The power balance method For aerodynamic performance assessment

Abstract

This thesis describes the use of the power balance method for performance estimation of aircraft configurations. In this method, mechanical power production and mechanical power consumption of the aircraft are balanced, rather than forces as in the conventional thrust and drag approach to aircraft performance estimation. It is shown that an approach based on mechanical power provides a substantial advantage in accuracy and a wider range of applicability for integrated configurations such as boundary layer ingesting (BLI) aircraft. The thesis provides evidence of the major benefits of the power balance method, and descriptions of its limitations, for three applications: 1) derivation of an analytic expression of profile drag estimates for conceptual design applications, 2) aerodynamic performance estimation for three canonical integrated configurations, and 3) performance quantification of a hybrid wing body (HWB) with BLI propulsion system. In the first application, an analytical expression for the form factor used in the wetted area method (a profile drag correlation employed in conceptual design, which is heavily based on empiricism) is derived from the power balance method. The developed estimation method uses only potential-flow surface velocities, and it can be applied to new geometries for which experimental drag data is not available. The accuracy of this analytical expression and its limitations are presented in terms of quantitative results and analysis of physical effects for two-dimensional and axisymmetric geometries in incompressible and transonic flows. In the second application, the mechanical energy loss is evaluated for three integrated configurations: a fuselage with a propelling fan at the rear, a nacelle-fan combination, and two interfering airfoils. Using the boundary layer mechanical energy equation, it is shown that the profile mechanical loss from potential field (pressure) interference scales according to ... The scaling is confirmed using computational fluid dynamics (CFD) calculations. The scaling is accurate to within 10% for configurations for which the average boundary layer kinematic shape parameter increases by no more than 0.04 from non-interfering to interfering configuration. The physical mechanisms responsible for the breakdown in accuracy are analyzed. The power balance method is also applied to a system-level optimization of fuel burn for an HWB with BLI propulsion system. The fuel burn of the HWB is shown to decrease monotonically with increasing BLI, lip to a maximuim fuel burn improvement of 11 % compared to a non-BLI aircraft.