Introduction To Aerospace

Engineering.Part-B

Written by…Venus Kumar (B.tech –

Aerospace).Supported by…

Sonu Kr. Gupta (B.Tech – Aerospace).

Reference by…Google and Wikkipedia

(Sometimes).

Aerospace Structures and Materials. General Types of

Construction and Structural Layout.

Flight Envelope and V-N Diagrams.Flight Evelope.In aerodynamics, the flight envelope, service envelope, or performance envelope of an aircraft refers to the capabilities of a design in terms of airspeed and load factor or altitude. The term is somewhat loosely applied, and can also refer to other measurements such as maneuverability. When a plane is pushed, for instance by diving it at high speeds, it is said to be flown "outside the envelope", something considered rather dangerous.Flight envelope is one of a number of related terms that are all used in a similar fashion. It is perhaps the most common term because it is the oldest, first being used in the early days of test flying. It is closely related to more modern terms known as extra power and a doghouse plot which are different ways of describing a flight envelope. In addition, the term has been widened in scope outside the field of engineering, to refer to the strict limits in which an event will take place or more generally to the predictable behavior of a given phenomenon or situation, and hence, its "flight envelope".

Flight envelope diagram.

Extra power.Extra power, or Specific Excess Power, is a very basic method of determining an aircraft's flight envelope. It is easily calculated, but as a downside does not tell very much about the actual performance of the aircraft at different altitudes.

Doghouse plot.A doghouse plot generally shows the relation between speed at level flight and altitude, although other variables are also possible. It takes more effort to make than an extra power calculation, but in turn provides much more information such as ideal flight altitude. The plot typically looks something like an upside-down U and is commonly referred to as a doghouse plot due to its resemblance to a doghouse. The diagram on the right shows a very simplified plot which shall be used to explain the general shape of the plot.

Altitude envelope.

The outer edges of the diagram, the envelope, show the possible conditions that the aircraft can reach in straight and level flight. For instance, the aircraft described by the black altitude envelope on the right can fly at altitudes up to about 52,000 feet, at which point the thinner air means it can no longer climb. The aircraft can also fly at up to Mach 1.1 at sea level, but no faster. This outer surface of the curve represents the zero-extra-power condition. All of the area under the curve represents conditions that the plane can fly at with power to spare, for instance, this aircraft can fly at Mach 0.5 at 30,000 feet while using less than full power.

In the case of high-performance aircraft, including fighters, this "1-g" line showing straight-and-level flight is augmented with additional lines showing the maximum performance at various g loadings. In the diagram at right, the green line represents, 2-g, the blue line 3-g, and so on. The F-16 Fighting Falcon has a very small area just below Mach 1 and close to sea level where it can maintain a 9-g turn.

Turn rate envelope. Flying outside the envelope is possible, since it represents the straight-and-level

condition only. For instance diving the aircraft allows higher speeds, using gravity as a source of additional power. Likewise higher altitude can be reached by first speeding up and then going ballistic, a maneuver known as a zoom climb.

Stalling Speed. All fixed-wing aircraft have a minimum speed at which they can maintain level flight, the

stall speed (left limit line in the diagram). As the aircraft gains altitude the stall speed increases; since the wing is not growing any larger the only way to support the aircraft's weight with less air is to increase speed. While the exact numbers will vary widely from aircraft to aircraft, the nature of this relationship is typically the same; plotted on a graph of speed (x-axis) vs. altitude (y-axis) it forms a diagonal line.

Service ceiling. Inefficiencies in the wings also make this line "tilt over" with increased altitude, until it

becomes horizontal and no additional speed will result in increased altitude. This maximum altitude is known as the service ceiling (top limit line in the diagram), and is often quoted for aircraft performance. The area where the altitude for a given speed can no longer be increased at level flight is known as zero rate of climb and is caused by the lift of the aircraft getting smaller at higher altitudes, until it no longer exceeds gravity.

Top speed. The right side of the graph represents the maximum speed of the aircraft. This is typically

sloped in the same manner as the stall line due to air resistance getting lower at higher altitudes, up to the point where an increase in altitude no longer increases the maximum speed due to lack of oxygen to feed the engines.

The power needed varies almost linearly with altitude, but the nature of drag means that it varies with the square of speed—in other words it is typically easier to go higher than faster, up to the altitude where lack of oxygen for the engines starts to play a significant role.

Velocity vs. Load factor chart. A chart of velocity versus load factor (or V-n diagram) is another way of showing

limits of aircraft performance. It shows how much load factor can be safely achieved at different airspeeds.

A V-n diagram showing VS (Stall speed at 1G), VC (Corner/maneuver speed) and VD (Dive speed)

Monocoque. Monocoque is a structural approach whereby loads are supported through an object's external skin, similar to an egg shell, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork. The term is also used to indicate a form of vehicle construction in which the body and chassis form a single unit.. The technique may also be called structural skin. The word monocoque is a French term for "single shell" or (of boats) "single hull".

Semimonocoque. The term semi-monocoque refers to a stressed shell structure that is similar to a true monocoque, but which derives at least some of its strength from conventional reinforcement. Semi-monocoque construction is used for, among other things, aircraft fuselages, car bodies and motorcycle frames.The Semimonocoque system uses a substructure to which the airplane´s skin is attached. The substructure, which consists of bulkheads and/or formers of various sizes and stringers, reinforces the stressed skin by taking some of the bending stress from the fuselage. The semimonocoque is the most often used construction for modern, high-performance aircraft. Semimonocoque literally means half a single shell. Here, internal braces as well as the skin itself carry the stress . The internal braces include longitudinal (lengthwise) members called stringers and vertical bulkhead.

ARV Super2 with semi-monocoque fuselage.

Semi-monocoque aircraft fuselages differ from true monocoque construction through being reinforced with longitudinal stringers.

Corrugated.

The term corrugated, describing a series of parallel ridges and furrows, may refer to the following:

Corrugated fiberboard also called corrugated cardboard. Corrugated galvanised iron , a building material composed of sheets of cold-rolled

hot-dip galvanised mild steel. Corrugated plastic , a wide range of extruded twinwall plastic-sheet products

produced from high-impact polypropylene resin.

Corrugated fiberboard.Corrugated fiberboard is a paper-based material consisting of a fluted corrugated sheet and one or two flat linerboards. It is made on "flute lamination machines" or "corrugators" and is used in the manufacture of shipping containers and corrugated boxes.The corrugated medium and linerboard board are made of kraft containerboard, a paperboard material usually over 0.01 inches (0.25 mm) thick. Corrugated fiberboard is sometimes called corrugated cardboard, although cardboard might be any heavy paper-pulp based board.

Corrugated fiberboard.

Properties. Corrugated fiberboard has a higher stiffness (resistance to bending) than flat

fiberboard of equal mass, especially parallel to the corrugations. This can be explained by the Theorema Egregium. The pleated board is roughly isometric to a flat plane, which has a Gaussian curvature of 0. Since the material is curved in one direction, perpendicular to the pleats, it must remain flat in the direction parallel to the pleats.

Corrugated galvanised iron or steel (colloquially corrugated iron or pailing (in Caribbean English), occasionally abbreviated CGI) is a building material composed of sheets of hot-dip galvanised mild steel, cold-rolled to produce a linear corrugated pattern in them. The corrugations increase the bending strength of the sheet in the direction

perpendicular to the corrugations, but not parallel to them. Normally each sheet is manufactured longer in its strong direction.CGI is lightweight and easily transported. It was and still is widely used especially in rural and military buildings such as sheds and water tanks. Its unique properties were used in the development of countries like Australia from the 1840s, and it is still helping developing countries today.

Corrugated galvanised iron roofing in Mount Lawley, Western Australia

A corrugated iron church (or tin tabernacle) in Kilburn, London

Typical corrugated galvanised iron appearance, with visible large flake type patterns. Thegalvanised sheet has been repaired or reinforced with a piece of angle iron (painted white).

Corrugated plastic.Corrugated plastic or corriboard - also known under the tradenames of Polyflute, Coroplast, FlutePlast, IntePro, Proplex, Correx, Twinplast, Corriflute or Corflute - refers to a wide range of extruded twinwall plastic-sheet products produced from high-impact polypropylene resin with a similar make-up to corrugated fiberboard. It is a light-weight tough material which can easily be cut with a utility knife.

Manufacturers typically offer a wide variety of colors and thicknesses (quite commonly 3, 4, 5 mm).Corrugated plastic made of polycarbonate is sometimes referred to as Twinwall plastic.Chemically, the sheet is inert, with a neutral pH factor. At regular temperatures most oils, solvents and water have no effect, allowing it to perform under adverse weather conditions or as a product component exposed to harsh chemicals. Standard sheets can be modified with additives, which are melt-blended into the sheet to meet specific needs of the end-user. Special products that require additives include: ultra-violet protection, anti-static, flame retardant, custom colors, corrosive inhibitors, static-dissipative, among others.This material is commonly used to erect commercial, political or other types of signs and for constructing plastic containers and reusable packaging. It is widely used in the signwriting industry for making signs for real estate sales, construction sites and promotions.The last decade has found its increasing use among guinea pig, rabbit, domesticated hedgehog and other small pet enthusiasts as components of DIY cages. Additionally, it is used by members of the remote-controlled aircraft community to build nearly indestructible SPAD model aircraft.

Corrugated plastic box used as reusable packaging and Corrugated plastic dividers used to packautomotive components.

Sandwich Structure. In combination with two skins applied on the honeycomb, the structure offers a sandwich panel with excellent rigidity at minimal weight. The behavior of the honeycomb structures is orthotropic, hence the panels react differently depending on the orientation of the structure. Therefore it is necessary to distinguish between the directions of symmetry, the so-called L and W-direction. The L-direction is the strongest and the stiffest direction. The weakest direction is at 60° from the L-direction (in the case of a regular hexagon) and the most compliant direction is the W-direction. Another important property of honeycomb sandwich core is its compression strength. Due to the efficient hexagonal configuration, where walls support each other, compression strength of honeycomb cores is typically higher (at same weight) compared to other sandwich core structures such as, for instance, foam cores or corrugated cores.

Composite sandwich structure panel used for testing at NASA.

Types of sandwich structures.Metal composite material (MCM) is a type of sandwich formed from two thin skins of metal bonded to a plastic core in a continuous process under controlled pressure, heat, and tension.Recycled paper is also now being used over a closed-cell recycled kraft honeycomb core, creating a lightweight, strong, and fully repulpable composite board. This material is being used for applications including point-of-purchase displays, bulkheads, recyclable office furniture, exhibition stands, and wall dividers.

To fix different panels, among other solutions, a transition zone is normally used, which is a gradual reduction of the core height, until the two fiber skins are in touch. In this place, the fixation can be made by means of bolts, rivets, or adhesive.

With respect to the core type and the way the core supports the skins, sandwich structures can be divided into the following groups: homogeneously supported, locally supported, regionally supported, unidirectionally supported, bidirectionally supported. The latter group is represented by honeycomb structure which, due to an optimal performance-to-weight ratio, is typically used in most demanding applications including aerospace.

Properties of sandwich structures.The strength of the composite material is dependent largely on two factors:

1. The outer skins: If the sandwich is supported on both sides, and then stressed by means of a downward force in the middle of the beam, then the bending moment will introduce shear forces in the material. The shear forces result in the bottom skin in tension and the top skin in compression. The core material spaces these two skins apart. The thicker the core material the stronger the composite. This principle works in much the same way as an I-beam does.

2. The interface between the core and the skin: Because the shear stresses in the composite material change rapidly between the core and the skin, the adhesive

layer also sees some degree of shear force. If the adhesive bond between the two layers is too weak, the most probable result will be delamination.

Application of sandwich structures.

The composite honeycomb structure of a helicopter nozzle.

Sandwich structures can be widely used in sandwich panels, this kinds of panels can be in different types such as FRP sandwich panel, aluminium composite panel etc. FRP polyester reinforced composite honeycomb panel (sandwich panel) is made of polyester reinforced plastic, multi-axial high-strength glass fiber and PP honeycomb panel in special antiskid tread pattern mold through the process of constant temperature vacuum adsorption & agglutination and solidification.

Theory.Sandwich theory describes the behaviour of a beam, plate, or shell which consists of three layers - two facesheets and one core. The most commonly used sandwich theory is linear and is an extension of first order beam theory. Linear sandwich theory is of importance for the design and analysis of sandwich panels, which are of use in building construction, vehicle construction, airplane construction and refrigeration engineering.

Reinforced and Honeycomb Structures.

Honeycomb structure.Honeycomb structures are natural or man-made structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal material cost. The geometry of honeycomb

structures can vary widely but the common feature of all such structures is an array of hollow cells formed between thin vertical walls. The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of-plane shear properties.

Man-made honeycomb structural materials are commonly made by layering a honeycomb material between two thin layers that provide strength in tension. This forms a plate-like assembly. Honeycomb materials are widely used where flat or slightly curved surfaces are needed and their high Specific strength is valuable. They are widely used in the aerospace industry for this reason, and honeycomb materials in aluminum, fibreglass and advanced composite materials have been featured in aircraft and rockets since the 1950s. They can also be found in many other fields, from packaging materials in the form of paper based honeycomb cardboard, to sporting goods like skis and snowboards.

A composite sandwich panel (A) with honeycomb core (C) and face sheets (B).

Application.Composite honeycomb structures have been used in numerous engineering and scientific applications.

Applicationarea

Industry Company/Product

Racing shells Sport Vespoli, Janousek Racing Boats.

Aerospacemanufacturing

Aerospace Hexcel, Neomet Ltd.

Gliders Aerospace Schleicher ASW 19, Solar Impulse Project.

Helicopters Aerospace Kamov Ka-25, Bell 533, Westland Lynx.

Jet aircraft Aerospace General Dynamics/Grumman

F-111B, F-111 Aardvark, all

commercial airlines since the Boeing 747.

Rocketsubstructure

Aerospace Saturn V Instrument Unit,Mars Exploration Rover, S-520.

LEDTechnology

Lighting SmartSlab.

Loudspeakertechnology

Audio Loudspeaker#Driver design, Woofer.

Telescopemirror

structure

Aerospace Hubble Space Telescope.

Automobilestructure

Automotive Panther Solo, Dome F105, Bluebird-Proteus CN7,

BMW i3 / i8, Koenigsegg Agera.

Snowboards Sports Snowboard.Furniture; woodworking furniture.RepulpableExhibitionstands;

Construction Repulpable Exhibition Stands.

More recent developments show that honeycomb structures are also advantageous in applications involving nanohole arrays in anodized alumina, microporous arrays in polymer thin films, activated carbon honeycombs, and photonic band gap honeycomb structures.

Aerodynamics.A honeycomb mesh is often used in aerodynamics to reduce or to create wind turbulence. It is also used to obtain a standard profile in a wind tunnel (temperature, flow speed). A major factor in choosing the right mesh is the length ratio (length vs honeycomb cell diameter) L/d.Length ratio < 1: Honeycomb meshes of low length ratio can be used on vehicles front grille. Beside the aesthetic reasons, these meshes are used as screens to get a uniform profile and to reduce the intensity of turbulence.Length ratio >> 1: Honeycomb meshes of large length ratio reduce lateral turbulence and eddies of the flow. Early wind tunnels used them with no screens; unfortunately, this method introduced high turbulence intensity in the test section. Most modern tunnels use both honeycomb and screens.

Honeycomb grille used on a computer fan to reduce noise.

Honeycombed, screened center for Langley's first wind tunnel.

While aluminium honeycombs are common use in the industry, other materials are offered for specific applications. People using metal structures should take care of removing burrs as they can introduce additional turbulences. Polycarbonate structures are a low-cost alternative.The honeycombed, screened center of this open-circuit air intake for Langley's first wind tunnel ensured a steady, non-turbulent flow of air. Two mechanics pose near the entrance end of the actual tunnel, where air was pulled into the test section through a honeycomb arrangement to smooth the flow.Honeycomb is not the only cross-section available in order to reduce eddies in an airflow. Square, rectangular, circular and hexagonal cross-sections are other choices available, although honeycomb is generally the preferred choice.

Properties.In combination with two skins applied on the honeycomb, the structure offers a sandwich panel with excellent rigidity at minimal weight. The behavior of the honeycomb structures is orthotropic, hence the panels react differently depending on the orientation of the structure. Therefore it is necessary to distinguish between the directions of symmetry, the so-called L and W-direction. The L-direction is the strongest and the stiffest direction. The weakest direction is at 60° from the L-direction (in the case of a regular hexagon) and the most compliant direction is the W-direction. Another important property of honeycomb sandwich core is its compression strength. Due to the efficient hexagonal configuration, where walls support each other, compression strength of

honeycomb cores is typically higher (at same weight) compared to other sandwich core structures such as, for instance, foam cores or corrugated cores.

Geodesic Construction. A geodesic (or geodetic) airframe is a type of construction for the airframes of aircraft developed by British aeronautical engineer Barnes Wallis in the 1930s. It makes use of a space frame formed from a spirally crossing basket-weave of load-bearing members. The principle is that two geodesic arcs can be drawn to intersect on a curving surface (the fuselage) in a manner that the torsional load on each cancels out that on the other.

Aeroplanes.The earliest-known use of a geodesic airframe design for any aircraft was for the pre-World War I Schütte-Lanz SL1 rigid airship's envelope structure of 1911, with the airship capable of up to a 38.3 km/h (23.8 mph) top airspeed.Barnes Wallis, inspired by his earlier experience with light alloy structures and the use of geodesically-arranged wiring to distribute the lifting loads of the gasbags in the design of the R100 airship, evolved the geodetic construction method (although it is commonly stated, there was no geodetic structure in R100). Wallis used the term "geodetic" to apply to the airframe and distinguish it from "geodesic" which is the proper term for a line on a curved surface, arising from geodesy.The system was later used by Wallis's employer, Vickers-Armstrongs in a series of bomber aircraft, the Wellesley, Wellington, Warwick and Windsor. In these aircraft, the fuselage was built up from a number of duralumin alloy channel-beams that were formed into a large framework. Wooden battens were screwed onto the metal, to which the doped linen skin of the aircraft was fixed.

Wellington Mk.X HE239 of No.428 Sqn. RCAF, illustrating the geodesic construction and the level of punishment it could absorb while maintaining integrity and airworthiness.

A section of the rear fuselage from a Warwick showing the geodesic construction in duralumin. On exhibit at the Armstrong & Aviation Museum at Bamburgh Castle.

The metal lattice-work gave a light structure with tremendous strength; any one of the stringers could support some of the load from the opposite side of the aircraft. Blowing out the structure from one side would still leave the load-bearing structure as a whole intact. As a result, Wellingtons with huge areas of framework missing continued to return home when other types would not have survived; the dramatic effect enhanced by the doped fabric skin burning off, leaving the naked frames exposed (see photo). The benefits of the geodesic construction were partly offset by the difficulty of modifying the physical structure of the aircraft to allow for a change in length, profile, wingspan etc.

Aerospace Materials. Aerospace materials are materials, frequently metal alloys, that have either been developed for, or have come to prominence through, their use for aerospace purposes.

Titanium support structure for a jet engine thrust reverser.

These uses often require exceptional performance, strength or heat resistance, even at the cost of considerable expense in their production or machining. Others

are chosen for their long-term reliability in this safety-conscious field, particularly for their resistance to fatigue.

Metallic and Non-Metallic Materials.

Uses of Aluminum Alloy. Titanium.

Stainless Steel. Composite and Ceramic

Material.Composite Material.

A composite material (also called a composition material or shortened to composite) is a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure. The new material may be preferred for many reasons: common examples include materials which are stronger, lighter, or less expensive when compared to traditional materials.

Typical engineered composite materials include:

Composite building materials, such as cements, concrete Reinforced plastics , such as fiber-reinforced polymer Metal composites Ceramic composites (composite ceramic and metal matrices)

Composite materials are generally used for buildings, bridges, and structures such as boat hulls, swimming pool panels, race car bodies, shower stalls, bathtubs, storage tanks, imitation granite and cultured marble sinks and countertops. The most advanced examples perform routinely on spacecraft and aircraft in demanding environments.

Composites are formed by combining materials together to form an overall structure that is better than the sum of the individual components.

Physical properties.

The physical properties of composite materials are generally not isotropic (independent of direction of applied force) in nature, but rather are typically anisotropic (different depending on the direction of the applied force or load). For instance, the stiffness of a composite panel will often depend upon the orientation of the applied forces and/or moments. Panel stiffness is also dependent on the design of the panel. For instance, the fibre reinforcement and matrix used, the method of panel build, thermoset versus thermoplastic, type of weave, and orientation of fibre axis to the primary force.In contrast, isotropic materials (for example, aluminium or steel), in standard wrought forms, typically have the same stiffness regardless of the directional orientation of the applied forces and/or moments.The relationship between forces/moments and strains/curvatures for an isotropic material can be described with the following material properties: Young's Modulus, the shear Modulus and the Poisson's ratio, in relatively simple mathematical relationships. For the anisotropic material, it requires the mathematics of a second order tensor and up to 21 material property constants. For the special case of orthogonal isotropy, there are three different material property constants for each of Young's Modulus, Shear Modulus and Poisson's ratio—a total of 9 constants to describe the relationship between forces/moments and strains/curvatures.Techniques that take advantage of the anisotropic properties of the materials include mortise and tenon joints (in natural composites such as wood) and Pi Joints in synthetic composites.

Ceramic Material.

Instruments and Navigation. Basic Instrumentation.

Electronics ( DC Electronics, AC Electronics, Semiconductors, Electro-Optics and Digital Electronics).

Electronics.Electronics is the science of how to control electric energy, energy in which the electrons have a fundamental role. Electronics deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive electrical components and interconnection technologies. Commonly, electronic devices contain circuitry consisting primarily or exclusively of active semiconductors supplemented with passive elements; such a circuit is described as an electronic circuit.

Surface-mount electronic componentThe nonlinear behaviour of active components and their ability to control electron flows makes amplification of weak signals possible, and electronics is widely used in information processing, telecommunication, and signal processing.Electronics is distinct from electrical and electro-mechanical science and technology, which deal with the generation, distribution, switching, storage, and conversion of electrical energy to and from other energy forms using wires, motors, generators, batteries, switches, relays, transformers, resistors, and other passive components.

DC Electronics.AC Electronics.Semiconductors.Electro-Optics.Digital Electronics.

Sensing Devices.

a sensor is an object whose purpose is to detect events or changes in its environment, and then provide a corresponding output. A sensor is a type of transducer; sensors may provide various types of output, but typically use electrical or optical signals. For example, a thermocouple generates a known voltage (the output) in response to its temperature (the environment). A mercury-in-glass thermometer, similarly, converts measured temperature into expansion and contraction of a liquid, which can be read on a calibrated glass tube.

Bridge Circuits. A bridge circuit is a type of electrical circuit in which two circuit branches (usually in parallel with each other) are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge was originally developed for laboratory measurement purposes and one of the intermediate bridging points is often adjustable when so used. Bridge circuits now find many applications, both linear and non-linear, including in instrumentation, filtering and power conversion.

Schematic of a Wheatstone bridgeThe best-known bridge circuit, the Wheatstone bridge, was invented by Samuel Hunter Christie and popularized by Charles Wheatstone, and is used for measuring resistance. It is constructed from four resistors, two of known values R1 and R3 (see diagram), one whose resistance is to be determined Rx, and one which is variable and calibrated R2. Two opposite vertices are connected to a source of electric current, such as a battery, and a galvanometer is connected across the other two vertices. The variable resistor is adjusted until the galvanometer reads zero. It is then known that the ratio between the variable resistor and its neighbour R1 is equal to the ratio between the unknown resistor and its neighbour R3, which enables the value of the unknown resistor to be calculated.

Wheatstone bridge 

Wien bridge 

Maxwell bridge 

H bridge 

Fontana bridge 

Diode bridge 

Kelvin bridge 

Lattice bridge 

Bridged T circuit 

Carey Foster bridge 

The Wheatstone bridge has also been generalised to measure impedance in AC circuits, and to measure resistance, inductance, capacitance, and dissipation factor separately. Various arrangements are known as the Wien bridge, Maxwell bridge and Heaviside bridge. All are based on the same principle, which is to compare the output of two potentiometers sharing a common source.In power supply design, a bridge circuit or bridge rectifier is an arrangement of diodes or similar devices used to rectify an electric current, i.e. to convert it from an unknown or alternating polarity to a direct current of known polarity.In some motor controllers, a H-bridge is used to control the direction the motor turns.

Optical Devices and Introduction to Computer Based Data Acquistion.

Optical Device. A device for producing or controlling light

Autofocus - an optical device for focussing a camera or other instrument automatically. Biprism - an optical device for obtaining interference fringes. Camera lucida - an optical device consisting of an attachment that enables an observer to

view simultaneously the image and a drawing surface for sketching it. Coelostat - optical device used to follow the path of a celestial body and reflect its light

into a telescope; has a movable and a fixed mirror. Collimator - optical device consisting of a tube containing a convex achromatic lens at

one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam.

Device - an instrumentality invented for a particular purpose; "the device is small enough to wear on your wrist"; "a device intended to conserve water".

Diffraction grating , grating - optical device consisting of a surface with many parallel grooves in it; disperses a beam of light (or other electromagnetic radiation) into its wavelengths to produce its spectrum.

Diffuser , diffusor - optical device that distributes the light of a lamp evenly. View finder , viewfinder, finder - optical device that helps a user to find the target of

interest. Kerr cell - optical device consisting of a transparent cell with two electrodes between two

polarizing media; passes light only if the two planes of polarization are parallel; used as a high-speed shutter or to modulate a laser beam.

Laser , optical maser - an acronym for light amplification by stimulated emission of radiation; an optical device that produces an intense monochromatic beam of coherent light.

Lens , Lens system, Lense - a transparent optical device used to converge or diverge transmitted light and to form images.

Nicol prism - optical device that produces plane-polarized light. Planetarium - an optical device for projecting images of celestial bodies and other

astronomical phenomena onto the inner surface of a hemispherical dome. Polarimeter , polariscope - an optical device used to measure the rotation of the plane of

vibration of polarized light. Optical prism , prism - optical device having a triangular shape and made of glass or

quartz; used to deviate a beam or invert an image. Projector - an optical device for projecting a beam of light. Rochon prism , Wollaston prism - optical device that produces plane-polarized ultraviolet

light. Stereoscope - an optical device for viewing stereoscopic photographs. Viewer - an optical device for viewing photographic transparencies.

Introduction to Computer Based Data Acquisition.

Measurements in Aerodynamics.

Aerodynamics Measurements. Measurements of ve locity, pressure, density, and temperature of moving air; also

measurements of the forces which act on the surface of a solid body relative to which the motion takes place and measurements of the heat transfer to this surface. The majority of practical problems which are being posed for aerodynamics by aviation, rocket technology, turbine construction, industrial production, and so on require for their solution the performance of experimental studies. In these investigations the flow being examined (for example, the motion of an airplane with given altitude and velocity values) is simulated on experimental installations (wind tunnels and testing units), and the force and thermal stresses on the model being studied are determined. The maintenance of the conditions which are dictated by the theory of modeling permits the conversion of results of experiments on the model to the full-scale object. The

measurement results are usually obtained as dimensionless aerodynamic coefficients that are functions of the basic similarity criteria—the Mach number, the Reynolds number, the Prandtl number, and so on—and in such a form are used for the determination of the lift and drag of the airplane, the heating of rocket and spacecraft surfaces, and so on.

Forces and moments. Measurements are made of the forces and moments which act on a body around

which there is a flow. In the solution of many problems, it becomes necessary to measure the total forces which act on the model. In wind tunnels, a wind tunnel balance is generally used for the determination of the magnitudes, directions, and points of application of aerodynamic force and moment. The aerodynamic force which acts on a freely flying model can be determined by measuring the acceleration of the model. The accelerations of flying models or full-scale objects in flight tests are measured by accelerometers. If the size of the model does not permit the installation of the necessary devices, then the acceleration is found by measurement of the velocity ν of the model along a trajectory.

The total aerodynamic force (moment) which acts on a body can be represented as the sum of independent normal and tangential forces on its surface. In order to obtain the value of the normal forces, the pressure on the surface of the model is measured by means of special apertures (called drainage apertures) which are connected to manometers by rubber or metal tubing (Figure 1). The type of manometer is chosen according to the magnitude of the pressure being measured and the required accuracy of the measurements.

Figure 1. Diagram of the measurement of static pressures on the surface of a model.

If the velocity of flow around the model is so high that the compressibility of the gas is affected, it is possible to find the density distribution of the gas near the surface of the model by optical methods, and then to calculate the pressure field and obtain the pressure distribution along the model’s surface. The forces tangent

to the surface of the model are usually determined by calculation; in certain cases special scales are utilized for their measurement.

Velocity of a gas flowing around a model. The velocity of gas in wind tunnels and during flow around airplanes, rockets, and

flying models is measured in the majority of cases by Prandtl tubes (probes). Manometers which are connected to Prandtl tubes measure the total (p0) and static (p) pressures of the flowing gas. The velocity of an incompressible gas is determined from Bernoulli’s equation

where ρ is the fluid density. If the velocity being measured is greater than the velocity of sound, a shock wave

develops in front of the probe, and the reading of the manometer connected with the Pitot tube will correspond to the magnitude of the total pressure behind the shock wave: p0‘ < P0 In this case the Mach number—rather than ν—is determined (by a special formula). In the measurement of supersonic velocities, sectioned probes are usually used for the measurement of the static pressure p and the total pressure p0’ behind the normal shock wave.

There are also methods which permit the measurement of gas velocity by the variation of the amount of heat which is discharged from the heated wire of a thermoanemometer, by the ratio of densities or temperatures in stagnated and flowing gas, or by the velocity of displacement of marked particles.

For the measurement of the relatively small velocities in industrial aerodynamics and meteorology, anemometers are used; the average velocity of the gas flowing in the tube can be obtained by measuring its flow rate with special flow meters. The velocity of a flying body can also be computed by measuring the traversal time of the body through a given segment of trajectory, by the Doppler effect, and by other methods.

Gas density. The basic methods of studying the density field of a gas can be divided into three

groups—methods based on the dependence of the index of light refraction on gas density, based on the absorption of radiant energy by the gas, and based on the afterglow of gas molecules in an electric discharge. The latter two groups of methods are applicable for the study of gas density at low pressure. From the first group, Töpler’s method (the “schlieren” method) and the interferometric method are used. In these methods, the dependency between the density ρ of the gas and the index of light refraction n is utilized for the measurement of the density:

In the flow of a compressible medium around a body, fields with nonuniform density distribution (density gradient fields) develop in regions where there are disturbances of the gas caused by the body around which the gas is flowing. Certain sections of the field with different density deflect light beams passing through them in different ways. A portion of the deflected beams does not pass

through the focal point of the Töpler device’s detector because it is cut off by an opaque plate—the so-called Foucault knife (Figure 2); as a result, there is a local variation of the illumination of the screen (photo plate). The photographs obtained permit a qualitative analysis of the nature of the flow around the model; on these photographs regions of significant variations of density are clearly visible—shock waves, zones of rarefaction, and so on.

Figure 2. Diagram of the Töpler device.

The shock waves, which are visible on the photograph in the form of thin lines, in reality are conical surfaces on which pressure, density, and temperature discontinuities in the air take place. During flow around the ring-shaped surface of the face of the cylinder, there is a takeoff of the boundary layer from the cone’s surface.

Quantitative data concerning the density of the gas and the magnitude of the variation (gradient) of density can be obtained by comparing by a microphotometer the variation in the illumination of the screen caused by the density gradient in the flow with the variation of illumination caused by a glass reference lens which is located outside the flow of the wind tunnel; points in the flow field and on the lens which have the same brightness have the same index of refraction. From the values of the refractive index in the flow field found in the above manner, the gas density and the magnitude of the density gradient are calculated for the whole field under investigation. In addition to the photometric method, other methods for the quantitative analysis of the density field are also used.

The interferometric method of investigation of gas flow is also based on the relation between the gas density and the refractive index. The Mach-Zender interferometer is usually used for this measurement. In the photograph obtained by this method, the regions of equal illumination correspond to regions of constant density. The interpretation of the photographs permits calculation of the density in the region of flow under investigation.

One of the important advantages of optical methods is the possibility of studying gas flows without using probes and tubes of various types which are sources of disturbance in the flow.

Temperature. In a high-velocity flow, two temperatures are usually considered—that of the

undisturbed flow T and that of the stopped flow T0 = T + v2 /2cp, where cρ is the specific thermal capacity of the gas at constant pressure in joules/(kg°K), v is in m/sec, and T and T0 are in 0k. It is obvious that T0— T as v — 0. In a viscous gas

flowing around a solid surface, the velocity on the wall is equal to zero and any immovable tube which is mounted in the airstream measures a temperature close to the stagnation temperature Ta. The readings of the device are corrected for the presence of heat leakage and such.

By means of tubes (Figure 3) in which the measuring element is usually a thermocouple or resistance thermometer, it is possible to measure a temperature T0 ≤ 1500 °K. For the measurement of higher temperatures of the stagnated or flowing gas, optical-luminosity and spectral methods are used.

Figure 3. Adaptor for measurement of the temperature of a retarded flow.

The static temperature T can be found from the relation of temperature and the velocity of sound, since a For the measurement of the velocity of sound, a source of acoustic vibrations of known frequency is mounted on the wall of a wind tunnel. The sound waves will be visible in the shadow photograph of the flow field. The velocity of sound is defined as a = fe, where e is the distance between waves and f is the frequency of the vibrations of the source (Figure 4).

Figure 4. Diagram of the measurement of gas temperature as a function of the speed of propagation of acoustical waves.

Tangential forces (friction) and thermal fluxes on the surface of a model.

For the determination of the shearing stresses τ and thermal flux q, it is possible to perform the measurement of velocity and temperature fields of a gas near the surface and find the unknown values by using the Newton equation for frictional stresses τ = μ (dvldy) and the equation of heat conduction q = λ (dT/dy) where μ and λ are the coefficients of dynamic viscosity and heat conductance of the gas, and dvfdy and dT/dy are the gradients of velocity and temperature near the surface of the body in the direction y, normal to the surface. Practically, it is impossible to obtain with sufficient accuracy the values dvldy and dT/dy as y → 0. Therefore, for the determination of the frictional forces and heat fluxes from the measurement of the velocity and temperature fields in the boundary layer, so-called integral methods are applied. In these methods the frictional force and the heat flow on the section of the surface under consideration are determined by measurements of the thickness of the boundary layer and the velocity and temperature profiles.

More accurate values for τ and q can be obtained by direct measurement. To do this, the tangential force ΔX on an element of surface ΔS is measured by special balances; the shearing stresses are defined as τ = ΔX/Δ S. Similarly, using calorimeters of various types, it is possible to measure the thermal flux q which flows through the element of the surface ΔS under consideration and to obtain the specific thermal flux q* = q/Δ S. To obtain the distribution of thermal fluxes along the surface of a body, one usually determines the rate of temperature rise dT/dt, which is measured by thermocouples mounted in special calorimeters built into the surface of the model, or by thermocouples which are soldered into the thin surface of the model with comparatively small heat conductance.

The increase of the altitude and velocity of flight and the necessity of modeling processes which develop behind intense shock waves and close to the body’s surface led to the widespread use in aerodynamic experimentation of other physical methods of measurement—for example, spectral methods applicable to shock tubes, radioisotope methods for the measurement of the breakdown rate of heat-shielding materials, methods of measuring the electric conductivity of a gas which is being heated by a shock wave, and so on.

Flight Structures. Flight Control. Principles of Navigation. Celestial.

Celestial navigation.Celestial navigation systems are based on observation of the positions of the Sun, Moon, Planets and navigational stars. Such systems are in use as well for terrestrial navigating as for interstellar navigating. By knowing which point on the rotating earth a celestial object is above and measuring its height above the observer's horizon, the navigator can determine his distance from that subpoint. A nautical almanac and a marine chronometer are used to compute the subpoint on earth a celestial body is over, and a sextant is used to measure the body's angular

height above the horizon. That height can then be used to compute distance from the subpoint to create a circular line of position. A navigator shoots a number of stars in succession to give a series of overlapping lines of position. Where they intersect is the celestial fix. The moon and sun may also be used. The sun can also be used by itself to shoot a succession of lines of position (best done around local noon) to determine a position.

A celestial fix will be at the intersection of two or more circles.

Marine Chronometer.In order to accurately measure longitude, the precise time of a sextant sighting (down to the second, if possible) must be recorded. Each second of error is equivalent to 15 seconds of longitude error, which at the equator is a position error of .25 of a nautical mile, about the accuracy limit of manual celestial navigation.The spring-driven marine chronometer is a precision timepiece used aboard ship to provide accurate time for celestial observations. A chronometer differs from a spring-driven watch principally in that it contains a variable lever device to maintain even pressure on the mainspring, and a special balance designed to compensate for temperature variations.A spring-driven chronometer is set approximately to Greenwich mean time (GMT) and is not reset until the instrument is overhauled and cleaned, usually at three-year intervals. The difference between GMT and chronometer time is carefully determined and applied as a correction to all chronometer readings. Spring-driven chronometers must be wound at about the same time each day.Quartz crystal marine chronometers have replaced spring-driven chronometers aboard many ships because of their greater accuracy. They are maintained on GMT directly from radio time signals. This eliminates chronometer error and watch error corrections. Should the second hand be in error by a readable amount, it can be reset electrically.The basic element for time generation is a quartz crystal oscillator. The quartz crystal is temperature compensated and is hermetically sealed in an evacuated envelope. A calibrated adjustment capability is provided to adjust for the aging of the crystal.The chronometer is designed to operate for a minimum of 1 year on a single set of batteries. Observations may be timed and ship's clocks set with a comparing watch, which is set to chronometer time and taken to the bridge wing for recording sight times. In practice, a wrist watch coordinated to the nearest second with the chronometer will be adequate.

A stop watch, either spring wound or digital, may also be used for celestial observations. In this case, the watch is started at a known GMT by chronometer, and the elapsed time of each sight added to this to obtain GMT of the sight.All chronometers and watches should be checked regularly with a radio time signal. Times and frequencies of radio time signals are listed in publications such as Radio Navigational Aids.

The Marine Sextant.The second critical component of celestial navigation is to measure the angle formed at the observer's eye between the celestial body and the sensible horizon. The sextant, an optical instrument, is used to perform this function. The sextant consists of two primary assemblies. The frame is a rigid triangular structure with a pivot at the top and a graduated segment of a circle, referred to as the "arc", at the bottom. The second component is the index arm, which is attached to the pivot at the top of the frame. At the bottom is an endless vernier which clamps into teeth on the bottom of the "arc". The optical system consists of two mirrors and, generally, a low power telescope. One mirror, referred to as the "index mirror" is fixed to the top of the index arm, over the pivot. As the index arm is moved, this mirror rotates, and the graduated scale on the arc indicates the measured angle ("altitude").

The marine sextant is used to measure the elevation of celestial bodies above the horizon.

The second mirror, referred to as the "horizon glass", is fixed to the front of the frame. One half of the horizon glass is silvered and the other half is clear. Light from the celestial body strikes the index mirror and is reflected to the silvered portion of the horizon glass, then back to the observer's eye through the telescope. The observer manipulates the index arm so the reflected image of the body in the horizon glass is just resting on the visual horizon, seen through the clear side of the horizon glass.Adjustment of the sextant consists of checking and aligning all the optical elements to eliminate "index correction". Index correction should be checked, using the horizon or more preferably a star, each time the sextant is used. The practice of taking celestial observations from the deck of a rolling ship, often

through cloud cover and with a hazy horizon, is by far the most challenging part of celestial navigation.

Radio. Radio is the radiation (wireless transmission) of electromagnetic energy through space. The biggest use of radio waves is to carry information, such as sound, by systematically changing (modulating) some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.

Radio navigation.A radio direction finder or RDF is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances "over the horizon", it makes a particularly good navigation system for ships and aircraft that might be flying at a distance from land.

Radio Navigation.

RDFs works by rotating a directional antenna and listening for the direction in which the signal from a known station comes through most strongly. This sort of system was widely used in the 1930s and 1940s. RDF antennas are easy to spot on German World War II aircraft, as loops under the rear section of the fuselage, whereas most US aircraft enclosed the antenna in a small teardrop-shaped fairing.In navigational applications, RDF signals are provided in the form of radio beacons, the radio version of a lighthouse. The signal is typically a simple AM broadcast of a morse code series of letters, which the RDF can tune in to see if the beacon is "on the air". Most modern detectors can also tune in any commercial

radio stations, which is particularly useful due to their high power and location near major cities.Decca, OMEGA, and LORAN-C are three similar hyperbolic navigation systems. Decca was a hyperbolic low frequency radio navigation system (also known as multilateration) that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings. As was the case with Loran C, its primary use was for ship navigation in coastal waters. Fishing vessels were major post-war users, but it was also used on aircraft, including a very early (1949) application of moving-map displays. The system was deployed in the North Sea and was used by helicopters operating to oil platforms.The OMEGA Navigation System was the first truly global radio navigation system for aircraft, operated by the United States in cooperation with six partner nations. OMEGA was developed by the United States Navy for military aviation users. It was approved for development in 1968 and promised a true worldwide oceanic coverage capability with only eight transmitters and the ability to achieve a four-mile (6 km) accuracy when fixing a position. Initially, the system was to be used for navigating nuclear bombers across the North Pole to Russia. Later, it was found useful for submarines. Due to the success of the Global Positioning System the use of Omega declined during the 1990s, to a point where the cost of operating Omega could no longer be justified. Omega was terminated on September 30, 1997 and all stations ceased operation.LORAN is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from three or more stations to determine the position of a ship or aircraft. The current version of LORAN in common use is LORAN-C, which operates in the low frequency portion of the EM spectrum from 90 to 110 kHz. Many nations are users of the system, including the United States, Japan, and several European countries. Russia uses a nearly exact system in the same frequency range, called CHAYKA. LORAN use is in steep decline, with GPS being the primary replacement. However, there are attempts to enhance and re-popularize LORAN. LORAN signals are less susceptible to interference and can penetrate better into foliage and buildings than GPS signals.

Inertial Navigation Schemes. Inertial navigation system is a dead reckoning type of navigation system that computes its position based on motion sensors. Once the initial latitude and longitude is established, the system receives impulses from motion detectors that measure the acceleration along three or more axes enabling it to continually and accurately calculate the current latitude and longitude. Its advantages over other navigation systems are that, once the starting position is set, it does not require outside information, it is not affected by adverse weather conditions and it cannot be detected or jammed. Its disadvantage is that since the current position is calculated solely from previous positions, its errors are cumulative, increasing at a

rate roughly proportional to the time since the initial position was input. Inertial navigation systems must therefore be frequently corrected with a location 'fix' from some other type of navigation system. The US Navy developed a Ships Inertial Navigation System (SINS) during the Polaris missile program to ensure a safe, reliable and accurate navigation system for its missile submarines. Inertial navigation systems were in wide use until satellite navigation systems (GPS) became available. Inertial Navigation Systems are still in common use on submarines, since GPS reception or other fix sources are not possible while submerged. Navigational and Guidance Requirements For Orbital.

Planetary and Atmospheric Entry Missions.