Sep 15, 2015
Chemical Bonding - 1
1. An ionic bond can be formed between two atoms when
1. An ionic bond can be formed between two atoms when
2. The cohesive energy of an ionic crystal is the energy
3. An ionic solid is a poor conductor of electricity because
4. A covalent bond is formed between two atoms when
5. Which of the following is required for the formation of an ionic bond?
Aug 5, 2015
Jul 24, 2015
NACA Series
1. In a NACA 4 digit series, what does the first number represent?
1. In a NACA 4 digit series, what does the first number represent?
2. Default maximum thickness of a 4 digit NACA series aerofoil is?
3. What is the maximum camber location of a NACA 2412 aerofoil from leading edge?
4. What is the theoretical optimum lift coefficient of a NACA 23112 aerofoil at ideal angle of attack?
5. What does the third digit in a NACA 5 series aerofoil represent?
Oct 16, 2014
INTRODUCTION
Leaf springs also referred to as semi-elliptical springs or cart springs are one of the oldest form of suspension used in vehicles, especially heavy vehicles. A leaf spring looks similar to a bow minus the string. It consists of a stack of curved narrow plates of equal width and varied length clamped together with shorter plates at the centre to form a semi-elliptical shape. The center of the arc provides location for the axle, tie holes are provided at either end for attaching to the body.
TYPES
There are different varieties of leaf springs namely mono-leaf springs and multi-leaf springs.
As the name suggests, the mono-leaf suspension consists of a single link. They are thick in the middle and taper out at the end. It doesn't offer much strength and suspension to towed vehicles.
MONO-LEAF SPRING
Multi-leaf springs are used for heavier vehicles which offer increased strength and suspension.
A more modern design is the parabolic leaf spring. It can have a mono-leaf or multi-leaf configuration. It has fewer leaves in comparison to the semi-elliptical multi-leaf springs whose thickness varies from centre to the end and it follows a parabolic path. This configuration not only saves weight but also gives greater flexibility which improves ride quality. A trade-off of using parabolic leaf spring is reduced load carrying capability.
MULTI-LEAF SPRING
ADVANTAGES and DISADVANTAGES
ADVANTAGES
1) The construction of the suspension is simple and strong as it acts as a linkage for holding the axle in position and thus a separate linkage isn't necessary.
2) As they locate the rear axle, the need for trailing arms and panhard rod is eliminated, thus saving cost and weight.
3) It supports the weight of the chassis
4) It controls axle dampening
5) It controls chassis roll more efficiently by utilizing a higher rear moment center and a wider spring base. The wider the springs are mounted apart, the lesser the roll tendencies. As the moment center height is high, this shortens the moment arm which in turn produces less roll.
DISADVANTAGES
1) The leaf-spring systems are not easy to install
2) The inter-leaf friction between the leaf springs reduces the ride comfort
3) The leaf springs may tend to lose shape and sag over time. If the sag is uneven, it alters the cross weight of the vehicle which changes the handling. It also changes the axle-to-mount angle
4) Acceleration and braking torque cause wind-up and vibration. Also wind-up causes rear-end squat and nose-diving
Feb 1, 2013
Thermogravimetric Analysis is a material characterization technique in which the mass of a substance is monitored as a function of temperature or time under controlled temperature and atmosphere.
This analysis is carried out primarily to determine
1) the composition of materials such as organic and inorganic content in the sample,
2) to predict their thermal stability at high temperatures such as vaporization, sublimation, absorption, adsorption, desorption, chemisorption, reaction kinetics etc.
A plot of mass change versus temperature, called thermogravimetric (TG) curve is plotted which helps in determining he extent of purity of analytical samples and the mode of their transformations within the specified temperature range.
A Thermogravimetric analyzer makes use of a thermobalance, whose basic components are,
1) Balance
2) Furnace
3) Programmer unit for temperature measurement and control
4) Recording unit for mass and temperature changes
The basic requirements of a Balance are accuracy, sensitivity, reproducibility and capacity. There are 2 types of balances
1) Null type balance which consists of a sensor which detects the deviation from the null point and restores the balance to its null point by means of a restoring force.
2) Deflection balance which converts the deflection of balance beam deflection into a suitable mass by means of photographic recording or recording electrical signals or using an electro-chemical device.
The Furnace provides linear heating over a wide range of operating temperaures, typically -150 deg. Celsius to about 2000 deg. Celsius depending on the requirement.
Temperature measurement and regulation is done with the help of thermocouples. Usually 2 thermocouples are used, where one records the temperature change, the other actuates the control system.
The recording unit makes use of a microprocessor which allows for digital data acquisition and processing using a personal computer.
The factors affecting the precision and accuracy of the TG curve are
1) Furnace heating rate
2) Sensitivity of the sensors
3) Recording speed
4) Amount of sample
5) Particle size
6) Heat of reaction etc.
All the above factors are to be taken into consideration and the instrument should be properly calibrated before performing experiments. It is a very efficient method in material characterization and is widely used in the analysis of polymers, plastics, composites, laminates, pharmaceuticals, rubber, petroleum, food, adhesives etc.
Jan 27, 2013
A Catalyst is a substance that is used in small amounts relative to the reactants that modifies the rate of reaction without it being consumed in the reaction and this process is called Catalysis. Catalysts that accelerate the reaction are called positive catalysts while the ones that slow down are called inhibitors. Catalysts react with one or more reactants to form intermediate compounds, which on further reaction gives the final products and regenerating the catalyst during this process.
The action of catalysts have been proposed by G. S. Pearson in 4 models.
1) by accelerating fuel decomposition
2) by accelerating HCLO4 decomposition
3) by accelerating the solid-fuel / HCLO4 reaction on fuel surface
4) by accelerating gaseous fuel reactions in gas phase
In addition catalysts may also enhance AP decomposition.
Catalysts for HCLO4 decomposition can be divided into 3 groups,
1) Highly effective oxides (Cr2O3, NiO, Al2O3, Fe2O3 and CuO)
2) Less reactive oxides (TiO2 and Cu2O)
3) Inactive oxides (CdO, MgO and CaO)
Copper Chromite accelerates the decomposition of HCLO4 but doesn't affect the binder degradation. The relative effectiveness of various catalysts in the ignition process depends on the surface area, particle size and quantity of catalyst used.
Ignition of propellants is an enormously complex process and a single rate determining step cannot explain it. One or more types of mechanism models such as gas-phase, condensed-phase and catalytic reactions contribute to the ignition process. A single reaction will depend on many factors such as pressure, local temperature, chemical and physical structure, local concentration, etc.
The action of catalysts have been proposed by G. S. Pearson in 4 models.
1) by accelerating fuel decomposition
2) by accelerating HCLO4 decomposition
3) by accelerating the solid-fuel / HCLO4 reaction on fuel surface
4) by accelerating gaseous fuel reactions in gas phase
In addition catalysts may also enhance AP decomposition.
Catalysts for HCLO4 decomposition can be divided into 3 groups,
1) Highly effective oxides (Cr2O3, NiO, Al2O3, Fe2O3 and CuO)
2) Less reactive oxides (TiO2 and Cu2O)
3) Inactive oxides (CdO, MgO and CaO)
Copper Chromite accelerates the decomposition of HCLO4 but doesn't affect the binder degradation. The relative effectiveness of various catalysts in the ignition process depends on the surface area, particle size and quantity of catalyst used.
Ignition of propellants is an enormously complex process and a single rate determining step cannot explain it. One or more types of mechanism models such as gas-phase, condensed-phase and catalytic reactions contribute to the ignition process. A single reaction will depend on many factors such as pressure, local temperature, chemical and physical structure, local concentration, etc.
Sep 19, 2012
INTRODUCTION
The aim of atomization is to substantially increase the surface area of the liquid to enhance vaporization, mixing and combustion. The end result is that the liquid jet becomes unstable which leads to the disintegration of the liquid surface into droplets. This surface area increase can be achieved in various ways and shear coaxial jet injector atomization process is one of them. The breakup of the liquid jet is a result of complex interactions between inertial, viscous and surface tension forces. Aerodynamic forces promotes disturbances on the surface while viscous forces have a damping effect. Surface tension tends to pull the liquids together. Turbulence and pressure oscillations in the injected fluids affect atomization. The non dimensional parameters such as Reynolds number, Weber number, Mixture ratio and Ohnesorge number help in characterizing the overall process. Le Visage, D. showed that both the momentum and density ratios determine the breakup length of the liquid core. By plotting the Ohnesorge number vs the Reynolds number, one can distinguish between
a) low-velocity region, where the breakup is due to the action of surface tension forces and
b) high-velocity region, where the influence of aerodynamic forces increases exponentially with Reynolds number at constant Ohnesorge number.
THE PROCESS OF DISINTEGRATION
1) The Primary Atomization zone: In the near field of the injector nozzle, the huge difference in the velocity between the gas and liquid, leads to a surface instability and formation of filaments or drops from the jet surface. This is the primary atomization zone.
2) The Secondary Atomization zone: In the far field of the injector nozzle, the fluid velocity decreases due to mixing with the external atmosphere which leads to instability. The large droplets and ligaments produced in the primary atomization zone and in the jet breakup zone, breakup further into smaller and more stable droplets, depending on the local weber number. The breakup time of the droplets can be expressed as a function of local relative velocity and ratio of gas to liquid density. The breakup time and initial droplet velocity determines the distance from the injector where secondary atomization takes place to the flame front.
CLASSIFICATION OF BREAKUP REGIMES
i) based on Weber number
Extensive experimental study on round liquid jets under conditions of with and without co-flowing gas stream were carried out by Farago, Z. and Chigier, N. They observed
1) a Rayleigh type breakup, which is further divided into two subgroups
a) axisymmetric breakup (We < 15)
b) non axisymmetric breakup (15 < We < 25) and
2) a Membrane type breakup (25 < We < 70), where the round jet develops into a thin sheet, which forms Kelvin-Helmholtz waves and breaks up into drops
3)a fiber type breakup (100 < We < 500).
ii) based on Mixture ratio
Based on previous studies and experimental work carried out by Gomi, N., the breakup regime is classified into three categories depending on the mixture ratio (MR).
1) MR < 0.2, relative velocity determines the drop size
2) 0.2 < MR < 1, relative velocity and mixture ratio determines the drop size
3) MR > 1, many parameters affect the drop size
The basic atomization phenomena that converts primary liquid jets into droplets is not yet fully understood. No unified theory is currently available and experimental investigations are the best way to characterize a given injection element.
Aug 19, 2012
FLUENT is a computational fluid dynamics (CFD) software which consists of modeling capabilities needed to simulate flow, turbulence, heat transfer and chemical reactions for a wide range of applications. It is an integral part in the design and optimization process of any product development.
FLUENT has a wide range of boundary conditions that allows the flow to enter and exit the domain. This article helps you in selecting the appropriate boundary conditions for your specific application.
There are about 10 different types of flow inlet and exit boundary condition options in FLUENT.
1) VELOCITY INLET - It is used to define the velocity and other properties of the flow at the inlet. Intended for in-compressible flows
2) PRESSURE INLET - It is used to define the total pressure and other properties of the flow at the inlet. Flow direction must be defined else non-physical results can occur. Suitable for both compressible and in-compressible flows. Outflow can occur at pressure inlet conditions.
3) MASS FLOW INLET - It is used in compressible flows to define the mass flow rate at the inlet. It is not necessary in in-compressible flows as the velocity inlet itself fixes the mass flow rate.
4) PRESSURE OUTLET - It is used to define the static pressure at the outlet. It often gives better rate of convergence when back-flow occurs. Back-flow can occur at pressure outlet conditions and is assumed to be normal to the boundary. This must be used when problem is set up with pressure inlet.
5) PRESSURE FAR-FIELD - It is used to model free stream compressible flow at infinity, with free stream mach number and static conditions specified. This is available only for compressible flows when density is calculated from ideal gas law.
6) OUTFLOW - It is used to model flow exits where flow velocity and pressure are not known prior to solution of the flow. It cannot be used for compressible flows, with pressure inlet boundary condition and in unsteady flows with variable density. Can be used with velocity inlet.
7) INLET VENT - It is used to model inlet vents with specified loss coefficient, flow direction and inlet pressure and temperature.
8) INLET FAN - It is used to model an external intake fan with specified pressure jump, flow direction and intake pressure and temperature.
9) OUTLET VENT - It is used to model an outlet vent with specified loss coefficient and discharge static pressure and temperature.
10) EXHAUST FAN - It is used to model an external exhaust fan with a specified pressure jump and discharge static pressure.
These boundary conditions help design and analyze the domain and model the flow through it.
Jun 20, 2012
Stealth or Low Observability is one of the most misunderstood concepts by the common man. Stealth aircraft are considered to be the invisible aircrafts that dominate the skies. But, in simple terms, stealth technology allows an aircraft to be partially invisible to Radar or any other means of detection. This is similar to the camouflage used by soldiers in jungle warfare. Unless he comes close to you, you cant see him. Before getting into the stealth technology, we must first know how a radar works. The radar sends out radio waves which is reflected back by any object it happens to encounter. The radar antenna measures the time taken by the wave to return and with that information it can tell how far the object is. The metal body of the aircraft is a very good reflector of radar signals, and this makes it easy to find and track planes with radar equipment.
The goal of stealth technology is to make an aircraft invisible to radar. It is a combination of technologies.
1) The aircraft should be shaped so that any radar signals it reflects are reflected away from the radar equipment.
2) The aircraft should be covered in materials that absorb radar signals.
3) Reducing visibility and infrared signature.
Usually conventional aircraft have rounded shape which makes them aerodynamic but it also creates a very efficient radar reflector. The round shape means that wherever the radar signal hits the plane, some of the signal gets reflected back. A stealth aircraft on the other hand has completely flat surfaces and very sharp edges which reflects the radar signals away from the radar antenna. The most efficient way to reflect radar waves back to the emitting radar is with orthogonal metal plates, forming a corner reflector consisting of either dihedral (two plates) or a trihedral (three orthogonal plates). This configuration is used in the tail of conventional aircrafts, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them. A more radical method is to eliminate the tail completely. In addition to altering the tail, the engines must be buried within the wing or fuselage, install baffles in the air intakes so that turbine blades are not visible to radar. A stealthy shape must be devoid of complex bumps or protrusions such as weapons, fuel tanks etc. and they must not be carried externally. These shaping requirements have strong negative effects on the aircraft's aerodynamic properties and hence they are inherently unstable and cannot be flown without a fly-by-wire control system.
In addition, surfaces on a stealth aircraft can be treated so they absorb radar energy and convert it into heat rather than deflecting them in other directions. Commonly used radar absorbent materials are iron ball paint and foam absorber. The simplest stealth technology is simply camouflage by using paint or other materials to blend with the environment or by resembling something else. Most stealth aircraft use matte paint and dark colors and operate only at night. With interest in daylight stealth, emphasis is on the use of gray paint in disruptive schemes. Usually planes are visible in thermal imaging systems because of the high temperature exhaust they give out. The exhaust plume contributes a significant infrared signature. This is a great disadvantage to aircrafts as they are vulnerable to missiles with IR guidance system. By minimizing the exhaust cross sectional volume and maximizing the mixing of hot exhaust with cool ambient air, IR signature can be reduced. Another way to reduce the exhaust temperature is to circulate coolants such as fuel inside the exhaust pipe.
The stealth aircrafts cannot fly as fast or are not maneuverable like conventional aircrafts. The reduced amount of payload it can carry and its sheer cost are the major factors that sharply reduced their research and development. As air defense systems are becoming more and more accurate and deadly, stealth technology can be a decisive factor for any country over the other. Nowadays the stealth technology is being incorporated in ships, helicopters and tanks as well and in the coming years we can see many more advancements in the field of military aviation.
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Jun 19, 2012
OVERVIEW
Everyone gets fascinated when it comes to Space, many of us couldn't afford to go to into space because it is just that expensive. But what if a commercial, cost effective spacecraft can take off from the ground on its own, travel into space and return? Well, this is exactly what XCOR Aerospace has managed to do with its EZ-Rocket. It is the first rocket plane to be built and flown by a private organization. It is modified from Burt Rutan's Long-EZ home-built fixed wing, canard aircraft manufactured by Rutan's Aircraft Factory. It has good gliding characteristics which makes it ideal for a rocket plane.
ENGINE
The four cylinder air cooled, piston aircraft engine Lycoming O-320 with constant speed propeller of the Long-EZ is replaced by a pair of 400 lbf thrust, pressure fed regeneratively cooled, non throttle-able, restart-able liquid fueled rocket engines. A pressurized fuel tank filled with isopropyl alcohol and two Styrofoam insulated aluminium tanks that hold liquid oxygen are placed at the bottom and top of the plane respectively.
PERFORMANCE
Thrust :- 800 lbf (both engines together)
Take-off roll :- 1650 ft (500 m) in 20 seconds
Max. climb rate :- 10000 ft/min (52 m/s)
Max. altitude attained :- 11500 ft
Never exceed speed :- 195 knots
Sound level :- 128 dB at 10 meters
SAFETY FEATURES
Just like any other plane, the XCOR EZ-Rocket has a lot of safety features installed for the safety of the pilot. The canopy is quick to open and the pilot has a parachute in case of emergency exit. The engine has its own Kevlar blast shield. An ultraviolet fire sensor illuminates a light on the instrument panel in the event of engine fire. The plane is equipped with large bottles of pressurized helium which are used as fire extinguishers when engine catches fire. The pilot can manually shut off both fuel and oxidizer supply to the engines if a fire is detected or engine fails to shut down. A burn through sensor signals the pilot when the fuel tank is empty.
After 26 successful flights of the EZ-Rocket, XCOR Aerospace is now focusing on the development of rocket racers and a suborbital spacecraft Xerus for space tourism and to launch micro satellites.
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