Engine & Cowl Mounting

Aug 21 , 2006

Open wide and say "aaaaah"

Before doing any cowl or engine alignment and installation, you're going to have to open up the front end of the JDM cowl. Just cut it out with a Dremel and you're good to go. Save that piece of fiberglass... you never know when it might come in handy!

Adding an engine standoff

I determined that in order to space out the cowl into a position that puts the back edge up against the leading edge of the wing, I'd have to have the front end land at about 7-3/4" from the firewall. Actually, scale position is about another 1/4" or so further back but this will do fine.

I want to avoid adding any more nose weight than I have to, so I'm trying to keep the engine as far forward as I possibly can (within reason). So, I'm shooting for a spinner backplate position of 8" from the firewall. That puts it a little farther forward than I'd like for scale purposes, but I'm more concerned with CG and all up flying weight, so it's worth the tradeoff.

So, with the RC Ignitions G62 and integrated cup mount, I would need some sort of standoff added to put the spinner at almost 8" from the firewall. I decided to laminate a number of pieces of 1/4" plywood together to build up a 3-1/2" square block. This G62 has a bolt pattern that is 2-3/8" square, so I drilled that pattern into the standoff block once it was ready.

I then marked the center and drew lines at about 12-14 degrees of rotation. The intent here was to turn the motor slightly to better align the muffler at the bottom, reducing how much I'd have to cut out the side of the cowl. I later found that 12-14 degrees was probably overkill, somewhere around 5-7 degrees probably would have been better.

Lastly, I used a large hole saw to cut a big lightening hole out of the center of the standoff block, tack-glued it into position on the firewall, and then drilled the through the four bolt holes into the firewall.

Mounting the engine

The RC Ignitions G62 has mounting bolt holes that are threaded for 1/4x20 bolts, so I had to mount the 3" stainless steel bolts from the inside of the fuse. I used large stainless fender washers on the inside of the fuse to minimize the amount of compression from the bolt heads into the backside of the firewall.

The engine mounted up very easily, although getting to those bolt heads inside the fuse may be a challenge after I have installed all the radio and fuel gear later. It will probably mean removing much of it any time I need to pull the engine for any reason. I may also decide to throw a couple lock nuts on the bolts protruding out of the cup mount to be sure they don't vibrate loose after final installation.

The pic below shows how everything lined up in relation to the fuse. The bottom of the cylinder head, the spark plug and the B&B exhaust diverter will definitely be hanging out of the bottom of the cowl. The "muffler" presented the most obstruction that would require a good deal of cutout in the side of the cowl, which is why I decided to rotate the engine slightly to get the muffler closer to the bottom center. After all was finished, I found I went a just a little too far, but not enough to warrant re-installing.

I cut just a bit off of the pipes in the B&B diverter so that I could slide the cowl on. But they are still hanging down pretty far, so I may end up cutting some more off later.

Aligning the cowl

In order to start aligning the cowl, I have to do the step we all hate... cutting that big gaping hole in the bottom of the cowl for engine clearance. I cut mine a bit larger than is needed for clearance as I want to make sure I have adequate exit airflow for cooling.

With the bottom hole cut out, I slid the cowl into place and began aligning it to the wing, the engine and the spinner. It's a bit of give and take to get it aligned perfectly square while going for accurate scale position and good fit to the engine and spinner.

While the JDM fuse is round at the nose, the 190 really isn't round at all. The cowl is a bit larger than the round nose, so you have some excess to work with here. This allowed me to shim the top and bottom of the cowl out a bit with some balsa while aligning. The gun hood that gets built on the top of the fuse will do away with that "round shape" and end up aligning flush to the raised portion of the cowl that I shimmed up with the balsa. On the bottom, the balsa shim allows for another point of exit airflow leaving the TE of the cowl.

The next challenge was to work out the function of the choke lever. Normally, I'd have to work out some sort of choke linkage and end up with a pushrod of some sorts sticking out of the cowl. But, with the engine rotated as I did, the choke lever ended up laying right against the inside of the cowl just below the teardrop blister.

So, I cut a horizontal slit in the cowl to reveal the choke lever right there at the cowl surface. One more smaller vertical slit intersecting the horizontal slit and I now have access to the choke lever via any piece of metal rod. In other words, with the choke off, it recesses flush to the slit, nearly closing it off entirely. When I want to turn the choke on, I reach through the vertical slit with a piece of metal pushrod, insert it into the choke lever linkage hole and pop it out. The lever sticks outside of the fuse while on, then pushing it back in shuts the choke off. I think this should work just fine and keeps things very simple with no additional linkage required.

The B&B exhaust diverter (muffler of sorts) has two long pipes at the bottom. I cut them shorter to allow me to slide the cowl on and off over them. However, I probably will end up cutting them even shorter eventually... this would only be for aesthetic reasons. My only concern is that this diverter will be quite loud as it is and shortening the pipes will only make it louder.

Adding cowl mounting blocks

With the cowl aligned I taped it in place with some masking tape. Next step is to figure out how I'm going to mount it to the fuse. The challenge of course is to have a mounting scenario that can handle the vibration of the G62, yet be light, and hopefully as inconspicuous as possible.

I decided to try to mount the cowl with blocks accessed from the inside as opposed to screws visibly going through from the outside of the cowl. I found that this would be possible in at the top positions and the bottom right, but the bottom left position was blocked from access by the carb. So, that one block would get the more conventional mounting method with a screw through the cowl. It's low and relatively out of site, so it's an acceptable compromise.

To start, I made 3 curved mounting plates from 1/4" plywood. I would need to epoxy them to the inside of the cowl (on edge) but didn't want them to stick to the firewall. So adding a piece of cellophane tape to the backside would ensure that the epoxy would not mate the blocks to the firewall when gluing in place. For the 4th block, I made a ply piece that would glue to the firewall, so the tape needed it to keep from sticking to the cowl.

Epoxying the first 3 blocks to the inside of the cowl, I pushed each block as far back and flush to the firewall as possible while curing. I did likewise with the fourth block, except the epoxy joined it to the firewall instead of the cowl. Once dried, I needed to get the screw holes in place. For the 3 cowl blocks, I chucked up a long "dead center drill" in my Dewalt drill and ran pilot holes through the blocks into the firewall. For the firewall block, I just used a small bit to drill a pilot through the outside of the cowl into the block.

With the pilots drilled, I removed the cowl, removed the tape, and drilled through the pilots with the correct final bit size. I'm using large 7/16" hex head sheet metal screws to go through the cowl blocks into the firewall plywood. For the 4th (firewall) block, I'm using a nylon 8-32 screw into a blind nut in the block. I'll probably end up adding a rubber grommet to the hole in the cowl later to reduce the wear between the nylon screw and the cowl fiberglass.

Reinforcing and sealing

It can be difficult to glue anything to the inside of a glass cowl and get it to stick well. The most important thing is to be sure to sand the inside of the cowl to roughen it up, and use some Acetone to clean any mold release residue. This will help the epoxy have any chance of sticking well.

Beyond those steps, the next tactic is to reinforce the blocks. To do this, I cut some triangular plywood supports that epoxy to the cowl and but up against the blocks. Then, using the heavyweight fiberglass cloth, I cut some patches and glassed the area with epoxy such that the cloth stretches out onto the cowl and up onto the face of the blocks. I did this to both the front and backside of the blocks.

Lastly, I hit the whole area with an extra coat of epoxy and microballoons. It seems to be quite strong now, but remember, with the cowl overlapping an inch and a half onto the fuse, you get a lot of support there. This really helps take a load off of the mounting blocks.

The only thing left to do at this time is to go ahead and paint a coat of thinned epoxy onto the entire firewall to help protect it from the gasoline it will encounter over time when in service. I have a few more things to do to the cowl, such as cut out the semi-circular areas where it wraps around the leading edge of the wing... and, fabricate the gun troughs into the top of the cowl so the nose gun barrels will have somewhere to reside. I'll do these steps later.

Why not a cowl ring?

You may be asking, "why cowl blocks and not a complete cowl ring" Tom? Well, good question. A cowl "ring" would have added more strength to the whole installation, but would also present a problem There are 3 large areas at the rear of the cowl that have to be kept unobstructed to allow for good airflow through the cowl. The two scale cooling vents on each side of the fuse and the shimmed out area at the bottom of the fuse all will provide a good deal of exit air that a cowl ring would have blocked off.

Also, the area at the top of the cowl (between the top two cowl blocks) would have to get cut out and modified greatly later to fabricate the scale "troughs" for the two nose machine guns to rest in.This meant that gluing a cowl "ring" over this are would not be feasible. So, with the reinforecement methods I utilized, I feel that going with four "blocks" instead of a "ring" was going to have to suffice. I'm confident it will work out fine.

"the gun deck"