Detail Design: Constructing Explicit Design Geometry
Next we turn to the direct creation of “explicit” geometry (as opposed to the “implicit” geometry we have mostly used so far, and which is generally used only during the early stages of design1 - implicit geometry is essentially a compact representation of a number of less abstract parameters). Ultimately the goal will be to declare an unambiguous, fully defined artifact that is referred to as detail geometry. This artifact is made up of a series of geometric parameters that explicitly define the shape. In this chapter, examples are illustrated using the Solidworks computer aided design (CAD) software package,2 which provides many useful capabilities including mass property analysis, cost modeling, and others.
The Generation of Geometry
A detailed description of the overall design stage logic is given in Appendix A. The detail design process is shown in Figure 17.1. It will be noted that, within the overall logic, generation of detailed geometry only starts after much preliminary activity/thinking/decision making has taken place. A premature start to the creation of detail geometry can be very inefficient, result in much wasted effort, and is likely to result in considerable unnecessary iteration and perhaps substantially suboptimal solutions.
In a typical low-cost or student design project, it is likely that the concept and preliminary design phases will make much use of spreadsheets of the sort described earlier, which hold a number of implicit design parameters. During the preliminary design phase, the geometry becomes sufficiently complex to justify the start of explicit geometry modeling (embodiment) for five reasons:
- 1. visualization (does the geometry look right; has there been a silly mistake - this is why our spreadsheets usually include very basic sketch views);
- 1 An example of implicit geometry is “wing area,” which is a key attribute for early hand or spreadsheet calculations and constraint analysis.
- 2 http://www.solidworks.com/.
Small Unmanned Fixed-wing Aircraft Design: A Practical Approach, First Edition. Andrew J. Keane, Andras Sobester and James P. Scanlan.
©2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.
Figure 17.1 Detail design process flow.
- 2. space allocation (do all the proposed systems fit within the fuselage/wing, etc.);
- 3. support for physics-based analysis of aerodynamic and structural performance calculations;
- 4. accurate calculation of mass properties;
- 5. start of detailing.
Figure 17.2 The structure of well-partitioned concept design models.
During the early design stages, flexibility needs to be preserved, as design parameters remain fluid. Therefore the goal is to construct geometry (assemblies) that can cope with large changes in parameter values. The “Holy Grail” is to achieve parametric geometry that can easily be modified externally in order to facilitate quick and easy search of a design space, as illustrated in Figure 17.2, see also Gudmundsson . As the definition becomes more complex and explicit geometry creation commences, this becomes increasingly difficult.
In what follows, it is assumed that certain initial design decisions have been made such as configuration, engine selection, constraint analysis, and basic payload/ range calculations. Until these decisions have been made, it is difficult to justify significant effort in explicit geometry modeling because it could change radically. Having selected a configuration, one has to make initial decisions concerning the following:
- • engine location (tractor, pusher)
- • main wing location
- • undercarriage configuration
- • control surfaces.
An example configuration study for a twin-engine UAV is shown in Figure 17.3. The rapid generation of reasonably realistic outline configurations is one of the great benefits of adopting the AirCONICS suite during preliminary design.