Structures and Materials Flashcards
Airframe
The aircraft without equipment and furnishment; The skin and frameworkd that provide aerodynamic shape; The load bearing parts; The parts that together protect the content from the environment
Primary and secondary structures
Primary structures are critical components, whose damage or failure could lead to failure of the entire aircraft. Secondary structures support aerodynamics, safety, comfort, but are not essential
Truss structures
Structures made from bars, tubes, wires; structural rigidity is obtained by placing elements in the diagonal (they can withstand compression or tension); later sheets were used for the load bearing function; Truss structures are still used today for weight saving;
Sheet stiffening
Geometrically manipulating sheet in order to carry compressive loads e.g. sheet profiles;
Shell structure
Sheet reinforced by a stiffener
L-shaped stiffener
Stiffener in the form of L; will bulge out more easily; can be easily inspected; lighter;
Hat stiffener
In the form of a parallelogram; attached to the sheet; very strong; can not be inspected on the inside; heavier;
Distance between stiffeners
A single stiffener has only local effect, thus optimal spacing between stiffeners shall be chosen
Thought about compression of sheets
Sheets actually do have compressive resistance. The problem is that they bulge before showing the resistance. Thus when reinforced, they do not bulge and actually contribute to compressive resistance.
Sandwich structure
Low-density core polymer core (usually honeycomb structure) with two thin alimunim or carbon fibre reinforced polymer sheets.
Polymer
A repeating chain or branch of molecular structure (monomer). Usually very lightweight, durable, flexible.
Advantages of sandwich structures
High strength-to-weight ratio. Good rigidity to weight. Absorbing impact energy. Abosrbing vibrations. Thermal and Acoustic insulation. Very high bending stifness.
Disadvantages of sandwich structures
Trap moisture which leads to corrosion in metals. Also moisture could freeze, then exapnds and delaminates the core form the sheets. Also moisture adds weight. Complex mechanical joints, due to the soft core which can not carry loads. Also the out of plane stiffness is very low, so no bolted joints.
Integrally stiffened structures
Structures that are made of thick sheet of metal with mechanically machined geometry.
Pros of integrally stiffened structures
The process is automated. Low amount of parts. The thickness could be regulated.
Cons of integrally stiffened structures
A lot of scrap metal (but nowadays parts come pre-miled and the supplier could re-use the metal). Complex stiffener concepts cannot be applied. Has no natural barriers (bolting or riveting) for cracks. Therefore, less damage-tolerant.
Fuselage
A thin-wall pressure vessel exploiting the stifenned shell concept
Fuselage skin
The outermost layer of the fuselage. Protects the content from the environment.
Frames
The are in the cross section of the fuselage skin. Provide and separate the cylindrical shape into structures and provid lateral stifness.
Stringers
Long stiffened elements providing the longtitudial stifness of an aircraft
Bulkheads
Separate the aircraft into sections. Abosrb impact forces. Keep the cabin pressurized. Absorb pressurization forces.
Splices and joints
Provide the mechanical connections between stringers, frames and fuselage skin.
Division of wing types
Kink or no kink in wing spars. Ribs parallel to flight direction or perpendicular to spars.
Disadvantage of sectioning large wings into sections
Highly loadead joints (rainbow fitting)
Functions of ribs
Maintain aerodynamic profile; transfer aerodynamic and fuel weight loads into the rest of the wing structure; provide stability against panel crushing and buckling; introducing local loads (landing gear, engines, flaps); sealing in case of integral tanks (fuel does not splash in the whole wing);
Desing of ribs
Depends on local loads, desing philosophy (allowed magnitude of stress), available equipment and experience, costs; low loads -> forming sheet or plate material into a rib; high loads -> forging or machining from a thick plate;
Spars
Usually I-beams which go spanwise the wing and provide the bending stifness; Must be thicker near root tip due to larger forces; Due to extrusion process varying cross-sections impossible, thus additional stiffeners (reinforcement) are added via riveting or bolting;
Differential bending
A single spar cannot withstand torsion. Therefore, we use differential bending (two spars) and the torsion is translated into two opposite bendings
Torsion box
A closed box, which can carry out torsional loads. The strength is provided by the shear resistance of the infinitesimal elements of the structure. Thicker skin for greater strength.
Advantages of torsion box
We can create longer wings for the same thickness. Also torsion stiffness and bending strength can be engineered seperately
Doublers
Due to the non-constant loads throughout the structure varying thickness is necessary. Thus second layer is added (doublers). Alternatively, thickness step is machined.
Joggling
To follow the thickness step stringer are joggled (shear deformation along the geometry of surface)
Splices
Used to connect different parts of the airframe (skin panels, stringers). Three tipes lap (overlapping elements), butt (next to each other connected via doubler), scarf (forming one into the other)
Stringer Frame connection
Two main methods, in both of them the stringer is uninterrupted. Either we interrupt the frame (local weakening) or we position it above the stringer.
Payload adaptor
Inserts the load provided by the launcher into the structure; Primariliy done by strut structures; main function is to insert load via compression, thus can be optimized for this (Space shuttle ribs)
Central cylindrical structure
A structure with a large cylindrical thrust-load-bearing member; it is used to transfer the loads from the thrust throught the entire structure up to the adaptor; all systems are attached to strong points via struts, platforms and shear webs;
Payload fairings
protects the payload from the agressive environment (heating and pressure) during launch; once outside the atmosphere the fairings are disposed; the accelerated mass is decreased, thus efficiency of thrust is increased;
Stage structures
Rockets are divided into stages; each has a propulsion system and subsystems; main stage forms the core; boosters are usually parallel to the main stage;
Stage structure design concepts
Either the fuel tanks are part of the load-bearing structure or they are not. If they are the different components are connected via interface structures to transfer loads between components and skin concept is semi-monocoque; If they are not, there is an external skin seperated by the fuel tanks via longerons and circular stiffeners;
Thrust structures
Extremely high loads (Ariane 5, 1500 tons); Very concentrated, thus introduces with conical structure;
