Britten-Norman Ltd (B-N) was founded in the late 1950’s by aeronautical engineers John Britten and Desmond Norman. The Islander is Europe’s most successful modern light twin-engined aircraft. Today Britten-Norman has a workforce of around 140 at its Bembridge factory on the Isle of Wight. Ninety percent of aircraft are built for export and to date over 1200 aircraft have been sold to customers in more then 115 countries. This particular piece of consultancy work was conducted as part of a research program funded by the Department of Trade and Industry (DTI). The photograph above illustrates an Islander at one of its more unusual destinations – on a beach in the Falkland Islands!

The analyses described here form the culmination of a five year research project involving a partnership between Britten-Norman Ltd, Queen Mary & Westfield College – University of London (QMW) and IDAC. The project has been funded by the UK DTI. The purpose of the research work was to correlate physical testing of an Islander fuselage conducted by QMW with both eigenvalue and non-linear large deflection buckling solutions performed by IDAC on a model assembled by B-N. The physical testing of the fuselage consisted of loading a fuselage and stub wing assembly up to failure (defined as several panels buckling) and evaluating the response of the structure with extensive strain gauge measurements. The test was designed to simulate a high-drag landing load case. The finite element analyses were then compared with the strain gauge solutions in order that a procedure could be developed for predicting the onset of buckling using computational techniques. This would allow B-N to use FEA in its certification procedure with such organizations as the CAA & FAA.

A finite element model was produced for both linear eigenvalue buckling and non-linear large deflection buckling analyses. The graphic to the right illustrates the geometric surface model.

The geometric model of the fuselage was constructed within the ANSYS program with shell thickness and stiffener beam cross sectional attributes attached to the lines and surfaces within the model. Having created the geometric model the finite element mesh was created with the ANSYS automatic meshing capability. The finite element mesh was created with the 4-noded structural non-linear SHELL43 element, the 2-noded structural BEAM4 and offset BEAM44 elements and 2-noded structural spar elements, LINK8. The active degrees of freedom in the elements are the translatory displacement in the x, y and z directions and the rotational displacements in the rotx, roty and rotz directions. The model consisted of 23,001 elements, 19,437 nodes resulting in a model size of 116,622 degrees of freedom.

A graphic illustrating the finite element mesh used in the analyses can be seen on the left

Having extracted the buckling critical loads and the mode shapes from an eigenvalue analysis it is possible to correlate these results with testing. It is also possible from this form of analysis to establish the loading increments that would cause the fuselage to start buckling which assists in the loading control of the subsequent non-linear large deflection buckling analysis.

A graphic showing the out of plane deflection distribution for the panels obtained from the large deflection analysis is shown below at a load increment of 20% of final load. This solution compares very favorably with the experimental data. The FE model actually predicts the onset of buckling at around 13% of final load.

The large deflection solution was run through to a loadof 60% of the final high drag landing load case at which point severe buckling of the fuselage structure was measured experimentally. It should be noted that the aluminium material was assumed to behave elastically during all of this work.

The graphic left shows a closer view of the buckling in the panels at 60% load. This graphic displays the out of plane displacement solution for the critical panels. The graphic shown right displys the same displacement solution at 60% load but from the other side.

It can be seen from this that the fuselage is developing buckled panels which are evolving progressively towards the tail. This matches precisely the shape of the panel buckling experienced during the test and also duplicates the magnitude of load at which this progressive buckling occurs. The analyses were run using SGI Intel based Pentium PC’s.

The stress office at Britten-Norman, led by their Head of Structural Engineering Alistair Macartney, was very impressed with the correlation of the analytical results with the measured response. “The analysis methodology, which has been proven with this work, will allow us to analyse the design changes that we make to our aircraft and hence assist us in obtaining CAA & FAA certification for these changes.”, said Alistair.

As these design changes are largely manufactured to order it is not viable for the company to conduct physical testing of high-value but low-volume designs. The company uses the ANSYS finite element analysis system to conduct simulations of their conceptual design models.