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Fitting and Installing the Cowling
By Tony Bingelis (originally published in EAA Sport Aviation, May 1986)
Fitting a brand new cowling single handedly is a bit difficult but it can be done. Naturally, it is better sometimes (not always) to have someone help hold the thing in place while you mark it. Working alone you will have to rely on inanimate help from rope, some sort of table or stand with suitable blocking, and certainly the generous use of grey duct tape. It goes without saying that a large cowling for a side-by-side two seater will be more awkward to handle than one to be installed on a single seater. The fitting procedures, however, are identical.
It is easy to see that a one-piece cowling would be a snap to fit and install, but it would become a hated thing any time you had to drain your oil or perform engine maintenance. The reason, of course, is that you would have to first remove the spinner and propeller to get the cowling off . . . and that takes a lot of time and a lot of work.
What Kind of Engine Access?
How much of your engine will you need exposed without resorting to the complete removal of the cowling? How often would you need this kind of access for:
- Oil inspection and servicing only?
- Checking the battery? Recharging it? Removing and replacing it?
- Changing the oil? Changing the filter? Cleaning the oil screen?
- Minor inspection of the engine for oil leaks?
- Firewall access to electrical accessories?
Most builders will settle for a simple daily access to the oil filler cap and little else, provided that they can do this quickly without having to locate and use some kind of tool to open the inspection door.
Other builders like to have a couple of sections of the cowling open up without the use of tools . . . sort of like the hood on an old Model A Ford. So you can see more of the engine, but can you drain the oil?
The Up Front Support Jig
If you had to make your own cowling you will probably still have the large plywood disc you will now need to support the front end of the cowling during its fitting. Those of you who bought a cowling from a commercial source will have to make an alignment jig. It will be secured to the crankshaft flange as a positioning aid to help you align the front end of the cowling with the spinner. This plywood disc's diameter should match that of the propeller spinner. Actually, it may be better to match the diameter to the inside recess of your cowling as a similar disc was undoubtedly used for fabricating the original mold and cowling. At any rate, since you have to have the front of the cowling blend into the propeller spinner and be assured that you will have sufficient spacing between the cowling and the spinner, this support disc size and exact positioning (fore and aft) will have to be determined before you cut off or trim any of the cowling.
Reassure yourself frequently that the front end of the cowling is snugly clamped against your plywood spacer as your trimming or cutting continues.
This up-front preoccupation is necessary because the cowling must be centered around the crankshaft without being fastened to the engine in any way. In other words, the cowling must be cantilevered from the firewall where its fit must be accurate and secure. As you know, the engine moves around quite a bit in its rubber shock mounts and that precludes any thought of fastening the cowling to the engine. If you should succumb to such an impulse, you will have a cowling that will not make it through its first engine start-up and shut-down without cracking in a dozen places.
Firewall Preparation For Cowling Installation
Since the cowling must be cantilevered from the firewall, its means of attachment to the firewall is important. The method most used relies on the integrity of a flange (or brackets) secured to the firewall. The cowling, in turn, is fastened directly to this firewall flange with appropriate fasteners.
If a flange is not an integral part of your firewall metal overlay, It will be necessary for you to fabricate and install a flange, or separate brackets, before you can fit and attach your cowling.
Make the firewall flange fairly wide so that you will not have to drill the cowl fastener holes too close to the edge of the cowling. A continuous flange around the perimeter of the firewall will provide the best support. This is not to say that individual brackets uniformly spaced around the firewall cannot do a good job. They can.
The firewall flange may be made of aluminum, or of fiberglass if composite builders are so inclined. However, metal flanges are much easier to make and are, in the long run, more durable and can better withstand the vibration and localized stresses imposed by metal fasteners . . . besides, aluminum is more fire resistant.
In order to obtain the continuous support you want, your flange may have to be formed from several pieces butted together. That poses no problem and actually it is easier to make the flange of several shorter lengths because of the changing angles that are encountered around the firewall perimeter. These short sections of angles are easier to adjust to a better fitting surface for the cowling.
A length of 6061-T6 aluminum angle (3/4" x 1 " x .040") should be suitable for the job. You could, if you prefer, form your own angle sections in a metal bending brake and over-bend them slightly as needed in the area to be fitted.
In order to curve that straight piece of aluminum angle so that it assumes the shape of the firewall, it will be necessary to:
- Form the bend or curvature in one leg of the flange with the help of a Metal Shrinker. A simple bench model costs about $90 . . . rather expensive for making only one curved flange, or
- Notch one leg of the flange, spacing the notches closer together where the curvature is severe, or
- Shrink one leg of the flange at intervals with a pair of fluting pliers. The spacing and depth of the flutes likewise depend on the severity of the curve. Fluting pliers cost approximately $18.
These options are illustrated in Figure 1.
How you secure the flange to the firewall Will be influenced by the type of structure behind the firewall metal. Ordinarily, you can rivet it or attach it with self-tapping sheet metal screws.
When determining how close to the outer edge of the firewall the flange should be installed, don't forget to allow for the thickness of the cowling if you want it to fit flush with the fuselage. It is better to install the flanges, or brackets, too low than it is to get them too high. A flange or bracket that is too low can always be shimmed up but one that is too close to the edge will cause the cowling to protrude in that area. The only fix, then, is to remove and relocate that bracket lower. Unfortunately, in the case of a continuous flange, that cure may not work because attempting to lower a point on the flange will be resisted by the adjacent fastened sections.
In addition to the firewall flange or brackets, you will need to make some sort of metal attachment plates for the front end of the cowling where the top and bottom halves join. A simple pre-bent metal strip riveted to the bottom half of the cowling is all that is needed here. Before installing these metal plates it may be better to attach the Anchor Nuts or the Camloc receptacles as the case may be. If there is severe compound curvature in this area it may be better to form the metal strips of soft aluminum so that they can be hammered into the proper contour.
How about the fasteners? What do you think your preference is in this regard?
- Anchor Nuts/Machine screws?
- Piano Hinges?
Dzus Fasteners - These are the old time standard quick release fasteners long used in cowlings. Sometimes they can be difficult to engage, but, as a rule, they are easily fastened and released with a coin or a Dzus Tool. Some Dzus fasteners have a wing type grip that you can twist and lock by hand (no special tool required).
Many builders do not use Dzus fasteners because they don't know how to select and order them and/or because their installation requires the use of special tools.
Note: One of our advertisers, Aircraft Spruce and Specialty, has an excellent illustrated treatise on the selection and use of Dzus and Camloc fasteners . . . and the tools required . . . in their famous "Educational" Catalog ($5).
The proper selection of the stud length is critical as the only spring action you have is a very stiff engagement piano wire that has very little give to it. As I recall, the old military cowling installations had some very obstinate cowling fasteners . . . don't know if the civilian Dzus are as ornery.
Maybe cost is a factor in the declining use of Dzus fasteners. A single assembly consisting of a Dzus stud fastener, a metal grommet and an engagement spring could cost you at least $2.00 per assembly. This means installing even a modest number of Dzus fasteners for your cowling could increase the cost of your project by about $30 . . . and this doesn't even include the cost of the special installation tools required.
Camloc Fasteners - Use Dzus fasteners if you prefer, however, most manufacturers and builders consider them to be inferior to the Camloc fasteners. Camloc fasteners are not much cheaper than the Dzus and will cost about $2.00 per unit installation, too. They are, however, easier to install as all you need is a special pair of pliers.
The stud spring of the Camloc fastener must be compressed during its insertion with 4P3 Pliers (about $19) so that the stud can slip into the drilled panel hole. Release the pliers and there you are. Well, if you don't want to shell out that much for a pair of pliers, you can make your own provided you have a pair of K & D (428) Hose Spring Clamp Pliers. Take a look at Figure 2 to see how easy they are to convert. An inspired builder could probably convert a cheapie pair of slip joint pliers in a similar fashion with a couple of pieces of steel and some brazing or welding.
Anchor Nuts/Machine Screws -Perhaps the most commonly used cowling fasteners among homebuilders are Machine Screws mated with almost any type of Anchor Nut. The screws used vary. Some install the Flat Head AN507 screws while others prefer the Washer Head AN525 or the Truss Head AN526 types. The flat head machine screws can be used with Tinnerman Countersunk Washers to provide a reinforced low-profile installation that also protects the cowling from the screw head. For that matter, most any other type of machine screw can be used. Try to obtain your preferred fasteners in stainless steel. The stainless steel screws cost more but they do not rust and will always look good. In this vein, I think it is a mistake to paint machine screw heads, or any other cowl fasteners, because they will look good only until the paint begins to chip. After that, you will have a battle scarred array of fasteners disgracing your pretty plane.
Piano Hinges - Piano hinges used as cowling fasteners have many applications, are effective and require no special tools for their installation. These fasteners may be installed so that their hinge loops are invisible. When installed in this manner you must provide access to one end of the hinge wire so that you can pull it out to quickly release and separate the cowling panels.
If your cowling split line is straight (not curved), you could use the piano hinge installation to make a hinged access door and there would be no need to remove the piano wire connector. Installed in that manner, the hinge loops are visible.
As with screw heads, I would not paint piano hinges if they show. They are especially prone to chip and peel after a short period of use.
Trimming The Cowling To Fit
Minor trimming of the cowl edges can be done with a coarse sanding block or a file . . . a bastard cut file at least 10 inches long. If more than 1/8" along the edges must be removed, you should probably cut off the excess with a hacksaw, or maybe with a fine tooth saber saw. Lay down a strip of masking tape along the line of cut to minimize chipping. Don't get so skillful that you are tempted to cut on the line . . . leave a smidgeon for final trimming with a file and/or sanding block. A little bit at a time... don't rush now.
Another important point. Do not strive for fits so close that you cannot even insert a knife blade in the joint. You must at least allow space for a coat of paint. Work to obtain a uniform 1/16" gap.
Don't forget that your cowl fitting work should be done with the engine and the exhaust stacks already installed - logical? If your installation is such that the exhaust pipes must protrude through the cowling, that could create another bit of a problem. Just where do you cut the holes and how big should they be? One helpful observation. Do the best you can to determine where to make the holes in the cowling with the assurance that if you miss the exact location, you can always make the hole a little bigger. Actually, you should provide at least 1/2" space around each exhaust pipe to prevent the pipes from abrading on the cowling.
Temporarily assemble your cowling by taping it together with duct tape. Check to see how it fits at the propeller end and how much you will have to cut off initially, and where. In particular, note how much has to be cut off at the firewall. You may have to remove and reinstall that cowling a half dozen times, trimming and checking, trimming and checking, until you get it to fit perfectly. The process can't be rushed if you want an excellent fit.
Put the cowling in place and insert all the fasteners. Work from the top of the cowling and down both sides along the firewall. Install the fasteners up front. If you are using machine screws do not torque them tight until after all of them are in place. Tighten all fasteners and check for uniformity in all the joints. Work a hand-held hacksaw blade all along each joint to even the gap and to obtain a uniform clearance. Finally, remove the cowling one more time and smooth and round all the edges. Yes, deliberately round them with a sanding block. This will provide space for the paint and also give it more area to cling to along the edges and will minimize the paint chipping problem so common on other cowlings. The finished results will be quite pleasing to the eye.
Do not locate fasteners too close to the cowling edges as they may tear out in time.
Do not use too many fasteners. After you decide where to locate yours, check again. Can you reasonably forego the installation of every other one in some areas?
Do not paint over the screw heads, washers or piano hinges. Your cowling installation will look much nicer without paint chipped screw heads, amigo