GoR AD OPEN TECHNICAL PUBLICATIONS

GARTEUR TP 126

Drag extraction for supersonic civil transports / S C McParlin. - GARTEUR TP 126. May 2001

Summary:

The potential for any future Supersonic Civil Transport (SCT) is entirely dependent upon its economic and environmental viability. The environmental and economic assumptions behind the first generation of SCTs were made at a time before many of the issues relating to SCTs were fully understood. There are a variety of aerodynamic issues critical to the satisfaction of both of these.

The mass of fuel required to transport a given payload over a given stage length contributes directly to the Direct Operating Cost (DOC) of the aircraft, while also being a key factor in theoverall size of the aircraft, which affects DOC indirectly through the useable payload fraction. Minimising the fuel burn for a given payload-range improves both the economic and environmental performance of any aircraft. However the fuel burn of SCTs is uniquely sensitive to variations in drag.

The DOC of a transport aircraft is a primary feature of performance guarantees from manufacturer to end user. The additional fuel burn resulting from excess drag levels can therefore undermine the competitiveness of an aircraft. Therefore accurate drag prediction is of immense value at all stages of an aircraft design project, from conceptual design to lines freeze.

This report summarises the work performed by GARTEur (AD) Action Group 31 (AG-31) in the period between its initial meeting in January 1999 and March 2001. The participants in the AG were Airbus UK, DERA, DLR, ONERA and Saab. The objectives of this AG were based on the results obtained during the EUROSUP project and the subsequent GARTEur (AD) Exploratory Group 35 (EG-35). These were threefold;

1. To approach as close as possible to industry required standards for drag prediction at transonic and supersonic Mach numbers.

2. To evaluate the relative merits of alternative combinations of CFD flow analysis and drag extraction procedures, and determine the minimum mesh density requirements for transonic and supersonic conditions.

3. To use field methods at transonic Mach numbers to understand the breakdown of vortex, wave and friction drag components.

It became apparent at an early stage in the project that the levels of drag accuracy required for the supersonic cruise condition were on the limit of current experimental practice. The levels of drag uncertainty provided by CFD methods are still an order of magnitude larger than required by industry for these cases. If these issues are to be addressed, significantly greater investment will be required in both experimental methodology and supersonic CFD application.

The greatest uncertainty in drag at both transonic and supersonic cruise conditions is in the level of skin friction, and hence viscous drag. The accuracy of skin friction prediction is dependent upon mesh quality, turbulence modelling and the dissipation levels in the solution schemes. The meshes generated for the 1/80 80th th scale EUROSUP model were for experimental Reynolds numbers. The flight Reynolds numbers based on chord are likely to be significantly higher than for any other application, with corresponding requirements for spatial accuracy in both the definition of the underlying geometry and the resolution of the extremely thin boundary layer and wake regions.

As the implications of supersonic drag accuracy on aircraft MTOW became apparent, it was obvious that the resources available within this project were inadequate for anything except a relatively limited assessment of shortfalls in current, relative to required, capability. Any serious further activity in this field will require significantly more effort and detailed planning. In particular, the experimental basis for the exercise will need a considerable amount of thought and effort to reduce the uncertainty in both measured drag and model geometry.

This exercise has highlighted deficiencies in virtually all of the meshes used. It seems that the reduction of uncertainty in viscous drag is the most suitable area for further effort, although this does not necessarily need to focus on a supersonic civil transport configuration. Issues of mesh quality, turbulence modelling and drag extraction are more general issues, although the SCT remains the most demanding likely application. Finer meshes still will be required for calculations at full aircraft scale, and these will exacerbate mesh generation issues found during this programme.

For a complete copy of the document, contact with GARTEUR Secretary.

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