Explore the vast range of titles available.

Ice accretion on rotary-wing unmanned aerial vehicles (RWUAVs) needs to be studied separately from the fixed-wing UAVs because of the additional flow complexities induced by the propeller rotation. The aerodynamics of rotatory wings are extremely challenging compared to the fixed-wing configuration. Atmospheric icing can be considered a hazard that can plague the operation of UAVs, especially in the Arctic region, as it can impose severe aerodynamic penalties on the performance of propellers. Rotary-wing structures are more prone to ice accretion and ice shedding because of the centrifugal force due to rotational motion, whereby the shedding of the ice can lead to mass imbalance and vibration. The nature of ice accretion on rotatory wings and associated performance degradation need to be understood in detail to aid in the optimum design of rotary-wing UAVs, as well as to develop adequate ice mitigation techniques. Limited research studies are available about icing on rotary wings, and no mature ice mitigation technique exists. Currently, there is an increasing interest in research on these topics. This paper provides a comprehensive review of studies related to icing on RWUAVs, and potential knowledge gaps are also identified.

The paper explains further developments of a new concept called TNC (Thickness Noise Control) of the application of surface ventilation to the reduction of helicopter rotor low-frequency in-plane harmonic (LF-IPH) noise. The TNC method is based on introduction of four cavities covered by perforated plates (connected to low and high pressure reservoirs) and positioned symmetrically at the front and rear extremities of the blade tip. Two operational modes are analyzed: constant (steady) and periodically varying (unsteady) transpiration mass-fluxes. For exemplary two-bladed model helicopter rotor of Boxwell et al. (Bell UH-1H Iroquois helicopter) in hover conditions, the results of numerical simulations, based on the CFD code SPARC (Spalart-Allmaras turbulence and Bohning-Doerffer transpiration models), suggest that the acoustic pressure fluctuations are significantly reduced in the near-field of the blade tip. Moreover, the unsteady approach (designed for forward located observers) is almost equally efficient to the basic (steady) operation mode, while substantially lowering penalties in terms of the aerodynamic performance and required transpiration flow intensity. Finally, it is proven that when TNC is activated, the twisted blade (operating in low-thrust conditions) exhibits lower deterioration of the aerodynamic performance compared to its straight (untwisted) counterpart.

This article aims at collecting and discussing the results reached by the research community regarding the study of the ground effect on small rotorcraft unmanned aerial vehicles, especially from the modeling and control point of view. Rotorcraft performance is affected by the presence of the ground or any other boundary that alters the flow into the rotors. Specifically, the ground effect can induce perturbations in the flight stability, when operating near the ground. For a rotorcraft, an accident is likely to happen when the vehicle leaves or enters the ground effect region, which may cause crashes and property damages. Today, the use of unmanned aerial vehicles has grown widespread, which raises safety concerns when they are flying at very low altitudes and near the ground. Consequently, studying the influence of the ground over rotorcrafts is of paramount importance for general safety. Also, these investigations can be used to design systems of guidance, navigation, and control. In this review, we break down the most relevant works to date. We discuss aspects related to modeling, control, and application of the ground effect for small-scale multirotors, as well as other aerodynamic proximity effects, such as the ceiling and wall effects. We conclude by mentioning potential avenues of research when studying the ground effect from the point of view of the robotics and artificial intelligence fields.

Purpose Dual-thrust hybrid unmanned aerial vehicle (UAV) technology offers a highly robust and efficient system that incorporates the take-off and landing capabilities of rotary-wing aircraft with the endurance capacities of fixed-wing aircraft. The purpose of this study is to model and control a hybrid UAV in three distinct flight modes: rotary-wing, fixed-wing and over-actuated model. Design/methodology/approach Model predictive control (MPC) along with linear models are applied to design controllers for the rotary-wing or vertical take-off and transition to the fixed-wing flight. The MPC algorithm is implemented with two approaches, first in its usual form and then in a new form with the help of tracking error variables as state variables. Findings Because the tracking error variables are more compatible with the cost function used in MPC, the results improve significantly. This is especially important for a safe and stable transition from rotary-wing to fixed-wing flight, which should be done quickly. The authors also propose a control allocation strategy with MPC algorithm to exploit the thrust and control inputs of both rotary-wing and fixed-wing systems for the transition phase. As the control system is over-actuated, the proposed algorithm distributes the control signal among the actuators better than the MPC alone. The numerical results show that the flight trajectory is also improved. Originality/value The research background is reviewed in the introduction section. The other sections are originally developed in this paper to the best of the authors’ knowledge.

Purpose Responsible for lift generation, the helicopter rotor is an essential component to protect against ice accretion. As rotorcraft present a smaller wing cross-section and a lower available onboard power compared to aircraft, electro-thermal heating pads are favored as they conform to the blades’ slender profile and limited volume. Their optimization is carried out here taking into account, for the first time, the highly three-dimensional (3D) nature of the flow and ice accretion, in contrast to the current state-of-the-art that is limited to two-dimensional (2D) airfoils. Design/methodology/approach Conjugate heat transfer simulation results are provided by the truly 3D finite element Navier–Stokes analysis package-ICE code, embedded in a proprietary rotorcraft simulation toolkit, with reduced-order modeling providing a time-efficient evaluation of the objective and constraint functions at every iteration. The proposed methodology optimizes heating pads extent and power usage and is versatile enough to address in a computationally efficient manner a wide variety of optimization formulations. Findings Low-error reduced-order modeling strategies are introduced to make the tackling of complex 3D geometries feasible in todays’ computers, with the developed framework applied to four problem formulations, demonstrating marked reductions to power consumption along with improved aerodynamics. Originality/value The present paper proposes a 3D framework for the optimization of electro-thermal rotorcraft ice protection systems, in hover and forward flight. The current state-of-the-art is limited to 2D airfoils.

Purpose The purpose of this paper is to advance the multiphysics analysis of helicopter rotors under icing conditions by coupling the iced rotor’s aerodynamics, analyzed by CFD, with the rotor’s structural characteristics, analyzed by CSD. Design/methodology/approach The current work introduces supercomputer-based computational approaches capable of assessing the impact of ice accretion on the aerodynamics, blade dynamics, vibrations and loading of a rotorcraft. The rigid and elastic motions of the blades are accounted for through a loose coupling of the flow solver to a multibody dynamics solver. The coupling framework allows for comprehensive aeroelastic simulations of iced rotors in hover and in forward flight. Findings The flow and structural modules were validated on a full helicopter configuration in forward flight using the ROBIN experimental model. The tip structural deflections were in very close agreement with the experimental measurements. Research limitations/implications The results of the CFD analyses are limited by the available experimental results they can be compared to. In dry air CFD, three-dimensional (3D) experiments occur first and CFD is then compared to them; in icing, the opposite is true: 3D experiments (if they are ever done, as they are very expensive) chase CFD and sometimes never occur. Practical implications This paper presents an outline of how CFD and computational stress dynamics (CSD) analyses can be linked and provides a toolbox for deeper investigation of the complex flows over helicopters operating under difficult in-flight icing conditions. Social implications More and more helicopters are designed to be able to operate in hostile environments such as rescuing and saving lives over the oceans or mountains, conditions under which icing encounters cannot be avoided. Originality/value A loosely coupled CFD/CSD framework that accounts for the rotor blades structural response to aerodynamic loading and ice accretion in hover and forward flight has been presented. This versatile and cost-effective framework provides a more accurate estimation of the helicopter rotor performance and its degradation due to icing encounters during the early design stages than traditional CFD tools.

Successful icing/de-icing simulations for rotorcraft require a good prediction of the convective heat transfer on the blade’s surface. Rotorcraft icing is an unwanted phenomenon that is known to cause flight cancelations, loss of rotor performance and severe vibrations that may have disastrous and deadly consequences. Following a series of experiments carried out at the Anti-icing Materials International Laboratory (AMIL), this paper provides heat transfer measurements on heated rotor blades, under both the anti-icing and de-icing modes in terms of the Nusselt Number (Nu). The objective is to develop correlations for the Nu in the presence of (1) an ice layer on the blades (NuIce) and (2) liquid water content (LWC) in the freestream with no ice (NuWet). For the sake of comparison, the NuWet and the NuIce are compared to heat transfer values in dry runs (NuDry). Measurements are reported on the nose of the blade-leading edge, for three rotor speeds (Ω) = 500, 900 and 1000 RPM; a pitch angle (θ) = 6°; and three different radial positions (r/R), r/R = 0.6, 0.75 and 0.95. The de-icing tests are performed twice, once for a glaze ice accretion and another time for rime ice. Results indicate that the NuDry and the NuWet directly increased with V∝, r/R or Ω, mainly due to an increase in the Reynolds number (Re). Measurements indicate that the NuWet to NuDry ratio was always larger than 1 as a direct result of the water spray addition. NuIce behavior was different and was largely affected by the ice thickness (tice) on the blade. However, the ice acted as insulation on the blade surface and the NuIce to NuDry ratio was always less than 1, thus minimizing the effect of convection. Four correlations are then proposed for the NuDry, the NuWet and the NuIce, with an average error between 3.61% and 12.41%. The NuDry correlation satisfies what is expected from heat transfer near the leading edge of an airfoil, where the NuDry correlates well with Re0.52.

Laminar flow aircraft may potentially save fuel and reduce the emission of pollutants and greenhouse gases. However, laminar flow aircraft face challenges caused by contaminations on the wings, such as insect impact residue. To study insect residue on an aircraft airfoil, a new setup was developed that used rotary wings and shot an insect toward the leading edge. This setup kept insects intact before impact while airflow was maintained throughout the experiment. Additionally, the setup enabled the long-term observation of the impact residue while the test speed was adjusted. Two experiments were carried out to investigate inconsistencies from past studies about insect rupture velocity and the effect of airflow on residue. Drosophila Hydei was the insect used, and aluminum was used as the baseline substrate, which was also coated with polyurethane, acrylic, and two superhydrophobic coatings. Instead of a threshold velocity for the minimum rupture velocity of the insect, a range from initial insect rupture to the velocity at which insects ruptured in all instances was determined (i.e., 17–30 m/s). Furthermore, the presence of a coating (polyurethane) on the airfoil did not affect the minimum rupture velocity. It was observed that airflow, which has been previously mentioned as a mitigation method, did not change the residue amount after coagulation for all coatings.

In the research field of rotorcraft aerodynamics, there are two fundamental challenges: resolving the complex vortex structures in rotor wakes and representing the moving rotor blades in the ambient airflow. In this paper, we address the first challenge by utilizing a third-order unstructured finite volume solver, which exhibits lower numerical dissipation than its second-order counterpart. This allows for sufficient resolution of small vortex structures on relatively coarse meshes. With this flow solver, the second challenge is addressed by modeling each rotor as an actuator disk (i.e., the actuator disk model (ADM)) or modeling each blade as an actuator line (i.e., the actuator line model (ALM)). Both of the two models are equipped with an improved tip loss correction, which is introduced in detail in the methodology section. In the section of numerical experiments, the numerical convergence properties of the two types of solvers have been compared in the case of two-dimensional infinite wing. In addition, the relationship between the ALM and the lifting line theory is discussed in the cases of fixed-wing calculations. Another goal of these cases is to validate the tip loss correction presented. The validation of the ALM/ADM and comparisons of computational efficiency are also demonstrated in simulations involving both hover and forward flight rotors. It was found that the combination of the third-order finite volume solver and the ALM/ADM with the improved tip loss correction presents an efficient way of performing the aerodynamic analysis of rotor-induced downwash flow.

In aerospace engineering, high-order computational fluid dynamics (CFD) solvers suitable for three-dimensional unstructured meshes are less developed than expected. The Runge–Kutta discontinuous Galerkin (RKDG) finite element method with compact weighted essentially non-oscillatory (WENO) limiters is one of the candidates, which might give high-order solutions on unstructured meshes. In this article, we provide an efficient parallel implementation of this method for simulating inviscid compressible flows. The implemented solvers are tested on lower-dimensional model problems and real three-dimensional engineering problems. Results of lower-dimensional problems validate the correctness and accuracy of these solvers. The capability of capturing complex flow structures even on coarse meshes is shown in the results of three-dimensional applications. For solving problems containing rotary wings, an unsteady momentum source model is incorporated into the solvers. Such a finite element/momentum source hybrid method eliminates the reliance on advanced mesh techniques, which might provide an efficient tool for studying rotorcraft aerodynamics.