STEALTH TECHNOLOGY(For Military Aircrafts using Composite Materials )

Paper Presented by:

Ashwin.V 2nd yr., B.E.MetallurgyAbirami.K 2nd yr., B.E.Production

P.S.G. College of Technology,Peelamedu, Coimbatore – 641004

Contact No: 9940152743, 9715652448Contact email id: ashwin.venkatesan@yahoo.com

ABSTRACT

The purpose of this paper is to provide the Indian aerospace research community with the information on the research activities related to “Aerospace Stealth System-Composite Structures and Materials” which can be implemented on the ageing aircrafts that exist in our armed forces’ inventory and to improve overall aircraft performance in terms of agility and weight reduction with the use of variety of composite materials. The other purpose is to share the recent achievements related to the advanced composite materials used on various aero-structures across the globe. Also discussed are the possibilities of achieving stealth capability on our existing fleet of fighter and bomber aircrafts of our Indian Armed forces using composite and smart materials.

The paper will focus on the two major categories of composites:

Composite Structures and Smart Structures.

INTRODUCTION TO COMPOSITES

There are four basic categories of structural material. They are: Metals Polymers Ceramics and Composites

Composites are combinations of two materials in which one of the materials, called the reinforcing phase, is in the form of fibers, sheets, or particles, and are embedded in the other materials called the matrix phase. The reinforcing material and the matrix material can be metal, ceramic, or polymer. Typically, reinforcing materials are strong with low densities while the matrix is usually a ductile, or tough, material. If the composite is designed and fabricated correctly, it combines the strength of the reinforcement with the toughness of the matrix to achieve a combination of desirable properties not available in any single conventional material.

One may conveniently speak of four generations of composites:

1 st generation (1940s):

Glass Fiber Reinforced Composites

2 nd generation (1960s):

High Performance Composites in the post-Sputnik era

3 rd generation (1970s & 1980s):

The Search for New Markets and the Synergy of Properties

4 th generation (1990s):

Hybrid Materials, Nano composites and Biomimetic Strategies

PROPERTIES OF COMPOSITES :

Composites offer many advantages over other materials. Within aerospace and marine markets, where exceptional performance is required but weight is critical, composites continue to grow in importance. The most distinct properties of composites may be summarized as:

Stronger and stiffer than metals on a density basis for the same strength, lighter than steel by 80% and aluminum by 60%.

Superior stiffness-to-weight ratios. Capable of high continuous operating temperatures Up to 250° Highly corrosion resistant. High energy absorption or high energy conductivity. Exceptional formability. Outstanding durability Well-designed composites have exhibited apparent infinite life

characteristics, even in extremely harsh environments.

TYPES OF COMPOSITES

The variety of composites reflects the endless search for materials that can withstand ever greater loads in increasingly hostile environments. For the sake of simplicity however, composites can be grouped into categories on the nature of the matrix each type possesses. Methods of fabrication also vary according to the physical and chemical properties of the matrices and reinforcing fibers.

Polymer Matrix Composites (PMC’s)

Polymer matrix composites are the most commonly used of the advanced composite materials. These materials can be fashioned into a variety of shapes and sizes. They provide great strength and stiffness along with a resistance to corrosion. The organic polymer matrix can be thermosetting or thermoplastic and gains its strength from fibers of carbon or boron.

Metal Matrix Composites (MMC’s)

Although heavier than PMC’s, metal matrix composites possess greater tensile strength, higher melting points, smaller coefficients of expansion, higher ductility and increased toughness. At the present time the focus seems to be on the development of matrices of aluminum, magnesium and titanium.

Ceramic Matrix Composites (CMC’s)

Naturally resistant to high temperatures, ceramic materials have a tendency to be brittle and fracture. Composites successfully made with ceramic matrices are reinforced with silicon carbide fibers. These composites offer the same high temperature tolerance of superalloys but without such a high density. The brittle nature of ceramics makes composite fabrication difficult. Usually most CMC production procedures involve starting materials in powder form.

Carbon Carbon Composites

Coveted for superior strength-to-density ratios at temperatures in excess of 1200K, carbon carbon composites are excellent structural materials.

Composition:

Composites are composed of:

Resins - The primary functions of the resin are to transfer stress between the

reinforcing fibers, act as a glue to hold the fibers together, and protect the fibers from

mechanical and environmental damage. The most common resins used in the

production of FRP grating are polyesters.

Reinforcements - The primary function of fibers or reinforcements is to carry load

along the length of the fiber to provide strength and stiffness in one direction. The

largest volume reinforcement is glass fiber.

Fillers - Fillers are used to improve performance and reduce the cost of a composite

by lowering compound cost of the significantly more expensive resin and imparting

benefits as shrinkage control, surface smoothness, and crack resistance.

Additives - Additives and modifier ingredients expand the usefulness of polymers,

enhance their processability or extend product durability

Each of these constituent materials play an important role in the processing and final performance of the end product.

APPPLICATIONS AND ADVANTAGES

Each year, composites find their way into hundreds of new applications, from golf clubs and tennis rackets to jet skis, aircraft, missiles and spacecraft. Composite materials offer designers an increasing array of as a material and system solution.

Advantages:

Defence applications

• Enhanced field effectiveness • Improved handling • High impact tolerance • Insensitive to climate and temperatures • Reduced maintenance costs

IMPACT OF COMPOSITES ON AIRCRAFT INDUSTRY

As is known, the development of high-performance aerospace vehicles gives rise to an increasing demand for advanced structures made of laminated composite materials and smart materials. Since the early 1990's, composites are mainly used on many different exterior panels, fairings, and structures for use on commercial and business aircraft. Composite aircraft structures provide improved strength and stiffness to weight performance, versus conventional fabricated aluminum structures. They offer improved fatigue

life. They can be primed and painted like the rest of the airplane, and then corrosion eliminated in those areas of the airplane.

Fiberglass is the most common composite material, and consists of glass fibers embedded in a resin matrix. Fiberglass was first used in the Boeing 707 passenger jet in the 1950s, where it comprised about two percent of the structure. By the 1960s, other composite materials became available, in particular boron fiber and graphite, embedded in epoxy resins.

COMMONLY USED COMPOSITES IN AIRCRAFTS:

FIBER REINFORCED PLASTICS (FRP):

CARBON FIBER REINFORCED PLASTIC(CFRP):

Carbon fiber or carbon fiber is a material consisting of extremely thin fibers about 0.0002–0.0004 inches (0.005–0.010 mm) in diameter and composed mostly of carbon atoms. Carbon fiber can be combined with a plastic resin and wound or molded to form composite materials such as carbon fiber reinforced plastic (also referenced as carbon fiber) to provide a high strength-to-weight ratio material.

CFRP is widely used in automotive and aerospace industries.

Much of the fuselage of the new Boeing 787 Dream liner and Airbus A350 XWB will be composed of CFRP, making the aircraft lighter than a comparable aluminum fuselage, with the added benefit of less maintenance thanks to CFRP's superior fatigue resistance. Due to its high ratio of strength to weight, CFRP is widely used in micro air vehicles (MAVs).

Some of the other FRPs used are

1. GFRP (glass fiber)2. AFRP (Aramid fiber)3. Nano Composites4. KFRP (Kevlar fiber) etc.

AGEING AIRCRAFTS, A MAJOR CONCERN:

According to a study by the India Strategic defence magazine ,IAF needs to replace and augment nearly 100 percent of its fighter, transport and helicopter fleet for the simple reason that all of them are around 20 years old or more. The rising number of air crashes in our Indian Armed forces, both because there were no advanced jet trainers (AJTs) and also as some of the aircraft were getting old. The air force now needs to replace as well as augment its aircraft and systems in line with modern technology. The IAF is embarking on a massive overhaul of its fleet of combat aircraft, including its Jaguars (background) and some MiG-21 fighters (foreground). A conservative estimate puts the total cost of upgrading some 300 of the service's existing aircraft at more than $3.5 billion over the next five to seven years.

IAF has some of the following aircrafts for specific roles apart from transport and helicopter fleets.

S.No ROLE AIRCRAFT1. Air superiority and Multi-role combat Su-30 MKI (48nos)

MIRAGE-2000 (46nos)MiG- 21 (+/- 400nos)MiG-29 (54nos)

2. Strike, attack and offensive support JAGUAR (84nos)MiG-27 (109nos)

3. Reconnaissance and AWACS MiG-23 (18nos)IL-76 (24nos)

The majority of fighter jets forming frontline air defence are the Russian made MiG-21s. The Indian Air Force's affair with the MiG-21 spans nearly forty years. The first MiG-21s, arrived in October 1963. The MiG-21 airframe has a 3,500-hour life. Frequent replacement of airframe is not practically possible due to the cost factors involved and also due to the non-availability of spare parts. The Mig-21 airframe is mainly made of aluminium and other metals which are comparatively heavier and weaker than the composite materials. With IAF’s fighter squadrons depleting fast, the need to maintain the operational readiness level is very much in need. So what is the solution?

COMPOSITES – THE ULTIMATE SOLUTION

Composites are the most important materials to be adapted for aviation since the use of aluminum in the 1920s. One useful feature of composites is that they can be layered, with the fibers in each layer running in a different direction. This allows materials engineers to design structures that behave in certain ways. For instance, they can design a structure that will bend in one direction, but not another.

The indigenous HANSA ,by HAL is India’s first all composite aircraft. Currently seven are flying in Indian sky. Also about 40% of the LCA- Tejas is made of composites.

The designers of the Grumman X-29(below) experimental plane used this attribute of composite materials to design forward-swept wings that did not bend up at the tips like metal wings of the same shape would have bent in flight.

The greatest value of composite materials is that they can be both lightweight and strong. The heavier an aircraft weighs, the more fuel it burns, so reducing weight is important to aeronautical engineers.

Modern military aircraft, such as the F-22, use composites for at least a third of their structures, and some experts have predicted that future military aircraft will be more than two-thirds composite materials. EADS’ Eurofighter Typhoon is made of almost 80% of Carbon fiber composites which gives them an airframe life span of 6000 hours which is almost double the lifetime of an MiG21 airframe. Hence by upgrading the MiG21’s airframe with composite structures, we can improve fatigue life of their airframe .

D RAWBACKS OF COMPOSITE AIRFRAMES

Aluminum is a very tolerant material and can take a great deal of punishment before it fails. It can be dented or punctured and still hold together. Aluminum, by contrast, is easy to manufacture and repair. Despite their strength and low weight, composites have not been a miracle solution for aircraft structures. Composites are hard to inspect for flaws. Some of them absorb moisture.

IS STEALTH AIRCRAFTS A DREAM TO INDIA? NO…!

India could soon be the third country in the world, after the US and France, to have a stealth bomber fighter aircraft in its armory. The Kolkata-based Indian Association for Cultivation of Science (IACS) has developed a technology to convert ordinary light combat aircraft into stealth jets that would go undetected on radar. The technology uses a special material to construct a shield on the plexi-glass canopies. It is the glass cover of the pit that usually betrays the presence of an aircraft as it reflects the laser beam that is emitted to catch them on the radar. The shield will cover the pit and deflect the laser beam on the shield in all directions.This will make sure the aircraft remains undetected on the radar. But this does not give the complete stealth capability.

Hence to achieve 100% stealth capability, the only solution is use of composites for building aircraft parts and airframes.

WHAT IS STEALTH TECHNOLOGY?

Stealth technology also known as LOT (Low Observability Technology) is a sub-discipline of electronic countermeasures which covers a range of techniques used with aircraft, ships and missiles, in order to make them less visible (ideally invisible) to radar, infrared and other detection methods. Stealth technology (often referred to as "LO", for "low Observability") is not a single technology but is a combination of technologies that attempt to greatly reduce the distances at which a vehicle can be detected; in particular radar cross section reductions, but also acoustic, thermal and other aspects .

WHY STEALTH?

Stealthy strike aircrafts are usually used against heavily defended enemy where enemy radar will cover the airspace around these sites with overlapping coverage, making undetected entry by conventional aircraft nearly impossible. Stealthy aircraft can also be detected, but only at short ranges around the radars, so that for a stealthy aircraft there are substantial gaps in the radar coverage. Thus a stealthy aircraft flying an appropriate route can remain undetected by radar.

The only aircrafts in service with full stealth capabilities are F-117 (Nighthawk), B-2(Spirit) and F-22 (Raptor) of the U.S.A.F.

HOW STEALTH CAN BE ACHIEVED WITH COMPOSITES ?

The F-117A and the B-2 derives it stealth properties in large part from composite materials. The exact composites used on the F-117A are classified, but it's possible to make some guesses. Many of the articles refer to carbon fibers and carbon composites. Furthermore, other synthetic fibers such as glass and Kevlar don't have the properties necessary for an aircraft structure. Metallic fibers, such as boron, aren't stealthy. Finally, carbon composites are black--the same color as the F-117A. The two most important things about the Stealth composites are: they contain small diameter fibers which won't burn at the crash temperatures; and these fibers are held together with a plastic that will burn at the crash temperatures.

Composite structure of an aircraft

Hence by using non-metallic dielectric composites, which are relatively transparent to radar we can achieve greater percentage of stealth features, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites used may contain ferrites to optimize the dielectric and magnetic properties of the material for its application.

Thus its no doubt that Indian can soon make a break through in stealth technology by using the carbon fiber and other kind of composites on its various weapon platforms and will become one of the most powerful nations in the world.

SMART MATERIALS

A smart structure is that which has the ability to respond adaptively in a pre-designed useful and efficient manner to changes in environmental conditions, including any changes in its own condition; the response is adaptive in the sense that two or more stimuli or inputs may be received as anticipated and yet there is a single response function as per design. Smartness ensures that the structure gives optimum performance under a variety of environmental conditions.

A smart configuration would be that in which normal loads are taken care of in normal conditions, and suitable actuation systems are activated to tackle abnormal loads.

A Smart structure consists of

Sensors Actuators and Neural Networks

which makes the structure to detect and respond to varying conditions. It contains many sensors embedded on the material.

Structural Health Monitoring System (SHM), is an upcoming technology in mechanical and aerospace engineering. SHM is a new approach to collect data about critical structural elements using sensors to provide indicators when some anomalies are detected in a structure. This approach will continuously update the data in a structure on current conditions of a structure including the detection of changes in chemical and electrical properties of materials related to deterioration, such as corrosion and chloride attack, steel corrosion and fatigue, alkali-silica reaction, and PH, humidity and changes in the service environment or exposure. SHM can also continuously monitor structure physical properties, such as loadings, stresses, strains, accelerations, cracks, etc. SHM refers to the broad concept of assessing the ongoing, in-service performance of structures using a variety of measurement techniques.

Thus smart-composite materials, if fitted on military and civilian aircrafts will definitely improve the life span of the airframes besides preventing air crashes that happens due to air frame fatigue and providing stealth capability.

FUTURE OF COMPOSITES

All-composite airframes are a reality today, especially in smaller airplanes. Components and entire sections of business aircraft and Transport category airplanes increasingly will be made of composites. There are still some hurdles to overcome before composites will replace aluminum completely, especially when it comes to larger airplanes. Improved manufacturing techniques, further education of the approving agencies, and a history of successes by composite airplane manufacturers will change airplane manufacturing techniques in the twenty-first century, delivering high performance aircrafts thus effectively increasing its overall performance with incredible features like stealth technology. Its no doubt that composites will take Indian military aviation into great heights making India the best among the rest.