WP 5: Design guidelines and new technology validation
The objective of workpackage 5 is to use the knowledge created in workpackages WP2 to WP4 to develop new design guidelines. The targets of WP5 are:
- Define new procedure for definition and certification of loads to be used during aircraft
- Quantify the potential weight saving of structural components due to the overloading
effect of static loads
- Develop design guidelines
WP leader: Tobias Wille, Deutsches Zentrum für Luft- und Raumfahrt eV
WP 5 Results
Conclusions regarding the results when the approach including dynamic analysis advocated in DAEDALOS is used and the possibility to integrate the approach within the design are summarized in the following. Firstly, the activities and results of the DAEDALOS industrial partners are described, and subsequently the contributions of SMEs, research institutes, and universities are presented.
IAI has investigated the possibility of obtaining a reduced set of static loads, by performing a dynamic response analysis using the full 3D coarse model of the aircraft. This dynamic analysis has been refined by calculating a more realistic forcing function. Damping effects on the analysis has also been explored as a means of obtaining lower stresses in the elements included in the study. Finally, for the case in study (dynamic landing response), the reduction in static sizing loads is shown as a function of the damping parameters which characterize the airplane structure under study. The results are summarized in a proposed method for redefinition of sizing loads.
The activity of AAEM regarded the analysis of the dynamic behavior of DAEDALOS aircraft when lateral gust is applied. Discrete gust approach has been followed in accordance with CS25.
AAEM has performed several analyses of dynamic loads acting on vertical tail according with current methodology which involves the usage of a reduced dynamic model (stick model) for aeroelastic analysis and the definition of a static load set for further stress analysis.
Besides, AAEM has investigated the possibility of a more reliable modeling of damping effects via viscoelastic material entry in Nastran SOL108 and the definition of an equivalent modal damping table to be adopted in NASTRAN SOL146.
Finally most of the activities regarded the development of a new methodology of analysis based on a modal approach aimed to avoiding the usage of the stick model but maintaining the definition of an equivalent set of static loads.
At AES, a new approach to determinate the equivalent static load from a dynamic gust excitation has been developed. A new combination of FEM model has been generated mixing the coarse model developed in WP2 with the 70% detailed wing part developed in WP3.
First the equivalent unsteady aerodynamic load set developed in WP3, coming from the stick model and NASTRAN SOL146, have been correctly checked running again the equivalent aerodynamic set into the stick model through a NASTRAN SOL109, direct transient response, and comparing the accelerations though the wing. The comparison shows that the equivalent aerodynamic set is good enough to trust in the computation.
Second the good aerodynamic set is applied to the new coarse-hybrid model through a RBE3 connections system and run with a NASTRAN SOL109, placing an equivalent global damping similar to the model used for the aerodynamic set generation. With these new results, we have the new dynamic response to be analyzed.
As the complete internal loading for the coarse-hybrid model is hard to compute, the internal loads due to inertia loading have been calculated supposing every partial wing box between ribs moving as a rigid solid and adopting rigid solid kinematics. The comparison of the static load with the dynamic case is very accurate. This demonstrate that this equivalent set of static loads reproduce accurately, in the whole wing at any critical time, the dynamic effect due to a gust considered as external actions at some discrete points.
SMR has been concerned with the introduction of loads into panels and fuselage sections, and with the dynamic post-buckling behaviour of stiffened curved panels, involving inertia effects and material dissipation. Such panels may exhibit locally severe stress fluctuations which are caused by the snap-back at first buckling and which are eventually damped out. A way to minimize the snap-back is proposed, resulting in considerably reduced stress fluctuations and making the structure less prone to fatigue. Guidelines to the FE analyst are also given.
The DLR focused in WP3 on determining the influence of dynamic loads on the stress distribution and fuselage sizing in comparison to the static loads, the current industry practice. For this, an automated sizing tool chain was developed to study the influence of different loading assumptions on the mass of the aircraft fuselage. Additionally, DLR developed a virtual test rig which was used to numerically simulate the experiments conducted within WP4. In WP4 unstiffened, cylindrical CFRP shells were designed and manufactured for static and dynamic buckling tests at DLR and POLIMI. In order to perform the necessary dynamic tests, DLR designed and set-up a new testing machine. This work is summarized and the conclusions and recommendations are presented.
BUT was involved in several DAEDALOS project activities, grouped around specific part of DAEDALOS structure – flat composite panel representing wing structure. Activities done by BUT included structural design and stress analysis of flat panel, as well as structural testing of the panel. In particular, BUT attempted to model dynamic loading on the wing structure (representing flight through the gust) and compare results of simulations and tests with results of conventional design procedures based on static loading of aircraft structure.
FE simulations of dynamic processes were confirmed by practical tests of 7 composite panels done in the lab. Results indicated that speed of loading (in the range of loading speeds considered – close to gust loading) does not have significant impact on force at first buckling.
If complexity of simulation and necessary time is taken into consideration, dynamic modelling using MSC.Patran/MSC.Dytran is significantly more complex and time consuming than static modelling using MSC.Patran/MSC.Nastran. Models used for DAEDALOS panels (solved in MSC.DYTRAN) were solved in the order of hours, while static models (solved in MSC.NASTRAN) were solved in the order of minutes. Although it is not major issue if we take into account model complexity used for DAEDALOS panels, results of past activities indicate there is no significant advantage from utilization of dynamic analyses. For described case, there is no significant increase in first buckling force (leading to no potential decrease in weight of designed aircraft structures). Significant increase in buckling force can be expected for higher loading speeds (faster loading that expected common dynamic loadings from gust).
The contribution of LUH to the establishment of the new design approach required when using the dynamic analysis procedure for aircraft structures advocated in DAEDALOS relates to the availability of appropriate analysis tools. For the new design approach for aircraft structures based on dynamic analysis, tools for high fidelity Finite Element analysis (linear static and linear buckling analysis, nonlinear static analysis, and nonlinear transient dynamic analysis) as well as fast design tools are indispensable. Essential steps involved in the high fidelity analysis procedure at panel level have been applied, and supporting tools have been developed within the DAEDALOS project. A Fast Tool (Reduced Order Model) for nonlinear structural analysis has been used to predict the buckling response of panels under dynamic loading.
The activity of POLIMI regarded the development of experimental and numerical techniques for the evaluation of damping effects in composite structures. The methods were applied to the analysis of subcomponent structures, whose response was examined in terms of dissipated energy and dynamic buckling. Considerations regarding the use of the proposed methods, as well as on the potential benefits due to the ability of accounting for dynamic effects during the design phase, are assessed. Guidelines and suggestions are summarized in this document with respect to material and panel testing, numerical modelling of damping and design considerations.
The Technion tested three different configurations of structures: a. aluminum stringered stiffened curved panels (with 2 and 3 bays); b. hybrid stringered stiffened curved panels (one bay) with skin made of laminated composite material and the stringer made of aluminum longerons; c. stringered stiffened laminated cylindrical shells (4 bays).
To be able to compare the results for the various specimens, both tested and calculated, the buckling loads were normalized by the specimens, weight and plotted vs. a geometric nondimensional parameter b/SQRT(R*s), where b is the distance between two adjacent stringers, R is the radius and s the skin thickness.
Based on this approach and the results of the other work performed by the Technion during the DAEDALOS campaign, various design guidelines were formulated and proposed.