Showplanes Cowl Exhaust area?

How are people finishing up the profile of the Showplanes lower Cowl where the exaust pipes emerge? As formed, that region of the cowl extends aft of the general line where the rest of the cowl ends at the firewall. Did you cut that bit off as if it met the firewall? Leave it extending aft of the firewall line? Is this only a cosmetic issue, or are there airflow concerns about the air venting from the lower cowling?

Following as I am in the same spot. My original thoughts are to leave the portion I think you are speaking of and blend ot in to the cowling at the lower connection points.

Left it extended…

Hello, I left mine extended. Just squared it up, it extends about .5 inch aft of the firewall line. Liked the look. Don’t have a good pic of that area.

Cowl exit pic

Found a pic…

This is a timely thread for me, because I was just getting ready to ask the same question about a standard vans cowl when I noticed you had already started this thread.

From an exit air velocity vs cooling perspective is it better to cut this straight up and down, swept fwd or swept aft?

My gut says swept aft like I've drawn with the red line might result in slightly higher exit pressure which is a good thing, and if it's not enough air flow I can always chop it off straight up and down.

But, if the result is micro knots worth of drag reduction then I don't think the juice is worth the squeeze andI'll chop it off straight up and down so that the cowl can stand up on the aft edge when I take it off the airplane.

any Aero guys out there care to comment?

resrob66 said:

Found a pic…

Click to expand...

Question for you, I am fitting the showplanes cowl now, if you used one, what size prop extension did you use?

Cowl/prop extension

Cwood2000, Sorry I don’t have the number. I measured where my cowling sat in relation to the prop flange and had MT make the custom extended hub to that dimension. Probably did it the hard way but it worked out. Seems like ShowPlanes specifies a 2.5” extension if I remember correctly. Double check your measurements for your plane.

resrob66 said:

Cwood2000, Sorry I don’t have the number. I measured where my cowling sat in relation to the prop flange and had MT make the custom extended hub to that dimension. Probably did it the hard way but it worked out. Seems like ShowPlanes specifies a 2.5” extension if I remember correctly. Double check your measurements for your plane.

Click to expand...

Got it, had to go with a 3"

I used a 2.5" extension. I got it from Sabre Manufacturing. I did not alter the aft end of the cowling where it meets the firewall and 2.5" was just about right. Just a bit of adding and subtracting around the backing of the spinner.