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Builders guide to safe aircraft materials
Aircraft fabric covering systems
Rev. 5 — page last changed 10 September 2011
Until the late 1950s, the fabrics used for aeroplane 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 used only 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 woven chiefly from polyester, with some from glass filament yarns.
This document chiefly deals with those synthetic fibre fabrics, associated coating systems, and laminated fabrics generally used for skins for homebuilt three-axis fixed-wing aircraft. Ram-air parachute wings are only factory manufactured, generally from very light-weight ripstop nylon fabrics. Hang glider and trike sails are also only factory or specialist manufactured but the polyester sail materials are mentioned in section 15.3. There is some reference to the fibreglass fabrics used in epoxy-fibreglass composite structures. 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. The more crimped the threads are, the more they will straighten out when pulled, and thus the more the fabric stretches and reduces its 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 is 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 — for example 4, 6 and 8-ounce sailcloth.
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 expressed in cubic feet per minute [cfm] per square foot of fabric surface. Suppliers tend to state porosity rather than permeability. Air permeability flows between 0 and 3 cfm are usually classed as 'zero porosity'. Permeability is of particular concern in the design of ram-air parachute wings and sections of such wings may be constructed from 0 to 3 cfm fabric coated to lower the permeability. Such coatings make the canopy more difficult to pack.
'Spectra' and 'Dyneema' are brand names of high-performance polyethylene fibres that are replacing polyester fibres in the manufacture of parawing suspension lines. 'Kevlar' is also used for parawing suspension lines.
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 (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; it may be expressed in 1/100ths of an inch per inch.
Drapeability 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 form a discernible pattern of small (perhaps 6 mm) squares, which restrict the spread of small tears.
A gore describes a component (of a parachute for example) that is cut as a long wedge-shaped piece of fabric. A number of gores (24 for example) are sewn together to form a circular parachute canopy.
polymeric material used for manufacture of strong, reliable, durable and economic fibres and films. It has proved most suitable for airframe covering use and for wing sails. Dacron is the registered trade name of the polyester fibre developed by DuPont from the original 1940s British patent. The name is now 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 (around 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 while 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 taut 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 melting point.)
This heat-set treatment significantly tightens the weave. Full or partial heat setting may be done at the mill by passing the fabric 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 and colour-dyed at the mill. Thus it can be cut and sewn to form an aircraft covering, with little further treatment required. Colour-dyeing processes could alter a sailcloth's elastic properties, which might affect 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. They 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 very 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 that 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. 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.
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 came 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 racing catamaran generates lift. Note the similarities between a Scout half-wing and the Hobie Cat mainsail. The sail structural design and fabrication techniques used in the sailing community are still utilised by the manufacturers of the machine-cut and sewn-together components and panels (known as sails) for the wings of trikes, hang gliders and the slower three-axis aircraft.
Sailcloth is also cut into panels, which are sewn together as close-fitting 'sleeves' that can be slipped over the wing and empennage structures, and mechanically secured at the root end of the unit, as in the photo below. The fabric can also be cemented to the rib cap strips. The sleeve forms the full aerofoil wing skins. Similarly, sailcloth envelopes 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. This also provides a harder finish and/or resistance to fabric stretch along the bias, and thus helps 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 and 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. They must be protected against deterioration from UV radiation by some form of UV blocking agent applied to the fabric. There are liquid blockers (for example, 303 Aerospace Protectant), which should be applied perhaps several times each year. Or there are two-part, clear lacquers that 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. 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, which is 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. 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.
Sailcloth wing sleeves have no ripstop capability and are usually not bonded to wing ribs, so care must be taken to avoid applying sudden and excessive aerodynamic loads; read this Bantam B22 fatal accident investigation report.
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 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 it will be first tautened at 250° F [120° C] where it will shrink about 5%, maybe again at 300° F [150° C] and, if required, a final tensioning at 350° F [175° C] where it will shrink to the maximum. The fabric cements used must withstand the high shear loads imposed in the tensioning process and also the subsequent aerodynamic loads. 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 to provide the tension necessary for flight without distorting lighter parts of the structure — wing ribs for example. So, for ultralight aircraft, it may be inadvisable to go much above 300° F [150° C]. The lightweight fabric is more subject to flexing under flight loads than the heavyweight. Excessive flexing will crack and peel the surface coating, and destroy the airworthiness of the covering.
If obtainable the use of the more expensive 45° bias cloth has the safety advantage that all rips would be stopped at a rib rather than possibly progressing along the full chord of the wing or tailplane.
Coatings. The fabric coating products and methods differ from those used to paint metal-skinned aircraft because fabric coatings form part of the aerodynamic load resisting structure. 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, additional fabric tension and protection. Nitrate cellulose, and later, cellulose acetate butyrate were historically used for doping cotton and linen fabrics. '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 tension 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 that block UV radiation develop less tensile strength, and the colour-pigmented finishing dopes are the weakest.
Coating methods. All cements and coatings do not adhere very well to woven polyester. The coating techniques used must ensure that the fabric weave is encapsulated within the cement (where it is attached to the airframe elements) and within the first applied coating elsewhere. 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 when adequately maintained.
After cementing and heat shrinking, primer/sealer coats are applied. These are 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 of the proprietary systems incorporate the UV-blocking function with the primer/sealer.
It is most important that the proprietary system 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.
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 with Vne less than 130 knots; but it is a good belt-and-braces approach that 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 from their websites.
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. The volumes are expressed in US gallons (3.8 litres).
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 ½ 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 burn characteristics included emission of thick black acrid smoke. An overall performance index and best performer is provided in the author's research search implications.
download in PDF format. |
The chapter covers:
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
2-11 Stitch spacing
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'
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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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