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Aerospace Structures- an Introduction to Fundamental Problems Purdue University

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Aerospace Structures- an Introduction to Fundamental Problems Purdue University 2011 Aerospace Structures- an Introduction to Fundamental Problems Purdue University This is an introduction to aerospace structures. At the end of one semester, we will understand what we mean by the “structures job” and know the basic principles and technologies that are at the heart of aerospace structures design and analysis. This knowledge includes basic structural theories, how to choose materials and how to make fundamental design trades, make weight estimates and provide information for decisions involved in successful aerospace structural design and development. Dr. Terry A. Weisshaar, Professor Emeritus School of Aeronautics and Astronautics Purdue University 7/28/2011 Preface leaves of absence at M.I.T., the Air Force For all of my 40 year plus career in aerospace Research Laboratory and at the Defense engineering I have been fascinated by design Advanced Research Agency (DARPA). I also and development of aerospace products and served as an advisor to the Air Force as part of fortunate to have participated in the the Air Force Scientific Advisory Board as well development of several of them. Design efforts, as serving on national panels. whether they are in the development of small components or large systems are at the heart of When I entered the working world (only briefly) the remarkable progress in aviation that has as a young engineer at Lockheed Missiles and occurred over the past 100 years. Space Company, the standard texts found on engineer‟s desk were the classic book by Bruhn To be a participant in this effort requires that one and the textbook by David Peery. However, in be literate in several different areas and a master the mid-1960‟s a new analytical method called of at least one or more areas. This set of notes the finite element method was talked about and was created for the class AAE 352, Aerospace even used by some of us who had been fortunate Structures, taught at Purdue University. to take classes at the Master‟s degree level. A Purdue‟s undergraduate aerospace curriculum is friend of mine from the University of California, different than most university curricula in that Berkeley and I were paid $11 a week more there is a great deal of freedom organizing an because we had taken such courses. In the late undergraduate plan of study. For instance, most 1960‟s the finite element method took the university undergraduate plans of study would industry by storm and, as they say, “the rest is require two one-semester classes in structures. history.” As a result, Chapter 4 provides an Purdue requires only one, leaving more introduction to this classic method and does just discretion in building a plan that may emphasize enough to introduce students to terminology and aerodynamics or control theory. above all, statically indeterminate structures. While the wisdom of doing this has been hotly Similar experiences interacting with other debated, I believe that it is a wise academic engineers, both in the working world and the choice. On the other hand to provide knowledge research world has convinced me that there is no about structures, over and above that found in a substitute for a blackboard/whiteboard, good standard Strength of Materials course, with a sketches of configurations and Free Body one-semester structures course is challenging. Diagrams. There is also no substitute for basic Creating this course is a design problem; each mechanics; you will find a review of mechanics instructor will have a different set of prioritized integrated into the truss analysis review in requirements. The results will differ depending Chapter 2. on the background of the instructor. External and internal load computations are The basic question that has to be answered in the extraordinarily important. “Loads” is a broad beginning is this: ” if I want my students to have area that could easily occupy an entire semester a skill set, a vocabulary and a level of literacy or two. One loads subject often overlooked in that will enable them to be competitive, the educational system is inertia loading. Inertia productive product team members for aerospace loads have destroyed more than one vehicle. employers, what must I absolutely include in This subject is considered in Chapter 3. this class?” My answers are contained in this set of notes. Knowledge of the problems and issues related to materials and material processing is important. My experience during the past four decades has An aerospace engineer must have knowledge of biased my vision. I have taught this course or materials, their properties, how they are used, similar courses at The University of Maryland, and how they are manufactured. Few aerospace Virginia Tech and at Purdue over a span of 38 curricula include such courses. Chapter 5 is a years. During this time I also spent time on summary of considerations and choices that i must be made when selecting aerospace majoring in structures is free to take any or all of materials. Chapter 5 draws very heavily on the these classes. But for the student who does not educational approach used by Michael Ashby feel a calling to the structures and structural and his colleagues in Great Britain. At Purdue mechanics world, this class will enable them to we use his CES Edupack computer aided design be conversant and aware of the problems we software. face in developing new structures and sustaining old structures. If this goal is reached then all of Stress analysis is important. The most important the work that it takes to write up this set of notes part of stress analysis is the development of (and wrestle with Microsoft Word) is worth it. simplified models that can be used independent of elaborate computer models. Beam analysis is briefly reviewed in Chapter 6. Thin-wall beam Terry A. Weisshaar approximations are excellent ways to describe these models and are included in Chapter 7. West Lafayette, Indiana Each of these seven chapters could be expanded August 2009 into an entire course. At Purdue we have semester long classes in matrix methods, materials, fatigue and fracture, composite Copyright T. A. Weisshaar, all rights reserved materials and structural vibration. A student ii Table of Contents CHAPTER 1 – AN OVERVIEW WITH A PLAN ............................................................................................................ 1 1.1 INTRODUCTION........................................................................................................................................................ 1 1.2 WHAT IS AN “AEROSPACE STRUCTURE?” SOME ESSENTIAL FEATURES OF AIRCRAFT AND SPACECRAFT SYSTEMS. .......................... 2 1.2.1 Spacecraft structures ................................................................................................................................... 3 Spacecraft structures summary ............................................................................................................................ 6 1.2.2 Aircraft structures - Background ................................................................................................................. 7 The legacy – the first two decades of flight ........................................................................................................ 13 A quick trip to the present day and a look to the future ..................................................................................... 15 The past is prologue – new materials for the future ........................................................................................... 19 1.3 THE STRUCTURES GROUP – WHAT DO THEY DO AND HOW DO THEY DO IT? ........................................................................ 21 1.4 SUMMARY AND COURSE GOALS ............................................................................................................................... 24 1.5 BIBLIOGRAPHY – CHAPTER 1 .................................................................................................................................... 24 1.6 - PROBLEMS TO CONSIDER ........................................................................................................................................ 26 CHAPTER 2 – A REVIEW OF STATICS AND MECHANICS OF SIMPLE STRUCTURES .................................................. 27 2.1 INTRODUCTION – MECHANICS, EQUILIBRIUM AND FREE BODY DIAGRAMS ........................................................................ 27 2.2 APPLIED, INTERNAL AND REACTIVE FORCES ................................................................................................................. 29 2.3 EQUIVALENT FORCES AND MOMENTS ......................................................................................................................... 30 2.4 DRAWING FREE BODY DIAGRAMS ............................................................................................................................. 31 2.5 TRUSS ANALYSIS REVIEW - THE METHOD OF JOINTS ....................................................................................................... 32 2.6 STATICALLY DETERMINATE AND INDETERMINATE STRUCTURES.......................................................................................... 33 2.7 THE METHOD OF SECTIONS ...................................................................................................................................... 34 2.8 A WORKED TRUSS ANALYSIS EXAMPLE ........................................................................................................................ 35 2.9 A SECOND
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