Seong-Wook Choi

Development and Conversion Flight Test of a Small Tiltrotor Unmanned Aerial Vehicle

T HE tiltrotor configuration was selected as the unmanned aerial vehicle (UAV) platform for the Smart UAV (SUAV) development program, which is one of the 21st Frontier research and development programs supported by the Korean government [1]. Some of the subsystems of the SUAV, such as the rotor system, drive system, andflight control system, were especially challenging for the Korea Aerospace Research Institute to develop, due to lack of previous development experience. To reduce the risk of failure during the development, the ironbird test of the rotor and drive system and flight simulation was conducted. Although the flight simulation mitigated the risk in the flight control system development, a downscaled model was constructed in an effort to ensure the safety in the flight test of the full-scale vehicle. The small-scale platform is shown in Fig. 1. It is expected from the flight test of the small scale model to enhance the understanding of the features of the actual tiltrotor vehicle. The size of the downscaledmodelwas determined to be 1=2:5 (2m overall length) of the full-scale SUAV (5 m overall length) so as to have large enough space for the simple flight control computer, the navigation system, and the other components such as the engine and actuators. The aerodynamic performance of the 1=2:5-scale tiltrotor was analyzed using the in-house performance code. The calculated performance data were used for scheduling the control devices, such as the collective pitch of the rotors and the flaperon deflection, in the flight control logic. In this Note, sizing and performance analysis of the downscaled tiltrotor are presented in which simple codes based on blade element and momentum theory were used. The conversion corridor of the tiltrotor was predicted and the nacelle tilt angle and air speed were compared with flight-test results. After a series of progressive flight tests, conversion flight from helicopter to fixed-wing mode was accomplished. This verified that the stability and control augmentation algorithmwork properly in the flight control software. This small tiltrotor flight test is expected to reduce risks in flight test of the 5-m-span full-scale tiltrotor UAV called Smart UAV. II. Sizing of the Tiltrotor

Envelop expansion flight test of flight control systems for TR-60 tilt-rotor UAV

Flight tests of TR-60 in order to evaluate the control performance in helicopter, conversion and airplane mode were completed. In order to reduce the development risk and schedule, single channel out of the dual flight control computer of Smart UAV (TR-100) was used without the other modification. Therefore the operational flight program (OFP) had the minimum changes only on control laws. Even though similar flight control system was applied to the scaled tilt-rotor aircraft, named as TR-60, many unexpected anomalies occurred during flight test from hover to airplane mode mainly due to the different bandwidth between TR-100 and TR-60. This paper presented the details of flight test data and anomalies which occurred during flight test of TR-60 as well as the trouble shooting results.

Trim Range and Characteristics of Autorotation(II): Advance Ratio Variation and Flapping Characteristics

The flapping characteristics and advance ratios at torque equilibrium state of autorotation were investigated when the airspeed, shaft angle, and pitch angle were varied. To simulate the airspeed increase, the aerodynamic data analyzed by using the compressible Navier-Stokes solver and Pitt/Peters inflow theory were used. Transient Simulation Method(TSM) was used to catch the torque equilibrium states. The maximum flapping angles at torque equilibrium state were correlated to the airspeed, shaft angle, and pitch angle. By comparing flapping behavior to the variation of advance ratio, the phenomenon that the extension of reverse flow area of retreating blade affects the characteristics of autorotation was qualitatively considered.

Ground test results of rotor governor and rate SAS for small tilt rotor UAV

A scaled flight model of a tilt rotor aircraft for the smart UAV program has been developed. Because the flight characteristics of tilt rotor are not well known, the developed scaled model would be helpful to evaluate flight control algorithm of full scale aircraft. Prior to conducting the flight test of this scaled model, we performed the ground test using the 4-degrees of freedom ground test rig in order to evaluate the flight control system of scaled model and control algorithms. The ground test rig allows freedoms in pitch, roll, yaw and heave motions. A rotor governor algorithm and rate SAS feedbacks were evaluated on the ground test rig under hovering power condition. A rotor governor is usually used for tilt rotor aircraft instead of engine governor due to the speed sensitivity caused by a collective pitch deflection in airplane mode. In order to increase the response of vertical axis in helicopter mode, a collective pitch gradient as a function of the throttle command is used for direct collective control similar to engine governor. Through a performance evaluation of the proposed rotor governor in the test rig, an optimal RPM was selected. The performance of rate SAS for pitch, roll and yaw axes were also evaluated qualitatively on the test rig. This paper describes the ground test results of the rotor governor and rate SAS on the 4-degrees of freedom ground test rig.

Aerodynamic Design of the SUAV Proprotor

The aerodynamic design of a proprotor for the Smart UAV adopting tiltrotor aircraft concept is conducted in this study. Since proprotor of tiltrotor aircraft is operated at both rotary and fixed wing mode with single configuration rotor, the proprotor has to be designed to meet performance requirements for both flight modes. The aerodynamic design of proprotor is accomplished by combining three sources of data - the proprotor performance data, the aerodynamic data of vehicle, and the performance data of engine. The performance analysis code for proprotor is based on the combined momentum and blade element theory and validated by comparison with the TRAM data. In order to design configuration for a proprotor satisfying requirements for both rotary and fixed wing mode, various kind of performance maps are constructed for many performance and configuration parameters. From the analysis the twist angle of 38 degrees and the solidity of 0.118 are decided to be the optimal geometric parameters for both operating conditions.

Trim Range and Characteristics of Autorotation(I): Rotor Speed Limit and Pitch Range

Numerical analysis has been performed to investigate the rotor speed and pitch range variations when the airspeed is increased in autorotation. Transient Simulation Method(TSM) was used to obtain the steady states of autorotation. The rotor blade was analyzed by the two-dimensional compressible Navier-Stokes solver in order to adapt to the airspeed increase and the results were used in the transient simulation method. Meanwhile, the Pitt/Peters inflow theory was used to supply the induced velocity fields. For the prescribed torque equilibrium state, the combinations of velocity, shaft angle, and pitch angle were produced to investigate the rotor speeds and variable ranges. The rotor tip Mach number and rotor speed were correlated and the trim range of pitch angle was observed with respect to the shaft angle decrease.

Ironbird Ground Test for Tilt Rotor Unmanned Aerial Vehicle

The power plant system of a tilt rotor unmanned aerial vehicle (UAV) was verified by the Ironbird ground test, which considerably reduces cost and risk during the developmental stages. The function and performance of the engine, drive line, nacelle conversion, and rotor systems were evaluated using a building block test approach. The Ironbird test concept facilitates the discovery of potential faults in earlier stages of the testing period. As a result, the developmental testing period could effectively be shortened. The measured test data acquired through a ground control and data acquisition system exhibited satisfactory results which meet the developmental specifications of a tilt rotor UAV.

Collision Avoidance Maneuver Simulation of Tilt Rotor Unmanned Aerial Vehicle

The collision avoidance maneuver flight simulation fo r tilt rotor unmanned aerial vehicle was performed by time -accurate numerical integration method based on wind tunnel test data. Five representative collision avoidance maneuver s were simulated under constraints of aerodynamic stall, propulsion power, stru ctural load, and control actuator capability. The co llision avoidance performance s of the maneuvers w ere compared by the computed collision avoidance time s. The sensitivities of initial flight speed and collision zone shape on the collision avoidance time were investigated. From these results, i t was found that the moderate pull -up turn maneuver defined using moderate pitch and maximum roll controls within simulation constraints is the most robust and efficient collision avoidance maneuver under the various flight speeds and collision object shapes i n the tilt rotor UAV applications.

Aerodynamic Design of SUAV Flaperon

Smart UAV, which adopting tiltrotor aircraft concept, requires vertical take-off and landing, long endurance and high speed capability. These contradictable flight performances are hard to meet unless the operation of flap system which should reveal optimal performance for each flight mode. In order to design SUAV flaperon satisfying the three performance requirements, various configurations are generated and their aerodynamic performances are analyzed using numerical flow computations around flap systems. Considering aerodynamic performance and structural simplicity, a final flap configuration is selected and the performance is validated through the wind tunnel testing for 40% scale model.

High Speed Wind Tunnel Test on the Aerodynamic Load Characteristics of Rocket Nozzle

The high-speed wind tunnel test of rocket model was performed to investigate the effect of skirt configuration on aerodynamic load characteristics of nozzle. Test parameters were the length and diffusing angle of skirt. Test results showed that the gimbals actuator power could be reduced to 1/10 of that without skirt. The normalized test result was proposed to be used as database for skirt design.