burned in trails on the skin consistent with paint lines. Also several carriers "Roll" paint on with paint rollers rather than spray. Additives are available for rolling polyurethanes as it is an accepted procedure but certainly not one that aesthetically works well or where fuel burn exists,...doesn't help. In the earlier days of the 787 Boeing suggested the engine nacelles/cowlings remain free of graphics, paint lines etc., as the airflow disruption, as minimal as it may be, effected engine performance. While I find that to be a stretch, it may be true. I guess the man hours involved in rubbing out paint lines, followed by clear coating would not be cost effective enough to warrant any benefit at least until jet A costs $20.00/gallon.

User currently offlineANITIX87 From United States of America, joined Mar 2005, 3309 posts, RR: 13
Reply 9, posted (1 year 10 months 1 week 4 days 5 hours ago) and read 3963 times:
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Quoting a380900 (Thread starter):
What about Concorde, did "skin" matter particularly for this plane?

Even more so, because of temperature. Any increase in friction would heat the skin more than anticipated, which could cause obvious problems. The aircraft elongated by some ridiculously large amount.

AFAIK, Concorde was polished from nose-to-tail rather frequently (I don't think it was for every flight, but maybe).

There are stories of the crew inviting passengers to the cockpit, the navigator putting his whole hand in a gap between his panel and the bulkhead, and telling the passengers that the gap was due to the aircraft's elongation - that gap did not exist on the ground. Pretty amazing.

TIS



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User currently offlinercair1 From United States of America, joined Oct 2009, 1348 posts, RR: 52
Reply 10, posted (1 year 10 months 1 week 4 days ago) and read 3854 times:
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Quoting DarkSnowyNight (Reply 7):
you can go to about 280 on LH aircraft), that wash would have to cost about $2500 before it becomes an issue.

Why?

I believe the colors available on the 787 engine are limited due to flow impact.



rcair1
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17171 posts, RR: 66
Reply 11, posted (1 year 10 months 1 week 3 days 20 hours ago) and read 3762 times:

Quoting ANITIX87 (Reply 9):
n a gap between his panel and the bulkhead, and telling the passengers that the gap was due to the aircraft's elongation - that gap did not exist on the ground. Pretty amazing.

Yepp. On one Concorde's final flight, one of the flight crew put his cap there. And there it remains, wedged in forever unless the plane is dismantled (or flies again).



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDarkSnowyNight From United States of America, joined Jan 2012, 1412 posts, RR: 3
Reply 12, posted (1 year 10 months 1 week 3 days 9 hours ago) and read 3623 times:

Quoting rcair1 (Reply 10):
Quoting DarkSnowyNight (Reply 7):
you can go to about 280 on LH aircraft), that wash would have to cost about $2500 before it becomes an issue.

Why?

Why what? Is the schedule different for LH vs SH Aircraft, is that what you're asking?


Simply put, the cost of washing a plane is significantly less than what they'd lose on extra fuel being expended to counter parasitic drag over that time period. It's why every major airline in the US has their planes on a wash schedule.

Much of the matter that accumulates on an aircraft tends to leak out of gear bays and control surface actuators. Thus a plane that has fewer daily cycles will not get as dirty as a short hauler that would have more.



Posting without Knowledge is simply Tolerated Vandalism... We are the Vandals.
User currently offlinebikerthai From United States of America, joined Apr 2010, 2191 posts, RR: 4
Reply 13, posted (1 year 10 months 1 week 1 day 7 hours ago) and read 3342 times:

Quoting DarkSnowyNight (Reply 7):
This is why we see button heads all over the place on an empennage. Even though they actually add weight, and protrude about 1/2 cm beyond the skin, they're a lot cheaper,

Button heads do not necessarily add weight. Depending on the situation, sometimes button heads allow for you to use thinner gauge skin at the fastener joint thus reducing over all weight.

They are cheaper to use if you have to drill the countersink by hand. If you use a robot to drill and install, then installation cost is not much less expensive than a countersink head.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlinePihero From France, joined Jan 2005, 4666 posts, RR: 77
Reply 14, posted (1 year 10 months 1 week 1 day 1 hour ago) and read 3244 times:
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As some posters have said, we are talking about boundary layer and skin friction drag here.
$The problem is rendered more complicated as different speeds cauise different skin drag coefficients ( the lower the speed, the higher the coefficient, which in part aids approaches as the skin drag coefficient, hence the total drag is higher, thus in some respect improving the lift )
For really slow moving foils / shapes, a clean surface is very important. That's why in the summer, we spend a lot more time polishing our gliders to a sheen as an insect covered surface would lose us many lift / drag points.

At airliners cruising speeds, the actual skin friction drag varies little from the computation data ( as an example, the highly polished 787 composite wing gains only some 2% of friction drag over an identical wing made' of metal ).

Of course, the less skin-draggy surface is with bare polished metal (Delta was right on this subject ), but the maintenace of these surfaces is a lot more expensive than a paint wash.

If one observes a wing, one could see on the leading edge bare metal, then a middle part that's rather roughly painted ; that roughness helps creating small eddies that keep the boundary layer attached to the wing, delaying airflow divergence.
Which proves again that life is a bit more complicated than theory.

[Edited 2013-02-13 12:41:27]


Contrail designer
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17171 posts, RR: 66
Reply 15, posted (1 year 10 months 22 hours ago) and read 2822 times:

Quoting Starlionblue (Reply 2):
I believe it was Singapore Airlines that was experimenting with a non-smooth "sharkskin" textured paint some years ago. Apparently it decreased drag significantly but the paint was more expensive. Don't know what became of that.

Speaking of which... http://travel.cnn.com/lufthansa-tests-shark-skin-881186?hpt=hp_bn6



"There are no stupid questions, but there are a lot of inquisitive idiots."
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