Aerodynamic drag area: Dirt vs clean paint vs polished metal

[Frank N. O.]

I recently noticed something called drag area in an article I saw online when airplane surfing and it seemed to describe that unlike cars that are generally just described by drag coefficient and frontal area then airplanes are also described by surface area that the wind has to go across. Ok of course this would probably cover anything going thrue wind and therefore also cars but it started me thinking about material friction values. In books where stallspeed for aircraft then the stall speed is listed as dirty which I gathered was with a dirty aircraft since dirt would decrease wing efficiency and I think this has even been mentioned for cars, but then I really started thinking (which would explain if anyone smelled smoke earlier ). Is there a significant different in aerodynamic drag when a plane is fully painted vs one that's in the buff? Or would new panels with fewer or no exterior rivets have a bigger impact? While on the subject of dirt then does more dirt grab a hold of bare metal panels then paint (possibly different types of paint and/or wax?).

In case I got something wrong or this is a dumb question then I'd like to defend myself that me and my mom are still waiting on the new car and the dealer was now on vacation until monday and we've both been sleeping badly so I'm not at peak efficiency but since you're so friendly and skilled and experienced then I took the chance.

Frank

[Fabo]

Well, clean and dirty are commonly used in aviation to say No Flaps or With Flaps...

[Frank N. O.]

oh man, I should've known it wasn't dirt on the wings since not many people fly thrue dust-storms. Well at least I asked this question amongs friends that won't hold it against me. Thank you for the friendly response

Frank

[Fabo]

Sure thing, I also enjoy the friendly atmosphere around here... it is sadly disappearing in many communities, even those around flying

[undatc]

You are correct to a degree though Frank.  Many new wing designs are only efficient when the wing is clean and highly polished.  I have a friend that is a co pilot down in MSP for 3M and basically his whole job is to polish the plane, every now and then he gets to fly.

I have a really good explanation of why it is, just don't have the time right now to write it all out.  I will after work tonight, unless someone else beats me to it.

[Franz]

oh man, I should've known it wasn't dirt on the wings since not many people fly thrue dust-storms. Well at least I asked this question amongs friends that won't hold it against me. Thank you for the friendly response

Frank

I also think in this case (stallspeeds listed in books) "dirty" refers to flaps ( I'm note sure wether gear / exterior loads count, too).
But "dirt", for example bugs, rain, ice or dust on the surface of planes has an impact on the performance.
This test of an ASW19 shows the difference in the flight polar for a clean (Figure 1) wing and a wing with simulated bugs (Figure 3).
There are even systems that allow you to remove bugs while flying to reduce the effects they have on the wing.
This video shows one of them in action.
Then I found another pdf which shows how uneven surfaces influence the performance of wings. But I do not know how much of this is true only for sailplanes. Maybe airfoils on powered planes are not as sensitive to uneven surfaces.

I hope I could provide some useful information.

CU Franz

[undatc]

Ok, instead of typing it all out, i did it the lazy way, and scanned copy from my book that describes how laminar flow airfoils work.  This is from the Jepp book, "Flight Theory for pilots".

What it is describing is the new wing design which makes use of the super smooth surface to reduce drag at a certain lift coefficient. 

[Fabo]

Franz: very interesting indeed.

And for that "one sted above pedestrian" - Were the flying language of men, soaring would be its poetry...

Still pedestrian hoping to be a poethe soon

[tundra_flier]

Or would new panels with fewer or no exterior rivets have a bigger impact? While on the subject of dirt then does more dirt grab a hold of bare metal panels then paint (possibly different types of paint and/or wax?).

The drag associated with dirt, rivets or other protrutions is part of the parasitic drag on a plane.  And I've sometimes heard the difference in a surfaces smoothness refered to as skin friction drag.  And yes, flush vs. domed rivets can make a big difference in a faster plane's performance.  The slower a plane flies the less important parasitic drag is.  Although I swear my C150 flies better with a fresh coat of wax, just seems much smoother in the air. 

As stated before, dirty vs. clean in the performance tables is actually landing configuration vs. cruise configuration.  So Dirty is with flaps, landing gear, speed brakes, spoilers, slats and anything else the plane has to add drag or increase lift hanging out.  Look at the wing of a jet on final approch sometime, there's all sorts of stuff hanging out all over the place making it very "dirty".  In cruise all that gets tucked up nice and flush in the wing.

Phil 

[Baradium]

As stated before, dirty vs. clean in the performance tables is actually landing configuration vs. cruise configuration.  So Dirty is with flaps, landing gear, speed brakes, spoilers, slats and anything else the plane has to add drag or increase lift hanging out.  Look at the wing of a jet on final approch sometime, there's all sorts of stuff hanging out all over the place making it very "dirty".  In cruise all that gets tucked up nice and flush in the wing.

Phil 

Sorry Phil.... spoilers/ speed brakes don't count for "dirty" when dealing with stall speeds.   A spoiler "spoils" the lift on the wing, essentially causing a stall.  The stall speed with spoilers deployed would be essentially anything the aircraft is capable of!

"Dirty" simply means in landing configuration.   Spoilers and speed brakes are generally used after landing for deceleration  (let's not get into the "that's really acceleration!" argument here ) and thus aren't considered part of the normal landing configuration (think the configuration you'd be in the landing flare).    While both items may be used in flight (although not neccessarily allowed to their full deployment),  they are not used for landing.   

Generally the numbers are just referred to as Vs and Vso    Vs being clean and Vso being in landing configuration (or 'dirty' if you'd prefer).   There's also Vs1 for a "specified configuration.


BTW,  anyone else notice that this month's AOPA magazine incorrectly refers to "blue line" as Vmc and talks about blue line as if it were?   

[airtac]

I have agree with Ryan that dirty refers to landing configuration....

Blue line as Vmc?  I haven't had time to read mine yet but I'm sure the'll get some letters about that

[Baradium]

I have agree with Ryan that dirty refers to landing configuration....

Blue line as Vmc?  I haven't had time to read mine yet but I'm sure the'll get some letters about that

Here's a quote:

"Don't ever lift off the runway until the indicated airspeed is above blue line is a rule of thumb drilled into every twin driver's head."

Man, if we tried to not lift off until above blue line in the 1900 I think we'd be sitting there riding the nosewheel!

[airtac]

I have agree with Ryan that dirty refers to landing configuration....

Blue line as Vmc?  I haven't had time to read mine yet but I'm sure the'll get some letters about that

Here's a quote:

"Don't ever lift off the runway until the indicated airspeed is above blue line is a rule of thumb drilled into every twin driver's head."

Man, if we tried to not lift off until above blue line in the 1900 I think we'd be sitting there riding the nosewheel!

Baradium, What article was that? I was looking through AOPA mag and couldn't find it.

[Baradium]

Baradium, What article was that? I was looking through AOPA mag and couldn't find it.

"Positive force" which starts on page 105.   The section I'm referencing starts on page 106.

It's actually an article about vortex generators, but it delves into this section because supposedly the VGs lower Vmc.

For later reference:  May 2007, page 105.

[airtac]

Baradium, What article was that? I was looking through AOPA mag and couldn't find it.

"Positive force" which starts on page 105.   The section I'm referencing starts on page 106.

It's actually an article about vortex generators, but it delves into this section because supposedly the VGs lower Vmc.

For later reference:  May 2007, page 105.

Been busy but finally got around to it, interesting stuff on VGs--   My former employer had a STOL equipped mixmaster with VGs being part of the kit and it cruised about 10K faster than the stock airplanes we had so the performance claims might be pretty accurate,

I'll bet when you're lightly loaded in the 1900 that you accelerate through Blue Line in the rotation--too much performance to be held on that long?  (or play like a wheelbarrow )
My guess i