Tiltrotor Research Articles

Determining allowable parametric uncertainty in an uncommon quadrotormodel for closed loop stability

In this article, control oriented uncertainty modeling of an uncommon quadrotor in hover is discussed. This quadrotor consists of two counter-rotating big rotors on longitudinal axis and two counter-rotating small tilt rotors on lateral axis. Firstly, approximate linear model of this vehicle around hover is obtained by using Newton--Euler formulation. Secondly, specific uncertainty is assigned to each parameter. Resulting uncertain model is converted into a linear fractional transformation framework for robustness analysis. Next, the most critical uncertain parameters in terms of robust stability in a proposed quadrotor model are investigated using $ \mu $ sensitivities. Finally, skewed-$ \mu $ analysis determines maximum possible uncertainty bounds for model parameters that are difficult to identify accurately.

A review of propeller stall flutter

AbstractResearch on propeller performance has been reinvigorated by the development of new classes of vehicles, ranging from electrically powered fixed-wing aircraft, to multi-rotor electrical Vertical Take-off/Landing (eVTOL) vehicles and tilt-rotor aircraft. These types of aircraft utilise a range of modern propellers, often with more advanced planforms and features such as anhedral, and operate in flight envelopes that are outwith the traditional bands of performance. The use of advanced materials (mostly composites), high geometrical sweeps and variable angular velocities are the source of unsteady aerodynamics, that is often coupled with the blade’s structural response. Data from experimental investigations is mostly historic, with the majority of studies conducted before 1960, when aviation shifted rapidly towards jet propulsion. These studies lack in flutter boundary assessment. Modern propellers are likely to be pushed toward their flutter boundaries, but the experimental database published to-date is insufficient to provide flutter boundary assessment. This review examines the value of the available experimental research and the status of the state-of-the-art numerical methods, in order to establish the requirements for modern research on propeller stall flutter.

Backstepping-based Super-Twisting Sliding Mode Control for a Quadrotor Manipulator with Tiltable Rotors

Designing a robust controller is very important in the control of outdoor unmanned aerial vehicles. This paper presents the design procedures and implementation of super-twisting sliding mode controller, which is a robust nonlinear controller. The robust controller is applied to an over-actuated quadrotor manipulator with four tiltable rotors. A serial manipulator with two links is mounted on the bottom of the quadrotor. The quadrotor possesses the property of decoupling its position and orientation. The main contribute of this paper is that a super-twisting sliding mode controller in vector form is designed and applied to the control of an over-actuated quadrotor manipulator. Another contribution of this paper is that the stability of the closed-loop system is proved by utilizing the Lyapunov stability theory. It is confirmed that the performance of the super-twisting sliding mode controller is superior to that of the conventional backstepping controller in terms of convergence rate and accuracy by simulations.

Trim Envelope Calculations for a Tiltrotor in Forward Flight, Hover and Transitions

Urban Air Mobility (UAM) is a future mode of transportation that will require revolutionary new electronic vertical takeoff and landing (eVTOL) vehicle concepts and operations. One of the challenges for these eVTOL vehicles will be the complexity of the flight control task which requires operation in hover, forward flight and the transition between the two. The trim envelope is an important piece of information for analyzing which airspeeds and either flight path angles or vertical speeds are safely achievable in each flight regime. The largest share of the vehicle concept designs for UAM are some kind of vectored thrust vehicle, highlighting the importance of analyzing these flight regime transitions. This papercalculates and compares the trim envelopes for a generic tiltrotor aircraft model for different rotor tilt angles. Preliminary results confirm a narrow transition corridor that is a well-known characteristic of tiltrotor aircraft

Research on a Fully Actuated UAV with Two 2-DOF Tiltable Rotors

Research on a Fully Actuated UAV with Two 2-DOF Tiltable Rotors

Determination of the aerodynamic characteristics of the tiltrotor with the wingtip-mounted coaxial rotors

The article describes the study of rotor-to-wing aerodynamic interaction for the wingtip-mounted coaxial rotors configuration of the tiltrotor aircraft.
 The influence of the rotor slipstreams on lift-to-drag ratio characteristic was determined. Obtained results were compared with similar characteristics of the equivalent in thrust conventional single rotor slipstreams impact.
 Using the computational aerodynamics methods (panel-vortex method) the flow around the tiltrotor model with the wingtip-mounted single and coaxial rotors has been simulated. A study of the basic model configuration with conventional single rotors, based on the technical characteristics of the AgustaWestland AW609 tiltrotor, was conducted. Further researches were conducted for a modified model where single rotors were replaced with equivalent in thrust coaxial rotors. The influence of the rotor slipstreams on the aerodynamic characteristics of the model for both directions of rotors rotation in coaxial combination is considered. Also, the dependence of the maximum lift-to-drag characteristic due to the coaxial rotor diameters change has been determined.
 The results show that the coaxial rotor slipstreams-to-wing aerodynamic interaction effect is the similar to the effect of conventional single rotor, but less intensive. Comparison of the results showed that a tiltrotor equipped with wingtip-mounted single rotors has approximately 20% greater maximum lift-to-drag characteristic than one equipped with coaxial rotors with the same thrust. However, the use of coaxial rotors allows getting higher maximum speed, when conventional single rotors lose the efficiency significantly. Therefore, it is advisable to conduct further research for the possibility of using coaxial rotors for tiltrotor aircrafts.
 The research results are presented in graphical form. The obtained data provides a basis for further studies of the described problem, and also will be useful for new tiltrotor design works.

Tilt-rotor aircraft fuselage wall aeroacoustics radiation suppression using Higher harmonic control

Aeroacoustics radiation is one of the main sources of tilt-rotor aircraft interior noise. In order to reduce the noise radiation on the surface of tilt-rotor aircraft fuselage, an active rotor control technology based on Higher Harmonic Control (HHC) is proposed. Its control effect is verified by simulation and the control mechanism is studied at the same time. Firstly, a tilt-rotor aeroacoustics prediction model is established based on FW-H equation and Beddoes unsteady aerodynamic model. And the accuracy of the above model is verified according to the flight test data of the XV-15 tilt-rotor aircraft. Furthermore, taking XV-15 as the object, a simulation research of rotor aeroacoustics active control is carried out in both airplane mode and helicopter mode. The simulation research reveals that: the HHC can suppress the aeroacoustics radiation of tilt-rotor to the fuselage through the principle of sound pressure cancellation. By applying appropriate control inputs, the maximum aeroacoustics radiation can be reduced by 9.2 dB in airplane mode; and the maximum aeroacoustics radiation can be reduced by 13.5 dB in helicopter mode.

Study of RCS characteristics of tilt-rotor aircraft based on dynamic calculation approach

To study the Radar Cross-Section (RCS) characteristics of the tilt-rotor aircraft, a dynamic calculation approach that takes into account rotor rotation and nacelle tilt is presented. Physical optics and physical theory of diffraction are used to deal with the instantaneous electromagnetic scattering of the target. The RCS of the aircraft in the helicopter mode, fixed-wing mode and transition mode is analyzed. The results show that in the fixed-wing mode, the blade has a weaker deflection effect on the head incident wave in the horizontal plane. The helicopter mode improves the scattering of the rotor in the horizontal plane, while it increases the scattering source on the surface of the nacelle. At a fixed tilt angle, the RCS of the aircraft under a given azimuth angle still shows obvious dynamic characteristics. Dynamic tilting significantly changes the scattering effects of blades, hubs, nacelles and wingtips. The proposed approach is shown to be feasible and effective to learn the electromagnetic scattering characteristics of the tilt rotor aircraft.

Numerical Investigations for Passive and Active Flow Control on Tiltrotor–Wing Aerodynamic Interactions

AbstractNumerical investigations for passive and active controls on aerodynamic interactions of the rotor/wing of a tiltrotor aircraft are conducted in hovering flight. The moving embedded grid met...

Coupling Mid-Fidelity Aerodynamics and Multibody Dynamics for the Aeroelastic Analysis of Rotary-Wing Vehicles

A mid-fidelity aerodynamic solver based on the vortex particle method for wake modeling, DUST, is coupled through the partitioned multi-physics coupling library preCICE to a multibody dynamics code, MBDyn, to improve the accuracy of aeroelastic numerical analysis performed on rotary-wing vehicles. In this paper, the coupled tool is firstly validated by solving simple fixed-wing and rotary-wing problems from the open literature. The transient roll maneuver of a complete tiltrotor aircraft is then simulated, to show the capability of the coupled solver to analyze the aeroelasticity of complex rotorcraft configurations. Simulation results show the importance of the accurate representation of rotary wing aerodynamics provided by the vortex particle method for loads evaluation, aeroelastic stability assessment, and analysis of transient maneuvers of aircraft configurations characterized by complex interactional aerodynamics. The limited computational effort required by the mid-fidelity aerodynamic approach represents an effective trade-off in obtaining fast and accurate solutions that can be used for the preliminary design of novel rotary-wing vehicle configurations.

Robust smooth transfer for tilt-rotor aircraft under the asynchronous switching

To achieve the flight mode transfer of the tilt-rotor aircraft, this paper develops a smooth switching method for an weighted L2 robust asynchronously switched system. Considering the asynchronous phenomenon that exists in the switching control of the tilt-rotor aircraft, first, a sufficient condition for the existence of the sub-controller is derived, which guarantees the exponential stability and weighted L2 performance. Besides, the on-line system and the off-line system are separately employed to realize the continuity of the control input signals, which further improve the transient performance and smoothness of the asynchronously switched system. The simulations on the tilt-rotor aircraft verify the effectiveness and applicability of the proposed method.

Crashworthiness of a Composite Bladder Fuel Tank for a Tilt Rotor Aircraft

The fulfilment of the crash is a demanding requirement for a Tiltrotor. Indeed, such a kind of aircraft, being a hybrid between an airplane and a helicopter, inherits the requirements mainly from helicopters (EASA CS 29) due to its hovering ability. In particular, the fuel storage system must be designed in such a manner that it is crash resistant, under prescribed airworthiness requirements, in order to avoid the fuel leakage during such an event, preventing fire and, thus, increasing the survival chances of the crew and the passengers. The present work deals with the evaluation of crashworthiness of the fuel storage system of a Tiltrotor (bladder tank), and, in particular, it aims at describing the adopted numerical approach and some specific results. Crash resistance requirements are considered from the earliest design stages, and for this reason they are mainly addressed from a numerical point of view and by simulations that treat both single components and small/medium size assemblies. The developed numerical models include all the main parts needed for simulating the structural behavior of the investigated wing section: the tank, the structural components of the wing, the fuel sub-systems (fuel lines, probes, etc.) and the fuel itself. During the crash event there are several parts inside the tanks that can come into contact with the tank structure; therefore, it is necessary to evaluate which of these parts can be a damage source for the tank itself and could generate fuel loss. The SPH approach has been adopted to discretise fuel and to estimate the interaction forces with respect to the tank structure. Experimental data were used to calibrate the fuel tank and foam material models and to define the acceleration time-history to be applied. Thanks to the optimized foam’s configuration, the amount of dissipated impact energy is remarkable, and the evaluation of tanks/fuel system stress distribution allows estimating any undesired failure due to a survivable crash event.

Research on near field aeroacoustics suppression of tilt-rotor aircraft based on rotor phase control

In order to suppress the near field aeroacoustics radiation on the fuselage wall of the tilt-rotor aircraft, a noise suppression method is proposed based on the rotor phase control. Firstly, based on the free wake model and panel method, a tilt-rotor near field aeroacoustics calculation model is established considering the interference between the rotors and the wing. Then, taking the XV-15 tilt-rotor aircraft as object, a simulation research is carried out in both hover and cruise conditions, and the influence of the rotor phase difference (RPD) on the near field aeroacoustics radiation is studied. The results show that the Overall Sound Pressure Level (OASPL) of aeroacoustics radiation on the fuselage wall can be effectively reduced by the rotor phase control, but the introduction of RPD will also aggravate the aerodynamic interference between the rotors, and have some negative impact on the noise suppression effect. In hover condition, the aerodynamic interference is most serious, and the noise suppression effect of the rotor phase control is relatively poor. In cruise condition, because of the distant inflow, the interference between the rotors and wing is relieved, and the rotor phase control can achieve better noise suppression effect. The OASPL of aeroacoustics radiation on the fuselage wall can be reduced by 10.2 dB at most in hover condition, and up to 15 dB in cruise condition. The simulation results demonstrated a significant effect of rotor phase control on near field aeroacoustics suppression of tilt-rotor aircraft.

Preliminary Design of Hybrid-Electric Propulsion Systems for Emerging Urban Air Mobility Rotorcraft Architectures

Abstract The increasing demands for air-taxi operations together with the ambitious targets for reduced environmental impact have driven significant interest in alternative rotorcraft architectures and propulsion systems. The design of hybrid-electric propulsion systems (HEPSs) for rotorcraft is seen as being able to contribute to those goals. This work aims to conduct a comprehensive design and tradeoff analysis of hybrid powerplants for rotorcraft, targeting enhanced payload-range capability and fuel economy. An integrated methodology for the design, performance assessment, and optimal implementation of HEPSs for conceptual rotorcraft has been developed. A multidisciplinary approach is devised comprising models for rotor aerodynamics, flight dynamics, HEPS performance, and weight estimation. All models are validated using experimental or flight test data. The methodology is deployed for the assessment of a hybrid-electric tilt-rotor, modeled after the NASA XV-15. This work targets to provide new insight into the preliminary design and sizing of optimally designed HEPSs for novel tilt-rotor aircraft. The paper demonstrates that at present, current battery energy densities (250 Wh/kg) severely limit the degree of hybridization if a fixed useful payload and range are to be achieved. However, it is also shown that if advancements in battery energy density to 500 Wh/kg are realized, a significant increase in the level of hybridization and hence reduction of fuel burned and carbon output relative to the conventional configuration can be attained. The methodology presented is flexible enough to be applied to alternative rotorcraft configurations and propulsion systems.

Anti-disturbance control of a quadrotor manipulator with tiltable rotors based on integral sliding mode control

Anti-disturbance control of a quadrotor manipulator with tiltable rotors based on integral sliding mode control

Challenges, opportunities, and a research roadmap for downwind wind turbines

AbstractThis study investigates the challenges and opportunities presented by downwind wind turbines and offers a roadmap of future research pathways to maximize their potential. Multidisciplinary design, analysis, and optimization comparison studies between upwind and downwind configurations on a modern 10‐MW offshore wind turbine are presented to support the discussion. On one hand, the downwind rotor is found to consistently have a smaller swept area under loading. As a result, the downwind design produces less annual energy production (−1.2%). On the other hand, lighter blades for the downwind configuration lead to lower capital costs (−1.7%), so there is little difference in the levelized cost of energy between the two. Key ultimate and fatigue loads are compared, with some values increasing in the downwind configuration, while others decrease. The impact of a downwind configuration on the tower and the impacts of cone and tilt angles and free‐yaw system on the levelized cost of energy are also investigated. The results show a mix of some advantages and disadvantages. Given these results, four areas of research in advanced controls, highly tilted rotors, higher fidelity aerodynamic models, and floating wind are proposed for downwind wind turbines.

The investigation on flight quality of tilt-rotor aircraft

Purpose This paper aims to present an investigation on flight quality analysis and design of tilt-rotor aircraft combined with corresponding flight quality specifications. Design/methodology/approach From the perspective of modal characteristics of tilt-rotor aircraft, it focuses on the analysis of the change rules of the longitudinal short-term motion mode, lateral roll convergence mode, spiral mode and Dutch roll mode. Then, the flight quality design research is carried out using the explicit model tracking control method. The quantitative relationship between flight quality requirements and explicit model is established. Accordingly, the closed-loop flight quality of XV-15 tilt-rotor aircraft is evaluated. Findings The stability of spiral mode is the result of the interaction of various aerodynamic derivatives and spiral instability occurs in helicopter mode. The other motion modes are stable in full flight mode and meet the requirements of level 1 specified in ADS-33E-PRF and MIL-F-8785C flight quality specifications. There is a quantitative relationship between flight quality requirements and explicit model, and the flight quality of tilt-rotor aircraft is improved through the explicit model tracking control method. Practical implications The presented analysis results showed the influence of motion modes and flight quality and the effectiveness of explicit model tracking control method in flight quality improvement, which could be considered as new information for further flight quality design of tilt-rotor aircraft. Originality/value The originality of the paper lies in the proposed design and analysis method of the flight quality of tilt-rotor aircraft from the direction of the influence of its aerodynamic derivatives and motion modes.

Development of a quasi-linear parameter varying model for a tiltrotor aircraft

The research presented in this paper focuses on the development of a quasi-Linear Parameter Varying (qLPV) model for the XV-15 tiltrotor aircraft. The specific category of qLPV modeling technique, known as the model stitching technique, is employed to model the time-varying dynamics of XV-15 tiltrotor aircraft over the entire flight envelope. In this modeling approach, discrete linear state-space models are interpolated through lookup tables as function of scheduling parameters with the implementation of nonlinear equations of motion. The XV-15 qLPV model is configured with four scheduling parameters: altitude, nacelle incidence angle, wing flap angle and velocity. Additionally, a computational complexity analysis is presented. In particular, computational sensitivity of qLPV models configured with lookup tables to number of states and number of scheduling parameters is demonstrated. This is done to show the feasibility of real-time implementation of qLPV models with increasing fidelity (number of states) and expanding dynamic flight envelope (number of scheduling parameters).

An analysis method of pressure characteristic for cylindrical intershaft gas film seal considering centrifugal expansion, rotor misalignment, and precession

To improve the weaknesses of large leakage and wear of dual-rotor intershaft labyrinth seal in aero-engines, the cylindrical gas film seal of metal rubber with a compliant feature is proposed to substitute this conventional seal. According to the dual-rotor operating condition, an analysis method of gas film pressure is presented considering the complex condition of rotor tilt, centrifugal expansion effect, and rotor circular precession. The characteristics of gas film pressure distribution are computed and comparisons are conducted in a complex operating state that the tilt rotor is in forward/backward circular precession under a homodromous/counter-rotating condition with/without the influence of centrifugal expansion. Besides, the formation mechanism of the gas film pressure caused by rotor rotational direction and circular precession direction is revealed. Also, the influence of rotor speed and seal ring speed on gas film pressure is analyzed when the rotor is tilted. The results indicate that for the dual-rotor cylindrical gas film seal with high rotating speed, the rotor tilt and centrifugal expansion effect have significant influence on the gas film pressure distribution, and the rotors’ rotational direction as well as rotor circular precession direction determines the maximum gas film pressure distribution area. The maximum pressure under a homodromous condition with backward precession is the highest; the maximum pressure under a homodromous condition with forward precession is the lowest. The former is 1.33% and 1.68% higher than the latter, respectively, with/without consideration of centrifugal expansion. The study method, which is generally suitable for the cylindrical gas film seal with single-rotor/dual-rotor, lays the foundation of performance analysis under complex operating conditions.

MV-22B Short Takeoff and Minimum Run-On Landing Tests Aboard Nimitz-Class Ships

Tiltrotor aircraft ship suitability and envelope expansion testing aboard a U.S. Navy aircraft carrier is discussed. The purpose was to validate and expand day/night vertical launch and recovery wind envelopes, and the development of short takeoff and minimum run-on landing wind envelopes. The preparation phase incorporated a six-degree-of-freedom motion flight simulator to evaluate flight dynamics prior to on-shore testing using V-22 developmental and production aircraft. The short takeoff and minimum run-on landing testing totaled 12.3 flight hours with 33 takeoffs and 32 landings. The gross weights ranged from 51,000 to 57,000 lb with of relative wind from 0 to 45 kt. The test pilots provided assessments of handling and flying qualities throughout the maneuvers using the deck interface pilot effort scale. Predictability tests were required to determine the pilot’s ability to land within the defined touchdown zone and speeds for safely stopping within the braking zone. The results show the integrated nature of shipboard flight testing and the process of combining pilot workload ratings, control strategies, and aerodynamic analyses to field a needed capability.