Aeroplanes Flashcards
Explain what is meant by a balanced symmetric layup?
Balanced
- equal amounts of +angle and -angle material
- no direct shear coupling
Symmetric
- same top to bottom and bottom to top
- no in plane/ out of plane coupling
List some of the advantages and disadvantages of using composite materials or GLARE in wing and fuselage panels, with reference to: 1) Structural performance 2) Manufacturing costs 3) Operating Costs 4) Environmental Impact
Advantages
- High stiffness & Strength
- Lower Mass leads to saving on fuel consumption and less atmospheric emission
- Not prone to corrosion
- Better Fire resistance
- Tailor the material during design , such that the number, alligment and type of layers suits the local stresses and shapes through the aircraft.
Disadvantages
- Complex and costly manufacturing process
- Prone to delamination - Prone to lightning strikes
- Difficult to reuse/ recycle
- Structural performance not as well understood as traditional materials
Outline some of the assumptions made in the analysis of stiffened wing panels by the exact strip method. Why might this approach be preferred to finite element analysis in preliminary aircraft design?
1) All plates are rectangular and prismatic
2) Plate properties, dimesnion and loads do not vary in x direction
3) Buckling mode sinusoidal in x direction
4) Linear Elastic Behaviour: buckling, initial postbuckling, undamped vibration
5) End conditions simple supports or same at each end
- In preliminary design, used to consider many alternative configuration and many load cases
- FE analysis is costly in data preparation and computation time
- Exact strip analysis offers a reasonably accurate, reliable alternative which is much faster to use
List Typical design variables, constraints and objective function for the optimum design of a composite stiffened panel. How must the continuous optimum be adjusted to satisfy practical manufacturing constraints?
Design Variables - Plate width, layer thickness, ply orientation
Constraints - buckling loads, max stress, max strain, max displacement
Objective Function - total mass of panel (or cost)
- Each layer thickness must be an integer multiple of the ply thickness
- This discrete constraint does not appear in the continuous optimisation so solutions must be adjusted by rounding up/down to nearest integer results
- Also constraints on which plies can be adjacent to each other, e.t.c
- Either adjust layer thickness to discrete multiple of ply thickness or formulate with fixed ply thickness and choose the orientation, perhaps using laminate parameters to reduce problems.
Sketch typical layups for the components of an aircraft wing panel made from
i) Carbon Fibre Composite Material
ii) GLARE
“Glass Laminate Aluminium Reinforced Epoxy”
Briefly discuss the factors influencing the choice of materials for the primary structures (e.g. wings and fuselags) of civil and militatry aircraft
1) High stiffness/ weight and strength/ weight ratios - composites good
2) Proven postbuckling strength, resistance to impact damage - metals good
3) Delamination, maintenance and repair (e.g. lightening strike)
- all issues that must be addressed in composites
4) Cost of materials - metals still preferable
5) Environmental impact - reduced cost of fuel with composites - Final disposal?
Briefly describe the main characteristics of carbon fibre composite material, ecplaining what is meant by the terms
(i) Stacking Sequence
(ii) Ply orientation
(iii) Ply thickness
(iv) Layer thickness
Illustrate your answers by reference to a sketch of a typical balanced symmetric laminate.
Many layers of material, each has fibres running in a different direction (alpha)
(i) Stacking sequence is order of the layers
(ii) Ply orientation is angle (alpha) for each ply
(iii) Ply thickness = (fixed) thickness of one ply
(iv) Layer thickness = total thickness of a layer which has the same orientation, i.e. an integer multiple of ply thickness
Balanced layup has equal thickness of +(alpha) and -(alpha)
Symmetric layup has the same stacking sequence and layer thickness reading top to bottom and bottom to top
Explain the exact stiffness approach to structural eigenvalue probelms
- Used for buckling and vibration problems.
- Solve exact governing differential equation to give a transcendentl eign value eigenproblem
- Need a special algorithm (e.g. Wittrich-Williams) to solve eigenproblem
What are the advantages and disadvanatges of FEM?
(+)
- Smaller model - faster
- Accurate - do discretisation
- Can find higher eigenvalues easily
(-)
- Limited range - need to solve DE’s for each memeber type
- Non-linear eigenvalues problems need special solution methods
- hard to include postbuckling, damping, e.t.c
Post 2001 strategies of Boeing and Airbus
- New markets in asia
- Boeing - point to point routes (e.g. USA) fast efficient aircraft (7E7)
- Airbus - hub and spoke routes (e.g. Europe) large capacity aircraft (A380) but also A350 to compete with 7E7
Discrete optisation method for design of structural CFC components
Each layer is made by stacking plies with a standard thickness (design thickness must be an integer multiple of ply thickness)
1) First solve continous optimisation problem
2) Adjust discrete variables to adjacent discrete solutions using modified branch - and - bound method
3) Then it may be feasible to expore further afield to seek global discrete optimum
How response surface methodology applie to structural optimisation
- Avoids costly evaluations of constraints (e.g. buckling loads
- Sample the design space to obtain a set of data points
- Obtain a best fit surface (e.g. polynomial function)
- Find optimium value on the surface analytically
- Can refine solution by taking more data points near the supposed optimum
- Implement by linking structural analysis software to standard software (e.g. MATLAB to fit surface, EXCEL to optimise it)
What is
i) Local optimum
ii) Global optimum
i) Better than all other feasible designs in immediate vincity
ii) Better than all other feasible designs
How can aeroplane design reduce atmospheric emissions?
1) Reduced mass
- advanced lightweight materials, improved modelling/design methods, designing into postbuckling
2) Smart Structure
- technology to reduce drag
3) Improved engine technology
- use if non fossil fuels
4) More use of IT/ control systems to improve efficiency of flight
5) Larger, more efficient aircraft (airport operations and route planning)
6) Alternative forms of transport
7) Technological alternatives to transport e.g. teleconferencing
How can exact stiffness method and finite method be combined
Graham’s notes
1) input -2) pre-procession- 3) FE software model of whole wing - 4)
3) I) Interface software ii) Panel software e.g. VICONOPT optimise each panel
CYCLE (roughly 4 times) to CONVERGENCE
