Aircraft Conceptual Design Using Vehicle Sketch Pad
Contents
1. aft facing projected area of the separated flow regions this number is divided by the reference wing area to yield an additional Cp increment The sum of the skin friction and the separated flow Cp 1s the Cpo for the entire aircraft Using Vorview one can build a drag polar that accounts for the induced and the profile drag of the aircraft The Cpo of this drag polar is subtracted from the Cp that is calculated for the entire aircraft to yield the net parasite drag This subtraction step is applied to prevent double bookkeeping the skin friction drag of the wing and tails that is included in the two dimensional airfoil drag polars As one can see in Figure 21 the net parasite drag is applied uniformly over the drag polar An extra 15 percent is also added to account for the cooling drag of the engine The total drag is the sum of the induced profile parasite and cooling drag For this drag build up the Cp was estimated to be 0 043 and the best lift to drag ratio was estimated at 10 4 12 American Institute of Aeronautics and Astronautics Skytote Drag Polar 1 6 1 4 1 2 j c z 1 Induced 0 8 i g E Profile O pe 0 6 Parasite 0 4 Cooling 0 2 gt Total 0 0 0 02 0 04 0 06 0 08 0 1 0 12 0 14 0 16 Drag Coefficient Figure 21 AeroVironment Skytote drag polar D Exporting to CAD Tools One of the most valuable features of VSP is its ability to export its geometry to ot2. in Inventor As was the case with the fuselage the end result is quite blocky and not very aerodynamic Extensive work with chamfering and filleting would still need to be done before this engine could be seriously considered for an aircraft design Furthermore unlike VSP Inventor does not have a symmetry feature built in as such creating multiple engines for the aircraft requires an extensive amount of measuring and re measuring to ensure that the location of each engine is symmetrical to the aircraft centerline Figure 9 Inventor jet engine 4 Turning Pipe VSP Turning pipes can serve a number of functions for instance they can be used inside the vehicle to show the routing of exhaust ducts for powered lift aircraft the location of control sticks and so on This is arguably the only component that can be modeled more easily in Inventor than in VSP because the turning pipe in VSP must be created by modifying a multisection wing with a circular airfoil However the pipe s diameter length and angle can be readily modified in either program Figure 10 shows a 90 deg turning pipe created in VSP Figure 10 VSP 90 deg pipe CAD Figure 11 shows the same turning pipe created in Inventor As previously mentioned this part was easier to model in Inventor than in VSP However if a pipe component were created in VSP independently of the aircraft it could be added to a parts list and then inserted and modified as needed in future
3. middle of the screen Then push the F6 button This will give you a view looking through the duct Go to the XForm tab of Strut 2 Change the X Rot to 90 and the Symmetry to XY Go to the Geom Browser and add a propeller with Aircraft highlighted so it loads as a child part Go to the Shape tab and change the diameter until just smaller than the diameter of the duct Change the number of blades to 8 Then go to the Shape tab and adjust the chord for each station until it looks appropriate Add another Prop with Aircraft highlighted Rename this part to Vanes Change the color to yellow to see the vanes better Then increase the diameter Under the Station tab change the Twist for each station to 90 degrees Change the Chord of the vanes to 0 3 for each station Now click on the model and hit F5 Change the X Location of the vanes and prop to place them inside the duct The final pieces to add to the model are the legs For these a fuselage will be inserted Change the length of the fuselage to 4 delete all but 3 cross sections move the X Location of the legs to the correct position Change to the F10 view adjust the Y Location to line up with the duct s edge about 13 66 for this case and change the Symmetry to XZ Rename the fuselage to Legs and copy and paste the legs under the Duct Change the Z Location to 10 and the Y Location to 0 and Change the symmetry to XY Zoom into the model pushing both mouth buttons at once and reshape t
4. one of the authors was already familiar with the program We discuss 4 of the 11 basic parts plus a pipe that was built in VSP 1 Wings VSP Two wings options are available in VSP the MS_Wing and the Hybrid Wing Body HWB These are modeled in different ways For a simple rectangular wing the Multi Section Wing 1s the best choice delete all but the innermost section and reset the sweep to zero The resulting wing is shown in Figure 1 To create a Multi Section Wing one can add as many sections as needed each with its own independent sweep offset chords and dihedral angles These parameters can be changed quickly in the parameters window for the part giving the user a great deal of flexibility in the type of wing they can design Even highly unconventional wing planforms can be created with ease and simplicity as shown in Figure 2 2 American Institute of Aeronautics and Astronautics _ Figure 1 Simple VSP wing Figure 2 Complex VSP multisection wings CAD In Inventor a simple wing is easy to create One need only draw the shape of the airfoil finish the sketch and extrude However in Inventor this piece has a squared off wing tip VSP automatically creates a rounded wing tip The edge of the wing can be rounded in Inventor however the time penalty is significant By the time the process is completed in Inventor a VSP user would already be well along in creating a very complex aircraft The extruded wing created in Inve
5. tail taper ratio also can be adjusted so that the model matches the background picture Secondly an MS_Wing part is inserted as the main wing of the V Bat The additional cross sections of the wing are deleted to better fit the wing shape of the V Bat The sweep and the dihedral of the wing are set to zero and the span and chord are adjusted to the proper lengths The airfoil shape can be selected from NACA 4 digit and 6 digit airfoils or a user input airfoil The next part to be added to the V Bat model is the duct The length inlet diameter and inlet outlet diameter ratio are easily selected as well as the thickness Propellers for both the fan and the vanes are inserted next For the fan the number of blades can be selected as well as the chord length of the blades at different stations The vanes are set to a uniform chord length and the twist is set to 90 deg The X locations of both parts are adjusted to fit the background image MS_Wings may also be inserted for the struts of the V Bat A MS_Wing with two sections should be created with one section having a sweep of 45 deg The symmetry of the wing can be set so that two pairs of MS_Wings are used as part of the model This reduces the workload and also ensures that the model 1s symmetric and balanced The final pieces to be added to the model are the legs Fuselage parts can be used to make legs for the V Bat The symmetry function is used for the legs to reduce the number of parts in t
6. 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition 4 7 January 2010 Orlando Florida Aircraft Conceptual Design Using Vehicle Sketch Pad William J Fredericks Kevin R Antcliff Guillermo Costa Nachiket Deshpande Mark D Moore Edric A San Miguel Alison N Snyder NASA Langley Research Center Hampton VA 23681 Vehicle Sketch Pad VSP is a parametric geometry modeling tool that is intended for use in the conceptual design of aircraft The intent of this software is to rapidly model aircraft configurations without expending the expertise and time that is typically required for modeling with traditional Computer Aided Design CAD packages VSP accomplishes this by using parametrically defined components such as a wing that is defined by span area sweep taper ratio thickness to cord and so on During this phase of frequent design builds changes to the model can be rapidly visualized along with the internal volumetric layout Using this geometry based approach parameters such as wetted areas and cord lengths can be easily extracted for rapid external performance analyses such as a parasite drag buildup At the completion of the conceptual design phase VSP can export its geometry to higher fidelity tools This geometry tool was developed by NASA and is freely available to U S companies and universities It has become integral to conceptual design in the Aeronautics Systems An
7. Avg Curve Fit 12 838 4 85 6 F 8 9 TW Ti P Number of models built Figure 15 Model Build Times A regression curve was built for each of the test subjects independently and the coefficients of each regression were averaged to obtain the coefficients of the curve fit These averaged coefficients were then used to build equation 1 which is denoted in light blue curve of Figure 15 X represents the number of models built by the test subject and Y represents the build time in hours 7 American Institute of Aeronautics and Astronautics y 1 2399 16 1523 x 1 As Figure 15 clearly shows the time required to model an aircraft in VSP was heavily dependent upon each subject s experience with the program The complexity of each successive model was on par with the previous one Once each subject s working knowledge of the software was established usually after two or three models were created the time to build each model decreased considerably While equation 1 does not properly portray this finding the build time does not drop below 1 to 2 hrs regardless of the user s experience Another finding was that that as each user obtained more experience 1 e built more than six models the ability to model more complex contours did not appreciably effect the modeling time Appendix A contains selected models that were created during this study III How to model with VSP The VSP tool was created specifically for modeling airc
8. North American Mustang fighter from WWII The designer of this model started learning Autodesk Inventor in 2006 and uses the program daily in his full time job He is a certified application specialist for an Autodesk reseller where he provides support and training for AutoCAD AutoCAD mechanical all modules of Inventor Showcase and 3ds Max This model took him more than 40 hrs to create The same model in VSP would take from 1 to 4 hrs with only minimal experience using the program This demonstrates how simple VSP is to use in comparison with Inventor If an accurate model requires more than 40 hrs of a skilled user s time then the same task will be much more difficult for an average user Figure 14 WWII Fighter Created in Inventor VSP and traditional CAD software differ significantly in functionality learning curve and overall ability to perform For CAD software the user must make separate drawings for each individual part and combine them i Plante Samuel http www youtube com watch v hwuewll3CBw 6 American Institute of Aeronautics and Astronautics together into a model In general the time required to create an aircraft in Inventor or any other CAD package is extensive compared with that required for VSP B VSP Learning Curve Study The key feature of VSP is the speed with which designs can be modified with each iteration This allows the designer to spend more time on tasks that are directly related to the developm
9. Space Station Ohio class submarine and the Space Shuttle Currently about 50 aircraft models are available and more are added with each study that is performed these models are very useful as baseline aircraft to provide a comparative benchmark of wetted area and performance or to calibrate specific design tools through detailed geometry data The VSP component library is a set of different premade parts of an aircraft This library includes seats galleys Load Device LD containers Figure 17 and so on These components are made based on current aircraft components with proper dimensions and spacing For example a full ten row economy seat section including aisles is modeled after the Boeing 767 aircraft Figure 16 The purpose of the component library is to simplify the process for incorporating wheels seat floor cross sections and so on into aircraft models When the components are inserted users are able to view a detailed three dimensional external and internal layout floor plan of the aircraft These components also can be used to assist in sizing the fuselage to fit the payload Figure 16 Boeing 767 economy seats Components are made within VSP in a process that is similar to modeling the actual aircraft The components are created by adding parts from the Geometry Browser and then modifying them For example a single seat is simply five modified fuselages grouped together After one seat is modeled it can be copied a
10. a the local C4 can be numerically integrated to provide the profile drag At this point only the drag from the wings and the tails has been taken into account The wetted areas for each component can be used to estimate a drag coefficient This process estimates the skin fraction drag by calculating the skin friction drag over a flat plate with the same Reynolds number and then makes a correction to account for the thickness of the body The Reynolds number can be computed by using a characteristic length for that part Then the engineer must estimate where the transition from laminar to turbulent flow occurs A good initial guess is the location at which the flow trips from laminar to turbulent in the two dimensional airfoil data 11 American Institute of Aeronautics and Astronautics at the same Reynolds number Then the flat plate skin friction coefficients C are calculated with equations 2 and 3 1 326 2 Cr_ taminar Ratt E 0 074 3 F Turbulent 5 Rett To obtain the equivalent flat plate area the wetted area of the component is multiplied by the weighted average skin friction coefficient C 4 The C4 is determined by the ratio of laminar to turbulent wetted area respectively For example say that Cy 0 002 Cyr 0 005 and 25 percent of the wetted area has laminar flow Then Cy 0 00425 Now the equivalent flat plate area must be scaled up to account for the super velocities that are generated as a r
11. al shape has been established Let s add the wing next In the Geom Browser select MS Wing from the drop down menu and click Add Change the background image to the top view Zoom out to re adjust the model to line up with the background Change the color of the wing to make it stand out from the fuselage This is done in the Gen tab by selecting the color desired Rename the fuselage and wing to something approiate by changing the name in the Gen tab This will help keep your parts list organized especially when multiples of the same part are used Click on the wing part in the Geom Browser and go to the Sect tab Change the Section ID by pushing the arrow to the right of the number Delete Sections 2 and then 1 Change the wingspan to 122 0 To do this go to the Plan tab and type in 122 0 for the span Next you will have to take out the wing sweep Go to the Sect tab Lower on the screen it will have a Sweep slider Simply highlight the 65 preset value and type in 0 Move the wing to its proper position on the fuselage Go to the XForm tab and move the X Loc slider all the way to the right When you reach the end of the slider track simply type in a larger number and it will allow the wing to move further For this vehicle 48 8 is sufficient Next change the chord length of the wing Go to the Plan tab and move the Chord slider until trailing edge of the wing lines up with the background image Add a duct In the Geom Browser choose Duct f
12. alysis Branch ASAB here at NASA Langley Research Center and is currently being used at over 100 universities aerospace companies and other government agencies This paper focuses on the use of VSP in recent NASA conceptual design studies to facilitate geometry centered design methodology Such a process is shown to promote greater levels of creativity more rapid assessment of critical design issues and improved ability to quickly interact with higher order analyses A number of VSP vehicle model examples are compared to CAD based conceptual design from a designer perspective comparisons are also made of the time and expertise required to build the geometry representations as well Nomenclature Local drag coefficient Parasitic drag coefficient Skin friction coefficient Laminar skin friction coefficient Turbulent skin friction coefficient Weighted average skin friction coefficient Lift coefficient Local lift coefficient Pressure coefficient I Introduction HIS paper addresses a number of topics to the capabilities and uses for Vehicle Sketch Pad VSP To this point the need to make frequent design iterations during the conceptual design phase has prevented significant use of geometry modeling early in the design process VSP has the capability to rapidly model aircraft thus making a geometry centered conceptual design process possible Its parametric based modeling makes this software easy to learn Examples ar
13. designs This step is possible in Inventor although the user is forced to search a complex and extended list of items in order to manipulate the pipe Figure 11 Inventor 90 deg pipe 5 American Institute of Aeronautics and Astronautics 3 Comparison of VSP to CAD Some of the aforementioned components were combined to create an aircraft in both programs The results are shown below Figure 13 shows a Boeing 787 model created in VSP This model took 1 hr 23 min to create Creation of a Boeing 787 was attempted in Inventor within the same timeframe Figure 12 shows how much progress was made Figure 12 Boeing 787 in Autodesk Inventor Figure 13 Boeing 787 in VSP As one can easily see the Boeing 787 that was created in VSP has much more detail than the model that was created in Inventor Because of the time constraint the landing gear engine pylon and flap tracks were not able to be included The wings that were drawn in Inventor have no airfoil shape these components are simply thin surfaces that define the wing planform The fuselage is lacking compound curves in its design The engines are inaccurate because they are simply extruded cylinders Lastly the ability to display a three view picture in the background of the VSP window enhances the model accuracy and reduces the build time 4 An Experienced Autodesk Inventor User Figure 14 shows a detailed model of an aircraft that is a hybrid between the Supermarine Spitfire and the
14. e given of studies that have used this capability to allow this geometry centered design process Aerospace Engineer Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Young Professional Summer Student Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Student Summer Student Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Student Summer Student Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Student gt Design Engineer Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Senior Member Summer Student Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Student Summer Student Aeronautics Systems Analysis Branch 1 North Dryden St Mail Stop 442 Student 1 American Institute of Aeronautics and Astronautics This material is declared a work of the U S Government and is not subject to copyright protection in the United States AIAA 2010 658 II Why Use Vehicle Sketch Pad Computer Aided Design CAD programs have been used for many years to develop accurate representations of objects for analysis and manufacturing These programs work quite efficiently and accurately for many kinds of odd objects across a wide variety of functions and industries However in creating an aircraft the shortcomings of these software programs make the process rather difficult and time consuming which precludes their use duri
15. e training modules some of which require weeks or months of constant learning on the part of the user 1 Functionality The user s initial reaction to VSP is the amount of freedom that the program permits Most of the common functions e g slide move zoom are accomplished with the mouse or a small number of hot keys More importantly the components that are included with the program can be modified in ways limited only by the user s imagination VSP contains 11 basic parts to model an aircraft These 11 parts can be modified to create any possible variation of that part including the creation of components that are not readily listed For instance although turning pipes and landing gear are not listed in the Parts window both can be created simply by altering aspects of the Multisection Wing MS_ Wing and Fuselage parts CAD software does not have the same degree of functionality Without the aforementioned instruction maneuvering through this software is challenging As a user attempts to create a simple model without instruction many errors are encountered error codes are frequently not helpful in fixing the problem This is in part due to the complexity of the program VSP is generally intuitive 2 Examples To illustrate VSP s unique characteristics a test model of a hypothetical air vehicle is created in both VSP and Autodesk Inventor 2009 reference 2 Inventor was chosen as a representative CAD software package because
16. ent of the concept vehicle and less time interacting with the software This benefit is primarily due to VSP s parametric nature because the program knows what a certain aircraft component is and what parameters affect its shape the designer is able to quickly create and modify parts without resorting to the draw extrude edit workflow of a traditional CAD program However as with all software VSP has an associated learning curve that must be overcome if the program is to be utilized to its fullest extent To measure this learning curve a time trial experiment with three subjects was conducted The subjects each possessed very little CAD and drafting experience and were considered beginners in the field These subjects were tasked with learning the software and were then asked to create several aircraft models each subject created an assortment of different aircraft in VSP that ranged from small unmanned aerial vehicles to subsonic transports and the build time for each model was logged for comparison The types of aircraft for these time trials were specifically chosen to provide a wide range of complexity levels in order to determine whether a linear relationship exists between a vehicle s complexity and the time required to model the vehicle in VSP Complexity in this case refers not only to the number of different components that are required to create a model but also to the intricacy of the shape that is being created Note that although t
17. esult of the thickness of the component this is referred to as the form factor Wings and fuselages have separate form factor regressions The wing form factor scales with wing thickness to cord and the fuselage form factor scales with the fineness ratio length diameter Figures 6 and 7 in reference 1 contain body and wing form factors respectively The final step in this skin friction calculation is to divide the scaled up flat plate area by the reference wing area to yield the Cp of that component Then the Cp are summed for each component Table 3 shows a sample skin friction buildup for the aircraft that is depicted in Figure 22 The aircraft that is being modeled in this example is the AeroVironment Skytote which is a vertical takeoff and landing VTOL tailsitter with counter rotating propellers Table 3 Sample Skin Friction Drag Build Up Wetted Mean Coo Component Area Chord AvgRe Laminar Turbulent T C Length Diam Component Wing 20 9 1 6 2 348 224 50 50 15 0 0063 Tails 13 261 0 75 1 100 730 60 40 10 0 0037 Legs 0 964 1 467 640 0 100 6 7 0 0004 Totals 60 6 018 Any areas in which the flow separates must be estimated and added to the drag buildup If we continue with the sample aircraft that is shown in Figure 22 separated flow is exhibited on the aft face of the landing gear legs shown in black and at the aft face of the fuselage The Cp is assumed to be 1 in the separated flow regions This Cp is multiplied by the
18. he legs To do this under the XSec tab move the location of Cross Section 2 to 0 999 and remove the interpolated cross sections on the Profile tab This will help to give the legs a flat base References Feagin R C Delta Method An Empirical Drag Buildup Technique NASA CR 151971 1978 Autodesk Inventor Autodesk Inc San Rafael CA 2009 Rhino 3D McNeel North America Seattle WA 2009 17 American Institute of Aeronautics and Astronautics
19. he model Parts such as fuselages and wings can be used to create other parts of the aircraft rather than just the actual fuselage and wing portions Models can be quickly assembled to aid in the design process of aircraft Appendix B gives more detailed step by step instructions for assembling the V Bat 8 American Institute of Aeronautics and Astronautics IV What You Can Do with VSP A Built In Libraries Libraries have recently been added to VSP as a way to sort aircraft components airfoils and complete vehicles These pieces or models can be opened or inserted into VSP inserting a model overlays it into the model that is currently open While NACA 4 series 6 series biconvex and wedge airfoils are already programmed in as parametric functions of the camber camber location thickness ideal C leading edge radius and series type an arbitrary airfoil can be used by providing a simple file that contains a title symmetry flag number of upper and lower points and the X amp Y locations of the upper and lower surfaces The airfoil library contains several examples of custom airfoils to guide the development of custom airfoil files The aircraft component library contains complete vehicles these are grouped according to market segment classification 1 e General Aviation Gliders Military Personal Air Vehicles PAVs Rotorcraft Supersonic Transports Unmanned Aerial Vehicles UAVs and Other which includes such models as the
20. her geometry programs for further manipulation At the start of the preliminary design phase engineers frequently want the qualities of a CAD package to further refine the design VSP can readily export its geometry to other CAD software The export method of choice depends on the purpose Choices include basic point data Xsec file computational fluid dynamics CFD geometry Felisa and NASCART solid modeling Stereolithography CAD Rhino 3D reference 3 or several standardized geometry formats Techplot Stechplot and POVRAY Typically Rhino is used extensively as an export medium to translate the geometry into many additional formats Rhino was selected for use here because it is one of the more affordable CAD programs and it has a relatively easy learning curve which enabled as many translation capabilities as possible Exported components maintain their grouping integrity so that each component can be manipulated within Rhino A typical use for Figure 22 AeroVironment Skytote Rhino export is translation of the geometry into a Maya model through a Rhino OBJ file or moment of inspiration file for engineering visualizations that are more aesthetically appealing 1 e animations and computer generated imagery CGI movies One piece of guiding advice is to increase the number of points on the Gen tab for each VSP geometry component prior to exporting to Rhino to increase the smoothness of the model and the quality of the parame
21. his latter factor is subjective it remains a contributing factor to the model s overall build time because as more work is required to create very intricate shapes For instance a simple delta wing planform and cylindrical fuselage are much easier to model than the contours of the SR 71 simply because more work is required to create the compound curves and resulting intersection surfaces During the course of the time trial strict control of communication between the subjects was required Casual interaction between the subjects was permitted however discussion of VSP modeling techniques or strategies and time data was strictly prohibited To ensure an accurate assessment of each subject s progression each subject was given different aircraft assignments to recreate in VSP The General Atomics Predator A was chosen as a control This control model was the first or second vehicle that was modeled in VSP by each subject after the subject had read through the user s manual once The intent in using the control was to provide an initial measurement of each subject s grasp of the program For each model subjects were told to adhere as much as possible to the exact contours of the source aircraft in order to create a sufficiently high fidelity VSP model for initial aerodynamic analysis The results of these time trials are shown in Figure 15 25 Subject 1 ho O1 Oo Time to build hrs 10 Subject 2 Subject 3 5
22. ics Oe ee Figure 20 Cabin floor layouts Table 2 Cabin Floor Area Breakdown B767 400 N2A Airframe ft 6 8 52 7 3 306 Aisle area per passenger ft pass 1 82 1 88 Total cargo capacity number of LD 2 containers 38 44 The final comparison between the two airframes shows that the HWB airframe is capable of accommodating the same number of passengers and nearly 16 percent more cargo than the B767 400 configuration while maintaining relatively the same floor space per passenger as well as the same galley lavatory and closet space per passenger The HWB airframe requires roughly 3 2 percent more aisle space Furthermore the N2 airframe exhibits a 25 percent greater span than the B767 but has only 15 percent more wetted area Thus the unconventional configuration can accommodate the same number of passengers but with a greater cargo volume relative to the conventional configuration This should yield a lower fuel burn due to its increased aerodynamic efficiency C Drag Estimation Example Another activity for which VSP is useful is building high resolution drag polars After a model has been created the CompGeom tool can be run to calculate the wetted areas of the model Then Vorview a vortex lattice software embedded in VSP can be run to calculate the local C at each location along the wing and tails for a given flight condition This tool also can provide the induced drag Using the two dimensional airfoil dat
23. lage 3 Jet Engine VSP Jet engines represent an important and difficult portion of design An engine must appear realistic for the design to be of value however often a design requires multiple engines which must be spaced evenly along a predetermined axis Figure 8 shows two default VSP engines By selecting one of four symmetry options 1 e XZ XY YZ None the user is able to duplicate the engines and separate each one some distance away from the aircraft s centerline Only a minimal amount of effort is required in this instance specifying the distance at which each engine is to be offset from the center The user is relieved from further tedious actions VSP permits great detail in the creation of the engines Every aspect of the engine is parameterized for example length diameter hub diameter inlet and exhaust nozzle area ratio external mold line shape factors and many more The user can even model the inlet duct like the center engine on the Boeing 727 If the user is unable to create the desired engine shape a fuselage or a duct can easily be adapted to look quite similar to an engine VSP also has a propeller component that is very flexible The default propeller settings resemble the propeller on a typical general aviation airplane a helicopter rotor can easily be modeled too Figure 8 Default VSP jet engine 4 American Institute of Aeronautics and Astronautics CAD The jet engine shown in Figure 9 was created
24. lowing the conceptual designer to make use of a geometry centered tool earlier in the design process 14 American Institute of Aeronautics and Astronautics Appendix A Selected Models from Learning Curve Study Figure A1 Aerosonde Laima Figure A2 AeroVironment Skytote Figure A3 General Atomics Predator A Figure A4 Lockheed SR 71 Figure A5 MLB V Bat Figure A6 VTOL Technologies Ltd Ducted Fan UAV 15 American Institute of Aeronautics and Astronautics Appendix B Detailed Instructions for the Creation of the MLB V Bat In building a model of an existing aircraft the first step in the process is to find a 3 view drawing of the aircraft For the case of the MLB V Bat a three view was not available but a SolidWorks Model was obtained In SolidWorks drawings it is possible to take screenshots of the model in different views so these images were saved as jpg files and inserted into the background of the VSP main window 2 3 4 10 11 12 13 14 15 16 17 18 19 20 2i Open VSP Go to Window at top Select Background In the Background window select JPEG Image This will bring up a window to find the jpg image you would like to insert as the background Select your model s side view and click OK The most common part to start with when building an aircraft model is the fuselage as this is what most other components connect to Go to the Geom Windo
25. nd pasted to make more seats in the proper X Figure 17 LD 2 and LD 8 containers Y and Z locations After a full seat cross section has been created more rows can be added by simply copying and pasting the entire row behind the existing row with the proper seat pitch Within the library components are organized into different folders based on the type of component and the aircraft for which they are modeled The component library clearly demonstrates that VSP is not limited to aircraft modeling alone With enough expertise nearly anything can be modeled in VSP B Aircraft Internal Layout Study The objective of this study was to compare the seating area per passenger and the cargo capacity for a tube and wing TAW and HWB aircraft This study involved the modeling of multiple HWB and TAW configurations as well as the passenger cabins and cargo bays for each airframe The seating galleys lavatories and cargo containers were arranged for each cabin and bay to form the internal layout of each configuration The Boeing 767 400 and the NASA HWB N2 Baseline airframe configurations were compared Each airframe accommodates the same number of passengers 306 in a multiclass arrangement with the same cabin floor galley lavatory and closet area per passenger the N2 aircraft required only 3 2 percent more aisle space Also the N2 airframe is able to carry almost 16 percent more cargo than the B767 400 This study demonstrates that this advanced c
26. ng conceptual design VSP was created to address the specific needs of aircraft conceptual design A Modeling Time with VSP as Compared to CAD Software The main difference between CAD software and VSP is ease of use Users who possess even a passing familiarity with aircraft components and terminology can learn 75 percent of the features of VSP without formal instruction by simply using the program The remainder of the features can be understood by reading the manual which may take an hour at most The learning curve for VSP is negligible relative to CAD This ease of use is possible because VSP parametrically defines each part For example a wing may be defined by its area taper ratio and aspect ratio If one of these values is changed the part simply adjusts without requiring any redrawing on the part of the user CAD programs are highly complex and require what can at times be a difficult learning curve Creating a model in these legacy programs is a complicated process that involves multiple steps per part This is due to the fact that legacy programs are not designed for any specific type of design while this may impart some measure of flexibility to said programs it also forces users who must operate within a specific design field 1 e aircraft to learn features and abilities for which they will have no use This is further seen in the training regimen for these programs in which portions of the software are compartmentalized into separat
27. ntor is shown in Figure 3 Creating a MS_Wing with the appropriate airfoil in Inventor is time consuming even planning how to tackle such a complex object takes time Although one can create a block with many sections and one section with an airfoil no readily apparent method exists for combining the two Figure 4 shows a multi section wing that was created in Inventor Figure 3 Simple Inventor wing Figure 4 Multisection Inventor wing 2 Fuselage VSP The fuselage is a key component of an aircraft VSP lets the user truly create a realistic representation of their idea Figure 5 shows the default fuselage shape in VSP However by simply moving points around on the fuselage one can create an unorthodox fuselage such as the Seabee built by Republic Figure 6 in a short amount of time Regardless of the desired shape the fuselage feature in VSP can create it 3 American Institute of Aeronautics and Astronautics Figure 5 Default VSP fuselage Figure 6 Republic Seabee fuselage in VSP CAD A fuselage is much more difficult to create in Inventor Figure 7 shows a slender cylinder attached to both a sphere and a semicone created to represent a fuselage shape similar to the default VSP fuselage Theoretically one could model any shape to the rear of the cylinder to create the tail cone however the procedure is complex and the end result is unrealistic to accomplish in the conceptual design phase Figure 7 Inventor fuse
28. onfiguration matches or exceeds the conventional one of the same size class in terms of passenger and cargo capacity 9 American Institute of Aeronautics and Astronautics An internal layout of the TAW aircraft was modeled by using internal dimensions of the B767 400 which are listed in Table 1 An interior also was laid out for the N2 configuration using similar constraints listed in Table 1 as well Table 1 Cabin Sizing Dimensions Dimension B 767 400 N2 HWB Floor height in 8 14 9 Window seat approximate headroom 18 seat top to cabin ceiling distance in Window seat approximate cabin height in 60 Seat distance from floor in 12 43 12 Window seat horizontal distance from edge distance from cabin edge to point of minimum headroom on seat in Centerline cabin height in The decision was made to design the initial cargo bay for the N2 airframe to accommodate LD 2 and LD 3 cargo containers each with a height of 64 in The cargo bay must be at least 66 in height The cabin modeling for the B767 400 and the N2 aircraft are shown in Figure 18 and Figure 19 respectively Figure 18 Boeing 767 400 interior layout Figure 19 NASA N2 interior layout Next the airframes were compared The cabin floor galley lavatory closet and aisle areas per passenger were calculated with the CompGeom function as shown in Figure 20 The results are presented in Table 2 10 American Institute of Aeronautics and Astronaut
29. raft Thus aircraft specific parts make up the list from which the user can select the specific part to be modeled Everything on the aircraft can then be modeled by manipulating these parts such as fuselages wings and propellers As evidenced by the previous data a complete model can be built with no difficulty This simplicity is further explored in the following sample process which documented the modeling of the V Bat built by MLB A Creation of the MLB V Bat In building a model of an existing aircraft the first step in the process is to find a three view of the aircraft For the case of the V Bat a three view was not available However top side and front views were obtained These images were saved as jpg files and inserted as the background in VSP This allowed the engineer to then model the aircraft directly on top of the picture to ensure that a reasonable representation was made A common starting place in building an aircraft model in VSP is the fuselage as nearly all other parts are connected in relation to it The V Bat model is no different After adding the fuselage the length may be adjusted to the correct length simply by typing the value into the length field The diameter and shape of the fuselage can also be specified The cross section shape can be a circle an ellipse or another shape or even a custom shape read from a user input cross section For a more detailed model additional cross sections can be added The nose and
30. rom the pull down menu Click Add Reposition the fuselage to match up with the background image Go to the XForm tab for the Duct and move the X Location to line up with the background image Tip you can store view angles on the F1 to F4 keys Hold down Shift then push one of the function keys If you make a mistake and it appears the view has locked up press the letter C key Change the Inlet Diameter of the Duct by going to the Shape tab and typing in 29 6 from the dimension drawing of the V Bat Next Change the Inlet Outlet Ratio to 1 22 29 6 24 25 1 22 by typing 1 22 into the field Finally change the length of the Duct until it lines up with the background picture For this case it is about 7 86 16 American Institute of Aeronautics and Astronautics 22 23 24 2S 26 oP 28 29 30 The Struts holding the fuselage to the duct will simply be made from MS Wings Insert an MS Wing and delete section 2 Move the wing to its proper X Location Change the Sweep of Section O to 0 degrees and Section 1 to 45 degrees Next set the span of section O until it lines up with the straight portion of the strut Change the TC and RC until the cord lengths line up Rename the MS Wing to Strut 1 Then highlight it in the Geom browser and click the Cut button Highlight Aircraft and hit Paste Then push the Copy button highlight Aircraft again and hit Paste Rename the second Strut 1 to Strut 2 Click on the
31. tric definition All of the surfaces in VSP are based on bicubic Bezier methods During Rhino output these are converted to bicubic NURB surfaces An example of a still picture from Maya is shown in Figure 23 This was created by using the VSP outer mold line geometry with the addition of texturing and lighting and slight modifications of cutouts fillets and other detail highlights Over the next year the addition of a structural layout geometry file that is consistent with the needs of finite element models FEM will be developed and added as an additional export option for VSP 13 American Institute of Aeronautics and Astronautics Figure 23 Sample Maya model taken from VSP geometry V Conclusion Having a geometry tool such as VSP as the center of a conceptual design methodology promotes a more rapid conceptual design cycle If all other tools such as weights aerodynamics performance and so on are able to pull their geometric inputs from one central geometry tool then for each design iteration subsequent tools have the most current geometry This reduces configuration version errors As a geometry model is built a degree of realism is added to the design because geometric constraints that have not been anticipated initially become apparent Because the conceptual designer can more rapidly visualize the concept problems can be addressed earlier in the design process This is all made possible by the short learning curve of VSP al
32. w and find the Fuse item from the drop down menu Click the Add button If the length of the fuselage is known go to the Shape tab in the Fuselage Geom window and type length into the length field the preset value is 30 For the case of the V Bat 66 17 is entered While pushing both mouse buttons push the mouse forward this will zoom out Pushing the mouse backward will zoom in Pushing only the right hand button will translate the model and pushing only the left hand button will rotate the model For now only use the zoom and translate functions Align the fuselage up with the fuselage of the background Next go to the XSec tab Change the height and width of Cross Sections 1 2 and 3 by entering the diameter of the fuselage into the field In this case 8 is entered To switch between cross sections push the arrows to the right of Cross Section ID located at the top of the tab Add XSections to increase the accuracy of the model To do this go to Cross Section ID 1 and click the Add button The new section will be added to the right of the selected cross section Obtain a total of 8 cross sections evenly spaced out To move a cross section slide the Location slider Next we will get the nose and tail cone shaped correctly Select the Shape tab and about mid way down two sliders will be found called Nose Rho and Aft Rho Slide these until the desired shape is achieved We will now leave the fuselage alone for awhile as the gener
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