Ch 23 Engine / Cowling
I got some great feedback from the Cozy Builders/Canard Fliers lists on making custom cowlings before starting on this. There have been some amazingly innovative approaches to the challenge of building a custom cowling. I chose to build custom for the following reasons:
1. Want the cowling to match the UL engine, which is a little smaller, and mounted slightly more aft than the Lycoming 360.
2. Want as clean an inflow to the UL prop as possible.
3. Want to run my exhaust directly out the bottom of the cowling as demonstrated by Nate Mullins. This is facilitated when the cowling is shaped for this approach.
1. Want the cowling to match the UL engine, which is a little smaller, and mounted slightly more aft than the Lycoming 360.
2. Want as clean an inflow to the UL prop as possible.
3. Want to run my exhaust directly out the bottom of the cowling as demonstrated by Nate Mullins. This is facilitated when the cowling is shaped for this approach.
Lower Cowling
Creating Custom Shape Inside Standard Featherlight Lower Cowling:
I started with the lower cowling since I need to figure out it's shape to determine where to trim/adjust the exhaust pipes to exit the cowling adequately. This little chain of dependency resulted in shift of focus as I was intent previously on getting everything done to get to first start of the engine, and shaking out any electrical, fuel, oil, etc issues prior to moving to hangar. That's just the way it is when you take on something new. You have to adapt to the need, and at the moment, I don't want to start the engine until I've got the exhaust system reasonably well set.
I did not want to turn the plane over at this point to build the cowling in place, and I could not figure out a way to build it without having gravity assist (see much smaller layups I had to do in this upside down configuration). So I decided to build my cowling inside a standard featherlight cowling and use foam to fill the gaps between the standard cowling and what I wanted for the UL engine. Note: this is no small undertaking. I'll go through what I did, and comment on things I would do differently the 2nd time, but it's not a quick process. Essentially, I'm building a one-time use mold inside the featherlight. Nelson Amen was gracious enough to let me borrow his featherlight cowlings for this process, as he's still a ways from using them, and I needed to return them to him undamaged. Here are the steps I went through, and a few pictures from the process:
Photos: Ugly up front building the form inside the Featherlight Cowling:
I started with the lower cowling since I need to figure out it's shape to determine where to trim/adjust the exhaust pipes to exit the cowling adequately. This little chain of dependency resulted in shift of focus as I was intent previously on getting everything done to get to first start of the engine, and shaking out any electrical, fuel, oil, etc issues prior to moving to hangar. That's just the way it is when you take on something new. You have to adapt to the need, and at the moment, I don't want to start the engine until I've got the exhaust system reasonably well set.
I did not want to turn the plane over at this point to build the cowling in place, and I could not figure out a way to build it without having gravity assist (see much smaller layups I had to do in this upside down configuration). So I decided to build my cowling inside a standard featherlight cowling and use foam to fill the gaps between the standard cowling and what I wanted for the UL engine. Note: this is no small undertaking. I'll go through what I did, and comment on things I would do differently the 2nd time, but it's not a quick process. Essentially, I'm building a one-time use mold inside the featherlight. Nelson Amen was gracious enough to let me borrow his featherlight cowlings for this process, as he's still a ways from using them, and I needed to return them to him undamaged. Here are the steps I went through, and a few pictures from the process:
- Obtain the standard cowling. I'll call this the "SC" for brevity in the following bullet list. Nelson's was a featherlight with a moderate boat tail shape to it.
- Position the SC in place on my fuselage and examine where I wanted to reduce the huge clearances I had. Note that I had mounted the oil cooler in the final position, and had removed the exhaust pipes. I did this by eye. It's not perfect, but there are adjustment opportunities later.
- Mark inside the cowling (removable ink) where I want to do which shaping. The emphasis for my lower were the following:
- Match the skin tangent at the fuselage. The match was good on the sides, but poor on the bottom, in part because I could not lift the trailing edge of the standard cowling as far as normally done with the Lycoming, because I have the oil cooler mounted low and forward of the engine. The pictures help show this point. It was not a big tilt difference, but did require some compensation.
- Pull in the lower rounded corners to create a better flow into the exhaust region, and reduce the overall volume of the lower cowling, while still tapering well to the prop.
- Bring the prop hub area up slightly, to account for the slight tilt of the standard cowling.
- Fill in the area in the lower cowling where the Lycoming exhaust normally flows out the rear
- These areas were then filled as shown with blue foam and pour foam and carved to achieve approximately the shape desired. I ran out of pour foam and it was somewhat hard to work with in this application, because I'm pouring into relatively unconstrained cavities. I chose to use drywall compound for fill small voids in foam and do final smoothing. This process involved MANY fill and re-fit to fuselage to check clearances and overall shape. Using standard joint compound is really heavy and the whole thing got really hard to lift into place for fit checks, so I found lighter drywall compound. The latter didn't sand quite as well but was adequate to finish what was becoming a big job...
- Once I had the shape I wanted I was worried about the epoxy absorbing into the joint compound so I brushed a couple of layers of polyurethane on it and waxed it heavily to provide a good release surface (this didn't work that well, keep reading).
- Just before glassing I became more nervous about the release and taped a layer of pallet wrap on the two flat wing sections, and a strip running up the centerline of the mold. This pallet wrap is very slippery, and I didn't use it in the highly curved areas, because I wanted the epoxy to still a little in these areas to hold the right shape.
- I then glassed the new shape as if it were a normal mold (2BID everywhere and an additional 2" BID on the periphery as a reinforcement for installation hardware. (more BID applied later).
- After 2 days of cure (using MGS 335/340 slow hardener) I performed the release. It was trivial in the wing sections with the pallet wrap, but pretty difficult in the highly curved areas, and then easy again in the centerline area where the pallet wrap was again used. The flexibility of my 2 ply layup helped to get the 1" scraper I used into the foam/joint compound to break these areas loose. Remember, it's a single use mold...
- Once released, I put a 3rd BID ply inside my piece with it just resting in the "mold" and let this cure another 2 days.
- Then I pulled my new lower cowling from the SC and used a scraper to clean 98% of the mess and wire brush for about 2%. on both components.
- buy more pour foam: I tried using Great Stuff again from local home improvement store and this stuff is really easy to spray on where you want it, but it's just terrible to try and sand because it's too compliant. With enough pour foam, you can avoid using joint compound, which I would not use again.
- Don't use polyurethane and wax it: The wax was not really effective in preventing the epoxy from sticking to the polyurethane, and the latter was just messy to get off of both my part, and the SC. Go with tried and true: duct tape.
Photos: Ugly up front building the form inside the Featherlight Cowling:
Starting with blue Foam and Great Stuff (don't use the latter where you want to finish sand the shape)
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Nearing completion of the shaping of the form inside the Featherlight Cowling
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Photos: Layup and fitting of the result:
3 ply BID in the form...
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My wife said she would fly with me if the Cozy had a potty, so I'm getting it ready for installation...
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A view of the lower cowling in the initial fit-up
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Modifications for oil cooler:
TBD. The main interface is the oil cooler exhaust duct, which needs to blend with the shape of the lower cowling. This will be a cowling cutout aft of the cooler with a duct walls isolating cooler flow from internal cowling air that's essentially from the downdraft cylinder cooling. I've had thoughts of tapping this exhaust stream for cabin heat, but still a long way from finalizing a design. |
View down into lower cowling, showing the duct from the belly NACA to the oil cooler.
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Modifications for exhaust:
TBD. I'm mainly expecting this to be figuring out where the cutout holes need to be for cylinders 3-6, and if any small allowance is needed for cylinders 1-2 exhaust pipes. I'm thinking I'll want some flow blending just in front of each exhaust tube exit to reduce direct impingement of the airstream on the exhaust pipe, and keep the pipe extension beyond the cowling small but enough to insure exhaust is clearing the cowling... When I get to this point, I'll be checking on Nate's latest experience.
TBD. I'm mainly expecting this to be figuring out where the cutout holes need to be for cylinders 3-6, and if any small allowance is needed for cylinders 1-2 exhaust pipes. I'm thinking I'll want some flow blending just in front of each exhaust tube exit to reduce direct impingement of the airstream on the exhaust pipe, and keep the pipe extension beyond the cowling small but enough to insure exhaust is clearing the cowling... When I get to this point, I'll be checking on Nate's latest experience.
Upper Cowling
The upper cowling shape was a completely new design because I wanted to keep the aft aperture small, fit the UL engine (which is mounted further aft than the Lycoming since it's lighter to meet the CG needs), and match the firewall/strake/wing root. I consulted Ryley Karl and Nate Mullins on how to set up foam in the engine area and ultimately used the method below, which borrowed most heavily from what I saw Nate doing with 1/2" pink foam from local home improvement store. In this case I'll annotate the figures more for description. I also show a visual comparison of the featherlight cowls to my custom cowls.
Use of 1/2" pink (Lowes had this in light blue) foam to start getting the shape. Note pieces: top, shoulder (front/back), and side. slits used to allow the foam to bend some. Duct tape pulling toward engine to bend the foam.
Refinement of the shaping: used some wedges inside to push the foam against the cowling support lips coming off of fuselage. Duct tape on the seams and over larger slits. Took a template off of my wing root (could have used plans, but more convenient to use the real thing) to make frames for the wing root extensions. I used cardboard over the cylinder cooling plenums as I wanted to keep this clearance about 3/8" vs 1/2" of the foam. The level wood piece mounted to the forward edge of the prop flange serves as a friction point on the trailing edge of the foam to allow me to "tuck" the foam profile down a slightly in this area.
After 3 plies BID and cure, I went in with pour foam and filled and sanded the larger low spots. Then micro'd them, and put on another 2 BID over the whole thing.
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Another view of early foam shape. Down draft ducts helped bend the foam to close to the firewall curvature. Duct tape between top and shoulder pieces are pulling the shoulder down towards engine to create twist. Pretty cool.
Everything got covered in clear packing tape as a release agent. It worked great in this case because the curves are very gentle and not overly compound, as in the lower cowling (should have used this on the "wing sections" of the lower cowling though). Here's what it looked like after 3 plies of BID
All foam removed and both cowlings attached with cleco's.
The exhaust tubes are visible here poking through the lower cowling.
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