Chang-Sun Yoo

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.

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.

Airflow angle and wind estimation using GPS/INS navigation data and airspeed

The aircraft autopilot needs the feedback of airflow angles, that is, angle-of attack and sideslip angle to control the aircraft. The airflow angles measured by a mechanical vane are not accurate and very noisy for low airspeed. Moreover, the vane has a mechanical failure risk. This paper describes the extended Kalman filter based method to estimate the airflow angles and three-dimensional wind speed under constant wind condition. In addition, it can correct the scaling error of the airspeed of an aircraft. It uses the airspeed measurements, constant wind condition and the sideslip angle computed from GPS/INS navigation data and stability and control derivatives estimated from flight data. Simulation results show that the proposed method works well in various conditions. It is expected that estimated airflow angles will be able to replace the airflow angles measured by a primary system. Estimated wind speed can also be used to reconstruct the GPS/INS navigation system by correcting the airspeed in the case of GPS failure.

Hardware-In-the-Loop simulation test for actuator control system of Smart UAV

The Smart UAV is the tilt rotor type of unmanned aerial vehicle that has been developed by KARI since 2002. According to the characteristics of tilt rotor aircraft, it has a flight envelope of VTOL(vertical take-off and landing) and high cruise speed. Because the conversion flight between helicopter and airplane mode is required, the actuator system of Smart UAV control surfaces has 12 actuators and 5 controllers for nacelle tilt as well as flaperon, elevator and rotor. Based on Fly-By-Wire system, all actuators except for elevator are an electric linear type with dual motors and RVDT sensors. Actuators for elevator are an electric rotary type with single motor and are equipped on each control surface of two elevators. Controllers for activating actuators were developed in domestic company by interfacing with flight control computer through ARINC 429 and with actuators through analog cable. To accomplish the safe conversion flight between helicopter and airplane mode, a role change of control surfaces according to tilting angle is also required on time. This requires the high reliable and accurate performance for actuator system that consists of actuators, controllers and flight control computer. Prior to actual flight test, this actuator control system was tested and evaluated by Hardware-In-Loop(HIL) simulation on the ground. This paper describes the configuration of actuator control system and HILS system of Smart UAV and gives the results of HILS test.

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.

Development of Simulation Program for Tilt Rotor Aircraft

VTOL(Vertical Take-Off and Landing) aircraft is attractive due to the reason that it is not necessary to have long runway. However a rotorcraft has a definite limitation to fly at the high speed due to the stall at the tip of rotor. To solve this problem, tilt rotor, tilt wing and lift fan were researched and developed. It was verified that the tilt rotor aircraft among them was more effective in disk loading. On this basis, the tilt rotor aircraft has been made into XV-15, V-22, BA-609 and Eagle Eye. This paper shows a nonlinear simulation program for general tilt rotor aircraft that was developed in order to validate the flight characteristics of tilt rotor aircraft and verified through the simulation analysis.

Simulation based on motion platform for tilt rotor UAV shipboard landing

Tilt Rotor UAV of KARI has been developed for ground surveillance, of which performance has been verified through flight test in 2013. Today the use of UAV becomes popular in the field of not only surveillance but also reconnaissance, rescue, science investigation, and home delivery etc. Tilt rotor UAV has a merit of vertical take-off and landing, so that it is convinient to be used in the oceanic operation of UAV with shipboard landing. However the operation environment of UAV in ocean is much severer than the one on the ground and very risk because of small landing area, nonuniform wind and moving landing point. Therefore it is necessary for a trial of shipboard landing to be prepared and evaluated in a simulated oceanic environment on the ground. KARI has developed the environment of flight simulation based on motion platform which provides the movement of ship according to sea conditions and evaluated the shipboard landing. This paper describes the simulation environment of shipboard landing based on the motion platform and gives the simulation results and the evaluation effectiveness.

Sea Wave Modeling Analysis and Simulation for Shipboard Landing of Tilt Rotor Unmanned Aerial Vehicle

The mission of UAV has been expanded from a land to an ocean based on an enhancement of its technologies. Korea Aerospace Research Institute (KARI) also tries to expand the mission of tilt rotor UAV to an ocean, in which the shipboard landing of UAV is required. However the environment of an oceanic operation is severer than that of land due to salty, fogy, and windy condition. The landing point for automatic landing is not fixed due to movement of shipboard in roll, pitch, and heave. It makes the oceanic operation and landing of UAV difficult. In order to conduct an oceanic operation of tilt rotor UAV, this paper presents that the sea wave modeling according to the sea state is conducted and the shipboard landing of tilt rotor UAV under the sea wave is tested and evaluated through the flight simulator for UAV.The mission of UAV has been expanded from a land to an ocean based on an enhancement of its technologies. Korea Aerospace Research Institute (KARI) also tries to expand the mission of tilt rotor UAV to an ocean, in which the shipboard landing of UAV is required. However the environment of an oceanic operation is severer than that of land due to salty, fogy, and windy condition. The landing point for automatic landing is not fixed due to movement of shipboard in roll, pitch, and heave. It makes the oceanic operation and landing of UAV difficult. In order to conduct an oceanic operation of tilt rotor UAV, this paper presents that the sea wave modeling according to the sea state is conducted and the shipboard landing of tilt rotor UAV under the sea wave is tested and evaluated through the flight simulator for UAV.

Sea wave modeling and shipboard landing simulation of tilt rotor unmanned aerial vehicle

Shipboard landing of Vertical Take-Off and Landing (VTOL) aircraft is an important consideration to operate aircraft in the ocean. The oceanic operation of aircraft has more difficult environment than the one on the ground due to salty, fogy, and windy condition. Especially the gust and the vortex around the structures on the ship make the approach of aircraft difficult. The movement of ship board in roll, pitch, and heave by wind and sea wave also makes the landing of aircraft severe. Tilt rotor UAV developed by Korea Aerospace Research Institute (KARI) basically aimed to be operated on the ground and to be expanded to an oceanic operation. The simulation environment of UAV to evaluate a various flight condition also considers to be applied to the oceanic operation. In this paper, the sea state conditions according to amplitude of sea wave and wind velocity was investigated and the mathematical description of sea state through sea wave modeling was conducted. In the condition of each sea state modeled with sea wave and wind, the shipboard landing of tilt rotor UAV was evaluated in simulation environment.