Aero 318 Flashcards
Ultimate Loads
The loads that a structure must be able to support without failure/rupture for at least 3 seconds. Often the limit load multiplied by 1.5.
Primary structure
Those that would endanger the aircraft upon failure.
Secondary structure
Those that do not cause immediate danger upon failing. Non load-bearing structure.
Spars
Carry loads by bending and provide foundation for attaching skins.
Spar web
Carry shear stresses and resist shear and torsional loads. Divide the skin into small panels, and increase the buckling and failure stress.
Spar caps/flanges
Capable of supporting large compressive loads from axial bending effects.
Ribs
Maintain the aerodynamic shape of the cross-section, and act with the skin in resisting the distributed aerodynamic load. Also distribute concentrated loads into the structure.
Formers
Establish the shape of the fuselage and reduce the column length of stringers to prevent instability.
Name three types of fuselage
Truss, monocoque and semi-monocoque.
Truss type fuselage
Made up of beams, struts and bars that all carry tensile and compressive loads.
Monocoque type fuselage
Rings (formers) establish the shape and prevent instability. The skin forms an impermeable surface for supporting the aerodynamic pressure and transmitting aerodynamic forces to the internal structure. There are no bracing members.
Semi-monocoque type fuselage
The skin is reinforced by longitudinal members called longerons and stringers, which prevent tension and compression from bending the fuselage. Most common for commercial aircraft.
What does the wing structure of an aircraft contain?
Spars, ribs, stringers and a skin.
Three types of aircraft fuel tank
Integral tanks, rigid removable tanks and bladder tanks.
Integral fuel tank
Non-removable. Areas in the aircraft structure that have been sealed, often in the wing.
Rigid removable fuel tank
A metal construction that doesn’t contribute to the aircraft’s structural integrity.
Bladder fuel tank
Constructed from two or more plies of rubber coated fabric.
Nacelles
Streamlined enclosures used to house the engine and its components. Have a rounded profile to reduce aerodynamic drag. Protect the engine from damage.
Four causes of loading on the airframe
Manoeuvres (inertial loads), wind gusts, cabin pressure and landing.
Load path
A path that links the applied load to equilibrium forces.
When do the maximum loads on the components of an aircraft’s structure generally occur?
When the aircraft undergoes some form of acceleration or deceleration.
Newton’s 2nd law vs D’Alembert’s principle
For Newton’s 2nd law, the acceleration should be measured with respect to an inertial axes system. For D’Alemebert, the reference axes system must be changed from an inertial one to fixed within the body and accelerating with it.
Load factor, n
Total lift/ weight = 1 + a/g
Symmetric manoeuvre
The motion of the aircraft initiated by movement of the control surfaces in a plane of symmetry.
Centre of pressure
Where the aerodynamic pressure field can be represented by a single force with no moment.
Aerodynamic centre
Where the pitching moment coefficient doesn’t vary with angle of attack.
Load factor during steady pull-out
n = (v^2)/Rg + cos(theta), where theta is the angle of the aircraft to the vertical plane and R is the radius of curvature of the flight path. Cos(theta) = 1 at lowest point of pull-out.
tan(phi)
(v^2)/Rg, n = sec(phi), where phi is the bank angle.
Primary certification
A new aircraft model requires certification by the airworthiness agency corresponding to the area of the world in which the manufacturer is based.
Secondary certification
If the aircraft is to be exported to other parts of the world, then secondary certification by the relevant agencies is also required.
New structure certification definition
Changes in the design philosophy regarding structures and loads have been made, or the manufacturer has not built an aircraft of this type before.
Similar new structure certification definition
Utilises similar design concepts to an existing tested aircraft.
Derivative/similar structure certification definition.
Uses structural design concepts almost identical to those on which analytical methods have been validated.
Two methods of classification of load cases for certification
Bookcase and rational
Bookcase load classification
Somewhat artificial in that applied loads are assumed and reacted by inertia loads, leading to a static equilibrium problem. Most useful early on in the design cycle.
Rational load classification
Used later in the design cycle. Attempt is made to model the loads and dynamics of the aircraft as realistically as possible.
Limit loads
The maximum loads to be expected in service and which the primary structure needs to support without detrimental permanent deformation.
Three types of wind gust
Sharp-edge, graded and 1-cosine.
Sharp edge gust
Instantaneous application of wind gust.
1-cosine gust velocity, u(t)
(u/2)(1-cos(πt/T)), where u is the max velocity and T is the time taken to reach max velocity.
What is a shear stress acting on a given plane always accompanied by?
An equal complementary shear stress acting on a plane perpendicular to the given plane and in the opposite sense.
Surface forces
Distributed across the surface area of the body.
Body forces
Gravitational and inertial effects that are distributed over the volume of the body.
What are longitudinal or direct strains associated with?
Direct stresses and changes in length.
What do shear strains define?
The changes in angle produced by shear stresses.
Shear strain at a point in a body
The change in angle between two mutually perpendicular lines at that point.
Plane stress
The strain rate at a material particle is such that the only non-zero strain components act in one plane only.
de Saint Vernant principle
sigmax = sigmay = tauxy = 0
Euler-Bernoulli beam model
Beam sections rotate in bending but remain plane, and they remain normal to the beam axis as it deforms.
Assumptions of Euler-Bernoulli (classical) bending
Plane cross-sections that remain plane after bending, and cross-sections that remain normal to the longitudinal axis of the beam after bending.
Symmetrical bending
Beams with either singly or doubly symmetrical cross-sections.
Strain in symmetrical bending, epsilon
-y/R, where y is the perpendicular distance from the neutral axis.
Stress in symmetrical bending, sigma
-E*(y/R), where E is the modulus.
Bending moment in symmetrical bending, M
-EI/R
Unsymmetrical bending
Neutral axis passes through the centroid of the cross section.
When can the product second moment of area, Ixy, be assumed to be zero in unsymmetrical bending?
When either or both Cx or Cy are an axis of symmetry, and Cxy is the principle axis.
Shear flow, q
tau*t, where t is the thickness
Two assumptions regarding the shear of beams
- The axial constraint effects are negligible.
- The shear stresses normal to the beam surface may be neglected since they are zero at each surface and the wall is thin.
What does a closed section beam subject to a pure torque in the absence of an axial constraint NOT develop?
A direct stress system.
What does the application of a pure torque to a closed section beam result in?
The development of a constant shear flow in the beam wall.
Relationship between the applied torque and shear flow for a closed section beam.
T = 2Aq
Condition of zero warping (closed section beam)
pGt = 2A/∂ = constant
Primary warping (open section)
Similar to that of a closed section beam - assumed to be constant across the wall thickness.
Secondary warping (open section)
Warping across the thickness. Much less than primary warping - usually ignored in the thin-walled sections common to aircraft structures.
First step in structural idealisation
Replace stringers and spar flanges with concentrations of area, known as booms, over which the direct stress is constant. Located along the mid-line of the skin.
Second step in structural idealisation
Assume that all direct stresses are carried by the booms, while the skin is only effective in shear.
In the method of successive approximation, what does ‘a’ represent?
The distance between the lift and the CG.
In the method of successive approximation, what does ‘b’ represent?
The distance between the drag and the CG.
In the method of successive approximation, what does ‘c’ represent?
The distance between the thrust and the CG.
Equation for the positive load factor according to CS-25
n = 2.1 + 24000/(W+10000), where W is in pounds. The load factor must lie in the range 2.5<n<3.8
Load factor for a graded gust
n = 1 ± (0.5rhovClalpha*Fu)/w, where F is the gust alleviation factor and w is the wing loading.
Longitudinal stress due to internal pressure
pr/2t, where p is the internal pressure, r is the radius of the vessel, and the is the vessel thickness.
Circumferential stress due to internal pressure
pr/t
Equation for maximum/minimum principle stresses
((sigmax + sigmay)/2) ± 0.5√((sigmax-sigmay)^2 + 4tauxy^2)
If section is symmetrical about x-axis
Ixy = 0
Second moment of area of rectangular section
Ixx = (bd^3)/12, where b is width and d is length.
Parallel axis theorem
I = Ic + Ab^2, where A is cross sectional area and b is distance from the reference axis to the centroid.
Torque of a closed section beam, T
2Aq, where A is cross-sectional area and q is shear flow.
Shear flow, q
tau*t, where tau is the shear force and t is the thickness.
Assumptions for thin-walled approximation
Neglect squares and higher powers of t, and take the section to be represented by the mid-line of its wall. Stresses may be regarded as constant along the thickness.
Ixx for thin walled rectangular section
2bth^2 + (t(2h)^3)/12
