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How to Make a Fiberglass Cowling - If You Have To - Part 1

By Tony Bingelis (originally published in EAA Experimenter, March 1986)

The title reflects a personal attitude. After having completed six projects and six cowlings it is hard not to conclude that I can forego any more of the creative opportunities that the design and the making of a cowling offers. It is sort of like raking leaves or shoveling snow. You know you can do it if you have to, so why not get someone else to do it... if you can.

I'm sure many builders share my sentiments. For example, why build a cowling when a purchased stock cowling will look good, fit good and will probably be lighter than one you could make. Most stock cowlings are lighter because they are vacuum formed in a female mold. Furthermore, a stock cowling most likely will have already proven itself to be properly designed and effective for the aircraft intended. And of no small value is the time that you can save by purchasing a cowling . . . time that you could better use for completing your project. Then, too, unless you have already tried your hand at making a cowling from scratch, you may not be fully aware that the process is a very messy, time consuming undertaking at best.

If this is true, why would anyone even consider developing and fabricating his own cowling?

Why? Well, for one thing, as much as you might like to buy a ready-made cowling, maybe you can't. It's just that nobody may have a suitable cowling for sale when you need it.

This non-availability situation is one you face when you choose to build a new design from plans that have just hit the market, or a one-of-a-kind design or even one of your own creation. In each of these instances you have no choice in the matter and must develop and build your own cowling.

Of course, the non-availability of a suitable cowling is not the only good reason for making your own. Perhaps you are among the good many types who would rather "do it all myself" and who believe that almost anything can be improved. Or maybe from your vantage point you simply do not like the looks of the cowling being marketed for your aircraft and you feel you can improve the appearance of the aircraft with a cowling of your own design. It could be, too, that you are using a different type engine that will not fit inside the stock cowling without drastic modifications. Whatever your reason for wanting to build your own cowling, I hope you don't make the mistake of building one primarily to save money. I doubt very much if you will. If you do save, it will probably be an insignificant pittance at best. There is more involved in making a cowling than buying a few yards of fiberglass cloth, a jug of resin and getting after it. Oh yes, much more.

Before you can make a fiberglass cowling you must make a male mold. The development of a good accurate mold over which you can form your fiberglass shell is the most difficult part of making a cowling. Of course, laying up a fiberglass shell over a male mold and finishing its external surfaces entails a lot of hours spent sanding, filling and finishing. Still, that is the most economical way to do it if you only need one cowling. The alternative is to make a second mold, a female mold, from your male mold.

A female mold can be built up over the male mold with layers of glass cloth and mat until you have a thick rigid shell. After removing it from the male mold, its interior surfaces must be carefully finished. Use a flashlight to help find imperfections that must be corrected. All irregularities and imperfections remaining will be cast in the finish of all the cowlings that may be made from that female mold.

The amount of work is, in my estimation, the same for either process. The female mold method, however, is far more expensive a process. There is little merit to making a female mold unless you intend to sell it or make extra cowlings from it. Anyhow, no matter which method you favor, you will still have to make that basic male mold first.

Before you can get started on making the mold you must obtain the spinner you intend to use or at least know its outside diameter measurement. A large spinner is quite effective, especially when used with Lycoming installations as its large diameter will help nullify the blunt front end with its bulky starter ring. A large spinner also minimizes the need for a propeller extension and permits developing a more effective aerodynamic shape up front.

You will want your cowling lines to flow smoothly from the spinner back to the firewall and there blend smoothly into the fuselage. To insure this, attach a plywood disc (the same diameter as the spinner's rear bulkhead) to the engine crankshaft flange. This will provide the necessary base and support for the front end of the mold and establish the correct spacing between the spinner and the cowling when completed. Use wood or metal spacers to establish about 1/2" clearance between the spinner and the mold if a constant speed prop is to be installed. Minimum spacing is very important, otherwise the completed cowling will rub.

Next turn your attention to the engine. An aircraft engine is a rather lumpy object having things sticking out here and there and not necessarily where you would prefer to see them. I have in mind items like the spark plug elbows, the starter ring (Lycomings), exhaust pipes, the carburetor and especially its heat box. These protrusions represent potential problem areas which can influence the shape and design of your cowling.

To be assured of adequate clearance under your cowling in these critical areas, tape 1/2" wood spacers to each spark plug elbow and to any other protrusion that might touch the cowling. Do the same thing to the exhaust pipes where they pass close to the cowling. Only here use 1" spacers as you will need a minimum of 1" clearance between the stacks and the cowling to prevent its burning or blistering the paint.

After you have all the clearance blocks taped in place wherever you think you might need them, you are ready to cocoon the engine. Wrap the engine in a plastic sheet to protect it from all the dust and debris that will be generated during construction and later during the destruction of the mold. Seal all openings with masking tape or duct tape.

At this point it would be very helpful to install what I call contour formers. Install one on the top centerline between the firewall and the plywood disc bolted to the engine flange. Shape this wood batten to establish the exact slope and curve you want to impart to the top line of the cowling. A similar former is shaped and installed on each side of the fuselage along the line of thrust. You can separate your cowling along this reference later. Fasten one end of these side strips at the firewall with blocks, clamps or other means, and the other end to wood braces tied into the plywood disc. These side reference strips must be curved to continue fuselage contours up to the front of the cowling where the air inlets have to be accommodated.

Establishing similar wood reference formers for the bottom contour of the cowling is more difficult because of the need to clear the alternator, starter, landing light or whatever it is you have there.

If you intend to have the air outlet on the bottom, the cowling profile will have to project below the firewall sufficiently to accommodate an air outlet opening. On the other hand, if you choose to exit your cooling air along each side of the fuselage through gills, the appropriate shape change will have to be made on the sides. With side gills, the bottom of the cowling can be made to fair smoothly with the bottom of the fuselage.

If your exhaust stacks exit on either bottom side, separate tunnels can be provided to serve as outlets for the exhaust pipes and the exiting air as well.

Lets face it... the foregoing discussion represents very important design determinations. The success of your cowling and engine cooling depend on properly providing for these essentials.

Examine other cowlings or at least some magazine photos to help you in developing your own cowling design.

After you have resolved your questions and are satisfied with the basic skeletal shape established by the wood formers, you are finally ready to build up the mold. Well, almost ready. First, another decision.


  1. The Old Way. Make the mold build-up of plaster of parts.
    If you use this method you should install a heavy screen wire base (over the plastic wrapped engine) between the wood formers to limit the thickness of the plaster build-up. Plaster is heavy and the front end of the airplane can easily pick up two or three hundred pounds. For this reason you must support the propeller end to take some of the load. Plaster of paris sets fast and the build-up can proceed almost as fast as you can mix the plaster. This is a messy process and the final sanding and shaping can be rather difficult because the plaster often develops hard spots. Sometimes plaster added to fill minor depressions or low areas will not stick too well or will dry extra hard, making the final finish results unpredictable. It is a lousy archaic process in my estimation. I did it that way three times because I knew of no other method at the time.
  2. A Better Way. Make the mold build-up with expandable foam.
    Some builders have used expandable foam with reasonable success. I am not one of them. Perhaps it is because I tried to save time by using the stuff that comes in spray cans. Anyway, the brands I got were totally ineffective, messy and a waste of time and money. I'm sure the type of expandable foam you have to mix and pour is better, but it too may be harder to control than you would expect, especially when you attempt to do the bottom surfaces of the mold.
  3. The Best Way. Use expanded polystyrene foam, Clark foam or most any kind of solid foam to build up the mold.
    My last cowling mold was built up with donated scrap hunks of light blue polystyrene foam left over from a local Vari-Eze project. I ripped the foam into 2" slabs on a bench saw and simply fitted and glued them edge to edge, beginning at each of the installed wood contour formers.

Any kind of glue can be used but I'm sure you would be the happiest using 5 minute epoxy because it dries so quickly. Other types of glue may have to be allowed to harden overnight. This would slow your progress considerably. Use 1/16" welding rods cut into 6" skewers to pierce and hold the fitted pieces of foam until the glue cures.

As each foam filler block or strip is fitted and glued in, roughly shape it with a surform plane, a wood rasp, or a coarse sanding block. This will help keep the mold contours under control and will make it easier to determine how to fit the next piece. The fit of the foam pieces need not be too precise as that will be taken care of later.

There is one useful precaution to observe. Try not to get any of the glue dabs close to the surface that has to be further trimmed and sanded. Cured adhesive lumps are hard and cannot be filed or sanded smoothly without causing pockets of foam to tear out. Actually, all you need are 3 or 4 small dabs of glue spotted away from the surface edges of each piece of foam. Applying a lot of glue is not wise. The less applied the easier your finishing job will be and the fewer repairs you will have to make.

After the entire engine area has been solidly overlaid with the foam layer, you can try your hand at sculpting. A good eye and an artistic bent can be helpful at this stage.

The location of the air inlet openings should be established before too much final shaping takes place. The faster the aircraft the smaller these inlets can be . . . providing that the baffling is well designed and properly installed. The air inlet openings should be just about on the thrust line of the aircraft, perhaps a bit more below than above. For my Falco, which is a fairly fast aircraft, I opted to make each opening approximately 3" high and 7" wide. This yielded a total air inlet area of approximately 39 square inches total after the corners were rounded. A lucky guess on my part as the cylinder head temperatures thus far range between 325 degrees F and 350 degrees F (actually a bit on the cool side). I think I could have made the inlets a bit shallower but with the width unchanged. However, I plan no such change unless I get bored for the lack of something to do.

Shape the air inlets to a nicely rounded airfoil cross section. Also try to form a ramp-like entry along the bottom lip to improve airflow entry when the aircraft is in a climb or nose high attitude.

A simple cardboard template can be made and used to duplicate the shape of each half of the mold. Shape the mold to fair in smoothly with the firewall area of the fuselage. Don't be concerned that the fiberglass shell, when laid on the mold, will be slightly bigger than the firewall dimension. When you cut the cowling to split it, this extra dimension will disappear and you can obtain your flush fit installation if that is what you want.

Step back and examine what you have done. View the molded nose from all angles - top, sides and bottom. Try to maintain symmetry as you continue with the final sanding. Don't worry if you should sand completely through the foam in one or two places. Simply take a knife and cut out a large wedge in that area and fit a fresh piece of foam into the hole. Glue it in and reshape the area as necessary. Do the same thing any place you have a soft weak spot as that indicates the foam is very thin there and might break through later.

Perfect the surface contours of the mold by checking the surface with a flexible chalk-coated slat (1/8" x 2" x 24"). Coat the wood slat with colored chalk and then rub it lightly all over the cowl mold surface to locate the high spots. The high spots will show up as starkly smudged areas. Sand these high spots lightly and continue the chalk and rub process until the entire surface smudges uniformly.

The final step is to develop a perfectly smooth surface over the entire mold. Do this by coating the foam mold with a thin layer of ready mix all purpose Joint Compound. This plaster dries quickly but keep each layer thin, less than 1/8", to insure that it does. Apply the plaster with a paddle (flat stick) and spread it smoothly with your hand. When the initial coating has filled the foam pores and dried, sand the mold surface lightly and repeat the treatment until you have the build-up you want and the mold surface is perfect. Remember to check here and there with the chalk stick to find imperfections you can't see. At this point the mold should look exactly like a perfect cowling. Make any last minute changes and allow the plaster to cure overnight or preferably for 24 hours.

The final treatment includes several coats of paint or varnish. Almost any kind will do. I took this opportunity to use up all the many left over cans of spray paint that accumulated over the years. After the paint dries sand the entire mold with 320 wet/dry paper to remove all the tiny bumps and dust particles. Blow the mold clean and it is ready to receive the fiberglass lay-up.

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