Formation Flight Aerodynamics of Oblique Flying Wing Type Aircrafts

Abstract

Due to the rapid development of military unmanned air vehicles and especially due to associated progresses in control and guidance technology it might be beneficial to use these technologies for civilian air transport, especially for unmanned cargo air frighters (UCAF). In particular, from the control and guidance technology side most of the challenges have been or will be mastered in the near future. Taking into account the potential advantages of unmanned flight in regard to safety issues and rules, significant economical and ecological potentials of UCAFs could be opened, when UCAFs would fly in close formation. Inspired by nature formation flight has been investigated by a various number of researchers and coworkers, it has been shown that in regard to fuel consumption certain formations show a great impact. This has been extensively tested by NASA. Other studies concentrated on wing shape optimisation in order to achieve best additional lift results at low admittable drag. Typically air frighters are only variations of civilian passenger aircrafts. For these manned aircraft the certification rules of the FAA would not allow to flight in a close formation. Even more, from the aerodynamic design point of view the current aircrafts are not able to make use of the positive aerodynamical effects of formation flight efficiently. Therefore, other completely different aircraft designs are necessary. One of the most promissing concept is the so called “oblique flying wing” (OFW), an aircraft which fuselage is integrated in the wing and which has a shock free wing shape design. This concept has been extensively investigated by Sobieczky. The speciality of OFWs is that due to its curved wing shape and its extremely slim camber shocks can be avoided even for high Ma number flows: The adjustable sweep angle and angle of attack allows shock free flow regimes over a wide range of high Mach numbers. In this numerical flow simulation study we are focusing on oblique flying wings in formation flight. In the first part of the work we will concentrate on the flow structure of formation flight, in particular, we will investigate the impact of the wing tip vortices of the front OFW on the flow structure at the following OFW. This is necessary in order to find best positions in the formation in regard to the feasibility to capture additional lift induced by the onflowing vortex. In particular, flying in the presence of trailing vortices is an unstable flight condition. Furthermore, since the vortex decays quickly, it is crucial to fly very close to the optimal lateral and vertical position in order to get the maximum additional lift and reduced drag. This is complicated by the fact that the vortex from the leading plane is deformed by the motion of the ambient air, including the movement induced by the presence of the trailing aircraft itself. Therefore, our further focal points are the manoeuvers in the vortical wake flow of the leading aircraft and the sensitivity against slight changes of the position in relation to the wing tip vortex generated by the leading aircraft. Before going into details the underlying grid generation, the used CFD code and the relevant CFD simulation setup will be explained. Additionally, the grid deformation technique, used for shape adaptation and positioning of the OFWs in the aircraft flight formation, will be reflected. Thereafter, the design concept of OFW will be briefly introduced. Special attention will be given to the design methodology, before the flow characteristics around an OFW will be exemplarily explained. In a short manner main aspects of the flow field will be presented Based on the CFD results the analysis will concentrate on the tip vortex OFW interaction. Thereafter, the position of the OFWs in the formation in relation to each other will be varied, the flow simulation results will be compared and discussed. Subsequently, an evaluation of the positioning and shape adaptation in regard to harvesting energy will be presented and summarized, followed by a discussion of OFW design variations. Finally, some design guidelines in respect to the aerodynamic shape will be given.