a compendium of tech stuff

Jun 20, 2012

On 5:03 PM by Lalith Varun   No comments



     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

On 12:10 PM by Lalith Varun   No comments



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

On 12:11 PM by Lalith Varun   1 comment





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

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