Coaxial Rotorcraft Research Articles

Conversion of a Coaxial Rotorcraft to a UAV—Lessons Learned

A coaxial helicopter with a maximum take-off weight of 600 kg was converted to an unmanned aerial vehicle. A minimally invasive robotic actuator system was developed, which can be retrofitted onto the copilot seat of the rotorcraft in a short period of time to enable automatic flight. The automatic flight control robot includes electromechanical actuators, which are connected to the cockpit inceptors and control the helicopter. Most of the sensors and avionic components were integrated into the modular robotic system for faster integration into the rotorcraft. The mechanical design of the control system, the development of the robot control software, and the control system architecture are described in this paper. Furthermore, the multi-body simulation of the robotic system and the estimation of the linear low-order actuator models from hover-frame flight test data are discussed. The developed technologies in this study are not specific to a coaxial helicopter and can be applied to the conversion of any crewed flight vehicle with mechanical controls to unmanned or fly-by-wire. This agile development of a full-size flying test-bed can accelerate the testing of advanced flight control laws, as well as advanced air mobility-related functions.

Physics-Based Airframe Design Approach Expanded from the Conceptual Design of Coaxial Rotorcraft

Physics-Based Airframe Design Approach Expanded from the Conceptual Design of Coaxial Rotorcraft

Individual Blade Control Approach for Active Vibration Suppression of a Lift-Offset Coaxial Rotorcraft

This study explores the best vibration reduction using an individual blade control (IBC) actuation scheme for a lift-offset coaxial helicopter in high-speed flight. The rotorcraft dynamics analysis model consists of coaxial, three-bladed counter-rotating rotors and a finite element fuselage stick model constructed based on the measured data of the XH-59A helicopter. The XH-59A helicopter is well-known for its severe vibration level encountered during the flight (over 0.5g), leading to the cancellation of the development program. A special focus is given to assessing the accuracy and efficiency of the integrated rotor–body vibration predictions evaluated between the one-way and two-way coupling methods in reference to the flight data of the vehicle. The two-way rotor–body coupled results show excellent correlations with the test data for rotor blade loads and airframe vibrations. The best actuation scenarios are then sought for the minimum vibration at the rotor hub and the pilot seat. The IBC pitch actuation effectively reduces the vibrations at both locations of the rotorcraft. Specifically, a multiple-harmonic IBC actuation enables to suppress the pilot seat vibration by 93% compared to the uncontrolled case, achieving a significantly reduced vibration level (below 0.05g).

Three-Dimensional Aeromechanical Analysis of Lift Offset Coaxial Rotors: A Helios Test Case

Three-dimensional (3-D) solid finite element analysis (FEA) is used to model and study coaxial helicopter rotors. The 3-D FEA is coupled with a lifting line aerodynamic model with free wake to capture rotor-rotor interactions. Two open-access models are developed: one is the Metaltail (a hingeless coaxial rotorcraft), and the other is the coaxial rotor built from articulated UH-60A-like rotors. The former is the main focus of this work and is developed as an example case for the U.S. Army/DoD rotorcraft simulation software CREATETM–AV Helios, while the latter is merely for a regression test. The analysis is performed on a low-speed transition flight for which qualitative data are available for the Sikorsky S-97 Raider aircraft for comparison. Predictions of performance, airloads, vibratory loads, and 3-D stresses are discussed. In the absence of available geometry or property data of the S-97, this study is primarily meant to serve as a capability demonstration of 3-D rotor structural dynamic modeling for such rotors.

The Application of Micro Coaxial Rotorcraft in Warfare: An Overview, Key Technologies, and Warfare Scenarios

The mode of future wars is mainly local wars and regional conflicts. This style of war is characterized by unpredictability and instability, which can happen anytime and anywhere. Once it happens, the battlefield is vast, short-range and fast-changing. In order to adapt to the characteristics of this kind of warfare, simple equipment and short reaction time have become two important factors in selecting weapons. Coaxial rotorcraft has been widely studied and applied in future wars due to its convenient carrying, strong environmental adaptability, and rich load capacity. This paper conducts an overview of the Coaxial rotorcraft. Furthermore, we also identify its key technologies, warfare scenarios and future directions. We anticipate that this survey can shed new light on the coaxial rotorcraft systems.

Aeromechanics of a Coaxial Ultralight Rotorcraft During Turn, Climb, and Descent Flight

This work presents the aeromechanics analysis and evaluation of an ultralight counter-rotating coaxial rotorcraft using a combined numerical and experimental approach with a focus on turn, climb, and descent flight conditions. The helicopter features a 2-by-2 bladed coaxial rotor system with teeter hinges, 6.5 m in diameter and uniform airfoil sections of the linearly twisted blades. Hover and forward-flight test data are shown with varying turn rates or climb and descent velocities. The predictive capabilities of a CAMRAD II numerical model are used to extend and support the flight test data and evaluate possible flight-envelope limitations in terms of ceiling limits and maximum climb rates. The paper focuses on rotor performance, thrust, horizontal-stabilizer loads, and blade-tip clearance, with special attention on the effects from interactional rotor aerodynamics. Results in terms of rotor performance, rotor thrust, horizontal-stabilizer loads, and rotorcraft aerodynamic angles of attack resulted in satisfactory correlation between the numerical model and the measurements. Rotor–rotor interactional effects contributed to the approaching rotor-blade tips, especially during low-speed descent flights. During turn flight, required rotor power increases with greater bank angles. Maximum axial climb rate of the ultralight coaxial helicopter was at , the hover ceiling at 2180 m, and the service ceiling at 4450 m. Ceiling limitations depended on the operating conditions and reduced with greater temperature and rotorcraft weight.

Modeling and control of lift offset coaxial and tiltrotor rotorcraft

The US Department of Defense has established an initiative to develop a family of next-generation vertical lift aircraft that will fly farther, faster, and more efficiently than the current fleet of rotorcraft. To accomplish these goals, advanced rotorcraft configurations beyond the single main rotor/tail rotor design must be considered. Two advanced configurations currently being flight tested are a lift offset coaxial rotorcraft with a pusher propeller and a tiltrotor. The US Army Aviation Development Directorate has developed generic, high-fidelity flight-dynamics models of these two configurations to provide the government with independent control-system design, handling-qualities analysis, and simulation research capabilities for these types of aircraft. This paper describes the modeling approach used and provides model trim data, linearized stability and control derivatives, and eigenvalues as a function of airspeed. In addition, control allocation for both configurations is discussed.

개별 블레이드 제어(IBC) 기법을 이용한 동축반전 회전익기의 진동하중 억제에 관한 연구

본 연구에서는 능동적인 블레이드 제어기법인 개별 블레이드 제어(Individual Blade Control, IBC) 기법을 적용하여 고속비행 시 동축반전 회전익기의 허브 진동하중을 억제하기 위한 최적 제어입력을 탐색하였다. 통합 공탄성 해석 프로그램인 CAMRAD II를 이용하여 동축반전 회전익기인 XH-59A를 모델링하고 다양한 IBC 입력 조건에 대하여 파라미터 연구를 수행하였다. 파라미터 조절 연구를 통하여 허브 진동억제 성능을 구한 결과, 3/rev 가진 주파수의 0.5° 진폭에 300° 위상각을 갖는 IBC 제어 입력을 적용할 경우 기준 대비 진동 수준이 최대 50% 감소하는 것을 확인하였다. 진동 억제 성능은 후류 간섭에서 보다 자유로운 상부로터에서 6% 가량 하부로터보다 크게 나타났다. 로터의 전진면에서만 IBC 입력를 가진하는 경우에는 조화 가진 입력과 동일한 입력을 가할 경우 진동 수준이 최대 17% 정도 추가적으로 감소하는 것을 확인하였다. 이러한 진동 감소는 전진면만을 대상으로 적은 에너지 투입 비용으로 달성한 특징이 있다.

Numerical and experimental investigation of an airfoil design for a Martian micro rotorcraft

The present paper aims at investigating the impact of an airfoil design on the propulsion system for a Martian rotary wing micro air vehicle. The main challenge for flying on Mars is the atmosphere’s density and speed of sound that are significantly lower than on Earth. It leads to compressible ultra-low Reynolds number ([Formula: see text]) flows for a coaxial rotorcraft with a 30 cm diameter . Since those flows are unknown in the biosphere, numerical tools have not been validated yet. Therefore, the test section from a known depressurized experiment is simulated in 3D for solver assessment. XFoil, a program for the analysis of subsonic airfoil, is also evaluated in the Martian flow conditions for evaluating its ability to be used in an optimization process. Based on the XFoil’s performance evaluations, both the camber line and thickness distribution are optimized in 2D incompressible and compressible flows. Optimal shape is a highly cambered airfoil shifting the boundary layer separation downstream. For optimization assessment, airfoils from each optimization step are numerically evaluated with the validated solver showing that small variations in the airfoil design has little impact on the 2D aerodynamic performance. A compressible optimal airfoil is also proven to be more aerodynamically effective than the experimentally effective airfoils from literature. Finally, the impact of airfoil shapes on the 3D rotor performance is evaluated with a depressurized experimental campaign recreating the Martian atmosphere in terms of kinematic viscosity. The impact of gas composition is also assessed in subsonic flows.

Comprehensive Analysis of a Coaxial Ultralight Rotorcraft and Validation with Full-Scale Flight-Test Data

The results of full-scale flight testing in combination with comprehensive analysis of an ultralight counterrotating coaxial rotorcraft are presented. The CoAX 2D was developed and manufactured by edm aerotec in Geisleden, Germany. The helicopter features two-bladed, 6.5-m-diameter upper and lower rotors in teeter configuration with uniform airfoil sections and linearly twisted blades. The focus was on the development of a comprehensive analysis model using CAMRAD II, its validation with measured flight-test data, and results evaluation from both measurements and analysis. The CoAX 2D's power consumption, tail loads, hub loads, teeter angles, and blade-tip clearance were evaluated for both hover and forward flight. Performance and thrust correlated well between the analysis and the measurements. Effects from rotor-fuselage interference dominated the measured 2/rev hub load and 1/rev teeter angle harmonics. The numerical model also matched well with the measured vibrations during forward flight in terms of hub loads, teeter angles, and resulting blade-tip clearance. Minimal rotor-rotor blade-tip clearance occurred at low flight speeds because of dominating effects from rotor-rotor interference.

Mission optimisation for a conceptual coaxial rotorcraft for taxi applications

This paper presents the development and an application of a multidisciplinary methodology for the preliminary design assessment of compound coaxial rotorcraft with a counter-rotating rotor system and a rear-mounted propeller. A comprehensive optimisation strategy is deployed to evaluate the environmental and operational benefits of the aforementioned rotorcraft architecture. The code is validated against experimental data prior to the application of the methodology to the evaluation of a conceptual vehicle for intercity taxi applications. Response Surface Models (RSMs) are generated to mimic the rotorcraft performance in order to accelerate the optimisation process. The effects of the defined mission input parameters such as cruise speed, altitude, climb rate or mission length are evaluated. Pareto fronts for fuel burn, NOx emissions and mission duration are obtained. The method was applied to a hypothetical scenario of mission length ranging from 50 to 300 km. Best estimate mission scenario are selected from the Pareto fronts, providing on average 23%, 20%, and 13% simultaneous reductions in mission duration, fuel burn, and NOx emissions when compared to a conventional flight procedure. The picked scenarios coincide with the fuel optimised mission scenarios for each mission length, thus the multi-disciplinary environment was not required. Besides, an “improved” mission procedure is outlined, defining the mission characteristics independently of the mission's length. This procedure yields on average 22%, 14%, and 8% reductions in mission duration, fuel burn, and NOx emissions, respectively.

Mission performance analysis of a conceptual coaxial rotorcraft for air taxi applications

The rotorcraft industry has recently shown a new interest in compound rotorcraft as a feasible alternative to tackle the rapid growth of civil aviation activities and associated environmental impact. Indeed, aircraft contribution to the global emissions of CO2, NOx, and noise are driving the development of innovative technologies and vehicles. At present, compound rotorcraft architectures are regarded by the industry as promising platforms that can potentially increase productivity at a reduced environmental cost. In order to quantify the benefits of compound rotorcraft, this paper details the performance analysis of a coaxial counter-rotating rotor configuration with a pusher propeller. A comprehensive approach targeting the assessment of the aforementioned rotorcraft design for civil applications is presented herein. The methodology developed encompasses a rotorcraft flight dynamics simulation module and an engine performance module, coupled with a gaseous emissions prediction tool for environmental impact studies. They have been integrated together to constitute a standalone performance simulation framework and verified with the performance calculations of Harrington's “rotor 1” and the Sikorsky X2TD. The method is then applied to evaluate the performance of a conceptual coaxial rotorcraft, during a notional inter-city air taxi mission, in terms of cruise altitude, speed, and range, overall mission time and environmental impact. The several trade-offs between these parameters highlight the need for an integrated optimisation process. Besides, the concept demonstrates the benefits of the compound rotorcraft architecture with a best range speed reaching 90 m/s leading to reduced response times and increase of round trips in a given time. As a consequence, operators will need fewer vehicles and heliports to cover the same areas. This outcome is highly attractive in the current growing market.

Conceptual Study of a Smart Docking System for VTOL-UAV

AbstractThis paper describes the conceptual design of a vertical takeoff and landing unmanned aerial vehicle (VTOL-UAV), mountable on an unmanned ground vehicle (UGV) for their combined operation as a team. The design aims to compensate for the weak points of each platform by providing carrying, launching, recovery, and recharging capabilities for the mounted VTOL-UAV through the host UGV. A sphere-shaped mechanical docking interface between the UAV–UGV is proposed to allow flexible and accurate positioning, and its operational concept with a coaxial rotorcraft UAV is suggested as one particular instance. A vision-based, target-tracking method to enhance the precision landing performance also was investigated and tested with a ground-based experimental setup.

Development of an Unmanned Coaxial Rotorcraft for the DARPA UAVForge Challenge

In this paper, we present a comprehensive design for a fully functional unmanned rotorcraft system: GremLion. GremLion is a new small-scale unmanned aerial vehicle (UAV) concept using two contra-rotating rotors and one cyclic swash-plate. It can fit within a rucksack and be easily carried by a single person. GremLion is developed with all necessary avionics and a ground control station. It has been employed to participate in the 2012 UAVForge competition. The proposed design of GremLion consists of hardware construction, software development, dynamics modeling and flight control design, as well as mission algorithm investigation. A novel computer-aided technique is presented to optimize the hardware construction of GremLion to realize robust and efficient flight behavior. Based on the above hardware platform, a real-time flight control software and a ground control station (GCS) software have been developed to achieve the onboard processing capability and the ground monitoring capability respectively. A GremLion mathematical model has been derived for hover and near hover flight conditions and identified from experimental data collected in flight tests. We have combined H∞ technique, a robust and perfect tracking (RPT) approach, and custom-defined flight scheduling to design a comprehensive nonlinear flight control law for GremLion and successfully realized the automatic control which includes take-off, hovering, and a variety of essential flight motions. In addition, advanced mission algorithms have been presented in the paper, including obstacle detection and avoidance, as well as target following. Both ground and flight experiments of the complete system have been conducted including autonomous hovering, waypoint flight, etc. The test results have been presented in this paper to verify the proposed design methodology.

Fully Autonomous Indoor Flight Relying on Only Five Very Low-Cost Range Sensors

Answering what is the bare minimum combination of avionics and vehicle platform design that enables useful autonomous indoor flight without a Global Positioning System (GPS) is required to understand the lower-cost bounds for indoor unmanned aerial system (UAS) technology. The GTLama indoor UAS, capable of autonomously exploring indoor areas in the GPS-denied environment discussed here, was designed in an attempt to answer this question. TheGTLamaweighs around 600 g (1.3 lb), has a diameter of approximately 70 cm (27.6 in.), and costs less than 900 U.S. dollars. Cost and weight savings are realized through development of an innovative indoor guidance, navigation, and control algorithm that uses only five small off-the-shelf low-cost range sensors rather than expensive and sophisticated sensors such as inertial measurement units, scanning laser range sensors, andGPS. The onboard software leverages the inherent stability of the coaxial rotorcraft platform and fuses information from the low-fidelity range sensors to detect and follow walls and other stationary obstacles. The algorithms are designed to encourage maximum exploration of the perimeter of an indoor environment in a reasonable amount of time. The avionics design and the guidance, navigation, and control algorithms used are discussed in detail and validated through flight tests.

DESIGN AND CONSTRUCTION METHODOLOGY OF AN INDOOR UAV SYSTEM WITH EMBEDDED VISION

This paper describes the design and construction of an autonomous indoor coaxial rotorcraft system with onboard vision processing capability. A radio-controlled (RC) helicopter was modified and upgraded with essential avionics and processing elements for autonomous flight. This paper shows that in an indoor environment, where GPS signals are unavailable, vision-based navigation system can be used as a substitution. An embedded vision system that is compact and light enough to be carried by indoor UAVs is integrated into the control loop. The system has been successfully tested and the UAV was able to follow a painted track on the ground autonomously.

무인지상차량과의 합동운용을 위한 동축반전 회전익형 무인항공기 개념연구

UAV-UGV teaming concept has been proposed that can compensate for weak points of each platform by providing carrying, launching, recovery and recharging capability for the VTOL-UAV through the host UGV. The teaming concept can expand the observation envelop of the UGV and extend the operational capability of the UAV through mechanical combination of each system. The spherical-shaped coaxial rotorcraft UAV is suggested to provide flexible and precise interface between two systems. Hybrid navigation solution that included vision-based target tracking method for precision landing is investigated and its experimental study is performed. Feasibility study on length-variable rotor to provide the compact configuration of the loaded rotorcraft platform is also described.

동축반전 헬리콥터형 소형 무인항공기 설계 및 제작

The rotorcraft-based unmanned aerial vehicle(UAV) capable of performing close-range surveillance and reconnaissance has been developed. Trade studies on mission feasibility led to the adoption of a coaxial rotorcraft with twin rotors counter-rotating in one axis and driven by electric motors. A commercial off-the-shelf flight control computer(FCC) and a radio frequency modem were adopted for autonomous navigation. In order to achieve an aerial view, commercial charge-coupled device camera was also integrated into the vehicle. The performance of the completed vehicle was proved with manual flight test, and mission capability was verified through waypoint navigation flight after being equipped with FCC. This paper treats the whole process of design and system integration for development of the coaxial rotorcraft UAV.

Modeling and Global Control of the Longitudinal Dynamics of a Coaxial Convertible Mini-UAV in Hover Mode

The aim of this paper is to present a configuration for a Convertible Unmanned Aerial Vehicle, which incorporates the advantages of the coaxial rotorcraft for hover flight and the efficiencies of a fixed-wing for forward flight. A detailed dynamical model, including the aerodynamics, is obtained via the Newton-Euler formulation. It is proposed a nonlinear control law that achieves global stability for the longitudinal vertical-mode motion. Indeed, we have performed a simulation study to test the proposed controller in presence of external perturbations, obtaining satisfactory results. We have developed an embedded autopilot to validate the proposed prototype and the control law in hover-mode flight.