一种多功能计算机辅助飞机外形造型原型系统

A Multi-Functional Computer-Aided Aircraft Exterior Shape Modelling Prototype System

基于多结点样条的理论研究与应用,研制了一种不同于目前商品化的用于飞机外形造型的多功能计算机辅助几何设计原型系统.文中简述了该系统中主要模块的计算格式及应用实例.理论研究及应用表明,该系统具有应用与开发前景.

A method known as multi-knot splines was proposed by Qi and Gao[2]. Now, a multi-knot spline-based multifunctional geometric modelling system has been developed by the authors to the point where it can be used all the way from initial sketch to complex geometric design to define the exterior curved shapes of an aircraft. Much CPU time (up to 40%) can be saved and more complex shapes can be generated. Multinot method proposed by the authors provides a local expression that is explicit. The multi-knot local computing representations of both tensor and boolean sum schemes developed have an order of geometrical continuity higher than those of interpolation computing representations obtainable with other methods (such as Coons[1]) under the same polynomial degree. The much better local and explicit expression of multi-knot method is the key to the saving of CPU time and the generation of more complex shapes. A multi-knot blending function interpolant scheme is provided to handle non-rectangular surface patches. The authors' scheme is a changing average of three interpolants, each of which provides continuity on two sides of a triangular patch. Such a scheme is believed to be better than the usual make-shift scheme of using a degenerate rectangular patch[4] to deal with non-rectangular patch. The authors' triangular patch scheme can handle nearly all non-rectangular patches. Only very special cases require the de- velopment of polygonal patch schemes where the number of sides of a polygon is 5, 6, etc. The authors' multi-knot method makes it possible for the designer to exercise convenient and direct control of shapes. The authors utilize polynomial interpolating splines with extra knots, which increase the degree of freedom in adjusting shapes of surfaces without destroying geometric continuity. The present widely used method for shape control[1] appears to be indirect and inconvenient. Another key function offered by the system is to support the construction of wider range of topological shapes. Though the need to handle polyhedral solids bounded by curved parametric surfaces for the design of such artefacts as machine parts has been widely recognized, there is so far no single modelling system which provides such capability with computing efficiency and is able to cope with topological complexity[5]. The authors develop an extended winged-edge data structure that so facilitates the interior description of objects in computer and so takes full advantage of multi-knot parametric surfaces that the system can be employed for some special areas where multicurved surfaces are predominant and that a faster rate of fundamental operations of shape-generation can be achieved. Although it is evident that the system requires further development, it can form part of fully integrated CAD system with a common database.