Weight Estimation Using CAD In The Preliminary Rotorcraft Design

Transcription

1 Weight Estimation Using CAD In The Preliminary Rotorcraft Design M. Emre Gündüz 1, Adeel Khalid 2, Daniel P. Schrage 3 1 Graduate Research Assistant, Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA , USA gtg128s@mail.gatech.edu 2 Systems Engineer, Avidyne Corporation 55 Old Bedford Road, Lincoln, MA , USA akhalid@avidyne.com 3 Professor, Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA , USA daniel.schrage@aerospace.gatech.edu Key words: Concept, rotor, design, weight, computer, CAD Abstract: Weight estimation of aircraft, including rotorcraft has always been a critical part of design process. Since most aircraft are designed based on a baseline similar to the concept at hand, current methods utilized for weight estimation rely on either extrapolation using statistics and historical data, or analysis software based on performance requirements. These approaches provide the designer with a rough approximation at the concept generation stage, but they may not always be adequate in the subsequent stages of design. They usually supply a total weight for each subsystem of the aircraft, such as transmission group, rotor group, etc. They do not assign a weight for each part in a subsystem assembly. A method to be applied using Computer Aided Design (CAD) software during vehicle engineering analysis for calculating weights of each component as well as the entire aircraft is proposed. It helps acquiring detailed weight assessment earlier in the design stage, and in turn, brings development costs down. This method depends on CAD drawings of all components of the aircraft. The CAD program is linked with design and analysis software. After any design change, the total weight can be recalculated automatically, enabling detailed weight information in every design iteration. A comprehensive weight optimization involving component size and shape modifications can also be performed. CAD software that permits assembly generation can also be utilized to obtain exact location of center of gravity of the aircraft, for every conceivable passenger or payload distribution during the mission. For proof of concept, a Schweizer TH 330 helicopter and several other helicopter rotors are modeled and calculated blade weights are compared with weights estimated using Prouty s equations [1]. It was found that even crude model of a helicopter rotor with correct dimensions and airfoil shape may result in a better approximation to the actual weight of the blades. 1

2 1. INTRODUCTION Estimating the helicopter weight has always been a challenge for weight engineers in the conceptual and preliminary design stages. Several techniques have been suggested in the past including the group weight estimation. Usually historical data from the helicopters of same or similar class is used to approximate weights of individual components. For example, the weights of avionics group can be obtained in the initial design stage using rough estimates of avionics based on historical data. Prouty et al [1] suggest lumping the weights of helicopter components into groups and using historical data to formulate approximate empirical models. These techniques yield results that are inaccurate and therefore do not provide satisfactory component weights for the weights engineers. Lack of correct or accurate weight information leads to incorrect estimation of cost and that results in several expensive design iterations. One of the objectives of concurrent engineering is to bring detailed design information early in the design stage. As shown in Figure 1 [3], more design information early in the design provides more design freedom and reduces committed cost. Weight engineers need the detailed design information as early as possible so that other disciplines that are dependent on the weights group can perform accurate analyses. Figure 1. Design freedom, Knowledge and cost relationship [3] Use of vehicle engineering early in the design stage is suggested in this paper for accurate weight estimation. Detailed Computer Aided Design (CAD) models can be linked with design and analysis software. Modern CAD models also help in determining the weights. By linking the design software with CAD packages, detailed and accurate weight information is brought to the design database, which is accessible by other disciplinary experts. This technique helps the designers to estimate helicopter weights in the preliminary design stage with a high degree of accuracy using the actual CAD drawings of the new helicopter under consideration. 2

3 Once a link has been established between the CAD software and the design package, updated weight information can be obtained as the design changes. Component weights, and subsequently overall empty and gross weights do not have to be recalculated every time the design changes. This approach guarantees automated weight estimation. Additionally, once the weight calculation is automated, it can also be optimized. This approach enables the designer to obtain detailed component weights as opposed to group weights. The level of detail and precision of weights depends on the level of accuracy and detail of the actual CAD model and component material properties. This approach is evolutionary in nature. If the existing helicopter weights information already exists, then this approach can be used to calculate and update the weight information for a new design where all the changes in the design are captured. This approach is also visual in nature. It is compatible with weight reports used in industry and military, such as MIL-STD Using the CAD model, the designer can also calculate the center of gravity and moments. 2. METHODOLOGY Helicopter design consists of several disciplines that interact with each other. In an optimization problem, design information flows between disciplines at every system iteration as shown in Figure 2 [2]. One of the key disciplines in the preliminary design process is the weight engineering. Traditionally, weights are estimated in the initial design stage both by extensive experience and by good judgment about existing and future engineering trends. Multiple linear regressions can be used to derive equations for each aircraft component from weights data of previous aircraft. This determines sensitivity with respect to every parameter that logically affects the weight of the component [1]. The resulting equations are continually modified as more modern helicopters are added to the database and as detail design of specific components is accomplished. Prouty [1] lists a set of regression equations for preliminary design weight estimates based on work done by Shinn et al [5, 6]. These equations are used to determine the initial system weight estimates of fuselage, landing gear, nacelle, engine installation, propulsion subsystems, fuel systems, drive systems, cockpit control, instruments, hydraulics, electrical, avionics, furnishings and equipments, air conditioning and anti icing, and manufacturing variations. This type of analysis can be used for prediction of almost all the group weights that comprise the helicopter empty weight, and is particularly applicable for the structural groups. The weight prediction can be refined as the design progresses and applicable values for increasing numbers of design parameters are determined. However in today s design environment where the design is iterative in nature and design variables change from one iteration to the next, it can be very time consuming and tedious to use the regression equations. Additionally the group weights obtained using regression equations are estimates that may have significant error. These estimates are often not suitable for new designs. The new approach proposed in this paper is to calculate weights by using a vehicle engineering or CAD package. CAD packages have improved significantly in the past decade. In this particular study, CATIA by Dassault Sytemes is used to calculate component weights. Detailed parameterized component CAD drawings are developed using CATIA V5R16. These drawings are then linked with the Phoenix Integration design software called ModelCenter. This link allows the designer to dynamically change the part designs parametrically from ModelCenter by changing the variables 3

4 as they get updated from one system level iteration to another. The updated empty weight information is then used by other disciplines, which are dependent on weights discipline for their calculations. This approach automates the weight estimation process while providing an accurate weight estimate. System Optimization Stability and Control Weight Iteration Aerodynamics Figure 2. Interdisciplinary dependency and design iteration [2] A wrapper is developed that helps establish a link between ModelCenter and CATIA. Important design variables are specified in ModelCenter. By changing these variables, the corresponding dimensions are updated in CATIA. This process is indicated in Figure 3. The wrapper facilitates this process. Corresponding material properties are specified in CATIA. These material properties are based on the best available information at the early design stage. In this research, historical data is used for the specification of material densities. In majority of new designs, the material details may not be available at the early design stage or the designer may decide to keep the material as a variable so the material information can be updated or new materials can be added as the design progresses. The design variables may change significantly from one iteration to another or over several iterations in a design process. For example during the helicopter sizing process, the designer may decide to start with a small rotor and then as the design matures, the rotor size may increase, as is the case in most designs. This change in the rotor dimensions is reflected in the CAD drawings. As the CATIA drawings get updated, the volumes and weights are calculated automatically in CATIA and the updated information is sent back to ModelCenter as shown in Figure 3. The component weights are then integrated in ModelCenter to find group weights. Group weights are added to find the vehicle empty weight. This updated weights information is then sent to various other disciplines or system level optimizer in an optimization problem, as indicated in Figure 2. The Component Weights (CW) are added in ModelCenter to get Group Weights (GW). For example rotor blades, flexures and hub weights are added to get the rotor group weight. Similarly group weights are added to get the vehicle Empty Weight (EW). The addition process in ModelCenter is shown in Figure 4. This entire process of sending new variable information from ModelCenter to CATIA, update of CAD drawings, new component weight calculations and addition of component and group weights to get the vehicle empty weight is automated, which enables system 4

5 iterations. With this methodology, the weights discipline s works load is significantly reduced. Users New Design Empty Wt. ModelCenter Change design variables CATIA Update drawings Calculate comp. Weights Component Weights Figure 3. Weight iteration between CAD (CATIA) and Design (ModelCenter) software CW Component Weights GW Group Weights EW Empty Weight Figure 4. Component, Group and Empty weight calculation in ModelCenter 3. IMPLEMENTATION The above mentioned methodology is implemented by means of modeling an entire helicopter in CATIA, and comparing its weight calculations with the actual values. Schweizer TH330 is modeled parametrically, as shown in Figure 5, and calculated rotor group weights are compared to provide rotor group weight of the rotorcraft by the manufacturer. Parametric design is crucial in this method because of the necessity to modify the various design parameters quickly when switching between various concepts. For instance, with the CATIA and ModelCenter models developed in this study, it is possible to modify the blade length, chord lengths at the root and tip of the blade, and number of blades easily within ModelCenter without 5

6 making any changes in CATIA. In addition it is relatively easy to switch between different airfoil cross-sections, such as NACA0012, NACA23012, or SC1095 as long as the airfoil shape is available in a table format of (x, y) coordinates in terms of chord length (c), as shown in Figure 6. The table values are fed into CATIA to obtain the exact blade cross-sections of unit chord length. It is then possible to scale the cross-section to the desired chord length at the tip or at the root. The number of blades can be changed from ModelCenter and the CATIA drawings get updated as shown in Figure 7. The wrapper file integrated in ModelCenter and the linkages between design variables and actual physical dimensions in CATIA are shown in Figure 8. The variables that are passed to CATIA are the same parameters that change and update drawings and are summarized in Figure 9. Figure 5. Isometric and parametric views of Schweizer TH 330 modeled in CATIA X/c Y/c

7 Figure 6. Airfoil geometry modeled in CATIA Figure 7. Main rotor group designed in CATIA with inputs from ModelCenter The model also allows changes in hinge offset. The possibility of making such changes easily on the rotor enables the authors to obtain a rough model of most conventional single-main-rotor helicopter rotors. For simplicity, potentially lowweight components such as control linkages are removed from the design to keep it more general, and the hinge-offset section is modeled as a single beam. Any forward or backward sweep of the blade, or any built-in twist angles throughout the blade span are also ignored, since they do not significantly affect weights. 7

8 Figure 8. CATIA wrapper shown in ModelCenter design environment Since there is very little knowledge about the details of the design in the conceptual stage, it is acceptable to make use of historical weight data to make a relatively reliable weight estimate. The main purpose of this research is to introduce high fidelity tools early in the design process. It is, however, highly possible to have little or no information about the design necessary to use those tools. For example, weight of a blade depends highly on its inner structure and materials. Exact weights of the blade can be calculated using CATIA if structural information and material distributions within the blade are well defined. However it is very unlikely to have knowledge of this level of detail in the beginning of design. Therefore as a first approximation, blade, hinge offset and rotor hub in this research are modeled as separate solid components, composed of one single material each. The density of each material is predefined by the user, based on weights of other similar helicopter rotors published by their manufacturers, i.e., historical data. A method for finding the densities is explained below. Since the volume of any rotor blade can be matched using CATIA, given necessary dimensions, the only data required is the density of the material of the blade, in order to calculate the blade weight. Volumes are calculated by CATIA, and weights are obtained from either published information or historical data. The densities to be entered into CATIA model are obtained by first calculating weight/volume information of several helicopters. These approximated densities for each helicopter are then inserted into the equation below: 1/ 5 s= d V V /100 (1) app tip c 8

9 where d app is approximated density, V tip is the tip speed of blades, and V c is cruise speed of the rotorcraft. V tip and V c are given in design specifications, and d app is calculated as weight of the component d app = (2) estimated volume from CAD design This expression of s combines the most important design parameters in blade design, implicitly including rotor radius and chord length within estimated density. The necessary values for each rotorcraft are given in Table 1. s value for each helicopter model is plotted against the years in which the particular models were first released for sale or operation, as shown in Figure 10. This plot is used for forming a regression equation to find the density for a helicopter component based on its first production year, to compensate for new technologies. Technological advances may not be identical for every component in a rotorcraft; therefore this method may be applied for each component separately in order to find different density estimations for each part. One may think that usage of historical data causes the weight estimation results to be same as the case where no high fidelity tools are utilized, thus those tools are futile. In the traditional approach of weight calculation, when the approximate size of the rotor is being estimated, usually a predefined weight equation based solely on statistical analysis of historical data is utilized. These equations still require specific data from the rotorcraft under development, but this data is not used for calculating an entity directly related to the helicopter; it is only needed for substituting the values in a regression equation. The proposed method is a step towards minimizing dependency on historical data in weight calculations. Although historical data is still employed, it is not the major source of information any more. It is only needed for a crude approximation to the overall density of the component under consideration. The design data of the newly-designed rotor is used for calculating component volumes of the same helicopter, rather than being used in a regression equation. Previous rotor data is a starting point for helicopter sizes in the same ballpark. Table 1. Helicopters considered in analysis Root chord (ft.) Rotor Radius (ft) No. of blades Hinge Offset ratio tip speed (fps) cruise speed (fps) Make and Model Year Aerospatiale/ Eurocopter AS 350B Eurocopter BO 105LS MBB/ Kawasaki BK McDonnell Douglas MD 500E Schweizer/ Hughes 300C Agusta A Robinson R (0.62)* Sikorsky UH-60A

10 Sikorsky CH-53E Sikorsky S-76A Bell JetRanger (0.098) * For rotorcraft with teetering rotors, the term hinge offset ratio represents the ratio of blade structure at the root enabling connections and controls to the blade structure with airfoil shape. Values for such rotorcraft are presented in parentheses. A unique blade weight is estimated using CATIA model for the new design, and the estimate is automatically updated for minute changes in blade geometry. As the design progresses, interior structure of the blade and other components will be formed eventually, thus crude density approximations will in turn be eliminated from the design without affecting CAD models. Using the same models provide a smooth transition between design, analysis and manufacturing stages of a product. 4. RESULTS & DISCUSSIONS Effect of production year is incorporated by fitting a linear curve for the points in Figure 10. The equation for that curve is used for finding approximate values of s. Estimated densities are then found by solving the following modified equation of s 1/ 5 s= d V V /100 (3) est to find d est. These estimated densities are then multiplied by CATIA blade volumes to obtain estimated weights. Results of these calculations are given in Table 2. These weights may be compared to Poruty s blade weights in terms of closeness to the published blade weight of the rotorcraft. For example, published total blade weight of Sikorsky CH-53E is 2120 lbs. Prouty s equations yield 2264 lbs, and estimation using the method described in this paper gives 2189 lbs. Although the CAD model crude and generalized for multiple rotorcraft, it still manages to approximate the actual data better than a statistics-based method. This approach, when implemented on new designs or design modifications, can greatly reduce the design time, cost and effort of the weight engineers and help the system designers for overall design optimization. In this research, ModelCenter is used to demonstrate the integration of different software including CAD, Spreadsheets and Design software. Information flow between these software is facilitated. It is shown that by linking the design software with CAD packages, detailed and accurate weight information can be brought back at the initial design stage. As the design matures and more information becomes available, the weights information is update automatically. This information is also passed to other disciplines dependent on weights. The entire process is automated, thus significantly reducing the weight engineers work. For future work, more parameterized components of helicopter, developed in the CAD package, can be linked directly with ModelCenter. This will ensure that as the user changes the design parameters or as the design information gets updated from other disciplines involved in the design cycle, precise updated weights information is calculated automatically. tip c 10

11 Figure 9. Updated CATIA Parameters s vs. year y = x s Year Figure 10. Plot of release year versus s values 11

12 Table 2. CATIA results and estimated densities CATIA blade volume (ft 3 ) Weight / Volume ( d app ) Estimated density ( d est ) Estimated Blade Weights (lbs) Prouty's Blade Weights (lbs) Make and Model Aerospatiale/ Eurocopter AS 350B Eurocopter BO 105LS MBB/ Kawasaki BK McDonnell Douglas MD 500E Schweizer/ Hughes 300C Agusta A Robinson R Sikorsky UH-60A Sikorsky CH-53E Sikorsky S-76A Bell JetRanger REFERENCES [1] Prouty, W. Raymond, Helicopter Performance, Stability, and Control, Krieger Publishing Company, Malabar, Florida, 1995 [2] Khalid S. Adeel, Development and Implementation of Rotorcraft Preliminary Design Methodology using Multidisciplinary Design Optimization, Ph.D. Dissertation, Georgia Institute of Technology, December 2006 [3] Mavris, D.N., DeLaurentis, D.A., Bandte, O., Hale, M.A., A Stochastic Approach to Multi-disciplinary Aircraft Analysis and Design, AIAA 36 th Aerospace Sciences Meeting and Exhibit, January 12-15, 1998 [4] [5] Shinn, Impact of Emerging Technology on the weight of Future Aircraft, AHS 40 th forum, 1984 [6] Schwartzberg, Smith, Means, Law, & Chappell, Single Rotor Helicopter Design and Performance Estimation Programs, USAAMRDL, SR 10, 77-1, 1977 [7] [8] 12