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Smilin' Jack

Well Known Member
Does anyone have a source for louvers, I don't have enough free airspace around the exhaust and it is causing pressure on the cowling between each skybolt fastner. While flying I can see a rise in the fiberglass about 1/8 inch between the skybolt fastners which are spaced 3 1/2 in apart.

Has anyone else noticed this?

Thanks
Smilin' Jack
 
Jack, I saw pictures here on VAF of one RV7 builder that installed louvers from an Audi.
It really looked really custom for sure.
 
Before cutting up your cowl, try this.

Turn off the lights in your hangar and put a trouble light under you cylinders. Plug any air hole that is not around a cooling fin. I did that recently and dropped my CHT's 20 to 25*F. Problem solved and no cooling louvers needed.
 
I am really not having a cooling problem as I only have 5 hours on the engine and the CHT are running about 370 at 82 % power. My Oil temp is At 186 so those temps are not bad. But I am assuming that if your packing 183 mph indicated air at 3000 feet into the upper cowling It has to find a place to escape and with a 4 pipe system and 3 drain tubes I don't believe there is enough exit space.
And thus causing pressure on the upper cowling, and the rise in the fiberglass between the skybolt fastners on the rear of the top cowling, we have checked all the baffling and it looks tight and all the holes between the engine and metal baffles are sealed.

I would like to try a smaller louvre than what Avery is selling and expand it if needed.
Jack
 
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If your temperatures are good then I would recommend adding a stiffener to the upper cowling to reduce the bulging. This can be made from aluminium or better yet with a bead of glass/resin/flox.
 
Tom Martin said:
If your temperatures are good then I would recommend adding a stiffener to the upper cowling to reduce the bulging. This can be made from aluminium or better yet with a bead of glass/resin/flox.
Click to expand...

Right on. The lower cowl pressure is A Good Thing; it is increasing cooling air exit velocity.
 
An easy way to add stiffeners to your top cowl is to remove the top cowl, turn it over, prep it by sanding and cleaning the underside, lay some vinyl tubing on it in the pattern you want, and then lay a few layers of BID fiberglass over the tubing. When it hardens, pull out the tubing, or leave it in, your choice.

One thought, after the fiberglass starts to harden, you might want to install the top cowl and secure it with all the fasteners you use in flight.
 
Smilin' Jack said:
Thanks for the input guys, I am definitely going to try the stiffeners before cutting the cowling. Jack
Click to expand...

Given internal air pressure, the cowling tries to assume a perfectly round shape rather than the rounded square we prefer. The curved sections above and inboard of the upper longerons already approximate the round shape, so there's less need for stiffeners there. The problem areas are the top of the cowl in front of the windshield (A), and the flat sections on the belly, on each side of the cowl outlet (B). The sides also bow outward at (C), but with less span they are less critical.

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If you want to try adding a rib for stiffening of the upper cowl edge, it should be as tall as practical. Looking at its cross section, the portion most distant from the inner cowl surface (red line in the sketch) will be loaded in compression, so you want the maximum fiber content there, with the fibers running lengthwise along the rib. Taper the ends. It would pay to use an epoxy with a higher Tg than West, perhaps Aeropoxy PR2032.

The flat panels on the cowl belly really load the fasteners at the outlet. Plenty of builders report broken hinge eyes there, for example. With quarter turns and higher-than-stock internal cowl pressures, I found it necessary to buttress the mounting tabs; they were bending outward and stressing the belly skin. The problem would be worse with a 7, as the flat sections are longer.

vimb5y.jpg
 
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