Optimum Aspect Ratio for Subsonic Air Vehicles

Abstract

This fundamental study considers airframes engineered to meet range and/or efficiency goals that span several orders of magnitude in size (from approximately 1000 lb to one million lb maximum flight weights). A simplified, analytical model is used to demonstrate the influence of key constraints (maximum ceiling and desired cruise speed) and design parameters (wing loading, wing aspect ratio, wing sweep, wing technology, and allowable excrescence drag) upon aerodynamic performance and fuel volume. This model illustrates factors that drive the optimum wing loading, technology, and aspect ratio for an airframe engineered to meet improved range and efficiency goals. The performance of airframes over a 200,000 lb flight weight is well represented by classical theory. For many airframes under 100,000 lb,flight at the typicalmission cruise point is dominated by zero-lift drag.A statistical design approach to refine these configurations reveals counterintuitive design insight. To simultaneously improve range and efficiency of a conventionally configured zero-lift–drag-dominated airframe, analysis of a full factorial design exploration suggests changes, such as increasing its service ceiling (though larger engines), increasing its certification ceiling, and revisions to the wing planform, that, in some cases, call for a reduction in aspect ratio and wingspan.