Can I paint a composite plaine a silver / grey color?

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autoreply

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The white seems a bit hot. Most non metallic paint has an emissivity around 0.9-0.95, while absorbtivity is highly dependent on colour, whites are down around 0.05, black is 0.9+. A bright white paint should be cooler than the chart indicates. Heat gain is determined by absorbtivity, heat loss by emissivity and convection. That colour curve gif looks like it only factors in convection. Experiment is the best way to decide as paint is highly variable.
To the best of my knowledge, that chart is extrapolated from measured data.

From experience (sailplanes in the sun) it's pretty close to reality.

Even in a home shop, having composite structures handle boiling water isn't that hard if it weren't for the foam cores. Swapping Divinycell for Rohacell is a pretty good marketing decision.

Obviously, practical issues quickly numb that idea. My last project could have easily dealt with black wings weren't it for the fuel tanks, which would be boiling off fuel even in our climate ;)
 

lr27

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You could always paint up some aluminum plates and put a thermocouple or thermometer on each one. This would give an idea of the relative merits of the specific paints you're considering.
 

ToddK

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I wonder if anybody has ever measured the temperature pre taxi and post. I suspect the surface temp of something sitting still is a bit higher then something that is moving. Put another way, I wonder how fast the skin exposed to the sun cools once it starts heading to the runway.
 

autoreply

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I wonder if anybody has ever measured the temperature pre taxi and post. I suspect the surface temp of something sitting still is a bit higher then something that is moving. Put another way, I wonder how fast the skin exposed to the sun cools once it starts heading to the runway.
I did some distinctly non-scientific testing. Up to 15 kts for about a 1 mile taxi didn't give a discernible reduction in surface temp for red painted parts that had been in the sun for a few hours.
 

BJC

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I'm thinking of usimg a Fluke 62Max IR Thermometer to measure temperatures of various paint colors on composite aircrft t Oshkosh.

Does anyone know what emissivity setting(s) might improve the measurement accuracy?

Thanks,


BJC
 

pictsidhe

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I'm thinking of usimg a Fluke 62Max IR Thermometer to measure temperatures of various paint colors on composite aircrft t Oshkosh.

Does anyone know what emissivity setting(s) might improve the measurement accuracy?

Thanks,


BJC
Most paints are between 0.9 and 0.96, if you have a contact thermometer too, you could also report back with emmisivities!
 

Aesquire

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The article that had that chart describes the testing. Light yellow and pink were available for Duckhawk gliders, which use no gel coat znd higher temperature cured resin.

Air flow has a much more limited effect on glass than skin. No evaporation.

Heat physics is sometimes not intuitive. Ask me how the Romans made ice in the desert.
 

Aesquire

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Dug a pit. Lay straw in it. Lay pans of water on straw. Cover pans with straw and tarp during day. Uncover pans at night. Radiation into the IR transparent dry air into space cools the water at night. Repeat.

This doesn't work in a humid environment. The water vapor greenhouse effect hold the heat in. In a basically zero humidity desert, you literally expose the water to the icy blackness of space.

See also Ruzic Cryostat. The first patent for a device to be used on the moon.
 

Aesquire

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If the EM drive works I'll kick start the first commercial solar powered steam spaceship.

But it will still have white paint to reduce over heating, just like a sailplane.

And brass. Lots of brass in the cgi promo pictures. And side wheels. Full steampunk.
 

choppergirl

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Solar absorbtion could be a problem, but I'm going to keep a 35 mph wind going across my wings which has a cooling effect. Something about flying the wing.

Do your sun temperature tests with a 50mph fan blowing on your color chart... :p

Boundary layer schmoundry layer. If you think things are getting too hot out there, fly upside down for a while...
 

pictsidhe

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The temperature that a surface reaches under the sun depends on heat gain, which is the solar absorptivity and heat loss, emmisivity, convection and conduction. I found more paint info here:
http://www.solarmirror.com/fom/fom-serve/cache/43.html

It seems that white paint absorptivity varies between 0.07 and 0.44, which is a huge range. The coolest white I've encountered is fresh whitewash, not on that page.
Waxing your paintjob can change the numbers.

Testing seems the order of the day. Easy enough to do.
 

Aesquire

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Back in the 1970's the U.S. weather service changed the paint spec on the wooden boxes that hold thermometers that gives us the numbers you hear on the news. The official temps.

The old spec was white wash. That went obsolete since they switched to latex paint.

The temperature in the U.S. went up over two degrees.

Yes, they put in a correction factor. But the dataset still uses the raw numbers. Further discussion violates site rules on religious argument.

So small changes in color can make a big difference.
 

pictsidhe

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Back in the 1970's the U.S. weather service changed the paint spec on the wooden boxes that hold thermometers that gives us the numbers you hear on the news. The official temps.

The old spec was white wash. That went obsolete since they switched to latex paint.

The temperature in the U.S. went up over two degrees.

Yes, they put in a correction factor. But the dataset still uses the raw numbers. Further discussion violates site rules on religious argument.

So small changes in color can make a big difference.
Stevenson screens. I redo whitewash annually, it generally needs it. The maintenance is a pita on whitewash, but it is cheap to make and very cool. That'll be why they switched to latex. I would add in 10% of white portland cement, which added durability.
 

autoreply

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Solar absorbtion could be a problem, but I'm going to keep a 35 mph wind going across my wings which has a cooling effect. Something about flying the wing.

Do your sun temperature tests with a 50mph fan blowing on your color chart... :p

Boundary layer schmoundry layer. If you think things are getting too hot out there, fly upside down for a while...
It'll take pretty long for especially composites and wood to conduct those lower temperatures into the structure. Too long to be comfortable about it; it takes hours for example to freeze water ballast that's directly against the inside skin of a sailplane, even if it's -20C or colder outside.

Back in the 1970's the U.S. weather service changed the paint spec on the wooden boxes that hold thermometers that gives us the numbers you hear on the news. The official temps.

The old spec was white wash. That went obsolete since they switched to latex paint.

The temperature in the U.S. went up over two degrees.

Yes, they put in a correction factor. But the dataset still uses the raw numbers. Further discussion violates site rules on religious argument.

So small changes in color can make a big difference.
Bigger change is urbanization:
Meetresultaten-Haaglanden-Rotterdam-TNO-onderzoek-2011-881x630.jpg

Notice over 15F in difference between built-up areas with little forestation and rural areas.

Very OT; many of the hot spots are container terminals or the airport (tarmac).
 

Aesquire

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Many composite structures of foam & fiberglass greatly resemble beer coolers, as far as heat flow goes. So I can well believe that the conduction heat loss on the surface is slow to cool the heated structure after all day in the sun.

I do know that on a sunny day, even wind swept rock and metal get hot, so I think we can skip airflow as a cooling technique. Sorry Choppergirl. Good idea, though.

Yeah, OT, there's a website that has pictures of weather stations that once were in an empty field at the airport, and are now in the industrial park next to the airport, in a black asphalt parking lot, next to the Air conditioning unit, by the gas grill, where the SUVs park after doing a service call. ( not all have all these features )

Also OT.

I like to mind fry a physicist buddy, and have for years insisted his textbooks were wrong. Gravity is considered to be a force that pulls on all matter, created by matter, with a bit of argument as to the nature of transfer particles, gravitons, and warped space. ( reminds me of Burnouli's principal vs. flow field dynamics for explaining airplanes )

I disagree. Gravity is a repulsive force, generated by the universe, and blocked by matter. So a planet has notable gravity by blocking the push from below, and you feel pressed down to the planet. When not near a planet it pressed equally from all sides, towards your center. But your own matter blocks that so you feel like you are floating.

The math works either way. The difference is philosophical. Attractive gravity says the Earth Sucks. Repulsive Gravity says the Universe Blows. ;)
 
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