Optimisation of Aerospace Fabrics


1.Safety systems are necessary for all military aircraft which are designed to operate on the fringes of safety envelope and are inherently vulnerable to internal or externally induced failures. The safety systems while ensuring safety of the aircraft and/ or aircrew, do not directly contribute towards operational capability of the aircraft. Their addition into the aircraft thus imposes a weight penalty and it is a challenging task for the designer to minimize their weights. It is in the use of fabrics for safety systems that the Russian and western design approaches converge. Fabrics are now universally and extensively used in the design of safety systems of military aircraft due to their light weight and a host of other favorable properties. Important examples are parachutes, dinghies, life jackets, survival packs, arrester barrier nets etc.

2. Fabrics are also used in other aerospace application like, protective systems viz; flying clothing, fire proof/ fire retardant zones, pressure suits etc and environment systems viz; passenger seats, cabin upholstery, pressure suits, camouflage covers etc.

3. Each of the above applications is quite diverse in its functionality, operating characteristics, load spectrum etc. These varied design requirements compel use of diverse range of fabrics which need to meet stringent aeronautical standards. Diversity in range of fabrics has been further enlarged, firstly by our strategic decision to indigenously design and manufacture the safety systems and secondly by the increased market competition. This enlarged diversity in the range of fabrics has led to technological and maintenance issues which need to be jointly addressed by the designer, manufacturer and the user.

4. The central theme of this paper is to communicate the user’s concerns to the designer and the industry and synergise their efforts to optimize the constraints in design, manufacture and supply chain management.

Application of Aerospace Fabrics
5. Aerospace fabrics find their application in three distinct systems of military aviation environment, viz; safety systems, protective systems and environment systems. Each of these systems has its specific role in the overall reliability of the weapon system viz; the military aircraft. They operate under specific conditions of physical and environmental loads and their design and materials used vary from application to application.

6. Typical applications of aerospace textiles in the military systems are:
Safety Systems Safety Systems, Environment Systems
Aircrew parachutes,Load lashing nets,Aircrew, PAX seats. Brake parachutes,Protective Systems,Pressure suits,Spin recovery parachutes, Flying Clothing,Cabin upholstery. Dinghies or Life rafts,Anti-G suits, Camouflage covers,Life jacketsFire resistant/ retardant barriers, Ballistic Control parachutes, Survival Packs, Arrester Barrier nets, Target Sleeves

7. The above are only generic and indicative uses to which aerospace textiles can be applied. The numbers and versions of applications are too many and any discussion on each of them separately is beyond the scope of this paper. However, fabrics used in safety systems in general and parachutes in particular are used as examples to drive home the point.

8. In parachutes, the aerospace fabrics are used in different forms, known as textile assemblies. Typically these assemblies are woven fabrics, woven narrow fabrics such as tapes and webbings, braided cordages, both cored and coreless and the sewing threads. The base yarns used in these textile assemblies may be nylon 6/66, capron, poly-aramids, cotton, polyester, rubberized nylons, neoprene fabrics, polyethylene etc. A broad summary of aerospace application of textiles is given in

Properties of Aerospace Fabrics
9. The decision of the designer to use a specific base fibre and textile assembly for an application is governed by the physical and environmental load conditions and properties of the base fibre as well as its textile assembly. Properties of aerospace textile materials are thus important as these are responsible for successful performance of a safety system. Besides, fabric dimensions such as width, thickness and roll length of the fabric, structural parameters viz; denier, tenacity, yarn twist, type of weave etc are also specified. Every physical, constructional parameter and the fabric property have their own significance with regard to their role in parachute construction, packing, deployment and service life. Any deviation from the specifications would have the potential to put life and other valuable assets into danger.

10.The properties which are important and generally specified for an aerospace textile material are listed below and their importance in design of aerospace applications is discussed in the succeeding paragraphs.
(a) Nature of Fibre
(b) Dimensions of Textile Assembly
(c) Mass and Volume
(d) Tensile Properties
(e) Air permeability

11. Nature of Fibre Each type of fibre has its own combination of properties and a parachute designer selects a fibre which could meet the minimum design requirements of a given parachute application. The use of a right fibre controls the properties of the materials namely mass, volume, strength, elongation, elastic behavior, thermal stability, resistance to UV light, abrasion, chemicals and moisture content etc. Use of wrong fabric or fabric with deviations in specified parameters would have serious effect on ultimate properties of the fabric. The choice of a fibre is, therefore, a major factor. Thus the intended performance of a parachute during operation is obtained if and only if the specified fibre is used in making the parachute materials.

12.Dimensions of Textile Assembly Width of fabric used in manufacture of parachute canopy is decided so as to minimize the wastages during mass manufacture. Higher thickness of a fabric increases the packed volume of a parachute and may create a difficulty in accommodating in the pack covers of specified size. In case of a tape or webbing, the width and thickness are important as these materials may have to pass through a metal component such as a pulley, buckle or a ring which have a definite gap to allow the tape to pass through. If the width is higher than the specified, it would not pass through the gap, and if it is less, then it would be loosely fit which is not desirable. Similarly, if thickness is on the higher side, the tape on fitment would go out of the dimensions of the metal component and may create problems in adjusting the extra volume.
13.Mass and Volume Mass of a parachute textile material is a highly critical parameter. An emphasis is always made to reduce the mass of each textile component to the lowest level so that the total mass of a parachute could be kept with in the given limits. Similarly low volume of the parachute system may vacate some space to allow some vital instrument to be installed. Thus, mass and volume are always kept at a minimum possible level. The mass of a range of packed parachutes are given in Table 2. Generally the mass of a man carrying parachute or a brake parachute is in the region of about 15 kg. Whereas, heavy drop parachutes and space recovery parachutes are substantially heavier as well as bulkier.

Man Carrying Parachutes Brake Parachutes Heavy Drop
Type Mass (kg) Type Mass (kg) Type Mass (kg)
Para trooper’s Main 14 MIG - 21 14 3-Cluster para 360
Para trooper’s Reserve 7 MIG – 23/27 17 P-7 PL form 700
SMK – 10 (HPT-32) 9 MIG - 29 8 G-12 D (64’ dia) 27
SMK-15 (Canberra) 10 Jaguar 14 G-11 A (100 dia.) 52
Sea Harrier / Mirage 11 Mirage 2000 14 G-11 B (100 Dia) 56
BMK 41 (Kiran) 12 Su - 30 24
C-3-3 (Iskra/ Mig 21) 12
Jaguar A/c 12 Space Recovery System
Ram – air – 7 cell 16 Apollo recovery system 100

Table 2 : Mass of Various Parachute Systems

14. Fibre Density Since the mass and volume are highly critical parameters for parachute materials, certain properties which could be used control these are considered essential. One such important property is the density of the fibre. Lower density of fibres can bring about significant reduction in packed volume of a parachute system. If this is not achievable, one would have to resort to pressure packing of a parachute. Pressure packing is often resorted to in one-time or infrequent applications like recovery parachutes, re-entry systems, ballistic control parachutes etc. However, in case of brake parachutes, which are deployed several times in a day, pressure packing would lead to maintenance difficulties. Thus higher density of a fibre used in parachutes not only adds to the weight of the parachute but also leads to packing problems. The density of various relevant fibres is given in table 3. From the data we can see that the ECPE fibres are the lightest of all fibres. Obviously the materials like fabrics and tapes etc manufactured from these fibres would also be the lightest. Some examples of these fibres are Dyneema and Spectra. But these fibres have a low melting point of 1470C and their elongation properties need modification for use in some critical aerospace application. This aspect is indicated in Table 4, which compares the percentage elongation of nylons with Dyneema (ECPE) fibre.
Sl No Fibre Density (gm/cm2)
1 Extended Chain Polyethylene Fibres (ECPE) 0.97
2 Nylons (6 And 66) 1.14
3 Silk 1.35
4 Polyesters (Terylene And Dacron) 1.38
5 P. Aramid (Kevlar-29) 1.44
6 Cotton 1.54

Table 3 : Density of some important fibres

Material Extension (%) Recovery (%) Permanent Set (%)
Immediate Delayed Immediate Delayed
Cordage Nylon 44 10.6 13.3 55.6 98.1 1.9
Cordage Nylon 66 7.4 8.6 75.4 98.6 1.5
DYNEEMA SK75 4.0 6.2 19.4 307 69.3

Table 4 : Comparison of Elongation Properties

15.Tensile Properties Textile fibres differ considerably in their response to an externally applied force. Most of them show elastic behavior at very low strains however, at higher strains, visco-elastic effects become prominent. Beyond the elastic region, the strains become hard requiring more stress. At still higher strains, certain degree of strain softening takes place followed by yield process. In this region small increments in stress give rise to large deformations. Finally, the fibre breaks. Fig 1 & 2 show the behavior of aerospace fabrics under tensile load.

16.The tensile strength of a textile material depends on many factors such as the processes it has undergone, the mechanical treatments it has been subjected to, the relative humidity and the temperature of the surrounding atmosphere. In parachutes, shock loading takes place in every deployment which involves loading of the textile materials at a very high speed or at a high strain-rate.

Fig 1 : Stress vs Strain curves of some aerospace fabrics.

17.Under these conditions sufficient time is not available for the viscous components of the fibre to react very fast, consequently the fibre undergoes a brittle fracture as soon as fracture stress is reached. Therefore under shock loading conditions the fibres such as nylon and polyester behave more ideally because these fibres show high tenacity and adequate elongation with sufficiently high melting point. Higher level of melting point is also necessary as temperature goes up during high rate of straining or shock load conditions.
Fig 2 : Elongation and Break characteristics at varied strain rates.

18.Table 5A gives a comparison of the tensile properties of nylon and polyester. The results shown in Table 5A are attributed to IH Hall who published results of very high strain rate testing up to 98000% extension per minute i.e. 330% per second where as the conventional testing in laboratory is done at 49.8% extension per minute i.e. 0.0083% per sec. It is has been reported that at higher rate of extension, the increase in tensile modulus is substantial and the increase in tenacity is also significant. The reduction in elongation and energy absorption is found more in polyester than nylon 6. Under high rate of straining, the visco-elastic effects do not play an effective role as they could play during slow rates of straining. The above data also indicates that under shock load conditions nylon 6 stands out superior. It is reported in literature that besides the positive attributes of nylon 6 indicated above, its recovery characteristics, abrasion resistance, frictional behavior, heat conductivity etc are relatively superior, and these are favorable for its applications where high strain rates are involved.

Table 5B gives a similar comparison between nylon and Kevlar.
Nylon 66 Polyester
Modulus (g/tex)
At 0.0083 Sec-1 510 1290
At 330 Sec-1 1900 2000
% change 272.5 % 55%
Tenacity (g/tex)
At 0.0083 Sec-1 84.9 60.8
At 330 Sec-1 110.0 77.7
% change 29.5% 27.8%
Elongation – to – break (%)
At 0.0083 Sec-1 20.1 12.8
At 330 Sec-1 12.7 7.3
% change 36.8% 42.9
Work of rupture (J / g)
At 0.0083 Sec-1 87.1 5.5
At 330 Sec-1 67.5 34.4
% change 22.5% 36.8%

Table 5A : Tensile properties of Nylon 6 and Polyester yarn at high strain rates

PROPERTIES NYLON KEVLAR
Straining Rate % Straining Rate %
10 100 290, 000 10 480, 000
Tenacity 7.0 7.7 9.0 18.0 15.5
Elongation at break 15.0 12.0 11.0 2.3 2.3

Table 5B : Effect of straining rate on Nylon and Kevlar

19.The elongation and thermal properties of ECPE fibres like Dyneema and Spectra, are not vary favorable in parachute applications. At the same time, their strength to weight ratio and density are extremely tempting for use in aerospace applications. Some research done so far do show some promise for ECPE fibres. The research is essentially aimed at modifying the fibre structure as well as weave structure to achieve the desired elongation properties and thermal durability. Salient properties of high performance materials (HFMs) are given in Table 6.

Type Of Fibres Density
Max. Use Temp Elongation At Break Tenacity
Modulus

gm/cm3 0C % gpd gpd
Poly-Aramids
Kevlar 29 1.43 250 3.6 23 550
Technora 1.41 250 4.6 28 590
Twaron 1.44 250 3.3 22 620
Aromatic Polyesters
Ekonol 1.4 150 2.6 31 1100
Vectran 1.47 150 - 25 700
Hetrocyclic Fibres
Zylon AS 1.54 350 3.5 42 1300
Zylon ZM 1.56 350 - 42 2000
PBZT 1.57 350 2.5 30 2366
Extended Chain -PE
Spectra 900 0.97 100 3.6 30 850
Spectra 1000 0.97 100 3.3 35 1246
Spectra 2000 0.97 100 2.9 38 1360
Conventional High Tenacity Fibres
Nylon 66 1.14 150 15-20 10 37
Polyester 1.38 150 12-18 9.5 120

Table 6 : Properties of High Performance Fibres
20. Use of HFMs in parachutes therefore has been very much restricted. So far tapes and cordages have found their use in limited parachute applications such as sport parachutes. However, with the development of improved fibres of this group like Tensylon, and some variants of dyneema may find increasing applications. Another important aspect is that in the parachute application, it is a shock load of a few milliseconds duration. Therefore the creep observed after loading for a long duration may not be occurring in very short time periods. Some more research is necessary to obtain desired properties from HFMs.

21.It is also very important for consideration that even though the fibre tenacities may be very high, but actually the woven textile material on conversion from fibres or yarns may have disproportionately reduced strengths due to low translation efficiency. Pal SK, Thakre Vikas B and Kamruddin, give a detailed report on the translation efficiency of yarn to braided cordages in their paper “High performance Braided cordages parachutes” [Journal of Man-Made textiles in India, Feb 2005]. The translation efficiency in case of nylon is as high as 87 to 92% followed by polyester (77.4%), Kevlar29 and Vectran (57%), Spectra 1000 (55%) and the least being Dyneema (48-54.5%). The advantage of higher strength is lost during conversion of fibre to textile assembly. However, the initial tenacity of the high performance fibres being quite high, the resultant strength even after conversion is still quite high compared to nylon and polyester fibres. Comparative results of strength to mass ratio of various freshly braided cordages obtained in the above study are shown in Fig 3.

Fibre Strength to Weight Ratio (x103)

Dyneema 1 144.50
Dyneema 2 134.50
Dyneema 3 119.10
Spectra 114.80
Kevalr 29 112.00
Polyester 48.00
Nylon 42.30

Fig 3 : Comparison of Strength to Weight Ratio
22.Air Permeability Air permeability is an important property of fabric used in canopies for parachutes. It is a measure of the volume of air passed per unit time through unit area of the fabric at a certain water head pressure. Air permeability is directly responsible for rate of descent of the parachute with the load. Higher porosity causes faster rate of descent and lower porosity causes slower rate of descent and increased snatch forces on the canopy of the parachute. While high rate of descent can cause damage to the load, especially human beings, during descent as well as at landing, a lower rate of descent would delay the delivery of load. This delay could have serious adverse affects in a hostile environment. Therefore it is essential to use fabric of right air permeability.

23.The above discussion on properties of aerospace textiles was aimed at bringing out the relation of properties with the performance of aerospace applications of such textiles. It is needless to say that careful selection of textile properties by the designer and an equally careful production of these textiles by the manufacturers would go a long way in assuring the performance of the safety systems used in aerospace applications.
Typical Problems encountered byusers

24.Premature withdrawal of brake parachutes.Brake parachutes are used to retard the landing speed of fighter aircraft. The life of these parachutes is specified in terms of number of deployments. In general, if the life assigned is 40 deployments, the brake parachutes are withdrawn after completion of as low as 25-30 deployments (average). The reasons are that after deployment of brake parachutes, they are jettisoned at the end of the runway. The parachute is then collected by ground crew for inspection and repacking. Most of the damages to the brake parachutes occur during the intervening period between jettison and collection. Some of the factors which could contribute the pre-mature failure are jettisoned parachutes falling into spillage of fuel, dirt and dust at the runway end, entanglement of parachute canopy with thorny shrubs, tears induced while transportation of the parachutes to servicing bays. While these are essentially environment induced failures, a re-look into design, especially the materials have a potential to reduce the pre-mature withdrawals. Another reason could be that most of the brake parachutes are pressure packed in manual rammers. Uneven ramming can causes creases or even pores to appear on the fabric. These areas then succumb to operating loads leading failure of fabric in the form of tear. Tear-proof and abrasion resistant materials with right mass density could provide possible answer.

25.Degradation due to moisture. Moisture and humidity are known to be having degradation effects on nylon, which is an essential material in all parachutes. Higher temperature like those encountered in desert regions accentuates the degradation. In order to prevent such degradations, parachutes are required to be dried at regular intervals. This requires withdrawal of the parachute from service for a minimum period of 48 hrs thereby increasing downtime of the aircraft, which could be otherwise serviceable. This process also requires massive infrastructure for drying the parachutes and man hours to undertake the job. Use of moisture proof material is certainly a solution to this problem however, such material needs to meet other specifications for parachute application.

26.Dirty Brake Parachutes. As indicated in para above, brake parachutes are jettisoned at the runway end. Dust, dirt, oil spillage would cause soiling of the parachute material. fungus and mildew formation or stains from shrubs at the runway end, chemicals in servicing bay also cause contamination on the parachute material. The design specifications do not permit any contamination or stains on the canopy of the parachute as they have the tendency to weaken the fabric. Therefore, any contamination or stains would lead to the parachute being withdrawn for repairs. Repair in such cases involves replacement of affected portion of the fabric by new fabric. The affected parachutes need to be sent to 16 BRD, Palam for repairs from as far flung places as J&K and Assam regions. The solution to this problem could be found in developing washable and/ or stain proof fabrics for parachute applications.

27.Degradation of Webbings. Webbings are generally used in harness of pilot parachutes. Harnesses are meant to strap the pilot to the ejection seat as well as provide connection to the parachute in case of any emergency. The harness is strapped to the pilot before each sortie and unstrapped after the sortie. The harness also provides for adjustment of lengths its various components to suit the size of the pilot. Therefore harnesses are the most frequently handled component of the parachute. In the process of strapping, adjustment and unstrapping, the harness material experiences tensile, bending, twisting and abrasive loads. It is also exposed to sweat, dirt and UV radiations. The result is fraying of harnesses at edges, hardening of the webbing due to action of sweat and dirt, folding of webbing leading to difficulty in strapping and adjustments. These problems could be overcome by using washable, abrasion-resistant, soft, UV stabilized webbings.

28.UV Degradation. As indicated earlier, the safety systems used in IAF are exposed to extreme weather conditions. Many of these systems are also exposed to direct sunlight for prolonged duration. The best examples of this situation are the arrester barriers installed at the runway ends of every flying station. Exposure to direct sunlight causes UV degradation of the textile materials and the life of these systems get curtailed. While a certain calendar based life (2 years) has been specified by the designer, the actual life achieved varied from 1 year to 4 ½ years. Such variation has been attributed to variation in exposure to UV radiations at different stations. As all arrester barrier nets need to be safety certified each day, a measure of abundance precaution is to test the strength of the arrester barrier straps at periodic intervals. UV stabilization of the material could not only add to life of the net assemblies but also their life could be predicted with better accuracy.

Conclusions
29.Textiles used in aerospace applications have been traditionally based on nylon as the base fibre. Nylon is very versatile fibre for use in such applications due to a host of favorable properties. Yet, certain drawbacks of nylon and feasibility of use of HFM like ECPE fabrics has given rise to a new debate in the design of aerospace safety systems, especially in the field of parachute design.

30.Optimum performance of safety systems and their maintainability should be viewed as a joint criterion in the design of safety systems. Such an integrated design approach was hitherto not considered due to lack of an alternative material. However, with advent of HFMs and availability of technology to optimize the textile properties, time has now come to look beyond traditional nylon stronghold in the design of safety systems for aircraft applications.

31.High performance materials like Spectra, Dyneema etc with their high strength to weight ratio are ideally suited for aerospace applications. Modifications of their thermal and elongation properties would make them strong contenders for application in parachute design.

32.While we continue our research in modifying the properties of HFMs, we also need to consider incorporating maintainability requirements into these materials. Some of the properties desirable from maintainability point of view are;
(a) wash ability
(b) abrasion resistance
(c) tear resistance
(d) moisture resistance
(e) UV stabilisation
(f) material standardisation

Bibliography and suggested further readings
1. J.H. Skinkle; Textile Testing; Chemical Publishing Co., Inc., Brooklyn, N.Y., 1949.
2. Man-made Textiles Encyclopedia; J.J. Press, Textile Book Publishers, Inc., New York, 1959.
3. Hall, I.H.; Journal of Applied Polymer Science, 54, 1961;
4. W.E. Morton & J.W.S. Hearle; Physical Properties of Textile Fibres; Textile Institute, Butterworths, London, 1962.
5. P.A. Koch; Microscopic and Chemical Testing of Textiles; Chapman and Hall Publisher, London, 1963.
6. Ward, J.M. and Pinnock, P.R.; Journal of Physics, 17, 1966, 3.
7. J.E. Booth; Principles of Textiles Testing; Butterworths, London, 1968
8. ASTM; ASTM D76, Annual Book of ASTM Standards, Part-32; Annual Book of ASTM Standards, Part-32, 1979.
9. S. Gupta, I. Husain and T. Tarafdar; Textile Materials for use in Parachutes; Man-made Textiles in India; Man-made Textiles in India, September, 1980.
10. William, B- Pepper Jr.; Evaluation of kevlar-29 vs Nylon for 3.8 m (12.5) dia ribbon parachute; Journal of Air craft, volume, 17, Number 3, March 1980.
11. BIS; Handbook of Textile Testing-SP:15-1981; Bureau of Indian Standards, 1982.
12. I.Husain and T. Tarafdar; Fabric Porosity and its control in Regulating Air Permeability of Parachute canopy; Seminar on Flexible Aerodynamic Decelerators & Balloon Technology, held at ADRDE, Agra, Mar, 1987.
13. BIS; Handbook on Glossary of Textile Terms-SP:45-1988; Bureau of Indian Standards, 1988.
14. Seminar; Production, Processing, structure, Properties and Applications of Man-made Fibres, vol-I & II;CEP organised by Deptt. of Textile Technology, I.I.T., Delhi, Dec., 1988.
15. Keith Benefield; Spectra High Performance Fibers for the Fabrication of Light Weight Parachute System; Proceedings of AIAA, 10th Aerodynamic Decelerator Systems Technology Conference, Cocoa Beach, Florida (U.S.A), April. 1989.
16. BIS; Handbook of Textile Testing, Part-1, Testing and Grading of Textile Fibres-SP:15 (Part-1)-1989; Bureau of Indian Standards, 1990.
17. I. Husain, S.K. Pal and Priti Saxena; Effect of Weaves and Yarn Constructions on the Resultant Properties of woven Nylon Tapes; Journal of Aeronautical Society of India, Vol. 47, no. 3, August 1995.
18. I. Husain & S.K. Pal; Effect of Constructional Parameters on the Properties of Nylon Braided Cordages; Indian Journal of Fibre & Textile Research, Vol. 22, December 1997.
19. Kothari, V.K.; Progress in Textile Science, vol 2, Textile Fiber: Developments and innovations; AFL PUBLICATIONS, New Delhi 2000.
20. S.K. Pal, Vikas B. Thakare & Kamruddin; Effect of repeated loading on the residual tensile properties of nylon load bearing members of parachute; Man-Made Textiles In India, May 2002.
21. S.K. Pal & Vikas B. Thakare; Development of High Performance Braided Cordages for Parachute Applications; Proceedings of the ‘National Seminar On Textile Ropes & Cordages’, held at IIT, Delhi during Dec 2003.
22. Pal S.K., Thakare Vikas B., Gaurav Singh; Creep Behaviour of cordages made of High Performance Fibers for Aerospace Applications; International Seminar 15-17 March 2005, ADRDE.;2005
23. S.K. Pal, Vikas B. Thakare & Kamruddin; High Performance Braided Cordages for Parachutes; Man-Made Textiles In India, 2005.;2005
25. B.W. White and D. Northey; Parachute Recovery System, Part-I Parachute materials, applications and Design; Technical Report No. A80-29657, AIAA Tech Information service, New York, USA.;
26. CEP; Technical Textiles for decelerators & Inflatables; CEP organized by ADRDE, Agra.;
27. Knacke, T.W; Parachute Recovery System-Design Mannual; Santa Bar

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