[Show abstract][Hide abstract] ABSTRACT: Recent investigations of a strut-braced wing (SBW) aircraft show that,
at high positive load factors, a large tensile force in the strut leads
to a considerable compressive axial force in the inner wing, resulting
in a reduced bending stiffness and even buckling of the wing. Studying
the influence of this compressive force on the structural response of
SBW is thus of paramount importance in the early stage of SBW design.
The purpose of the this research is to investigate the effect of
compressive force on aeroelastic stability of the SBW using efficient
structural finite element and aerodynamic lifting surface methods. A
procedure is developed to generate wing stiffness distribution for
detailed and simplified wing models and to include the compressive force
effect in the SBW aeroelastic analysis. A sensitivity study is performed
to generate response surface equations for the wing flutter speed as
functions of several design variables. These aeroelastic procedures and
response surface equations provide a valuable tool and trend data to
study the unconventional nature of SBW. In order to estimate the effect
of the compressive force, the inner part of the wing structure is
modeled as a beam-column. A structural finite element method is
developed based on an analytical stiffness matrix formulation of a
non-uniform beam element with arbitrary polynomial variations in the
cross section. By using this formulation, the number of elements to
model the wing structure can be reduced without degrading the accuracy.
The unsteady aerodynamic prediction is based on a discrete element
lifting surface method. The present formulation improves the accuracy of
existing lifting surface methods by implementing a more rigorous
treatment on the aerodynamic kernel integration. The singularity of the
kernel function is isolated by implementing an exact expansion series to
solve an incomplete cylindrical function problem. A hybrid doublet
lattice/doublet point scheme is devised to reduce the computational
time. SBW aircraft selected for the present study is the
fuselage-mounted engine configuration. The results indicate that the
detrimental effect of the compressive force to the wing buckling and
flutter speed is significant if the wing-strut junction is placed near
the wing tip.
[Show abstract][Hide abstract] ABSTRACT: Flutter of T-tail configurations is caused by the aeroelastic coupling between the vertical fin and the horizontal stabiliser. The latter is mounted on the fin instead of the fuselage, and hence the arrangement presents distinct characteristics compared to other typical empennage setups; specifically, T-tail aeroelasticity is governed by inplane dynamics and steady aerodynamic loading, which are typically not included in flutter clearance methodologies based on the doublet lattice method. As the number of new aircraft featuring this tail configuration increases, there is a need for precise understanding of the phenomenon, appropriate tools for its prediction, and reliable benchmarking data. This paper addresses this triple challenge by providing a detailed explanation of T-tail flutter physics, describing potential-flow modelling alternatives, and presenting detailed numerical and experimental results to compensate for the shortage of reproducible data in the literature. A historical account of the main milestones in T-tail aircraft development is included, followed by a T-tail flutter research review that emphasises the latest contributions from industry as well as academia. The physical problem is dissected next, highlighting the individual and combined effects that drive the phenomenon. Three different methodologies, all based on potential-flow aerodynamics, are considered for T-tail subsonic flutter prediction: (i) direct incorporation of supplementary T-tail effects as additional terms in the flutter equations; (ii) a generalisation of the boundary conditions and air loads calculation on the double lattice; and (iii) a linearisation of the unsteady vortex lattice method with arbitrary kinematics. Comparison with wind-tunnel experimental results evidences that all three approaches are consistent and capture the key characteristics in the T-tail dynamics. The validated numerical models are then exercised in easy-to-duplicate canonical test cases. These parametric studies illustrate the impact of well-known factors in T-tail flutter, namely horizontal tailplane dihedral, flexibility and static deformations. In addition, scenarios are exposed in which the stability behaviour is dictated by typically second-order effects, such as chordwise forces and quadratic modes, revealing drastically different qualitative flutter curves. It is also shown that there is a distinction between angle of attack of the whole tail assembly and incidence of the horizontal tailplane relative to the fin, which might yield very counterintuitive trends depending on the configuration parameters. The paper concludes with flight test results of the Airbus A400M, epitome of modern T-tail aircraft. Tests performed in a wake-vortex encounter campaign complement the virtually nonexistent literature in the topic, demonstrate how T-tail effects can be measured in flight and restate the adequacy of potential-flow models for T-tail flutter prediction.
Progress in Aerospace Sciences 08/2014; · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The catalytic cracking of methylcyclohexane over an industrial FCC catalyst(containing Y-zeolite) has been studied in the Temporal Analysis of Products (TAP) reactor. High selectivities towards toluene were observed (Sto1≈70 %). The unusual product distribution originates from a predominant protolytic cracking that is favored by the low pressures and high concentration of acid sites applied in the TAP reactor. The formation of the other cracking products (benzene and smaller paraffins and olefins) is well accounted for by the relative stability of the secondary and tertiary carbenium ions. The catalytic cracking of methylcyclohexane can be adequately modeled by a reaction scheme by which the secondary and tertiary carbenium ions are formed in parallel. The activation energies for the formation of toluene and benzene are 144 kJ mol−1 and 220 kJ mol−1 respectively. This is in line with thermodynamic considerations concerning the stability of carbenium ions.
Studies in surface science and catalysis 01/2001; 133:341-348.
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