What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Inside Top Cowl paint

I used a high build primer and then a battleship grey single stage paint. Any light color should work.



Ross Scroggs
RV4. #3911
Paint and finishing stage
 
I used West Systems Epoxy and West Systems White Tint for epoxy. The tint can be obtained at your favorite Marine Supply company. Brush or roll it on with a few coats and you'll have a nice hard candy shell that is easy to wipe down and clean. Paint can chip. This stuff is never going to chip off. It has held up great over 7 years and almost 900 hours.

IMG_3257-M.jpg
 
Many production aircraft use intumescent paint to help the cowling avoid falling apart in the event of a fire. Mostly water soluble and difficult to use, and need to be over coated with 2K PU/acrylic to provide any kind of hard wearing surface. Something that seals the weave and prevents oil seeping into the composite seems like the best bet, perhaps straight to 2K top coat with a good primer/undercoat?
Pete
 
It also absorbs less heat than a dark color.

Carl

Kirchhoff's Law (of Radiation). An extension of the second law of thermo, IIRC.

Simplified, bodies emit and absorb radiation at the same rate. In general for applications: if you're trying to protect something from heat, white surfaces work better. If you're trying to shed heat, black surfaces work better.

Same referenced physicist has some electrical laws named after him as well. Quite the intellectual stud.

Regarding the origin of the common reference of electronics boxes to "black boxes", many/most think it came came about from the perceived mysterious inner workings of such. They were coated black (and kept away from radiant energy) to help them shed heat. The name stuck.

So, your fiberglass should be white and your cylinders black (if coated). There is your semi-worthless stuff for the day
 
I got a reference from this forum for Pettit boat primer and paint. Very easy to apply and seems to be working well. I put it over a thin (not diluted) layer of epoxy.

Tried the tinting with poor results. I was heavy on the white tint and it still seemed to be very clear.

Like to try the Rhino stuff for the next RV.

FWIW for me the bottom cowl was more heat critical. I also added a port side heat muff.
 

Attachments

  • IMG_20200823_154056.jpg
    IMG_20200823_154056.jpg
    305.2 KB · Views: 264
Best to prep and paint 2k urethane, just like the outside.
Sealed, tough, easy to clean, and stick on reflective foil stays put.
 
Plus one for tinted West Systems epoxy poured on and squeegeed off, followed by a "roughing up" with a paint roller. The first coat was mostly clear but after three coats you can no longer see through the epoxy.
 

Attachments

  • LKRV8-FWF569.jpg
    LKRV8-FWF569.jpg
    205.5 KB · Views: 265
Kirchhoff's Law (of Radiation). An extension of the second law of thermo, IIRC.

Simplified, bodies emit and absorb radiation at the same rate. In general for applications: if you're trying to protect something from heat, white surfaces work better. If you're trying to shed heat, black surfaces work better.

Same referenced physicist has some electrical laws named after him as well. Quite the intellectual stud.

Regarding the origin of the common reference of electronics boxes to "black boxes", many/most think it came came about from the perceived mysterious inner workings of such. They were coated black (and kept away from radiant energy) to help them shed heat. The name stuck.

So, your fiberglass should be white and your cylinders black (if coated). There is your semi-worthless stuff for the day

thats not the physics. the color of the paint has little to do with heat dissipation. true, a light color will reflect more radiant heat than a dark color, thats why you paint the exterior a light color. the problem is the inside of the cowl is not dealing with radiant energy in the visible spectrum. its is dealing with the energy radiated from the heat source ie the engine. that energy is in the IR specturm. when they talk about black bodies in heat transfer they are not talking about color in the visible spectrum. they are talking about the IR spectrum, snow is actually black in the IR spectrum. take light out of the equation, as inside the cowl, the color has really no effect on heat dissipation. now, yes you will see oil on a light color better, thats why i painted mine light gray.

bob burns
RV-4
N82RB
 
thats not the physics. the color of the paint has little to do with heat dissipation. true, a light color will reflect more radiant heat than a dark color, thats why you paint the exterior a light color. the problem is the inside of the cowl is not dealing with radiant energy in the visible spectrum. its is dealing with the energy radiated from the heat source ie the engine. that energy is in the IR specturm. when they talk about black bodies in heat transfer they are not talking about color in the visible spectrum. they are talking about the IR spectrum, snow is actually black in the IR spectrum. take light out of the equation, as inside the cowl, the color has really no effect on heat dissipation. now, yes you will see oil on a light color better, thats why i painted mine light gray.

bob burns
RV-4
N82RB

Thanks Mr. Burns,

Been a long time since I've been in school but it sounds like I was just taken to school. I'll have to bone up on it again. To be clear, you're talking about the white paint only. The black body radiation effects stated are still true. Anyway. someone else made reference to a radiant barrier which makes sense to me at this point. I'm so far away from ever getting to that point of this build.

Thanks again. If even up the state near ORL, let me know.

msf
 
Kirchhoff's Law (of Radiation). An extension of the second law of thermo, IIRC.

Simplified, bodies emit and absorb radiation at the same rate. In general for applications: if you're trying to protect something from heat, white surfaces work better. If you're trying to shed heat, black surfaces work better.

Our engines don't shed a significant fraction of energy through photons. Most of it leaves by convection, i.e. contact with air molecules passing between the fins. This process is not affected by the color of the engine. Also, the photons emitted by the fins simply get reabsorbed by the opposing fin surface.

Most of the photons that want to melt your cowl come from the exhaust pipes, which are significantly hotter than the engine (remember, fourth power of T). That's why a radiative barrier is the best way to keep the heat off the cowl: a piece of sheet metal in between that catches those photons and is kept at a much lower temperature through convective cooling. Infrared emissivity of visibly white or visibly black paint doesn't make that much difference to cowl temperature. That paint is for ease of cleanup only. Any epoxy will do.
 
Our engines don't shed a significant fraction of energy through photons. Most of it leaves by convection, i.e. contact with air molecules passing between the fins. This process is not affected by the color of the engine. Also, the photons emitted by the fins simply get reabsorbed by the opposing fin surface.

Most of the photons that want to melt your cowl come from the exhaust pipes, which are significantly hotter than the engine (remember, fourth power of T). That's why a radiative barrier is the best way to keep the heat off the cowl: a piece of sheet metal in between that catches those photons and is kept at a much lower temperature through convective cooling. Infrared emissivity of visibly white or visibly black paint doesn't make that much difference to cowl temperature. That paint is for ease of cleanup only. Any epoxy will do.

Sticking with my previous mea culpa-ish post. Wrong on the absorption/wrong part of the EM spectrum. Still think I'm right on the emission. Never mentioned exhausts where any coating would be short lived. If you're going to paint cylinders (paint is an insulator) black would be the wisest choice. There's basically no forced air convection without airspeed and none at shutdown. You have to corrosion protect them. Conti used to gold alodine their jugs (no paint). It was not an option for the build they just completed for me.
 
Most of it leaves by convection, i.e. contact with air molecules passing between the fins. This process is not affected by the color of the engine. Also, the photons emitted by the fins simply get reabsorbed by the opposing fin surface.


More thread drifting. Full throttle and tire smoke. :D


The above reminded me of an exercise we did in Eng skool regarding heat rejection from fins. We were plotting the shape of the fin that would have the maximum heat flow due to convection.


It was not a straight fin with parallel sides. Not terribly surprising. It was tapered to a point, but not triangular. Rather, the remains between two parabolic looking curved sides. Fine. BUT... And here's the surprise, there were a couple of spurious points not on the curve. We increased the resolution and found MANY points not on the curve. Further resolution showed the shape to be essentially a coarse Fern leaf kind of shape. Surely a fractal but it was long enough ago that fractals were not the common knowledge they are today and we probably did not go deep enough with resolution to see more of the fractal nature. We took it to be a sinusoidal shape at the time.
 
Last edited:
Does anyone have some idea of ADDED weight of the different finishes?

Epoxy Paint
Epoxy resin w/ tint
Metalized tape

The top of the cowl and parts of lower COOK...

Which finish has best heat, fuel and oil resistance?
 
Back
Top