A flexible, subsonic high altitude long endurance UAV conceptual design methodology

Abstract

High Altitude Long Endurance (HALE) Unmanned Aerial Vehicles (UAVs) have been considered for use in both civil and military applications for some years. Their advantages, such as low cost, high survivability, long endurance, easy maintenance etc. relative to alternatives, make them useful to scientist and military personnel. In this thesis, a flexible conceptual design methodology for HALE UAVs has been developed. This has been implemented as a FORTRAN computer code. However, it is unlike some commercially available general aircraft design software which appears to the user as a ‘black box’. In this case, the code is broken-down into several subprograms which deal with the different aspects such as parametric study, drag, performance etc. Each of these can be either used as it stands, tailored to the user needs in a particular case or by-passed if more accurate methods or known values are available in a particular area. During production of the methodology problems were encountered in a number of areas due to the unusual operating regime and configurations of HALE UAVs. Obtaining engine data for the high altitude of interest was a problem. This was addressed through use of an existing engine modeling code to generate data. The high altitude also leads to low Reynolds numbers and along with the high aspect ratios typical of HALE UAV configurations, these place such vehicles beyond the validity limits of data sheet methods for prediction of a number of important parameters. Improved methods for the prediction of Oswald efficiency and maximum lift coefficient, in particular, are recommended to be sought. Accepting the above difficulties, an analysis of the Tier-II Plus, Global Hawk, was carried out using published data to provide some validation of the methodology and program. The results obtained provide confidence in the usefulness of the program in the analysis and investigation of HALE UAVs.