A conceptual design methodology for low speed high altitude long endurance unmanned aerial vehicles

Abstract

A conceptual design methodology was produced and subsequently coded into a Visual C++ (GUI) environment to facilitate the rapid comparison of several possible configurations to satisfy High Altitude Long Endurance (FIALE) unmanned aircraft (UAV) missions in the Low Speed (propeller driven aircraft) regime. Several comparative studies were performed to verify the applicability of traditional design methods. The traditional computational design methodologies fail in several areas such as high aspect ratio wing weight estimation and design, low Reynolds number wing design, high altitude engine performance, low Reynolds number drag estimation, unmanned aircraft design, and the conceptual design of unconventional configurations. The methodology developed for this thesis was robust enough to allow not only for consideration of these areas of inadequacy in traditional methods, but also to allow for the inclusion of advancements in the relevant technologies as they become more widely available. The following configurations were evaluated for suitability to the Low Speed HALE UAV application: conventional, canard, twin boom, multiple fuselage (conventional or canard), tandem wing, multiple fuselage tandem wing or flying wing configuration. The configurations were compared on the basis of aircraft endurance for takeoff weights ranging from 2,000 to 20,000 pounds and wing loadings ranging from 5 to 25 lbs1fe. Initial drag estimates were made using traditional parabolic drag estimation techniques. A more refined drag buildup was performed using a vortex lattice drag estimation for the lift induced drag (for all lifting components) and calculated skin friction coefficients for the parasite drag. Statistically based methods were used for other components of drag having much smaller contributions. In addition, a statistical approach was taken to the weight estimation of the major aircraft components. However, this approach made comparison of alternative configurations more difficult. Thus wing bending moments trends were evaluated and utilized in the development of weight saving values for multiple fuselage wing weight estimation. The comparative performance of each configuration is justified with direct reference to the terms in the Breguet Endurance equation. Validation was performed where possible on all modules and segments associated with the methodology, as well as for the macroscopic results. In addition, parametric studies on endurance were performed for the conventional configuration for geometric characteristics and operating conditions directly and indirectly effecting the calculated endurance and generalized results presented. Finally, a case study was performed to demonstrate this capability. A new relation was developed for aircraft empty weight prediction, a low speed airfoil figure of merit was proposed, and new constants were offered for UAV fuselage length prediction. In addition, horizontal and vertical tail volume coefficients were proposed for all of the Low Speed HALE UAV configurations considered. It was determined that the multiple fuselage configurations showed comparatively superior endurance performance across a range of takeoff weights, with several other configurations demonstrating marginal endurance improvements. Finally, a highly flexible and robust computer based conceptual design methodology was developed and validated enabling the quick comparison of a greater number of possible configurations to satisfy a given mission for Low Speed HALE UAV's and providing detailed drag and weight breakdown data.