Abstract:

This Data Item covers the extension of the VFP program to predict the effect of exterior discontinuities such as steps, ridges and grooves on lifting devices such as wings, horizontal and vertical tail surfaces.

From the early stages of design to service there are external discontinuities known as excrescences such as steps due to skin joints, repair patches and grooves/steps between slats and flaps and the main wing. It is important to rapidly be able to obtain a reasonably accurate assessment of the impact of these excrescences on performance. For example, during the design phase this will provide the information needed to make a better judgement of trading manufacturing cost/timescales with performance. Further to this, for an in service repair an assessment of the impact on not only on performance but on possible deterioration of flight handling is important.

The excrescence models in VFP offer the capability to provide this information needed to make a judgment on the impact on performance and handling without the need to modify the surface profile to model the excrescence. This is done by either introducing a step change in the form of a local increase in boundary-layer momentum thickness or a local addition of a normal to surface velocity increment at grid point/points close to the excrescence. The VFP program can model forward and backward facing steps with and without edge treatments such as chamfered or rounded edges, square and rectangular ridges and grooves. It can deal with single or two excrescences. It is recommended that VFP calculations are performed with the momentum thickness and normal to surface velocity increase along with estimates using the excrescence Data Items listed in the Aerodynamic Series to give a range of data and to check the validity of the results.

See also:

• ESDU ASTEROID : Aircraft Surface Tolerances for Enhanced Repair, Operations and Design