Jonathan Fleming

Improving Control System Effectiveness for Ducted Fan VTOL UAVs Operating in Crosswinds

A valuable new resource being developed for today’s soldier are small ducted fan VTOL UAVs. Although beneficial in many ways, a major operational problem of ducted fan vehicles is precise control when flying in crosswinds and turbulent conditions in general. There are two significant, inherent issues associated with ducted fan control in crosswinds; 1) lateral momentum drag and 2) a duct stabilizing torque which resist tipping into the wind. For vehicles designed for flight over large ranges in angle of attack (i.e. “transition to high-speed cruise”), trim over the flight envelope is strongly affected by the duct moment characteristics. Poor CG placement can lead to control saturation at intermediate flight speeds. These are characteristics of all ducted fan vehicles designed to hover as well as transition to horizontal flight. Techsburg, in collaboration with AVID LLC. and Honeywell Labs, has recently completed a research program to investigate and test new aerodynamic control devices to improve the handling of small ducted fan UAVs in crosswinds. This project consisted of both computational analysis and modeling of the vehicle aerodynamics and control system, as well as wind tunnel experiments using a powered ducted fan VTOL UAV model. This work leveraged methodologies and testing resources developed as part of a larger DARPAfunded VTOL UAV program. The research focused on a better understanding of the baseline vehicle and control vane aerodynamics, and selection and aerodynamic modeling and testing of new VTOL UAV auxiliary control device concepts. The analytical results are then used as input into an existing control system modeling framework, which in turn can be used to predict the vehicle’s dynamic response to turbulence. In addition to an improved flight control system, insights into establishing disturbance rejection criteria for VTOL UAVs are sought for use in future vehicle development efforts. This paper focuses on describing the baseline vehicle aerodynamics in a crosswind, as well as characteristics and design considerations for traditional control vanes used for this type of vehicle. Some limited, initial results from analysis of auxiliary control devices are presented as well.

Thermoelectric Power Generation for UAV Applications

There is an increasing need for and development effort in small UAVs (<100 lbs) that can be deployed at the platoon or small unit level in the military. These new vehicles need to be implemented as lightweight, man-deployable systems, yet still retain the essential mission capabilities that larger, more complex UAV systems offer. Since UAV power requirements do not scale linearly with the vehicle’s physical size and weight, there is a need to develop high energy density power systems to aid in the development of smaller UAVs. A promising technology for small UAVs is based on thermoelectric power generation, a process by which thermal heat flux is converted directly to electric power via the Seebeck effect. In previous work, Techsburg and Hi-Z Technology have partnered to develop and integrate thermoelectric generator (TEG) modules onto unmanned micro air vehicle propulsion systems as part of the DARPA MAV (Micro Air Vehicle) program. Techsburg, as the prime contractor, was responsible for TEG system modeling, integration, and testing, while Hi-Z developed and fabricated new high temperature, high efficiency miniaturized TEG modules. In more recent work as part of DARPA’s OAV (Organic Air Vehicle) program, Techsburg has performed system analysis and experiments in support of future application of TEG modules to small UAVs. Techsburg and Hi-Z have demonstrated that with proper TEG module design and system integration, high efficiency thermoelectric generator modules (TEGs) offer a viable alternative source of electrical power for UAVs. The future of thermoelectric technology is very promising. Due to advances in module design, a 300% increase in module efficiency (and power output) is expected within the next 3 to 4 years. With optimal thermal management, systems producing 400 to 500 mW/g or more may be possible. These advances promise a significant enabling technology for extending the endurance and mission capabilities of small, lightweight UAV systems.

Development of an Air Data System for Ducted Fan Unmanned Aircraft

Analysis and wind tunnel testing performed as part of a development program for a family of ducted fan unmanned aircraft has generated a significant database of wind tunnel data. These include force and moment data as well as duct lip surface pressures over a range of flight velocities, fan speeds, and vehicle tilt angles. In recognition of the need for a simple, lightweight, and robust air data system for this class of UAV, a study was undertaken to correlate duct pressure data to freestream flow velocities. Sample pressure data and a simple correlation methodology is presented in this paper. Results from this study indicate a strong correlation is present between duct pressures and freestream flow velocities, with very good system sensitivities at flow speeds at and well below 5 knots for the vehicles studied.