shashank mishra -golden hawk mav

Symposium on Applied Aerodynamics and Design of Aerospace Vehicle (SAROD 2009)December 10-12, 2009, Bangalore, India

Conceptual Design and Analysis of Airframe for Fixed Wing MAV

Shashank Mishra* G. Ramesh+, Sajeer Ahmed+

( [email protected] ), ([email protected] , [email protected])

National Aerospace Laboratories (CSIR), Bangalore, India

ABSTRACT

This paper presents the design and development of an airframe for a fixed wing Micro Air Vehicle that has a maximum linear dimension restricted to 300 mm. The preliminary estimate of the weight is based on the commercially available components including the autopilot and payload. Based on the wing loading and power loading data, a flying wing configuration has been chosen as the most optimum solution for the airframe. Numerical codes X-Foil and XFLR5 available as a freeware were used to analyze and optimize the airfoil and planform in terms of static stability and aerodynamic efficiency. The Eppler 61 was chosen as seed airfoil for optimization as it provided the highest aerodynamic efficiency in low Reynolds number flow regime. However, the higher pitching moment values and its oscillatory nature necessitated the modification of the airfoil geometry to obtain a better longitudinal stability. The new airfoil thus designed (SM-4308) using the seed airfoil is found to provide a better stability at a marginal loss of aerodynamic efficiency. The planform optimization carried out using the numerical codes and the cropped delta planform with the above airfoil showed to provide the optimum aerodynamic coefficients. Eigen mode analysis carried out for lateral and directional stability using Athena Vortice Lattice method (AVL) showed the stability in Dutch roll and Spiral mode. The prototype MAV with the designed wing flown in fully autonomous mode is found to perform well against high wind gusts (up to 13m/s) and has a typical endurance of 20 minutes. The Designed MAV is named as Golden Hawk.

Key words: MAV, Low Reynolds Number, VLM and 3-D Panels, Longitudinal stability, Lateral-Directional stability.

1

mailto:[email protected]

mailto:[email protected]

2 Shashank Mishra, G. Ramesh, Sajeer Ahmed

NOMENCLATURECd = Sectional drag coefficient (2D-Airfoil) Cl = Sectional lift coefficient (2D- Airfoil)CD = Drag coefficient (3D-Wing)CL = Lift coefficient (3D- wing)Cdmin = Minimum drag CoefficientCLmax = Maximum lift coefficientCLmin = Minimum lift CoefficientCm = Pitching moment coefficientCmo = Zero Angle Pitching moment coefficient

= pitching moment about the quarter-chord

CLα = Lift-Curve slope L/D = Lift-to-Drag Ratio CG = Center of Gravityt/c = Thickness to Chord Ratioα = Angle Of Attack

* Project Engineer.+ Scientist, Experimental Aerodynamics Division © Shashank Mishra, G. Ramesh, Sajeer Ahmed SAROD 2009Published in 2009 by Macmillan India Ltd.

1. INTRODUCTIONInterest in Micro Air Vehicle (MAV) systems

arises from both military application and scientific intrigue [1]. As relatively an emerging field it provides great opportunities and challenges in many disciplines of aeronautical engineering. The small size and weight and the relatively high atmospheric gusts under which the vehicle need to fly in fully autonomous mode make the development of MAVs a technological challenge. The development of MAV is a highly multi disciplinary and a highly technology driven activity. In the area of airframe design the aerodynamics plays an important role. Due to the limited size and low flying velocity, it operates at significantly lower Reynolds number ranging between 50,000 – 200,000 based on the mean aerodynamic chord (MAC) and cruise velocity. Wings of such low aspect ratios exhibit unique aerodynamic properties such as high stall-angles of attack and nonlinear lift versus angle of attack curves. Due to low Reynolds number, complex flow phenomena such as laminar separation bubbles, laminar to turbulent transition and bubble bursting may arise. The aerodynamic characteristics of the wing and other components in turn affect the static, dynamic and aero elastic stability of the entire vehicle. The highly three-dimensional low Reynolds number flows and

lack of experimental database to understand the flow around the wing body requires fundamental research of the phenomena.

Many of the researchers and designers use the aero prediction codes such as XFOIL [2] and AVL [3] for the design and development of airfoil, wing, and airframes for the low Reynolds number flow regimes as the codes are being constantly improved. The behavior of the flow around the wing body can be predicted using aero-prediction code based on 3-D Panel method called XFLR5 [2]. It is an open source code modified by many developers and is an excellent tool for the preliminary aerodynamic analysis of the airframe with the selected airfoil. It uses two different methods namely: Vortex Lattice method and 3D Panel method which gives reasonably good results in the Low Reynolds number regime.AVL [3] is also an open source vortex lattice code to analyze the Longitudinal and Lateral-directional stability characteristics of the airframe in the preliminary design stages. The above free wares have been used in the design and analysis of the airframe to meet the mission performance of a Fixed Wing Micro Air Vehicle.

The aim of the present work is to develop MAVs of 300 mm span with about 30 minutes endurance and to operate in a fully autonomous mode. This work addresses the development of aerodynamically efficient and stable airframe that meet the mission requirement. The prototype is populated with the commercially available off the shelf components (COTS) including the autopilot and flown in Radio Control (R/C) and autonomous mode. Future work involves wind tunnel validation of aero predictions and detailed structural analysis to get the natural frequency, strength and weight optimization. The fabrication of the final flight model will be with high strength, light weight material.

2. AIRFRAME DESIGN

2.1 Design Methodology

The schematic outline of the methodology of the design process for the fixed wing micro air vehicle is shown in Figure 1.

The methodology has three levels; basic level consists of aerodynamic force models and profile selection, the second level consists of design optimization and third level consists of flight testing of the prototype. The tests can be used as a feedback module for the design optimization.

Angle of Attack

Coefficicentoflift(Cl)

-4 0 4 8 12 16 20 24 28 32

-0.4

0

0.4

0.8

1.2

1.6

Eppler 184Eppler 61NACA 4415Selig 4083Selig 5020MH 45

Angle of Attack

Cm

-8 -4 0 4 8 12 16 20 24 28 32

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05


Cd

Cl

-0.1 0 0.1 0.2 0.3 0.4

-0.5

0

0.5

1

1.5


Angle of Attack

Cl/Cd

-4 0 4 8 12 16 20 24 28 32

-20

0

20

40

60

80Eppler 184Eppler 61NACA 4415Selig 4083Selig 5020MH 45

Conceptual Design and Analysis of Airframe for Fixed Wing MAV 3

Figure 1 Schematic Outline of Design Methodology

2.2 Aerodynamic Force models

The wing, vertical tails, control surfaces and the propeller are the main subsystems that are subjected to the aerodynamic forces in any symmetrical or asymmetrical flight condition. The efficiency of the model is defined by the aerodynamic forces acting on the wing which will be mainly affected by the propeller slipstream in any flight condition. The effect of propeller slipstream is an ongoing research topic as it has a significant influence on the wing drag and lift, which is out of scope of this paper. The aerodynamic force model consists of efficient airfoil and planform with weight estimation of the commercially off the shelf components used in the models to find the aerodynamic forces acting on the wing. Numerical methods, such as 3-D panel method and vortex lattice methods are used to investigate the various aerodynamic forces acting on the wing.

2.3 Airfoil Selection

Selection of airfoil for the MAV is the crucial task as the flying wing design has to provide inherent stability as there is no conventional tail that balances the moment. The factors for the selection of airfoil include high aerodynamic efficiency, increased Clmax, low Cdmin, higher stall angle, negative and near zero constant pitching moment etc [4 and 5]. The airfoil should have minimum thickness and moderate camber. Various low Reynolds number airfoils such as Eppler 61, MH45, NACA 4415, S4083, Eppler184 and the S5020 airfoil were analyzed and aerodynamic characteristics has been obtained through the XFLR5. Analysis was carried out under the viscid mode at Reynolds number of 160000 corresponds to cruise

velocity of 11m/s for an AOA ranging from -5o to 3o

and aerodynamic characteristics are plotted in Figure 2.

(a) Coefficient of Lift Vs angle of Attack

(b) Pitching Moment Coefficient Vs Angle of Attack

(c) Drag Polar Curve

(d) Lift-to-Drag Ratio Vs Angle of Attack

Figure 2 Aerodynamic Data of Different Low Reynolds Number Airfoils

Aerodynamic Force Models

Weight Estimation

Airfoil Selection

Planform Selection

Control Surface Sizing

Design Optimization

Stability Analysis Wind Tunnel Testing Structural /CFD

Test Environments

Endurance Testing Prototype / Autonomous Flight Testing

Angle of Attack

CoefficientofLift(Cl)

-4 0 4 8 12 16 20 24 28 32

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Eppler 61SM-4308

Angle of Attack

Cm

0 10 20 30

-0.2

-0.15

-0.1

-0.05

0

Eppler 61SM-4308

Angle of Attack

Cl/Cd

-4 0 4 8 12 16 20 24 28 320

20

40

60

80

Eppler 61SM- 4308


As seen from the above plots though the Eppler 61 airfoil provides a high L/D the undesirable oscillation in the pitching moment and higher negative value necessitates the modification and tweaking of this airfoil to achieve better performance. Eppler 61 is modified (leading edge radius, the camber, thickness ratio and its position) using inverse design method in XFLR5 to obtain the new airfoil named SM-4308 is shown in Figure 3. The airfoil has nearly 0.04c (0.0369c) camber at 0.28c (0.30c) with maximum thickness of 0.0784c (0.08c), so we name the airfoil as SM-4308.

SM-4308 AirfoilFigure 3 Modified Airfoil for better stability

This new airfoil has camber reduced nearly by 50%

that improved . The new modified airfoil SM-

4308 has been found to significantly improve the stall angle and longitudinal stability compared to the seed airfoil as shown in Figure 4. The aerodynamic characteristics of the analyzed airfoils can be seen in Table 1.



Table1Aerodynamic Characteristics of Low Reynolds Number

Airfoils

Table2 Geometric Properties of Designed MAV wing

(c) Lift to Drag Ratio Vs Angle of Attack

Airfoil Camber (%) t/c (%) Cd0 Cm,c/4 (Cl/Cd )max Clmax

Eppler 61 6.69 5.67 0.02381 -0.1897 87 1.5S4083 3.45 8.00 0.00912 -0.0896 67.22 1.221

Eppler 184 1.20 8.32 0.01102 -0.0093 36.79 0.849S5020 2.62 8.40 0.0114 -0.007 59.67 1.19

NACA 4415 4.01 14.99 0.0134 -0.107 63.69 1.401MH45 1.71 9.84 0.0.1102 -0.0093 53.48 1.114

SM-4308 3.69 7.84 0.02432 -0.0078 56.67 1.26

Span 300mmArea 0.06m2

Aspect Ratio(AR) 1.5 Mean Aerodynamic chord (MAC)

201.67mm

Root Chord 240mmTip Chord 140mmSweep Angle 20o

C.G Location 45mm from LE*

Aerodynamic Centre 60mm from LEWinglet/Fin area 0.01m2

Height of winglets 60mm


Figure 4 Comparison of SM4308 airfoil with Eppler 61 Airfoil

2.4 Planform Selection

The main design consideration for the MAV is the weight and dimension i.e. the chord and span of the wing. The calculations were performed for different aspect ratio, velocity, lifts coefficient, etc., and optimized result was chosen as the design requirement specification of the MAV. Increasing the aspect ratio could lead to a very low wing area permitting the MAV to carry only less weight, so the aspect ratio was varied between 1 and 1.5. The various planform like Zimmerman, Inverse Zimmerman, Modified 3-Circle, Delta Planforms were analyzed to choose optimally best planform for the Golden Hawk (GH). The cropped delta planform was selected due to high aerodynamic efficiency and moderate stall [6]. Single vertical fin/tail is added to counter the moment caused by propulsion system. NACA0006 airfoil is used in fin/tail because of symmetry and thickness of the airfoil. The geometric properties are tabulated in Table 2

*LE = Leading Edge of the Wing

2.5 Modeling and Meshing of wing in XFLR5

The designed MAV is modeled in XFLR5 with the help of 3-D panel method. In 3-D panel method wing is defined by a set of panels and each panels is defined by its length, root and tip chord, leading edge offset at root and tip, mesh for VLM/3-D Panels analysis. The 1000 numbers of VLM Panels and 2000 numbers of 3D Panels can be modeled in XFLR5. The spanwise length of a Panel should be at least equal to the minimum length of the VLM elements on other panels which can be seen in the Figure 5

Figure 5 VLM AND 3-D Panels Arrangement

for Wing The Golden Hawk MAV is modeled in XFLR5 with the help of 3-D panel method which include VLM mesh also. The airframe has been modeled by giving the hypothetical sections in X and Y direction respectively. The spacing between the hypothetical sections is filled by 3-D panels and VLM panels as shown in Figure 6. The spacing between the panels can be defined either linearly or using a sinusoidal function. The cosine function is generally used to resolve the flow accurately at all the desirable points by increasing the density of mesh at the root, tip, leading and trailing edge. As panel distribution is consistent with the wing geometry so the density of the mesh can be increased at the geometrical breakdown points and at the root and tip of the wings. The wing is "meshed" into a number of panels distributed over the span and the chord of the planform, and a vortex is associated to each panel. The MAV wing is meshed with VLM panels as well as 3-D panels with the help of XFLR5. The mesh disposition on the designed MAV can be seen in Figure 6. The details of the VLM panels and 3-D panel are given in the Table 3.

Table 3 Distribution of VLM Panels and 3-D panels over

the MAV Wing

Distribution VLM Panel 3-D PanelWing 976 1968

Vertical fin/tail 66 144

Figure 6 Mesh disposition on the MAV

An initial estimate of the aerodynamic characteristics of the Golden Hawk has been obtained through the XFLR5. The computation is done with the help of 3-D panel method. The 3-D panel method is more accurate for low Reynolds number and it takes into an account of wing thickness for analysis. 3D Panel Method is to model the perturbation generated by the wing by a sum of vortex distributed over the wing's top and bottom surfaces. The strength of the vortex is calculated to meet the appropriate boundary conditions

Control points in 3-D Panels

Horseshoe Vortex

VLM Panels

3-D panels and VLM panels are distributed in cosine function to resolve the flow accurately in X and Y direction.

x

y Hypothetical Section 1

Hypothetical Section2

Coefficient of Drag (CD)

CoefficientOfLift(CL)

0 0.2 0.4 0.6 0.8

-0.5

0

0.5

1

1.5

GH_SM-4308 AirfoilGH_Eppler 61 Airfoil

Angle of attack

CL/CD

0 10 20 30 40 50

-2

0

2

4

6

8

10 GH_Eppler 61GH_ SM-4308 Airfoil

Angle of attack

Cm

-4 0 4 8 12 16 20 24

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

GH_Eppler 61 AirfoilGH_ SM-4308 Airfoil

Angle of Atttack

Coefficientoflift(CL)

-10 0 10 20 30 40 500

0.4

0.8

1.2

GH _Eppler 61 airfoilGH_SM-4308 Airfoil


which can be Neumann or Dirichlet type. In Neumann type of boundary condition, velocity's component normal to the surface must be zero. In the Dirichlet case the velocity's potential on the panel's inside surface is zero, so that the total potential inside the body is equal to the free stream velocity's potential.

The 3-D Panel analysis carried out at 11m/s as a typical cruise condition of the vehicle for an angle of attack ranging from -5 to 50 degrees. The static longitudinal stability is much important to freeze the airfoils and planform. The curve shows the cropped delta planform with seed airfoil E61 having the highest negative pitching moment and constant about the C.G but this is not much desirable to control the MAV. The pitching moment for the planform with SM-4308 shows the desirable result which gives the controllable flight of Golden Hawk. Golden Hawk shows the high lift-to-drag ratio close to 10 at an optimal angle of attack of 8o can be seen in Figure 7. The aerodynamic characteristics of the golden hawk are tabulated in Table 4.



(c) Lift-to-Drag Ratio Vs Angle of Attack

(d) Drag polar Curve

Figure 7 Comparison of aerodynamics performance of cropped delta planform

Table 4Aerodynamic Characteristics of Designed MAV

Wing


The

climb angle and rate of climb calculated using empirical formulae based on the take-off velocity, thrust available, power required at the take off and all-up weight of the vehicle.

2.4 Numerical Flow visualization using XFLR5

Flow field and pressure distribution over the MAV wing is numerically visualized using XFLR5 that uses 3-D panel method. The range of the angle of attack was varied from -5 to 30 degree. The surface pressure distribution, streamlines and tip vortices are obtained. It was found that the results are in general agreement with our earlier observations. The streamlines and surface pressure distribution showed attached flow for a large range of angle of attack and the tip vortices influence is also low for the angle of attacks up to 10 degree. The streamline distribution over the wing creates the weak trailing edge vortices which create less drag over the operating range of angle of attack which is shown in the Figure 8.

(a) Pressure Distribution at α = 50

(b)

Vorticity distribution at α = 50

(c) Streamline distribution at α = 50

Figure 8 Flow Visualization with the help of XFLR5

2.5. Dynamic Stability Analysis using AVL

Dynamic stability analysis is performed to analyze the controllability and maneuverability of the micro air vehicle. The longitudinal and lateral modes are analyzed with the help of Athena Vortice Lattice (AVL) method [3]. The longitudinal modes are studied using the general equation of motion around the cruise condition with the help of Athena Vortices Lattice method. The dynamic modes are computed for 30cm Golden Hawk MAV with mass of 245grams. The Eigen mode analysis is done on different combinations of wing with vertical tail and winglets. The golden hawk with vertical tail shows an impressive result in Lateral-Directional mode. The Eigen mode analysis shows that the Dutch roll and spiral are stable throughout the operating range. The Dutch roll mode has a slow oscillation. The damping factors depend on the fins area. The short period mode and roll mode are dampened very fast. They are comparatively less sensitive to operating CL of 0.5 it can be seen in Table 5. Roll mode is too far in left plane which roll mode is highly stable for the Golden Hawk. The Phugoid mode is slow damped mode. The damping factor is increased if the CD/CL ratio is increased but this ratio is imposed by the endurance optimization, here is also an influence of the propulsion system. The detailed analysis is given in reference [6]. The various modes of lateral-directional stability for the wing with single vertical tail are tabulated in Table 5.

Table 5 Stability Modes of Wing with Vertical Tail

Eigen Value Damping Frequency (rad/s)

CLmax 1.34CDmin 0.133Cmo -0.007Stall angle 30o

(CL/CD)max 9.075CLα 0.006 / deg(CL

3/2/CD)max 7.40Cruise Angle 8o

Rate of climb 2.5 m/sAngle of climb 15o


Short period

-7.61±12.5i 0.521 14.6

Phugoid -0.212±1.05i 0.197 1.07Roll -24.4 1 24.4

Spiral -0.351±13.1i 0.0267 13.1Dutch roll -0.0128 1 0.0128

3. Prototype fabrication and Flight trials

A prototype of the Golden Hawk (Figure 10) was made using the light weight foam and the control surfaces are made of balsa wood. The structural weight of the airframe is about 65 grams. A 25 watt brushless DC motor along with APC 7 x 5, 2 bladed propellers was used in tractor mode. A Li-Po battery (1350mAh, 11.1 V, 3 cells) is placed ahead of the wing to have a good static margin. Two servos fixed on the top surface were used for the deflecting the elevons. The all up weight of the MAV is found to be 245 grams. The MAV was flown in R/C mode and is found to be controllable and stable. The subsequent flight in autonomous mode using commercial autopilot exhibited the stability and endurance of the MAV. The initial tests provided endurance of just over 20 minutes and in a gust wind as high as 13m/s.

Figure 10 Prototype used for R/C and Autonomous flight4. Conclusions and Future work

An airframe with a flying wing configuration was designed and developed for the micro air vehicle. A new airfoil designed using inverse method from a numerical code using the Eppler 61 as the seed airfoil. The modified airfoil is found to provide inherent longitudinal stability thereby reducing the load on control surfaces. The modified airfoil with cropped delta planform analyzed using AVL code is found to give a good stability against Dutch roll and Spiral mode. The numerical flow visualization carried out using XFLR5 exhibited that the influence of tip vortices to be moderate in the operational range of the flight of MAV. A prototype of the MAV was made and flown in both R/C and autonomous mode and is found to perform well both in terms of endurance and stability under high cross winds. Further improvements are expected with the detailed wind tunnel validation

studies and optimized component placement. It is proposed to include the effect of fuselage and propeller in the future studies planned.

References

[1] Grasmeyer, J.M and Keennon, “Development of Black Widow Micro Air Vehicle”, AIAA paper 2001-127.[2] XFLR5 –http://xflr5.sourceforge.net/xflr5.htm.[3] AVL - http://web.mit.edu/drela/Public/web/avl/[4] Abdulrahim, M., Cocquyt, J. B., “Development of

Mission-Capable Flexible-Wing Micro Air Vehicles”, 53rd Southeastern Regional Student Conference 4–5April 2002, Alabama.[5]Mueller T.J., “Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle, Applications”, AIAA, Virginia, 2001.[6]Shashank Mishra, G. Ramesh “Design and Development of Airframe for Fixed Wing Micro Air Vehicle- GOLDEN HAWK”, NAL PD

EA 0908, March 2009.

http://xflr5.sourceforge.net/xflr5.htm

shashank mishra -golden hawk mav

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