tony-howard

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Sometime back I bought a Great Planes 1/4 Scale Spacewalker after I'd seen how well a friend's flew. His looked like a relaxing plane for a lazy afternoon to wind down with after flying more demanding airplanes.

After opening the kit I realized it just begged for a set of cylinders to set it off. After researching the market for engine kits I decided I'd have to do my own if I wanted a truly realistic set. Another consideration was that I had in my "inventory" a Sig 1/3 scale Spacewalker. I rationalized I could use the design and drawings for a 1/4 scale engine for 1/3 scale. (In reality the 1/3 scale were almost a completely new effort - I'm guilty of rationalizing more than I should frequently!) The ultimate decision to start the project came when my friend agreed he'd put a set on his Spacewalker if I came up with the parts.

Looking around for scale sources I eventually purchased 3-views and pictures of a Continental A-65 from Bob Banka. Using those and other references I made up a CAD drawing of the full sized engine concentrating primarily on the cylinders and the manifold parts which are visible outside the cowl. The cylinder bank offset and the position of the prop flange was also located on this master drawing.

When I started to design the scaled cylinders it was clearly more formidable than I imagined. There were times I thought I should bag the project, but the idea had me hooked. Eventually I had a drawing of the 1/4 scale cylinder components which I then sent off to have a single cylinder laser cut as a proof. Good thing I did - there were several corrections needed. Another laser set and I though "aha, I'm almost done".

Wrong!

Getting the mounts fitted and aligned accurately inside the cowl was a major effort. I went through several ideas and spent lots time making templates before I got it right.

tony-howard

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Eventually I assembled the first cowl, adding vacuum formed valve covers and manifolds made from off the shelf plastic parts to the laser cut parts. Seeing it all together made the effort worthwhile - although as always there were a few areas that I think could be improved. The alignment jig needed more thinking too – it was still just too complex.

By then my friend had crashed his Spacewalker and I was off onto other projects. I did buy an aftermarket glass cowl for my Spacewalker. But I put completing it into my "someday" pile of projects along with the bigger Sig Spacewalker. However several people who'd seen my Spacewalker cowl suggested I should kit it. My main worry about producing kits was the price. I'd found early on that the cylinders needed to be assembled on a very accurate jig or they would be skewed. That meant the cylinders would have to be pre-assembled taking quite a bit of my time. And the laser cutting for all those parts - well over 100 - was not cheap. So the idea was shelved in indecision.

Note: the pictures are of the original GP Spacewalker ABS cowl and the first engine set - which is now pretty dusty. When I do fly that plane I'll use my aftermarket glass cowl with the "improved" engine set. And the correct shape valve covers!

tony-howard

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So when the question was asked in another thread in this forum a couple of weeks ago about a realistic engine kit for a Balsa USA J-3 Cub, I decided to see if there was any interest what I have. This thread will illustrate the installation process for the cylinders in my Sig 1/3 scale cowl. If there's no kit interest at least I'll wind up with the cowl for my Sig kit being done. (Another rationalization.)

Before I started on the actual installation I needed to develop the correct outline for the mounting plates. Each cowl requires mounts with its own width spacing and distinct shape as a result of its taper and contour. Here I used a trick I've used for several times to get a close approximation of the outline of interior parts. It "works a treat" as an English friend would say. After transferring the Sig drawing of the cowl area into CAD (and correcting for a different overall length shown on the plans – wish I'd caught that earlier!) the horizontal spacing of the plates was determined.

Using that dimension I drew a line that approximated the outline of mount on the outside of the cowl using a marking pen. The cowl centerline was used to position a parallel line for the top and front part of the mount – the rear and bottom lines were measured from the rear of the cowl.

Next I went around outside my line with 1/4" masking tape. The area inside the tape was cleaned and waxed with mold wax (any carnauba based wax will work). Then the area was sprayed with polyvinyl alcohol (PVA). No worry about overspray here – PVA is water soluble.

(The wax is to make the part release more easily from the cowl and the PVA prevents any chance of the part sticking to the cowl. I've been using fiberglass for models since the early 60's and I still use PVA rather than relying on just wax. My philosophy is why take a chance when PVA is so easy to use?)

When the PVA was dry four layers of 4 oz cloth was layed over the area using polyester resin (I like the fast cure time of polyester for this kind of work). When the resin was cured, the part was removed and trimmed back to the tape line.

tony-howard

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Now I had a reasonably accurate outline of the mounting plate in 3D – just as if I'd cut out that portion of the cowl. A small scrap of wood was placed at the aft rear corner of the cutout that propped it up so I could trace the outline of the 3 parts that make up the mounts on a piece of paper. Then I scanned my tracing into my CAD drawing of the cowl.

This is a very easy – and fast - way to get the shape for formers and other attachments for glass fuselages and any other 'solid' parts.

Next the mounts – along with the necessary holes for the cylinders – were drawn, printed and rubber cemented onto plywood, cut out and glued together.

tony-howard

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The alignment jig holds the two mounting plates the correct distance apart as well as aligning the mounts with the thrust line. It also positions the mounts the correct distance back from the front of the cowl. The mounts bolt to it using existing holes in the mount. As stated earlier, the jig is the essential tool to getting the cylinders mounted so they look right.

With the mounts bolted to the alignment jig, it was time to see how they fit in the cowl. The mounts must slip into place without pressing on the cowl. A small edge gap is fine – distorting the cowl is not. Many production and aftermarket cowls have minor layup thickness differences that require either trimming the edges of the mounts or sanding the interior of the cowl – sometimes both. Normally very little sanding is required when that happens before the assembly will drop into place without being forced.

When I was satisfied with the fit I traced the adjusted outlines onto paper. Then I scanned them into the CAD drawing of the cowl and adjusted the drawings for future reference.

tony-howard

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Before removing the mounts for the last time I traced around them on the outside of the cowl (marked on the inside, the lines could get sanded away.) Then two holes were drilled on the outside of the cowl - located by eye - so I could remove the socket screws holding the mount to the jig after it was attached to the cowl. (A small single rectangular hole would work too – neither has to be very accurate. This portion of the cowl will be removed later since it lies well within the basic cowl opening)

I then washed the interior of the cowl with acetone and then with soap and water. Next I sanded the area outside of where the mounts will be positioned with #50 sandpaper to make sure the fiberglass that will be used to attach the mounts adheres well. (The picture shows the sanding process about half done.)

Meanwhile the mounts were removed from the jig and the outside faces were glassed with a single layer of 1-1/2 oz cloth. In this case all three parts of the mount are covered with a single piece of cloth. A 45° slit makes the lower rear corners easily lay flat. Glassing provides additional strength and it weather and fuel proofs them.

I 'paint' the area to be glassed first with a light layer of resin, then lay the cloth into it. Then just enough resin is added to turn the cloth translucent but leave the weave pattern showing uniformly. As I'm wetting the cloth I very lightly resin over the holes and slightly beyond the edges of the parts. When the resin has set, this makes trimming the cloth a very simple matter using a sharp #11 X-Acto knife. It helps make a sharp cut if the cutting edge of the knife is kept at an angle so the cloth is sheared downward slightly against the wood.

tony-howard

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With the glass trimmed the mounts are ready to be installed. It works best to install them one at a time. But both are needed for proper alignment so they were bolted to the alignment jig and slipped back into place in the cowl. After making doubly sure everything was lined up properly, the left mount was tacked in place with thick CA, using 6-8 "spot welds". Once the CA was set I carefully unbolted the left mount from the jig and then gingerly removed the jig and right side mount from the cowl.

Fiber (West Microfibers in this case) was added to epoxy resin to make a putty about the consistency of peanut butter. The putty was then forced into the joint around the edges of the mount using a popsickle stick. (I like to paint the surfaces where the putty will be used with a thin coat of resin first - I think it helps adhesion.) A small fillet of putty was left to help fair the cloth. After cleaning up the area the cowl was laid on its side with the mount facing down and left for the epoxy to cure.

Next morning I repeated the process on the other side.

The next step is to cover the seams and the exposed plywood surfaces with cloth. Here again 1-1/2 oz cloth was used. One large piece was used to cover the plate and lower flange and another was used on the rear flange. These pieces need to overlap at least 1/2" onto the cowl. 1" wide double strips were added to the seams, using short overlapping pieces at the upper front corner.

(It's important to get the mounts glassed into place to both seal them and to insure they are bonded well to the cowl. And glassing the backside of the mounting plate insures it's fuel proof and it strengthens it.)

tony-howard

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The next step is to rough cut openings in the cowl so the cylinders can be fitted to the mounting plates. A jig composed of a 3/4" spacer, a tracing pattern and two short pieces of 1/4" aluminum tubing is used. The spacer and tracing pattern have two 1/4" holes that match cylinder alignment holes in the mounting plate. These tubes are inserted thru the alignment jig assembly and key into the mounting plate holes to hold the tracing pattern in position. (These parts – laser cut and pre-drilled - would be included in the kit.)

It's important to remember that the right side cutout is further back from the cowl face than the left. This is because the cylinders themselves are offset.

As can be seen in the picture below, the square hole in the cowl for the spacer doesn't need to be very precise. However it's important not to make the opening wildly oversize or the hole may wind up outside the final cut. I positioned the spacer on the cowl by eye looking thru the screw access holes drilled earlier, marked around it and then and trimmed the hole until the spacer fit flush against the mounting plate.

I realized after I was done marking the two cuts I could have simply measured and marked the cut out using the bottom and rear mounting flanges as a reference point – they're plainly visible through the cowl. This would not work with a gel coated cowl though.

tony-howard

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With the cut-out jig mounted, the outline of the cut-out in the cowl can be marked. The cowl was sanded first so the marks would show up well. I made sure to allow for the taper of the pencil as I traced. Once both sides were marked, the jig was removed and the cowl opening cut-out made. I cut short of the marked line by about 1/16" around the ends and the top of the opening. The objective here is to just be able to mount the cylinders with the barest minimum of clearance. When the cylinders are in their final position I'll make the final trim to the cut-out. I want the final opening to be equally spaced around the cylinders and to have a neat and symmetrical appearance.

The cylinders are keyed to the mounting plates by two 1/4" aluminum tubes. They are each held in place by a single socket head sheet metal screw. Pilot holes for these screws are pre-drilled by laser in the cylinders. The corresponding holes in the mounts are the same holes used for bolting them to the alignment jig.

It's rewarding to see the cylinders actually in place in the cowl!

tony-howard

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I'm finally able to get back on this project again.

(I've had two inquiries as to whether I will sell just the cylinders. I don't feel this is in my best interests. Without the alignment jig there is a chance they wouldn't be installed correctly which might influence any kit sales from modelers who see the installation. Secondly, the major expense putting the kits together is in the 'jugs' themselves. So selling them individually would cost almost as much as the full kit.)

The first order of business now that the cylinders were mounted was to check that the cylinders are square to each other. There are two areas to check:

* The flat area on the bottom of the cylinders where the intake and exhaust manifolds attach should be parallel and in-line with each other. This makes sure the cylinders are in line with each other as viewed from the front. Adjustments are done by filing the 1/4" alignment tube holes . With laser cut mounts there shouldn't be any required, but my hand cut and drilled prototype mounts required a very slight adjustment.


* And the fin gap between cylinder pairs should be uniform. This usually is the result of something caught underneath the base of the cylinder and the mount.


With the cylinders aligned, the cowl was marked for the final trimming of the cowl cut out. A gap of about 1/8" looked about right. I find that laying out the straight portions of the cut with tape makes adjusting the lines much easier. Once the tape lines are OK I then trace along the cutting edge with a marking pen, remove the tape and then hand sketch the large curves. A circle template is used for small curves.

Final trimming was done with the MotoTool and a dull 1/8" bit (a dull bit helps prevent digging in and having the bit 'take off cross-country"). A relatively slow speed, very light passes and a good working position that keeps the cowl steady and allows a comfortable grip on the MotoTool allows trimming to within a pencil line width of the cut line with relative safety. Then it's time for hand sanding.

It usually takes at least a re-mount or two of the cylinders and small adjustments before the cut out looks right. Then it's on to the second side of the cowl to repeat the process.

abufletcher

Those cylinders look very nice! I don't mean to intrude on your thread but I happened to have taken this shot of a Continental engine recently and thought it might be of interest (though clearly it's a 6-cylinder model).

tony-howard

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Nice picture.

When I was laying out the cylinders I used a series of pictures of a Continental 4 cylinder engine in addition to the drawing.

SDCrashmaster

Your work looks awesome.
Question,
Why not just mold and resincast each mount, cylinder halves, mufflers, etc.
No need to build each cylinder in all those layers with a jig everytime.
I'd use the lasercut pieces as a template to cut the pieces out of thinner more scale styrene sheet.
Spend the time perfecting 1 cylinder then silicon mold and resincast. Plus you can cast the cylinder parts in black, brush silver or aluminum powder into the valve cover molds prior to casting, etc. No painting afterwords...just weathering.
It's easy and fun...and you certainly have the skills!
The attached pics are some samples of my resincast work done that way.
Paul

SDCrashmaster

The photos I posted are parts done for the large 1:7 Yellow Aircraft F/A-18 Hornet. And, although a large model, the pieces are fairly small, yet have a lot of detail, the amount of realism/detail that can be acheived is astounding, and would be even easier to work on some parts for a larger 1:4/1:5 plane. Lot of small nuts, bolts, and rivets, etc. built on the masters. The mold process pics up every detail, from sloppy CA gluing, minute sanding marks, primer texture, and smooth glossy glass-like surface. On the landing gear, the detail is added to the stock Yellow AC gear for the Twin (see pic of Stock Yellow gear below). Major components of the struts are put into molds and resincasted directly onto, capturing the piece within the resin. The rest of the parts are bolted on using scale bolts and nuts. These pieces are cast with bolt holes in them by inserting rod into the part that when removed, leaves the appropriate diameter for tapping with either .090, .080, or .072 threads, or on something your size in 2-56 range. This really save a lot of time not having to drill the many, many holes during assembly. I work on sets of these gear 4 at a time, so anything that can be built into the mold is a real timesaver later.
Anyway, sorry to ramble.
Give it a shot. Every modeler should acquire these skills. It's uses are endless, if your a scale modeler, especially RC-sized models.
Paul

tony-howard

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Very nice Paul. You need to start a thread to showcase your accomplishments and provide some techniques.

Actually you're preaching to the choir here ... I've been using curative RTV silicone molds and casting resin for several years. In fact the vacuum forming plugs for these cylinders were done using that process.

As I mentioned in the beginning of this thread, these cylinders were initially created for my own use. Once I had the CAD drawings it was easiest to have them laser cut. Setting up to resin cast them in a kit form is an option I may pursue if it turns out there is a market. Judging by the limited interest so far, that may be a moot question.

SDCrashmaster

Right On Tony. I did notice the extremely limited interest in this thread, the Spacewalker just might not be the scale subject guys think about adding much detail too. If you did it for a Cub, might be different.
Glad to see that you knew exactly what I was talking about re: resincasting.
Look forward to seeing the cowl painted and your finished application. I think once she's done and the improvement is clearly visible in pics, it may jar some interest.
However, I found your posts very interesting. You certainly made sure you had things aligned and true. I don't think most would approach it as committed as you.
Go big or go home.
Good luck,
Paul

Cub Man

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so what size of continental did you make yours after... 60< 75, 85, 90???

tony-howard

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Cub Man

The dimensional information I have covers the A50, A65, A75 & A80 series engines. The primary pictures I used for additional small details are of an A65.

BTW I expect to have a 1/3 Balsa USA Cub cowl soon which I will fit a set of mounts for a cylinder set.

Tony

hopkimf

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I have a 1/3 Spacewalker waiting toi be built. In reading threads here I'm wondering if anyone has considered stereo lithography? The process is pretty simple. You need to build a computer model of the part with software like PRO/E or something similar. Then the file can be emailed to a stereo lithography firm. They make the model right off your file. If you can model it they can make it. No concerns over draft angles or any of the considerations usualy needed for castings. The process builds the part by continuous layers of an epoxy material. Each layer is a few thousandths thick. Dimensional control is excellent. It is relatively inexpensive. I've used it for a few non-RC jobs. All I had to do was create the model in PRO/E and send it off.

If you have some good photographs and access to solid modeling software, it is relatively easily done. Even if you don't have the ability to model it, there are thousands of college kids trained in PRO/E, Catia, or other software that could knock it out in no time. If anybody needs the source I used for the stereo lithography supplier, I can look it up. But there are plenty of them out there.

If anyone wants to make these parts in quantity, the stereo lithography firms have low cost methods for making patterns from the origional part and could put out several hundred or more at relatively low cost.

SDCrashmaster

Interesting information.
Thanks,
Paul

U-Bird

U-Bird wrote:
Did you ever finish your scale A-65 flat four and did you do both scales, one quarter and one third?? What became of all your aquired information,drawings and castings if any were ever made?
Thank You
U-Bird

tony-howard

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I didn't go any further with commerical development. There wasn't enough of a market to continue by making instructions, brochures and the like. One reason is that the mounting method - which is crucial to getting the cylinders aligned properly - won't work with a Cub with an IC engine. There just isn't enough room left inside the cowl for the engine.

Another drawback was the estimated cost of a kit with the jugs jig assembled (they need to be done on a jig or they get quickly out of line as they're stacked and glued together). In view of the limited market, fabricating the cylinders from molds or by stereo lithography is moot too.

I still may revisit selling kits at sometime in the future, but at the moment I'm very busy making accessories for Mustangs.

I appreciate your interest.

GregG7

Another option is to order the optional scale engine kit from Balsa USA for their 1/3 scale cub. It is ABS plastic and included the cylinders, valve covers, and eyebrow cooling shroud.