SUBJECT INTRODUCTION The term flight vehicle is used to describe a board class of craft for transporting payloads through the atmosphere and space. It includes aircraft, missiles, and space craft. While the configurations of these vehicles differ greatly, their structures have notable similarities. The most important being the consideration of “Minimum structural weight”. For this consideration they frequently employ similar types of constructions and materials.

STEPS IN STRUCTURAL DESIGN :

STEPS IN STRUCTURAL DESIGN The determination of the critical-combinations of applied loads and temperatures to which structure is subjected.
The layout of the design in which the arrangement, size, and materials of the components parts of the structure are tentatively decided upon.
The determination of the actual stresses and deformations in the structure due to the applied loads and temperatures.
The determination of the allowable stresses or deformations of the structure.
The comparison of steps 3 and 4 to determine weather the design of step 2 is adequate and efficient. If the design is either inadequate or over designed ( and therefore inefficient), steps 2 to 5 must be repeated until a satisfactory design is obtained.

APPLIED LOADS AND TEMPERATURES :

APPLIED LOADS AND TEMPERATURES The loads imposed upon flight structures may be divided into three categories
Ground Loads
Loads imposed during fabrication, assembly, shipping, storage and handling.
Launch operations (missiles)
Taxing and landings for aircraft
Flight Loads
Loads applied to the structure during its flight phase and loads imposed by maneuver, gusts, and wind shear.
Temperature Effects
Temperatures are usually not significant in the ground operations phase, but during the flight phase they are often of equal or greater importance than the mechanical loads.

APPLIED LOADS AND TEMPERATURES :

APPLIED LOADS AND TEMPERATURES Flight structures have to withstand the aerodynamic loads imposed by passing through the subsonic, transonic, and supersonic, and hypersonic phases of flight.
At the same time it may be subjected to temperatures ranging from the extreme lows of cryogenic fuels to the highs associated with aerodynamic heating, heat from propulsion unit, and radiation from the sun.

AERODYNAMIC LOADS :

AERODYNAMIC LOADS

LOAD SIMULATIONS :

LOAD SIMULATIONS

LOAD SIMULATIONS :

LOAD SIMULATIONS

LOAD SIMULATIONS :

LOAD SIMULATIONS

THERMAL LOADS :

THERMAL LOADS

HOW LOADS ACTS ON A STRUCTURE :

HOW LOADS ACTS ON A STRUCTURE Loads may also be categorized according to how they act upon the structure.
Surface forces are those forces which act upon the surfaces of the structures, e.g. aerodynamic or hydrodynamic pressures, aerostatic or hydrostatic pressures, or contact pressures from other bodies.
Body forces are those forces which act over the volume of the structure, e.g. gravitational, magnetic and inertial forces.

LIMIT LOADS :

LIMIT LOADS Limit Loads are the largest loads that the structure is anticipated to be subjected to, during its life. It is usually impossible to specify the largest loads that a particular vehicle will be subjected to, but it is often possible to predict statistically the number of times that an average vehicle will encounter certain loads. Important factors to be considered are
Over design / Inefficient Design
Calculate acceptable low level of failures
Failure rate of inhabited vehicles is to be low as compared to uninhabited vehicles.

LIMIT LOAD FACTORS :

LIMIT LOAD FACTORS The limit loads are often prescribed by giving a limit load factor or the factor by which basic loads are multiplied to obtain limit loads.
As an example, the loads of 1g level flight are often taken as a basic load condition for aircraft. In a maneuver that imposes inertial and gravitational forces upon the structure that are six times greater than those caused by the gravitational force in level unaccelerated flight, the limit load factor nlim would be 6

LIMIT LOADS :

LIMIT LOADS

FACTOR OF SAFETY :

FACTOR OF SAFETY In order to provide for a separation between the limit loads and the load at which the structure fails, a factor of safety is specified. This factor, which may vary according to the mission of the vehicle, is usually 1.5 for the inhabited vehicles and as low as 1.25 for missile. These factors are considerably lower than those used in civil or machine structures.
The use of such low factors of safety requires considerable substantiation by analysis and test.

ULTIMATE LOAD :

ULTIMATE LOAD The ultimate load (Sometimes known as design load) is defined as the product of the limit load and the factor of safety.
The failing load (Ultimate strength) of the structure should be only slightly greater than the ultimate load.

ULTIMATE LOAD FACTOR :

ULTIMATE LOAD FACTOR The ultimate load factor is often specified by giving an ultimate load factor, nult, which is equal to the product of the limit load factor and the factor of the safety. The ultimate loads are then obtained by multiplying the basic loads by the ultimate load factor.

DYNAMIC LOADS :

DYNAMIC LOADS Airplane structures are not completely rigid, and aeroelastic phenomena arise when structural deformations induce changes on aerodynamic forces.
The additional aerodynamic forces cause an increase in the structural deformations, which leads to greater aerodynamic forces in a feedback process. These interactions may become smaller until a condition of equilibrium is reached, or may diverge catastrophically.
Aeroelasticity can be divided in two fields of study:
Steady aeroelasticity
Dynamic aeroelasticity.

STEADY AEROELASTICITY :

STEADY AEROELASTICITY Steady aeroelasticity studies the interaction between aerodynamic and elastic forces on an elastic structure. Mass properties are not significant in the calculations of this type of phenomena.
Divergence
Divergence occurs when a lifting surface deflects under aerodynamic load so as to increase the applied load, or move the load so that the twisting effect on the structure is increased. The increased load deflects the structure further, which brings the structure to the limit loads (and to failure).
Control surface reversal
Control surface reversal is the loss (or reversal) of the expected response of a control surface, due to structural deformation of the main lifting surface.

DYNAMIC AEROELASTICITY :

DYNAMIC AEROELASTICITY Dynamic Aeroelasticity studies the interactions among aerodynamic, elastic, and inertial forces. Examples of dynamic aeroelastic phenomena are:
Flutter
Dynamic response
Buffeting

FLUTTER :

FLUTTER Flutter is a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion.
Flutter can occur in any object within a strong fluid flow, under the conditions that a positive feedback occurs between the structure's natural vibration and the aerodynamic forces.
The vibrational movement of the object increases an aerodynamic load which in turn drives the object to move further. If the energy during the period of aerodynamic excitation is larger than the natural damping of the system, the level of vibration will increase. The vibration levels can thus build up and are only limited when the aerodynamic or mechanical damping of the object match the energy input, this often results in large amplitudes and can lead to rapid failure.

FLUTTER :

FLUTTER

DYNAMIC RESPONSE :

DYNAMIC RESPONSE Dynamic response or forced response is the response of an object to changes in a fluid flow such as aircraft, to gusts and other external atmospheric disturbances. Forced response is a concern in axial compressor and gas turbine design, where one set of aerofoils pass through the wakes of the aerofoils’ downstream.

BUFFETING :

BUFFETING Buffeting is a high-frequency instability, caused by airflow separation or shock wave oscillations from one object, striking another. It is caused by a sudden impulse of load increase. It is a random forced vibration. Generally it affects the tail unit of the aircraft structure due to air flow down stream of the wing

SUMMARY :

SUMMARY In most cases, and especially if the structure is unconventional, tests are performed to substantiate the analysis and prove the strength and stiffness of the structure. A reduction in the structural weight of a flight vehicle permits an increase in payload or performance. It is therefore economically feasible to use expensive materials and fabrication methods and to expend many man hours of analysis and testing if it results in a decrease in structural weight.

Slide 26:

IDEALIZATION OF STIFFENED-SHELL STRUCTURES

STRESSES AND DEFLECTIONS :

STRESSES AND DEFLECTIONS Major effort in aircraft structures is devoted to methods of analysis for predicting stresses and deflections of structural components under applied loads and temperatures.
In mechanics of deformable bodies simplifying assumptions are introduced to arrive at solutions. The results are therefore approximate. It is very necessary to understand theses assumptions.

LEARNING OBJECTIVES :

LEARNING OBJECTIVES Types of aircraft structure
Terminology
Idealization of aircraft structures
Assumptions

IDEALIZATION OF STIFFENED-SHELL STRUCTURES :

IDEALIZATION OF STIFFENED-SHELL STRUCTURES The structure of a flight vehicle usually has a dual function:
It transmits and resists the forces which are applied to the vehicle
It acts as a cover which provides aerodynamic shape and protects the contents of the vehicle from the environment
This combination of roles is fortunate since, from the stand point of structural weight, the most efficient location for the structural material is at the outer surface of the vehicle

TYPES OF AIRCRAFT STRUCTURE :

TYPES OF AIRCRAFT STRUCTURE The structure of most flight vehicles are thin shells. The two types are
Monocoque
Semi monocoque

SEMI-MONOCOQUE :

SEMI-MONOCOQUE In semi monocoque structures the cover or skin has the following functions
It transmits aerodynamic forces to the longitudinal and transverse members by plate and membrane action
It develops shearing stresses which react the applied torsional moment and shear forces
It acts with the longitudinal members in resisting the applied axial loads
It acts with the longitudinals in resisting the axial load and with the transverse members in reacting the hoop, or circumferential load when the structure is pressurized.

LONGITUDINAL MEMBERS :

LONGITUDINAL MEMBERS Longitudinals
Stingers
Stiffeners
Longerons (Larger cross section areas)
Functions are
They resist bending and axial loads along with the skin
They divide the skin into small panels and thereby increase its buckling and failure stresses
They act with the skin in resisting axial loads caused by pressurization

TRANSVERSE MEMBERS :

TRANSVERSE MEMBERS Frames
Rings
Bulkheads
Functions
Maintain the cross sectional shape
Distribute concentrated loads into the structure and redistribute stresses around structural discontinuities
Establish the column length and provide end restraint for the longitudinals to increase their column buckling stress
Provide edge restraint for the skin panels and thereby increase the plate buckling stress of these elements
Act with the skin in resisting the circumferential loads due to pressurization

IDEALIZATION; ASSUMPTIONS :

IDEALIZATION; ASSUMPTIONS The behavior of these structural elements is often idealized to simplify the analysis of the assembled components. The following assumptions are made
The longitudinals carry only axial stresses
The web (skin or spar webs) carry only shear stresses
The axial stress is constant over the cross section of each of the longitudinals, and the shearing stress is uniform through the thickness of the webs
The tranverse frames and ribs are rigid within their own planes, so that the cross section is maintained unchanged during loading. However they are assumed to possess no rigidity normal to their plane so that they offer no restraint to warping deformations out of their plane

IDEALIZATION; ASSUMPTIONS :

IDEALIZATION; ASSUMPTIONS When the cross sectional dimensions of the longitudinals are very small compared to cross sectional dimensions of the assembly, assumptions 1 and 3 result in error
The webs in an actual structure carry significant axial stresses as well as shearing stresses, and it is therefore necessary to use an analytical model of the structure which includes this load carrying ability

IDEALIZATION: METHOD :

IDEALIZATION: METHOD This is done by combining the effective areas of the webs adjacent to a longitudinal with the area of the longitudinal into a total effective area of material which is capable of resisting bending moments and axial forces.
The fact that cross sectional dimensions of most longitudinals are small compared with those of the stiffened shell cross section makes it to assume without serious error that the area of the effective longitudinal is concentrated at a point on the middle line of the skin where it joins the longitudinal
The locations of these idealized longitudinals will be indicated by small circles as shown on next slide.

IDEALIZATION IN MONOCOQUE STRUCTURES :

IDEALIZATION IN MONOCOQUE STRUCTURES

IDEALIZATION IN SEMI-MONOCOQUE STRUCTURES :

IDEALIZATION IN SEMI-MONOCOQUE STRUCTURES

LIMITATIONS :

LIMITATIONS In thin aerodynamic surfaces the depth of the longitudinals may not be small compared to the thickness of the cross section of the assembly, and a more elaborate idealized model of the structure may be required.
The fewer the number of longitudinals, the simpler the analysis, and in some cases several longitudinals may be lumped into a single effective longitudinal to shorten computations.

CONCLUSIONS :

CONCLUSIONS The simplification of an actual structure into an analytical model represents a compromise, since elaborate models which more nearly simulates the actual structure are difficult to analyze.
Once the idealization is made, the stresses in longitudinals due to bending moments, axial load, and thermal gradients can be computed from the equations developed in literature.

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