Burnt Cowl

I had a customers RV-10 in the workshop last month for some upgrades and repairs. (new windscreen and Planaround door upgrade )
This particular RV-10 is 10 years old and has flown approx 1100 hrs.

The lower cowl exhibited external heat damage and on closer inspection the heat shielding was no longer doing its job and not covering enough of the cowl.
The heat shielding had been allowed to get stained over time and was no longer able to reflect any heat.




New to me was how much heat damage there was from No1 Cylinder,the heat shield only partially covered this area.


and once the old foil was removed the cowl sub-strait crumbled away,



The damaged sections were cut away and multiple 9oz cloth was applied to the affected areas. The cowl was then cleaned and sealed with West epoxy before 1/8" Fibrefrax was applied under the new adhesive reflective foil covering all of the lower cowl.








The rest of the cowl is to be painted white after I had finished with the repairs.

So Keep those lower cowls CLEAN , Reflective foil heat shields don't reflect heat when they are dirty or oil stained.

Great write up! Thanks for sharing.

+1

Thanks for sharing. I think this is something that should have been caught during annuals, or even oil changes.

Minor thing....the self-stick aluminum foil shielding from Vans really sticks well if the inside of the cowl is finish painted before application.

I recall a few reports of it coming loose, with at least one blocking the cowl exit.

Dan is correct about sealing the cowl.

This repair was sanded and squeaky clean with acetone then sealed with a coat of West epoxy.

After the foil was applied ,all edges where sealed down with another coat of West epoxy to prevent any chance of edge lifting.

Yep

DanH said:

Minor thing....the self-stick aluminum foil shielding from Vans really sticks well if the inside of the cowl is finish painted before application.

I recall a few reports of it coming loose, with at least one blocking the cowl exit.

Click to expand...

Happened to me...showed in the CHT's.

Best,

Long prep question

A good friend of mine has a beautiful RV7 with a unique paint scheme. It has been on the plane for about 10 years and the upper cowl painted surface is showing cracks in the clear coat form the excessive heat. He suggested heat resistant paint on the underside of the cowl to prevent this type of damage. I ordered some Extreme Temp Coating from KBS coatings to apply to our newly painted RV14A. (Pictures to come as soon as we get MoJo home from Tampa this week.) Should I apply a coat of epoxy to the underside of the upper cowl before putting on the paint or just apply it directly to the new cowl? I like the idea of paint as opposed to heat barrier because there is no danger of anything coming loose and cutting off airflow.

woodmanrog said:

A good friend of mine has a beautiful RV7 with a unique paint scheme. It has been on the plane for about 10 years and the upper cowl painted surface is showing cracks in the clear coat form the excessive heat. He suggested heat resistant paint on the underside of the cowl to prevent this type of damage.

Click to expand...

"Heat resistant paint" isn't going to be very useful in this application. Hopefully we have some pros who can expand on the subject, but here is my understanding.

The properties of interest are reflectance and emissivity. Reflectance is what is sounds like, a measure of how much radiant energy is reflected rather than absorbed by the surface. Clean, shiny aluminum has very good reflectance (ballpark 90%) across a wide range of wavelengths, meaning most of the infrared radiation from exhaust headers and hot engine parts is reflected back into the engine compartment.

The small percentage of energy absorbed by the aluminum is re-emitted from both the front and back sides of the sheet. Our interested is the energy emitted by the side facing the fiberglass. Shiny aluminum foil has very low emissivity, on the order of 0.03 if a perfect black cube is treated as 1.

Here we back the high reflectance, low emissivity material with an insulator, a material with low conductivity (thermal energy moves through the material by conduction at a slow rate). It also blocks the free movement of air, and thus heat transfer by convection. As a result, a low percentage of the original infrared radiation reaches the fiberglass.

Now compare with white paint. Reflectance in the infrared range is not as good as shiny aluminum, but it's not bad, with a best of about 80% if I'm reading the right papers. In addition, emissivity of paints is quite high at around 0.9, or about 30 times the emissivity of the aluminum foil, plus the paint is in full contact with the underlying fiberglass. Bottom line is that at least twice as much energy is absorbed, and it's all transferred to the fiberglass. In general, dark colors or rough surfaces would be worse.

I suggest that the best performance is found by finishing the inside of the cowl with a glossy white, then adding the ceramic felt insulator and aluminum foil reflector. The gloss white is easy to clean, reflects a high percentage of the radiant energy, and the foil sticks very well. The angle at which the radiant energy strikes the surface has a lot to do with the total energy absorbed per unit area, so cowl areas struck at highly obtuse angles get less energy; white paint should be adequate. The felt and foil greatly reduces the energy reaching the glass in those areas struck at closer to right angles, i.e nearer the pipes. Below I've simply shadowed the pipes, but the actual minimum or maximum widths, thickness, etc is a subject for experimentation. This lower cowl appears to be be healthy at 680 hours. The upper cowl gets nowhere near as much radiant energy.

Thanks for the info Dan. The paint is glossy aluminum. Here is the new plan. I Will use the paint and measure surface temperature after a flight. Then do the same with the heat material that came with the kit. Of course I will take a measurement without any insulation as well. Complete comparison report to follow.

woodmanrog said:

Thanks for the info Dan. The paint is glossy aluminum. Here is the new plan. I Will use the paint and measure surface temperature after a flight. Then do the same with the heat material that came with the kit. Of course I will take a measurement without any insulation as well. Complete comparison report to follow.

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You know I love a good experiment.

Measuring after three flights can introduce variables. Perhaps put the cowl on the work table, and point a heat lamp bulb at the subject area? Probably get all three data points in an hour.

Will do, great idea.