INTRODUCTION TO AIRCRAFT STRUCTURAL REPAIR TECHNIQUES & SUBSTANTIATION
Target audience is A&P mechanics and young engineers. This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Formatting has been lost during cut and paste.
Aircraft composites have been around since Orville, Wilbur, and Chuck. That’s Orville and Wilbur Wright, and Charles Taylor with Taylor being the first aircraft and power plant mechanic, although not certified since there was no regulatory agency at the time.
The Wright brothers used wood natures composite and plywood being a man-made solid laminate composite.
I heard Wilbur told Orville, “this has never been done before.” kitty hawk
Fiberglass is a lightweight, extremely strong, and robust material. Although strength properties are somewhat lower than carbon fiber and it is less stiff, the material is typically far less brittle, and the raw materials are much less expensive. Its bulk strength and weight properties are also very favorable when compared to metals, and it can be easily formed using molding processes.
Carbon fibers are lightweight and initially twice as strong as the same weight of steel. Carbon fibers find their way into bulletproof vests, high performance aircraft, automobile tires, and sports equipment.
A composite is when two or more different materials are combined together to create a superior and unique material. The first uses of composites date back to the 15
00s B.C. when early Egyptians and Mesopotamian settlers used a mixture of mud and straw to create strong and durable buildings. Straw continued to provide reinforcement to ancient composite products including pottery and boats.
Plywood has been made for thousands of years; the earliest known occurrence of plywood was in ancient Egypt around 3500 BC when wooden articles were made from veneers glued together crosswise.
Wood was among the first materials used to construct aircraft. Most of the airplanes built during World War I (WWI) were constructed of wood frames with fabric coverings. Wood was the material of choice for aircraft construction into the 1930s.
will wood return and replace aluminum in aviation?
figure 9.2 langley twin
Engineers at the Owens Illinois Glass Company and Corning Glass Works develop several means to make glass fibers commercially viable. Composed of ingredients that constitute regular glass, the glass fibers produced in the 1930s are made into strands, twirled on a bobbin, and then spu
n into yarn. Combined with plastics, the material is called fiberglass.
Fiberglass, Fiber-reinforced polymer (FRP), Glass-reinforced plastic (GRP), Glass-Fiber Reinforced Plastic (GFRP) is a fiber reinforced polymer made of a plastic matrix reinforced by fine fibers of glass. It is also known as Glasfaserverstärkter Kunststoff (GFK in German), Gurasufaibā (in Japanese), زجاج مغزول (in Arabic), fibra de vidro (in Portuguese), fiberglas (in Turkish), and fibreglass (in English, the Queen’s).
Fiber-reinforced polymer (FRP) composite materials date back to the early 1940’s in the defense industry, particularly for use in aerospace and naval applications. The U.S. Air Force and Navy capitalized on Fiber-reinforced polymer (FRP) composites high strength-to-weight ratio capability, resistance to weather, and salt air corrosive effects.
British engineers in 1964 develop carbon (graphite) fiber by stretching synthetic fibers and then heating them.
“In the world of aviation, composites may be the most significant materials. Around 1960’s saw the need of carbon fiber composite embedded in resin. None other than U.S. Air Force and U.S. Navy used this material in many different applications. Major applications are particularly seen in aircraft control surfaces such as rudders, primarily controlling the movement of the airplane’s nose to the left or right directions; and ailerons, responsible for the longitudinal movement of the airplane. The pinnacle of success for the material’s application was being brought to known in public with its major participation in the production of Boeing 787 Dreamliner.
Boeing 787 Dreamliner prides itself as the most innovative airplane ever invented in the history of mankind. This is the first ever airplane that uses carbon fiber in most of its parts. It is in this regard that its developer, Boeing Commercial Airplanes, boasts that the state-of-the-art airplane is the most fuel-efficient among all airliners in the world since it only consumes twenty percent less fuel than any similarly-sized planes. Since it is lighter in weight compared to other airliners, it has smaller engine, thus the need for minimal amount of fuel.” Wes Collins
figure 9.3 boeing 787
Good stuff but say good bye to your old expectations of emergency field repairs of primary structure like pressurized fuselage skins keeping revenue aircraft in service. Damaged structural plies may not have simple temporary repairs. Heard of boiler plate proposals over the damaged plies but that may require specific approval meaning even more time.
What are the downsides of carbon (graphite) fiber repairs that mechanics need to be aware of compared to fiberglass repairs? There is no difference except when it comes to primary structure. We don’t use fiberglass in primary structure and primary structure is critical therefore the repair has less margin for error. Carbon (graphite) fiber repairs on primary structure are critical and therefore must be done right.
Poor workmanship can not be remedied by any repair and every subsequent poorly repaired structure may require another repair approval costing more money and time.
The following list lays out some common concerns for composite repairs:
Repair facilities are expensive
Repair facilities must remain uncontaminated
Repair facilities must be environmentally controlled
Repair materials have a shelf life
Repair materials must remain uncontaminated
Repair materials are not cheap
Repair materials may be hazardous
Repairs take time, much more time
Repairs require holding fixtures
Repair technicians require training
Repair technicians require protective clothing
Repair technicians require protective gear
Repair processes are definitive and compliance is a must
But carbon (graphite) fiber is good stuff just no “metal hammers,” greasy fingers, or hap hazard slap it together attitude. That comment about greasy fingers is anyone’s fingers no matter how washed – use gloves! Humans are greasy, some more than others but we are all greasy.
Working the B-1B bomber’s flaps, I was directed to go through the flap assembly process and figure out why the flap couldn’t pass its leak test. Titanium ribs, aluminum honey comb core, carbon fiber spar and carbon fiber skins assembled on a steel tool and baked for hours led to warped ribs. All materials had a different coefficient of thermal expansion so each part expanded at different rates while in the autoclave or oven and then cured into place. But that different cause the leak.figure 9.4 USAF b1-b
My investigation continued into the clean room and discovered the source. Prior to any assembly, cutting, or ply orientation the geniuses picked up the individual plies with their greasy index finger and thumb at one corner. You guessed it, at the corner that leaked. After laying each ply down carefully they to put their gloves on.
figure 9.5 cirrus sr22
“The iconic SR22; the world’s first choice of general aviation aircraft. Specifically engineered to give you the lifestyle you want -freedom to go where you want, when you want, without waiting or hassles- and to do it with advanced technological innovation, exhilarating performances and unprecedented style. Make your life possible. For ten years and counting, the SR22 series has been the best selling aircraft in its class.”
Just thought that would be nice.
Cirrus SR22, SRM 51-20 available for free online at http://servicecenters.cirrusdesign.com/techpubs/pdf/AMM/SR22/pdf/51-20.pdf
Carbon (graphite) fiber material varies significantly from aircraft make and model. Do not make an unapproved material substitute for plies or resin. Composite material is determined by the airframe or power plant manufacturer and not the material manufacturing.
Prepreg plies in lieu of ply and resin combination is not a material substitution but a process change. In most cases this is not only acceptable but preferred Check the OEM manual in case it states otherwise.
This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Stay tuned for:
Structural Aircraft Repair Substantiation for non-Engineers
How to fix or not fix what’s broke and justify it
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