Numerical Investigation of the Rotor-Rotor Aerodynamic Interaction for eVTOL Aircraft Configurations

The rotor-rotor aerodynamic interaction is one of the key phenomena that characterise the flow and the performance of most of the new urban air mobility vehicles (eVTOLs) developed in the recent years. The present article describes a numerical activity that aimed to the systematic study of the rotor-rotor aerodynamic interaction with application to the flight conditions typical of eVTOL aircraft. The activity considers the use of a novel mid-fidelity aerodynamic solver based on vortex particle method. In particular, numerical simulations were performed when considering two propellers both in side-by-side and tandem configuration with different separation distances. The results of numerical simulations showed a slight reduction of the propellers performance in side-by-side configuration, while a remarkable loss of thrust in the order of 40% and a reduction of about 20% of the propulsive efficiency were found in tandem configuration, particularly when the propeller disks are completely overlapped. Moreover, the flow field analysis enabled providing a detailed insight regarding the flow physics involved in such aerodynamic interactions.

Impact of a Wind Turbine Blade Pitch Rate on a Floating Wind Turbine During an Emergency Shutdown Operation

During the shutdown of a wind turbine, the turbine blades rotate from their typical operating angle to their typical idling angle (approximately 90 degrees) at a specific speed, called the blade pitch rate. This operation leads to rapid loss of thrust force on the turbine resulting in a corresponding heel response of the floating structure. This rapid variation of loads at the turbine also leads to large nacelle accelerations which are transferred to the bottom of the tower and consequently to the floating structure, making the turbine shutdowns, and specifically emergency shutdowns, of significance in the design and certification of the turbine, tower and floating structure. In case of an emergency shutdown (for instance due to a grid loss), the blades typically pitch from 0 degree to 90 degrees in approximately 20–35 seconds, whereas this time period can be more than 100 seconds in the case of a normal shutdown [6]. For fixed-bottom wind turbines, increasing the blade pitch rate leads to an increase of instantaneous loads at the nacelle and tower, leading to the emergency shutdown pitch rate being usually chosen to be as low as possible. In the case of a floating wind turbine, however, water/platform interaction effects such as wave induced damping on the floating platform, challenge this approach. Indeed, increasing the blade pitch rate can increase the effect of wave-induced damping on the floater and therefore reduce the loads on the overall structure. On the other hand, reducing the blade pitch rate during an emergency shutdown can reduce this damping effect and increase those loads, meaning that an optimal blade pitch rate for a fixed bottom turbine is not necessarily optimal for a floating wind turbine. This paper will examine the behavior of a floating offshore semi-submersible platform, the WindFloat, during turbine shutdown operations, with an emphasis on the blade pitch rate during an emergency shutdown.

Auto-landing guidance for unmanned aerial vehicle with engine flame-out

Landing is the most dangerous phase of the entire flight phases. If the total loss of thrust occurs during flight, a powered aircraft converts to a glider, which can use kinetic and potential energy only. For this reason, a proper scheme is needed for safe landing in cases of the total loss of thrust. This paper presents three-dimensional unpowered auto-landing guidance based on trajectory generation, expanding the concept of the energy-to-range ratio. We develop the terminal velocity estimation method for a horizontal plane applied to three-dimensional space; this method is based on the previously suggested terminal velocity estimation method for a vertical plane. Then, we show trajectory generation for landing guidance combining vertical with horizontal waypoints. The proposed auto-landing guidance with trajectory generation is evaluated by numerical simulation.

Modified Dubins parameterization for aircraft emergency trajectory planning

This paper presents a trajectory parameterization method for calculating emergency flight paths with variable airspeeds under conditions of constant wind. The method is based on the Dubins curve; however, it has been modified to allow for acceleration along the path and finite rate of change in turn rate. The aircraft’s planar trajectory from an initial condition to a terminal condition is parameterized into a small set of path-defining variables. The method uses a number of closed-form solutions and simple iteration schemes to efficiently calculate a path that meets the specified constraints. The parametrized path can then be optimized to minimize a performance objective for real-time emergency path planning. For emergency flight planning, the vertical degree of freedom is treated as a function of the aircraft state and parametric controls, and the optimization is formulated to ensure touchdown at a desired location and aircraft state. The performance of the proposed method is investigated using several test cases, including landing of a commercial jet following total loss of thrust and autorotative recovery of a utility helicopter following total loss of power.

A KINEMATIC APPROACH TO SEGMENTED-TRAJECTORY GENERATION FOR THE TOTAL LOSS OF THRUST EMERGENCY

Contemporary twin-engine airliners are more vulnerable to total loss of thrust than yesterday‘s three and four engine airliners, due to reduced engine redundancy. In the event of a total loss of thrust, flight crews have only one chance for landing, because the aircraft cannot gain altitude. Therefore, there is a pressing need to explore the idea of an engines-out landing trajectory optimization for commercial jets. A few past studies addressed this safety issue for general aviation aircraft and fighter jets but not commercial jets, primarily because the essential aircraft-specific aerodynamic data are not publicly available. To fill in this gap, this study adopts a kinematic approach to aircraft trajectory optimization. Unlike conventional trajectory optimization methods, the kinematic algorithm requires minimal amount of aircraft-specific aerodynamic data that can be effortlessly collected in a full flight simulator. The paper describes the kinematic algorithm and applies it to a realistic bird strike scenario. Flight simulation tests are conducted in a full flight simulator to verify the accuracy of the algorithm. The results demonstrate that the algorithm can compute the optimum trajectory with a less than 3.0 percent error. Since the algorithm is accurate and computationally-undemanding, it is promising for real-world applications.

A Method of Height Adjustment for Unpowered Emergency Landing

The aircraft needs to adjust its heading angle to go to the landing window after the loss of thrust. However, the actual height of the aircraft within the landing area may be higher than that of the landing window. In order to ensure a safe landing, a spiral drop is needed to lose height. Through some certain integer rings of spiral, the aircraft will fall into the landing window. On this stage of height loss, this paper proposes a method of height adjustment to make the aircraft land exactly into the window. This method can get the desired bank angle through calculating the height difference between the height in the landing area, where the aircraft is, and the height of the landing window. This bank angle ensures the aircraft just to go through exactly the landing window in integer spiral rings of falls. Finally through simulation, this paper tests the effectiveness of the method.