Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/126719
S a f e t y W ir e completed, a fine sandblasting and epoxy primer will work well. Remember that all oil must be removed in order to sandblast. You cannot sandblast over oil as the sand will not remove the oil and will cake up. a massive hangar fire caused by linseed-oil–soaked rags thrown in a plastic trash can. Lacquer thinner or an appropriate solvent also works fine, but use gloves and work in a ventilated area. Let everything dry completely prior to sandblasting. The sandblasting should be done on clean, dry steel and should be free of any substance after completion. Once blasted the part must be handled with clean gloves. The natural oils on your skin contain salt and will quickly cause rusty fingerprints. Remember that aircraft manufacturers building airplanes with steel tube fuselages seldom sandblasted the parts. They were generally built very quickly, wiped down with a cleaner, and primed. In 1940, these manufacturers used a zinc chromate primer and then only a thin coating was used. Sheet metal construction can have all sorts of problems with corrosion and foreign materials. One of the most common is dissimilar metal corrosion or galvanic corrosion. If bare, dissimilar metals are in contact, the different electrical values can cause corrosion in the presence of moisture. One example is steel and aluminum, but it can happen with different aluminum alloys as well. Using a primer or paint barrier between these parts is a common practice. Powder coating also has been used on both steel and aluminum, but it is not recommended in aircraft structure for a number of reasons. It looks nice and can be really hard shell coating, but it also hides cracks, particularly on engine mounts. It's quite possible for the tubing to crack and the powder coating to remain intact. This being said, it is used; I'm just not fond of it. Amateur-built parts are constructed over a longer period of time and may require sandblasting. Fine sand should be used so the peaks between the sand pecks do not stick through the primer. Glass beads or soda blasting are other possible methods. Do some research if you are sandblasting, as you can destroy something that took you a long time to build. For example, a large industrial sandblaster can actually cut through a tubular structure in a very short time. Some auto restoration shops do nice work with sandblasting. Their goals are similar to aviation folks with delicate parts and high quality. Check around if you hire out sandblasting. Remember, you're building an aircraft, not a bridge. Line oil or "tube seal" may be used on the inside of tubular structures and is optional. I have sawed up old fuselages with airtight welds and found no rust. Open-end tubing such as a strut should be oiled. Some line oils are progressive, meaning they will creep above the oil level to coat the whole piece of tubing. Some wise old mechanics will drill a small hole inside a longeron cluster. Then line oil can be injected in the longeron and run through the holes into the diagonals. Since the cluster weld surrounds the holes, there is no strength penalty. (See photo reference in Sportplane Construction Techniques by Tony Bingelis, pages 66 and 70.) This practice also allows a pressure relief during the welding process. You can buy a plastic oiler and oil through a very small hole. AN driven rivets can be used to plug the holes and are driven in with a small hammer. An alternative is a self-tapping screw. I do not recommend plugging a 1/8-inch hole with a pop rivet. Such a hole is much too large and the rivet is not airtight. Ask the plans or kit vender before drilling any holes in the primary structure. Follow the instructions for the use of line oil to the letter. One last warning is that some line oils are boiled linseed oil and are very likely to spontaneously combust on a rag. I know of 34 Vol.2 No.5 / May 2013 Sheet Metal Construction When building with wood, there should always be a barrier between wood and metal parts. Most builders use epoxy varnish on wood and epoxy primer on steel parts. The use of cadmium-plated aircraft hardware on aluminum structures is commonplace. The cadmium acts as a barrier between the steel hardware and the aluminum, a classic location for galvanic corrosion. Cadmium is not only a dissimilar metal barrier but a sacrificial metal to help prevent corrosion of the part. A cadmium-plated fastener can easily be replaced when the cadmium corrodes and thus allows longevity of the aircraft part. Some builders install hardware with wet primer much like bolts in wood structures. Also, the simple turning or tapping of a bolt in tightly reamed holes can wear through the cadmium plating. The final bolts and screws installed should be new. Another problem with aluminum airplanes is simply washing with the wrong soap or cleaner. Remember that dishwashing detergents usually contain citric acid that will wick into lapped joints and start corrosion. Use an aircraft soap or cleaner and rinse thoroughly. Some spray cleaner products admonish users to rinse thoroughly. Only spray cleaners marked for aircraft should be used; do not use common household cleaners. If you read the fine print, you may be surprised to find that many household cleaning products say, "Do not use on aluminum."