UWE Bristol Engineering showcase 2015

Alistair Montgomery
BEng Aerospace Systems Engineering
Project Supervisor:
Dr. Rui Cardoso
An Investigation into the affects of Structural Idealisation on an Aircraft
Wing Box
Introduction
Aircraft structures consist of a range of highly
complex, detailed components, and as a result
require a high level of structural analysis and
testing to ensure the structure is able to
withstand the many aerodynamic loads
encountered in flight.
In order for such high level models to be
analysed efficiently both in terms of calculation
complexity and computational hardware, in the
preliminary stage of the analysis, models are
highly idealised. This is the process of modifying
a fully detailed model into a simple geometry for
practical applications such as an FE analysis via
the removal of unnecessary features and
typography alterations.
The wing acts as both a beam and torsion
member. The internal structure of a wing
consists of the main bending member know as a
spar, lateral cross sections called ribs, and
smaller axial members known as stringers or
stiffeners.
Idealisation Methods
Two idealisation methods are conducted on the
wingbox as part of this investigation. The
equivalence area method, and the linear stress
distribution method.
In order to perform the idealisation assumptions
are made to the geometry. Firstly the wingbox is
considered to consists of only two load bearing
elements; the wing skin, and longitudinal stiffeners
or stringers.
In a fully effective model we will consider the skin
to carry shear stresses (τ) and direct stresses (σ)
However when we simplify the model we assume
the skin to have a zero thickness and as a result is
only responsible for the shear stresses. It will also
be assumed that for the stringers that the load
carrying capabilities will be concentrated to a
single point, this allows the assumption that the
stress is constant for the entire cross-section of
the stringer. This area of concentration is
represented on the idealised geometry as a
circular point and is known as a boom.
Analysis and Results
The analysis has been run on the wingbox profile
of a cessna 172 skyhawk. The section spans from
the front spar to the rear spar and consists of a
section of length 2.54m from the wing root.
After idealization it was concluded that the area
equivalence idealization method provides a less
evenly distribution of shear stresses with the
wingbox. There are also significantly larger areas
of stress at the location of the stringers. This is due
to the area idealisation method creating booms at
a greater distance from the original skin location
that the linear stress idealisation method.
Overall it can determined that the axial stress
distribution idealisation method provides more
accurate and consisted results, especially over the
location of stringers and skin reinforcing members.
Project summary
This investigation examines the affects that
idealisation processes present to an aircraft wing-box
via two alternate idealisation methods compared to
the fully effective geometry. The analysis concludes
how the idealised model’s results differ to the fully
effective model with respect to their individual load
paths, shear stress, principal stresses, displacement
and areas of highly concentrated stress, allowing a
review of the overall reliability and accuracy of each
idealisation process.
Project Objectives
The objectives of the investigation will be to
conclusively determine the which of the idealisation
methods, linear axial stress distribution, or the
triangular area equivalence method provide more
reliable and accurate results to a comparison of an
FEA model.
Project Conclusion
There is a positive correlation between the maximum
shear stresses within the panels of each idealised
structure. The area equivalnce method has shown to
consist of a less evenly distrubuted shear flow. With
areas of concentrated shear stress at the location of
idealised stringers. In conclusion, it has been
determined that idealisation via linear axial stress
distribution provides more accurate and reliable
results compared to the triangular area equivalence
method.