A structural glass repair has an accepted minimum of a 12:1 scarf/bevel. Thickness times 12 means a 0.040-inch-thick glass edge should have about a 0.5-inch bevel from the edge. A 12:1 bevel could be much more in aerospace applications, but what about nonstructural applications where an edge needs to be extended? A good solid scarf joint will suffice. If no stress will be applied, the 12:1 can be challenged with a greatly reduced bevel. Nonstructural might be window joggles, door joggle edges, a fuselage cowling joggle, or an over-trimmed cowling—anywhere additional glass is needed to extend an edge. If the bevel can accept several layers of glass in a staggered fashion, the glass should bond (with proper cleaning and sanding as surface prep) and be strong enough to handle being exposed to the minimum stresses.
In this example, a section of the vertical stabilizer trailing edge was over-trimmed, and the tail spar was riveted in place. There was no way to achieve the 12:1 without removing the spar. This edge was purely cosmetic, so removing the spar was not needed for the addition of 1/8 inch. A very short bevel was acceptable to clear the rivets.
The bevel was created as large as practical, about 4:1. A piece of waxed aluminum was used as a backup mold and attached to the inside of the tail fin using double-sided carpet tape. If there was real estate, Clecos installed outside of the scope of work could have been used to hold the aluminum, with the holes countersunk and filled later. The glass strips were cut much larger than the needed 1/8-inch end result. For ease of handling, 1/2-inch pieces of glass were applied starting at the very edge of the bevel, with each subsequent layer of cloth applied slightly farther up on the bevel. It is always easier to add more than needed and trim back as opposed to trying to add close to what is needed. Five layers were applied in this application and peel-plied. Once cured, the repair was trimmed to length and the surface sanded smooth. A final fairing putty wipe was applied for a careful finish sanding.
If the situation allows, extending the bevel back enough to get a good overlap and closer to 12:1 will always be better. 8:1 or even 6:1 will suffice in a nonstructural application.
That is one way to do it but it is harder and much heavier than necessary. If you work with composites, you must have faith in bond strength so rivets should never be used. They are also prone to cause distortion.
Here is a simple way:
Measure the thickness of the skin to be extended. Use a strip of
“Freeman sheet wax” of that thickness on a flat surface, typically a vacuum table. You can also use a strip of aluminum of the right thickness.Make the strip twice as wide as the width of the area you have for bonding. Laminate Bidirectional cloth on the 45 covering the strip and at least 1/2″beyond on each side. Vacuum bag if possible.
After cure and removal of the wax strip, you have a “hat section”. Cut down the middle to have two lengths of a joggle strip. Bond in place using an aluminum angle for alignment and straightness and cleco clamps.
Yes that is a great way to add extra length pretty easy in a down dirty way. The intended extending of the skin is just that, extend a skin that is a set thickness and end up with the same thickness. In the example shown, the tail spar was riveted to the skin of the vertical stab. There was about 1/4″ of fiberglass extending beyond the spar. What isn’t shown in the photos is the aft of the extension where the aluminum was just held against the inside of the skin.
There were no rivets used in this repair. The short scarf and adding of glass to the scarf yielded an extension of the skin in a fashion that looked and acted like a factory part as opposed to a glass repair fix. There would be no room for your application here as there is a clearance that has to be maintained for the rudder to bypass the skin at full deflection.
As for cowl, firewall joggles or door joggles, they all have thickness requirements that an offset flange glued to it would most likely interfere with the door seal, camloc receivers or even nutplates if they fell on the edge… not to mention the upper cowl’s need to fall into the lower cowl joggle. All of the above would likely be best suited with a scarf extension. Oddly enough, I have made your “hat section” glass parts and cut them down to a little less than 3/16″ and used them, uncut, to hold wires in a nice neat orderly line on the inside of a glass fuselage.