AIR LOAD CALCULATION FOR ISTANBUL TECHNICAL UNIVERSITY (ITU), LIGHT COMMERCIAL HELICOPTER (LCH) DESIGN Adeel Khalid *, Daniel P. Schrage + School of Aerospace Engineering, Georgia Institute of Technology Atlanta, GA 30332-0150 ABSTRACT Aerodynamic load calculation is one of the most important aspects of any aerospace vehicle design. As long as all the requirements are met, attaining the best possible aerodynamic shape is one of the objectives of the design of all flying bodies. Since the very beginning of air vehicle design, aerospace engineers and aerodynamicist have been trying to optimize the vehicle design for maximum possible lift and minimum drag. The objective of this research is to find the drag and lift of the fuselage designed by the ITU-LCH team. Since it is an active design, the fuselage design is kept flexible until a later stage in design. Aerodynamic analysis dictates the final shape to a great extent. Dr. Stephen M. Ruffin 1 has developed Cartesian grid flow solver called NASCART-GT. This home made software is used for the first cut analysis of air loads on the prescribed helicopter fuselage design. Introduction Helicopter aerodynamics is much more complex than any fixed body configuration design. The rotating wing makes it hard to fully analyze the helicopter aerodynamics. Computational fluid dynamics (CFD) is used today for the analysis of rotor fuselage interference. This research however focuses only on the fuselage lift, drag and pressure distribution. NASCART-GT is capable of geometry inputs from simple Cartesian coordinate surface nodes, CATIA TM V5 files, or PLOT3D TM surface files. This makes the modeling part easy and quick as compared to other CFD programs that require significant expertise for grid setup. NASCART-GT s automated grid generation and flow * Graduate Research Assistant + Professor School of Aerospace Engineering, Georgia Institute of Technology
solver minimizes these issues while conducting high fidelity flow analysis. CATIA V5 was used by the ITU team for the generation of helicopter model. Before the actual analysis of the complete helicopter fuselage, a small Bullet model was tested to compare the results with experiments and literature. The idea was to make sure that NASCART-GT not only produces the desired results but also that all the input files and variables are set at the correct settings for the desired flight condition. NASCART-GT has three options for flow analysis i.e. Euler equations, Euler and integral boundary layer equations, and Navier-stokes equation. Navier-stokes equations give the most reliable analytical results. Following section describes the details of the analysis. Flow Analysis of a Bullet A simple bullet shape is generated in CATIA V5 as shown in figure 1. Figure 1: Simple bullet shape geometry generated in CATIA for flow analysis. The CATIA meshing tool is used to mesh the body. This is done using Static Analysis module. There are two options for the cell shapes i.e. triangular and quadrilateral. NASCART-GT supports both of them. However the quadrilateral cells require less memory and CPU time than the triangular cells. Thus quadrilateral cell shapes are normally recommended 1. The mesh size should be small enough to follow the curvature - 2 -
of the body. Some experimentation was necessary to come up with the final mesh size that adjusts the body curvature. Figure 2: Bullet mesh It is usually a tradeoff between the accuracy and time. Large number of cells results in more detailed analysis but also take up greater computing time. Figure 2 shows the final grid size selected. Note that quadrilateral cells of 5mm dimensions are selected. The actual dimension of the bullet is 175mm long and 45mm radius. 5% constraint sag was used. Angles between the faces and angles between curves were all set at 0 as per recommendations 1. With these dimensions, 2030 nodes and 2031 elements were created. This mesh is then exported from CATIA as a *.dat file. NASCART uses the *.dat file as an input for analysis on any profile. This makes the interface between CATIA and NASCART fairly simple. The first analysis is performed in air and both upper and lower surfaces of the body are calculated. The free stream Mach number is set at xminf=1.85, Density at rhoinf=1kg/m 3 and pressure at pinf=1.0 5 N/m 2. The bullet is given a pitch angle of 1 degree. All the units are in International System (SI). For this particular analysis, invicid flow is selected. NASCART also does viscous flow analysis but that again requires a large amount of computer time and higher grid resolution. 3-D analysis is performed. In other words, the pressure, drag and lift is created at each and every node on the surface of the body. The minimum number of grid points along the length is set at 64 because the grid is relatively coarse. This saves some of the computing time. The maximum of 5000 iterations are selected. One of the important considerations that are necessary while doing the analysis in NASCART is that the body should be enclosed (i.e. airtight 1 ). In addition the body must have finite thickness in every location. Analysis - 3 -
of slender bodies requires more grid points than thick bodies. NASCART is executable on a PC but requires high processor speeds for fast computations. In the case just mentioned above, the NASCART can easily take up to two hours to complete the 3D flow analysis for the given grid resolution. Once the results are obtained, they can be stored as a text file. NASCART computes the aerodynamic loads at all the nodes. So information at any given node can be obtained from the text file. Figure 3: NASCART-GT analysis result on bullet There are two options for visualizing the results of NASCART. They can be either viewed using the windows based application called FIELDVIEW TM or VISCART application that is included in the NASCART-GT code and uses the VISUAL 3 interactive graphics environment developed at MIT 1. Figure 3 shows the results of the CFD analysis. As mentioned above, the pressure, drag and lift are calculated at all the nodes. NASCART-GT writes out the key solution variables (x, y, z, C p, T, C f ) for each surface panel. It also writes out computed force coefficients, moment coefficients and all freestream quantities used. With the bullet analysis complete, it was learnt that NASCART-GT is not only an easy tool to use but it performs very nicely for full Navier stokes equation. This lays a foundation for the full blown analysis of the ITU-LCH design aerodynamic analysis. ITU-LCH Design Aerodynamic Analysis The very first aerodynamic analysis done on the ITU-LCH design is on the first fuselage shape designed by ITU team in CATIA V5. It is a simple design with no empennage or - 4 -
details of auxiliary components on the fuselage. The first analysis is also done without the main rotor, hub, landing gears doors or windows. Figure 4 and 5 show the basic hulk shape. Figure 4: 3-D view of the first ITU-LCH design Figure 5: 3-D rear view of the first ITU-LCH design Notice that the fuselage has a void at the back indicating an open system. Since it is an active design, all the parts are designed independently. The idea is to design every part independently and then bring them together as an assembly. That gives high degree of design freedom. Once all the parts are put together, another aerodynamic analysis will be performed on the entire helicopter system. - 5 -
A coarse mesh is generated using advanced meshing tool in CATIA. Static analysis is performed. The mesh size is set at 150mm with 5% constraint sag. Quadrilateral elements are selected for the analysis. Angle between faces, angle between curves and minimum hole sizes are all kept at zero. CATIA creates 1861 nodes over the entire fuselage surface. The meshed body and body s cutaway section are shown in figure 6, 7 and 8 respectively. Figure 6: Meshed surface of ITU-LCH fuselage The cutaway sections show the details of the mesh along y-z and x-z plane. The correct size of the mesh is achieved by few trials. It is important to note that if the mesh size is too small, or there is large number of panels or nodes in the mesh, it may take long hours to do the complete CFD analysis. The final mesh is exported as bodyin.dat file for NASCART analysis. Figure 8: Fuselage cut along x-z plane - 6 -
Before running NASCART, few important parameters are set at their proper values. As mentioned earlier, these can be changed in the input.dat file. The first analysis is done at Mach 0.4. The free stream density is set at 1kg/m 3 and the free stream pressure at 1*10 3 N/m 2. The fuselage is given a pitch angle of 1 degree and all the other orientation angles are kept at zero. Since this is the first analysis, the flow is considered inviscid. More detailed and accurate analysis can be done after the complete helicopter design is available. Since a CATIA V5 meshed structure is used, the igrid is set at 3. The Prandtl number is set at 0.72. Most of the important information is stored in the output file called bodytab.dat. It writes out table of key solution variables (x, y, z, C p, T, C f ) for each surface panel. It also writes out computed force coefficients, moment coefficients and all free stream quantities. Figure 9 and 10 show the CFD visual results from the front and rear respectively. Figure 9: CFD result of ITU-LCH design at Mach 0.4 Figure 10: Rear view of CFD analysis of ITU-LCH design - 7 -
Table 1 lists some of the key parameters obtained from the bodytab.dat file as output. Free stream density 1 Kg/m^3 Free stream pressure 100000 Pascal Free stream temperature 348.37 C Free stream Mach No. 0.4 Free stream Reynolds No. 100000 Angle of Attack 1 degree Free stream viscosity 1.49E-03 Pa.sec Surface area 11693.66 m^2 Table 1: Some important data obtained as a result of CFD analysis Sample output file shows the pressure coefficient, temperature and coefficient of force. Figure 11 shows a snapshot from the output file. x y z Cp -5.729-2.540 4.763 1.121-5.688-3.109 4.763 1.081-5.641-3.548 4.763 8.184-5.664-3.334 4.763 2.842 Figure 11: Snapshot of the output data stored in bodyin.dat file Similarly the lift and drag coefficient of the entire body is stored in a separate file called loads.dat. This information is stored as a function of number of iterations. So the iteration history can also be investigated. This file also outputs the force coefficients along x, y and z-axes. Figure 12 shows the drag and lift convergence history as a function number of iterations. - 8 -
Drag and Lift coefficient vs. No. Iterations Drag, Lift 1.40E+03 1.20E+03 1.00E+03 8.00E+02 6.00E+02 4.00E+02 2.00E+02 0.00E+00-2.00E+02-4.00E+02-6.00E+02 0 200 400 600 800 1000 1200 No. of Iterations Drag Lift Figure 12: Convergence history of lift and drag as a function of number of iterations All the outputs from the NASCART-GT can be stored in text files for easy retrieval. NASCART-GT also provides an option of parsing the data points along a selected plane. So if it was desired to cut the body along certain axis and extract all the information on the cut away section (see Figure 7 and 8), NASCART-GT allows us to create another file called xyzplane.dat file. It is actually an input file that can be created before running NASCART-GT. Location of the area or the cut plane can easily be specified in this file. Conclusion and Recommendations The objective of this study was to lay the foundation for the aerodynamic analysis of ITU- LCH design. It has been shown that lift and drag coefficients and pressure points along the 3-D surface can be calculated using a home build tool NASCART-GT. NASCART- GT is compatible with CATIA-V5. With all the necessary tools available, it is now very easy to perform the complete and detailed analysis on the design. It is also worth mentioning that as the design progresses and further changes are made in the shape, these changes can be accommodated in the aerodynamic analysis along the way. Note that the analysis shown in this study is simple and quick. A full Navier stokes analysis in real viscous flow may need more computer memory and time. - 9 -
It is recommended for the future research that the body mesh file bodyin.dat be used directly from CATIA-V5. For generating the mesh, the number of nodes should be kept less than 50,000. Otherwise it may cost extensively long computing time and computing resources. The attitude angles of the aircraft can also be changed to simulate the yaw, pitch and roll flight conditions. Free stream flight conditions can also be changed to simulate the real life environment in Turkey. As mentioned earlier, future studies will include the complete Navier stokes analysis on the completed ITU-LCH design in a more realistic and accurate environment. The complete design will be an assembly of fuselage, empennage, main rotor, tail rotor, landing gears and all other main external components. This detailed analysis, can include rotor and fuselage interference. Reference: 1. NASCART-GT website: http://www.ae.gatech.edu/~sruffin/nascart/ by Dr. Ruffin - 10 -