AERODYNAMIC TESTING OF A CIRCULAR PLANFORM CONCEPT AIRCRAFT Except where reference is made to the work of others, the work described in this thesis is my own or was done in collaboration with my advisory committee. This thesis does not include proprietary or classified information. ___________________________ Bryan David Recktenwald Certificate of Approval: ______________________ _____________________ Roy Hartfield Anwar Ahmed, Chair Professor Associate Professor Aerospace Engineering Aerospace Engineering ______________________ _____________________ Gilbert Crouse Joe F. Pittman Associate Professor Interim Dean Aerospace Engineering Graduate School AERODYNAMIC TESTING OF A CIRCULAR PLANFORM CONCEPT AIRCRAFT Bryan David Recktenwald A Thesis Submitted to the Graduate Faculty of Auburn University in Partial Fulfillment of the Requirements for the Degree of Masters of Science Auburn, Alabama August 9, 2008 iii AERODYNAMIC TESTING OF A CIRCULAR PLANFORM CONCEPT AIRCRAFT Bryan David Recktenwald Permission is granted to Auburn University to make copies of this thesis at its discretion, upon request of individuals or institutions and at their expense. The author reserves all publication rights. ______________________________ Signature of Author ______________________________ Date of Graduation iv THESIS ABSTRACT AERODYNAMIC TESTING OF A CIRCULAR PLANFORM CONCEPT AIRCRAFT Bryan David Recktenwald Masters of Science, August 9, 2008 (B.S., Aero. Eng., Auburn University, 2006) 86 Typed Pages Directed by Anwar Ahmed Auburn University has entered into collaboration with Geobat Flying Saucer Aviation Inc. for aerodynamic and flow visualization studies of the Geobat aircraft. The aircraft model that was tested consisted of a circular planform with a central opening. A circular disk with an airfoil cross-section in the streamwise direction can offer distinct advantages of a circular planform configuration such as the reduced influence of tip vortices and hence lower induced drag. The aerodynamic challenges of such planforms include longitudinal and lateral stability, controllability and handling qualities partly due to the unique dynamics of wake vorticity. Wind tunnel testing was conducted to study the longitudinal stability of the Geobat aircraft. Studies include analysis of both a solid flat disk and one with similar geometric characteristics of the Geobat. The Geobat was tested with and with out a v leading edge transition strip to determine the difference between laminar and turbulent flow over the model. Multiple flap and elevator deflections were tested for both cases to help determine longitudinal stability characteristics. For comparison, a highly stable and conventional aircraft model, a Cessna 172, was also tested under the same conditions. After comparing, it was found that the Geobat model yielded much better stall characteristics than the Cessna 172 while pitching moment trends show a far less stable aircraft. Comparing the laminar and turbulent testing, aerodynamic data shows that the transition strip does not affect the longitudinal characteristics below the stall region. This illustrates that the flow over the model is already turbulent in nature. This can be seen in the flow visualization tests where a crescent shaped separation bubble was located at the leading edge tripping the flow to turbulent. Also distinct recirculation near the cockpit and trailing edge of the control surfaces was also observed. vi ACKNOWLEDGMENTS The author would like to thank Randy Pollard and Jack Jones of the Geobat Flying Saucer Aviation Inc. for their continued involvement during the research period. The author would also like thank Dr. Anwar Ahmed along with his committee for providing him with guidance and support. vii Style manual or journal used: Modern Language Association Style Manual Computer software used: Microsoft Office Word 2003 Microsoft Office Excel 2003 UGS Solid Edge V19 Labview V8.2 Tecplot 10 xiii TABLE OF CONTENTS NOMENCLATURE ........................................................................................................... x LIST OF FIGURES ........................................................................................................... xi LIST OF TABLES........................................................................................................... xiv 1 INTRODUCTION ...........................................................................................................1 1.1 Effects of aspect ratio.........................................................................................1 1.2 Low aspect ratio aircraft ....................................................................................4 1.3 Problem statement..............................................................................................7 2 OBJECTIVES ................................................................................................................9 3 WIND TUNNEL MODELS ........................................................................................10 3.1 The Geobat.......................................................................................................10 3.1.1 Description........................................................................................10 3.1.2 Model preparations ...........................................................................11 3.1.3 Transition strip..................................................................................14 3.2 Flat disks ..........................................................................................................15 3.3 Cessna 172 .......................................................................................................16 4 EXPERIMENTAL SETUP..........................................................................................18 4.1 Description of test facility................................................................................18 4.2 Test methods ....................................................................................................18 4.2.1 Force and moment.............................................................................18 ix 4.2.2 Wind tunnel flow visualization.........................................................20 5 RESULTS AND DISCUSSION..................................................................................21 5.1 Uncertainty analysis.........................................................................................21 5.2 Geobat aerodynamic data.................................................................................21 5.2.1 Aerodynamic characteristics without transition strip .......................22 5.2.2 Comparison with and without transition strip...................................28 5.3 Geobat and flat disk comparison......................................................................34 5.4 Geobat and Cessna 172 comparison ................................................................38 5.5 Flow Visualization...........................................................................................43 5.5.1 Flat disks ...........................................................................................43 5.5.2 Geobat...............................................................................................47 6 STABILITY ANALYSIS ............................................................................................52 6.1 Theory ...........................................................................................................52 6.2 Stability Results ...............................................................................................55 7 CONCLUSION ...........................................................................................................57 8 RECOMENDATIONS ................................................................................................59 REFERENCES ..................................................................................................................60 APPENDIX A: Coefficient Plots................................................................................62 APPENDIX B: Flow Visualization............................................................................69 x NOMENCLATURE ? Angle of attack, deg ?CDmin Angle of attack at minimum drag, deg ?L=0 Angle of attack at zero lift, deg c.g. Center of gravity CD Drag coefficient CD0 Drag coefficient at lift = 0 CD0? Minimum drag angle, deg CDmin Minimum drag coefficient CL Lift coefficient CLmax Maximum lift coefficient CL? Lift-curve slope, ??? LC CL?=0 Lift coefficient at zero angle of attack CM Pitching moment coefficient CM0 Pitching moment coefficient at zero angle of attack CM? Pitching moment curve slope, ??? MC ?CM Change in pitching moment coefficient ?e Elevator deflection, deg ?F Flap deflection, deg h Center of gravity location, ft hn Neutral point location, ft Kn Static margin, ft L/Dmax Maximum lift to drag ratio NP Neutral point xi LIST OF FIGURES Figure 1.1 Illustration of induced and effective angles of attack .....................................2 Figure 1.2 Schematic of the Sack AS-6 ...........................................................................4 Figure 1.3 U.S. Navy?s XF5U-1.......................................................................................5 Figure 1.4 Wind tunnel model of the LRV .......................................................................6 Figure 1.5 Model views of the Geobat.............................................................................8 Figure 3.1 Top schematic of the Geobat ........................................................................11 Figure 3.2 Geobat mounting location and mount...........................................................12 Figure 3.3 Elevator hardware construction ....................................................................13 Figure 3.4 Attached elevator with hardware ..................................................................13 Figure 3.5 Application of transition strip .......................................................................14 Figure 3.6 Final transition strip ......................................................................................14 Figure 3.7 Flat solid disk model.....................................................................................15 Figure 3.8 Flat cutout disk model...................................................................................16 Figure 3.9 Cessna 172 wind tunnel model .....................................................................17 Figure 4.1 Wind tunnel test setup...................................................................................19 Figure 5.1 Lift coefficient for ?F = 0 deg .......................................................................23 Figure 5.2 Drag coefficient for ?F = 0 deg .....................................................................24 Figure 5.3 Drag polar for ?F = 0 deg ..............................................................................26 Figure 5.4 Pitching moment coefficient for ?F = 0 deg..................................................28 xii Figure 5.5 With and without TS comparison of lift coefficient for ?F = 0 deg; solid symbols- with TS, open symbols- without TS.....................................29 Figure 5.6 With and without TS comparison of polar for ?F = 0 deg; solid symbols- with TS, open symbols- without TS.....................................30 Figure 5.7 With and without TS comparison for pitching moment coefficient for ?F = 0 deg; solid symbols- with TS, open symbols- without TS ..................31 Figure 5.8 With and without TS comparison of lift coefficient for ?e = 0 deg; solid symbols- with TS, open symbols- without TS.....................................32 Figure 5.9 With and without TS comparison of drag polar for ?e = 0 deg; solid symbols- with TS, open symbols- without TS.....................................33 Figure 5.10 With and without TS comparison for pitching moment coefficient for ?e = 0 deg; solid symbols- with TS, open symbols- without TS ..................33 Figure 5.11 Lift Curve- Geobat and disk comparison......................................................35 Figure 5.12 Drag Curve- Geobat and disk comparison....................................................36 Figure 5.13 Drag Polar- Geobat and disk comparison .....................................................37 Figure 5.14 Pitching moment- Geobat and disk comparison ...........................................38 Figure 5.15 Lift curve- Geobat and Cessna comparison for ?F = 0 deg; solid symbols- Geobat, open symbols- Cessna 172......................................39 Figure 5.16 Drag curve- Geobat and Cessna comparison for ?F = 0 deg; solid symbols- Geobat, open symbols- Cessna 172......................................40 Figure 5.17 Drag polar- Geobat and Cessna comparison for ?F = 0 deg; solid symbols- Geobat, open symbols- Cessna 172......................................41 Figure 5.18 Pitching moment curve- Geobat and Cessna comparison for ?F = 0 deg; solid symbols- Geobat, open symbols- Cessna 172......................................42 Figure 5.19 Pitching moment curve- Geobat and Cessna comparison for ?e = 0 deg; solid symbols- Geobat, open symbols- Cessna 172......................................42 Figure 5.20 Solid disk flow visualization at ? = 0 deg.....................................................43 Figure 5.21 Cutout disk flow visualization at ? = 0 deg ..................................................44 xiii Figure 5.22 Solid disk flow visualization at ? = 5 deg.....................................................45 Figure 5.23 Cutout disk flow visualization at ? = 5 deg ..................................................45 Figure 5.24 Solid disk flow visualization at ? = 10 deg...................................................46 Figure 5.25 Cutout disk flow visualization at ? = 10 deg ................................................47 Figure 5.26 Geobat flow visualization at ? = -5 deg........................................................48 Figure 5.27 Geobat flow visualization at ? = 0 deg .........................................................48 Figure 5.28 Geobat flow visualization at ? = 5 deg .........................................................49 Figure 5.29 Geobat flow visualization at ? = 10 deg .......................................................50 Figure 5.30 Geobat flow visualization at ? = 15 deg .......................................................51 Figure 5.31 Geobat flow visualization at ? = 20 deg .......................................................51 Figure 6.1 Effects of c.g. location on Cm curve..............................................................54 Figure 6.2 Total lift and moment acting on aircraft .......................................................54 xiv LIST OF TABLES Table 3.1 Geobat geometric parameters .......................................................................10 Table 3.2 Cessna 172 geometric parameters.................................................................16 Table 5.1a Lift coefficient (?F = 0 deg) ..........................................................................23 Table 5.1b Lift coefficient (?F = 10 deg) ........................................................................24 Table 5.1c Lift coefficient (?F = 20 deg) ........................................................................24 Table 5.2a Drag coefficient (?F = 0 deg) ........................................................................25 Table 5.2b Drag coefficient (?F = 10 deg) ......................................................................25 Table 5.2c Drag coefficient (?F = 20 deg) ......................................................................25 Table 5.3a Drag polar (?F = 0 deg) .................................................................................27 Table 5.3b Drag polar (?F = 10 deg) ...............................................................................27 Table 5.3c Drag polar (?F = 20 deg) ...............................................................................27 Table 5.4 Lift characteristics- Geobat and disk comparison.........................................36 Table 5.5 Drag characteristics- Geobat and disk comparison.......................................37 Table 6.1 Model values for NP calculations with ?F = 0 deg .......................................55 Table 6.2 Models NP and static margin for ?F = 0 deg.................................................56 1 1 INTRODUCTION 1.1 Effects of aspect ratio Unconventional aircraft of disk shaped planform configurations have been studied for several decades including the Vought V-173 (?The Flying Pancake?) and the aircraft in this study, the Geobat. These disk shaped aircraft as well as most fighter aircraft have a very low aspect ratio wing designs. Aircraft with low aspect ratio wings behave differently than high aspect ratio aircraft, such as the USAF B52 bomber and sail planes. The aspect ratio, AR, is determined by the wing span, b, and the wing area, S, as shown in equation 1.1. SbAR 2 = (1.1) In comparison with high AR aircraft, low AR aircraft typically have higher structural integrity, are more maneuverable, have lower parasitic drag and better space efficiency. A large span wing will have to overcome a larger moment of inertia in order to roll therefore a lower AR will have a higher roll rate which is very important in fighter planes. Another advantage is the structural weight of a low AR aircraft. The larger the AR, the larger the wing bending moment at the wing root and therefore the stronger the wing structure has to be, increasing the weight of the aircraft. This increase in weight will affect the performance of the airplane. For example, the thrust required will increase which will increase the fuel consumption and reduce the range of the [1]. 2 In designing an aircraft, the AR is one of the most important design features. It strongly affects the maximum lift to drag ratio at cruise conditions impacting the range of the aircraft [1]. Although low AR wings have advantage of higher roll rates and lower structural weight, there are other performance penalties, one being the increase in induced drag. Induced drag plays an important roll in determining the efficiency of the aircraft. Induced drag is a pressure drag that is a result of wing-tip vortices that induce changes in the velocity and pressure over the wing. These vortices induce a downward component of velocity called downwash, w, which causes an induced angle of attack, ?i, and results in the wing seeing an effective angle of attack, ?eff, which is smaller than the geometric angle of attack, ?g, as seen in Figure 1.1. Moreover, since lift is perpendicular to the local relative wind, the downwash tilts the lift vector aft and results in a component of lift in the drag direction. This is referred to as induced drag. Figure 1.1 Illustration of induced and effective angles of attack [1] As mentioned above, the downwash results in a lower CL value at a given geometric angle of attack. Estimation of the aerodynamic coefficients for finite wings can be done by using Prandtl?s lifting line theory. Using this theory, the lift slope for finite wings can 3 be written as shown in Equation 1.2, where a0 is the slope for a given infinite airfoil and e is the span efficiency factor [2]. An elliptical lift distribution over the wing minimizes the induced effects and how close the lift distribution is to an elliptical shape is the span efficiency factor, e [1]. Most aircraft have an efficiency factor from 0.90 to 0.97 [1]. This equation will yield a hn) yielding a positive Cm? or negative pitching stiffness. For the same range of ?, hn is almost equal to the c.g. for ?e = 20 deg (h = hn) showing the boundary between positive and negative pitching stiffness. For ? of 5 degrees and higher the Geobat model shows positive stiffness for every ?e with a maximum Kn = 0.0203 ft at ?e = 20 deg. The Cessna 172 is stable through its entire range of ? up to stall. The NP is behind the c.g at every ?e location and has a static margin much larger than the Geobat with a maximum Kn = 0.0376 ft. The calculations further show that the Geobat model is slightly unstable to neutrally stable below the 5 degrees with increasing stability above 5 degrees. The Cessna 172 model shows that it is much more stable with double the static margin of the Geobat. The Geobat could have the same static stability as the Cessna 172 model by moving the c.g. location forward. This would increase the static margin, therefore increasing the stability of the aircraft. To have similar longitudinal stability characteristics to the Cessna 172, the Geobat c.g. would need to be moved to a point 0.621 ft aft of the nose versus its current position at 0.645 ft. 57 7 CONCLUSION Wind tunnel tests confirmed acceptable aerodynamic characteristics for the Geobat airplane. The Geobat was able to produce lift curves that agreed with the theoretical value from Prandtl?s lifting line theory. Both flow cases with and without a transition strip showed drag polars that were similar however the model with the transition strip exhibited a more gradual stall at higher ?. The lift curve shows a higher stall angle for the Geobat with relatively the same CLmax as the Cessna 172. The lift curve slope of Geobat however was lower than that of Cessna 172 model decreasing the L/Dmax while drag data revealed a lower minimum drag for the Geobat model and better stall characteristics again following low aspect ratio characteristics. The pitching moment coefficient for the Geobat indicated neutral stability in the lower ? range and higher stability with increasing angle of attack while the Cessna 172 has good stability characteristics through the entire range of angles of attack. This was confirmed by analysis of the NP and static margin of both aircraft. Comparison of the Geobat to the flat disks showed that the cutout disk had trends similar to the Geobat, but with much higher drag. All models showed trends agreeing with low AR designs. Flow visualization revealed a crescent shaped laminar separation bubble near the leading edge followed by turbulent reattachment. This visualization confirmed results 58 that were noted from the aerodynamic data, that the addition of the transition strip did not improve longitudinal characteristics in the lower angles of attack range because the flow was already turbulent in nature. Additional flow structures observed on the Geobat cockpit, control surface trailing edge and pylon mountings may decrease overall performance. 59 8 RECOMMENDATIONS This research has allowed for a wide range of applications to be applied to the Geobat model in the search for more promising aerodynamic qualities. By moving the c.g. forward on the model, testing for a more stable aircraft can be done. After viewing the flow visualization it would be a good idea to place more turbulent strips at areas of separation, in particular the cockpit and leading edges of the tail section. This would hopefully keep the flow more attached on the aircraft and may improve the lift as well as the pitching moment of the aircraft. There are also sections of the aircraft that can be ?fine tuned? in order to improve aerodynamics. The outer edges, or wing tips, seem to be too thick. The aircraft already possesses strong structural integrity due to the design. Unless engines were to be mounted at this region, reduction in thickness may improve its overall characteristics. Water tunnel tests should also be conducted in depth to further understand the wake of the aircraft. What is the strength of the vortices at the wing tip? How does the flow off of the front of the aircraft affect the horizontal and vertical control surfaces? The novelty of this aircraft has a great potential for improvement and better understanding in low aspect circular planform designs. 60 REFERENCES [1] Anderson, J.D., Jr. Aircraft Performance and Design. New York: McGraw-Hill, Inc., 1999. [2] Anderson, J.D., Jr. Fundamentals of Aerodynamics 3rd ed. New York: McGraw- Hill, Inc., 2001. [3] Raymer, D.P. Aircraft Design: A Conceptual Approach 3rd ed. Virginia: American Institute of Aeronautics and Astronautics, Inc., 1999. [4] ?Sack AS-6 Luft ?46 entry.? Sack AS-6. 1997. 8 Jan 2008 . [5] ?VoughtV-173/XF5U-1.? Daves war birds. 1999. 3 Jan 2008 . [6] Wainfan, B. ?Zimmerman?s Flying Flapjack: Designs Ahead of their Time.? Flight Journal. (2005): 89-93. [7] Gudaitis, F. ?Charles Zimmerman and his ?Skimmer?.? Flight Journal. (2005): 65- 71. [8] Wilson, T. ?Americas Nuclear Flying Saucer.? Military. 15 January 2008 . [9] Ware, G. ?Investigation of the Low-subsonic Aerodynamic Characteristics of a model of a modified Lenticular Reentry Configuration.? NASA TM X-756, 1962. [10] Stilley, G.D. ?Aerodynamic Analysis of the Self Sustained Flair.? RDTR no 199, Naval Ammunition Depot. Indiana. 1972. [11] Stilley, G.D., and Carstens, D.L. ?Adaptation of Frisbee Flight Principle to Delivery of Special Ordnance.? AIAA Paper 72-982. In Proceedings of the 2nd Atmospheric Flight Mechanics Conference, Palo Alto, California. 1972. [12] Mitchell, T.L. The Aerodynamic Response of Airborne Discs. MS thesis, University of Nevada, Las Vegas, NV. 1999. 61 [13] Yasuda K. ?Fight and Aerodynamic Characteristics of a Flying Disk.? Japanese Soc. Aero. Space Sci. (1999): 16-22. [14] .Potts, J.R., and Crowther, W.J. ?Frisbee Aerodynamics.? AIAA Paper 2002-3150. In Proceedings of the 20th AIAA Applied Aerodynamics Conference, St. Louis, Missouri. 2002. [15] Ali, W., ?Aerodynamics of Rotating Disc Wings.? Division of Aerospace, Undergraduate Report, School of Engineering, University of Manchester, UK, April 1998. [16] ?Geobat Flying Saucer, Future Horizons.? 2005. 10 November 2007 [17] Etkin, B., and Reid, L. Dynamics of Flight: Stability and Control 3rd ed. New York: John Wiley and Sons, Inc, 1996. [18] Abbott, I., and Von Deonhoff, A. Theory of Wing Sections. New York: Dover Publications, Inc, 1958. 62 APPENDIX A COEFFICIENT PLOTS 63 ? C L -5 0 5 10 15 20 -0.5 0 0.5 1 1.5 ?e = -20 deg ?e = -10 deg ?e = 0 deg ?e = 10 deg ?e = 20 deg Figure A1: Laminar lift coefficient for ?F = 10 deg ? C L -5 0 5 10 15 20 -0.5 0 0.5 1 1.5 ?e = -20 deg ?e = -10 deg ?e = 0 deg ?e = 10 deg ?e = 20 deg Figure A2: Laminar lift coefficient for ?F = 20 deg 64 ? C D -5 0 5 10 15 20 -0.1 0 0.1 0.2 0.3 0.4 0.5 ?e = -20 deg? e = -10 deg? e = 0 deg? e = 10 deg? e = 20 deg Figure A3: Laminar drag coefficient for ?F = 10 deg ? C D -5 0 5 10 15 20 -0.1 0 0.1 0.2 0.3 0.4 0.5 ?e = -20 deg? e = -10 deg? e = 0 deg? e = 10 deg? e = 20 deg Figure A4: Laminar drag coefficient for ?F = 20 deg 65 CD C L -0.1 0 0.1 0.2 0.3 0.4 0.5 -0.5 0 0.5 1 1.5 ?e = -20 deg ?e = -10 deg ?e = 0 deg ?e = 10 deg ?e = 20 deg Figure A5: Laminar drag polar for ?F = 10 deg CD C L -0.1 0 0.1 0.2 0.3 0.4 0.5 -0.5 0 0.5 1 1.5 ?e = -20 deg ?e = -10 deg ?e = 0 deg ?e = 10 deg ?e = 20 deg Figure A6: Laminar drag polar for ?F = 20 deg 66 ? C M -5 0 5 10 15 20 -0.03 -0.02 -0.01 0 0.01 0.02 ?e = -20 deg ?e = -10 deg ?e = 0 deg ?e = 10 deg ?e = 20 deg Figure A7: Laminar pitching moment coefficient for ?F = 20 deg ? C M -5 0 5 10 15 20 -0.03 -0.02 -0.01 0 0.01 0.02 ?e = -20 deg ?e = -10 deg ?e = 0 deg ?e = 10 deg ?e = 20 deg Figure A8: Laminar pitching moment coefficient for ?F = 20 deg 67 ? C L -5 0 5 10 15 20 -0.5 0 0.5 1 1.5 2 ?F = 0 deg?F = 10 deg ?F = 20 deg ?F = 0 deg ?F = 10 deg ?F = 20 deg Figure A9: Lift curve- Geobat and Cessna comparison for ?e = 0 deg; solid symbols- Geobat, open symbols- Cessna 172 ? C D -5 0 5 10 15 20 -0.1 0 0.1 0.2 0.3 0.4 0.5 ?F = 0 deg? F = 10 deg? F = 20 deg? F = 0 deg? F = 10 deg? F = 20 deg Figure A10: Drag curve- Geobat and Cessna comparison for ?e = 0 deg; solid symbols- Geobat, open symbols- Cessna 172 68 CD C L -0.1 0 0.1 0.2 0.3 0.4 0.5 -0.5 0 0.5 1 1.5 ?F = 0 deg ?F = 10 deg ?F = 20 deg ?F = 0 deg ?F = 10 deg ?F = 20 deg Figure A11: Drag polar curve- Geobat and Cessna comparison for ?e = 0 deg; solid symbols- Geobat, open symbols- Cessna 172 ? C M -5 0 5 10 15 20 -0.06 -0.04 -0.02 0 0.02 ?F = 0 deg ?F = 10 deg ?F = 20 deg ?F = 0 deg ?F = 10 deg ?F = 20 deg Figure A12: Pitching moment curve- Geobat and Cessna comparison for ?e = 0 deg; solid symbols- Geobat, open symbols- Cessna 172 69 APPENDIX B FLOW VISUALIZATION 70 Figure B1: Rear view at ? = 0 deg Figure B2: Rear view at ? = 0 deg 71 Figure B3: Rear view at ? = 5 deg Figure B4: Rear view at ? = 10 deg 72 Figure B5: Rear view at ? = 15 deg