Reduce weight, is an important pursuit of aircraft design and manufacturing, can give military aircraft stronger flight performance, improve the fuel economy of civil aircraft. However, if the thickness of the plate-like parts on the airplane is too thin, it will face the problem of insufficient strength and stiffness. Adding lightweight, rigid sandwich material between two layers of panels can significantly increase load-carrying capacity without significantly increasing weight, as opposed to adding a supporting frame.
This is akin to the construction of wind turbine blades that this site has shared before:
A layer of balsa wood or foam core is filled between the inner and outer surfaces of a skin made of fiberglass-reinforced epoxy resin (FRP). Balsa was also the first core material used in airplanes, such as the British Mosquito Bomber, a famous WWII wooden airplane made of two layers of birch sandwiched by a layer of balsa plywood.
In the modern aerospace industry, the core materials used include honeycomb structures and foam. The seemingly soft honeycomb can withstand the crush of heavy trucks because the stable honeycomb grid structure prevents flexural deformation, the same principle that makes cardboard boxes made of corrugated paper highly resistant to compression.
Aluminum is the most used metal in aircraft, and the structure of aluminum alloy panels + aluminum honeycomb core is a natural fit.
This is a section of a VFW614 aircraft wing, with the trailing edge of the wing and spoiler made directly from the aluminum honeycomb structure.
Incidentally, this VFW614 was an oddball with an engine mounted on top of the wing, developed by West Germany's United Aero-Technologies-FOKKER (VFW-FOKKER) Aircraft Company as a short-range 40-passenger airliner, which made its first flight in 1971 and was the first development of a jet civil airliner in West Germany after the Second World War (East Germany's attempt was the Bard 152, which made its first flight in 1958 ). The airplane was not a commercial success, with only 13 built, but it is still a milestone for the German aviation industry. An even more famous story was the Franco-German partnership between Airbus and Boeing.
As composites are increasingly used in aircraft construction, can aluminum honeycomb be replaced as well?
The answer is yes. The most common application in modern aircraft is aramid paper honeycomb composites, and made of carbon fiber reinforced resin and other composites to cover the honeycomb panels, used in aircraft floors, hatches, interior parts, tails, rudder surfaces, fairings, paddles, etc., but also used in high-speed trains, the interior walls, ceilings, baggage racks and so on.
As the name suggests, this honeycomb is made from a special kind of paper, aramid fiber paper.
First, the aramid paper is coated with adhesive strips, the width of which is equal to the length of the sides of the honeycomb holes (in the case of a hexagonal honeycomb), and the spacing between the strips is four times the length of the sides. The adhesive strips are applied by gravure printing, whereby grooves of a specific width and spacing on the applicator rollers transfer the adhesive strips onto the aramid paper.
Two adjacent layers of aramid paper are stacked in a staggered position and the adhesive strip is cured by hot pressing.
Next, the aramid stack is stretched like an accordion to form a honeycomb. The honeycomb is also baked to release the rebound stresses and finalize the honeycomb.
The honeycomb is cured by impregnation (usually with phenolic resin) to fill the pores between the aramid fibers. The resin plays a role in bonding the fibers and transferring the load.
Honeycomb can be made into various shapes by cutting process.
The above honeycomb manufacturing process comes from AVIC Composites, a subsidiary of Aviation Industry Corporation of China (AVIC), which currently produces aramid paper honeycomb with aperture lattice lengths ranging from 1.8 to 5.5 mm, densities ranging from 29 to 144 kg/m3, and maximum dimensions of 3,900 x 1,850 x 900 mm.
However, the key raw materials for the production of aramid paper honeycomb - aramid fibers and aramid paper - were once the "necklace" problem we faced for a long time.
Aramid is usually referred to as para-aramid and meta-aramid.
Para-aramid is made from the polycondensation of terephthaloyl chloride and p-phenylenediamine, and is also known as "aramid 1414" according to the position of the substituent group on the benzene ring, and of course, its most famous trade name is "Kevlar" of the U.S. DuPont company (Kevlar). Meta-aramid is made by polycondensation of m-toluene dicarbonyl chloride and m-phenylenediamine, and is also known as "Aramid 1313", which is sold under the trade name of Nomex by DuPont.
In comparison, para-aramids have higher strength and modulus, resulting in better mechanical properties of aramid paper and honeycomb, while meso-aramids have better heat and flame resistance.
The aramid filament with a diameter of about 10 microns obtained from spinning is cut to obtain one of the important raw materials for aramid paper - short-cut fiber, with a length of 5~6 mm. Short-cut fibers of high strength, but the surface is smooth, less active groups, the lack of intertwining force between the fibers, can not be caused by the paper alone, but as a "rebar", and "concrete" - aramid Pulp meal or precipitation fiber combination, become a paper with high strength. Pulp meal is a short cut fiber after pulping to get, the original fiber structure is partially broken up, the surface is fluffy, exposing more amino acids, can form more hydrogen bonds, so that the short cut fibers bonded together; precipitation of fibers is a polymer solution in the vigorous stirring precipitation precipitation of fluffy short fibers.
Aramid fibers dispersed in water are copied into paper (lab test shown), similar to the traditional papermaking process. However, the aramid paper has an additional step of hot pressing to bond the aramid fibers together. The lower softening temperature of mesoaramids makes it easier for the fibers to bond to the paper, and thus they are used more often in aramid paper honeycomb composites. In addition, aramid paper is used as insulating paper in the power and electrical industry.
Foreign countries in 1967 began industrialized production of aramid, while China's aramid industry started in the 1980s, there was a large gap with foreign countries, faced with the technical embargo of foreign enterprises, aramid fibers and aramid paper for a long time dependent on imports. Yantai Minsda (production of spandex and aramid Tahe New Material subsidiary) and Shaanxi University of Science and Technology of pulp and paper engineering professor Zhang Meiyun cooperation, to break the foreign monopoly, to achieve the localization of aramid paper, there are still Chaomeisi new materials, Shenzhen Haotian Longbang composite materials company can mass production of aramid paper.
It is the efforts of contemporary Cai Lun, so that China's airplanes have a light and strong body!
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