Airframe Technology: Repair on Thin Skinned Aircraft

The business of shrinking and stretching


Using this process not all of the shrink lines need to be in the same plane, and they can be varied 45 degrees to break up the lines and remove the shrink marks. It is also better to regularly study the damage and the effect that you are having on the aluminum rather than go too far and buckle the skin. Trying to stretch metal that is already stressed with lines stamped in the surface can very easily result in a crack in the skin. It’s best to proceed slowly and with caution.

Take your time

Let’s say this takes care of the damage on the left side of the stabilizer, but what about the crack and the stop drilled hole on the right side? A typical example is a crack that also happened from being over stressed sometime in the past with a pair of pliers or wrench used to remove the elevator hinge bolts. If you want to make a flush repair that looks nice you may have a dilemma. This skin is too thin to use flush rivets, so dimpling will serve the purpose but some shrinking may be required for the skin to lay flat. Dimpling also shrinks the material even more but it also adds rigidity to the repair.

If you spend a little time to consider the known elements of a repair before forging ahead, then you will be less likely to paint yourself into a corner because you were rushing. When using any shrinker-stretcher designed for thin metals it’s important to check that it is adjusted properly. In the case of the Minimizer this is done by loosening up the lock nuts on the eccentric bolts and moving them until both edges of the beveled blade edges are parallel and touching. This adjustment will ensure that the pressure on the dies is equal which is important on thin metal. The more that you cold-work the material the harder it gets, so try to complete your shaping or repairs in as few moves as possible.

Hopefully I’ve been able to provide you a few new ideas to achieve better results the next time you have a thin sheet metal repair or fabrication job. Make it fun and be creative, but above all make it as easy as possible by thinking the job through first and then use the right tools.

Don’t be afraid to ask questions and to share knowledge because we are all interdependent upon one another. Be proud to learn something new everyday as our industry is a learning process that never stops! AMT

 

Robert Behrend holds an A&P and IA. He founded his business Aerostructures of the Palm Beaches Inc. in 2003 which specializes in structural repair work on corporate use and general aviation aircraft, government prototype design, and commercial use aircraft. Behrend graduated from Teterboro School of Aeronautics and has 25 years experience in the aeronautics engineering and fabrication business. For more information visit www.metalminimizer.com.

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