SECTION 12:
FITTING THE COWL, SPINNER & COOLING BAFFLES
12-5
12 11/29/12
MODIFYING THE COWL FOR THE RV NOSEGEAR LEG
A slot is cut in the lower cowl, or induction air scoop (depending on the engine) just wide enough to fit around the
gear leg. When the lower cowl is installed, the lower brace is fitted as shown on View U-U, DWG 48 (RV-8A), View
T-T, DWG 45 (RV-7A-9A), DWG 61 (RV-6A).
The cover plate is fabricated from two pieces of aluminum sheet. The narrower sheet fits on the inside of the slot
cut into the lower cowl, and the wider sheet spans the gap. Screws and nutplates hold the cover plate to the cowl.
RV-8 note that one screw in the center of the cover plate attaches it to the U-816 brace.
Those using the 200 hp IO-360 will find that the cluster of tubes at the bottom of the engine mounts hangs slightly
below the bottom of the cowl, so an intersection fairing will improve the appearance noticeably. While this is not
true for those using the O-320/O-360 induction scoop, an intersection fairing will still clean up the junction both
aesthetically and aerodynamically. Fabricate a fairing using the same techniques as the gear leg intersection
fairings described in Section 10 and DWG C3.
INSTALLING THE INLET DUCTS
A detail on the drawing illustrates the installation of the cooling air inlet ducts. These are essential for proper engine
cooling. While the inlet ducts and the cowl itself tend to be flexible by themselves, when bonded together they
become quite rigid. Thus, the cowl should be installed on the airplane, and all fasteners installed, the ducts are
bonded on. Otherwise, the upper and lower cowls may not match each other once the ducts are in place.
Fit the ducts to cowl, trimming and sanding as necessary for a good fit. Drill and cleco them in place. Clean the
area to be bonded by sanding with a coarse (40-60 grit) disc sander. Sand away the glossy surface of the resin
until the fibers of the glass cloth are exposed on all the mating surfaces.
The only way to hold the ducts to the cowl while bonding them is to drill holes and use clecos. These holes can later
be filled with resin. Enough clecoes should be used to assure contact at all points. Once the ducts are in place, the
cowl should be pinned in place on the airframe before the resin cures. This prevents any possibility of shifting out of
alignment.
After the resin cures, use coarse sandpaper to remove the rough edges and oozes. Sanding with progressive
grades of sandpaper will result in a smooth and pleasing surface.
INSTALLING THE OIL FILLER DOOR
A recess for the oil filler door is molded into the upper cowl. The door itself is supplied as a separate molded piece
with trim lines scribed in the surface.
A scribe line on the cowl marks the shape of the opening. Cut this out with a disc or hacksaw blade. Make sure the
edges are smoothly finished or you will regret it later. The upper cowl pins are installed though this opening, which
requires sticking your arm in a lot farther than just reaching the dipstick.
Trim the door to its finished shape and tape it in place on the cowl.
The hinge and fasteners are installed as shown on the drawing.
FINISHING THE SURFACE OF THE COWL
Epoxy pre-preg structures like the cowl are pressed into the mold during curing by vacuum pressure. This process
precludes the use of the traditional gel coat, so the outside surface must be filled after the cowl is removed from the
mold. If this step is omitted, the painted surface will be covered with pinholes -- not a pretty sight.
The following is the process that the prototype shop has found to be a fast and efficient way to prep the prepreg cowlings for paint.
This is not the only way, it is just what works for us.
STEP 1:
Fit the cowling fit to the fuselage, with all the hinges, retaining screw holes, and nut plates installed. Leave the oil door
installation for later. Sand the entire surface with 80 grit sandpaper , being careful not to remove too much material and damage the
core.
STEP 2:
When the cowling is completely sanded, use an air hose to blow off the dust, paying close attention to the voids in the
finish. These areas must be clean for the resin to adhere. Mix a small amount of epoxy resin (about 1 oz. We use West Systems
105 resin and 205 hardener, but any good epoxy will work.) Thin the resin about 1 to 1 with acetone (or approved product
recommended by the epoxy manufacturer). This may seem thin but it works well. Use a cheap bristle paintbrush (not a foam one)
to cover the cowling with a thin coat and let it dry overnight. If the resin begins to thicken while you are applying it, just thin it with
more acetone.
SECTION 12:
FITTING THE COWL, SPINNER & COOLING BAFFLES
12-6
12 11/29/12
STEP 3:
When the resin has dried, block sand it with 80 grit. At this point you are just trying to remove the high spots. Blow the
cowl off and look closely for any large voids or depressions. Most will be found at outside corners and where the core and the foam
end. Mix a marble sized portion of polyester (“bondo”) body filler -- the filler will dry fast so do not mix too much. Use a popsicle stick
to fill the larger areas and use a new single edge razor blade as a squeegee to fill smaller voids and pinholes.. Come back across
the area with the razor blade at a 90 degree angle to remove extra filler. Work a small area at a time. After the filler has dried sand
the areas with 80 grit and blow off the cowl. Extra time spent on this part will pay off later. Now repeat step 2.
STEP 4:
If you are not going to paint or if you are going to wait until after you fly the plane( this is understandable if the weather is
nice and you don't have something else to fly) stop here. The resin coat will protect your cowl from grease and grime until you are
ready to paint. When that day comes, sand again with 80 grit and apply a filler-surfacer. We use Featherfill made by Evercoat.
There are other brands and they will probably work fine. Just follow the directions on the can. After the feather fill has dried, sand
down to 400 grit and apply your paint system.
PROTECTING THE INTERIOR OF THE COWL
The fiberglass cowling can be damaged by excessive heat radiating from the exhaust pipes. Van’s recommends covering the
inside of the cowl with reflective, heat-resistant material similar to the EA EXHST/COWL SHIELD supplied in Van’s Firewall Forward
kits. Make paper patterns for the areas on the inside surface of the cowl where protection is required. The cowl shield will also
protect the fiberglass from oil/fuel drips as well as heat, so consider applying it to the majority of the lower cowl.
Prepare the cowl by sanding the interior smooth with 80-100 grit sandpaper. Clean the sanding residue away and roll, sponge, or
brush a layer of thinned epoxy resin over the sanded area. The goal is a lightweight glossy smooth surface – but brush/roller marks
are not significant. Once the resin has cured, cut the adhesive backed cowl shield material to match the paper patterns. Peel the
backing from the shield and begin applying the shield to the cowl. Make sure it is exactly where it needs to be -- the adhesive is fast-
acting and strong, so re-positioning the shield once it’s attached is impractical. Work carefully and, as much as possible, avoid
wrinkles. Rub the entire surface with a stiff plastic squeegee, working the inevitable wrinkles and bubbles to an edge.
SPINNER INSTALLATION
Before installing the spinner, be sure you have the correct parts for YOUR propeller. Constant-speed, wood or fixed
pitch metal propellers require different parts. On the constant-speed propeller, the back plate mounts to the hub of
the prop and is reinforced with an aluminum ring. The front plate is a different size, as well. Follow the installation
instructions included in the constant-speed spinner kit and DWG C4.
The following instructions describe mounting the spinner on a fixed pitch propeller. If you are installing a Sensenich
fixed-pitch metal prop, use the appropriate front plate.
The S-601-1 spinner is a translucent cone shaped fiberglass molding. Cut-outs are made for the prop blades. The
cone is fitted over the prop hub and fastened to two flanged aluminum mount plates, installed on the front and rear
faces of the prop.
The rear spinner mount plate is the main structural mount, and the front plate provides alignment and stability to the
spinner. Because of the tapering cone shape of the spinner, the required diameter of the front plate will vary with
the thickness of the prop hub. The prop cut-outs in the spinner must be sized to the cross section of the prop being
used, and will vary considerably with props supplied by different manufacturers. Sometimes there are noticeable
differences between props of the same manufacturer. Since it is desirable to achieve a close fit of the spinner cut
out to the prop, it is suggested that the cut out be custom tailored to the specific prop being used.
Before installing the S-602-1 and S-605 (or S-605SEN) spinner mount plates to the prop, fit them to the spinner.
This will verify that the prop hub thickness corresponds to the relative diameters of the mount plates. It is
imperative that the mount plates be placed in the spinner perpendicular to its center line. This can be checked by
measuring equal distances from the plate to the rear skirt of the spinner all around. When both the front and rear
plates are aligned with the spinner and with each other, the distance between them will be the required thickness of
the prop hub.
If the distance is greater than the thickness of your prop hub, spacers may be installed between the front plate and
the forward surface of the prop. These spacers are available through Van's Accessories Catalog. If the distance is
less than the thickness of your prop hub, a smaller front plate will be required. Sometimes, if the mis-fit is very
slight, sanding the inside of the spinner will allow the front plate to fit. Keep this sanding to a minimum to avoid
weakening the spinner bowl.
Mount the rear plate on the back side of the prop and the front plate on the forward face of the prop or whatever
spacers the prop requires, and under the crush plate. Install and tighten the prop bolts to the prop manufacture's
recommendations.
