16
$\begingroup$

This news article links to a 2019 academic study: Thermal impacts of wing coloration on surface temperature during bird flight.

Most of birds have dark upper wing feathers. Dark colors keep a sunlit surface hotter compared to white colors.

Do black color or hotter surfaces improve the aerodynamics of wings?

A key consideration might be airspeed: fast air over a plane wing doesn't allow time to heat up the boundary layer so much. Birds and sailing boats operate at much slower airspeed so maybe those get some benefits?

$\endgroup$
8
  • 5
    $\begingroup$ There could be something to it. Maybe the lowered density of the heated boundary layer reduces skin friction, or maybe it affects the pressure distribution in beneficial ways. Perhaps it only works at really low Reynolds Numbers. I don't think it's ever been studied on full size aircraft. There are black painted airplanes out there, but I don't think anybody has ever detected any difference, or everybody would be doing it. Black paint creates its own problems as skins over expand and make wrinkles in the sun. On composite airplanes it's a no-no because the heat can soften the epoxy. $\endgroup$
    – John K
    Jan 15 at 18:07
  • 2
    $\begingroup$ The weight of the paint is more important, also the surface finish. If you can use a thinner, lighter paint coat then you save fuel. If you can get the finish glossy smooth, you save fuel. $\endgroup$
    – J...
    Jan 16 at 8:33
  • 10
    $\begingroup$ Colours of bird wings are probably more influenced by camouflage aspects rather than aerodynamics, I think. But still an interesting question! $\endgroup$ Jan 16 at 14:01
  • 2
    $\begingroup$ @Toby Speight: And some of the more aerodynamically efficient birds are white, or light colored - seagulls, albatrosses, &c. $\endgroup$
    – jamesqf
    Jan 16 at 17:13
  • 3
    $\begingroup$ Also look at (string)rays (Batoidea). Always light colored underneath, dark colored on the top side, for camouflage reasons in water. $\endgroup$
    – alecail
    Jan 16 at 18:29
27
$\begingroup$

For the bird analogy, you're thinking about this wrong. Birds have dark upper wings for camouflage, not for aerodynamics. A predator above them is less likely to see a dark top against a darkish background (the ground). This is the same reason that many birds have lighter colors under their wings - that looks a bit more like the sky.

$\endgroup$
2
  • 3
    $\begingroup$ @CamilleGoudeseune This answer has value as a frame challenge to the original question. meta.stackexchange.com/questions/263661/… $\endgroup$
    – barbecue
    Jan 16 at 20:02
  • $\begingroup$ As the question now stands, this answer to the original Q has unfortunately become obsolete. $\endgroup$
    – Jpe61
    Jan 17 at 2:11
16
$\begingroup$

Heating the boundary layer has been shown to decrease drag:

B. Kramer, B. Smith, J. Heid, Gregory K. Noffz, D. Richwine, Terry Ng, 1999: Drag reduction experiments using boundary layer heating

Unfortunately the research paper is not freely available, but the abstract reads as follows:

A new technique for reducing the drag of existing aircraft configurations was investigated. Heating the surface under a turbulent boundary layer reduces the turbulent skin friction. An experimental program was conducted to determine the feasibility of applying this technology to military and commercial aircraft. Three different experimental investigations were conducted to measure the extent of the drag reduction. The first experiment was a low speed wind tunnel test. Very significant drag reduction was measured while only heating the forward portion of the wind tunnel model. In order to determine the effects of higher Mach and Reynolds number, a flight test program was conducted using the NASA Dryden F-15B Flight Test Fixture. The [second higher mach] experiment measured the skin friction over a flat plate using a momentum deficit technique. The magnitude of the drag reduction was less than measured in the wind tunnel, but showed a trend of increasing effectiveness with decreasing Reynolds number. The third experiment confirmed the overall drag reduction on a business jet class aircraft. This work has shown that significant drag savings can be achieved using boundary layer heating.

As to why this method is not (yet, at least) utilized, might be because of the adverse effects of heat on structural integrity and / or longevity of airframe; added fuel consumption if heating is active, negating fuel savings; added systems complexity if active system is used; and possible negative effects on other aerodynamic properties of planes, such as controllability (I personally doubt this, but it is possible).

Note that he research is "only" about 20 years of age, so things might change.

$\endgroup$
3
  • 6
    $\begingroup$ I'd assume it takes pretty significant power to do any useful / meaningful heating to that much air. So it's a question of whether that energy would help you more by reducing drag this way, or by directly pushing air backwards with the engines. Possibly a black paintjob (including infrared absorbing) could have a slight benefit for an airliner or fighter, for passive solar heating. $\endgroup$ Jan 16 at 20:17
  • 5
    $\begingroup$ But OTOH heat-absorbing paint will increase thermal expansion stresses over the life of an airliner when it goes from parked at a sunny airport on an already hot day to 36000 feet repeatedly, and white makes oil leaks and so on easy to see, as Captain Joe explains (youtube.com/watch?v=659vqjkwvP0). Also other reasons, like resale of aircraft between airlines. So there are reasons why non-white paint schemes might not be worth it if the fuel savings are tiny. (Or outweighed by the extra weight of paint, if they follow the normal airline model of a white base coat.) $\endgroup$ Jan 16 at 20:20
  • 1
    $\begingroup$ @PeterCordes and added complexity of installing/maintaining the heating apparatus in such a confined space which stores a lot of fuel. $\endgroup$
    – RonJohn
    Jan 18 at 14:57
15
$\begingroup$

This has actually been tested, but results were not particularly positive.

During the Vietnam era, B-52s were painted with an "SEA" (South East Asia) camouflage scheme, which was black on the bottom, and fairly light green/brown colors on top:

enter image description here

During the cold war, the SEA camouflage scheme was replaced with the SIOP (Strategic Interdiction Operations Plan). The SIOP camouflage scheme for the B-52 had the bottom of the aircraft painted glossy white, and the top side in darker, flat colors (not black, but fairly dark greens and browns).

enter image description here

This picture looks pretty faded to me though--in real life, the top side was rather darker than shown here.

Around the mid-to-late 1980s, the SIOP camouflage scheme was replaced with the "new strategic" camouflage scheme, which was lighter grey, with minimal contrast between top and bottom (sorry, I haven't been able to chase down a picture of this one).

I never heard even the slightest suggestion that there might be even a slight difference in fuel usage depending on the camouflage scheme in use. It certainly wasn't mentioned in the documentation ("technical orders") and I never heard a pilot or navigator say anything even hinting at such a possibility either.

At least as I see things, the basic problem here is pretty simple. During a typical flight, you have an air speed in the range of hundreds of miles per hour, and you're typically flying in air that's at -40 degrees (or colder), so it's next to impossible for the light falling on the aircraft to heat it up to any noticeable degree at all.

In the specific case of a B-52 (or other strategic bomber) you do have the possibility of flying at terrain avoidance level (low altitude, so the air isn't nearly as cold), and you might be flying at somewhat lower speed, but your air speed is still going to be in the range of hundreds of miles per hour so you still have almost no possibility for light falling on the plane to raise its temperature to any significant degree. In any case, most aircraft don't fly at terrain avoidance level anyway.

$\endgroup$
1
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Farhan
    Jan 19 at 14:53
8
$\begingroup$

How black color/hotter surface increase wings aerodynamics?

We don't yet know. The study Thermal impacts of wing coloration on surface temperature during bird flight that the question points to made a few guesses about aerodynamics, but the only thing that it measured was how color affects the temperature of a wing in flight:

In white wings with black tips, the temperature differential produced convective currents towards the darker wing tips that could lead to an increase in lift. Additionally, a temperature differential between wing-spanning warm muscles and colder flight feathers could delay the flow separation above the wing, increasing flight efficiency.
...
Future studies should directly explore to what extent wing coloration affects lift and drag when exposed to solar radiation.

$\endgroup$
4
  • $\begingroup$ It looks like you meant to include a link, but there isn't one. $\endgroup$
    – barbecue
    Jan 16 at 19:59
  • 1
    $\begingroup$ @barbecue: I assume this is quoting the academic study the question linked to, pointing out that the question is making a stronger assertion than the study actually did. $\endgroup$ Jan 16 at 20:03
  • $\begingroup$ You may be right, I saw the words "this study" and assumed it was supposed to be a link. $\endgroup$
    – barbecue
    Jan 16 at 20:04
  • $\begingroup$ I'd edited the question itself to drill down to the actual study. Good catch - now I've added that link to this answer so it's more self-contained. $\endgroup$ Jan 16 at 21:07
6
$\begingroup$

In both natural and artificial flight (birds, airplanes), wing color is more about camouflage than anything else. Dark upper surfaces and light lower surfaces usually make the bird (or plane) harder to see against the background. Countershading is common in many birds and warplanes for this reason. If there was a demonstrable efficiency advantage to dark upper wing surfaces, we'd already see them as a commonplace thing on commercial aircraft where even small improvements matter. There are two drawbacks to painting the top surface of an aircraft wing dark:

  1. It's an additional heat load where most of the fuel is stored. While it won't create enough heat to ignite the fuel, increased temperature will increase vapor pressure which could make it more likely that a combustible mixture will find its way to an ignition source.
  2. Paint adds weight which means the aircraft has to burn more fuel to do the same job, which adds more weight and so on.

Warplanes are painted because the need for camouflage is a greater concern than the weight penalty or thermal considerations.

$\endgroup$
2
  • $\begingroup$ Dark paint also increases thermal expansion stress cycles by getting hotter when sitting parked in the sun. (While still cooling to very nearly the same low temp in flight, when cooled by -40C air in somewhat drafty conditions... over 200kts IAS. So the total temp swing is larger.) That's one of the factors cited by Captain Joe (youtube.com/watch?v=659vqjkwvP0), along with weight for non-white paint. (Although if this was a standard thing, probably they could use black paint as the base layer instead of only putting it over a white base coat as an extra layer.) $\endgroup$ Jan 17 at 16:34
  • $\begingroup$ "In both natural and artificial flight (birds, airplanes), wing color is more about camouflage than anything else." - For manned aviation, this is generally only true of military aviation, not civilian. In civilian aviation, it's generally preferred for aircraft to be easier to see, not harder. $\endgroup$
    – reirab
    Jan 18 at 16:34
5
$\begingroup$

As a data point, I would mention that gliders, the most efficient aircraft out there, have been regularly painted white for decades. Though the main reason for doing this is protection from the sun, I'm not aware of any case (in over 5 decades of fiberglass/carbon gliders) where darker wings were used, not even for a single competition or a single record flight.

enter image description here

$\endgroup$
3
  • 10
    $\begingroup$ Isn't that because of material issues? $\endgroup$
    – Abdullah
    Jan 16 at 7:32
  • 7
    $\begingroup$ @Abdullah: Exactly right. As JohnK mentions already, white is chosen to limit the surface temperature. Red is allowed at low-stress parts like the wingtips or the fuselage nose, but white is mandatory on highly-stressed parts. When the SB-10 spent a few months in Australia (winter 79-80), it came back with permanently lowered dihedral. The Australian sun heated the epoxy matrix too much even though the airplane was painted white. $\endgroup$ Jan 16 at 8:33
  • 3
    $\begingroup$ It is mostly because of the sun. But if there was an advantage to be gained by painting the wings a different color, you bet it would have been done for a competition/record. $\endgroup$ Jan 16 at 22:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy