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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