A Research On A Small Electric-powered Tailless UAV Using Propeller Thrust Trimming

For the fixed-wing aircraft,flight performance is traded for longitudinal static stability and trim.Without an empennage,this is especially true for a tailless configuration.As a result,in the tailless aircraft design process,more trade-offs are needed among longitudinal stability,balance and performance.In comparison with a conventional aircraft,a well-designed tailless aircraft is difficult.In the conceptual design,there are many aleatory uncertainties encountered in both aircraft shape and its flight condition.Especially for a tailless configuration,some uncertain factors are sensitive to flight performance and stability.If these uncertainties are not considered in the conceptual design process,the design requirements of aircraft could not probably be satisfied.To deal with these two problems,a new tailless configuration using propeller thrust trimming(PTT)is proposed in this thesis.It is expected that propeller thrust can help to provide a nose-up pitching moment and improve the flight performance of tailless aircrafts.Focusing on this configuration,a small electric-powered tailless unmanned aerial vehicle(UAV)is taken as a case study.The performance benefits and longitudinal stability are studied for the PTT configuration.In addition,the endurance robust optimization is implemented for a small tailless UAV using PTT.The main research issues and achevements in this dissertation are as follows:(1)The PTT configuration is proposed,and the preliminary design is studied for the small tailless UAV using PTT.Firstly,the principle of PTT that improves the flight performance of tailless aircrafts is illustrated.Then,the aerodynamic configuration of the small tailless UAV using PTT is investigated,and the parameter selections of the wing,winglet and control surface are qualitatively analyzed on the base of stability,trim and aerodynamic performance.(2)To evaluate the aerodynamic benefits of the small tailless UAV using PTT,an experimental study is conducted using the wind tunnel.For comparison,two tailless configurations,namely the conventional configuration trimmed by the elevon and the PTT configuration trimmed by propeller thrust are tested.The wind tunnel data indicate that with the help of PTT,the lift to drag ratio and maximum lift coefficient are increased by 15% and 16% in case a moderate pylon height is achieved.Moreover,the aerodynamic computational method for the small UAV is also verified by the wind tunnel test.(3)To evaluate the endurance performance,the conceptual design model of a small electric-powered UAV using PTT is established.Considering the particularity of the PTT configuration and electric-powered UAV,the following four aspects are focused.Firstly,taking into account the effects of the thrust line offset,center of gravity(CG)offset and propeller thrust vectoring,the mathematical models of longitudinal stability and trim are developed for the PTT configuration.Secondly,in accordance with the models of the brushless motor and propeller at angles of attack relative to the propeller axis,the electric propulsion system is designed with a high efficiency.Thirdly,the mathematical model of battery discharge time is established assuming a constant power consumption.The endurance formula of the electric-powered UAV is deduced based on this model.Fourthly,according to the aircraft dimension,the structural weight is estimated by the sum of all components,and then the weight model of the electric-powered UAV is builded.(4)To determine the endurance benefits of PTT configuration,the conceptual parameters of a small electric-powered tailless UAV using PTT are optimized based on the conceptual design model.Two other conventional tailless configurations which are trimmed by the elevon are designed for comparison.One employs a cambered airfoil and the other employs a reflexed airfoil.The optimization results suggest that using propeller thrust to trim the pitching moment,the tailless aircraft is lighter weight and achieves higher lift to drag ratio under the condition that satisfies the design specifications.The PTT tailless configuration results in 24.2% and 40.4% endurance increase compared with the two other conventional tailless UAVs.Flight tests of a 2.5kg built aircraft demonstrate the aerodynamic feasibility of the PTT configuration as well as the validity of the conceptual design model for small electric powered UAVs.(5)According to the optimal geometry of the small tailless electric-powered UAV,the effects of conceptual parameters on the longitudinal static stability,longitudinal trim and longitudinal dynamic stability are explored for the PTT configuration.The analysis results show that the CG offset has significant influences on longitudinal static stability.When the CG location is lowered below the aircraft neutral point,the longitudinal static margin of PTT configuration is increased and varies nonlinearly with respect to the lift coefficient.While increasing the CG offset results in an increased longitudinal static margin,it increases a nose-down pitching moment with the associated performance penalty.The thrust line offset and propeller thrust vectoring have negligible impacts on longitudinal static stability.However,these two factors increase the period of phugoid mode and decrease the damping of phugoid mode.Meanwhile,they also determine the zero-lift pitching moment and longitudinal trim of PTT configuration,respectively.(6)The endurance robust optimization is presented for the small electric-powered tailless UAV using PTT.Through Sobol' global sensitivity analysis,13 geometric design variables and flight condition parameters that have notable effects on endurance and constraints are selected.These parameters are divided into 5 uncertain problems for the small electric powered tailless UAV using PTT.Based on the surrogate models,the robust design optimization theory is applied to solving 5 uncertain problems through multi-objective genetic algorithm.After an optimization considering the uncertainties,the UAV endurance is insensitive to the variations of geometric design variables and flight condition parameters.The constraints in the UAV design process are also satisfied with much higher probabilities.Finally,according to the robust optimization results,the design principle of endurance and constraints robustness is summarized for the small electric-powered tailless UAV using PTT.