Sam-Ok Koo

Flight test results of automatic tilt control for small scaled tilt rotor aircraft

A small scaled flight model of the tilt rotor aircraft for the Smart UAV Program at KARI(Korea Aerospace Research Institute) has been developed and tested. Since the flight characteristics of a tilt rotor aircraft are new to KARI, the scaled model was used as a test bed to evaluate the flight control algorithm for the full scale smart UAV. The flight test of the small scaled model was performed after various ground tests including tethered hover test. The control laws in initial phase consist of rate SAS feedbacks, control surface mixers, a rotor governor and a manual tilt command path. A rate feedback SCAS control law was used in order to evaluate the flight characteristics of the tilt rotor aircraft. As the flight test proceeds, an attitude SCAS was added because the attitude of aircraft was not clearly recognized due to the small size and fast speed of the aircraft. The first full conversion to a fixed wing mode was made through the manual tilt command by the external pilot. And then the automatic conversion was successfully performed by speed hold command in compliance with a pre-defined conversion corridor. Several problems unexpected were found during flight tests including oscillation of long period mode near helicopter mode, a delayed response to the altitude command and etc. The flight test results of the small scaled tilt rotor aircraft using an automatic tilt control is described in this paper and the solutions of the problems noticed in the flight test are presented.

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

Performance test and simulation of a reciprocating engine for long endurance miniature unmanned aerial vehicles

Abstract Development of an engine with good fuel economy is very important for successful implementation of long endurance miniature UAVs (unmanned aerial vehicles). In the study, a four-stroke glow-plug engine was modified to a gasoline-fuelled spark ignition engine. Engine tests measuring performance and friction losses were conducted to tune a simulation program for performance prediction. It has been found that excessive friction losses are caused by insufficient lubrication at high speeds. The simulation program predicts that engine power and fuel economy become worse with high altitude, due to an increasing portion of friction losses. The simulation results suggest quantitative guidelines for further development of a practical engine.

Control Law Modification According to Flight Test of Small Scaled Tilt Rotor UAV

A small scaled flight demonstration model of a tilt rotor aircraft for the Smart UAV Program has been developed. The tethered hover test following the ground test on a test rig with 4 degrees of freedom has been performed prior to the flight test of the scaled model. During the flight test and tethered hover test, the performances of a rotor speed governor and a rate SAS have been evaluated. Because the pilot could not confirm the aircraft attitude at high speed flight condition at low tilt angle, the control structure was modified to attitude SCAS system in pitch and roll axis. All conversion flight tests were performed by pilot’s manual control and the attitude SCAS control law and rotor governor were fully engaged. The pilot had to input the tilt commands during coordinated turn flight in order for the aircraft not to exceed pilot’s visual range. The rate SAS feedback system was also enough to meet handling qualities requirement at all axis so that it would be work as a manual mode in emergency condition. During flight test, the control law has been modified to improve many unexpected aircraft responses. This paper presents the conversion flight test results of small scaled tilt rotor model and also shows the control law updates according to unexpected flight characteristics of tilt rotor aircraft.

Ground test results of flight control system for the Smart UAV

The ground test results of Smart UAV are described. The ground test consists of HILS, iron-bird test, 4-DOF (degrees of freedom) ground rig test, and tethered hover test. One of the main purposes of ground test is to evaluate the integrated flight control system including a digital flight control computer, GPS/INS sensor, air data sensors, etc as well as ground control system (GCS). The performances of whole flight control systems were evaluated. Rotor speed was kept constantly by the rotor governor even during collective sweep and pitch/roll attitude SCAS (Stability and Control Augmentation System) showed good handling qualities performance during 4-DOF and tethered hover test. These control laws and guidance law will be evaluated during the flight test of envelope expansion planed for this year. This paper presents the ground test results and the procedure for Smart UAV.

Development of Flight Control System and Troubleshooting on Flight Test of a Tilt-Rotor Unmanned Aerial Vehicle

The full results of troubleshooting process related to the flight control system of a tilt-rotor type UAV in the flight tests are described. Flight tests were conducted in helicopter, conversion, and airplane modes. The vehicle was flown using automatic functions, which include speed-hold, altitude-hold, heading-hold, guidance modes, as well as automatic take-off and landing. Many unexpected problems occurred during the envelope expansion tests which were mostly under those automatic functions. The anomalies in helicopter mode include vortex ring state (VRS), long delay in the automatic take-off, and the initial overshoot in the automatic landing. In contrast, the anomalies in conversion mode are untrimmed AOS oscillation and the calibration errors of the air data sensors. The problems of low damping in rotor speed and roll rate responses are found in airplane mode. Once all of the known problems had been solved, the vehicle in airplane mode gradually reached the maximum design speed of 440km/h at the operation altitude of 3km. This paper also presents a comprehensive detailing of the control systems of the Tilt-rotor unmanned air vehicle (UAV).

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.

Ground and flight test of ADS-B system in Goheung area

Todays the integration of UAS (Unmanned Aircraft Systems) in the national airspace system becomes issued. In Goheung area of South Korea, there is the Aeronautical Center of KARI (Korea Aerospace Research Institute) that the flight test of UAS and small aircraft has been conducted. To support a tracking of ground movement and flight trajectory of a test vehicle, ADS-B (Automatic Dependent Surveillance - Broadcast) was considered. 1090ES (Extended Squitter) ADS-B ground system was implemented in 2013 and UAT (Universal Access Transceiver) ADS-B is planned in 2014. This gives the result that ADS-B system has been implemented and evaluated through small aircraft on the Goheung Aeronautical Center (GAC), and its additional applications.

Test and Simulation of An Engine for Long Endurance Miniature UAVs

Development of an engine with good fuel economy is very important for successful implementation of long endurance miniature UAVs (unmanned aerial vehicles). In the study, a 4-stroke glow-plug engine was modified to a gasoline-fueled spark-ignition engine. Engine tests measuring performance and friction losses were conducted to tune a simulation program for performance prediction. It has been found that excessive friction losses are caused by insufficient lubrication at high speeds. The simulation program predicts that engine power and fuel economy get worse with high altitude due to increasing portion of friction losses. The simulation results suggest quantitative guidelines for further development of a practical engine.