static tests on complete structures

Testing of Aerospace Structures

Testing of Aerospace StructuresStatic Tests of Complete Structures1TopicsTypes of testReference StandardsTolerance RequirementsChoice of the loading conditionsChoice of the test loadsApplication Method of the loadsMode constraintParameters measured during the test22Types of testContingency testsStrength testsBreaking tests

The type of test is closely related to the load condition, which should be defined;In particular, while in the first two types of test, load is known a priori, for the third it is the main unknown;Definition according to the regulations3Contingency is something unknown?3Types of testICAO International Civil Aviation OrganizationFAA Federal Aviation Administration (FAR)EASA European Aviation Safety AgencyJAA Joint Aviation Authorities (JAR)MIL

Technical standards defined by EASA regulationsCS 22 approval requirements for sailplanes and powered sailplanes.CS 23 type-approval requirements for aircraft weighing less than 5670 kgCS 25 type-approval requirements for turbine aircraft weighing more than 5670 kg.CS VLA approval requirements for single-engine aircraft weighing less than 750 kg.4Examples from Reference Standards

5Examples from Reference Standards

6Tolerance Requirements

7Classification of Acting LoadsAerodynamic loadsManeuver, gustLoads resulting from the propulsion systemThrust, torque, other effectsLanding LoadsGround LoadsSpecial loadsPressurization, impact, loads underwing refueling

Type: distributed or concentrated 8Choice of the loading conditionsFundamental question: what is the size of the load?

9Choice of the loading conditionsWe try to make a distinction between:Static loadsDynamic loadsBut when a load must be considered dynamic?

10Choice of the loading conditionsWe try to make a distinction between:Static loadsDynamic loads

But when a load must be considered dynamic?The answer depends on:The variation of the load (DL)The time in which it occurs (Dt)The properties of the structure in which applied

And then there is a simple answer! 11Choice of the loading conditionsBut why dynamic load is important?

Because the structural response is not' intuitive!

12Choice of the loading conditionsBut what are the typical dynamic loading conditions?

13Choice of the loading conditionsApply the following constraintsMust be limited in number in order to be realized in a reasonable timeMust be representative, thus constituting the correct set of loads of contingency (dimensionanti)

Sources from which to drawV-n DiagramSymmetrical loads to maneuver and gustSimilar diagrams for non-symmetric loads and details

OriginNumerical simulationsMeasurements on real aircrafts 14Choice of the loading conditionsV-n Diagram for maneuver and gustApply the following constraints

Must be limited in number in order to be

realized in a reasonable time

Must be representative, thus constituting the set

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correct load contingency (dimensionanti)

Sources from which to draw

Diagram V n

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Symmetrical loads to maneuver and gust

Similar diagrams for non-symmetric loads and details

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Origin

Numerical simulations

Measurements on real aircrafts

15Choice of the loading conditionsEach point inside the area defined by the V-n diagram defines a load condition: n2 conditions;The outline defines the envelope of the most critical conditions: still too many conditions to be verified in the test;Conditions to definitely check: points ADEFG;Does it makes sense to choose other intermediate points on outline? Apply the following constraints





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Diagram V n

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16Choice of the loading conditionsApply the following constraints





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Diagram V n

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17Choice of the loading conditionsEnvelope of the most stressed parts for the wing structure in relation to the point of the V-n Diagram considered Apply the following constraints





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Diagram V n

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18Calculation of dynamic loading conditionsApply the following constraints





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Diagram V n

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19Calculation of dynamic loading conditionsApply the following constraints





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Diagram V n

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20Dynamic LoadingApply the following constraints





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Diagram V n

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21Taxi Loads

22Taxi Loads

23Gust Loads

24Structural Dynamics and Aeroelasticity

25Sample Load ConditionsExample:Terms of load used for static testing of B777

26Choice of test loadsThe test loads are the set of the loads applied during the experimental testing that best approximate the actual loading conditions;The main simplification applied for passing the load conditions to test loads is given by the practical impossibility to apply the real distributed loads (aerodynamic loads and mass);Not being able to properly reproduce, the actual load is chosen to approximate the best performance of internal actions;The test loads are therefore all concentrated loads that allow you to better approximate the performance of the internal actions corresponding to a known load condition.The test loads vary with the load condition and the configuration of the aircraft27Methods of the application of the loads

28Methods of application of the loadsThere are different rules for the application of the test loads to the structure, and often depend strictly on past experience and the know-how of the industry that makes tests.

The two most representative mode, from which descended several variations, are based on the use of:Load cellsRubber pads

29Masting LoadDiagram example of masting load

30Masting LoadThe test loads introduced in the structure are generated by a single cylinder and distributed through linkages.AdvantagesReduced number of cylindersRigging easily achievablePrecision of test loads only linked to the precision with which the distances are realized in linkageDisadvantagesThe levers must behave 'rigidly' with respect to the test structure, which means that they must be sized to stiffness and resistance;The weight of spars, in a vertical configuration, fell to the structure: screw jacks must be preloaded in the absence of the test load;Can reach very large dimensions, requires sufficient space, generally above the structure31Masting Load

32Masting Load

33Masting LoadsSome images taken from static tests full-scale of the new aircraft Aermacchi trainer M346

34Masting LoadIn the absence of the space necessary above the test structure for the development spars, it is possible to use a configuration in horizontal development. In this case, the masts must be supported by an appropriate structure, it is no longer possible to do so directly with jacks.

Tests carried out at the Department on the wing of the aircraft from basic training F1300 shark at the end of the 80s.35Mode ConstraintThe modes with which to constrain a structure during the static tests depends on the type of the structure itself. We can distinguish:

Tests on complete structuresSince the load condition acting on the aircraft is completed real instant by instant in equilibrium, the resultant of the loads is zero. From the theoretical point of view, but only theoretical, it would be possible to perform the test by applying the aerodynamic loads and the corresponding inertial discharges without the need to constrain the structure. In doing so, however, a minimum load imbalance on the jacks would be enough to bring the way. The structure is constrained, usually in areas where they are already predisposed strengths (trucks, engines, lifting hooks) and occurs through direct measurement, the reaction forces are negligible during the test, testifying that the applied load conditions is in equilibrium.36Mode ConstraintTests on componentsDuring the development and validation of a structure are also carried out static tests on components (e.g. A wing). In this case the load is not in equilibrium and the constraint reactions must of course balance the resultant of the loads. In this case need to be measured, the more that the constraint reactions, their sagging, to avoid that such rigid movements associated with constraint settlements are lead to erroneous assessments of structural rigidity.37Mode ConstraintMeasurements of subsidence of the constraints, through linear potentiometers, at the attack of a wing, during the static testing of the aircraft F1300 Shark.

38Parameters measured during the testThe parameters to be measured during the static testing of components or complete structures are those necessary to demonstrate that the structure under test meets the requirements imposed by the regulations. As often happens, given the complexity and the cost of this type of test, the tendency is to measure as many parameters as possible, more than are necessary to demonstrate the overcoming of the test.Among the most important parameters measured include:Forces introduced in correspondence of the jacks (load cells)Displacements in different parts of the structure (linear or angular potentiometers, LVDT)Reaction forces (load cells, strain gauges)Constraint settlements (linearo or angular potentiometers, LVDT)39The Loading SequenceThis is a static test: the behavior of the structure must therefore be studied and characterized in static conditions. This is achieved through successive increments or load, which is followed by the measurement phase to balance achieved, or by continuous load increments, with continuous measures, provided that the speed of the load is sufficiently slow to be able to neglect the inertial effects.

The three seconds of pause required to load robustness have precisely this meaning40

static tests on complete structures

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