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Etude sur le choix des fibres composites sur un hydravion
NOT ALL FIBERGLASS IS CREATED EQUAL
There is a reason that sailboats and yachts use polyester resin, and it's not just because it's a little less expensive. Epoxy resins take on considerable moisture (in the range of 4% to 6%). That moisture can cause blistering or, when frozen, can cause delaminating and cracking. If that isn't enough, when the moisture is heated by solar radiation, it severely reduces the allowable strength of the structure. It is called the hot wet condition and it comes from solar heating or exhaust from turbines over a wide area. Epoxy is not as fuel compatible, and requires special coatings or liners. Epoxy is also known for causing contact skin rashes. Once susceptible, a person can never work with epoxy again.
Vinyl ester resin is a hybrid resin with nearly the strength of epoxy, but without the disadvantages. It takes on virtually no moisture (less than 2%). It is compatible with all the fuels and requires no special treatment for the fuel tanks. There have been no recorded health problems in either the aviation or the much larger boating industry.
So, the main structural item remaining is the fiber.
KEVLAR is a high priced, hard-to-work-with fabric that is poor in compression. It is not recommended for amphibians unless you are concerned about bullets being shot at your plane.
CARBON FIBER is a high-priced, lightweight fabric. It has a very limited market and limited weaves. As a result, the weaves commonly used in aircraft are very coarse compared to those weaves available in E-glass. So, to acquire a smooth surface, much more filler is required for carbon fiber. That severely reduces the weight advantage.
THERE ARE MANY MORE DISADVANTAGES
Carbon fiber is not compatible with vinyl ester resin. It must be used with epoxy resin, which is not compatible with moisture, i.e., boating.
Carbon fiber requires sophisticated electronic inspection because defects are not visible due to the color.
Every bolt hole requires an isolated patch of E-glass to reduce catalytic corrosion between the carbon fiber and the bolts. The presence of moisture and salt make this even more serious.
Overly stressed carbon fiber will splinter, emitting many little spears that have been known to cause injuries.
Carbon fiber, when hit by lightning, will sustain much more damage than E-glass.
Finally, you cannot build in an antenna beneath carbon fiber as you can with E-glass. So, that beautiful bird just became a porcupine with antennas as quills.
E-GLASS OR S-GLASS
S-glass is about 15% stronger and stiffer than E-glass and costs almost twice the price. Like carbon, since its use is limited, there are few weaves available.
E-glass fabric is the most commonly used in the industry, and the most cost effective. For flying boats, it is the best for a water environment.
To a great extent, the resin and fiber criteria apply to all aircraft, not just amphibians, even though the moisture condition is greatly reduced with land planes.
SPECIFICALLY FOR SEAPLANES
All aircraft require attention to maintenance. All boats require attention to maintenance. With flying boats, you guessed it. In this case it is the worst of all worlds, especially if you enjoy the salt water environment.
WHAT DO YOU LOOK FOR?
Make sure that stainless steel hardware is available.
Are all aluminum parts annodized?
Are all the bolt holes annodized?
Are all the cables stainless steel?
Are all the nav and landing lights protected behind plexiglass?
Are the exhaust pipes stainless steel?
Are the wheels and brakes designed for salt water use? Some amphibs carry eight to 10 pounds of grease to protect wheel bearings and they only last 100 hours. The Seawind special design system has logged 740 hours between bearing replacement.
Autre article pour la revue "Experimental Aircraft".
CARBON/GRAPHITE VS. FIBERGLASS
We occasionally receive inquiries form potential customers about having their Seawind parts fabricated in carbon fiber/graphite due to the emphasis placed by the aerospace, military and high performance manufacturers. A recently published article by Martin Hollmann, comparing wet lay-up to prepregs, has brought a few more calls. Although Martin's article has placed some emphasis on his use of graphite prepregs and his comparative refers to non-vacuum bagged, wet lay-up of fiberglass, the carbon/graphite issue is most regularly addressed by our callers.
The Seawind, as with every other aircraft, is the result of compromises that the designers and engineers toiled over to attain their ultimate creation. The Seawind team correctly chose to apply a very strong, but economical approach to material selection. The materials and the fabricating processes now used for the Seawind are considered Advanced Composites.
Surface finishes are applied to reinforcing fibers to allow handling with minimum damage and to promote the fiber to matrix (resin) interfacial bond strength, water resistance and optical clarity. Since graphite is almost exclusively used for aerospace and military markets, where expensive epoxies are utilized, the manufacturer's finishes have been optimized for epoxy.
This is the reason that vinyl ester resin is not normally recommended for use with carbon fabrics. Exceptions would have to be tested and close attention to the Manufacturer's Certification should be specific regarding the applied finish.
It is very difficult to properly wet out graphite woven cloth because there is little change in appearance when resin is introduced. Since carbon is opaque even when completely wetted out, visual inspection for air inclusions is impossible. This makes inspection either very difficult or expensive (ultra-sonic equipment) and reject rates are unquestionably high. The danger is in what you can't see.
For that reason (and others related to aerospace requirements), the most common fabrication process for carbon parts is elevated temperature curing, performed with expensive epoxy prepregs.
To fabricate a graphite part, epoxy resin impregnated, graphite fabric reinforcements, core, peel-ply, breather ply (perforated sheet for resin control), and bleeder cloth (for excess resin and a vacuum path) are placed into or on a prepared mold, covered and sealed with a thin impervious sheet of plastic material, drawn down with vacuum, placed into an oven, and cured at high temperature (typically 250°-270°F) for several hours under precise controls. A standard for many years in the aerospace industry, this process presents many negative issues relating to cost, handling and quality control, making it practical only for the military, the aerospace industry and a few other specialized and high priced products (sporting goods, race cars, etc.).
After manufacture, shipping and storage of the prepregs in a frozen state also requires a high degree of quality control. The material has a limited storage life but, most importantly, a short out-life. This means that after bringing the material up to room temperature in sealed storage bags (to avoid moisture, one of prepreg's major problems), the clock starts ticking. After the required material is cut from the roll, it is returned to the freezer and the elapsed time is recorded. When either the out-life or storage life has been used up, the material is not suitable for use and then must be disposed of.
Carbon fiber has the highest specific stiffness of any commercially available fiber and very high strength in both tension and compression. It's impact strength, however, is lower than glass with particularly brittle characteristics being exhibited by high modulus fibers.
The graphite laminate tends to shatter, with very sharp, stiff needles and shards around damaged edges. The racing industry must provide crash "cages" of Kevlar to protect the drivers from dangerous pieces.
Like metals, graphite is opaque to radio signals. Antennas cannot be installed within the carbon skins so they must be attached outside, interrupting the smooth, flowing, composite surface and, of course, causing drag. Fuselage and wing skins of carbon are normally electrically grounded to each other with jumper wires, as are the control surfaces to the wing, so that the hinge bearings are not damaged if forced into service as electrical conductors.
Since carbon can be greatly affected by corrosion due to galvanic reaction, special care and time must be taken to insulate dissimilar metals e.g., aluminum, steel, brass, etc., from the carbon. This would involve placing a sacrificial piece of fiberglass between the graphite laminate and all metal hinges, brackets, tracks, etc., and dipping rivets, bolts screws, and bushings in primer resin before installation.
Surface finish of a prepreg is extremely porous. Epoxy resin has an affinity for moisture, as does the freezing and thawing process, and any moisture lay-up will produce water vapor (steam) under vacuum and elevated temperature, which is evident in the finished part as porosity, a rejection factor. The solvents used in the manufacturing (prepregging) process can also produce voids during cure. The predetermined resin quality is sufficient to wet-out the fibers but not to fill in the coarser graphite fabric weave patterns. That process is left for the builder to do — squeegee filler into the porous surface and sand. Then repeat the process for any remaining pinholes. Some of the weight savings is certainly lost with the addition of fillers.
Expensive honeycomb core materials and film adhesives, used to bond the core to the face sheets, require additional labor and associated expense. Any assembly of carbon prepreg parts must be accomplished with more expensive structural adhesives, compatible with the graphite/epoxy components. If the ribs, bulkheads, webs, etc. are attached or reinforced with a wet lay-up process, graphite fabric and epoxy must be used and, again, a good visual inspection will be impossible. Be careful — epoxies are toxic and could cause serious and lasting reactions.
Dry graphite fabric must be handled with more care than fiberglass. Fibers can break and loose particles can be inhaled. Also, cured carbon splinters will not work their way out of the skin as would glass or wood.
Hollmann stated that vinyl ester resins are most suited for water environments and that "aircraft such as the Seawind" are made of these resins for that reason. He points out that porous prepregs must be fuel proofed, coated with acceptable sealants or Derakane 411, or they will leak fuel. For corrosion resistance, of course, vinyl esters are the choice of the chemical industry. He stated that he has learned from experience that a wet lay-up of graphite does not make sense because the coarse weave has a high resin content and is so heavy that the advantages of the lighter and stronger graphite are not realized or offset by its higher cost. Hollmann shows that in-plane shear strength of graphite and fiberglass panels, when processed in a similar manner, are nearly identical (18,00 psi vs 17,500 psi). In the article, the fabricated wing skins Hollmann quoted on were $16,000 in prepreg material @ $53.33/lb and $2,100 in fiberglass @ $3.50/lb.
The bottom line for graphite vs. fiberglass is cost. Material costs, freezers, oven, additional processing, additional training, quality controls, rejection rates, etc., bring the price of graphite composite prepreg parts to prohibitive levels. Consider this especially when graphite is used in excess of, or adversely to, an aircraft design's requirements — or simply to give the aircraft an appearance of being more advanced…