This is my 4th change to the outlet opening and I've run out of room to make it bigger. If this doesn't fix the problem I have an air leak somewhere reducing the pressure drop across the cylinders. I think I've found a leak around the carb heat box, but I'm not touching it until I fly with this change.
The original opening was 61.4 sq. in. minus the outlet for the cabin heat box for when heat is off, of 3.1 sq. in. gave a net area of 58.3 sq. in. The holes for the exhaust pipes were only about 1/4" bigger than the pipes so they really didn't add any outlet area. They do now.
The other day I drew a pattern of the opening for the cowl outlet on my C-140, also powered by a C-85. It measured 120 sq. in. It does not have a skirt but extends below the fuselage about 2".
This final version has a net area, in the center opening, of 130.9 sq. in. plus 28.5 sq. in. around the exhaust pipes for a total of 159.4 sq. inches, enough already. Literally, if it were any bigger the cowl would be split into 2 pieces.
I followed the same procedure as last time. I drew a guide line on the cowl, then made a cardstock pattern of the skirt. Unriveted the old skirt and made up the skirt, using the ends of the old skirt to get the mounting holes at each end to match the holes in the cowl. The new shirt was fitted to the cowl, on the plane. The new holes were only drilled to 3/32" so I would have more freedom to get the final 1/8" holes aligned, if needed. I brought all that back to the attic to trim the cowl to fit the new skirt and then rivet it all together. The plane under the cowl is my
1926 WACO NINE project.
I made new stiffeners for the 2 narrow webs of cowling beside the exhaust holes. I have enough opening area without the couple inches lost to them and they do stiffen it all up.
Ready to go back on the plane.
Each time I've changed it I like it better.
There is a lot of opening area. Now I just need the weather to calm down. Spring is here.
Each time I've changed it I like it better.
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