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lta_envelopes Airship Envelopes: Requirements, Materials and Test Methods Tim Miller Mathias Mandel ILC Dover, Inc Zeppelin Luftschifftechnik GmbH One Moonwalker Road Allmannsweilerstrasse 132 Frederica DE, 19946-2080 88046 Friedrichshafen Tel: (302) 335-3911 ...

lta_envelopes
Airship Envelopes: Requirements, Materials and Test Methods Tim Miller Mathias Mandel ILC Dover, Inc Zeppelin Luftschifftechnik GmbH One Moonwalker Road Allmannsweilerstrasse 132 Frederica DE, 19946-2080 88046 Friedrichshafen Tel: (302) 335-3911 Tel.: 07541-202 05 Fax: (302) 335-0762 Fax: 07541-202 516 E-mail: millet@ilcdover.com E-mail: mmandel@zeppelin-nt.com Abstract: Current airships all employ the pressure envelope design principle. Thus the envelope must be considered as a main structural element of the airship. This paper will provide information on the design requirements of airship envelopes and materials from a designer’s point of view and material development and qualification information from a manufacturer’s point of view. Finally special consideration is given to material tear resistance and test results are presented in detail. Introduction For non-rigid and semi-rigid airships, the envelope is one of the major structural elements. It is, therefore, required that this part of an airship deserves special attention. Materials, design and workmanship must be of the highest standard possible. Additionally, material performance and overall cost need consideration. Since these requirements are in some aspects contradictory, the challenge is to find the best compromise. SECTION 1 - Envelope Specification At the beginning of development, it is necessary to specify all requirements. Form, Fit and Function require detailed investigation and analysis to provide the basis for the materials specification. Additionally, airworthiness regulations must be considered, as these will provide a guideline for the designer. The FAA - ADC (Airship Design Criteria) or the German LFLS (Lufttüchtigkeitsforderungen für Luftschiffe) are very similar and provide the minimum requirements for non-rigid and semi-rigid airships. The following is a list of relevant paragraphs taken from the LFLS, which specifically need to be addressed when establishing the means of compliance for the airship envelope. Photo 1 – Zeppelin LZ N07 § 601 General The suitability of each questionable design detail and part having an important bearing on safety must be established by tests. § 603 Materials and workmanship (a) The suitability and durability of materials used for parts, the failure of which could adversely affect safety must.… (b) Workmanship must be of a high standard. § 605 Fabrication methods (a) The methods of fabrication used must produce a consistently sound structure. If a fabrication process requires close control to reach this objective, the process must be performed in accordance with an approved process specification. (b) Each new aircraft fabrication method must be substantiated by a test program § 609 Protection of structure Each part of the airship must (a) Be suitably protected against deterioration or loss of strength in service due to weathering, corrosion, abrasion, or other causes; (b) Have adequate provisions for ventilation and drainage. § 613 Material strength properties and design values (a) Material strength properties must be based on enough tests of material meeting specifications to establish design values on a statistical basis. § 627 Fatigue strength The structure must be designed, as far as practicable, to avoid points of stress concentration where variable stresses above the fatigue limit are likely to occur in normal service. § 881 Envelope design (a) The envelope must be designed to be pressurized ........ while supporting the limit design loads for all flight conditions and ground conditions...... The effects of all local aerodynamic pressures ...... must be included in the determination of stresses to arrive at the limit-strength requirements for the envelope fabric. (b) The envelope fabric must have an ultimate strength not less than four times the limit load determined by the maximum design internal pressure combined with the maximum load resulting from any of the requirements speci fied herein. (d) It must be demonstrated by test in accordance with the section Tearing Strength of the appendix that the envelope fabric (in both the warp and woof (fill) directions) can withstand limit design loads without further tearing. (h) Internal and/or external suspension systems for supporting components such as the car must be designed to transmit and distribute the resulting loads to the envelope in a uniform manner for all flight conditions. The fabric parts of such systems and their connection with the enve- lope must be designed and constructed in such a manner that the bond are not subjected to peeling loads. ...... SECTION 2 - Compliance Aspects The following paragraphs will address each relevant paragraph and explain the relationship between the design aspects and the means of compliance. Suitability of Materials (§ 603) For the Zeppelin LZ N07 (Photo 1) a laminate of polyester basecloth and poly-vinyl fluoride (PVF or Tedlarâ) film was selected for the main hull material (Photo 2). ILC had historical data available from similar laminates used on large aerostats, which had been operational for many years. This data and experience helped to reduce the risk during certification and minimized the efforts to show compliance with the requirement of suitability and durability. Full qualification testing was performed on the new LZ N07 and was compared with available data from ILC’s aerostat material. In the same manner, the ballonet material was selected as a flexible coated nylon fabric (Photo 3). Due to the movement of the ballonet curtain within the envelope it was necessary to develop a lightweight material which provided high Photo 2 - LZ N07 Envelope flexibility without leakage. Again ILC used extensive historical data from aerostat ballonet material to define the new LZN07 material. Photo 3 - Upper forward ballonet installed in frame Protection of structure (§609) The outer cover of PVF film serves as an excellent environmental barrier to protect the structural member, here the load carrying polyester fabric, as required by §609. Environmental tests to verify behavior were carried out and the data analyzed. Since, as in many other tests, there is no specific pass/fail criteria available, only comparison to materials in use in similar products could be made. Additional data will be analyzed using the standard practice of checking “in service” material, either by installing a weather patch or testing removed envelope material. Material strength properties and design values (§ 613) The material construction needs to be designed to fulfill the specified material strength requirements. Material strength is directly related to the selection of the base fabric. Strength data for various types of fabrics is readily available. However, since material performance and cost needed to remain within defined limits, a series of qualification tests were necessary to collect a representative data base from which both compliance and economic justification could be verified. To gain confidence in the selected material strength, all the requirements of § 613 need to be considered. Enough material samples need to be tested to establish statistically sound design data. The reason for this requirement is to minimize the probability of any structural failures due to material variability. Data for the LZN07 envelope was obtained and analyzed from several lots of production material. Fabrication method § 605 The processes used in airship envelope fabrication must be properly defined to guarantee an airship envelope of consistently high quality. The strength of the airship envelope is dependent not only on the strength of the material but on the design and strength of its seams and accessories, as well as the procedures for fabrication, acceptance, packing and final assembly. ILC’s experience as a Lighter Than Air (LTA) envelope supplier and its ISO 9000 quality assurance (QA) system provided a good basis for establishing all appropriate QA System functions during airship envelope production. New design details must be established by test § 601 In addition to “standard” airship design features, the Zeppelin LZ N07 incorporated many new design features such as the integration of a rigid structure within a pressurized hull (Photo 4). Additionally, many unique subassembly features are incorporated within an airship hull and each needs special qualification testing. Table 1 shows a partial list of design details that were tested during the qualification of LZN07. Subassy Ballonet Attachment Aft Endcap Attachment Tie Tab and Cord Tab Access Port installation Longeron Lacing to Hull Hull Sleeve assy Small V-Patch Doubler Installation Tie Patch on Ballonet material Ballonet Catenary Loop Tape Ballonet Kevlar Grommet with Sleeve Installation Clear Vinyl Material Ballonet Kevlar Grommet Longeron Lacing to Hull Pressure Sense Bulkhead Attachment Manline Patch Grommet Insert with Flexible Passthrough Pressure Sensor Assy attachment Hull Grommet Installation Table 1 - List of Design Details Photo 43 - Example of Design Details Workmanship must be of high standard (§ 603) The manufacturing of an airship envelope requires a high degree of craftsmanship. Therefore it is necessary to insure that the production team is properly trained and adequate test and inspection methods are utilized. This is accomplished by establishing a set of manufacturing procedures, which defines a controlled, repeatable process. Quality assurance is provided to document and control these established manufacturing processes. Envelope design (§ 881) Specifying the anticipated loads on the envelope is mandatory. To properly define limit load (the maximum load the envelope will see in operation) the following must be considered: - Static loads resulting from the overpressure of the lifting gas. - Dynamic loads under all operational conditions (including aerodynamic loads). Helium Filling Port Access Port Landing Gear Sleeve - Additional system loads. (including local loads introduced by means of patches and accessories). Due to the rigid structure of the LZ N07 the load on the envelope is very evenly distributed. Areas of stress concentration are minimized as the main elements of the car, fins, engines, and aft wheel are all interconnected by the internal structure. There are no large suspension system loads or other features, which directly load the envelope. Because of this, strength requirements for the LZN07 hull material are primarily driven by the internal pressure of the lifting gas. Special factor of safety for envelope materials § 881(b) The current airworthiness requirement, § 881, requires a safety factor of 4 on envelope materials. This is to provide equivalent safety to that required for rigid structures where a fatigue evaluation for major parts must be demonstrated. Fatigue analysis on flexible envelope material is generally not practiced as on rigid structures. This lack of hard data and analytical methods requires other means of compliance resulting in a higher safety factor (based upon historical experience). Also it must be considered that material degradation is a function of load cycles and environmental exposure. Tearing strength § 881(d) In the same way that rigid airframe parts need to be analyzed for cracking and crack propagation, the airship envelope needs to be analyzed for tear and tear propagation. Today’s practice is to follow the Cut Slit Test Method according MIL-C-21189 which will be described in detail later. Unfortunately, analytical methods like those utilized on rigid components are not readily available for fabrics. Fabric tear and tear propagation behavior is still a fairly unexplored field. For this reason it was decided not only to collect the data required for compliance by the LBA but also to conduct additional testing in an effort to relate lab test data to real world envelope performance and increase our knowledge of tear propagation. In the next sections, both the standard and the additional test methods and data will be described. SECTION 3 - Material Development and Qualification The above-mentioned specification on envelope material and certification requirements helps define the material development and qualification process. However, different requirements including performance, cost, risk, and service life have to be considered. Therefore the material becomes a delicate balance between often competing demands such as: - Highest tensile strength vs. lowest possible mass - Maximum tear strength vs. maximum adhesion - Maximum material life vs. ease of field repair - Minimum price vs. everything. To satisfy all these demands, extensive development work and testing must be accomplished. Table 2 provides a partial test matrix of required testing for airship qualification. When multiplied times several materials (hull, ballonet), several test directions (warp/fill/bias), several environmental regimes (hot, cold, humid, high UV) the amount of testing for qualification of an airship material becomes daunting. TEST TEST METHOD Weight FED-STD-191 TM5041 Bow and Skewness ASTM D 3882 Surface Finish – Interior Visual Inspection Surface Finish - Exterior Visual Inspection Water Release - Exterior FED-STD-191 TM5504 Blocking at Elevated Temperature FED-STD-191 TM5872 Surface Polymer Characterization Infrared Spectrophotometry Tensile Modulus ASTM D 751 Breaking Strength/Elongation - Strip Method Ultimate Tensile FED-STD-191 TM5102 Breaking Strength/Elongation - Strip Method, Ultimate Tensile after Weather Exposure (QUV Chamber) FED-STD-191 TM5102 Seam Tensile Strength - Heat Seal FED-STD-191 TM5102 Seam Tensile Strength at Elevated Temperature Heat Seal FED-STD-191 TM5102 Base Cloth Breaking Strength - Ravel Strip Method Ultimate Tensile FED-STD-191 TM5104 Creep/Hysteresis Evaluation Vendor Test Method Tear Strength - Cut Slit MIL-C-21189 Para 10.2.4 FAA P-8110-2, Appendix A Tear Strength -Tongue FED-STD-191 TM5134 Coating Adhesion -Heat Seal Seam, Back/Structural Tape FED-STD-191 TM5970 Coating Adhesion - Heat Seal Seam, Cover Tape FED-STD-191 TM5970 Coating Adhesion - Cement FED-STD-191 TM5970 Film Ply Bond Adhesion (Dry) FED-STD-191 TM5970 Film Ply Bond Adhesion (Elevated Humidity) FED-STD-191 TM5970 Seam Deadload - Elevated Temp (Underwater) Heat Seal Vendor Test Method Seam Deadload - Elevated Temp (Hot Air) Heat Seal Vendor Test Method Seam Deadload -Elevated Temp (Underwater) Cement Vendor Test Method Seam Deadload - Elevated Temp (Hot Air) Cement Vendor Test Method Cylinder Deadload - Elevated Temp (Underwater) Vendor Test Method Inflated Cylinder Flex Testing Vendor Test Method Low Temp Flex ASTM D 2136 Helium Permeability ASTM D 1434 or Vendor Test Method Helium Permeability after Weather Exposure (QUV Chamber) ASTM D 1434 or Vendor Test Method Seam Helium Permeability ASTM D 1434 or Vendor Test Method Table 2 – Sample Test Matrix for Airship Hull Material As previously discussed one critical parameter for airship envelope material is its ability to resist tearing after it has been damaged. As this parameter is a function of the overall design of the fabric system, it is important to appreciate the consequences of varying fabric attributes relative to performance properties. To aid in the understanding of these trade-offs, Table 3 was constructed. It shows the effect on selected properties as the fabric attributes are varied for the same given mass of yarns. In general, these trends hold true for most coated/laminated woven fabrics. Fabric Attributes PROPERTY Tensile Strength Tear Strength Amount of Coating Requires (Mass) Fabric Stability Smaller Yarn Denier Same Decreases Decreases Increases Plain Weave Same Decreases Decreases Decreases Ripstop Weave Same Increase Increases Decreases Higher Yarn Count Same Decreases Decreases Increases Table 3 – Fabric Attributes vs Properties This table shows the delicate balance in materials design. For example, to minimize mass, you would pick a small denier, high count, plain weave fabric. To maximize tear strength you might choose exactly the opposite, a high denier, low count, rip stop fabric. SECTION 4 - Cut Slit Tear Testing Cut slit tear testing is one method of measuring the ability of a fabric to resist tearing after it has been damaged. This test was developed and utilized by U.S. Navy in the 1950s as an acceptance test for the airship hull materials of that era. It is specified in MIL-C-21189, “Cloth Laminated, ZPG2 and ZPG2W Type Airship Envelope”, Amendment 1, 15 July 59, and original 13 December 57, Para 10.2.4. It was developed because it better simulated the tearing action of a damaged inflatable than did other standard tear methods of the time (tongue/trapezoid). The Federal Aviation Administration adopted this test in FAA P-8110-2, “Airship Design Criteria”, 10 Oct 86, Appendix A as did the German LBA in “Airworthiness Requirements: Normal and Commuter Category, Airships”, 15 Sep 95, Page 42. Description of the Cut Slit Tear test This method is used to determine the tearing strength of the fabric. The fabric sample is 102mm (4”) wide x 152mm (6”) long having a 32mm (1¼”) wide razor cut slit across the center of the sample at right angles to the longest dimension (See Photo 5). Photo 5 – Cut Slit Tear Sample The specimen is placed symmetrically into clamps of a universal tester (See Photo 6) with the longest direction parallel to the direction of load applic ation. The clamps must be 25mm (1”) wide and must grip the yarns that are cut. At the start of the test the distance between the clamps (gage length) must be 76mm (3”) with the slit an equal distance from each clamp. Breaking force is applied to the sample at a rate of 305 mm/min (12”/min) (See Photo 7). The tearing strength is determined as the average load of the highest recorded peaks of five specimens recorded in pounds. Photo 6 – Cut Slit Tear Testing Initial Photo 7 – Cut Slit Tear Testing In Progress SECTION 5 – Hull Material Slit Testing On Inflated Cylinders While the cut slit tear testing provides a valuable tool for comparison testing of two fabrics and quality control testing, it has no direct correlation to tear propagation in an operational airship. Dr. A. D. Topping investigated critical slit length vs. stress levels in a paper titled, “ The Critical Slit Length of Pressurized Coated Fabric Cylinders” published in October 1973. Dr. Topping utilized inflatable cylinders in sizes ranging from a diameter of 69mm (2.7”) to 152mm (6”). J. R. Thiele furthered this investigation by attempting to correlate cut slit tear strength with “critical slit length.” Critical slit length was defined as the point at which the threads at the ends of a tear can no longer hold the stress and break. The tear becomes larger and puts increased load on the next yarns until they in turn break and the tear rapidly
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