Upper cowl fasteners

I'm getting closer to working on the cowling and need to make some decisions.
I had a good look recently at a well finished 7a which used screws and nutplates for the upper cowl rear attach. Would be much cheaper than skybolt though not as pretty or convenient. It would seem to be easier to build than the hinge method.
Anyone with experience who could comment?

Cowl

jimgreen said:

I'm getting closer to working on the cowling and need to make some decisions.
I had a good look recently at a well finished 7a which used screws and nutplates for the upper cowl rear attach. Would be much cheaper than skybolt though not as pretty or convenient. It would seem to be easier to build than the hinge method.
Anyone with experience who could comment?

Click to expand...

Skybolt! In the overall price of the end product they won't even be a blip on the radar.

OBTW----much more convenient!

I used screws (ten #6's) on the cowl across the top of the firewall.

I also used eight #6 screws and two #8 screws to secure the cowl across the bottom of the firewall.

I have eight years and 600 hours of good experience with this installation.

Highly recommended.

jimgreen said:

I'm getting closer to working on the cowling and need to make some decisions.
I had a good look recently at a well finished 7a which used screws and nutplates for the upper cowl rear attach. Would be much cheaper than skybolt though not as pretty or convenient. It would seem to be easier to build than the hinge method.
Anyone with experience who could comment?

Click to expand...

Too many years ago, about the first thing I did after I bought my C-150 was to drill out all those cheap steel Tinnerman nuts used with sheet metal screws to attach both cowls. I replaced all with floating nutplates. In 17+ years and uncounted on/off cowl cycles, I never had to replace a single nutplate.

Based upon that experience, I fabricated a (standard width) 48" long .050 aluminum strap. Scalloping the relatively thick .050 material was very quick and easy to do. Using a band saw, I cut out a basic "V" notch between each fastener location to remove most of the material, then rounded out and finished each scallop by using a rotary file mounted in the drill press and "walking" the strap around on the drill press table quickly shaping and finishing each individual scallop. Using approximately 22-24 #8 screws and floating nutplates to hold the upper cowl onto my -6A, this arrangement has worked out well. To attach and/or remove the cowl, I sometimes use a cordless drill, sometimes not. If a screw becomes even slightly worn, I simply toss it away and replace with new. Some time ago, I even stopped the practice of routinely placing 100° SS washers under the screw heads.

Sure, you can use more user friendly Skybolt fasteners but a Skybolt fastening system has a higher profile "look" which I personally do not find as pleasing to the eye as many other builders do. That is merely my personal taste and as such, pointless to argue about. What is NOT personal taste or opinion is that a kit of Skybolt fasteners can easily cost hundreds of dollars more than a system of off-the-shelf screws and nutplates with a standard piece of 48" wide sheet metal stock. In the end, it is your airplane to build and this is not the first or last decision you will ever have to make.

Ditto what Rick said!

Back in 2005 I priced the 1/4 turn fasteners from SkyBolt and several others and, after trying several combinations for appearance, convenience, and price, selected #8 SS flush phillips screws with #6 SS tinnermans and nutplates. I used a #8 dimple die set and squeezer to enlarge the #6 tinnerman flush washers to accomodate the #8 screws. I used this combination across the top rear attach for the cowl to firewall and across the bottom rear cowl attach to firewall. The bottom is the recipient of a large amount of the vibration and needs to be secure to avoid repairs. I also used a strip of 050 as the mount point for the nutplates, with scallops and some fluting to fit the cowl taper--similar to Rick. This combo has been on my plane since early 06 and over 250 hrs----absolutely no problems.

BTW, I did use the hinge system on the top to bottom cowl horizontal seam and the vertical side firewall attach.

Cheers,

db

I've installed Sky-bolt/Camloc types in three upper cowls and they work just dandy. Hinges elsewhere except the bottom and those are ten #8 screws into plate nuts.

The current project has a hinge on the top and I hate the thing before the plane has even flown. Getting the intersecting horizontal and vertical hinge lines to engage is - on paper - impossible, and in reality a twisting, pounding, cussing activity. I've tapered the dickens out of them, too. The pins are pulled through an external-access center top door, so that's worlds easier than reaching through the oil door per Van's, but it does nothing to help the engagement. I'm seriously thinking of replacing the top hinge with friendly stud fasteners.

John Siebold

I've looked at the various screw-in and 1/4 turn systems. After 4 years and 450 hrs. using Van's hinge pin setup on my -7A, I'd have to say that his design does the job adequately. I've had some hinge loops break on the bottom of the cowl, but overall, I'll just stick with the original design hinges for now.

RE upper cowl/firewall fasteners

Thanks for the replies everyone.
I'm pretty well decided to go with the screw / nutplate method. I could spend the extra cash on something else.
I rather like the look of the screws also.
A couple more queries;
Do floating nutplates do a better job in this application? Seems they might allow enough sideways movement to make the cowling seam open up slightly.

What sort of backing should be used to prevent the screws pulling through or elongating the hole in the cowl?

Exactly how do tinnerman washers work and how does one make them flush?

The aircraft I looked at used 3 1/4" spacing for the top cowl. Is that about right?

BTW Rick, great pics of your handiwork. Did you need much fluting or tweaking to get that thick old strap to do what you wanted?

Thanks for the help. I'm reasonably confidant with aluminum now but find the cowling and fiberglass a bit daunting.

Consider How Often the Cowling Comes Off

You take the cowling on & off a lot, during building and phase I. Couple hundred bucks now will save you hours over the life of your plane. I can take both of mine off in less than a minute.

Hans

jimgreen said:

......Do floating nutplates do a better job in this application? Seems they might allow enough sideways movement to make the cowling seam open up slightly.
What sort of backing should be used to prevent the screws pulling through or elongating the hole in the cowl?
Exactly how do tinnerman washers work and how does one make them flush?
The aircraft I looked at used 3 1/4" spacing for the top cowl. Is that about right?
Did you need much fluting or tweaking to get that thick old strap to do what you wanted?

Click to expand...

Jim,

In any application in which an assembly has a large number of nutplates holding it together, a floating type nutplate is usually a better choice because the element allows for some small amount of movement to help get the screws centered and started with much less chance of crossthreading. Not necessary for small jobs like access panels, I did use floating nutplates for most other jobs including making the floors removable. Since a cowling is curved over a large area, a floating nutplate makes it easier to find the center of each nutplate element. Certainly, you can use fixed element nutplates with success, but a floater is significantly easier to work with. Once attached, there is virtually no chance the cowling can or will shift in any way. That concern is simply not an issue.

Tinnerman washers are used as a "beauty ring" but mostly to help prevent holes in fiberglass from elongating. This picture shows SS Tinnermans installed underneath the screws on the same cowling.


Over time, I started asking myself "Why?" and then I decided I really did not need or desire their higher profile look after all. I began reinstalling the wheel pants, empennage fairing, and cowling without Tinnermans with no apparent down side. Of course, if a hole is already elongated (which is more likely to happen if you use fixed element nutplates) a Tinnerman washer is the ideal way to compensate.

For spacing, I used 24 equally spaced screws over the length of the 48" long .050 strap. As I recall, Skybolt recommended that thickness so that's what I ordered from Wicks in a 4' X 4' sheet. I eventually had three straps sheared off that stock, at first for my use and then two other budget-minded local RV builders who chose to do the same thing.

I did not experience any unusual problems installing the .050 strap and found no need to flute or tweak it in any way either. Basic sheet metal practice dictates the edge of the strap that nests up against the firewall be sufficiently rounded/beveled and polished for a good fit. I chose to merely alodine the strap for corrosion protection. I then found the centerline of both the strap and the firewall and worked my outboard drilling and clecoing, alternating from the left and right one hole at a time.

nucleus said:

You take the cowling on & off a lot, during building and phase I. Couple hundred bucks now will save you hours over the life of your plane. I can take both of mine off in less than a minute. Hans

Click to expand...

No question Hans is right. If you want to be able to remove the cowl in a minute or so, nutplates and screws attaching the upper cowl and hinges installed per plans everywhere else will not give you that option. We must all decide for ourselves if our choice is driven by convenience or money. With a cordless drill, I can easily remove all 24 screws holding the upper cowl down in approximately 2 minutes but then I have all those hinges to deal with. Pick your poison.

Hi Jim,

The use of floating nutplates would work fine and would compensate for any mismatch between cowl and fw flange attach points. After they are tightened down they won't move so the gap thing is not an issue with their use. They are expensive relative to standard nutplates so I did not use them. If you fit your fw flange, cleco it in place, flute it, mark your cowl for the holes (mine are, speaking from memory here, 4" or less between fasteners), and then match drill through the cowl and the flange, THEN attach the nutplates you won't have a problem with fit. If you are concerned with match, using the floaters just on the side curves of the upper cowl will address the area most likely to be a problem.

No "backing" material was necessary on the back of the cowl. The Tinnermans require you to countersink the cowl holes, the metal of the Tinnerman "nests" into the countersink hole to hold the cowl in position, and the flat portion of the Tinnerman provides a larger radius "clamping" force on the surface of the cowl---the cowl will not move and if it does not move it has no opportunity to oval out the holes. Can't tell you how many times my cowl has been on and off (literally hundreds of times) and no change in hole match.

To make the Tinnerman flush with the cowl I used a small flat blade cutting tool, can't remember the proper name for it (any help out there?) that looks like a bit for a rivet shaver. The only exception is that it has a changeable "center pin" that goes in the #8 hole to maintain center. The outer diameter of the cutting bit is the same as the outer diameter of the #6 Tinnerman. A couple of turns on the fiberglass cowl and you now have totally flush fasteners---be careful and do this VERY slowly or you will over cut!!! I used a hand device vs a drill motor.

Unlike Rick, I did have to do some minor fluting to make the fw attach strap follow the natural taper of my James cowl. The geometry is such that after the strap is riveted to the firewall flange it extends straight out but my cowl tapers down. Without the fluting (one between each nutplate attach point) the cowl would be resting only on the forward edge of the flange. Minor fluting and it follows the taper. Send me a PM if you want pictures of the tinnermans or the flange install. I also have a couple of pics of the cowl with several combo's of #6, #8, tinnermans/screw as well as finish washer/screw combos that I took while trying to decide on fasteners--as mentioned I decided on #8 SS screws and #6 SS tinnermans.

Hope this helps.

Cheers,

db