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 Builders guide to aircraft materials |
Aircraft fabric covering systemsRev. 2c — module updated June 25, 2006 [minor terminology changes to bring in line with the composites module] |
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Until the late 1950s the fabrics used for aircraft fuselage, wing and/or control surface coverings were invariably woven from natural fibres — linen or cotton — in various grades. The fabric was glued, sewn or laced to the wood or metal airframe, soaked with water to initially shrink the skin and remove wrinkles then 'doped' to further tauten, seal and protect it.
Nowadays the natural fibre fabrics are only used in repair or restoration of vintage aircraft and the covering fabrics for recreational aircraft have been adapted from other uses — yacht sailcloth for example — and are chiefly woven from polyester with some from glass filament yarns. Powered parachute wings are only factory manufactured, generally from very light weight ripstop nylon fabrics. This document deals only with those synthetic fibre fabrics, associated coating systems and laminated fabrics generally used for skins for homebuilt HGFA and RA-Aus trikes, and three axis RA-Aus aeroplanes. There is some reference to the fibreglass fabrics used in epoxy-fibreglass composite structures. 15.1 Fabric design and terminologyThe continuously drawn filaments produced in the initial stages of synthetic fibre manufacture [or manufacture of the glass, aramid or carbon reinforcing fibres used in composites] are formed into yarn. When yarn is woven into fabric the yarns running through the length of the roll of fabric are the warp, the transverse yarns are the fill or weft. The fill threads may be of different dimensions to the warp. In plain weave fabrics the warp and fill are woven over and under each other so if viewed in fabric cross-section each thread would appear as a series of 'waves'. This is the crimp and the more crimped the threads are the more they will straighten out when pulled and thus the more the fabric stretches and reduces mechanical properties. You could say that the crimp is the difference in length of an individual thread as part of the fabric compared to its length if extracted from the fabric and pulled taut. By manipulating the relative diameters and spacing of the warp and fill yarns the crimp in the warp and fill can be made the same [i.e. balanced] or with significant orientational differences, the choice affects the amount of stretch under load in the warp, fill and bias directions.The selvage is the outer edge(s) of the fabric formed by the reversing of the fill yarn during weaving. Bias is a diagonal across a piece of fabric generally at 45 degrees to the warp and the fill. A rectangular piece of cloth cut 'on the bias' from a bolt of material will have the warp and fill running at 45 degrees to the edges, somewhat akin to 45° plywood. Woven fabrics tend to stretch most along the bias and the designer generally aims to reduce that by tightening the weave. The thread count is the number of threads per inch of material, usually expressed as the warp count × fill count e.g. 65 × 58. The count is dependent on both the thickness of the yarns and the tightness of the weave. Denier values are the units of weight often used for very fine yarns; the value is the weight in grams of 9000 metres of the filament or yarn. The threads in women's everyday nylon stockings are around 15 denier. The higher the denier value of the yarn the thicker and stronger the woven material. Tex values are the weight in grams of 1000 metres of yarn. The weight of woven fabric is usually expressed in ounces per square yard or grams per square metre. Polyester sailcloth and some nylon fabrics may be expressed as ounces per sailmaker's yard the latter being 36 inches long but only 28.5 inches wide — a carry-over from the days of sailing ships and cotton sails — and equivalent to 0.79 square yards, so a fabric described as 1.1 ounce material may in fact weigh, on average, 1.4 ounces per square yard. [To convert ounces per sailmaker's yard to grams per square metre multiply by 45]. Also the weight may refer to a generic class rather than a specific average weight — 4, 6 and 8 ounce sailcloth for example. Porosity is the amount of open space within the fabric which is dependent on the fibre/yarn thickness and the tightness of the weave; a porous fabric would tend to be lighter but more permeable. Permeability is the rate of air flow through the fabric's surface. It is measured in laboratory conditions using a suction fan to produce a standard slight differential pressure and the result reported in cubic feet per minute [cfm] per square foot of fabric surface. Suppliers tend to state porosity rather than permeability. Air permeability flows between zero and three cfm are usually classed as zero 'porosity'. Permeability is of particular concern in the design of powered parachute wings. The tensile strength or breaking strength of fabrics is expressed as a force per linear inch or centimetre rather than the force per unit area used for metals. It is the tensile stress necessary to rupture a strip of fabric of the stated width e.g one inch or one centimetre and expressed as pounds force per inch or newtons per centimetre. Tenacity is the tensile stress at rupture of a fabric or yarn expressed as force per unit of the cross-sectional area or perhaps force per denier. The tear strength is the force needed to start and/or maintain a tear in a fabric under particular conditions. The modulus is a measure of initial stretch or elasticity of a fabric usually expressed as load per unit of stretch for a certain amount of fibre weight, the higher the value the less the stretch. Elongation is the difference between the length of a stretched sample and its initial length; may be expressed in 1/100ths of an inch per inch. Drape-ability is a term mainly associated with the woven cloths used in composite construction and refers to the readiness of a cloth to conform to a compound curve during layup. Ripstop fabrics have heavier, stronger threads woven at fixed intervals into the fabric and forming a discernible pattern of small [perhaps 6 mm] squares which restricts the spread of small tears. |
15.2 Polyester fabric propertiesPolyester in its polyethylene terephthalate form is a petroleum derived synthetic thermoplastic polymeric material used for manufacture of strong, reliable, durable and economic fibres and films proved most suitable for airframe covering use. Dacron is the registered trade name of the polyester fibre originally developed by DuPont but the name is often used as a generic term for fabrics woven from polyester yarn, particularly sailcloths.The fabric manufacturing process starts with molten material being extruded through spinnerets and air cooled; the very fine [5-10 micron?] filaments are then heated and drawn [extended perhaps five times original length] so that the molecular chains are arranged lengthwise and packed together in a regular manner i.e become crystalline. This increases strength, decreases stretch and improves elasticity, producing a filament of the desired denier. Both the diameter and the cross section of extruded fibres are varied according to intended use. A number [perhaps 50-100] of filaments are formed into a continuous filament yarn. When woven into fabrics the yarns will react in a particular way to the controlled application of heat. At 250° F [120° C] the fabric will shrink about 5% while at 350° F [175° C] the fabric will shrink around 10% – 15% (the maximum obtainable) and will remain at that tautened condition if it is not subsequently exposed to higher temperatures. (Above 375° F [190° C] the fibres start to soften and the fabric starts losing tension, at 450° F [230° C] the fibres are nearing the melt point.) This heat set treatment significantly tightens the weave. Full or partial heat setting may be done at the mill by passing through heated rollers after weaving or the fabric may leave the mill without heat setting. Sailcloths may be passed through the heated rollers under high pressure [calendered] which also imparts a high sheen to the surface and minimises porosity and stretch. Both heat treated and untreated fabric categories have airframe use. Sailcloth, generally used for mechanically attaching [rather than chemically bonding] the covering to trikes and slower speed three-axis aircraft, is normally heat set [perhaps calendered], stabilised [see below] and colour dyed at the mill thus it can be cut and sewn to form an aircraft covering with little further treatment necessary. Colour dyeing processes could alter a sailcloth's elastic properties which might effect the behaviour of a trike wing incorporating multicolour panels. The fabrics for chemical bonding come 'unfinished' from the mill, they are neither dyed [usually slightly transparent near-white] nor heat-set when received by the homebuilder and require considerable further work to produce a finished airframe covering. It may be difficult to obtain these unfinished or greige fabrics other than through a few specialist aviation suppliers. The foregoing are generalisations, there are many types of polyester based fabrics produced, each with particular attributes. Various substances are used as lubricants during the yarn making and cloth weaving processes. These substances may still be in the fabric delivered to the end user. Polyester fabric, polyester resins and polyester sewing threads are quite durable but will be deteriorated by exposure to ultraviolet radiation probably losing sufficient strength to become unusable after 400-500 hours exposure to full sun. However there are products and complete coating systems which will fully protect the covering for the life of the airframe, if properly maintained. Salt will also deteriorate polyester though it is generally resistant to chemical attack and while also resistant to direct micro-organism attack any organic substances (bird droppings, dirt, animal dung] allowed to remain on the surface are themselves subject to biological attack and the chemical by-products may be harmful to the fabric or fabric coating. The foregoing also applies to the manual and machine sewing threads and lacing cords used to assemble the panels to form the covering. Unprotected polyester is susceptible to oil staining, even from fingers. |
15.3 Sailcloth and laminate sails and skins The wings of the first three-axis minimum aircraft designed in Australia were very much based on the sail technology used in the yachting industry, as can be seen in the picture of the 1977 Wheeler Scout. It utilised an aluminium tube leading edge spar and a single surface sailcloth wing rather than a full aerofoil wing. The camber is formed by curved aluminium tubing battens inserted into pockets sewn into the wing fabric. If you are wondering about the VULA designation the image comes from the Vintage Ultralight and Lightplane Association collection.
 Single surface or part single surface sailcloth wings generate lift in exactly the same way as the mainsail of a Hobie Cat 16 feet racing catamaran [left] generates lift — note the similarities between the Scout wing and the Hobie 16 mainsail. The sail structural design and fabrication techniques used in the sailing community are still utilised by the manufacturers of the sewn-together panels [known as sails] for the wings of trikes and slower three axis aircraft and, of course, hang gliders.
Sailcloth is also cut into panels which are sewn together as close fitting 'sleeves' which can be slipped over the wing and empennage structures and mechanically secured at the root end of the unit as in the photo below; or the fabric can also be cemented to the rib cap strips. The sleeve forms the full aerofoil wing skins. Similarly sailcloth envelopes or blankets are used for fuselage enclosures.
Sailcloth is very tightly woven [perhaps 150-250 threads per inch] but sometimes also structurally stabilised by impregnation of polyester resins or some other polymer to further limit porosity, to provide a harder finish and/or to provide resistance to fabric stretch along the bias and thus help to maintain the aerodynamic shape under flight loads. However the resin is also subject to UV deterioration. The sailcloth weight used for ultralights is typically 4 ounce but 6 or 8 ounce fabric classes are used in wings for the heavier trikes. Sailcloth is very economical and there are many types available, generally they display good strength, low stretch and good durability but must be protected against deterioration from UV radiation by some form of UV blocking agent applied to the fabric. There are liquid blockers [303 Aerospace Protectant for example] which should be applied perhaps several times each year or there are two-part clear lacquers which provide protection for a longer period, particularly so if the aircraft is kept out of the weather when not being flown. Sailcloth covering is the lightest, least costly and easiest to apply of all the covering methods including metal, plywood, glass fibre and chemically bonded polyester fabric. It must be borne in mind that the thread and stitching used in fabricating the cover must have mechanical and UV resistance properties that are at least equal to those of the fabric. A more costly form of sail material is produced in a simple laminate form. Such laminates are low stretch, zero porosity materials made by bonding a polyester film to one or both sides of a polyester scrim or layers of scrim. Scrim is a loose open unwoven grid, perhaps 10 threads per inch of 500-1000 denier yarns, used as the load carrying material with polyester film heat and pressure laminated to one or both sides. Films have low stretch in all directions, near zero permeability and excellent adherence to scrim but low tear resistance. Mylar is the registered trade name of the extruded polyester sheet film developed by DuPont. X-Lam and GT-foil are brand names for polyester or Kevlar scrim/ Mylar film laminates and there are similar laminated fabrics with names such as Trilam and Ultralam from the UK. Some of the fabrics may incorporate a UV resistant coating. |
15.4 The chemically bonded fabric covering processFabrics. There are a number of companies who produce packaged aircraft fabric and fabric coating systems. The polyester fabrics supplied in their proprietary covering systems are similar although they are likely to have had differing post-weaving treatment. Generally there are three fabric weights offered — light [~1.6 ounce] is used for the medium speed ultralights and for protective covering of plywood skins, medium [~2.7 ounce] is suitable for all RA-Aus aircraft, heavy [~3.6 ounce] is intended for agricultural, high speed and aerobatic aircraft. Medium fabric probably costs 25% more than light fabric while heavy fabric is 10% more costly than medium. Fabrics of different weights may be used on the same aircraft, light weight on the top and sides of the fuselage and medium on the wings, under fuselage and empennage for example.The medium [~2.7 ounce] and heavy [~3.6 ounce] fabrics sold by the covering 'system' companies will most likely be marked as certified materials and, as such, much more expensive than similar weight non-certified material possibly available from other sources. All light [~1.6 ounce] fabric is non-certified. There is no Australian requirement to use certified materials in homebuilt CAO 95.10 and CAO 95.55 aircraft. When unshrunk fabric is to be bonded to the airframe the heat shrinking property of polyester allows the builder to cement the fabric to the parts of the structure with which it comes in contact then to tauten it in two [possibly three] stages during the covering process, perhaps once at 250° F [120° C], maybe again at 300° F [150° C] and, if required, a final tautening at 350° F [175° C]. The heat is applied to the fabric with a full size domestic clothes iron pre-calibrated using a thermometer to ensure accurate setting of the thermostat control. Normally the fabric will be tautened as much as is possible without distorting lighter parts of the structure — wing ribs for example, so for ultralight aircraft it may be inadvisable to go above 300° F [150° C].
Coatings. The fabric coating products and methods differ from those used to paint metal skinned aircraft; there are three basic types — polyester-vinyl, two-part polyurethane [urethane] and aircraft dope although some flexible acrylic enamels and lacquers might be used. Aircraft dopes are plasticised lacquers used to treat woven fabrics while on the airframe to provide adherence, sealing, fabric tautening and protection. Nitrate cellulose and later cellulose acetate butyrate were historically used for doping cotton and linen fabrics and 'non-tautening' versions of those lacquers are now used with polyester fabrics. Cellulose acetate butyrate dope must not be applied directly to polyester but it can be used as a build-up coat if clear nitrate cellulose dope is first worked into the polyester fabric. Non-tautening dopes will still shrink somewhat as they age so an allowance must be made for this during the initial heat shrinking of the fabric, otherwise excessive tautness developing later may pull ribs and similar light structures out of line. Dopes are highly flammable and, if ignited, freshly doped fabric will flash burn. Clear dopes produce a strong coating film, the aluminium powder pigmented dopes which block UV radiation develop less tensile strength and the colour pigmented finishing dopes are the least strong. Coating methods. All cements and coatings do not adhere very well to woven polyester and the coating techniques used must ensure that the fabric weave is encapsulated within the cement — where attached to the airframe and within the first applied coating otherwise. Thus, apart from the strength and flight characteristics, the big difference between sailcloth covering and these chemically bonded systems is that the brushed and sprayed-on cement and coating chemicals form a load carrying film [if correctly applied] and transfer the aerodynamic loads to the airframe — the fabric carries little load unless the coating is damaged. The coating adds weight to the aircraft, perhaps 15–20 kg for a completely fabric covered two seat RA-Aus aircraft and requires considerable outlay but, if done well, is aesthetically pleasing, handles the weather and is very long lasting if adequately maintained. After cementing and heat shrinking primer/sealer coats are applied, followed by build-up coats then coats containing UV blocking/reflecting solids [usually aluminium flakes and called silver coat, aluminium undercoat or similar] then final colour finish coats. Some systems incorporate the UV blocking function with the primer/sealer. It is most important that the coating methods and materials are not inter-mixed otherwise the finished coating will not be a single strong monolithic structure bonded at the molecular level but rather two or more loosely conjoined and much weaker covering layers. The process outlined in the covering system supplier's manual should be followed otherwise the results are most unlikely to meet expectations of strength, appearance and continuing airworthiness. For more information see Surface coatings and finishes. | |||||||||||||||||||||||||||||||||||||||||||||||||
15.5 Trade names for fabrics and covering systemsThe US Federal Aviation Administration issues Parts Manufacturing Authorisations [FAA/PMAs] to companies who have proven they can consistently manufacture designated aircraft parts [fabrics or fabric adhesives for example] that meet required standards and can also maintain traceability of all production lots. The best known brands of FAA/PMA approved polyester fabrics are Ceconite, Poly-Fiber [Stits] and Superflite.
The table provides a rough guide to the sequence of operations necessary to apply these fabric coatings. Step 4 includes rib lacing which is unlikely to be necessary for non-aerobatic aircraft or for aircraft with Vne less than 140 knots; but it is a good belt and braces approach which doesn't entail that much extra effort, see section 2.10 in AC 43.13-1B below. The systems represented all have pdf manuals downloadable, for $10–15, from their web sites.
The following is an estimate made by Aircraft Spruce & Specialty Co for the amount of material needed to cover a Piper J-3 Cub using the Poly-Fiber process. The Cub is all fabric and fits into the LSA category. 45 yards of Poly-Fiber fabric 6 rolls of 2 inch medium finishing tape 1 roll of 4 inch medium finishing tape 1 roll of rib lacing cord 2 rolls of 1/2 inch reinforcing tape 2 rolls of inter-rib bracing tape 1 roll of cloth anti-chafe tape 100 plastic or aluminum drain grommets 30 inspection rings 25 inspection ring covers 8 gallons of Poly-Brush fabric sealer 1 gallon of Poly-Tak adhesive 11 gallons of Poly-Spray aluminium undercoat for UV protection 5 gallons of reducer for thinning Poly-Brush 11 gallons of Poly-Tone top coat colour A general conclusion regarding cost SEEMS to be that the finished costs, for each of the chemically bonded covering systems, are much the same. A comparison of design and performance properties Daryl Irving Hammond, Oklahoma State University, published in 1999 the results of a study of design and performance properties for selected aircraft fabric covering processes. Of course the material compositions and recommended techniques may have changed since 1999 but the following is the abstract: Scope and method of study. The purpose of this study was to examine the design and performance properties of aircraft fabric covering using the Grade-A cotton with Randolph dope, Ceconite with Randolph dope, Cooper Superflite II, Air-Tech Coatings, and Stits Poly-Fiber processes. The design properties studied were characteristics of the base fabric and nonweathered coated material. The performance properties investigated were coating surface changes of gloss and yellowing, strength degradation, and response to heat and flame throughout an accelerated weathering cycle. The hypotheses were written to answer questions about how a selected fabric covering method performs over its intended life and in a variety of functional areas. Findings and conclusions. There were significant differences in design and performance properties among the five Federal Aviation Administration (FAA) approved covering processes. An investigation of the design properties pointed out differences in elongation, weight, and breaking strength. Ceconite 101 was thicker and stronger, yet stretched more than the other samples. Air-Tech and Superflite processes had higher than average weight per square foot values. An investigation of the performance properties indicated that Superflite and Air-Tech had excellent gloss retention over the accelerated weathering cycle. Ceconite with Randolph dope was the least stable in yellowing degradation while Stits was the best performer. Strength degradation was most pronounced in the Superflite process, decreasing rapidly during weathering. Thermal stress testing showed all processes exhibited heat and flame resistance loss due to weathering. Ceconite with Randolph dope was a volatile combination, bursting into flames with the application of heat, while Air-Tech and Stits resisted sustained burning after ignition. Superflite bum characteristics included emission of thick black acrid smoke. An overall performance index and best performer is provided in the author's research search implications. |
15.6 AC 43.13-1B Chapter 2If you are interested more detailed information can be found in the FAA advisory circular "Acceptable metods, techniques and practices — aircraft inspection and repair. I have placed the complete Chapter 2 'Fabric covering' of AC 43.13-1B on this Web site for download in PDF format — 486 KB.The chapter covers: 2-1 General 2-2 Problem areas 2-3 Aircraft fabric synthetic 2-4 Aircraft fabric natural 2-5 Recovering aircraft 2-6 Preparation of the structure for covering 2-7 Fabric seams 2-8 Covering methods 2-9 Reinforcing tape 2-10 Lacing 2-11 Stitch spacing 2-12 Fasteners 2-13 Finishing tape 2-14 Inspection rings and drain grommets 2-20 Application of dope - general 2-21 Dope application procedure 2-22 Covering over plywood 2-23 Coating application defects 2-30 Inspection and testing - general 2-31 Fabric identification 2-32 Coating identification 2-33 Strength criteria for aircraft fabric 2-34 Fabric testing 2-35 Rejuvenation of dope film 2-42 Repairs to fabric covering - general 2-43 Repair of tears and access openings 2-44 Sewn patch repair 2-45 Doped-on patch repair |
The next module in this fabrics, composites and coatings group is 'Plastics and thermosets'
Builders guide to aircraft materials – fabrics, composites and coatings modules
| Guide contents | [Aircraft fabric covering systems] | Plastics and thermosets |
| Reinforcing fibres and composites | Surface coatings and finishes |
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