飞机曲面造型中的自由变形方法

Free-Form Deformation in Aircraft Geometry Modeling

介绍了CAD／CAM系统中的一种最新的几何造型方法──自由变形方法（FFD）。该方法在飞机曲面外形设计初期，可以直观、简洁、方便地获得高阶连续的曲面实体，同时本文还系统论述了自由变形方法的特点，最后给出了利用该方法和有理Bezier方法在构造飞机翼身融合体曲面时的特征对比，从应用中得出自由变形方法具有广阔的发展前景。

In the open literature, no more than mentioning of aircraft geometry modeling was found. Our research revealed that in applying FFD(free - form deformation) to geometry modeling, one should fully understand two limitations, which had barely been mentioned in the open literature.We applied FFD to aircraft blended wing - body condguration design. Eqs. (3) and (4)were the two important relationships derived for the purpose of this application. The deformation tool was the parallelepipedical control lattice (Fig. 1) defined by eq. (3). The FFD formula, eq. (4), was based on tri - variable Bernstein polynomials. Eq. (4) was used to obtain data and parameter equations. In the derivation of eqs. (2), (3), and (4), we came to understand fully the following two limitations:(1) The parameter equations derived were complicated.(2) FFD could be used only for preliminary configuration design.The blended wing - body surface shown in Fig. 3c was obtained with FFD. Table 1 compared the characteristics of Fig. 3c with those obtained with rational Bezier method. From Table 1, we can conclude that FFD is fast, simple and convenient.