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One of the many defining scale features of a Merlin engined Mustang are the covers with a series of holes that are located on the lower forward cowl. They are covers over a pair of air filters that are connected to the air intake duct leading to the carburetor. The pilot can switch the normal carburetor air supply from the chin scoop to the filters in dusty conditions.
(Some combat aircraft had solid covers installed when the filters weren't needed. And some restored Mustangs with replacement lower cowls now don't have them at all. All the race planes don't have them. However, virtually all service aircraft had provisions for them.)
These covers can be made by hand, but that can be very hard to well with their 32 very closely spaced holes. They're darned hard to get evenly spaced (I know - I've tried it several times). And if the holes aren't near perfect, they look like the devil. However, laser cutting provides a nice solution.
To be continued …
< Message edited by tony-howard -- 5/8/2008 2:04:11 AM >
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This thread demonstrate one way to install these covers on a Mustang cowl.
The covers used here are available from Pacific Scaled Aircraft. They are available in several sizes and come in two kit styles. Each size cover has been scaled up from the original North American production drawings so they're accurate.
The SURFACE MOUNT kit has a flush mounted a pair of 1/64" plywood covers that are intended to be surface mounted on the lower cowl. Easy to install, the plywood is so thin that they appear to be flush. An advantage is of this style is that they can be installed virtually any cowl, even if already painted.
The FLUSH MOUNT kits uses identically shaped covers cut from 1/32" plywood and are designed to be flush mounted in fiberglass cowls. A bit more work, they're the ultimate in scale appearance.
Note that each kit has illustrated instructions and includes a template for initial positioning of the covers in the scale position.
A Ziroli cowl will be used to show both installations. The process and finished appearance is identical to all of the kits. You may have seen the cowl's exhaust installation detailed in an earlier thread: Ziroli exhaust installation To be continued …
< Message edited by tony-howard -- 5/6/2008 5:48:33 AM >
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First let's look at the installation of the surface mount covers.
The position template in the kit is used primarily to position the covers fore and aft and has multiple cutting lines for various kit scales. For the Ziroli it's the 1/4.5 outline. After cutting it out, the template is positioned on the cowl with its upper edge on the engine thrust line and its front at the front of the cowl. (Since the base of the exhaust manifolds on this cowl were installed on the thrust line, that's used here for alignment.
Using the template, the location for the front and top of the cover is marked on the cowl on one side only. Next one of the covers is taped into place using the marks. This is an initial position and will usually need to be adjusted.
Since model cowls are often not perfect scale, the position of the covers usually need to be adjusted to look right. The illustration drawing shows the correct scale orientation and can be used to eyeball how they should be positioned vertically on the cowl. On the Ziroli cowl the covers are about the right height vertically but need to angled to make them parallel to the bottom profile of the cowl. When the cover looks right, mark its new top and front outline closely on the cowl.
Then repeat the process for the other side, this time measuring the changes made from the positioning template lines.
Note: The cover's location can be moved slightly fore and aft if they conflict with a hatch or cowl cut-out. If moved more than about 1/2 their length will be obvious.
To be continued …
< Message edited by tony-howard -- 5/7/2008 10:10:29 PM >
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Before going further, the covers need to be pre-curved. Skipping this step runs the risk of breaking them while curving them to the cowl. Pre-curving also makes them easier to install.
It works best to form them with a slightly more radius than needed – it makes them easier to hold in position during installation. Using the curve of the cowl as a guide, find a bottle, pop can, spice bottle – anything cylindrical and water proof – that has about right radius.
Drop the covers into a cup of boiling water and let them continue to boil for 3 minutes, then rubber band them carefully around the forming tool. Make very sure they are on straight and that there is a right and left hand cover. Don't ask me how I know about that too!
The covers will need to dry for at least 2 days – 3 is better. If you take them off too soon, they probably will look fine but may warp later. (If you have a digital oven that will go as low as 100°-120° you can put them in the oven to speed up the drying. Just make sure whatever you’re using to form them can take the heat!)
(In the pictures below, both covers are rubber-banded one on each side of the can and bottle. The covers on the prescription bottle are for a smaller size application. Note the nice curve on the right hand cover in the last picture. This technique really works!)
The pre-molding process can be repeated if necessary.
< Message edited by tony-howard -- 5/8/2008 2:11:28 AM >
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When the covers are dry, place them in position on the cowl and carefully trace around their perimeter.
The cover's holes can be used to bleed off engine compartment air by cutting a hole in the cowl behind them. It's easy to do and now's the time to do it. Trace inside the four corner holes in the covers as shown, then draw the outline of a rectangle that is about 1/32" outside the corner holes and cut it out. (I like to drill 4 holes in the corners as it makes cutting the straight lines easier using a Dremel cut off wheel.)
Sand the covers to remove any roughness from the forming, then finish to suit. Sanding a slight radius into the top edges before installation helps make them look even thinner. If you didn't cut an opening in the cowl, paint the area behind the holes flat black after the cowl is painted and before installing the covers. If the cowl is painted, sand or scrape the finish off first, staying just inside the cover's outline (and outside the black paint when used).
(Using the kit's aluminum foil is an easy way to seal the covers prior to painting. Check them for smoothness, then wipe the cover surfaces clean again. Cut the foil piece in half and put the oversize patch of foil on the cover and smooth it down lightly with your finger. Next burnish it with a smooth pen barrel or any smooth burnishing tool. When it’s smooth, use a new X-Acto #11 blade to trim around the outside and cut out each hole with a twirling motion. Prime the covers with a good metal primer and paint to suit.) Epoxy the covers in place.
(In the picture with the cover on the cowl, the top and bottom of the cover are slightly blurred from the tape holding it in place…this cowl will also be used to illustrate flush mounting the covers)
That's all there is to it. For a very reasonable amount of work you now have two realistic air filter covers that will enhance the appearance of your Mustang.
Next – Flush Mounting ….
< Message edited by tony-howard -- 5/10/2008 10:22:45 PM >
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Flush mounting the covers provides the ultimate in scale realism. The same size covers - cut from 1/32" plywood instead of 1/64" - are used along with an inside-the-cowl mounting plate and an optional spacer. The opening in the cowl is now the same size as the cover. (see drawing below)
To prepare the flush mount covers, the same positioning and pre-curving process described for the surface mount covers is used. The mounting plates are easier to work with if they're pre-curved also … use the same process, making their radius slightly less than the cowl's curavature.
Don't forget …MAKE A RIGHT AND LEFT PAIR FOR BOTH THE COVERS AND THE BACKING PLATES!
Mark the complete outline of the covers on the cowl using the same process for the surface mounting process above. Do an accurate job here and keep the lines as close as possible to the cover.
To be continued ...
< Message edited by tony-howard -- 5/12/2008 7:37:38 PM >
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Guess who though he already had a set of pre-curved mounting plates and didn't? I'm back on track now.
A couple of catch up points first:
* One way to be sure the plywood is wetted completely is to look for pale spots, particularly if the parts lay on top of one another during the boiling process.
* It has been suggested that a picece of gauze or loose fabric underneath the plywood parts when they're wrapped around a form will help hasten the drying process. That sounds like a good idea and one I'll try next time. Any wrinkles would need to be smoothed out underneath the parts before leaving them to dry.
* Note in the picture below that popsicle sticks were needed to keep the 1/64" plywood mounting plates flat against the vase that was used. The hole in the center tends to cause them to bulge. The point is to make sure they are laying flat before leaving them to dry.
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Start the opening in the cowl by carefully cutting out the center opening with a Dremel tool, staying well inside the lines. Then slowly enlarge the opening to the inside of the outline on the cowl. Tools used here were a well used and small diameter Moto-Tool reinforced disk, a small carbide bit in the Moto-Tool (to release the corners) and a fine tooth 1/4" rat tail file in that order. Then I switched to 150 grit sandpaper wrapped around a narrow sanding block to work the edges and the file for the corners. When the cover is just barely short of fitting, it's time to stop.
Tip – trying to hold the cowl steady while making the cut-out can be frustrating … try putting some weight inside the cowl with it laying it on its side. I used a couple of peanut cans that I keep nuts and bolts in.
Another Tip - it's easy to get the corners too small. So concentrate on keeping them a bit oversize until the rest of the edges are good, then carefully match the corners to the covers.
Making the cut-out is the only critical part of the installation - it’s not hard, it just takes a bit of patience. Several check fittings are usually needed, each followed by a smaller and smaller adjustment. (The cut-out for one cover on this cowl took less than 15 minutes.) The more accurate the opening is, the better the installation will look.
The trick here is to get the fit very close - then sand the cover edges later after the backing plates are in place for the final fit and gap.
To be continued ...
< Message edited by tony-howard -- 5/21/2008 2:22:22 AM >
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Installing Air Filter Covers on a Mustang - 
