Inertial navigation units INUs

The most precise dead-reckoning system is an inertial navigation unit in which accelerometers measure acceleration while aviation gyroscopes measure the orientation of the accelerometers. An on-board computer resolves the accelerations into navigation coordinates and integrates them to obtain velocity and position. The gyroscopes and accelerometers are mounted either fastened directly to the airframe, in which case the sensors are exposed to the maximum angular rates and accelerations, or on a servo-stabilized platform with gimbals that angularly isolate them from rotations. Attitude is then measured directly from the gimbal angles.

Inertial gyroscopes

Older aviation gyroscopes contained metal wheels rotating in ball bearings or gas bearings. More recent aviation gyroscopes contain rotating, vibrating rings whose frequency of oscillation measures angular rates. The newest inertial gyroscopes are evacuated cavities or optical fibres in which counter-rotating laser beams are compared in phase to measure the sensor’s angular velocity relative to inertial space. Inertial navigation units or gyroscopes are used aboard airliners and in most military fixed-wing aircraft.

Inertial measurement units

Inertial measurement units using mems-based sensors like accelerometers and gyroscopes are becoming more and more commonplace for tactical applications requiring short term navigation. In a practical inertial navigation system, there is very little actual movement of the mass. The relative displacement between the mass and housing is sensed by an electrical pick-off signal. A closed loop servomechanism feedback signal (proportional to acceleration) is then amplified and used to restrain the mass in the null position. The amount of feedback required to maintain the null position is proportional to the sensed acceleration; this becomes the accelerometer’s output signal. By combining two accelerometer outputs in the directions N-S and W-E, we can sum the vector outputs and calculate distance and velocity in the horizontal plane. By comparing the distance travelled, with the starting position, we can calculate our present position.
Since the aircraft will be operating through a range of pitch and roll manoeuvres, it is vital that when measuring acceleration in the N-S and WE directions, we do not measure the effects of gravity. The original inertial navigation systems maintained a physical platform such that it was always aligned with true north, and always level with respect to the earth’s surface. Electromechanical gyros and torque motors mounted within gimbals in each of the three axes achieved these requirements. The platform is aligned with true north and levelled at the beginning of the flight; this condition is maintained throughout the flight.
A by-product of aligning and levelling the platform is that attitude information is available for use by flight instruments and other systems. Modern day commercial aircraft inertial navigation systems are equipped with strapdown devices including solid-state gyros and accelerometers. The alignment process and attitude compensation is now achieved in the computer’s software, i.e. there is no physical platform.

Find out from the experts featuring in aviation-database. http://www.aviation-database.com/Inertial_Aerosystems_UK_inertial_sensors.htm is all about  MEMS-based sensors available from Inertial Aerosystems UK.

as9100 rev b composites manufacturers

Basic knowledge for as9100 rev b composites manufacturers is that the structural efficiency of aerospace composite structures is established, with very few exceptions, by joints, not by basic structure. The possible joining method for composites is as broad as with metals, namely riveting. bolting, pinning, and bonding. The only metal joining processes that are not suitable for thermoset composites are welding and brazing. However, thermoplastic and metallic matrix composites can be joined by welding or brazing. Composites are mechanically fastened in a manner similar to metals. Parts are drilled, countersunk, and joined with a fastener. Rivets, pins, two-piece bolts, and blind fasteners made of titanium, stainless steel, and aluminium are all used for composites.

The properties of as9100 composite components

Aluminium and stainless fasteners expand and contract when exposed to temperature extremes, as in aircraft applications. In carbon-fibre composites, contraction and expansion of such fasteners can cause changes in clamping load. Pressure within the joint is often critical. Drilling and machining can damage as9100 composite components. Several techniques exist for producing quality holes in composites. Carbon, aramid, and boron fiber reinforced materials each require different drilling methods and tools. When composites are cut, fibres are exposed. These fibres can absorb water, which weakens the material. Sealants can be used to prevent moisture absorption in the clearance hole. Sleeved fasteners can also provide fits that reduce water absorption as well as provide tightness. Fastener holes should be straight and round within the limits specified. Normal hole tolerance is 0.075 mm (0.003 in). Interference fits may cause delamination of the composite. Holes should be drilled perpendicular to the sheet within one degree. Special sleeved fasteners can limit the chances of damage in the clearance hole and still provide an interference fit. Fasteners can also be bonded in place with adhesives to reduce fretting. Galvanic corrosion may occur in carbon fibre composites if aluminium fasteners are used, due to the chemical reaction of the aluminium with the carbon. Coating the fastener guards may prevent corrosion but adds cost and time to the assembly process. As a result, aluminium fasteners are often replaced by more expensive titanium and stainless steel fasteners in carbon fibre composite joints.
When joining as9100 composite components with mechanical fasteners, special consideration must be given to creep. There are two kinds of creep: creep of the fastener hole and long term material compression. The greater the material modulus, the lower the creep. There are mechanical ways to reinforce the hole or distribute the load so that the creep problem is minimized. For fasteners that rely on inserts, the ability of the composite to retain the fastener must be considered.
Like mechanically fastened metal structures, composite components exhibit failure modes in tension, shear and bearing but, because of the complex failure mechanisms of composites, two further modes are possible, namely cleavage and pullout. Environmental degradation of a bolted joint, after exposure to hot, wet environment is most likely to occur in the shear and bearing strength properties. The evidence shows that for fibre reinforced epoxies, temperature has a more significant effect than moisture, but in the presence of both at 127°C, a strength loss of up to 40 percent is possible.
Evidence suggests that the failure behaviour of thermoplastics is much the same as for thermoset composites. High joint efficiencies can be obtained with suitable consideration to the joint design, fastener type, and environmental factors. In addition to mechanical and adhesive joining, thermoplastic composites can also be heat welded depending on the concentration and type of fillers within the composite.

For a technical article on advanced composites and removable aircraft structures see http://www.aviation-database.com/aviation_database_pages1/Aerospace-Composites-in-airline-service.html

Aircraft structures and stress.

The significant parts of an aircraft structure are the wings, the fuselage or hull, the landing gear and the moving operational parts, and control surfaces such as flaps, rudder, elevators and ailerons.
The wings are subjected to the highest levels, and also the most complex variation, of stresses. When the plane is on the ground the wings hang down due to self-weight, the weight of fuel stored inside them and the weight of the engines, if these are wing-mounted. The upper wing surfaces are then in tension and the lower surfaces in compression. This loading pattern continues whilst the plane is taxiing, which represents a significant contribution to the service life of the plane. However, the largest forces on the wings occur when the plane is airborne. Since the wings must then support the whole weight of the aircraft the steady stresses are high, and with the wings bending upwards the upper surfaces are in compression and the underside in tension. Each wing therefore acts as a cantilever with the maximum bending moment occurring at the wing roots. If the engines are mounted on the wings, then engine weight, together with weight of undercarriage and fuel, oppose the lift force and in a small way reduce its effects. Superimposed on to the steady stresses are fluctuating stresses which are complex in form and origin. They occur mainly at low altitude where the air is dense, as a result of manoeuvring or gusty weather conditions, but may also be due to clear air turbulence at higher altitudes. In general, however, aircraft cruising at high altitude should not be subject to extreme stress fluctuations: these will tend to be restricted to take-off, climb, approach and landing. In military combat aircraft fluctuations of stress are liable to be higher due to the requirement for frequent and fast manoeuvring. In contrast, a high-altitude military reconnaissance plane should be subject to few fluctuations. Since fatigue crack growth is favoured by tensile, but less so by compressive, stresses the upper and lower wing surfaces have different materials requirements. The main requirement of the upper wing surface is for resistance to compression. In the lower wing surface, whilst there is clearly a requirement for static tensile strength, the more critical need is for resistance to fatigue. Additional properties are, of course, required in both surfaces but these depend to some extent upon the method of manufacture. The earlier method for manufacturing wings is to thread two wing spars, or girders, through the fuselage. These may stretch from wing-tip to wing-tip, but in larger aircraft they may be attached, as separate spars for each wing, to the fuselage which is then strengthened in the appropriate position by a special bulkhead. The aerofoil surface is provided by a skin which is attached by rivets to the spars and ribs. Modern methods of construction aim at diffuse load paths which are more easily achieved when the wings are formed by machining from a solid plate. In this case there is an additional requirement for good resistance to stress corrosion since the parent plate must be thick and this, together with the fact that it has to be heat-treated, means that complete freedom from residual stresses cannot be guaranteed. The tensile nature of the stresses also introduces a requirement for fracture toughness. There is also a need for stiffness to resist bending and buckling.
The fuselage, or body, is a long approximately cylindrical shell, closed at its ends, which carries the whole of the payload. The effect of the payload, acting vertically downwards and supported by the wings at a nearly mid-length position, is to subject the shell to considerable bending. The lower part of the body, especially beneath the wings, is therefore subject to compression whilst the upper fuselage correspondingly experiences tension. In addition, when the aircraft rolls, this applies torsion to the fuselage. In aircraft which fly at altitude the cabin must be pressurized, and this subjects the shell to additional longitudinal and circumferential tension. Since the fuselage then becomes a pressure vessel there is not only a requirement for static tensile strength but also for fracture toughness and damage tolerance. Furthermore, the need for pressurization and de-pressurization once per flight establishes a critical requirement for resistance to low-cycle fatigue.
In aircraft designed to fly at very high speeds, airframe surfaces and especially leading edges are heated by interaction with the air. Stresses in landing gears are high on take-off when the aircraft has a full load of fuel. Fatigue loads can be generated during taxiing and there are high static loads on the undercarriage components of military aircraft standing on an airfield at a high state of readiness with a full fuel load. The greatest stress, however, occurs on landing as the weight of the whole aircraft hits the ground on touchdown. Whilst the vertical descent velocity of more than 1 m/sec (>2 mph) is not high enough to rate as shock loading, the stresses are very high. During flight, the landing gear must be retracted into the wings or body so as not to spoil the aerodynamic performance of the aircraft, and it follows that these parts must take up as little space as possible. Performance criteria must therefore be referred to the minimum volume condition. Because density-compensated property values are not then appropriate, landinggear components must be manufactured from materials with the highest commercially available levels of static strength, low-cycle fatigue strength and fracture toughness. Since the critical components are often heat-treated forgings there is also a need for good resistance to stress corrosion.

Aircraft control services.

The control surfaces consist of the rudder, elevators, ailerons and flaps. These parts are; in general, lightly loaded so that static strength is not a major requirement although the flaps must be sufficiently robust to withstand flying debris from the runway. Control surfaces are rather thin components which are still commonly of skin and stringer construction, and in view of the function they have to perform must be provided with adequate stiffness. Any control surface in the vicinity of an engine exhaust will also need to possess temperature resistance. Further, if there is any direct influence from engine noise the resulting acoustic loading of structural panels will demand good resistance to high cycle fatigue.

Manufacturers of aircraft structures are listed and further technical editorial on this subject is to be found at http://www.aviation-database.com/aviation_database_pages1/Aircraft-structures-and-the-use-of-aerospace-composites.html

With current commercial airliner sales showing a marked upward trend, and operators replacing ageing aircraft that have served them well for the last 15 to 20 years, the market for furnishing equipment, notably galley units, has rarely been so buoyant. Old stainless steel structures, heavy but reliable, are being replaced by incredibly lightweight units made from immensely strong honeycomb bonded structures, in materials such as Fibrelam made by Ciba-Geigy, which conform fully to the flammability and toxicity regulations which were introduced by the Civil Aviation Authority two or three years ago, to improve the chances for passenger escape in the event of aircraft fires.
The most obvious economic benefits to the airlines are in the weight saving and increased seat count.

Aircraft galleys

When Driessen-Zodiac Aerospace and Airbus signed an agreement on progressively introducing aircraft galleys and stowages as single-source supplier furnished equipment (SFE) on the A320 family of aircraft, the offer was based on a modular concept. This, optimising the initial configuration and easing later re-configurations, facilitated the re-marketability of the aircraft. Airbus A320 airline customers will also benefit from pre-assigned customer support conditions, which will complement the existing support of the Airbus customer services for all SFE cabin equipment. The advantage is perceived to be that SFE will improve the robustness of the supply chain, supporting the increase in production rates. The aircraft galleys and stowages will offer flexibility for airline-specific configurations. The new SFE approach will start with deliveries from mid-2012.

Galley Stowage Modification KSSU To ATLAS

Airlines use either Atlas or KSSU sized trays (KSSU stands for KLM, Swissair, Sabena and United). KSSU is used by very few operators now only about 10% and not just limited to the airlines listed but is also used by other Airlines, SAS for example. The ATLAS system is used for 80 to 90 % of aircraft trolleys.

To avoid aircraft galley equipment hassle, many airlines outsource catering and it is not unusual to see trolleys and galley boxes belonding to Airline A being installed on Aircraft B as all Atlas and KSSU equipment is interchangeable amongst operators using the same system. Trolleys of the KSSU type as well as trolleys of the ATLAS type are thus both standardised products. Both types of trolleys are provided along facing internal side walls with rail systems with horizontal rails parallel to one another. The rails are separated by a certain spacing in the vertical direction. In the case of trolleys of the KSSU type the spacing is a standard 30 mm and in the case of trolleys of the ATLAS type the spacing is a standard 60 mm. It will be clear that as a result of the smaller spacing, the KSSU trolleys can be provided with more drawers than the ATLAS trolleys for, apart from that, the same height of the trolleys. In the case of trolleys of the KSSU type, it is, moreover, possible as a result of the shorter spacing to achieve a higher degree of loading when using drawers that are shallow in the vertical direction than with an ATLAS trolley. If a shallow drawer, also termed a tray, with a height of approximately 5 mm is slid into an ATLAS trolley, approximately 55 mm of empty space, or at least space not occupied by a drawer, then remains above said tray in the trolley. In the case of a trolley of the KSSU type this distance will be approximately 25 mm.

Identification of Aerospace Metals.

In the US, The Society  of Automotive Engineers (SAE) and the American Iron and Steel Institute (A.I.S.I.) use a numerical index system to identify the composition of various steels. The numbers assigned in the combined listing of standard steels issued by these groups make it possible to readily identify the principal elements in the material. The first digit of the four number designation indicates the type to which the steel belongs. Thus “1″ indicates a carbon steel, “2″ a nickel steel, “3″ a nickel chromium steel, etc. In the case of simple alloy steels, the second digit indicates the approximate percentage of the predominant alloying element. The last two digits usually indicate the mean of the range of carbon content. Thus the symbol “1020″ indicates a plain carbon steel lacking a principal alloying element and containing an average of 0.2 percent (0.18 to 0.23) carbon. The symbol 2330 indicates a nickel steel of approximately 3 percent (3.25 to 3.75) nickel and an average of 0.30 percent, (0.28 to 0.33) carbon content. The symbol “4130″ indicates a chromium-molybdenum steel of approximately 1 percent (0.80 to 1.10) chromium, 0.20 percent 0.15 to 0.25) molybdenum, and 0.30 percent) carbon. The four digit series are for carbon and alloy steels.
 

Aerospace Aluminium.

To provide a visual means for identifying the various grades of aluminium and aluminium alloys, such metals are usually marked with symbols such as Government Specification Number, the temper or condition furnished, or the commercial code marking. Plate and sheet are usually marked with specification numbers or code markings in rows approximately 5 inches apart. Tubes, bars, rods, and extruded shapes are marked with specification numbers or  code markings at intervals of 3 to 5 feet along length of each piece. The commercial marking consists of a number which identifies the particular composition of the alloy.
  
Clad aluminium alloys have surface layers of pure aluminium or corrosion-resistant aluminium alloy bonded to the core material to inhibit corrosion. Presence of such a coating may be determined under a magnifying glass by examination of the edge surface which will show distinct layers.
It is possible to distinguish between some heat-treatable alloys and some nonheat-treatable alloys by immersing a sample in a 10 percent solution of caustic soda (sodium hydroxide). Those heat treated alloys containing several percent of copper (2014, 2017, and 2024) will turn black due to the copper content. High copper alloys when clad will not turn black on the surface, but the edges will turn black at the center of the sheet where the core is exposed. If the alloy does not turn black in the caustic soda solution it is not evidence that the alloy is not heat-treatable, as various high strength heat treatable alloys are not based primarily on the use of copper as an alloying agent. These include among others 6053, 6061, and 7075 alloys. The composition and heat-treatability of alloys which do not turn black in a caustic soda solution can be established only by chemical or spectro-analysis.
 

AS9120 approved suppliers

Companies supplying aerospace metals seek to be AS9120 approved suppliers. These companies and their contact details are available from www.aviation-database.com