Lateral Control of Tailless Aircraft With Reduced Directional Stability

Abstract

The work in this thesis aims to create and develop an improved method of lateral control for finless flying wing aircraft using only conformal control effectors. This improves on existing methods that typically use additional non-conformal aerodynamic surfaces such as spoilers and split flaps to generate yaw control primarily through modulation of profile drag. The method is based on the design and allocation of a suite of lift based aerodynamic controls to modulate the spanwise lift distribution in such a way to provide independent control of pitch, roll and yawing moment, with the yawing moment principally produced through laterally asymmetric induced drag. Previous relevant work in the literature has mainly focussed on the development of sophisticated control allocation methods for over-actuated suites of aerodynamic controls and development of innovative non-conformal control devices. Alternative conformal yaw control schemes based on forebody flow control have shown some promise, however to-date, all known successful finless flying wing aircraft have used non-conformal surfaces, or exceedingly large deflections of conformal control surfaces as to negate the benefits of conformity, to provide closure on the yaw control problem. Initial investigations concerning the use of fluidic yaw control show that with the fins removed, the opportunities to actuate tailless aircraft at low angles of attack with no significant coupling are scarce and require large changes in pressure over the surface of the aircraft to achieve a useful authority without modifying the thrust line. The primary contribution of this thesis is a design method based on the use of a series of control mode shapes which are defined from the response of the aircraft in lift, pitching moment and rolling moment to a given control input. By forming this response as a linear system and taking the null space, the mode shapes are defined such that they produce a yawing moment with no coupling. Mode shapes are obtained using a low order aerodynamic analysis and can be derived for a range of flying wing geometries and control layouts of practical interest. The proposed method is evaluated through two case studies on a generic finless flying wing aircraft. One considering steady sideslip relevant to a cross wind landing case and one concerning a coordinated rolling manoeuvre to initiate a banked turn. For steady sideslip at an angle of attack of four degrees, a given yawing moment can be produced with up to a factor of 2.5 reduction in the aggregate control deflection and 10% less overall drag when compared to a comparable conventional non-conformal control solution. For the roll case, there is at least factor 1.25 reduction in aggregate control deflection for roll rates meeting mil-spec requirements. The greatest benefit in aggregate control deflection can be up to a factor of 5 depending on the angle of attack. The method is more effective with decreasing angle of attack as the effect of adverse yaw is reduced. As part of an assessment for three-axis fluidic control, the method was also applied to an aircraft case study that used fluidic effectors in the form of circulation control actuators in place of conventional geometric flight control surfaces. In doing this, a framework was developed to assesses novel control actuators against mission requirements. The methods developed within this thesis show that is possible to generate yawing moments sufficient for independent three axes control of tailless aircraft using only lift based controls over a useful range of angle of attack and sideslip, though the maximum authority achievable will typically be lower than that for a comparable drag-based control implementation. The benefits demonstrated include a reduced drag increment and reduced aggregate deflection when compared to conventional, non-conformal profile drag based controls. These benefits make the application of the mode shaping technique particularly useful during

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	william crowther (Supervisor) & M Nabawy BSc, MSc, PhD, MRAeS, SMAIAA, FHEA (Supervisor)