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.
Jun 10, 2012
A Cycloidal Rotor consists of several blades that rotate about a horizontal axis that is perpendicular to the direction of flight. Blade span is parallel to the axis of rotation and the pitch angle of each of the blades is changed periodically as the blade moves around the azimuth of the rotor.
Blades at the top and bottom produce a vertical lifting force while those at the left and right produce very little force because of their small angle of attack. When resolved into vertical and horizontal directions, the sum of horizontal components is zero, resulting in a vertical thrust. A unique and desirable characteristic of cycloidal blade system is its ability to change direction of thrust enabling any vehicle utilizing this system to take-off and land vertically, hover and to fly forward or reverse by changing the direction of thrust. For implementation on a vehicle, two cycloidal rotors would be necessary, one on each side of the fuselage.
Lateral motion and roll control is achieved through differential control of the magnitudes of the two vectors.
Yawing motion is accomplished through directional control of thrust vectors.
ADVANTAGES:
It provides the same hover capability as a conventional rotor. However unlike a conventional rotor, the blades on a cycloidal rotor operate at constant speed along the entire blade span, allowing all the elements operate at their peak efficiencies.
Cycloidal rotors operate at much lower rotational speeds than conventional rotors, and as such the acoustic signature should be significantly lower.
The greatest advantage of this design is the possibility of greater thrust to power ratios than can be achieved by a conventional rotor.
DISADVANTAGES:
The mechanism required to achieve the periodic pitch changes for each of the blades is by nature more complex than what is required for a conventional rotor.
The complex flow surrounding the rotor makes analysis of cycloidal propulsion difficult.
Weight of the rotor is another problem. The huge no. of components necessary for operation, i.e. multiple blades, bearings and linkages incur more weight penalty compared to a conventional rotor.
REFERENCES:
http://www.inderjitchopra.umd.edu/projects/proj10.html
http://drum.lib.umd.edu/bitstream/1903/3068/1/umi-umd-2875.pdf
May 22, 2012
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Mar 30, 2012
The pintle injectors enjoy several advantages over other types of liquid bipropellant injectors. The design is inherently simpler than the face type injectors in the sense that there is only one injector element, but the single element can have multiple holes. In any case the pintle injectors have lesser number of injection sites than the face type injectors. The second advantage is it's inherent combustion stability. The pintle engines have never reported any cases of combustion instability which reduces risk and the need for stability aids such as baffles. The third attractive feature of the pintle injector is its throttleability. Throttling ratios of 10-20 : 1 have been demonstrated with hypergolic propellants.
The injector flows and combustion have been much less studied than those of flat face type injectors and all the designs and analysis are within the industry and not available for general public. The major issues of concern that tend to complicate the development of these injectors are as follows. Manufacturing issues related to maintaining the required gap between injector holes. As the pintle tip lies in the re-circulation zone, it is subjected to high heat flux. In engines using hypergolic propellants, because of local combustion, the pintle tips can get damaged. To alleviate this, size of re-circulation zones should be minimized.
Very little research work has been carried out on these injectors and much information related to them is not available in open literature. By integrating the design of the pintle injector with that of the combustion chamber, better results can be achieved and it even helps reduce the size of the combustion chamber.
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Jan 3, 2012
INTRODUCTION
The cost of launching spacecrafts using expendable vehicles is very high and efforts are being made to reduce these costs significantly. This reduction in costs can be achieved either by cutting down the overall weight and cost of the vehicle by selecting suitable low cost materials and optimize the design of the components or by opting for reusable vehicles so that the initial high development costs can be recovered over number of flights.
The main issues pertaining to reusable aerospace vehicles are a) recovery and reflight, b) maintainability, c) reusable materials, d) thermal management etc. The intense aero-thermal loads to which the vehicle is subjected during its flight, reentry and stringent mass budget make selection of materials for reusable aerospace vehicles a challenging task.
The main considerations for the development of Reusable Launch Vehicle's (RLV) are
1. they can bring down the launch cost substantially as compared to that of expendable vehicles.
2. their design having in-built abort and emergency landing capabilities would fructify mission success probability and overall safety.
3. retrieval of payloads for overhaul and reuse, in-orbit servicing of space systems and re-fueling of satellites are possible.
4. they can avoid debris in orbit and loss of pricey materials, and can cut down environmental pollution.
MATERIALS FOR REUSABLE AEROSPACE VEHICLES
The materials for reusable aerospace vehicles can be classified as
1. Air-frame materials
2. Thermal protection materials
AIR FRAME MATERIALS
Sizeable cost savings can be obtained by minimizing the overall weight and part count. Air frame weight reduction can be normally achieved by the use of lighter materials and using efficient structural designs.
Concisely, materials for constructing air-frame should have high
1. stiffness
2. stress corrosion resistance
3. fracture toughness
4. fatigue strength
5. creep resistance
6. ease of fabrication and repair.
Aluminium Alloys
Most commonly used Aluminium alloys are AA2024 and AA7075. Modifications to the base alloy composition resulted in higher fracture toughness alloys such as AA7175 and AA7475. AA7150, AA7055 and AA2524 have higher compression yield strength, corrosion resistance and fatigue crack growth resistance.
Composite Materials
They have very high strength and resistance to corrosion and fatigue. Their properties can be tailored to meet the specific needs and they can be formed to complex shapes. High strength composites such as Carbon Fibre Reinforced Polymer (CFRP) and Graphite-Epoxy are used for making Air-frame structures.
THERMAL PROTECTION SYSTEMS
The factors which lead to heating of the external surface of the reusable vehicle are aerodynamic heating and the thermal properties of the materials used in making the air-frames. The aerodynamic heating is dependent on flight profile of the vehicle, i.e. the angle of attack, mach number, body geometry, pressure etc. The thermal properties such as emissivity, absorptivity, catalycity and conductivity of the external surface decides the heat load acting on the reusable vehicle.
Reinforced Carbon-Carbon (C-C) composites have operating range of -150K to about 2000K
Carbon/Silicon Carbide Ceramic Matrix Composites (C/SiC) can operate up to 1800K
Silicon Carbide/Silicon Carbide Ceramic Matrix Composites (SiC/SiC) are resistant up to 1600K
Alumina borosilicate (ABS)-silica ceramic tiles can withstand temperatures up to 1600K
Toughened Unified Fibre Insulation (TUFI) tiles can operate up to 3000K
Carbon aerogels up to 3200K and tiles coated with carbides of hafnium, zirconium and titanium can withstand temperatures as high as 3250K
Since the prime requisite of the materials selected for fabrication is re-usability, the testing and quality requirements have to be very rigorous and precise. The development of reusable aerospace vehicles can result in lower launch costs of satellites as compared to the use of expendable launch vehicles.
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Mar 19, 2011
Once a system has been started by some external power source, the working fluids sustain the process on a self sustaining basis, such a system is known as a bootstrap system. A gas turbine engine once started, sustains a bootstrap operation, because the turbine continues to drive the compressor that feeds air to the engine. Another bootstrap system used by aircrafts is the bootstrap air conditioning system which provides adequate cooling to the cabin and electronic equipment.
The bootstrap air conditioning system mainly consists of two units, the cold air unit and a heat exchanger, both of which are mounted on a common shaft. This assembly was supported on ball bearings until they were replaced by air bearings which provide a lighter solution, requiring less maintenance. The compressor increases the air pressure with a corresponding increase in temperature. The increased temperature is now reduced in the heat exchanger which is cooled by ram air. This reduction in temperature might lead to formation of water, especially when the aircraft is flying in humid conditions. The water extractor placed at the turbine inlet removes most of the water thus preventing damage to the turbine blades and spraying water into the cabin and electronic equipment. As the air expands across the turbine, the temperature can drop below 0o and hence a cold air bypass line is used to vary turbine outlet temperature to get the required temperature for cabin and equipment cooling.
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Mar 15, 2011
One amateur rocket society which has operated for many years on the Pacific Coast has worked out a formula for determining the fin area required for the average amateur rocket which it claims it has used with good success in building most of its rockets. This formula gives the area of one side of one fin, regardless of the shape of the fin and applies only to a three fin arrangement.
Here is the formula, ((d+0.5) X L)/6
where d is outside diameter of the rocket tube
and L is length of the rocket without nose cone
the value of 0.5 is always added to the diameter of the rocket.
The design formula further provides that width of each fin should be at least 1.25 times the diameter of the rocket body.
The cut and try method of fin design is probably the most widely used and the most successful. Even if you hit upon some formula for fin design you would still have to test the result. Most groups build small scale models of the rocket they are designing and test them with a variety of fin arrangements until they hit upon one that seems to give the rocket the best stability.
Steel and aluminum are the most widely used materials for fin construction as they are both strong and easy to work with. Wood is used to a certain extent, but wood cannot be bent without breaking and is not strong enough to withstand stresses involved. Lightness, is an important consideration for any component of the rocket, but do not sacrifice the strength in the fins of your rocket in order to cut down weight. The most common problem is that fins fall off or bend by the force of blast off. To avoid this fins are made of steel or aluminum 1/16th or 1/8th of an inch thick.
For greatest stability in flight it is important that the fins be inexact longitudinal alignment with the rocket body. Your fins will never stand the stress of takeoff if they cant stand being dropped on the floor a few times. Whatever method of attachment you use, be sure that it is strong and secure. There should be no wobble or flutter of the fins. Do not drill holes into your rocket combustion chamber in order to bolt a flange. The only place that bolts or screws can be used are the points where the nozzle and forward bulkhead are attached.
There you have it, try out all the fin designs shown, test them on your rocket and see your rocket sour to greater heights.
Dec 4, 2010
A hubless wheel is a wheel lacking a central hub, first designed by an Italian mechanical engineer, Franco Sbarra especially for motor vehicles. but it is also been seen in the application of many bicycle prototypes over the years. The Orbital wheel, a type of hubless wheel was designed in 1990 by Dominique Mottas. Hubless wheels lack a central hub and an axle which form the major part of conventional wheels. The concept behind this is to reduce the number of rotating parts in the wheel to a bare minimum. Many scientists argue that the efficiency of the wheel is compromised by using hubless and spoke less wheels but others see its advantages as it claims to make the bike lighter and striking in appearance, not many prototypes have actually become commercially viable.
It consists of 3 parts, 1) a bearing with a thin section and a large diameter, 2) rotating part consisting of a tyre, a centre free rim and a brake ring all integrated with each other to form a rotating outer ring of the bearing and 3) fixed part consisting of the non rotating inner ring of the bearing on which the vehicle’s steering system is connected. With this arrangement we get an empty wheel that is free from midwheel structural constraints.
Due to the lack of a central hub this wheel is more advantageous than the conventional wheel. Steering becomes all the more easier due to the elimination of effects of deformation of stub axles and forks of the motorbikes and it is that kinda design that allows for more rational steering systems to be attached in the future. An automobiles stability is dependent on its roll angle, the greater the angle, the more stable the vehicle. But the manoeuvrability will be reduced on turns and tight bends. The hubless wheel comes with a roll angle variator. The greater the speed, the higher the roll angle and hence maximum stability is maintained at all times and when the speed is reduced, the roll angle is reduced thus increasing manoeuvrability. For any vehicle, be it an automobile or a railway car braking is a major source of concern. Conventionally braking occurs at the centre of the wheel which requires a complex braking system. As the hubless wheel has no central hub, braking power is applied close to the ground by using large diameter brake rings or discs and also the braking system has excellent ventilation.
In a traditional wheel, the dynamic forces acting on the wheel at the road-tyre interface passes through the midwheel causing damage to the forks of the wheel. But with the advent of hubless wheels, these forces are transmitted directly to the suspensions. As vibration and jarring are reduced by 50%, the driver is much more comfortable and can drive longer. Goods can be carried in better conditions as they undergo less vibration. There is less strain on the straps used for fastening the goods so road safety is also increased. As each tyre is subjected to the same load, the wear is evenly spread and is greatly reduced and hence its useful life is increased. Thus newer and cheaper tyres with an equal performance level can be used.
Although hubless wheels are striking in appearance and advantageous compared to the conventional wheel, their numerous practical disadvantages have impeded their widespread use as an alternative to conventional wheels. They are difficult and expensive to manufacture and require a great deal of precision while machining. And the design also leaves the bearings and other mechanical parts exposed, the drive system is also very difficult to design and is problematic hence conventional wheels are preferred to hubless ones though they are more efficient.
Oct 16, 2010
An Inertial Navigation System (INS) is a navigational aid that makes use of a clock, accelerometers (sensors for measuring acceleration), gyroscopes (sensors for measuring rotation) and a computer to continuously calculate the position, orientation and velocity of an object on the move without any external references. Inertial navigation is a self-contained navigation technique in which measurements provided by accelerometers and gyroscopes are used to track the position and orientation of an object relative to a known starting point, orientation and velocity. Inertial measuring units (IMUs) typically contain 3 orthogonal rate-gyroscopes and 3 orthogonal accelerometers, measuring angular velocity and linear acceleration respectively. These signals are processed with the help of the on board computer and the position and orientation of the object can be tracked.
All the IMUs fall into 1 of the 2 categories outlined below. The difference between the 2 categories is the frame of reference in which the gyroscopes and accelerometers operate. The 2 categories are stable platform systems or gimballed platform systems and the strap down systems.
In stable platform type of systems, the inertial sensors are mounted on a platform which is isolated from any external rotational motion. This is achieved by using gimbals which allow the platform freedom to move in all the 3 axes. The gyroscopes on the platform detect rotations of the platform if any and the signals are fed back to the motors which rotate the gimbals in order to cancel out such rotations and hence keeping the platform aligned with the global frame. To track the position of the object, the angles between adjacent gimbals is calculated with the help of angle pick-offs and to calculate the position, the signals from the accelerometers mounted on the platform are double integrated. To get accurate results, we need to subtract acceleration due to gravity from the vertical channel before performing integration.
In strap down systems the sensors are mounted rigidly onto the device and hence the output is measured in the body frame and not global frame as in the case of stable platform systems. The orientation of the object can be determined by keeping track of the signals from the gyroscopes and integrating them while the position is determined from the accelerometer signals which are resolved into global co-ordinates and then integrated.
Both these types of systems are based on the same principles. Strap down systems are less complex and are physically smaller compared to stable platform systems. Due to this the strap down systems are the more dominant type of INS.
INS is autonomous and does not rely on external aids or visibility conditions and can operate in tunnels or underwater and anywhere. It is immune to jamming and inherently stealthy. It neither receives nor emits detectable radiation and requires no external antenna.
Errors increase with time as they are mean-squared. Cost of acquisition is high, the maintenance costs are also higher and so are the power requirements when compared to GPS receivers. With integration with GPS, these systems have become cheaper, maintenance costs have also gone down, the sensor errors have dropped to acceptable values and when there’s loss of GPS signals this integrated INS can be used.
Oct 7, 2010
The NAVSTAR-GPS (NAVigation System with Timing And Ranging Global Positioning System) commonly known as GPS, is a U.S. space based radio navigation system that provides reliable positioning, navigation and timing services to users on a continuous worldwide basis, freely available to all. GPS was originally intended for military applications, but in the 1980’s the government made the system available for civilian use. GPS works in all weather conditions, anywhere in the world, 24 hours a day. GPS was created by the U.S. Department of Defence (DOD) and was originally run with 24 satellites and there are currently 28 operational satellites orbiting the earth at a height of 20,180 km on 6 different orbital planes. Their orbits are inclined at an angle of 55° to the equator, ensuring that at least 4 satellites are in radio communication with any point on the planet. Each satellite orbits the earth in approximately 12 hrs and has 4 atomic clocks on board.
Before getting into the details of how the GPS works we should first know what signal transit time is and how it is measured. At some point or the other on a stormy night, you might have wondered how far away you are from a flash of lightning. This distance can be measured quite easily:
Distance = time the lightning flash was first perceived (start time) until the thunder is heard (stop time) multiplied by the speed of sound. The difference between the start and stop time is called transit time.
Hence distance = transit time * speed of sound
The GPS system exactly works under the same principle. In order to calculate one’s position, all that needs to be measured is the signal transit time between the point of observation and four different satellites whose positions are known.
Each satellite transmits its exact position and its precise on board time to earth at a frequency of 1575.42 Mhz. These signals are transmitted at the speed of light (3,00,000 km\s) and therefore require approx. 67.3 milli seconds to reach a position on the earth’s surface. The signals require a further 3.33 micro seconds for every extra km of travel.
Measuring the signal transit time and knowing the distance from a satellite isn’t enough to calculate one’s own position in 3-D space. To achieve this, 4 independent transit time measurements are required. It is for this reason that signal communication with 4 satellites is needed to calculate one’s exact position. All this time we have been assuming that the signal transit time that we measured was precise and highly accurate, but an error of 1 micro second can produce a positional error of 300m. As the clocks on all the satellites are synchronised, the transit time in all the measurements is inaccurate by the same amount. From the basic knowledge of mathematics we know that to solve a problem with n no. of unknowns, we need n no. of equations. To precisely determine the position of a person we need 4 equations i.e. one each for longitude, latitude, height and time error hence 4 satellites are required.
The most important applications of GPS are to determine one’s exact location and the precise time anywhere on the earth. The traditional fields of application for GPS are surveying, shipping and aviation. However with the increasing demand and popularity of the GPS, now it is being used in archaeology, geology, cartography, forestry and agricultural sciences, planning and managing of plantations, fleet management, navigation systems, trekking and sports activities and most importantly military services for which it was mainly developed for.
Oct 2, 2010
Ekranoplan is a Russian word which means 'screen craft' or 'skimmer'. It is not a plane nor is it a ship, moreover its a mixture of both. It is a transition between a hovercraft and an aircraft. It is a vehicle that attains level flight close to the surface of the earth which is possible by the cushion of high pressure air created by the aerodynamic interaction between the wings and the surface of the earth generated by its forward movement called as ground effect. It is also known as a flarecraft, sea skimmer, wing in surface effect ship etc.
In the 1970's when this vehicle was first photographed by the American satellite, it was believed to be a normal Jumbo jet sized aircraft under construction but when the vessel was later photographed sitting in water the American’s were puzzled! These crafts are big, fast and were kept secret from the west until the fall of communism in the early 1990's, when information about them was slowly revealed.
It can quite easily be envisaged that a hundred of such crafts zooming across the ocean, flying undetected by the radar as they fly underneath it and invading a country! By the time that country realized what was going on, it would have been too late. The front of the fuselage would have opened to allow soldiers, tanks, trucks, jeeps, and armoured cars to deploy and invade. You have to imagine several hundred of these craft, all flying at over 290 knots, all under radar in a mass invasion. We could then fully realise the element of surprise that they would have had.
It’s one hell of a ship, fast, manoeuvrable, and heavily armed but maybe not so well protected. Unlike a warship that has armor plating, the Ekranoplan would be susceptible to enemy firepower, a couple of rocket or bazooka hits could seriously damage or render the Ekranoplan inoperable. As the Ekranoplan picks up speed, the air is compressed under its short stabilizing wings until enough pressure produces lift and this supports the craft. Ekranoplan's as they are bigger, have more power and travel just a few feet over the water surface. Infinitely faster than any sea vessel ever designed the Ekranoplan is a marvel to behold. The terrific speed that these crafts can reach is mainly due to the fact that there is no friction between the hull of the craft and the water. This of course is a factor that keeps normal ships at such a comparatively slow speed, as they must push themselves through the water and not over it. They have better fuel efficiency than an equivalent aircraft flying at low level due to the close proximity of the ground, reducinglift-induced drag. There are also safety benefits for the people travelling in the craft in flying close to the water as an engine failure will not result in severe ditching. However, this particular configuration is difficult to fly even with computer assistance. Flying at very low altitudes, just above the sea, is dangerous if the craft banks too far to one side while making a small radius turn.
All Ekranoplan's are amphibious and will easily fly over land just as well as the sea, and at high speed, just as long as the surface is relatively flat. The last Ekranoplan from the former Soviet Union was the 400 ton LUN. It was built in 1987 with 100% military application in mind as a missile launcher and troop carrying ship. It was equipped with six large missiles on top of the fuselage, and could carry tanks and trucks in its large cargo hold. Also it must be reiterated here that it would not have been detected by radar as it would have travelled under the radar waves but the modern radar now has the ability to scan vessels on the surface of the sea so the surprise element of the Ekranoplan can be negated.
At the time when the Soviet Union fell apart, there was a second LUN under construction, it was about 90 percent finished when the military funding stopped. This was because of Russia's poor financial situation and also the end of the cold war. Later the eventual collapse of communism was enough to create further financial upheavals in much of Russia's economy. Most countries have now built prototypes of the Ekranoplan design as it has very useful application both in the military and commercial fields. As yet no commercial flights have been instigated but there are reports that a lot of private ventures are being developed, mostly in the USA.
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