Custom Cowling

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This page is for builder's tips on making custom cowlings.

Al Gietzen

Here is some information from Al's website on building his awesome custom cowling for his Mazda motor.


The stock Velocity cowling is designed to accommodate a Lycoming or Franklin aircraft engine. It's appearance is also a bit lacking with just an unfinished open back end. The use of the rotary engine allows for a narrowing of the cowling and a bit more streamlining for better airflow into the prop. In my installation the Propeller hub is about 2" higher than stock, requiring the cowl to be changed, and I wanted to finish off the aft end to provide for controlled airflow for reduced drag.

Rather than start completely from scratch, I decided to retain the cowl-to-fuselage flange and a portion of both the upper and lower cowl going nearly all the way to the back. So it was basically a matter of cutting out both sides of the existing cowlings, and making a mold which would form the new portion while allowing incorporation of the remaining part of the original.

I had previously made molds for scoops and small parts using urethane foam from 2” thick construction “sheathing” that you can buy at Home Depot (or other home improvement supply). It is inexpensive (about $14 for a 4’ x 8’ sheet) and is very light weight (about 2.5 lbs per cu. ft.) and easy to form. After considering other options, I decided to use the same material for this larger job.

The first step was to laminate together a few thicknesses of the urethane and start sculpting the aft end of the cowling. Before gluing the layers together with a glue gun, the foil paper covering needs to be removed so it doesn’t interfere with the shaping. The urethane can be easily shaped with a sharp knife or hacksaw blade, and sandpaper.

The exits at the back of the cowling are a 10” diameter center hole for the prop shaft with two 5” diameter ports on either side that accommodate airflow and exhaust exit. The prop shaft hole accommodates a 6” diameter prop hub which transitions to a 10” diameter prop spinner with a fairing that covers a 6” prop hub extension. The spinner fairing (mold seen at the left of the photo) gives a 2” wide annular space for cowl air exit around the prop hub.

Once this aft portion was nicely shaped, it became a matter of adding more layers and continuing the shape. This first portion had to be done with the engine on the airplane, and hollowed to fit over the engine redrive. That way it could be formed and hard-coated, and attached to the portion of the original cowl be used to assure accurate alignment of the cowling to the prop hub. I first laid up the fiberglass over the back end of the cowling mold, and then aligned it on the airplane, and glassed it to the existing portion of the cowl.


For hard-coating I use about 50/50, by volume, alphapoxy and microballoons, and apply with paint brush. Mold release is also needed before laying on the fiberglass.


Then with the cowl/mold combination off the airplane and on a firewall mockup to maintain the shape; additional layers were added (glued together) and shaped as well as possible until the entire open area was closed in with the mold. To conserve the use of urethane, the mold is actually hollow - made up of annular pieces that were cut at the top and bottom into left and right hand pieces. This allowed getting the entire mold made out of three 4 x 8 sheets of 2” urethane. Total cost - under $45. Much less than buying bigger blocks from Aircraft spruce, and probably a lot easier to deal with than trying to make it up with expandable pour foam.


Rather than use the stock approach of making the cowling, and then fitting transition pieces to fit the wing flange, I decided to mold the transitions right along with the cowling. This, of course required removing the engine and putting on the wings in order to get the proper alignment. I had peviously extended the wing roots inboard to accomodate the narrower cowling. This was necessary in order to bring the aileron torque tube bearing bracket further inboard to keep the aileron bellcrank inside the cowling. Then it was a matter of finalizing the shape, getting the whole thing hard-coated, applying mold release, and doing the necessary lay-ups.

The main body of the cowling is made from one ply of triax with one ply of fine BID over the outer surface. This was then transitioned to the portions of the existing cowling and to the aft end of the new cowling with BID tapes. Main reinforcing ribs of 2-plys of BID over microballoons were formed along the transition joint to the original cowling, and additional stiffening ribs of 2 plys of triax under BID were added at a couple of intermediate points to give it a bit more rigidity. You can see these ribs in the photo taken with a light inside the cowling.

After cutting the cowl into top and bottom halfs, connecting flanges were added. The resulting structure is quite strong and rigid, and the total weight of the entire cowling is only 16 lbs.

Making the mold and cowl was probably less than half the job. Filling and sanding to get nice smooth consistent surface, and forming and fitting the flanges of top and bottom halves, to the wings, and around the induction and raditor intake scoops seemed to take again as long, maybe just because it was such tedious work.