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2010 results in Aerospace engineering

Page 43 of 101

  • Appendix B - Sources of Experimental Data for Code Validation

  • Russell M. Cummings, William H. Mason, Virginia Polytechnic Institute and State University, Scott A. Morton, David R. McDaniel, University of Alabama, Birmingham
  • Book:
    Applied Computational Aerodynamics
    Published online:
    28 May 2018
    Print publication:
    27 April 2015, pp 766-775

  • Summary

    Some sources of aerodynamic geometry and experimental data for use in code evaluation are listed here. They are invaluable for making sure that you are using a computational aerodynamics code correctly. Always check a code that is new to you against known results, which we already discussed in Chapter 2. Note that rigorous validation of codes requires very careful analysis and an understanding of possible experimental, as well as computational, error (which was also discussed in Chapter 10). Most of the NASA and NACA reports cited here are available from the NASA Technical Reports Server, http://ntrs.nasa.gov/; a mirror website for NACA reports is available from Cranfield University at http://naca.central.cranfield.ac.uk/. Some of the reports listed here will also be provided at the book website: http://www.cambridge.org/aerodynamics. Most of the results are presented graphically, so a utility such as DataThief or Engauge is needed to digitize the data for comparison with calculations.

    Books

    Abbott, I.H. and von Doenhoff, A.E., Theory of Airfoil Sections, New York: Dover, 1959: This is a book every aerodynamicist should have. Look in the references for the original NACA airfoil reports. The aerodynamic descriptions contained in the reports are unsurpassed. However, beware of the actual data presented prior to 1939, which is when they discovered that they had to apply a different support interference correction to the measured results (see NACA Report 669 by Jacobs for details). Note that pressure distributions for airfoils are fairly rare. See also NACA Report 824, which is an earlier compendium similar to this book.

  • Appendix A - Geometry for Aerodynamicists

  • Russell M. Cummings, William H. Mason, Virginia Polytechnic Institute and State University, Scott A. Morton, David R. McDaniel, University of Alabama, Birmingham
  • Book:
    Applied Computational Aerodynamics
    Published online:
    28 May 2018
    Print publication:
    27 April 2015, pp 731-765

  • Summary

    Aerodynamicists control the flowfield through geometry definition and are always interested in possible geometric shapes that would be useful in design. This appendix provides the detailed definition of many of the classic shapes frequently specified in aerodynamics. It is not intended to be encyclopedic, but will provide a good starting point for where to obtain geometric definitions for aerodynamic shapes.

    The NACA Airfoils

    The NACA (National Advisory Committee for Aeronautics) airfoils were designed during the period from 1929 through 1947 under the direction of Eastman Jacobs at the NACA’s Langley Field Laboratory (now NASA Langley Research Center). Most of the airfoils were based on simple geometrical descriptions of the section shape, although the 6 and 6A series were developed using theoretical analysis and don’t have simple shape definitions. Although a new generation of airfoils has emerged as a result of improved understanding of airfoil performance and the ability to design new airfoils using computational methods, the NACA airfoils are still useful in many aerodynamic design applications. A number of references have been included to allow the reader to study both the older NACA literature and the new airfoil design ideas. Taken together, this literature provides a means of obtaining a rather complete understanding of the ways in which airfoils can be shaped to obtain desired performance characteristics.

  • Preface

  • Russell M. Cummings, William H. Mason, Virginia Polytechnic Institute and State University, Scott A. Morton, David R. McDaniel, University of Alabama, Birmingham
  • Book:
    Applied Computational Aerodynamics
    Published online:
    28 May 2018
    Print publication:
    27 April 2015, pp xv-xviii

  • Summary

    Aren’t there already plenty of excellent books on the topic of CFD? Yes, there are … if you are a graduate student who wants to learn the intricacies of numerical methods applied to solving the fundamental equations of fluid dynamics. However, we believe that a paradigm shift has taken place in CFD, where the development of algorithms and codes has largely been replaced by people applying well-established codes to real-world applications. While this is a natural progression in any field of science and engineering, we do not believe that the paradigm shift has filtered into the academic world. In academia, undergraduates learning about aerodynamics are still going through theories and applications that were being taught 40 or 50 years ago. We believe that it is time to write a book for people who want to be “intelligent users” of CA, not for those who want to continue developing CA tools. We strongly endorse the perspective of David Darmofal and Earll Murman of MIT (see AIAA Paper 2001–0870):

    Within aerodynamics, the need for re-engineering the traditional curriculum is critical. Industry, government, and (to some extent) academia has seen a significant shift away from engineering science and highly specialized research-oriented personnel toward product development and systems-thinking personnel. While technical expertise in aerodynamics is required, it plays a less critical role in the design of aircraft than in previous generations. In addition to these influences, aerodynamics has been revolutionized by the development and maturation of computational methods. These factors cast significant doubt that a traditional aerodynamics curriculum with its largely theoretical approach remains the most effective education for the next generation of aerospace engineers. We believe that change is in order.

    We agree completely and believe that CA needs to be brought into the undergraduate classroom as soon as possible. That is why we have written this book!

    The target audience for Applied Computational Aerodynamics is advanced undergraduates in aerospace engineering who want (or need) to learn CA in the broad context of learning to do computational investigations, while also learning engineering methods and aerodynamics. In addition, we believe that working engineers who need to apply CA methods, but who have no CA background, will also find the book valuable.

  • Appendix D - Computational Aerodynamics Programs

  • Russell M. Cummings, William H. Mason, Virginia Polytechnic Institute and State University, Scott A. Morton, David R. McDaniel, University of Alabama, Birmingham
  • Book:
    Applied Computational Aerodynamics
    Published online:
    28 May 2018
    Print publication:
    27 April 2015, pp 790-801

  • Summary

    Codes Available from Book Website

    Several types of programs are used to provide insight into aerodynamics, such as airfoil, wing, and aircraft analysis, as well as various aerodynamic design programs. The number of programs available has grown a great deal over the past few years, and a number of the programs have been discussed in Chapters 2 and 5 especially. This appendix will list and describe some of the programs that are either available from the book website (www.cambridge.org/aerodynamics) or that could be used to complete various projects listed at the end of chapters in the book. In this section we will just list and briefly describe some of the programs:

  • FOILGEN; provides ordinates for NACA 4-digit, 4-digit modified and 5-digit airfoils

  • LADSON; provides ordinates for NACA 6- and 6A-series airfoils

  • PANELV2: an airfoil panel method

  • THINFOIL; an airfoil CFD program

  • LIDRAG: a lift-induced drag program

  • LAMDES; a program that finds wing camber and twist to obtain a given splanload

  • FRICTION; a skin friction and form drag estimation program

  • VLMpc; a two-surface vortex lattice program

  • DESCAM; an inverse design airfoil program

  • TRIDAG; the Thomas algorithm solution approach for a tridiagonal matrix

    • FOILGEN

    This program is used for airfoil geometry generation. For airfoils with analytically defined ordinates, this program produces airfoil definition data sets in the format required for PANELV2. This includes NACA 4-digit, 4-digit modified and 5-digit airfoils. In addition, the NACA 6 and 6A camber lines are available. The user can combine any combination of thickness and camber lines available within these shapes. This provides a wide range of airfoil definitions. The program runs interactively, and a sample session, with inputs and outputs, is provided at www.cambridge.org/aerodynamics.

  • Epilogue

  • Hema Singh, Rakesh Mohan Jha
  • Book:
    Active Radar Cross Section Reduction
    Published online:
    05 April 2015
    Print publication:
    02 March 2015, pp 283-284

  • Summary

    The main objective of this book is to establish the feasibility of a novel concept of active radar cross section reduction (RCSR). In this context, the phased antenna array theory and algorithms are discussed.

    A discerning reader may conclude that this book does not directly lead to the realisation of low-observable platforms, which is correct. The book in hand is not a handbook or a reference manual for achieving active RCSR over low-observable platforms.

    Design and realisation of active RCS low-observable platforms through this route (i.e. the theory and algorithms of phased antenna arrays) would also involve on-board/airborne antenna analysis. As mentioned in the Preface of this book, the methods and techniques for on-board/airborne antenna analysis are now well-established across all the three (low, resonance and high) frequency regions. References are numerous and readers may have their own preferences.

    Another important aspect is that of conformal arrays. Again, the modern conformal antenna theory now appears to have been systematised (Josefsson and Persson, 2006). Incidentally, the analytical core of the conformal array theory in this reference (Josefsson and Persson, 2006) relies extensively on the work of one of the authors (Jha et al. 1993), (Jha et al. 1994) or (Jha and Wiesbeck, 1995). Analysis of conformal phased arrays over the airborne platform would be required towards achieving the low-observable platform. The conformal antenna array RCS is summed up with the structural RCS of the platform to obtain a quiescent RCS pattern. Following the sensing of the direction of arrival (DOA) of the incoming radar wave, the overall structural plus antenna mode RCS pattern is adapted to obtain the active RCSR in the DOA, thereby leading to the active low-observable platform.

    It may be noted that other techniques to realise low-observable platforms are also prevalent and have been discussed in this book. Broadbanding of RCSR is important and the basic theory of RAM design (Vinoy and Jha 1996) should not be lost sight of. Likewise, plasma stealth is emerging as yet another feasible option in the context of low-observable platforms.

  • 8 - Emerging RCSR Techniques

  • Hema Singh, Rakesh Mohan Jha
  • Book:
    Active Radar Cross Section Reduction
    Published online:
    05 April 2015
    Print publication:
    02 March 2015, pp 268-282

  • Summary

    Introduction

    The active radar cross section reduction (RCSR) and the algorithms discussed in this book relate to the real-time RCSR and control of aerospace structures. The antenna array mounted over a platform actively adapts the pattern in coherence with the overall structural radar cross section (RCS) of the platform. The pattern adaptation depends on parameters such as the number of radar sources, their frequencies, the angle of arrival, the power levels, and the bandwidth. The RCS of aerospace platform with phased array mounted over it is reduced towards each of the hostile probing sources while simultaneously maintaining communication links with friendly radars.

    Apart from the concept of active RCSR, several other techniques and ideas have been proposed towards achieving low observability through passive means. These include the usage of engineered materials, i.e. frequency selective surfaces (FSS), artificial magnetic conductor (AMC), metamaterials, conformal load-bearing antennas, etc. The research is emphasised towards the miniaturisation, bandwidth enhancement and conformality of the structures. This is especially true for the antennas mounted over the surface of aerospace structures. Typically, RCS of a non-stealthy aircraft may vary from 10 dBsm to 30 dBsm, whereas missiles have relatively lower RCS (–20 dBsm to 10 dBsm) (Doan et al. 1995) (Paterson 1999). Moreover, various sensors and antennas are mounted over these platforms owing to applications such as surveillance, tracking and reconnaissance and, hence, contribute to the overall RCS of the platform. Extensive studies have, therefore, been done to reduce the antenna RCS (Volakis et al. 1992), (Zhang et al. 2012). The solutions proposed include the shaping of the surface, application of radar absorbing material (RAM) (Pozar 1992), (Wang et al. 2011), using tapered resistive sheets in antenna (Gustafsson 2006), and (Genovesi et al. 2012) in radome, passive RCSR (Knott et al. 2004) and active RCSR. Scattering at the edges can be suppressed effectively by using tapered loading.

  • 6 - RCS of Dipole Array Including Mutual Coupling Effects

  • Hema Singh, Rakesh Mohan Jha
  • Book:
    Active Radar Cross Section Reduction
    Published online:
    05 April 2015
    Print publication:
    02 March 2015, pp 216-244

  • Summary

    Introduction

    Radar cross section (RCS) of an aerospace vehicle significantly depends on the scattering of the on-board mounted antenna array. This antenna mode scattering is dependent on (i) the geometry of the phased array, (ii) the nature of the feed network employed to excite it, and (iii) the Mutual Coupling effect. A phased array, in general, requires a feed network to function as an effective transmitter or receiver. This feed network comprises radiating elements, phase-shifters, couplers and terminating resistors. Depending on their mode of arrangement, the feed network may be categorised as series feed, corporate-feed and space-feed (Volakis 2007).

    To design a complete phased array system with specific radiation characteristics, several factors like the number of elements, their inter-element spacing, beam scan angle, arrangement of phase shifters, couplers, and loads have to be considered. Further, if the spacing between the antenna elements is less than half-wavelength, the effects of mutual coupling become predominant. These parameters of the array design are found to affect the RCS of the array considerably.

    Mutual coupling affects the impedance of the array elements, reflection coefficients, radiation pattern and, thus, the RCS of phased array. Every element in the antenna array is of a definite physical size and shape. The incident wave on each of these elements will reradiate due to various reasons, such as the mismatches in the network, manufacturing defects, etc. The fields radiated from one antenna element interact with those of the surrounding antennas, giving rise to mutual coupling. These reradiated fields when scattered along different directions affect the RCS of the target significantly.

    Further, the effect of mutual coupling on the array performance is influenced by (i) the feed network, (ii) array scan angle, and (iii) the geometry of the antenna array, viz. side-by-side, collinear and parallel-in-echelon configuration. The literature available in the open domain presents several methods to estimate the antenna RCS.

  • 3 - Radar Cross Section of Phased Antenna Arrays

  • Hema Singh, Rakesh Mohan Jha
  • Book:
    Active Radar Cross Section Reduction
    Published online:
    05 April 2015
    Print publication:
    02 March 2015, pp 65-125

  • Summary

    Introduction

    Radar cross section (RCS) is the measure of detectability of a target which, in turn, depends on its external features and electromagnetic (EM) properties. It relates the EM energy reflected from the target at the receiver to the incident EM energy coming out from the source. Alternately, it is the power scattered by the target towards an illuminator. The RCS of a target depends on its physical shape, frequency, polarisation properties, orientation, constitutive materials and subcomponents such as antennas and other sensors.

    Phased antenna array is an array of antenna elements such as aperture, slot, horn, microstrip patch, spiral, dipole, etc. used to achieve high directivity (Mallioux 1994). It also affects the RCS of a platform on which it is mounted. Thus, for the estimation followed by reduction of RCS of a phased antenna array, scattering for both in-band (i.e. the operational frequency of radar signal) and out-of-band threat frequencies should be considered.

    Scattering analysis of phased antenna arrays includes two scattering modes: antenna (or radiation mode) and structural mode (Hansen 1989). The antenna mode occurs when an induced current gets reflected at the antenna feed point and then re-radiated. On the contrary, the structural mode is generated from the antenna surface induced currents. The two modes are difficult to identify and separate, when the array is mounted on a platform (Wang et al. 2010). Further, the radiation and scattering characteristics of an antenna array depend on the relative operational frequency range of an antenna array with respect to that of the radar. If the frequency of the radar signal falls within the operating band of the antenna array, then the impinging signal will match that of antenna elements, and then it will penetrate into the feed structure. This signal results in multiple reflections within the antenna system and hence contributes significantly to RCS of the antenna array (Jenn 1995). of the impinging signal falls outside the operating band of the antenna array, then there will be no match between the radar signal and that of the antenna elements, and reflections inside the feed will be insignificant.

  • 1 - Introduction to Radar Cross Section Reduction

  • Hema Singh, Rakesh Mohan Jha
  • Book:
    Active Radar Cross Section Reduction
    Published online:
    05 April 2015
    Print publication:
    02 March 2015, pp 1-14

  • Summary

    Introduction

    The concept of stealth or radar cross section (RCS) reduction and control has been a topic of interest since World War II. Attempts were initially made to reduce the detectability of the aircraft by employing wood and other composites as aircraft materials since they were less reflective to the radar waves than a metal. Following the initial systematisation, one realised that shaping and coating [by radar-absorbing materials (RAMs)] emerged as the primary techniques for the RCS reduction (RCSR).

    RCSR through shaping is readily apparent in case of stealth fighter aircraft F-22. The edges at principal and drooping ends of wings and rear end of the aircraft have similar angular sweep. Further, the fuselage and canopy are smooth-surfaced with slopes at sides. The shapes of the surface interfaces, such as the doors at bay and the seam of the canopy, are saw-wave-type. The vertical airfoil of aircraft tails is slant. The front side of its engine is obliterated and includes a serpentine-shaped engine duct. Finally, all the weapons are stored within the aircraft itself. These alterations in the conventional shape of the aircraft resulted in considerable RCSR of the aircraft.

    In contrast, RAM coatings have been used since 1950s to achieve low-RCS aircraft design. RAM was also useful in mitigating the coupling effect and cross talk between the antennas mounted on the surface of the aircraft. The reconnaissance airplane Lockheed U-2 and the fighter aircraft F-117 are few examples where RAM has been used for RCSR.

    Sufficient knowledge base was created over time regarding the scattering behaviour of aircraft structures. The parameters that played a significant role in overall scattering characteristics of these structures were identified. For example, flat plates and cavities were observed to result in large radar return at normal incidence. Similarly, the inlet and exhaust systems of the fighter aircraft were identified as significant contributors towards the aircraft RCS in front-on and rear-on angles, while its vertical tail dominated the radar signature from other angles at the sides.

Page 43 of 101