K. C. Wong

Flight Testing of the T-Wing Tail-Sitter Unmanned Air Vehicle

Since October 2005, the T-wing tail-sitter unmanned air vehicle has undergone an extensive program of flight tests, resulting in a total of more than 50 flights, many under autonomous control from takeoff to landing. Starting in August 2006, free flights with conversion between vertical and horizontal flight modes have also been undertaken. Although the latter flights have required some guidance-level ground-pilot input, significant portions of them were performed in autonomous mode, including the transitions between horizontal and vertical flight. This paper considers the overall control architecture of the vehicle, including the different control modes that the vehicle was flown under during the recent series of tests. Although the individual controllers for each flight mode are unremarkable in themselves, it is notable that the aggregate system allows the vehicle to fly throughout its entire flight envelope, which is considerably broader than that of conventional fixed- or rotary-wing vehicles. The performance of the controllers for the different flight modes will also be considered, with a particular focus on hover dispersion results, in differing wind conditions. The majority of these flights were performed on a tether test rig during autonomous control development, to ensure vehicle safety with minimal impact on vehicle dynamics. The demonstration of autonomous flight under the constraints imposed by the tether system in winds up to 18 kt is a significant achievement Results from the more recent horizontal flight tests with conversions between vertical and horizontal flight are also presented. Most important, these results confirm the basic feasibility of tail-sitter vehicles that use control surfaces submerged in propeller wash for vertical flight control.

Realization of Morphing Wings: A Multidisciplinary Challenge

Morphing-wing research is growing in significance, as it is driven by the need to improve aircraft performance. There are many aspects to consider when designing a morphing wing, making the task a multidisciplinary challenge. This has led to a multitude of approaches to morphing-wing research. This paper provides an overview of the field, drawing together these different approaches. Morphing wings can be classified in terms of shape parameters (what to morph), performance benefits (why morph), and enabling technologies (how to morph). Regarding the structural system, the majority of morphing-wing concepts have consisted of distinguishable substructure, skin, and actuator components. However, these components need to be integrated to such a level that all share the functions of carrying loads and changing shape, thus blurring the distinction between these components. The trends include shifts from using conventional mechanisms and actuators to smart-material-based systems to topology-optimized compliant-mechanism designs. Furthermore, concepts found in nature may offer potential morphing solutions, and the working principles of muscles and plants may be emulated in a morphing wing. The focus of this paper is on morphing for traditionally fixed-wing aircraft and on the structural system in particular.

Attitude stabilization in hover flight of a mini tail-sitter UAV with variable pitch propeller

In this paper, the modeling and control design of a vertical flight attitude control for a mini tail-sitter with variable pitch propeller is discussed. Tail-sitters VTOL-UAVs have operational flexibility of typical helicopters while having the cruise performance of fixed wing airplanes. The configuration proposed in this work is highly unstable in its natural flight state in vertical mode. A dynamic model is developed and a linear control strategy has been developed to stabilize the platform. The results are supported by experimental tests.

Robust control design based on sliding mode control for hover flight of a mini tail-sitter Unmanned Aerial Vehicle

In this paper, the modeling and robust control design of a vertical flight attitude control for a mini tail-sitter with variable pitch propeller is discussed. Tail-sitters Vertical Take-of and Landing — Unmanned Aerial Vehicles (VTOL-UAVs) have the operational flexibility of a typical helicopter while having the cruise performance of a fixed wing airplanes. The VTOL-UAVs configuration used in this paper is highly unstable in its natural flight state in vertical mode. First of all, a simplified attitude dynamic model that includes interval parametric uncertainty is obtained; then a control law based in the sliding mode control (SMC) technique is applied to stabilize the decoupled attitude control systems. The results are supported by simulation tests.

Attitude Stabilization with Real-time Experiments of a Tail-sitter Aircraft in Horizontal Flight

This paper focusses on the attitude stabilization of a mini tail-sitter aircraft, considering aerodynamic effects. The main characteristic of this vehicle is that it operates in either the hover mode for launch and recovery, or the horizontal mode during cruise. The dynamic model is obtained using the Euler–Lagrange formulation, and aerodynamic effects are obtained by studying the propeller effects. A nonlinear saturated Proportional-Integral-Derivative (SPID) control with compensation of aerodynamic moments is proposed in order to achieve the asymptotic stabilization of the vehicle in horizontal mode. In addition, a homemade inertial measurement unit (HIMU) is built for operating the complete operational range of the vehicle (including vertical and horizontal modes). Finally, simulation results are presented for validating the control law, and practical results are obtained in real-time during the flight.

Hyperion UAV: An international collaboration

The Hyperion aircraft project was an international collaboration to develop an aerial vehicle to investigate new technologies with a focus on performance efficiencies. A delocalized international team of graduate and undergraduate students conceived, designed, implemented, and operated the aircraft. The project taught essential systems engineering skills through long-distance design and manufacturing collaborations with multidisciplinary teams of students located around the world. Project partners were the University of Colorado Boulder, USA, The University of Sydney, Australia, and the University of Stuttgart, Germany. The three teams are distributed eight hours apart; students can relay select work daily so that developments can “Follow-The-Sun”. Select components are manufactured and integrated both in Stuttgart and Colorado, giving the students an opportunity to learn multifaceted design tactics for manufacturing and interface control. Final flight testing was conducted by the global team in Colorado during the month of April 2011.

Single and multi–objective UAV aerofoil optimisation via hierarchical asynchronous parallel evolutionary algorithm

Unmanned Aerial Vehicle (UAV) design tends to focus on sensors, payload and navigation systems, as these are the most expensive components. One area that is often overlooked in UAV design is airframe and aerodynamic shape optimisation. As for manned aircraft, optimisation is important in order to extend the operational envelope and efficiency of these vehicles. A traditional approach to optimisation is to use gradient-based techniques. These techniques are effective when applied to specific problems and within a specified range. These methods are efficient for finding optimal global solutions if the objective functions and constraints are differentiable. If a broader application of the optimiser is desired, or when the complexity of the problem arises because it is multi-modal, involves approximation, is non-differentiable, or involves multiple objectives and physics, as it is often the case in aerodynamic optimisation, more robust and alternative numerical tools are required. Emerging techniques such as Evolutionary Algorithms (EAs) have been shown to be robust as they require no derivatives or gradients of the objective function, have the capability of finding globally optimum solutions amongst many local optima, are easily executed in parallel, and can be adapted to arbitrary solver codes without major modifications. In this paper, the formulation and application of a evolutionary technique for aerofoil shape optimisation is described. Initially, the paper presents an introduction to the features of the method and a short discussion on multi-objective optimisation. The method is first illustrated on its application to mathematical test cases. Then it is applied to representative test cases related to aerofoil design. Results indicate the ability of the method for finding optimal solutions and capturing Pareto optimal fronts

Multidisciplinary Aircraft Conceptual Design Optimisation Using a Hierarchical Asynchronous Parallel Evolutionary Algorithm (HAPEA)

In this paper we present some results of continuing research into improving robustness speed and application of Hierarchical Parallel Asynchronous Evolution Algorithms (HAPEA) to multidisciplinary design optimisation (MDO) and aircraft conceptual design problems. The formulation and implementation of the HAPEA-MDO algorithm is described and can be regarded as an architecture that is applicable to either integrated or distributed system optimisation design for complex, non-linear and non-differentiable problems. In this paper the formulation for HAPEA-MDO will be described and applied to single and multi objective MDO problems. Two cases related to aircraft design are analysed. We compute the Nash and Pareto optimal configurations satisfying the specified criteria in both cases and show that the HAPEA approach provides very efficient solutions to the stated design problems.

Aerodynamic Shape Optimisation of Unmanned Aerial Vehicles using Hierarchical Asynchronous Parallel Evolutionary Algorithms

One of the challenges in Unmanned (Combat) Aerial Vehicles (UCAV) is the improvement of aerodynamic performance to complete diverse missions, increase endurance and lower fuel consumption. Recent advances in design tools, materials, electronics and actuators have opened the door for implementation of transonic flow control technologies to improve aerodynamic efficiency. This paper explores the application of a robust Multi-Objective Evolutionary Algorithm (MOEA) for the design and optimisation of aerofoil sections and wing planform of UAVs and UCAVs. The methodology is based on a canonical evolution strategy and incorporates the concepts of hierarchical topology, parallel computing and asynchronous evaluation. For the design and optimisation of UCAV wing planform shape, an aero-diamond planform shape with a jagged trailing edge is considered like saw tooth. Results obtained from the combination between the approach and the aerodynamic analysis tools show the improvement of the aerodynamic efficiency, a set of shock-free aerofoils and the supercritical aero-diamond wing. Results also indicate that the method is capable to produce non-dominated solutions.

Design of a Fuel Cell Powered Blended Wing Body UAV

Small-scale electrically powered Unmanned Aerial Vehicles (UAVs) are currently in use for a variety of reconnaissance and remote sensing missions. For these missions, electrical propulsion is generally preferred over small internal combustion engines because of the low noise and IR signature, low vibration levels, ease of operational support, and physical robustness. A desire for longer endurance than is available from the current generation of batteries has motivated the development of fuel cell based hybrid electrical propulsion systems. These advanced powerplant designs often include implementation challenges that will require new development methods and tools. Fuel cells generally lead to very low fuel weight at a high specific energy (Wh/kg) but have low specific power (W/kg). A high specific power is required to improve aircraft performance and manoeuvrability. Aircraft concepts powered solely by fuel cells therefore require both extremely lightweight airframes with a large internal volume and low-power payloads, which remains a challenge for conventional airframe designs. A blended-wing-body (BWB) airframe has high aerodynamic and structural efficiencies, which therefore seem ideally suited for this new generation of power-plants.This paper presents the development and testing of a novel BWB fuel-cell powered UAV. The paper first describes the initial design steps that led to the current airframe design. The Mark 1 platform has been developed, with a half-scale model built and currently being flight-tested. Based on the flight test results, the airframe will be scaled up and optimised to accommodate the fuel-cell and its associated systems. This aircraft will then be tested with a standard electrical propulsion system to determine the airworthiness with the restricted fuel cell power output as well as the design of the take-off boost system. This paper reports on the design, analyses, and preliminary testing of a fuel cell powered BWB UAV.Copyright