INVESTIGATION ON STRUCTURAL ASPECTS OF UNMANNED COMBAT AIR VEHICLE FOR AEROELASTIC ANALYSIS P N Vinay *, P V Srihari *, A Mahadesh Kumar

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1 Research Article INVESTIGATION ON STRUCTURAL ASPECTS OF UNMANNED COMBAT AIR VEHICLE FOR AEROELASTIC ANALYSIS P N Vinay *, P V Srihari *, A Mahadesh Kumar Address for Correspondence * Dept. of Mechanical Engineering, R V College of Engineering, Bengaluru Aeronautical Development Agency, Bengaluru ABSTRACT This paper presents the investigation of an ideal configuration of the UCAV structure by means of strength, stiffness and aircraft stability. As a basic approach the wing is geometrically modeled by focusing on the spar and rib locations and as a second step the wing is analyzed by changing the sweepback angle to 45 and 60 degrees so as to identify the efficient Planform shape within the selected planforms. The different wing configurations are compared by conducting the static aeroelastic and flutter analysis using MSC/NASTRAN analysis tool. The geometric and finite element models were developed by the MSC/PATRAN and the analyses were carried out at 5g maneuver for Mach number 0.8 for the symmetric model of the UCAV. The comparison of the results obtained showed that the wing with spar located at 30% from leading edge has given moderate results in terms of the static stresses, stability and natural frequencies. Furthermore, it is concluded that the wing with smaller sweepback angles have indicated significant effects. KEY WORDS: Spar, Planform, Aeroelasticity, Flutter 1. INTRODUCTION The design, development and use of Unmanned Combat Air Vehicles(UCAV) are in the significant progress for years because of their various capabilities such as military reconnaissance, border monitoring and combat. With growing interest in UCAVs, the flying wing configuration of the Stealth Fighter is gaining popularity in the aircraft industry, based on several requirements there is a need for the design of the efficient and ideal UCAV configurations. The initial stages of the aircraft design consumes enormous amount of time and costs to bring out the ideal configuration of the aircraft that best suits the required expectations. Starting from the selection of airfoil, materials to internal structural arrangement, several studies and methods have to be carried out before finalizing the design. Only a limited number of published articles have been focused on the structural design aspects of the unmanned combat air vehicles[1-2]. This study is mainly focused on analyzing the response of the structure to change in the spar and rib arrangements as well as to investigate 3 different planform shapes. This involves determining the static response of the structure, dynamic stability of an aeroelastic system to obtain flutter frequencies and velocities, Evaluation of the natural frequencies and mode shapes for all UCAV configurations. 2. APPROACH Firstly, it was decided to design the configurations that approximately matches the existing UCAV s with considerable modifications on the outer profile. The parameterization similar to the one used in the aircraft concept optimization[3] is considered to derive the outer geometry with the addition of few more parameters such as secondary sweep angle for wing and inner wing span. The next section illustrates the three configurations which have been derived by compromising on the present configurations like NG X-47B, neuron and Boeing X-45C. As a second step, In order to study the behavior of the structure at different spar locations, The four locations of the front spar are randomly identified so as to investigate the ideal offset distance from the leading edge of the wing. The modeling approach will be discussed in the following paragraphs. 2.1 Structural Model A geometric model closely approximating the conventional UCAV configuration having a overall span of 10m and fuselage length of 8m is generated. A simple symmetric NACA four digit airfoil is used to develop the cross-section of the aircraft. The structural model with three fixed spars are described in the UCAV optimization[1] where as in this case, the number of spars used are two just enough to sustain the continuity of the wing and focused to study the effect of front spar located at 20, 30, 40 & 50% of the chord whereas the rear spar is fixed at 70% of the chord. The main reason behind this is to keep the best option that improves the aeroelastic characteristics of the wing. In order to prevent the skin panels from buckling, It was decided to have twelve ribs with a span wise distance of 0.25m is maintained between the rib segments from root section of the wing. Out of these twelve, two ribs are used to support the actuators for control surface mechanisms. Along with the Longerons and Frames, the other components that are being modeled are Elevons, Engine, Fuel, Missile, Main and Nose landing gears[4]. The thickness and areas of these components were chosen based on the rational guess. The spar and rib arrangements being considered in this study are shown in the Figure 1.

2 Two more UCAV planforms are considered keeping all the parameters unchanged except maintaining the sweep angle at 25, 45 and 60 so as to generate three different Planform configurations. The strategy of fixing the rear spar and allowing the front spar for variable distance offset is maintained for all the three configurations. The geometric and FE models for various configurations of UCAV were generated by using the MSC/PATRAN program package[5]. Most of the structural components are modeled by using the Quad and Tria elements whereas the engines, missile & landing gears are made up 1D beam elements. The three UCAV planforms and the parameters defining the shape for one of these configurations is shown in the Figure 2 and Table 1 respectively. As far as the material is concerned, the structural model and its components are assumed to be made up of Aluminium alloy 2024-T3 due to its Figure 1: Spar and Rib arrangements wide usage in the aircraft industry because of several reasons such as light weight, high strength, fatigue resistance, etc., The material properties such as Young's Modulus, Poisson's Ratio, Density are assigned and applied to the appropriate elements using MSC/PATRAN tool. Maximum amount of fuel is accommodated based on the internal volume available inside the structure and modeled as local mass using 0D type of point elements. In total the aircraft model is assumed to have an approximate overall weight of 4800 Kg s including a payload of 2200 Kg s and it was variable from one configuration to other. The FE models under consideration are symmetric about X-Z plane of the aircraft and hence only half symmetric models are used through the study. For one of the three selected configurations, the FE model is as shown in the Figure 3. Figure 2: Three UCAV planforms with varying wing sweep angles (A) 25, (B) 45 and (C) 60 Where, AR=Aspect Ratio, b=overall Span length, C=Mean aerodynamic chord, C t =Tip chord, L=Chord length(fuselage), λ 1 =Major Sweep angle (Fuselage), λ 2 =Wing Sweep angle, b 1 =Inner wing Span Table 1: Parameters defining the Planform b L C C t AR b 1 λ 1 λ 2 10m 8m 4m 2m m 45 25, 45 and 60

3 Figure 3: Half symmetric FE model for Configuration `B without top skin Figure 4: 2D Aeromesh for 45 Swept wing UCAV Configuration 2.2 Aerodynamic Model Aerodynamic model is a collection of lifting surfaces that provides the primary lifting capability for the wing surface [6]. The Doublet Lattice Method in MSC/Flight loads & dynamics tool was incorporated to define the aerodynamic geometry including wings and control surfaces. For the present study, two control surfaces i.e., Elevons have been used on the trailing edge of the wing. Overall four aero-groups were created each one for fuselage, wing and other two for control surfaces. The interpolation from structural to aerodynamic model has been done using the available splining concepts. 2D Aeromesh for Configuration B is as shown in the Figure ANALYSIS AND CALCULATIONS 3.1 Dynamic Analysis Prior to Static and Dynamic aeroelastic analysis, a normal modes analysis was done for the finite element models to evaluate the natural frequencies and mode shapes of the structural model which characterize the basic dynamic behavior of the structure. The first out-of-plane bending natural frequency is considered as the best parameter to examine stiffness characteristics of the structure. As a preliminary step, it was decided to conduct the normal modes analysis for two configurations i.e. A and B to study the effect of changing spar positions. Table 2 shows the natural frequencies for configurations A and B with different spar locations. Table 2: Comparison of Natural frequencies for the selected configurations Configuration A B Spar Location 20% 30% 40% 50% 20% 30% 40% 50% First 5 Frequencies (Hz)

4 Noticing all the frequencies of Configurations A and B, the frequencies for Spar at 30% of the chord have shown significant results than other. Henceforth, it was decided to focus only 30% Spar position for Configuration C and the natural frequencies for the same is shown in the Table.3 Table 3: Comparison of Natural frequencies for Configuration C Mode Frequency (Hz) Static Aeroelastic Analysis The main purpose of this section is to generate the outputs containing the internal loads and stresses, structural deflections, aerodynamic stability derivatives and trim states. The aerodynamic data was provided through the MSC/NASTRAN code based on linear panel method such as Doublet-Lattice Method (DLM) for 0.8 Mach subsonic speed. The general forms of the aeroelastic trim problem is utilized in the static aeroelastic problem [7]. The dynamic pressure (45 KPa) and the required control parameters (e.g., accelerations and rates) were inputted to meet 5g Pull-up maneuver conditions. The same boundary conditions used in the modal analysis were incorporated to constraint the model. Figure 5 shows the stress concentration obtained for three selected configurations for the case of 30% spar location. It was observed that maximum stress is located near the root portion of the spar and resulted values are tabulated in the Table 4. Figure 5: Stress Concentration for three configurations for the case of 30% Spar location Table 4: Maximum Vonmises Stresses and Model weights Configuration A B C Spar Location 20% 30% 40% 50% 20% 30% 40% 50% 30% Stresses [Mpa] Weight [kg] Figure 6(a): V-g plot for Configuration A for different Spar locations

5 Figure 6(b): V-g plot for Configuration B for different Spar locations 3.3 Flutter Analysis A MSC/NASTRAN SOL 145 dynamic flutter analysis was performed using the DLM approach and PK-Method [8] for selected test cases and different spar positions. This approach introduce the aerodynamic loads into the equations of motion as frequency dependent stiffness and damping terms [9]. The dynamic characteristics such as geometry of the structure, flight conditions(density and velocity) and natural frequencies are used as basic inputs for flutter solutions. Fundamental modes of the lifting and control surfaces are considered to carry out flutter computations. The Velocity(V)- Damping(g) plots for the configurations A and B are shown in the Figure 6(a) and Figure 6(b) respectively. The velocity value where the velocity vs. damping curve is passing from negative value to a positive value is considered as the flutter speed and it can be noticed from the V-g plots that the frequency jump occurs between Mach for all the test cases which is much above the 15% margin value. The flutter speed for Configuration C was found at 2.2 Mach which is not shown in the figure. It can be observed from the flutter plots that the damping for 50% spar case have shown high variation as compared to the other cases. 3.4 Stability Margin Calculations Having performed static and dynamic analysis, the next task chosen is to calculate the aerodynamic centre (AC) of each model to verify the stability of the aircraft by means of comparing the values of aerodynamic centre with the respective centre of gravity (CG) values. It was known that for stable aircraft the CG should be forward of AC and as expected the results have shown that the most of the flying wing aircrafts are unstable due to the absence of the vertical stabilizers. The pitching moment and lift coefficients for specific configuration was taken directly from the MSC/NASTRAN output file. The aerodynamic centre is evaluated by using the Equation 1, AC=Cm a L /CZa (1) Where, Cm a =Pitching moment coefficient, Cz a =Lift coefficient Table 5: AC and CG comparisons for three configurations Configuration Ac CG Margin A % B % C % The instability of the aircraft is expressed as percentage of the chord length L and is shown in the Table 5. It was observed that the margin for each of the configuration A and B exhibits almost the same values for different spar locations and based on this experience, only a single case of 30% spar location is selected to tabulate the margin for all the Configurations. 4. RESULTS AND DISCUSSIONS Modal analysis of the finite element model was performed to analyze the different bending modes and their natural frequencies. After examining the obtained results, it showed that frequencies goes on decreases with the increase in sweep angle. Also it was found that the frequencies increases only up to

6 30% spar position and starts decreases as the position of the spar forwards to the middle of the wing. Results from the Static aeroelastic analysis indicated the maximum stresses on the rear spar that was used to support two control surfaces and it was observed that the stress levels in the spars increases with the increase in sweepback angle (λ 2 ) and decreases when the spar is moved away from the leading edge. By comparing the resulted Vonmises stresses with the yield strength of the selected material ( MPa), it was found that the structure is safe. Furthermore, the plots shown in Figure 6 indicates that there is no significant variation in the flutter speeds for the selected cases though it exhibits differences in damping when the spar is moved from forwards to aft. 5. CONCLUSION This paper presents an approach to structural design highlighting on the spar positions and various Planform shapes resulting in the selection of ideal design within the limited number of chosen configurations. This tradeoff study was started with the creation of three planform configurations and random selection of front spar locations with the rear spar fixed. In order to investigate the three configurations, the models were subjected to normal modes, static aeroelastic and flutter analysis. Considering the different analysis performed on various test cases, the following conclusions have been drawn. (1) Configuration A with spar located at 30% of the chord exhibits low stress levels, higher frequencies and considerably lesser weight among the selected cases. (2) No flutter occurs within the flight envelope for the selected configurations.(3) It was observed from the stability margin computations that the lower margin is obtained for the Configuration C and it increases with the decrease in the sweep back angle. REFERENCES 1. Hu Tianyuan and Yu Xiongqin, Aerodynamic/Stealthy/Structural Multidisciplinary Design Optimization of Unmanned Combat Air vehicle, Chinese Journal of Aeronautics, 16 September Dr. Wolf R. Krüger, D. Hoffmann, Design Considerations for a UCAV Wing for Subsonic and Transonic Aeroelastic and Flight Mechanic Wind Tunnel Tests Meeting proceedings RTO- MP-AVT-145, France, Amadori K and Jouannet C, Use of Panel Code Modeling in a Framework for Aircraft Concept Optimization, 11th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference, Portsmouth, VA, Christian Anhalt, Hans Peter Monner and Elmar Breitbach, Interdisciplinary Wing Design Structural Aspects, German Aerospace Center, MSC/PATRAN User s Guide and reference manuals (2005), MSC Software Corporation, MSC.Flight Loads and Dynamics User s Guide Version 2001 (r1), MSC Software Corporation, Rich Jacobs, Erwin Johnson, John Ausman, A CFD/Doublet Lattice/MD NASTRAN Calculation Of Static Aeroelastic Trim And Structural Loads, The Boeing Company, Rodden W.P, Johnson E.H, User s Guide of MSC/NASTRAN Aeroelastic Analysis, MSC/NASTRAN v68, Aleš Kratochvíl and Svatomír Slavík, Aeroelasticity analysis of wing UL-39,Czech Technical University in Prague, Students Conference 2011.