Military aircraft

A variety of military aircraft projects have occurred over the years — far more than can be summarized in a short space here. Thus, only some of the significant milestones will be described. 

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Hydraulic fluid resistance makes fluorosihcones the preferred military aircraft choice for the manufacture of the flexible bellows (12) between the hydraulic fluid reservoir and the suction pump on Northrop Corp. s T-38 trainers and T-5 fighters. Its use allows for fluid continuity during normal and inverted flight attitudes. 

Polyimides (PI) were among the eadiest candidates in the field of thermally stable polymers. In addition to high temperature property retention, these materials also exhibit chemical resistance and relative ease of synthesis and use. This has led to numerous innovations in the chemistry of synthesis and cure mechanisms, stmcture variations, and ultimately products and appHcations. Polyimides (qv) are available as films, fibers, enamels or varnishes, adhesives, matrix resins for composites, and mol ding powders. They are used in numerous commercial and military aircraft as stmctural composites, eg, over a ton of polyimide film is presently used on the NASA shuttle orbiter. Work continues on these materials, including the more recent electronic apphcations. 

For a three-axis system, three triangles in mutually perpendicular planes may be used. For navigational purposes, the output of the laser gyroscope may be integrated to determine the heading of an aircraft. Laser-based navigation systems have been in use on commercial and military aircraft since the early 1980s. 

Initially, DADC polymers were used in military aircraft for windows of fuel and deicer-fluid gauges and in glass-fiber laminates for wing reinforcements of B-17 bombers. Usage in impact-resistant, lightweight eyewear lenses has grown rapidly and is now the principal appHcation. Other uses include safety shields, filters for photographic and electronic equipment, transparent enclosures, equipment for office, laboratory, and hospital use, and for detection of nuclear radiation. 

Titanium is the ninth most abundant element ia the earth s cmst, at approximately 0.62%, and the fourth most abundant stmctural element. Its elemental abundance is about five times less than iron and 100 times greater than copper, yet for stmctural appHcations titanium s aimual use is ca 200 times less than copper and 2000 times less than iron. Metal production began in 1948 its principal use was in military aircraft. Gradually the appHcations spread to commercial aircraft, the chemical industry, and, more recently, consumer goods. 

Oxide films on aluminum are produced by anodi2ing in a chromic acid solution. These films are heavier than those produced by chemical conversion and thinner and more impervious than those produced by the more common sulfuric acid anodi2ing. They impart exceptional corrosion resistance and paint adherence to aluminum and were widely used on military aircraft assembHes during World War II. The films may be dyed. A typical anodi2ing bath contains 50 to 100 g/L CrO and is operated at 35—40°C. The newer processes use about 20 volts dc and adjust the time to obtain the desired film thickness (184). 

Applications. Initial appHcations have been largely in military and aerospace areas. These include hydrauHc seals for military aircraft and fuel seals and diaphragms for both military and civiHan aircraft. Shock mounts for EZ are used on aircraft engines. Large fabric-reinforced boot seals are used in the air intake system on the M-1 tank. The material s useful temperature range, fuel and fatigue resistance, and fire resistance were determining factors in this appHcation. 

It is stated that modem passenger and military aircraft each use about 1000 lb of silicone rubber. This is to be found in gaskets and sealing rings for jet engines, ducting, sealing strips, vibration dampers and insulation equipment. 

In primer formulations for adhesive bonding of metals, the coupling agents that are most frequently used are those based on epoxy and amine functionalities. Aqueous solutions of aminosilanes have been successfully used for obtaining stable adhesive bonds between epoxy and steel [10] and epoxy and titanium [11,12], while epoxy functional silanes are preferable for applications involving aluminum substrates [13,14], A simple solution of % epoxy functional silane in water is currently used for field repairs of military aircraft [15] where phosphoric acid anodization would be extremely difficult to carry out, and performance is deemed quite acceptable. 

Baker, A.A., Bonded Composite Repair of Metallic Aircraft Components — Overview of Australian Activities, AGARD (Advisory Group for Aerospace Research and Development) Conference of Composite Repair of Military Aircraft Structures, 3-5 October 1994, Seville. 

Although the acrylate adhesives are readily available and studies have shown that they can produce reasonable bonding properties, they have the disadvantages of having high shrinkage, high fluid absorption, and low service temperatures. Acrylate adhesive applications would be limited. The development of EB-curable epoxy adhesives would have applications in the aerospace and automotive industry and potential wider uses. The most immediate application for these resin systems is composite repair of commercial and military aircraft. 

The aerospace field is a broad one and has a complex history. A comprehensive review of structural adhesive applications on currently flying aerospace vehicles alone could fill its own book. Hence this chapter will concentrate on the aerospace commercial transport industry and its use of adhesives in structural applications, both metallic and composite. Both primary structure, that is structure which carries primary flight loads and failure of which could result in loss of vehicle, and secondary structure will be considered. Structural adhesives use and practice in the military aircraft and launch vehicle/spacecraft fields as well as non-structural adhesives used on commercial aircraft will be touched on briefly as well. 

It is clear that European civil aircraft manufacturers adopted adhesive bonding for major structural elements much more rapidly than their American counterparts, but it is difficult to determine exactly why. Certainly a number of contributing factors are obvious. One was a history of success in incorporating adhesively bonded structure in military aircraft such as the Mosquito. Although the Mosquito was the most unusual and extreme example of adhesively bonded structure, other European wartime aircraft contained bonded structure as well. American military craft of the time were almost exclusively riveted aluminum structure. 

The final section in this volume deals with applications of adhesion science. The applications described include methods by which durable adhesive bonds can be manufactured by the use of appropriate surface preparation (Davis and Venables) to unique methods for composite repair (Lopata et al.) Adhesive applications find their way into the generation of wood products (Dunky and Pizzi) and also find their way into the construction of commercial and military aircraft (Pate). The chapter by Spotnitz et al. shows that adhesion science finds its way into the life sciences in their discussion of tissue adhesives. 

The advent of advanced fiber-reinforced composite materials has been called the biggest technical revolution since the jet engine [1-4], This claim is very striking because the tremendous impact of the jet engine on military aircraft performance is readily apparent. The impact on commercial aviation is even more striking because the airlines stwitched from propeller-driven planes to all-jet fleets within the span of just a few years because of superior performance and lower maintenance costs. 

Boron itself has been used for over two decades in filament form in various composites BO3/H2 is reacted at 1300° on the surface of a continuously moving tungsten fibre 12/tm in diameter. US production capacity is about 20 tonnes pa and the price in about 80(. The primary use so far has been in military aircraft and space shuttles, but boron fibre composites are also being studied as reinforcement materials for commercial aircraft. At the domestic level they are finding increasing application in golf shafts, tennis rackets and bicycle frames. 

Another example of the importance of the VI is the need for a high viscosity index hydraulic oil for military aircraft, since hydraulic control systems may be exposed to temperatures ranging from below — 65°F at high altitudes to over 100°F on the ground. For the proper operation of the hydraulic control system, the hydraulic fluid must have a sufficiently high VI to perform its functions at the extremes of the expected temperature range. 

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