Aerodynamics and Design for Ultra-Low Reynolds Number Flight

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Chapter 7 160 TABLE 7.4 Summary of physical and performance data for three prototype electric rotorcraft. �� Three areas of technology play the largest roles in determining the feasibility of a design: aerodynamic efficiency, electro-mechanical efficiency, and the energy storage density at the necessary current levels to drive the rotors. 7.5.1 Emerging Battery Technologies A detailed discussion of battery technologies is beyond the scope of this work but comparisons of several widely available chemistries and cell sizes is provided by Kroo [43]. As a footnote to their discussion of power storage issues for micro-air-vehicles, lithium polymer cells have now become widely available in the consumer market. Due to the current physical dimensions of these cells, this development does not immediately affect the 15g vehicle, but for the larger vehicles, this technology offers significant performance gains over the previously considered chemistries. At approximately 50mAh/g, current consumer lithium polymer cells offer a higher energy density than the 43mAh/g 780mAh Tadiran lithium / manganese-dioxide cells. In addition to the benefits of increased energy density, further gains are possible due to the increased base voltage of 3.7V and a 4C to 6C discharge rate, versus 3.0V and a 3C rate for the Tadiran cells. This results in a power density of 185mWh for the lithium polymer cells versus 129mWh for the Tadiran cells, a 43% increase. The term ‘C-rate’ refers to the one hour discharge current, equal to the cells mAh capacity.
Using the 150g prototype as an example, conversion to lithium polymer technology Chapter 7 would permit a reduction from four to three cells resulting in roughly a 50% increase in endurance at the same total battery mass. There is a one volt drop in the no-load voltage, but the operational voltages of the two cell types would be much closer due to the additional headroom in discharge rate and associated reduction in internal losses provided by the lithium polymer cells. The higher discharge rate also permits application of these cells to higher current draw situations such as the 65g prototype, where previously NiCd and NiMh chemistries, with significantly lower energy densities, provided the best solution. 7.5.2 Electro-mechanical Efficiency While the primary focus of this work has been the aerodynamics of ultra-low Reynolds number flight, the design and development of a complete vehicle cannot be undertaken without consideration of the electro-mechanical efficiencies of the supporting systems. This issue, in conjunction with energy storage issues, poses a significant impediment to the development of extreme micro-rotorcraft such as the 15g prototype. This area encompasses not only the motors and speed controllers, but additional equipment required for voltage conversion and regulation, communications, and control. As discussed here, this does not include the battery system, which is considered separately, or the power required for mission-based payload such as cameras or other sensors. The 150g vehicle exhibits an overall power efficiency of 52%, this is considerably lower than the experimentally determined motor efficiency of approximately 65% to 70%, but in addition to the motors, this system includes a radio receiver, four motor controllers, and four solid-state piezo gyros. Although the 15g prototype’s rotor system requires less than half of the power of the 150g vehicle’s rotors, the total system power required for hover increases by 60%. This results in a total system efficiency of only 15%. This is once again lower than the experimentally determined electro-mechanical efficiency for the motor, in this case combined with the closed-loop controller, of approximately 17%, but there are no additional system components in this case. The 40% drop in power plant 161
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AERODYNAMICS AND DESIGN FOR ULTRA-L
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I certify that I have read this dis
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Abstract Growing interest in micro-
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Nomenclature A Rotor disk area B Nu
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τ Shear stress ω, Ω Rotor angul
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Contents Acknowledgments...........
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Chapter 5..........................
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List of Tables 3.1 Effect of Airfoi
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List of Figures 2.1 Comparison of I
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5.6 Small test fixture in the large
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6.38 Predicted spanwise thrust dist
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Chapter 1 Introduction 1.1 Looking
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Chapter 1 wind-tunnel facilities an
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1.3 Summary of Contributions Chapte
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Chapter 2 Two-Dimensional Computati
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Chapter 2 The artificial compressib
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Chapter 2 The control volume method
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2.2.4 Comparison with Experiment Th
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Chapter 2 to diverge if significant
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2.4 Flow Field Assumptions The INS2
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Chapter 2 The results of these time
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Chapter 3 Analysis and Design of Ai
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Chapter 3 adverse gradient in the p
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The canonical pressure coefficient
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Chapter 3 reaches α=5.0 and a lift
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��
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Cl 0.50 0.40 0.30 0.20 0.10 0.00 -0
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Re=6000 result due to Reynolds numb
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FIGURE 3.10 NACA 0002 and NACA 0008
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C l C l 0.8 0.7 0.6 0.5 0.4 0.3 0.2
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Chapter 3 The maximum L/D for all n
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3.5 Effect of Leading Edge Shape an
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C l 0.60 0.50 0.40 0.30 0.20 0.10 0
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Chapter 3 prominent droop near the
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Chapter 3 Small improvements in lif
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Chapter 4 Hybrid Method for Rotor D
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Q/r D L Chapter 4 FIGURE 4.1 Typica
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4.2.3 Elimination of Trigonometric
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Chapter 4 applications of interest
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4.2.7 The Distinction Between the A
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Chapter 4 Two input parameters dete
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Chapter 4 simplification, particula
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4.3.3 Conservation of Angular Momen
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Chapter 4 moves downstream is less
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Chapter 4 of the pitch angle at the
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FIGURE 4.6 Converged wake streamlin
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Chapter 4 idealized models. The met
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��
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Chapter 5 Overview of Experimental
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1. Substrate Preparation 2. Polymer
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Chapter 5 surface finish and aids i
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FIGURE 5.5 Small test fixture in th
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Chapter 5 The direct influence of t
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FIGURE 5.9 Large test fixture in th
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5.4.1 Motor Power Supply All tests
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Chapter 5 or roughly (+/-) 25% of t
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Chapter 5 newly forming blade tip v
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FIGURE 5.14 Near-body grid geometry
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Chapter 6 Design Examples and Compa
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Chapter 6 capable of speeds up to 1
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FIGURE 6.2 The Astro Flight Firefly
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t/c 0.5 0.4 0.3 0.2 0.1 0.0 FIGURE
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c/R 0.0 0.2 0.4 0.6 0.8 1.0 r/R Cha
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FIGURE 6.10 Carbon-fiber two-blade
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Power Req. (W) 3.5 3.0 2.5 2.0 1.5
Page 133 and 134: Chapter 6 TABLE 6.1 Comparison of p
Page 135 and 136: Electro-mechanical Efficiency 0.20
Page 137 and 138: these variations and possible solut
Page 139 and 140: upper and lower surface corners of
Page 141 and 142: Chapter 6 The Sample-1 rotor exhibi
Page 143 and 144: The static deformations described i
Page 145 and 146: Q( r) This problem is solved as a l
Page 147 and 148: Structural Analyses Chapter 6 The m
Page 149 and 150: Aero-Structural Twist (deg.) 0.00 -
Page 151 and 152: Thrust (g) 5 4 3 2 1 0 10000 20000
Page 153 and 154: Thrust (g) 4.5 4.0 3.5 3.0 2.5 2.0
Page 155 and 156: Torque per unit span (g cm) 0.75 0.
Page 157 and 158: Thrust per unit span (g) 3.5 3.0 2.
Page 159 and 160: V total / ωr 1.0 0.9 0.8 Classical
Page 161 and 162: Torque per unit span (g cm) 0.75 0.
Page 163 and 164: Chapter 6 Recall that this rotor wa
Page 165 and 166: Incidence (deg.) 30 25 20 15 10 5 0
Page 167 and 168: Thrust (g) 9 8 7 6 5 4 3 2 38000 40
Page 169 and 170: x/R 0.4 0.2 0.0 -0.2 Chapter 6 FIGU
Page 171 and 172: Cl 0.64 0.62 0.60 0.58 0.0 0.2 0.4
Page 173 and 174: C f 0.125 0.100 0.075 0.050 0.025 0
Page 175 and 176: Chapter 7 Micro-Rotorcraft Prototyp
Page 177 and 178: This vehicle does not currently inc
Page 179 and 180: FIGURE 7.2 The 65g prototype electr
Page 181 and 182: 7.4 The 150g Prototype Chapter 7 Th
Page 183: At hover, with a mass of 153g, the
Page 187 and 188: Chapter 7 Basic rotor theory has be
Page 189 and 190: most of the blade. These values are
Page 191 and 192: Chapter 7 For large scale helicopte
Page 193 and 194: Chapter 8 Conclusions and Recommend
Page 195 and 196: the significant performance gains a
Page 197 and 198: Chapter 8 are also opportunities to
Page 199 and 200: Chapter 8 formation flight, and man
Page 201 and 202: References [1] Spedding, G.R., Liss
Page 203 and 204: [28] Lamb, Sir Horace, Hydrodynamic
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