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Preliminary Geometry Design

Once the spreadsheet analysis has been completed and a set of preliminary dimensions has been found, a more realistic series of analyses can be undertaken before the project is passed on for detailed design. The purpose of these analyses is to check that a number of the key assumptions made when using spreadsheets to size the aircraft will be borne out in practice. The primary checks that need to be completed before proceeding on to detail design are typically the following:

  • • that the lift and drag estimates for the aircraft in cruise, turn, takeoff, and landing configurations are sufficiently accurate that the desired aircraft performance will be achieved.
  • • that the flight stability of the aircraft will be acceptable.
  • • that acceptable stress levels in the primary aircraft structure can be achieved within the allowable weight budget.
  • • that the aircraft can be made sufficiently stiff to avoid control surface operational and aeroe- lastic problems within the allowable weight budget.

The majority of the calculations underpinning these checks depend on the quality of the geometrical description available, so we begin by first building a much more realistic model of the airframe. Such models can be generated in a variety of ways but, if they are to include more than just the lifting surfaces, they almost always depend on some form of computer aided design (CAD) program. Here we continue to use the AirCONICS system1 that drives the Rhino CAD platform.2 Typical outputs from this process have already been shown in the previous chapter; here we give more details on how this works and the steps involved in producing realistic wetted surfaces for the airframe, including control surfaces and a series of components sufficient for preliminary structural analysis.

Once this definition is in place, more detailed experimental aerodynamic data and computational fluid dynamics (CFD) approaches can be used to check the lift and drag performance and stability of the airframe against the assumptions already made in the spreadsheet analysis. If necessary, the spreadsheet process can be refined in the light of such information and the

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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.

geometry updated: generally any deficiencies in the aerodynamic characteristics will lead to changes in the overall planform shape. Then attention can be turned to the internal definition of the airframe structure and more refined hand calculations, backed up by finite element analysis (FEA), applied to check stressing and stiffness, including aeroelastic effects. When an adequate structural model is also in place, a relatively rigorous weight and center of gravity check can be made. Again, if the spreadsheet assumptions are inadequate, refinements can be made and the whole process updated - structural problems typically lead to increases in the estimated weights, which will adversely impact on wing loading and thus may require modifications to the planform shape. Finally, and only when all is well, the detailed design that ultimately leads to the final definitions needed for manufacture can begin. Sometimes a series of experimental models will be built and tested before final production is committed to; these can be used to validate any computations carried out as well as providing data for future design studies.

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