Airframe Technology: Repair on Thin Skinned Aircraft

The business of shrinking and stretching

Sheet metal fabrication is considered a blend of art and science, requiring finesse to achieve desirable results. Sometimes the experience to achieve the desirable results can be costly.

If you have ever worked with aircraft sheet metal you have undoubtedly done something to that metal and then noticed that it had an unexpected effect somewhere else. Getting unwanted shrinking and stretching where you didn’t intend is a costly and time-consuming mistake. The ability to straighten thin skin before riveting can make all of the difference to achieve a professional result.

One of the more common hand operated forming tools is the shrinker-stretcher. This may be one of the first metal forming tools you get to use, but you may not fully understand its operation. It works by applying tremendous pressure on dies that grab the surface of the metal to push the metal together or apart.

Always practice first on a scrap piece of metal the same thickness and temper as the aircraft skin being repaired in order to learn how the jaws will affect the material. Over time, the jaws become clogged with aluminum dust and do not grab the same way as they did the last time they were used. It’s a good idea to keep the jaws clean with a file card, not a wire wheel, and remove material buildup in the textured surface. After cleaning and testing on scrap material of the same thickness you are using, you’re ready to do some shrinking or stretching.

Hangar rash is common on aircraft, both fixed wing and rotary wing alike. They are both built from the same thin aluminum skins and difficult to work with. Often times you will find metal on an aircraft has been bent and subsequently stretched. If you take a hammer and dolly trying to straighten this you will just be chasing your tail. You may reach for the steel hammer next but hitting the metal will probably stretch it even more. In this case the metal needs to be shrunk not stretched.

The correct process is to take the part or metal over to the vice where the shrinker is located. But what if this damage is on the horizontal stabilizer of a general aviation aircraft and removal from the aircraft to facilitate bench work is labor intensive and costly? The vise-mounted shrinker-stretcher has been around for a long time and the value is undeniable. However, it was designed to be strong, heavy, immovable, and cumbersome.

The right tool for the right job

When initially looking for the right shrinker-stretcher tools for my shop I became frustrated. While tools were available I felt they were heavy in terms of weight and cost. The cost of acquisition now exceeds some $7,000 when buying all the required tooling. I felt a better tool was needed for specialized spot repairs so I set out to design and build my own tool which I call the “Minimizer™” shrinker-stretcher. The hand-held shrinker-stretcher is a fraction of the cost, much lighter and easier to use, handcrafted in the USA, cadmium plated, about half the weight, and uses the same interchangeable dies that fit in a standard shrinker.

“The right tool for the right job,” I remember these words from A&P school. After you have the right tool you need to be able to use it properly. This does not start with the tool, but with studying both the metal and the damage you want to repair.

My method is to mark the high spots with a pencil on the stretched material. This is where you will use the tool and center the jaws. Smaller shrinks over a larger area is much better than a large shrink in one area which results in buckling the skin. This is difficult to remove and might crack the skin; either way buckling the skin makes the job much harder. Using the small shrink method will get the metal to lay straight. Push on the aluminum to see if it wants to spring back. If it does spring back you may need a small shrink or two in the right place to remove the “oil canning.”

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