Aircraft Structures Flashcards
Types of safety design
Safe-life design
- never fails in fatigue during a planned lifetime
- used where it is impractical to provide a redundancy , or where damage would propogate rapidly (e.g. a spinning helicopter blade)
Fail-safe design
- has adequate strength and stiffness in prescence of structural damage
- goes hand-in-hand with crack propogation resistance
- inpsection needed to detect damage
Damage-tolerant design
- structure is capable of withstanding service loads with failed elements + cracks in adjacent structures
Fuselage stresses
Hoop stress and longitudinal stress
Model fuselage as a cylindrical pressure vessel of thickness t
- cut in half to expose hoop stress along the fuselage:
from equilibrium, pressure * area: 2pRL = 2σ_h tL
σ_h = pR/t
longitudinal stress:
pπR^2 = 2πRtσ_L
σ_L = pR/2t
Hoop stress is twice the longitudinal stress, so we take hoop stress as the limiting load case
Fuselage stress + deflections
After finding SF + BM diagrams
σ = My/I
δ = PL^3/3EI (point load)
δ = WL^4/8EI (UDL)
deflections are linear so can be summed
External loads on aircraft
Tension: structurally efficient, consider fracture and fatigue loading
Compression: often leads to buckling failure
- recall P_e = Kπ^2EI/L^2
Shear is a combination of tension in some direction and compression in the other, so the structure can still efficiently carry applied loads
Different structural frames
Stressed-skin construction:
- monocoque design
- loads were all carried internally, the skin only carried air pressure
Semi-monocoque:
- all components optimised to carry loads => lightweight design
- load is divided between skin and stringers
- stringers and frames/ribs divide the skin into smaller panels to increase buckling resistance
Fuselage structural components and their function
Skin:
- provide smooth aerodynamic surface
- creates an enclosed envelope
- carries tensile and compressive (bending of fuselage) and shear (torsion of fuselage) loads
Stringers: travel along the fuselage
- carry tensile/compressive loads by adding extra CSA, increasing EI of the skin, against buckling
- divides skin into smaller panels agains buckling (reduces L^2 so increases P_E)
- reduces local deformations
Frames:
- help maintain cross sectional shape
- transfers local loads into the structure
Wing structural components and their function
Spars: I-beam structures that carry majority of bending + shear loads
- form the wingbox with the top+bottom cover to carry torsion loads
- spars have webs/flanges (spar caps), and are tapered from root to tip as bending moment is greatest at root
Skin:
- contains fuel carried inside wing
- same as fuselage: smooth aerodynamic surface
Stringers:
- similar to fuselage: carry tensile/compressive load, increase I of skin against buckling
- divide skin into smaller panels
Ribs:
- help maintain cross sectional shape like the frame
- transfers local loads to overall wing structure
