AERSP 301 Fall 2008

1. AERSP 301 Fall 2008 Introduction to Flight Vehicle Structures Jose L. Palacios The Pennsylvania State University Department of Aerospace Engineering August 2008

2. Syllabus • Course Objectives • Students who successfully complete this course will be able to: • Identify design features of aerospace structures, and calculate load factors and margins of safety • Analyze the behavior of thin-walled beams subjected to combine loads, including bending, torsion, • and shear • Analyze the stability of structural elements and determine critical buckling loads • Develop structural finite element models and use them to predict deformations and stresses under • external loads • Times and Locations • M W F 9:05 am – 9:55 am, Room 220 Hammond • Additional optional evening problem and exam review sessions will be scheduled • Text Book (Optional – Highly Recommended) • Megson, T.H.G., Aircraft Structures for Engineering Students, 4th ed., Elsevier, 2007

3. Syllabus (Cont’d) - Tentative • Introduction to Flight Vehicle Structures • Aerospace Structures Design Concepts • Loads, structural members and roles, types of failure, damage tolerance, conservatism in design, margins of safety, stress reports • Analysis of Aircraft Structures Review • Elasticity (stress, strain, plane stress, solutions, stress concentrations) • Structural materials (conservative eqn’s, temp. effects, failure, composites) • Extension and bending of beams; deflections and stresses • Torsion and Shear of Thin-Walled Beams; Deflections and Stresses • Work and Energy Principles • Finite Element Analysis (Bars and Beams) • Structural Stability and Buckling (intro to plate theory) • Intro to Structural Vibration (If time allows) Homework 8 week-long assignments 20% Participation Attendance, contributions to class +5% Midterm I Time, Location TBD 25% Midterm II Time, Location TBD 25% Final Exam Time, Location TBD 30%

4. Office Hours TAOlivier Leon • M W F: 11:15 – 12:05 • Somewhere in Hammond (TBD) • Email: OUL103@psu.edu

5. Important Dates • Drop/add Period: August 25 - September 3 • Late Drop Deadline: November 14 • Withdrawal Deadline: December 12 • Classes End: December 12 • Final Exam: December 15 - 19

6. What does a structures engineer due? Example: Flap Actuator for Helicopter Vibration ControlINVERCON LLC. – PSU

7. Piezoelectric Stack Buckling Beam Concept • Buckling Beam Actuators • Very simple and easy to manufacture • Compact and lightweight • Easily scaled for use in bench top tests, small-scale prototypes, and full-scale production rotors

8. Buckling Beam Concept Static Analysis of Buckling Beam Spring Load

9. Buckling Beam Concept Preliminary Dynamic Experiments

10. Phase I Review Phase I Design Active Biasing

11. Phase I Bench Top Actuator 1 Hz 5 Hz 20 Hz 10 Hz

12. Actuator Development Phase I Option Design Single Crystal Stacks Outer Casing 1” Linear Bearing for Beam Base Beam Motion Amplifiers Precompression Fine-Tuning Beam Bracket Output Shaft Half Roller Bearing Beam Base

13. Experimental Testing Bench Top Testing Adjustable Steel Rod to Simulate Variable Aerodynamic Loading Flap with Equal Weight of 36” Composite Flap Actuator Output Actuator Rotational Output Sensor

15. Actuator Development Centrifugal Force Testing Actuator Fixture

16. Actuator Development Centrifugal Force Testing • Detailed FE analysis was conducted on hub and fixtures to ensure integrity during spinning • In addition, detailed stress analysis of all critical parts was conducted

17. Experimental Testing Bench Top Testing

18. Experimental Testing Bench Top Testing Buckling Beam Performance with 2 Actuators Buckling Beam Performance 2X-Frame Performance MD 900 Aerodynamic Flap Stiffness

22. Another Example:Dynamic Roughness for Boundary Layer Control

23. Please Read • Chapter 12 (in detail), 13, 14 • In Angel web: - Loads Vocabulary (Angel – Structural Verification and Loads) • Structural Verification Process

24. Aerospace Structures • Space Shuttle. Why does it look like it does and not something else? • Structural members THIN WALLED BEAMS • Bulkheads, skin, spars, ribs, etc… • Materials • Aluminum alloys, composites • Design vs. Analysis • Design  invention, definition • Analysis  suitability • Functions and Important Features of Structures • Hold everything together under loading • Lightweight • Cost • Life (“every” part critical under some condition)

25. Aerospace Structures Design Goal: Lightest adequate structure • Avoid failure – how can a structure fail to perform adequately? • 3 S’s of design • Strength • Rapid (“static”) • Gradual (Fatigue) – Slow cycles – Vibration • Stiffness • Deflections • Nat’l Frequencies • Stability • Buckling

26. Loads (Operating-conditions, Environments) • Structure • configuration of joined structural members • Size (cross-section) • material Performance Analysis evaluate acceptability Common Steps in Design (Overview) • Get one good design, then improve it. • Iterative refinement • Concept • Prelim • Detail “load factors” (V-n envelope) Deflections, Strains, and Stresses Vibration, Fatigue, Buckling, Cost Analysis

27. Conservatism Reduce risk of failure to an acceptable level • Factors of safety • Statistics (materials) • Testing • Redundancy Flight Vehicle Loads • Operating Conditions and Environments • Mechanical (Forces, Accelerations) • Thermal (space, fast vehicles) • Chemical • others

28. Mechanical Loads • Mechanical loads – Surface (pressure) – Body (weight, inertial) • Aircraft • Ground (taxi, land) • Air (cruise, climb, descend, turn, gusts, control) • Spacecraft • Ground (handling, transportation) • Launch (boost, orbit transfer) • On orbit

29. Loads Nomenclature Basic Load Case • Load under some nominal condition(s) • Vehicles  often mass-related (acceleration, weight) • 1g load case (cruise, steady level flight) for aircraft • n, Load Factor – multiple of basic load • Limit Load – maximum load expected in operation • Ultimate load – limit load x design ultimate factor of safety • Typically 1.5 – human rated vehicle • Can be higher or lower • Flight critical • Missiles • Proof Load – sometimes used, 1.0 < FOS < ULT

30. Loads Nomenclature • Structure should be designed to carry • Ultimate load w/o failure • Limit load w/o permanent distortion/damage • Weight is important – ideally, every member is on the verge of failure at some expected load condition! Load case vs. Member load • Individual members loads (forces) are the result of a combination basic load cases

31. Aircraft Flight Loads • “Flight Envelope” (V-n diagram) -- Level Flight, n= 1

32. Aircraft Flight Loads • Margin of Safety = Factor of Safety - 1 What it must carry = Allowable load - Design load Design Load What it can carry

33. Aerospace Structural Concepts • Aircraft Structures • Aero shapes and loads • Launch, Missiles • Propulsion • Satellites • Deployable • Trusses, frames (stiffness) • Typically, arrangements of thin, load-bearing skins and stiffeners • Made from lightweight, strong materials “Stiffened-Skin” or “Stressed-Skin”

34. Aircraft Structures Wings / Fuselages / Internal Structure Consider general loads acting on these structures • Wings --- beam like behavior L(x) Bending & Shear M(x) mg/L V(x) Mg L Torsion D L2 T T(x) mg/L Distributed along wing

35. Aircraft Structures • Fuselage --- Bending, Shear, Torsion & Internal Structure • Need not maintain aero shape • For a vehicle in equilibrium, external forces are balanced, but not collocated • Internal forces develop to transfer load: • Term “load path” used to describe how structure carries/transfers load Pressurization  axial + hoop stress

36. Aircraft Structures Consider a wing or fuselage consisting of skin alone (monocoque) • Skin (plate shell) • Pressure  uniform stretching (2 directions) • Bending  non-uniform stretching • Torsion  ~uniform shear • Shear  ~non-uniform shear σxx σxx σxx σxx σxs σxs

37. Loads on Structural Components • Ground loads • Encountered during moving and transportation • Air loads • Loads encountered during flight (maneuvers and gust) • Other loads: particular role of components • Cabin pressure, turbulences, crash safety… Example of Air loads

38. Loads on Structural Components • Loads can be divided into: • Surface loads, such as aerodynamic and hydrostatic pressure. Surface loads can generate direct loads, bending, shear and torsion, in addition to local, normal pressure loads to the skin • Body forces, which act over the volume of the structure (gravitational and inertial effects)

39. Principal Aerodynamic Loads on an Aircraft • Wings, tailplane and fuselage are subjected to bending, shear and torsion loads, and must be designed to withstand them at minimum weight

40. Airframe Structure • Strong • Rigid • Durable • Light • SAFE • Multifunctional

41. Airframe Structures Disciplines • Materials – Wood, metal, laminates, composites, high-temperature • Structures – Cable-stiffened truss, stressed-skin, sandwich – Wing, swept-wing, fuselage, control surfaces – Aeroelastic tailoring, active stability augmentation – Crashworthiness • Durability – Materials; Safe-life design; Fail-safe design • Manufacturing – Assembly, one-piece machining, molds, reduced fasteners, CAM – Nails, welds, rivets, adhesive; composites • Analysis methods – Hand, matrix, finite elements – Solid modeling

42. Aircraft Structural Components • Basic component: • Wings (rotors), fuselages, tail units and control surfaces • Each component has a specific function and must carry it safely Aerospace components must withstand safely those loads to which they are subjected to at a minimum cost, and weight. Analysis and modeling guides the selection and design of aerospace components

43. Types of Primary Structures • Trusses: • A structure that can withstand loads applied to its joints with its members loaded on axially (No shear and no moments for pinned members) • Stability is recognizable by triangular arrangement of its members • Weight-efficient for rectangular and triangular structures

44. Types of Primary Structures • Skin-Frame Structures: • Has skin (sheets or panels) surrounding a skeletal framework made of stringers (members oriented in the vehicle’s axial direction) and lateral frames which introduce shear into the skin • Versatile in shape

45. Types of Primary Structures • Monocoque Structures • A monocoque cylinder is an axi-symmetric shell without stiffeners or ring frames. Its length is limited by its buckling strength • To be effective, monocoque cylinders must have loads introduced uniformly over its cross-section • Concentrated loads can cause local failure • Simplest and lower cost structure • Difficult to mount components without overloading the shell

46. Skin/Monocoque • In-plane stretching and shear most important • Also bends locally, carries transverse shear • Skin transfer aero loads • Problems with monocoque construction? • Lightweight  Thin Skin • Maintain Aero Shape  Transverse Reinforcement [ribs] • Skin Buckling  Transverse and longitudinal reinforcement [ribs stringers] (also shares bending and axial loads with skin (non shear, torsion) • High Torsion, Shear Loads  Internal Spar Web (plate) reinforcement (shares shear and torsion loads with skin, can buckle)

47. Types of Primary Structures • Skin-stringer • Stiffened-skin • Semi-monocoque • Have members that help carry loads and stabilize the skin • Members allow for the introduction of loads from other structures at the cost of additional weight

48. REINFORCED SKIN (Semi-Monocoque) or SANDWICH PANEL Common in aircraft structures Sandwich panel may also contain reinforcements Transverse Reinforcement Ribs (beam) --- bending Frame Bulkhead (plate) --- in plane shear Used to: Maintain cross-section shape Resist and transfer aero loads Increase buckling stress of skin and longitudinal reinforcement Spacing End restraint Provide “hard points” for attachment of other structure (landing gear, engines, etc…) Provide load paths around discontinuities Semi-Monocoque

49. Function of Structural Components • RIBS: • To withstand all combinations of loads while maintaining an aerodynamic shape of a wing, ribs are normally used • They increase buckling loads of the longitudinal stiffeners and the plate buckling loads of the skin • At the root area the transmit and absorb large concentrated applied loads, while at the tip of a blade are mare formers of the airfoil shape • Transmit and resist the applied loads • Provide aerodynamic shape • Protect cargo from environmental conditions and potential crash • Commonly used: • Thin shell semi-monocoque

50. Function of Structural Components WING SKIN: - To form an impermeable surface for supporting the aerodynamic pressure distribution of the wing - Forces are transmitted into the ribs and stringers through membrane action - They resist to shear and torsional loads - Axial and bending loads are reacted by a combination of skin and stringers