Conceptual Design of an Airship using Knowledge Based Engineering

Transcription

1 18th AIAA Lighter-Than-Air Systems Technology Conference<BR> 4-7 May 009, Seattle, Washington AIAA Conceptual Design of an Airship using Knowledge Based Engineering Ravindra Joshi * and Amool A. Raina Indian Institute of Technology Bombay, Mumbai , Maharashtra, India Rajkumar S. Pant Indian Institute of Technology Bombay, Mumbai , Maharashtra, India The aim of this paper is to demonstrate the benefits of Knowledge Based Engineering (KBE) in conceptual design of an airship. The design methodology helps calculate the envelope volume required to carry a user-specified payload, and arrives at the mass breakdown by incorporating the optimum usage of multiple fabric and performance estimates. Alternatively, the payload that can be carried by an airship of a specified envelope volume under a given set of operating conditions can also be estimated. The first half of the paper discusses the basics of KBE beginning with the concept of Engineering Design. The later half of the paper discusses in detail the process of KBE as applied for designing an airship. I. Introduction n engineering design aims at finding a physical relation between requirements, behavior, and properties that Ameets the customer requirements. A designer finds it very difficult to handle traditionally used design tools like Computer Aided Design (CAD), Computer Aided Manufacturing (CAM), Computer Integrated Manufacturing (CIM) for designing complex products like airships. He faces too many options in the beginning of a design and has very little knowledge about the final customer requirement. As the design work on a product progresses, it becomes increasingly difficult to make suitable changes in the design. The designer therefore ends up repeating the initial noncreative design work several times before reaching a solution. Hence there arises a need to manage the design tools and resources right from the beginning in order to automate the repetitive or non-creative design work, and allows the designer more time for the creative design work. In the current competitive environment, products are designed with high complexity, making it increasingly difficult to capture the life-cycle intent of the product, despite using highly sophisticated tools. The traditionally used tools like CAD/CIM are not requirement capture tools and they come into play after critical design decisions have already been made. The CAD models are robust and inflexible. They are developed after the engineering activity is complete and are validated via a series of design reviews. In later stages of the life-cycle process, configuration changes recommended by the designer or driven by customer needs cannot be incorporated easily in such static representations of geometry. As a result, the data from various iterative runs is lost, leading to loss of configuration control, ambiguous design and proliferation of changes to fix the errors caused by other modifications. Hence, the need has emerged to emphasize on a method for capturing the life-cycle intent that allows easy incorporation of modifications. Changes in configuration could be inflicted and controlled more effectively if we lay more importance on design intent as opposed to a static geometry. Since most designed models use either a parametric scheme or an iterative variation scheme, the CAD models thus designed will be reusable. Hence, these models will have some level of intelligence built in them. These models will be built by instantiating the old ones and validating them through simulation, optimization and sensitivity analysis. Knowledge Based Engineering (KBE) approach is one such way to do this work. 1 This paper describes how the KBE approach was applied to the design of an airship. To understand the KBE model, one needs to have a fair understanding of the basic principles of engineering design. * Dual Degree Student, Aerospace Engineering Department, IIT Bombay, Mumbai , India, Non Member. Project Engineer, Aerospace Engineering Department, IIT Bombay, Mumbai , India, Member Associate Professor, Aerospace Engineering Department, IIT Bombay, Mumbai , India, Member. 1 Copyright 009 by Rajkumar S. Pant. Published by the, Inc., with permission.

2 A. Engineering Design In principle, an engineering design aims at finding a physical relation between requirements, behavior, and properties that meets the customer requirements. Traditionally, to solve an engineering design problem, the design process encompasses multiple design phases like conceptual, preliminary and detail design, apart from multiple design disciplines like control and structures, and multiple levels of details like macro, micro, etc., as shown in the three dimensional design cube in Fig. 1. Initially, a designer faces the complexity, since he has to choose from a lot of design options and tools to find a proper set of design options that describe a feasible final solution. Apart from that, little or no information is available on the product at the beginning of the design process; and at a higher level, no mathematics is available that describes the problem. To address these problems, a designer uses problem simplification and problem decomposition. However, in order to apply problem decomposition, the designer must have an in-depth idea of what the solution could look like. He should therefore, define all the relevant model properties. His basic aim can be considered to be - to use the given resources to their full potential and get an optimized solution with minimal possible efforts. In the current engineering scenario, where availability of resources is limited, one has to take recourse to optimization techniques to achieve this aim. Figure 1. The design cube Based on the basic design principles and the base-line specifications 3 of airship design, a modified GNVR shaped model was chosen as the initial reference point for commencing the design process. II. KBE Model Layout As discussed earlier, the envelope geometry is considered to be a modified GNVR shape, by introducing an insert of length l between the section where it achieves maximum diameter. This insert would help achieve a greater range as the airship would travel at the same speed while consuming lesser power. The tail arm would also increase thereby providing better stability margin. The High Level Primitives created during this study are described in brief in this section. As more complex models of airships are planned, number of HLPs may increase. B. The Hull A cylindrical insert of length α times D is added to the above design. Hence, the envelope would now be made out of a combination of four different profiles as mentioned below. It is assumed that the value of D would have to be appropriately decreased so as to almost satisfy the geometrical constraints specified. The equations in Table 1, when drawn and merged together would look like the curves shown in Fig. ; where D and α are the only two parameters that need to be changed for optimizing the design. Rotating this profile about a fixed axis, results in geometry as shown in Fig. 3. Figure. Sketches of the profiles to be used for Hull Figure 3. Geometry of the proposed GNVR Hull shape

3 Table 1. Profile definition Profile 1 Ellipse X1 1.5Dt 5 Y1 (.5DX 1 X 1 ) Z1 0 Profile Cylindrical insert X 1.5 D+( α Dt) Y ( D ) Z 0 Profile 3 Circle X3 1.5D+ α D+ 1.65Dt Y3 16 D ( x3 1.5 D α D) 3.5D Z3 0 Profile 4 Parabola X4 (.875+ α t) D Y D[( α ) D x4] Z4 0 C. Gondola The gondola shape used in this study is a design modification of Standard size Pax/cargo gondola from. 4 The shape and size of gondola is based on existing airship in the same class i.e. Skyship 600. The HLP of gondola has been designed considering the following constraints: 1) The height of the gondola should remain constant even on increasing the capacity of cargo or number of passengers. ) The distance between seats of passenger would be almost equal hence proportionate increase in length of gondola has to be ensured in order to accommodate more seats on capacity expansion. 3) A window of standard size per passenger has been provided assuming that in majority of the cases the airship will be used for tourism purpose. Once the number of passengers in the gondola increases beyond a certain number, an additional escape window comes into picture taking into account the safety norms. When the width of the gondola increases, the dimensions of the front window in the flight compartment increases proportionately so as to provide a better view to the pilot for operational ease. The power-plant that is attached with gondola is a standard one used by Skyship 600. Several options can be made available by working in layers. For e.g. if a petrol engine is chosen, while defining the initial requirement, a particular layer corresponding to petrol engine will become visible and similarly things can be worked out for a diesel engine case and for a default case. Threads for all the independent variables have to be made available in one single file so that it gets easier for the user to control the input parameters. The design procedure for gondola involves following steps: 1. Sketching The basic sketch of the gondola is generated using the equations shown in Table. Table. Profile definition Curve Name Equation Elliptical A Cylinder B Elliptical C z z z ( a ( x a ) ) b = a 1 1 = b 3 3 ( a ( x a mid _ por ) ) b = a 3

4 . Mirroring and Offsetting The above discussed basic curves are mirrored along the axis and then the whole set of curves are offset by a distance of say gon_t units. This distance accounts for the thickness of the gondola material and thus the default value of this variable can be altered even when strength analysis is incorporated in such designs as shown in Fig Extrusion and Subtraction Figure 4. Law curves for governing the basic sketch of gondola The two curve loops are extruded with relevant dimensions and tapering is also provided so as to meet the design requirements. The internal extrusion is subtracted from the net solid form to get the required design shape later in the process. 4. Projection and further extrusion for flooring The edge of the lowermost parts of the extrusion is used to extract new curves. These curves are used for further extrusion in order to create the floor of the gondola. This variable has initially been categorized as a controlled variable, but could be modified if the role of the gondola is changed; in which case the thickness of this extrusion becomes a dependent variable. 5. Creation of windows and doors The number of windows required inside the gondola would strictly be a function of number (for standard type gondolas) of passengers inside the gondola. It is a dependent variable if only the payload is specified (for cargo) or number of passengers is given. The location of the door is fixed for a gondola of any length. A standard size dimensioning is done for the door and the windows. The requisite number of windows needs to be made as per the different types of input parameters. So it has been done by using the Array formation function in Unigraphics NX3 Software (UG). One window shaped curve is made, extruded and then subtracted from the main gondola body. This extrusion and subtraction is repeated in form of a rectangular array along the length as shown in Fig. 5. Figure 5. Creation of door and windows via subtraction of extruded features In this case, we assume one window per passenger and thus, add 1. meter additional length per passenger along with a window. Once the length of the gondola goes beyond certain value, it might get difficult to enlarge it further due to strength specifications. In such a case, we can widen the gondola and therefore add two more seats in each passenger row. Such cases may require creation of an additional escape window (as number of passengers would be more) towards the end of the gondola. This feature can very easily be provided with bigger gondolas. 6. Attachment fins for power plant The attachment fins for the power plant are made using the standard shapes from Skyship 600. Their purpose is to bind the power plant to the gondola. The shape on the two ends of such a fin will depend on the taper given to the walls of gondola and the exterior radius of the power plant casing as shown in Fig. 6. 4

5 Figure 6. Attachment fins for the power plant D. Power Plant The power plant is separately joined to construct the gondola. Figure 7 shows vof the power plant. The shape used for the power plant has not been drawn using any specific guidelines, and hence the shape has not been discussed in this study. Various options can be made available to the user while specifying the inputs for the independent variables. Figure 7. Front and the rear view of the power plant used 5

6 E. Attachment Disc The attachment disc has a very simple role yet indispensible for the design under consideration. The hull has a circular rotated geometry which cannot be easily attached to the relatively flat roof of the gondola. The attachment disc used has full knowledge about the dimensions of the gondola roof and the surface geometry of the hull. As and when the user specifies new values to the independent parameters of the gondola or the hull, this attachment disc updates its knowledge and changes its own shape so as to fit well between the two, as shown in Fig. 8. Figure 8. The attachment disc F. Fins Usually the fins of a lighter airship are attached to the skin itself while those for the heavier ones are attached to the internal solid skeleton framework of the hull. Since the study undertaken here is just a technology demonstrator, the internal skeleton structure of the hull is not considered. All four fins are joined together to handle all four collectively while attaching them during the assembly as shown in Fig. 9. The actual case would be less complex but time consuming. The dimensioning of the fins is done using the values from a previous study. 5 G. Assembling the HLP s The assembly of all the HLPs is an equally important procedure in the design process. A typical assembly is made by importing all HLPs, one at a time and correlating them by defining at least three constraints. All the independent variables have to be moved to the final assembly, to demonstrate their impact on all the HLP s together. The independent variables, which actually drive the design, have to be controlled from the assembly file. The values punched into these variables are shared by all the HLPs and the resultant is a design assembly of fully compatible parts. In the shape considered here; there are in all five major HLP s namely gondola, Attachment disc, Power plant, the Hull and the Fins. The steps used for combining these HLP s are: Figure 9. The fins used for the modified GNVR shape 1) A set of datum planes and datum axis are created to be used as a reference platform for the HLPs that are to be added. ) One by one, the components are added by selecting the suitable mating conditions for the components. Adding the initial components usually is quite easy because very few constraints exist initially. As we keep adding new components to the assembly, things get a little complicated. After the hull, the fins have to be added. Before the mating conditions are decided, a new expression fin_lo (decides the distance of the fin from front nose 6

7 of the Hull) has to be imported from the fin design file. This expression helps us mate the Fin with the Hull. Similar few other expressions also have to be imported and few new have to be created as and when required. One by one each component of the airship is assembled and the final assembly looks as seen in Fig. 10. Figure 10. Assembly of an airship After assembling all the HLP s, a sequencing of expressions has to be done. This sequencing would control all the HLP files and provide them with vital inputs. The control threads of the independent variables from all the HLP files have to be shifted to the assembly file. Apart from this, the existing methodology, 3 has been integrated in this file of assembly. So now the user can feed in some of the parameters and alter the basic design of the airship as per the requirements. III. Results A trial run for demonstrating the results of the study was carried out. Three specific cases were generated by altering the basic parameters in the parent file. The comparison of the three cases is shown in Table 3 and Fig. 11. Table 3. Comparison between 3 cases Case 1 Case Case 3 Passengers = 8 Passengers = 0 Passengers = 30 Volume obtained using integrated methodology = 5,000 m 3 α = 0 Max. Diameter = m Volume obtained using integrated methodology = 15,000 m 3 α = 1 Max. Diameter = m Volume obtained using integrated methodology = 5,000 m 3 α = 1.3 Max. Diameter = 1.54 m 7

8 Figure 11. Assembly of an airship IV. Conclusions The basic HLP for airship Hull has been developed for the GNVR envelope shape. Self contained HLPs for gondola, fins, disc and power-plant were also made using Unigraphics NX3 software. All the HLPs were successfully assembled using the principles of Knowledge Based Engineering. This assembly is capable of giving a 3D passenger airship model for any specified volume and payload requirement of the customer in a fraction of a second. Strength analysis of the structures is done using Unigraphics NX3 software. Thus, when inputs are specified by the customer, only structurally feasible solutions would be suggested by the support files. This study has effectively shown that KBE is indeed a very powerful technique that should be incorporated into the design process where in several combinations of a base product are required. Acknowledgments We would like to thank Directorate of Scientific and Industrial Research (DSIR) for sponsoring this study as a part of the Trans-Brahmaputra Ferry Project. We would also like to thank Dr. R. R. Abhyankar for his invaluable help and support given during the course of this study. References 1 Tooren, M.J.L. van, Nawijn, M., Berends, J.P.T.J. and Schut, E.J. Aircraft Design Support using Knowledge Engineering and Optimization Techniques, 46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, Austin, Texas, USA,

9 Tooren, M.J.L. van, La Rocca, G., Krakers, L., and Beukers, A., Design and technology in aerospace. Parametric modeling of complex structure systems including active components, 13th International Conference on Composite Materials, S. Diego, CA, Pant, R. S., A Methodology for Determination of Baseline Specifications of A Non-Rigid Airship, Technical Note, AIAA Journal of Aircraft, Vol. 45, No. 6, Nov-Dec Rao, G.N.V, Note on optimizing of the body of tethered aerostat/airship, Indian Institute of Science, Bangalore. 5 Pant, R. S., Methodology for Determination of Baseline Specifications of a Nonrigid Airship, Technical Note, AIAA Journal of Aircraft, Vol. 45, No. 6, Nov-Dec