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Working With Ready Made Fiberglass Parts, Part 2

By Tony Bingelis (originally published in EAA Sport Aviation, December 1991)

Of the several ready-made fiberglass components furnished with the more complete aircraft kits, the cowling is by far the most visible and eye catching of the lot.

A well fitted cowling will enhance the appearance of any airplane while a poorly fitted and finished one will detract from the appearance of an otherwise beautifully completed aircraft.

Only too evident are irregular joints and seams, large gaps, and mismatched upper and lower portions of the cowl.

Another attention grabber, even to the most casual of observers, is the space between the spinner and the cowling. If it is excessive, the installation simply will not look good.

Furthermore, should there be a very obvious mismatch vertically between the spinner and the cowling, an increased drag penalty can be anticipated in addition to the degraded appearance. So, what can you do to avoid problems like that?

When In Doubt . . .

Read the instructions. What instructions? I'm sure all kit aircraft have a construction manual of sorts. These manuals are useful, especially to first-time builders, even though some manuals may not provide much guidance for a non-structural subject like installing the cowling.

With this in mind, I offer a few seldom chronicled procedures for fitting most any ready-made fiberglass cowling. Included are certain important sequential preparations that need attention and perhaps completion before you can happily start cutting here and there.

For example, if you intend to install a propeller extension, do so before you start on that cowling.

After you bolt the prop extension in place, install the spinner backplate so it can serve as a ready reference for obtaining that important symmetrical alignment of the cowling and the spinner.

In addition, you may want to make a temporary plywood cowl centering jig (disc) to support the front end of the cowling while you are trying to position it . . . many builders do.

The diameter of that plywood disc must be the same as the inside nose opening in the cowling . . . which will probably be about 1-1/2" to 2" less than the diameter of the propeller spinner (one of the photos shows it in use).

Cut the plywood disc in half or slot it at the bottom so you can slip it on and attach it to the prop extension face (or to the crankshaft hub if no extension is installed).

With the plywood disc in place, the symmetrical positioning of the front end of the top cowling around the prop hub and spinner is automatically assured. In addition, the plywood support will permit you to accurately set the clearance you want between the front end of the cowling and the aft edge of the propeller spinner.

Make whatever minor adjustments you need in the plywood jig's diameter to achieve a perfect streamlined match between the nose of the cowling and the propeller spinner.

A ¼" gap between the two is considered to be a good average clearance. However, I would consider anything less than 1/8" gap to be marginal.

With a dynafocal engine installation, a gap somewhat greater than ¼" could be provided if the airplane is to be used more for aerobatics than for fuddy duddy flying.

Bracket Cowl Supports vs. Piano Hinges

Since your cowling must enclose the entire engine installation, including the baffling, carburetor, and the exhaust system, it is advisable that the exhaust system and baffling be in place before you begin to install the cowling . . . it is simpler and easier to do it that way.

One of the most important things you can do – for appearance’s sake – is to trim and true the firewall edges of the fuselage to make it easier to match fit the cowling to it. Do this before making any cowl trim cuts.

After you are satisfied with the trimmed edges, you must decide how the cowling is to be attached to the firewall and/or the fuselage.

Incidentally, cowlings are not directly attached to the engine in any manner. They are, instead, cantilevered from the fuselage structure.

Many older aircraft designs have cowlings that lap over the front end of the fuselage about an inch or so. Fitting this type of cowling is easy as you don't have to match its aft end to anything.

However, most builders now prefer to install the cowling surface so that it is flush with the fuselage surface. You can see that such a butt joint requires a good match fit or it will look lousy.

The next decision is yours to make. That is, you must decide whether you want to fasten the cowling to the fuselage with piano hinges or with brackets and anchor nuts or camlocs . . . or both.

Piano Hinges vs. Cowl Support Brackets

Using piano hinges to support the cowling completely around the firewall edges usually results in a very clean installation because no fasteners will be visible. However, it won't be easy because it is impossible to run a single continuous piano hinge all the way across the top of the cowling and still be able to insert the hinge wire. Instead, you will have to install the piano hinges in two or three segments.

Even then, you will find it a bit difficult to match fit and install the piano hinges around the top curvature of the fuselage primarily because the connecting, pre-bent, piano hinge wires must be inserted from inside the cowling . . . but you can do it.

This will require the construction of a rather generous access door for your oil inspection routine. One that is large enough for you to get your hand in easily . . . and still leave room for a peek inside to see what your hand is doing:

Avoid locating accessories like the voltage regulator, over-voltage regulator, fuel and oil senders and, perhaps, an air/oil separator (for the engine breather) on the firewall close to the cowling where any of them can interfere with the insertion of the piano wire connectors.

If on the other hand you were to install brackets around the upper curvature to support the cowling, they would allow you more flexibility in locating the accessories where you want them.

Thanks to cordless electric screwdrivers, removing a few cowl attaching machine screws is no greater chore than blindly trying to insert piano hinge wires inside the cowling.

My current cowl installation preference is for piano hinge connectors along the sides and bottom of the cowling, and screw fasteners or camlocs along the top firewall curvature.

I also prefer using sturdy brackets with nutplate fasteners, in lieu of piano hinge segments, to connect the top and bottom halves of the front center air inlet areas of the cowling.

Whether you install piano hinge connectors or brackets, don't forget to install them recessed below the skin line, where necessary, to allow for the thickness of the fiberglass cowling . . . if your heart is set on a nice flush outer surface.

Install the anchor nut brackets on the firewall with solid or pop rivets. In wood aircraft you may have to use screws. However, do not rivet anchor nuts to the brackets until after the cowling has been drilled with the pilot holes for them. Otherwise you might have a bit of a problem trying to find where to drill holes through the cowl and into already installed anchor nuts underneath.

Now, About Fitting That Top Cowl . . .

Fortunately, the cowling, as received, will undoubtedly be slightly longer than the installed length. This will allow trimming for a good fit at the firewall.

Look closely and you may be able to detect an embossed trim line in the surface near the edge of the cowling.

However, do not depend on this. It may or may not be correct for your installation.

Instead, mark the top cowling yourself using a reference line you can draw on the fuselage from which to measure the location of your trim line. Getting an accurate flush fit at the firewall is rather simple when you work from this line (see Figure 1 and Figure 2).



First, draw the reference line, with your trusty Sharpie pen, spacing it exactly 1" back from the edge of the firewall.

Next, lay the top cowl on the fuselage for a preliminary fitting. For the moment, the aft end of the cowling will rest on the fuselage and the front end will rest on the plywood alignment disc. If the cowling is much oversize and covers the 1" reference line, use a 2" reference line instead.

Shift the cowling around until you have a 1/4" clearance between its nose and the spinner/spinner backplate. The plywood disc will keep the front end perfectly centered around the spinner.

Carefully measure onto the cowling exactly 1" from the drawn reference line. This will accurately establish where the edge of the firewall is located under the cowl. Mark this distance at several points. Connect your marks with a flexible straight edge and you will know exactly where to cut for a perfect fit with the firewall.

Finally, remove the top cowl and trim its aft end along this line. But, don't cut on the line, leave a little margin for fine trimming the fit.

When you replace the trimmed top cowl, its aft edge should butt perfectly flush against the fuselage resting on the installed piano hinge or bracket supports . . . if not, do a little more filing wherever needed to true up the matching edges.

Next, trim and true the horizontal side edges of the top cowling and install the upper piano hinge half on each side of the cowling. Bond and rivet them in place.

Before applying the adhesive, scuff both the cowl surface (in the area of contact) and the piano hinge with coarse sandpaper. For a stronger bonded joint, drill 1 /4" holes in the hinge spaced between the rivet holes to allow some of the epoxy adhesive (or polyester resin) to ooze through the holes.


Incidentally, it is easier to keep piano hinges straight while drilling the rivet holes when the hinge wire is inserted to connect both halves of the hinge assembly.

The Oil Inspection Access Door

Before proceeding with the fitting of the bottom half of the cowling, determine the location for the oil filler door. If the location for this door is not embossed in the fiberglass, establish its best location by using the oil dipstick housing as a guide.

Do this by holding a felt marker pen parallel against the oil filler tube and projecting that angle up to the cowling surface.

This will locate the oil filler door in the proper position for easy access to the dipstick and will simplify checking the oil. If you install piano hinges to secure the upper cowling, you will need a slightly larger oil inspection door so that you can reach in and insert the connecting piano hinge wires (mine is 5" x 6" but 6" x 6" would be better, I think).

Note: A good way to cut out the inspection door opening is with a hand-held hacksaw blade. Score along the cutting line with the tip of the blade using short back and forth strokes until the blade slices through the fiberglass. With this starting slot you can proceed to saw along the line. You will have to repeat the starting slot technique each time you have to change the direction of the cut. An alternate method is to use a 3" abrasive disc chucked in your electric/air drill to start the slot.

You will be more pleased with an aluminum inspection door than you would be with one made from the cut-out fiberglass piece . . . provided there is no compound curvature present.

An aluminum door will not warp or distort with the passage of time. Furthermore, it is more durable and is easy to make.

Fitting the Bottom Cowl Section

The fitting of the bottom cowling section is handled in much the same way as the top half. Only now you must trim the bottom cowl at the firewall only as much as is necessary to get a match fit in the front inlet area with the top cowling. Trim the sides of the bottom cowl as much as is necessary to match the top cowl half and have it align with the bottom of the fuselage.

You may need someone to help hold the bottom half of the cowling up in position while you adjust a cinch strap around the cowling to hold it in place. This will give you more time to check and adjust the accuracy of the fit.

Remember, when you rivet the second half of the piano hinges to the bottom cowling, the hinge loops will have to mesh and match the opposite (upper cowl) hinge segment already installed.

I find the easiest way to do this is to remove both cowling sections and stand them on their aft ends on a level surface.

Then, assure yourself that the piano hinge wires are in place connecting the as yet unattached hinge half for the bottom cowling.

Keep the cowling ends and edges aligned while you carefully assemble cowl halves with duct tape.

Now, the trick is to drill a couple of rivet holes along your marked rivet line - through the bottom cowl surface and into the as yet unsecured hinge.

To do this, raise the duct taped cowling assembly up on blocks high enough for you to get your hand under it. Then you should be able to reach in and press the loose hinge half against the cowling while you drill the rivet holes . . . use a block of wood (not your finger, Wilbur). Insert a cleco in each drilled hole.

You can also do the same thing by reaching inside the cowling from the top. Drill a couple of rivet holes in the hinge at this end, too, inserting a cleco in each hole drilled.

With the hinges locked in place, you can now drill additional rivet holes as far as you can easily reach from either end.

After you have drilled and clecoed several rivet holes, remove the duct tape, pull the hinge wires, and separate the cowl. You can now finish drilling the bottom hinges on both sides with assurance that the cowling will fit well.

What's Up Front Does Count . . .

The front end of the cowling is subject to high air inlet pressures that try to force it apart.

I have experienced piano hinge failure in this area even though my rugged extruded hinge segments seemed to be

securely bonded and riveted. Henceforth, I believe, I will install at least two anchor nutplates on each side, or extruded piano hinge, well bonded and riveted with 1/8° rivets spaced no further than 3/4" apart, to take the loads.

In this regard, some high performance aircraft suffer from cowling bulge, especially at higher speeds. Often a lightly constructed cowling will lack the rigidity needed.

As the inlet air pressures build, the cowling bulges and may even pull away from the engine baffle seals, allowing the cooling air to leak past the open gap. Consequently, engine cooling often suffers.

The cure is to bond in reinforcing triangular wood strips with extra layers of fiberglass to stiffen the overly flexible areas of the cowling.

There is another problem common to fiberglass cowlings. Sometimes, the front inlet areas become warped and you are stuck with a bad fit.

You might try to straighten it by clamping a straight edge across the front and heating it with a heat lamp. But in time it will probably return to its original warped condition.

I, therefore, believe that the permanent cure is to rebuild the affected areas with more layers of glass to correct the distortion.

Do the corrective work with the cowl halves assembled. Cut them apart only after you have completed the job and the fiberglass has cured.

One final reminder. Retaining the long side piano hinge wires in the cowling during flight is essential. Should one of the side hinge wires work its way forward, it would ruin a wood propeller before you could land. For that matter, it could be just as detrimental to a metal prop, and the unidentified sound effects could be frightening.

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