Large passenger aircraft produced in the European Union are designed to meet the regulatory requirements of Certification Specification 25 (CS25), as defined by the airworthiness authority – the European Aviation Safety Agency (EASA).
The wings of such aircraft are highly flexible structures, which can deform significantly when, for example when atmospheric turbulence or gust is encountered. Typical limit deformations can reach 20% of the wing span. Wings also house the fuel tanks, and generally carry an amount of fuel comparable in weight to that of their structural components. For a typical wing of a single aisle aircraft, with maximum take-off weight of 100t and 3000nmi range, both structural and fuel weights are of the order of 4 to 6t.
The standard engineering practices for wing (and aircraft) design do not consider the effect of the fuel movement within the tanks for the determination of the aircraft design loads. This is due to the lack of maturity of the current toolsets available to industry.
The typical aircraft development cycle includes two major tests to identify/verify the dynamic characteristics of the aircraft structure and its characteristic excitations:
- Ground Vibration Test (GVT), performed before first flight. The aircraft is made to vibrate by mechanical means so that its structural behaviour (including damping) can be measured. It is standard practice to repeat the test for the two extreme conditions of either empty or full tanks.
- Flutter Flight Test (FFT), usually performed via prescribed oscillatory motion of the aircraft control surfaces (such as ailerons) when flying at speed close to the design envelope of the aircraft. Results from the FFT campaign are generally used to adjust the unsteady external aerodynamic model used to excite the structural model.
Both above tests are limited to small amplitudes and therefore not likely to excite significant sloshing modes, the GVT to avoid damaging the specimen (usually the first production aircraft), and the FFT to avoid endangering the safety of crew and aircraft. Also significant is that these qualification tests are generally performed after the development of the fuel tank architecture has reached an advanced stage, which severely limits the alterations that can be effected to the design.
In contrast with these industrial practices, the CS-25 Acceptable Means of Compliance (AMC) 25.341 6. c. (Gust and Continuous Turbulence Design Criteria) state:
“Structural dynamic models may include damping properties in addition to representations of mass and stiffness distributions. In the absence of better information, it will normally be acceptable to assume 0.03 (i.e. 1.5% equivalent critical viscous damping) for all flexible modes. Structural damping may be increased over the 0.03 value to be consistent with the high structural response levels caused by extreme gust intensity, provided justification is given.”
The focus of SLOWD is therefore to quantify the extent to which liquid sloshing in the aircraft tanks affects the “structural damping” mentioned in the AMC above, or more, in general, the structural dynamic behaviour of an airliner, at “high structural response levels”. In addition, the identification of the architectural parameters to maximise fuel slosh induced damping will overcome the major shortfalls of the current industrial practises: unnecessary conservatism in the design loads calculation (and consequent oversized structure) and maximisation of the benefits related to sloshing-induced dissipative effects (via informed design of the fuel tank layout).