[american institute of aeronautics and astronautics 13th lighter-than-air systems technology...

(c)l999 American Institute of Aeronautics & Astronautics

A99-33322 ANA-99-3910

STRUCTURAL DESIGN ASPECTS OF THE CARGOLIFTER CL 160

Ingolf Schafer’ CargoLifter Development GmbH, CargoLifter Werl? Briesen-Brand 1, D- 15 190 Krausnick

Abstract The CargoLifrer CL I60 project is described with its primary background and the actual status of the project. Furthermore, the approach for developing the planned series of airships is shown. Major design aspects will be briefly highlighted before the challenges presented by the structural configuration are shown in more detail. The two major design drivers for the structural geometry are the high aerodynamic loaa!s on the tailfins, and the large concentrated weight of the payload under the hull. These demands have led to an essentially classic semi-rigid design incorporating a structural keel coupled with a pressurised soft envelope, Nearly 50% of the weight of the payload and its attached exchange installations will be transferred to the top of the envelope through a central catenary curtain which gives the envelope a heart-shaped cross- section. Fin forces are carried by a wire-braced internal frame and transferredforward using the load paths offered by the pretensioned envelope and the keel. The structural design problems and the strategies developed to deal with them will be illustrated by graphical interpretations of finite element programs and test results.

Introduction Based on a market survey in 1995 for heavy and outsized goods [1] and an analysis of technical solutions, it was found that for the special needs of these transport problems an airship is the most promising solution. This led into the CargoLifter program with the aim of developing an alternative transportation technology. As a first step, the prototype of a series of airships is now under development. The CL 160 is planned to carry loads of up to 160 metric tons over a range of 10000 km (5400 mn) at an average airspeed of 80 km/h (43 hots).

The key operational feature of the CL 160 will be its ability to collect and deliver its payload on site without landing, using a patented load exchange procedure [3], [4] . In order to achieve the goal of starting commercial service in 2004, the development program is based on

* Dr.&g. in Aerospace Engineering, Head of Research and Conceptual Design, Member AIAA

Copyright Q 1999 by CargoLifter Development GmbH. Published by the American Institute of Aeronautics and Astronautics with permission.

the design and production of two prototypes, CL160 Pl and P2 before a pre-series of ships will be built. The aim is to come to a production rate of at least 4 airships per year. The first prototype is now under development with a conceptual design frozen in November 1998 and close to preliminary design review. It is now fixed to a size of 550.000 m3 (19.4 mill. ft3) at a length of 260 m (850 fit>. Differing from the initial concept, a single gas cell with ballonets was chosen as individual gas cells do not increase safety but complexity of the design. This ship shall be brought into air as fast as possible without increasing technical risks in order to gain operational experience which should then help to improve the design of the following ships. Therefore, PI will be used mainly for development risk reduction, while P2 will include more sophisticated techniques especially for lift control and maneuvering with a longer development time. Therefore conceptual design for P2 is right under way to start these long-lead items soon. Due to the size of the ship new challenges have to be addressed. One of these is the structural layout, dealing with loads which are unusual to airships so far. It is clear that this leads to new concepts in areas, where existing solutions are not able to fulfill the demands. This paper covers the various aspects which have been investigated in order to come to a balanced solution for the different design demands of a fast developable, low weight, high reliability, cost effective and low-risk design.

Loads As a first step, the different loads acting on such an airship were summarized and quantified. The main load groups are (Fig. 1): . static loads (mass distribution, lift distribution) . dynamic loads (acceleration forces) . aerodynamic loads (gust loads, maneuvers) . ground loads (masting and mooring) . operational loads (especially load exchange) . assembly loads This grouping allowed to pre-define the design- driving loads for the different parts and areas of the airship. It is clear that for example the nose arrangement will be highly affected by the masting and mooring loads. Due to the highly concentrated load in the middle of the ship, the hull bending moment differs from those of more conventional applications.

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\, Mas 200

(front 8 aft) U 2000 kN

vertical I ----

1000 kN vertical

1 Useful Load 2000 kN

Fig. 1: Overview of the primaq forces acting on the airship

The high-loaded area is broader than usual, requiring more load carrying structure in order to spread the loads out. Another important point is the internal pressure of the envelope, which is defined by the pressure at the lowest point. During pitched conditions, serious higher gas pressure is existent at the higher points of the shape, increasing the loads there to a reasonable amount. These changes in pressure are much higher than the required pressure at the bottom of the ship and do define in part the limit loads of the envelope material.

Eauilibrium Shapes With the high concentrated load at the bottom of the ship and the lift created at the top, an efficient load path has to be developed. As an initial step, 2- dimensional analysis of the cross section has been carried out. Several ways of internal suspension have been investigated and the load fraction between the suspension and the outer envelope has been varied. As shown in Fig. 2, the tension in the outer envelope is lowest if all loads are carried by the internal suspension. Beside the problems of the notch on the top also a very poor volume/ surface ratio has been achieved. The other extreme of no internal suspension does not only lead to a much higher load in the envelope but also the overall height of the ship is increased. In order to reach an optimum volume/ surface ratio, about 50% of the loads should be carried by the single internal suspension. This also gives an acceptable height of the envelope and the loads are reduced by about 20% compared to the unsuspended case (Fig. 3), the maximum height is reduced by about 10%. Other versions with a trilobed shape were also investigated but found to be not as efficient. Due to the long-range capability of the CL 160, the ballonets

have to be quite large (approx. 32% of the hull net volume), which restricts also space for the internal suspension. A bilobed shape with the single internal suspension has higher breathing stresses than a trilobed one, but initial FE-calculations have shown that these could be handled with an accurate tuning between the keel and the envelope.

Envelope Stress

30000 , I I I

~*ooooLJ+p

iri

0% 25% 50% 75%

Load carried by the internal suspension

100%

Fig. 2: Envelope stress for various loadfiactions carried by the internal suspension

Envelope - Keel Connection Differing from the classic Italian semi-rigid design which had an unpressurized envelope, CL 160 requires internal pressure due to two aspects: 1. Increasing load-carrying capacity for bending and

shear forces 2. Reducing breathing stresses

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0,900 i 0% 25% 50%

Load Fraction Envelope 75% 100%

Fig. 3: Area of equilibrium shape compared to circle with same circumference. Referring shapes for comparison with identical scale.

As bending, especially around a vertical axis, is restricted by the pre-tension of the envelope, a pressurized envelope is able to carry higher bending loads. Preliminary calculations of the tail loads and the related bending moments have shown that this was the most effective way to handle this task. Breathing stresses, primarily based on the changes of the shape with changes in pressure and ballonet inflation are much higher at a low pressure level because under these circumstances the shape changes much more than at a higher pressure level. At the size of a CL 160 with a maximum diameter of 65 m, changes are comparatively small from a pressure level of 450 - 500 Pa (1,8 . ..2.0 inch w. g.) on. Additional to this, it was decided to reduce the interactions between the keel and the envelope by reducing the keel in its length, giving also the advantage of reduced interaction between the keel and the tail section. Calculating the interaction between the envelope and the keel is a difficult task. Very early it was found out that the real behavior has to be expected between the two theoretical extremes called “suspended beam” and “hybrid beam”. The model of a suspended beam

states that no shear forces are transferred between keel and envelope due to bending and that the only way of interaction is based on geometrical restrictions li-om change in curvature of the keel and the envelope. A hybrid beam is understood as transferring all shear forces between the keel and the envelope, resulting in a structural element that acts as a single beam. Both models are restricted in some way by the assumptions that the envelope is acting similar to basic theory of bending. Although it was shown in the 20’s that this is in principle the case [5], effects form the non-linearity of the envelope material and the low shear-stifIhess of a woven fabric leaves a reasonable amount of uncertainty. An ideal hybrid beam would end up in a neutral axis for bending very close to the upper beams of the keel, taking advantage of the lower envelope stiffhess by having greater distance. This would finally give a much better relation of loads carried by the keel and carried by the envelope. Unfortunately, this will only solve the problems in a vertical plane but not in a horizontal plane, where the keel is lying very close to the neutral axis and will therefore not carry major fractions of the loads.

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For better understanding the interaction between keel and envelope and for improved adaptation, a finite- element model was build up. For this, it was required to use a nonlinear approach with geometrical nonlinearities and material nonlinearities. After some preliminary investigations, the ABAQUS@ system was chosen due to its capability to handle the requirements. The envelope was initially built up from isotropic membrane elements and later changed to an orthotropic material behavior. The use of shell elements due to its simpler handling is now under investigation, knowing in far more detail the general behavior of the envelope, giving the chance to interpret the results getting with shell elements. Several values of the keel and the envelope were’ varied to get a better understanding how to reach a tuned design. One aspect was to change the stiffness of the envelope material. Increasing the stiffness by a factor of four (which is basically the move from a polyester based fabric to a high-modulus fabric such as Twaron@, Technora@ or Vectran@) will increase the bending moment in the keel just by about 10% (Fig. 4).

Fig.4: Bending moment in the keel with envelope stiffness increase by a factor offour.

Due to the higher stiflhess of the envelope, two effects are taking place: The first is a reduction of the volume increase due to material elasticity, which also reduces the breathing stresses and changes in c.b. in pitched conditions, the second is higher local stress concentrations, which should be avoided as much as possible. Compared to the weaker material, local stresses at the envelope- keel interface go up by a factor of three. It is assumed that this can be reduced during detailing the design of this connection.

5 120%

5 J 100%

g 3

80%

iz 60% v)

; 40%

z 20% z

Ill 0%

-20% 40% 60% 80% 100%

Envelope Stiffness

Fig. 5: Envelope stresses with variation of envelope material stiffness

Further variations with changes in the envelope stiffness of 15%, 30% and 60% reduction showed, that these will reduce the local loads, but not as much. These results are important as they give first information about the limits to be set for quality assurance.

175% , I 1

!/I 1 Stresses / -Minimum Stresses

i (Compression) ~ ,

50% ! I I

50% 100% 150% 2Db%

Envelope Shear Stiffness

Fig. 6: Envelope stresses with variation of envelope shear stiffiess

Another variation deals with changes in shear stillness. Although the initial shear stiffness was stated as reasonably low (l/40 of the longitudinal stiffness), the effects of changes had to be studied. Going down to a shear stitiess of 50% of the initial value reduces the maximum stresses of the envelope, but the distribution is not as symmetrical as before. The greater disadvantage is that also the minimum

200


stresses will be reduced, causing a much higher risk of envelope wrinkling. Doubling the shear stiffness would give nearly the same lowest values but the maximum stresses are increased by nearly 30%. Another point is to investigate the effects of varying the stiffness of the keel. Therefore this value tias changed by +15%, +30% and +60%. Surprising enough, this has nearly no effect on the envelope stresses. Both, maximum ‘and minimum stresses remain nearly constant. This gives a much higher flexibility for the keel designer in the next design steps. The picture becomes somewhat different if also the length of the keel is varied. The basic concept was here to reduce the stress concentrations by simply enlarging the keel with elements of reduced stiffness, giving a smoother transition to the ends of the keel. The maximum stresses could not be reduced as much, but the minimum stresses (which are locally still compression stresses) can be handled in this way much better. Further investigations are under way in order to improve this approach further.

102% i -Maximum Stresses L

5

6

-Minimum Stresses / (Compression)

-

96% -i--- I I 100% 125% 150%

Keel Stiffness

Fig. 7: Envelope stresses with variation of the keel stiffness

Tail section

201

One of the common problems in airship design is the integration of the tail section. Due to the instability of the bare hull, fins are required for stabilizing the ship. Additional, the control surfaces are also used for steering to the intended flight path. All airships are statically unstable but due to the specialties of LTA systems, most of them are dynamically stable. Another advantage is that due to the size of the ships and their flight speed, also unstable ships are controllable by a human pilot or an automatic one. This leads to other definitions for the required size of the fins. Finally, flight mechanics is been asked to

propose the smallest fins at the most forward positions they can accept from their perspective. Nevertheless, this still remains a challenge for the structural designer. Historically, very often the fin forces were underestimated. This is also the case for the greater rigid ones [6]. During creating reasonable load assumptions, several approaches have been made. As a rule of thumb it was found out that an average lift coefficient of 1 ,O is of good assistance for comparing more complex models with what has to be expected. It should be noted that this rule of thumb could not be understood as a worst case approach for all circumstances. One of the key issues is how to transfer the loads from the fins into the envelope. One approach which has been followed here was to reduce the fin size and to move them forward. This has two effects: 1. More load will be carried directly by the

envelope and must therefore not be transmitted into it.

2. The greater diameter increases the load carrying capability of the envelope itseif

As a starting point it was clarified that the conventional way of fixing fins to the envelope is a potential technical solution, but due to the effect in drag increase and the expected difficulties during assembly, investigations were made about alternatives possible at the size of CargoLifter. A comparison of different designs of the fins, from full rigid one covered with fabric up to an full inflated one is under way. For better weight comparison, also the required installations for pressure control and the fuel required have to be taken into account. The whole amount of tail loads, including the fraction carried already by the envelope, has to be balanced with its counterpart, the suction area in the nose region of the airship. This means that the load has to be carried over nearly % of the length of the airship. This gives several aspects to be covered: 9 The envelope has to be able to carry the loads,

e.g. the bending stresses and the shear stresses, n The deformations have to be small enough in

order to reduce the effects horn this deformation. The load carrying capability is mostly affected by the internal pressure of the envelope. Due to bending, the internal pressure defines a pretension, and the worst bending moment should create compression forces somewhat lower than that. The maximum shear load a fabric can take is based on the pretension and can be simply computed at a small representative element [7]. For CargoLifter it was found out that the shear carrying capability is the limiting factor. A basic analysis of scalability effects has show-n that size is not affecting the required pressure level, but it is highly depending on the speed range under which the airship should be operated.

1 (c)l999 American Institute of Aeronautics & Astronautics

The other aspect is deformation of the tail under load. Due to the low shear stiffness of a woven fabric, the tail can deform in the range of more than 8 m (25 ft), resulting in a complete different aerodynamic behavior. Several solutions were discussed for overcoming this problem. This includes a stiffer connection between the keel and the tail, a second fabric layer in bias direction and local stiffening using battens lying in 45’ direction.

1 Fig. 8: model of the tail section with shear battens

The latter shall be discussed here. Basically, the idea was to transfer major amount of the tail loads directly into these battens in order to distribute them over a much larger portion of the envelope. Therefore, these battens started at the fin connection points to the envelope (Fig. 8). FE- analysis showed that the capability of this design carrying shear loads was nearly tripled compared to the unstiffened envelope. Further increase can be expected if the number of battens and the design of their endpoints is improved. Also longitudinal and circumferential stiffeners were investigated and it was observed that these will increase the load-carrying capability of the envelope to a much higher level than the original stifhress of these battens would allow to expect. The model assumes so far that battens and envelope are untensioned at the begin of envelope inflation. This gives some lacing at the location of the battens. The load carrying capability of the batten- envelope

system could be further improved by putting the battens on the pre-tensioned envelope.

Conclusion It was shown that the structural design of an airship of the size of CargoLifter CL 160 is in several areas different to existent designs. This is in some part based on the size, in some part on the environment under which this ship shall be operated. Starting with some simple, 2-dimensional analysis getting a preliminary design of the envelope’ cross section, the two major areas were identified in a second step: envelope-keel connection and the tail integration. Several calculations and investigations were shown in order to verify basic models and assumptions and to optimize the overall structural layout. This process has not been finished yet but is well under way. It was the intention of this report to show explicitly those results which could be of general interest for future airship designs.

References [I] Verein des deutschen Maschinen- und

Anlagenbaus e. V: (VDMA): Initiativgruppe CargoLifter: Die Probleme des Schwerlasttransports Studie September 1995

[2] Ardema, M.D.Economics of Modem Long-Haul Cargo Airships, AIAA Lighter Than Air Systems Technology, Melbourne, Fla., Aug. 1 l-12 , AIAA Paper 77-l 192 p.89-98 (1977)

[3] E. Mowforth: Technical Concept of the CargoLifter. Annual Meeting of the Airship Association, Nov. 1997

[4] I. Schafer: Technische Daten zum CargoLifter. DGLR Workshop Flugsysteme Leichter als Luft (25./26.4.1997), Tagungsband

[5] R. Haas, A. Dietzius: Stoffdehmmg und Formandenmg der Htille von Pralluftschiffen. Untersuchungen im Luftschiffbau der Siemens Schuckert-Werke. Luftfahrt und Wissenschaft 1913

[6] J. Cook: An Engineering History of the ZRS4/5 Fin Design, self publicized 1997

[7] C.P. Burgess: Airship Design, p. 134. Ronald Press Company, 1927

[ 81 Luftfahrtbundesamt: Transport Airship Requirements (TAR)

[93 E. Mowforth: An introduction to the Airship Airship Association Publication Nr. 3

[lo] E. S&rem, T. Maier, M. Ruiz-Almendinger: CL 160: Vergleich der Hiillenwerkstoffe. Bericht ISD5/98-CLD July 1998

[ 1110. Reinhardt: Keel Design Report CargoLifter CL 160 Steinbeis-Transfer Zentrum 1998

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[ 121 lngolf Schafer: Joey - The small CargoLifter. Proceedings 2”d International Airship Convention Bedford (UK) Airship Association 1998

[ 131 B. Kroplin, 1. Schafer: Experiences by Design and Operation of the Solar Powered Airships ,,Lotte 1 -3”, Proc. of the 11 th AIAA Lighter- Than-Air Systems Technology Conference, Clear-water, FA, May 15 - 18, 1995

[ 141 Ingolf Schafer: Em Verfahren zur Enhvurfsoptimierung von Luftschiffen, Dissertation, Universitat Stuttgart 1997

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[american institute of aeronautics and astronautics 13th lighter-than-air systems technology...

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