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Cowling Installation Notes
By Tony Bingelis (originally published in EAA Sport Aviation, October 1996)
Just how important is a cowling to your airplane? Never thought about it that way, have you? I’ll bet you simply thought it was good looking, kind of streamlined, and maybe you even wondered how difficult it would be to install - and that’s just about it.
Well, I suppose you are partially right about the installation difficulty bit, but a cowling has more important attributes than just its good looks. For example, consider the following:
- It is true, of course, a well designed, nicely finished, cowling will really enhance the appearance of any airplane.
- Imagine how much drag an uncowled engine compartment would cause in flight. Actually, so much that some low-powered homebuilts might just barely be able to take off and fly without a cowling. There is no doubt about it, even a poorly fitted cowling will reduce drag.
- Just about any kind of cowling will improve engine cooling. How well it does this depends, naturally, on basic design and how well you install it.
- A cowling provides protection for vulnerable engine controls, wires, hoses, tubing and engine accessories, all of which are jammed in most any firewall forward engine compartment.
Cowlings in General
If you have to build your own cowling you will naturally dimension and shape it to fit your engine/propeller installation.
On the other hand, if you are a kit builder you have to be sure that the cowling provided will fit your particular installation. That is, if you plan to use a constant speed instead of a fixed pitch propeller, you might have to modify the cowling you received.
Of course, if the kit manufacturer has optional cowlings to choose from there should be no problem provided you order the correct one.
Never A One Piece Cowling
A one-piece cowling installation is a poor choice. True, the one-piece installation would be lighter and can be made using the minimum of hardware. However, nullifying those obvious attributes is the one terrible disadvantage of a one-piece cowling. It is difficult to remove because the propeller and spinner installation have to be removed first.
This means that, because access to the engine compartment is complicated and difficult, you might be tempted to slight some important engine compartment maintenance and inspections.
Cowlings Are Never Attached To The Engine
This is worthy of your attention. Cowlings are never supported by or connected directly to the engine. Rather, they are cantilevered from the firewall area of the fuselage.
The reason for this is understandable when you visualize what would happen when one end of the cowling is secured to a rigid unmoving fuselage while the other end is bolted to an engine that "floats" on rubber shock mounts. A cowling secured like that to both the engine and the fuselage would soon be damaged.
Cowl Flaps For Better Engine Cooling
Faster homebuilts will sometimes have cowl flaps incorporated as part of their cowling installation. These cowl flaps allow the pilot to control the flow of cooling air by varying the size of the cooling exits. This permits better control over engine temperatures, especially during climbs.
Sounds good but cowl flaps complicate the cowl installation. You must remember they will have to be disconnected before you can remove the cowling. On reinstallation, the cowl flap mechanism will again have to be reconnected and tested for operation.
A cowling with well proportioned air inlets and outlets may not need cowl flaps to maintain acceptable engine temperatures. I suggest you defer the installation of cowl flaps until after a number of test flights have been completed . . . you may not need them.
That Oil Inspection Door
Many builders make their oil access door much too small and often locate it poorly - directly over the oil dipstick. Since the dipstick housing is slanted rearward, it is better to locate the oil inspection door somewhat further aft to make the addition of oil less messy and easier.
Making your oil access door larger is especially important when your cowling is to be secured along the firewall by a piano wire hinge installation. A larger oil inspection door will increase your reach when trying to inset the curved hinge wires along the firewall . . . even so it can be a frustrating experience with a new installation. Substituting a slightly smaller diameter piano wire for the regular hinge wire helps.
Incidentally, most builders try to reuse the cut-out fiberglass portion of the cowling for the oil access door. That’s O.K but, even though that small fiberglass cutout may have the correct contour, it will in time surely warp and fit poorly.
Anyhow, it is more practical, by far, to make the oil access door from .030"-.040" aluminum. A metal door will retain its shape indefinitely.
Engine Cooling Problem?
Here’s one possibility often overlooked. The top portion of your cowling is rather thin and flexible and may need to be stiffened by riveting or epoxying in reinforcement strips. This is especially true for some of the faster homebuilts where higher inlet air pressures develop with higher airspeeds.
Air entering the cowling pressurizes the top side of the engine compartment creating a tendency for the cowling to bulge up - so much so that some of the pressurized cooling air leaks out between the baffles and the cowling. This can seriously affect the engine’s cooling because essential cooling air is escaping without being forced down through the engine’s cooling fins to help cool the engine.
Fitting The Cowling
You have two basic options:
- Butt the cowling against the firewall so that its outer surfaces are flush (streamlined) with the fuselage skin. Or . . .
- Lap the cowling over the fuselage skin. This simpler installation is more commonly seen on slower aircraft than it is on high performance homebuilts.
A cowling is best fitted with the engine installed. This will make it much easier for you to assure yourself that the front end of the cowling will be symmetrical with the crankshaft and will clear the propeller spinner.
Providing a support, or jig, for the front of the cowling while you are fitting it can be tricky. It is important to make and secure this jig so the cowling will be positioned correctly behind the spinner. A spinner-to-cowling clearance of 1/4" is about right for most homebuilts and "store boughts."
Make the alignment and support jig of 3/4" plywood or particle board and bolt it to the engine’s propeller flange.
If a propeller extension is to be used it should be installed before fitting the cowling.
With the plywood jig in place, it will support the front end of the cowling and automatically align it with the engine. One more thing must be done. Small wood spacer blocks may have to be taped to the forward face of the plywood jig to establish the proper clearance between the cowling and the propeller spinner. Remember the spinner bulkhead has a 1/2" or 5/8" flange that faces toward the engine. The blocks are to provide clearance for the spinner bulkhead flange plus 1/4" clearance.
Proceed with your jigging preparations by marking a short centerline on the top of the fuselage deck at the firewall. It will serve as a reference aligning the aft end of the cowling.
Most cowlings furnished with kits come in two pieces with the cowl being split on the sides. That is, you are furnished a top piece and a bottom piece.
Some builders try fitting and installing the bottom half of the cowling first. I prefer fitting the top half first because it is smaller and lighter than the bottom half. Besides, you will have gravity working for you because you can rest the top part on the fuselage and the plywood jig up front while you work.
The scariest part is determining exactly how much to cut off the aft end of the cowling so that it perfectly matches the firewall in a flush installation.
A commonly used trick is to measure back on the fuselage from the firewall, exactly 1" and draw a reference line all around the fuselage paralleling the firewall. This will give you an accurate reference from which you can measure your cowling trim line. Simply measure exactly 1" from your reference line onto the cowling. Because you know the firewall is exactly 1" away from that line, your trim line will automatically be established.
A small 3" abrasive disc does a good job of trimming away the excess. Do not cut exactly on the trim line. Allow yourself about 1/16" for final trimming with a sanding block fitted with #80 grit black floor sanding paper.
How well your cowling installation will look will depend greatly on achieving a close fit and an even joint. Don’t fit the cowling so tightly that there is no gap. Remember, you will have to allow for paint.
After I fit a new cowling and fasten it in place, I carefully work on a handheld hacksaw blade (a new 18 tooth blade works good) along the joint to precisely even the cowling’s edge with that of the firewall . . . be careful that you do not cut into the underlying cowling support flange.
The next time you attend a fly-in inspect a number of aircraft to see how well the spinners align with the cowlings. Here are four common and highly visible goofs:
- A spinner that is not centered with the nose of the cowling due usually to attempting to fit the cowling when the engine is not installed. (Some engines are offset to help compensate for torque.)
- The spinner is not streamlined with the top of the cowling. Quite often the spinner-to-cowling’s vertical alignment is mismatched by as much as 1/2" . . . who needs more drag?
- The spacing between the cowling and spinner is excessive or so small that the spinner bulkhead is wearing a groove into the cowling. A 1/8" spacing should be considered to be the minimum with a preferred spacing of at least 3/8" for an aircraft intended for serious aerobatics.
- A spinner that is obviously too small or too large for the cowling.
Incidentally, some builders think it is necessary to allow for the settling of the engine somewhat in its shock mounts. They deliberately mismatch the spinner-to-cowling fit in anticipation that the spinner, in time, will "sag" by about 1/8". It may never happen. If your engine has been installed for a number of months during construction, it is quite probably that a permanent set has taken place in the shock mounts and no allowance needs to be considered in aligning the spinner with the cowling.
The cowling is subjected to two large forces or pressures. It must handle the rush and pressure of the incoming (inlet) air and it must withstand the pressures exerted by the heated exiting air. These two forces impose considerable pressure from inside the cowling and only the cowling fasteners prevent it from bulging or blowing off.
At the cowling inlet, the fasteners holding the two halves of the cowling must be strong enough to withstand the air pressures and yet be fairly easy to assemble and disassemble.
The same is true at the outlet areas of the cowling. A number of builders have been surprised to see their fasteners fail before the very first condition inspection rolls around.
Piano hinges riveted along both sides of the cowling provides a simple and effective method for the installation and removal of a cowling.
One disadvantage to this type of installation is the necessity to find some way to secure the front ends of the connecting piano hinge wires.
When first fitted, the cowling connecting wires are most difficult to insert. However, after a few flights, the assembly will loosen up a bit. Until it does, be prepared for some vexing cowling installations sessions.
Finding a good method for securing the cowling wire ends is important because to have one of them vibrate out and contact the propeller would be dangerous if not disastrous. Believe me, it does happen even when you think your wires are too hard to get in. Anyway, you think they couldn’t possibly work out. Oh yeah!
The most commonly used method for securing the wires is to form a closed loop (or eye) on the end of the wire and fasten it to the cowling with a pan head stainless machine screw. It will be necessary to install a plate nut behind it.
If you are not ready to install your cowling you may have plenty of time to examine a few homebuilts to see how other builders installed theirs. Study the accompanying photos and you may pick up a few ideas.