The purpose of the bottom cowl baffles is to direct the incoming air (from the NACA scoop) to the engine cylinders and oil cooler such that all cylinders are being cooled at the same rate. I made a set of templates per Plan (Chapter 23, Page 17) out of cardboard paper. Then I transferred the templates to a 1/16" foam instead of 1/4" because I have the 1/16" handy. I don't think the thickness of the foam makes any difference because it will be glassed onto the bottom cowl permanently. If it vibrates too much under pressure, I shall add more structure to keep them rigid. The other benefit with the thinner foam is that I do not have to shape the bottom edge of the baffles to conform to the slope of the cowl surface. Here's a picture of the Plan's baffles. I ended up not using the middle and the aft baffles (discussion later).
I pre-glassed the side baffles with 1 BID each side and then shaped them to fit. Even with one layer of BID, the baffles are pretty stiff. I secured the side baffles in place (onto the bottom cowls) with 5-minute epoxy first. Then I glassed the outboard sides of the baffles, while the lower cowl is buttoned onto the fuselage. I did not do the inner baffle sides because I do not have good access to them in this configuration. Once the outboard glass was cured, I removed the cowling from the fuselage. I floxed and glassed the inboard edges of the side baffles. I made sure there is a nice rounded edge for the incoming air to flow through with minimum obstruction and turbulence.
Note the forward edge of the baffles were notched and not to Plan? Here's the story... I was told that the Plan's middle baffle is not that effective in diverting all the necessary air to the engine cylinders 3 & 4 and the oil cooler (if you choose to mount it under the engine). Instead, a diffuser mounted at the inlet of the NACA is more effective in pressure recovery, resulting in better cooling. I found a picture of the diffuser in Terry Schubert's PowerPoint presentation and decided do the same for my cooling needs. Unfortunately, my fuel injection throttle body and air filter sits smack in front of the NACA inlet, therefore it is not possible to copy Terry Schubert or Vance Atkinson's diffuser exactly. I decided to improvise - at least its better than none!
A couple key factors I learned from Schubert's presentation (confirmed later via e-mail) is that the included angle of the diffuser sides should be less than 11 degrees (while others sited 14 degrees). In addition, the diffuser outlet area should be at least 2x of the inlet area. What is puzzling to me was that if I project the 5.5 degrees outwards on each side, it will take ~24" to get the 2x outlet area. In addition, the WxH dimensional ratio of the inlet should be between 3 to 5. Since my width is 15" and height is 4", my ratio is 3.75 - somewhat in the middle range.
Regardless, I decided to charge forward and do my best to duplicate their diffuser. As Terry pointed out, if the engine runs hot when time comes, I can always make further modifications then. I took a compass and measured the spread of the Plans side baffles and Surprise!, they were ~ 5 degrees outwards from the NACA inlet - coincidence or intended? Maybe Nat already had the diffuser concept in mind. I decided to shorten the side baffles just a bit to make them 6 degrees spread.
The one part that is not in the Plans is the top cover across the side baffles. I decided to add one - duplicating Schubert and Atkinson diffusers. As I mentioned earlier, I have quite a bit of obstruction at the inlet, therefore, I have to do some surgery to accommodate. Terry mentioned that I just have to make a cutout to accommodate the air filter (though not optimal)...
I first traced out a 6 degree slope on the baffle and projected out 5 inches such that I can duck under the fuel supply line. Then I drew a curved line to expand the diffuser exit area quickly. After trimming the side baffle notches out (as shown in the above picture), I made a couple templates for hot wiring. The picture shows the hot wired blue foam in place - ready for glassing. This forms the basic diffuser structure that requires many subsequent steps for all the attachment ears and stiffening supports.
Here's the picture of the diffuser. This includes an extended diffuser exit (aft end), two side tracks (Upside down 'U') to seat the diffuser onto the side baffle walls, & an attachment fence (fore) for attaching the diffuser to the firewall. Now I have to cut up this carefully made diffuser to accommodate the fuel injection throttle body and air filter. I showed Terry this picture for his comments. He felt that my extended lip at the aft end of the diffuser may not be of much benefit and that it may not provide added pressure recovery benefits. So, trimming it off may just save a little bit of weight. I decided to hold off on the trimming until later.
Here's another picture of my diffuser that is not permanently glassed to the side baffles. Due to the location of the fuel injection throttle body, it is not possible to glass the diffuser onto the side walls permanently. Most importantly, I do not know if this 'modified' diffuser will work of not? Therefore, I can easily remove it during test flight to determine its functionality.
A few noteworthy features include rounded underside corners (~1/4" radius), slotted (or upside down 'u' tracks) to fit tightly on top of the side wall edges, a wide fore fence to bolt the diffuser to the firewall. This fence is set-back ~1/16" to accommodate the firewall protection sheet and fibrarax (to be added later).
As I mentioned earlier, my air filter and the fuel injection throttle body sits smack in front of the NACA inlet. There's just not enough space to 'steer' the air filter 'out of the way'. I just have to cut a large hole in the middle of the diffuser/baffle. With clearances around the air filter, I suspect it will leak air like a sieve. I decided to build a cover over the air filter - at least to keep the incoming air from escaping up through the clearances.
The picture (left) shows the initial cut. I added 3/8" clearance all around to accommodate engine vibration down the road.
I decided to make an air filter cover to limit the inlet air from escaping through the 3/8" clearance around the air filter. I made 2 half circle templates with diameter just a hair larger than the clearance foot print. Then I tacked the templates at both ends of a blue foam block and hot wired a foam core. The picture shows the blue foam block between templates, ready for hot wire.
Once the foam block is made, I added one BID on the foam and peel-plied. Here's a picture of the resulting air filter cover, the hot wire templates, next to the air filter.
Since the surface of the diffuser is curved, integrating the air filter cover onto the diffuser is not as straight forward. After quite a bit of trimming and fitting, I finally got it seated in place satisfactorily. The next step is to anchor it in place with hot glue at the underside. Then I floxed, glassed and peel-plied the top surface of the cover with 1 BID. Once completed, I flipped the cover over, rounded off the inside edges, glassed with 1 BID and peel-plied.