Trimming NACA duct aft edge

(S7 with aluminum radiator, if it matters)
The manual states to trim the aft edges of the duct to have "a constant gap" between the duct and the face of the radiator.
My question is, How close is close enough? Are most folks trying to minimize the gap (or even use a seal to fill it) to force air thru the radiator or going pretty generous on the gap.
My thought was to go for a 1/4" gap to the end pieces (would be 3/8" or so more to the actual face), but would like to hear from others as to your setup and results.

I think something on the order of 1/4" would be fine. Main thing is you don't want it rubbing and vibrating against the radiator. Trying to seal that gap or making it very close is probably not worth the effort; I doubt any measurable difference in cooling would be noticed.

I agree with Jim on this - it took a lot of trial fitting to get this right - too close and it rubs the oil cooler when installing the lower cowl. I have more problem keeping temps up than down so I don't think the gap is that critical. My installation is a 912is sport - not sure if the same on others - but I noticed at the factory they fit a heat foil on that narca inlet to protect against heat from the muffler which is close to the fibreglass - I did the same - I found the sheet of heat shield on line and it wasn't expensive.

cheers

r

Thanks for the feedback guys. I was trying to preserve as much to the final straight portion of the duct as possible by only trimming about half of it off and locating the duct itself a bit further forward since the aluminum radiator is thicker. Appears that won't work well as the duct has a "sweet spot" for matching the cowl curvature. I will probably need to trim all of that straight portion away to get even that 1/4" clearance. I do not have the spinner backing plate on currently, but can see where that will really restrict the cowl installation/removal. Setting the spinner/cowl gap to minimums, as I did, may bite me there.
Ross, thanks for the heads up on the exhaust proximity. I'll have to get some heat shield material ordered too.

You are right about the spinner back plate restricting lower cowl installation and removal. Mine is very tight and requires the prop to be in a certain position and the cowl to be slightly sprung to get past the backing plate. No big problem, just requires some care or you will be scuffing some paint.

I am curious about the heat shield you talk about to protect the NACA duct from exhaust heat. I did not use any shielding material and have not noticed any heat problems for 3 years now. I would like to hear from anyone who has actually observed any heat related problems to the fiberglass duct or cowl. The build manual says nothing about this, unless its a recent addition.

I have the same scenario as Jim with the lower cowl clearing the backing plate for the prop. I normally remove the spinner, place the prop in the position that gives the least restriction then twist the cowl a bit to clear the prop blades - no problem.

I am also interested in the heat shielding material used on the NACA duct.

I currently do not have the heat shielding material used on the NACA duct; however, I have had no discoloration or other evidence of heat influence on the inside of the cowl. I did do two things in the interest of handling under cowl heat. 1) I painted the lower cowl on the inside with white epoxy primer with the theory that white would reflect radiant heat better than the somewhat natural yellow color of the of the fiberglass. 2) I fabricated a small aluminum heat shield between the back & top side of the muffler primarily to deflect radiant heat away from the lower rubber engine mounts. Before I made the engine mount heat shield, I noticed a little color bleed developing on the metal surface of the engine mount immediately adjacent to the lower rubber engine mount compared to the top engine mount.

Sincerely,

Jim,
During prop balance static runups the right front header got the upper cowl hot enough to bubble the paint! I had never run it up over 5,000 RPM on the ground before. The exhaust header is less than an inch from the cowl only on the right side. After repairing, I used automotive header wrap on the exhaust pipe as a preventive measure.

I have to kink the lower cowl slightly as well to get it past the spinner. To prevent scratching the paint, I loop a towel between the cowl and spinner.

Re the heat shield - remember mine is the 912is sport and the tolerances may be different - the one I saw it on at the factory was also a 912is sport installation. I remember seeing both Jim and Floog's spinner prop set up - mine has a bigger gap i think - a full inch - more a product of my bad fitting of cowl and having to cut a few more time than ideal! But that does make it easier to fit the bottom cowl!

cheers

r

When Ross brought up heat shield material, this is what crossed my mind to use:
http://www.aircraftspruce.com/catalo...p?recfer=13170

Yep, that's the stuff.

r

Floog, so you bubbled the paint on the UPPER cowl? I didn't think the exhaust headers were up that high. Or did you mean the upper part of the LOWER cowl?
I ran my engine several times on the ground at WOT to check static rpm after fussing with prop pitch (ground adj.) at around 5200-5300, but did not keep it at this hi speed for very long. That's probably why I got no paint blisters.

Did you have to repaint the cowl, or just a small patch?

Another question Floog: I thought prop balance runs had to be with cowl off so you could mount the equipment-sensors etc.

My bad, Jim. You are right: it bubbled on the UPPER part of the LOWER COWL. Only the upper cowl had to be removed for the prop balance. I sanded it out, touched up with a little Aerothane, and then wet sanded/polished. You can't see it anymore. Good catch, Jim!! Thanks.