Aerodynamic Benefits Research Articles

Distributed feather-inspired flow control mitigates stall and expands flight envelope

Multiple rows of feathers, known as the covert feathers, contour the upper and lower surfaces of bird wings. These feathers have been observed to deploy passively during high angle of attack maneuvers and are suggested to play an aerodynamic role. However, there have been limited attempts to capture their underlying flow physics or assess the function of multiple covert rows. Here, we first identify two flow control mechanisms associated with a single covert-inspired flap and their location sensitivity: a pressure dam mechanism and a previously unidentified shear layer interaction mechanism. We then investigate the additivity of these mechanisms by deploying multiple rows of flaps. We find that aerodynamic benefits conferred by the shear layer interaction are additive, whereas benefits conferred by the pressure dam effect are not. Nevertheless, both mechanisms can be exploited simultaneously to maximize aerodynamic benefits and mitigate stall. In addition to wind tunnel experiments, we implement multiple rows of covert-inspired flaps on a bird-scale remote-controlled aircraft. Flight tests reveal passive deployment trends similar to those observed in bird flight and comparable aerodynamic benefits to wind tunnel experiments. These results indicate that we can enhance aircraft controllability using covert-inspired flaps and form insights into the aerodynamic role of covert feathers in avian flight.

Studies on V-Formation and Echelon Flight Utilizing Flapping-Wing Drones

V-Formation and echelon formation flights can be seen used by migratory birds throughout the year and have left many scientists wondering why they choose very specific formations. Experiments and analytical studies have been completed on the topic of the formation flight of birds and have shown that migratory birds benefit aerodynamically by using these formations. However, many of these studies were completed using fixed-wing models, while migratory birds both flap and glide while in formation. This paper reports the design of and experiments with a flapping-wing model rather than only a fixed-wing model. In order to complete this study, two different approaches were used to generate a flapping-wing model. The first was a computational study using an unsteady vortex–lattice (UVLM) solver to simulate flapping bodies. The second was an experimental design using both custom-built flapping mechanisms and commercially bought flapping drones. The computations and various experimental trials confirmed that there is an aerodynamic benefit from flying in either V-formation or echelon flight while flapping. It is shown that each row of birds experiences an increase in aerodynamic performance based on positioning within the formation.

Effects Of Wing Elasticity On Tip Vortex: 3D Deformation And 2D3C Flow Field Measurements

As aircraft embrace broader applications, the flexible wing presents a promising avenue to enhance aircraft performance across a wide range of flight conditions. Although the aerodynamic benefits of wing deformation are widely recognized, the present understanding of the tip vortex is relatively limited. This work experimentally examines the effects of wing deformation on the tip vortex of a simplified aircraft model with two kinds of elastic wings and a rigid wing. The 3D deformations and 2D3C flow fields are measured using a synchronous measurement system, which includes the high accuracy deformation measurement (HADM) method and the stereoscopic particle image velocimetry (SPIV). It is shown that increased wing elasticity leads to greater bending, with the wing tip moving along the streamwise, vertically upward, and spanwise inward directions. These deformations are closely linked to aerodynamic forces and influence the tip vortex positions. The more elastic wing, EW2, exhibits the highest vortex core position, more than 0.5 times the length of the wing root, while the other one, EW1, shows the strongest vortex in the lift-increased regime. In the early wakes, the tip vortex of the elastic wing is elongated along the spanwise direction, resulting in an asymmetric shape. As it convects downstream, the vortex core disperses via viscous diffusion, resulting in a relatively more circular shape with decreased vorticity.

How much can roof-mounted bicycles on a following team car reduce cyclist drag?

Earlier research demonstrated that a car following a cyclist can provide an aerodynamic benefit to this cyclist. This effect could be large enough to potentially influence the outcome of individual time trials. This incited the International Cycling Union (UCI) in 2023 to raise the minimum distance from 10 to 25 m. However, this previous research work did not consider the bicycles typically mounted on the roof of a team car. Indeed, some teams have mounted up to ten bicycles on the roof, possibly in an attempt to optimise the aerodynamic benefit by their team car, even in individual time trials. This study employs CFD simulations validated by wind tunnel measurements to assess the benefit provided by different rooftop bicycle configurations. It is shown that the extra benefits are substantial and extend up to a distance of at least 25 m between cyclist and car, which could trigger new rules by the UCI. This study also shows that the cyclist drag reduction by a following car and for any car-cyclist separation distance d, can be estimated by the static pressure coefficient at position d in front of the isolated car, which can be obtained by a single CFD simulation.

The aerodynamic performance of a platoon of lorries in close-proximity during an overtaking manoeuvre

For the first time, this paper examines the aerodynamic forces that arise in a platoon of Heavy Goods Vehicle lorries travelling in close proximity during an overtaking manoeuvre. Through an in-depth programme of wind tunnel experiments it is demonstrated that there is a complex relationship between the drag and side force coefficients, with a possible hysteresis-like behaviour shown. The influence of the overtaken lorry on the overtaking lorry is shown to be key in terms of the aerodynamic-induced forces experienced by the latter. Furthermore, the overtaking lorry is shown to benefit aerodynamically from the overtaken lorry, and this is attributed to the elliptical nature of the Navier-Stokes equations. In addition to examining the forces that arise during overtaking, the variation of the wind-induced forces with respect to yaw on a single lorry is compared to those on a middle lorry in a 3-lorry platoon. The aerodynamic benefits arising from platooning/vehicles travelling in close proximity are again demonstrated, although it is shown that these benefits may only exist over a limited range of yaw angles, raising questions about the real-world application of this approach. This has important implications for platooning in crosswinds and the drag force reduction that is assumed due to platooning.

A Preliminary Evaluation of Morphing Horizontal Tail Design for UAVs

Morphing structures are a relatively new aircraft technology currently being investigated for a variety of applications, from civil to military. Despite the lack of literature maturity and its complexity, morphing wings offer significant aerodynamic benefits over a wide range of flight conditions, enabling reduced aircraft fuel consumption and airframe noise, longer range and higher efficiency. The aim of this study is to investigate the impact of morphing horizontal tail design on aircraft performance and flight mechanics. This study is conducted on a 1:5 scale model of a Preceptor N-3 Pup at its trim condition, of which the longitudinal dynamics is implemented in MATLAB release 2022. Starting from the original horizontal tail airfoil NACA 0012 with the elevator deflected at the trim value, this is modified by using the X-Foil tool to obtain a smooth morphing airfoil trailing edge shape with the same CLα. By comparing both configurations and their influence on the whole aircraft, the resulting improvements are evaluated in terms of stability in the short-period mode, reduction in the parasitic drag coefficient CD0, and increased endurance at various altitudes.

Aerodynamic performance of a flyable flapping wing rotor with dragonfly-like flexible wings

Drawing inspiration from insect flapping wings, a Flapping Wing Rotor (FWR) has been developed for Micro Aerial Vehicle (MAV) applications. The FWR features unique active flapping and passive rotary kinematics of motion to achieve a high lift coefficient and flight efficiency. This study thoroughly investigates the aerodynamic performance and design of a bio-inspired flexible wing for FWR-MAVs, emphasizing its novel backward-curved wingtip and variable spanwise stiffness resembling a dragonfly's wing. The research departs from previous aerodynamic studies of FWR, which focused predominantly on rectangular and rigid wings, and delves into wing flexibility. Employing Computational Fluid Dynamics (CFD), Computational Structural Dynamics (CSD), and experimental measurements, the study demonstrates the aerodynamic benefits of the dragonfly-inspired FWR wingtip shape and its reinforced structure. Fluid-Structure Interaction (FSI) analysis is used to examine the effects of elastic deformation encompassing twist and bending on aerodynamic forces. The results underscore the importance of bending deformation in enhancing lift and power efficiency and propose a method for analysing variable stiffness along the wingspan using a vortex delay mechanism that is induced by delayed flapping motion. By comparing modelled and measured stiffness, the study validates the flexibility of the FWR wing, revealing optimal aerodynamic efficiency is achieved through moderate flexibility and spanwise stiffness variation. The curving leading-edge beam forming the sweep-back wingtip offers a practical approach to obtaining variable stiffness and aerodynamic benefits for FWR-MAVs. Using the same pair of dragonfly-like flexible wings, FWR-MAVs have effectively exhibited VTOL and hovering flight capabilities, spanning from a 25-g single-motor drive model to a 51-g dual-motor drive model. This research provides valuable insights into flexible wing design for FWR-MAVs, leveraging biomimicry to improve flight efficiency.

Experimental and numerical study of different wing number and flapping-rotation decomposition of a flapping wing rotor unmanned aerial vehicle

The flapping wing rotor (FWR) is a novel aerial vehicle that combines the aerodynamic benefits of both a flapping wing and a rotary wing. By utilizing the passive rotation effect resulting from the flexible deformation of the center symmetric flapping wing, the FWR can enhance its lift force. However, previous research has neglected to explore the mechanism behind the flapping-rotation motion of a flyable FWR, which elucidates its lift advantage compared to conventional flapping motion. Additionally, the impact of varying wing number on the flapping-rotation motion and performance of the FWR has not been taken into consideration. Therefore, it is imperative to conduct an experimental analysis to ascertain the impact of flapping-rotation decomposition and varying wing quantities on FWR. In this study, our prior vehicle design, which exhibited consistent stable hovering and maneuvering capabilities, is employed to construct the flapping wing rotor experiment system. Through this unique experimental system, the effects of flapping-rotation decomposition and different wing quantities on FWR are individually investigated. Additionally, computational fluid dynamics simulation is utilized as an auxiliary and supplementary approach to analyze the aerodynamic characteristics of the flapping-rotation motion. The result proves that the stable flapping-rotation motion does produce a more significant lift increase than the normal flapping motions. Under the premise of stable flapping-rotation motion, more wings will not only produce more lift but also require more driving power. The interactions between the wings also affect the flapping-rotation motion.

Space Efficiency in Tapered Super-Tall Towers

In modern skyscraper architecture, the preference for incorporating tapered building configurations is on the rise, constituting a prominent trend in the industry, particularly due to their structural and aerodynamic benefits. The efficient utilization of space is a critical consideration in the design of tapered skyscrapers, holding significant importance for sustainability. Nevertheless, the existing body of scholarly work falls short in providing an all-encompassing investigation into the space efficiency of super-tall towers featuring tapered configurations, despite their prevalent adoption. This research endeavors to rectify this notable void by undertaking an exhaustive examination of data derived from 40 case studies. The key findings are as follows: (1) average space efficiency was about 72%, with values fluctuating between a minimum of 55% and a maximum of 84%; (2) average ratio of core area to the gross floor area (GFA) registered about 26%, encompassing a spectrum ranging from 11% to 38%; (3) most tapered skyscrapers employed a central core design, primarily tailored for mixed-use purposes; (4) an outriggered frame system was the prevailing structural system, while composite materials were the most commonly used structural materials; and (5) significant differences in the influence of function and load-bearing systems on the space efficiency of tapered towers were not observed. The author anticipates that these results will offer valuable direction, particularly to architectural designers, as they work towards advancing the sustainable development of tapered skyscrapers.

Characteristics of Vortices around Forward Swept Wing at Low Speeds/High Angles of Attack

The forward-swept wing (FSW), one of the wing planforms used in aircraft, is known for its high performance in reducing wave drag. Additionally, a study has shown that this wing planform can mitigate sonic booms, which pose a significant challenge to achieving supersonic transport (SST). Therefore, FSW is expected to find applications in future SST aircraft owing to aerodynamic advantages at high speeds. However, there is a lack of sufficient knowledge and systematization to improve aerodynamic performance at low speeds and high angles of attack during takeoff and landing. These are crucial for practical implementation. Although the aerodynamic benefits of an FSW in high-speed flight can be harnessed using advanced structural and control technologies, the realization of SST using an FSW is challenging without enhanced research on low-speed aerodynamics. This study explores the practical aerodynamic knowledge of FSWs. We utilized a numerical simulation based on the Navier–Stokes equation and focused on investigating wake vortex phenomena. Our simulation included various wing planforms, including backward-swept wings (BSWs). The results revealed the presence of vortices with lateral axes emanating from the FSW, while longitudinal vortices were observed in the BSW. Based on these results, we developed a theoretical hypothesis for the vortex structure around an FSW.

Potential Propulsive and Aerodynamic Benefits of a New Aircraft Concept: A Low-Speed Experimental Study

The aerodynamic design of a new aircraft concept was investigated through subsonic wind-tunnel testing using 1:28-scale powered models. The aircraft configuration integrates a box-wing layout with engines located at the rear part of the fuselage. Measurements involved a back-to-back comparison between two aircraft models: a podded version whose engines were assembled on pylons and a boundary-layer ingestion (BLI) version that provided several system-level benefits. The flowfield was investigated through the power balance method and a variety of pressure flowfield and inlet flow distortion metrics. The results proved that the BLI configuration enhances the propulsive efficiency by reducing both the electrical power coefficient and the kinetic energy waste due to lower jet velocities. Furthermore, there was a reduction of the total pressure recovery due to pressure gradients inside the duct, thereby causing high distortion. Overall, this research highlights the importance of wind-tunnel testing to bring any aerodynamic technology to a sufficient level of maturity and to enable future new aircraft concepts.

Influence of Upstream Sweeping Wake Number on the Unsteady Flow Mechanism in an Integrated Aggressive Intermediate Turbine Duct

This paper focuses on the dynamic internal flow in the integrated aggressive intermediate turbine duct (AITD) with different HPT wake numbers, using CFX Solver with dynamic Reynolds-averaged Navier–Stokes equations (RANS), the shear stress transmission κ-ω turbulence model (SST) and the γ-θ transition model. The HPT wakes are simulated using sweeping rods, with the number of rods ranging from 14 to 56 and a reduced frequency of 1.07. The increasing wake number reduces the radial pressure gradient in the integrated AITD, and then decelerates the radial migration and dissipation of wake vortices, so that some residual wakes can reach the integrated low-pressure turbine guide vane (LPT-GV) to enhance the suppression of flow separation to a certain extent. On the other hand, the increase in wake number can also weaken the skewness and stretching of the wake, thereby increasing the duration of flow separation suppression. When there are too many wakes, the mixing between adjacent wakes accelerates the dispersion of wake vortices, leading to increased total pressure loss and an enhanced turbulence intensity. This enhanced turbulence intensity promotes bypass transition on the suction surface of the LPT-GV in advance, thereby completely eliminating flow separation on the LPT-GV in the entire spatiotemporal domain, which is beneficial for reducing separation loss, but also increasing turbulent viscous loss. When N ≤ 28, the gross loss of the integrated AITD studied in this paper reaches a minimum value (around 0.22), as the benefits brought by the wake suppression of flow separation can offset the wake dissipation loss and the turbulent viscous loss caused by the wake-induced transition. Considering that wake loss is inherently present, using sweeping wakes to inhibit the flow separation on the integrated LPT-GV can bring certain aerodynamic benefits when the wake number is less than 28.

Assessment of the aerodynamic benefits of collocating horizontal- and vertical-axis wind turbines in tandem using actuator line model

The actuator line model is used to study the vertically staggered wind turbine cluster composed of horizontal- and vertical-axis wind turbines (HAWTs and VAWTs) in a tandem layout. We consider three simple configurations, including VAWT upwind of HAWT (V + H), VAWT downwind of HAWT (H + V), and VAWT between the two HAWTs (H + V + H). A VAWT installed upwind of the HAWT can not only generate power by itself but can also enhance the power generation of the HAWT, and the total power increases by about 100 kW. When installed downstream the HAWT, the presence of the VAWT slightly reduces the power generation efficiency of the HAWT. However, the VAWT utilizes the increased wind speed between the HAWT and the ground and generates more power. The total power increases by about 60 kW. When installed between the two HAWTs, the beneficial effects of the VAWT on the downstream HAWT are not manifested. Nevertheless, the wind turbine cluster still generates 50 kW more power than that without the VAWT. Overall, even in the tandem layout where the wake effects are most pronounced, the collocation of VAWTs can still utilize the otherwise wasted wind resources, thus increasing the power generation density of wind farms.

Incipient wing flapping enhances aerial performance of a robotic paravian model

The functional origins of bird flight remain unresolved despite a diversity of hypothesized selective factors. Fossil taxa phylogenetically intermediate between typical theropod dinosaurs and modern birds exhibit dense aggregations of feathers on their forelimbs, and the evolving morphologies and kinematic activational patterns of these structures could have progressively enhanced aerodynamic force production over time. However, biomechanical functionality of flapping in such transitional structures is unknown. We evaluated a robot inspired by paravian morphology to model the effects of incremental increases in wing length, wingbeat frequency, and stroke amplitude on aerial performance. From a launch height of 2.8 m, wing elongation most strongly influenced distance travelled and time aloft for all frequency-amplitude combinations, although increased frequency and amplitude also enhanced performance. Furthermore, we found interaction effects among these three parameters such that when the wings were long, higher values of either wingbeat frequency or stroke amplitude synergistically improved performance. For launches from a height of 5.0 m, the effects of these flapping parameters appear to diminish such that only flapping at the highest frequency (5.7 Hz) and amplitude (60°) significantly increased performance. Our results suggest that a gliding animal at the physical scale relevant to bird flight origins, and with transitional wings, can improve aerodynamic performance via rudimentary wing flapping at relatively low frequencies and amplitudes. Such gains in horizontal translation and time aloft, as those found in this study, are likely to be advantageous for any taxon that engages in aerial behavior for purposes of transit or escape. This study thus demonstrates aerodynamic benefits of transition from a gliding stage to full-scale wing flapping in paravian taxa.

Propulsion integration study of civil aero-engine nacelles

AbstractIt is envisaged that future civil aero-engines will operate with ultra-high bypass ratios to reduce the specific fuel consumption. To achieve the expected benefits from the new engine cycles, these new powerplants may mount compact nacelles. For these new configurations the aerodynamic coupling between the powerplant and the airframe may increase. For this reason, it is required to quantify and further understand the effects of aircraft integration for compact aero-engine nacelles. This study provides an insight of the changes in flow aerodynamics as well as quantification of the most relevant performance metrics of the powerplant, airframe and the combined aircraft system across a range of different installation positions. Relative to a conventional architecture, there is an aerodynamic benefit in net vehicle force of about 1.2% for a compact powerplant when installed in forward positions. This is the same improvement that was identified when the aero-engine nacelles were in isolation. However, for close-coupled installation positions, the aerodynamic benefit in net vehicle force erodes to 0.44% due to the larger effects of aircraft integration on compact nacelles.

Aerodynamic Exploration for Tandem Wings with Smooth or Corrugated Surfaces at Low Reynolds Number

Skin corrugation and tandem configuration are two distinct features that characterize the flow around dragonfly wings. In contrast to the smooth airfoil and single pair of wings of conventional airplanes, corrugated surfaces and tandem wings influence aerodynamics both locally and globally. In this article, several kinds of doubly- tandem wing configurations were designed, then computational investigations based on wind tunnel experiments were conducted to investigate the aerodynamic characteristics of these models. Computational simulations using in-house codes were carried out with a freestream velocity of 20 m/s at an angle of attack from −4° to 16°. Based on these computational results, the effects of airfoil thickness, surface waviness and hindwing decalage on aerodynamic characteristics were compared and presented quantitatively. Final results demonstrate that a tandem wing configuration could eliminate separation close to the trailing edge at angles of attack 8°~10°, or delay the trailing edge separation at angles of attack greater than 10°. Thus, the aerodynamic efficiency of tandem configurations could provide significant improvement compared to configurations with a single wing. The greatest percentage of aerodynamic efficiency improvement for a tandem thick configuration compared to a single thick configuration is 1376% at angle of attack 0°. Surface waviness will stall at a lower angle of attack, but will gain some aerodynamic benefit from the standing separated flow. Hindwing decalage has obvious lift enhancement for the tandem configuration. Therefore, it is concluded that the tandem configuration is attractive and promising for MAVs with flexible structures in the near future.

A BEM theory adaption for inclusion of hub ratio effects in HAWT rotor design and analysis

A large hub ratio in a horizontal axis wind turbine (HAWT) rotor can provide aerodynamic and structural design benefits, but the flow effects are not accounted for in the classical Blade Element Momentum Method (BEMM). Research into the effect of HAWT rotor hub ratio, necessitated development of an adaption to the BEMM, so that rotors with large hub ratios, could be designed to be aerodynamically efficient, and so that the flow effects could be included in performance prediction. Results from the BEMM with large-hub adaption were compared with results from using the classical BEMM and with results from CFD simulation. Two sets of rotors were analysed, one set in a viscous flow regime and the other in a turbulent flow regime. The BEMM with large-hub adaption, was found to provide rotor designs with better performance and provided a more accurate prediction of relative rotor power than the classical BEMM.

An efficient flow control technique based on co-flow jet and multi-stage slot circulation control applied to a supercritical airfoil

Abstract Circulation control is a kind of efficient flow control technology which can improve aircraft aerodynamic performance and reduce fuel consumption. However, improving the aerodynamic efficiency of a circulation device to enhance flight endurance and achieve environmentally flying is a challenging problem for the application of circulation control. This paper presents an efficient flow control technique that combines co-flow jet and multi-stage slot circulation control. The combinational flow control technique is applied to a supersonic airfoil to test its energy consumption and aerodynamic benefit achievement. Results show that both the single and double slot circulation control can improve the maximum lift-drag ratio of the baseline airfoil, with an increment of 11.3% and 19.1%, respectively. Compared with the single application of co-flow jet control which can increase the lift-drag ratio of the baseline airfoil by 16.3% and extend the stall angle of attack from 6° to 8°, the combinational flow control can obtain a more significant lift-drag ratio increment by about 27.3% and eliminate flow separations at high angle of attack. The stall angle of attack can even be increased to about 10°. Additionally, the blowing efficiency of the circulation control airfoil has been comprehensively analyzed. The results show that the maximum effective lift-drag ratio and highest blowing efficiency can be achieved at a blowing coefficient of 0.00235.

Robust Control for UAV Close Formation Using LADRC via Sine-Powered Pigeon-Inspired Optimization

This paper designs a robust close-formation control system with dynamic estimation and compensation to advance unmanned aerial vehicle (UAV) close-formation flights to an engineer-implementation level. To characterize the wake vortex effect and analyze the sweet spot, a continuous horseshoe vortex method with high estimation accuracy is employed to model the wake vortex. The close-formation control system will be implemented in the trailing UAV to steer it to the sweet spot and hold its position. Considering the dynamic characteristics of the trailing UAV, the designed control system is divided into three control subsystems for the longitudinal, altitude, and lateral channels. Using linear active-disturbance rejection control (LADRC), the control subsystem of each channel is composed of two cascaded first-order LADRC controllers. One is responsible for the outer-loop position control and the other is used to stabilize the inner-loop attitude. This control system scheme can significantly reduce the coupling effects between channels and effectively suppress the transmission of disturbances caused by the wake vortex effect. Due to the cascade structure of the control subsystem, the correlation among the control parameters is very high. Therefore, sine-powered pigeon-inspired optimization is proposed to optimize the control parameters for the control subsystem of each channel. The simulation results for two UAV close formations show that the designed control system can achieve stable and robust dynamic performance within the expected error range to maximize the aerodynamic benefits for a trailing UAV.

Aerodynamic Performance Benefits of Over-the-Wing Distributed Propulsion for Hybrid-Electric Transport Aircraft

The goal of this study is to analyze how the aeropropulsive benefits of an over-the-wing distributed-propulsion (OTWDP) system at the component level translate into an aeropropulsive benefit at the aircraft level, as well as to determine whether this enhancement is sufficient to lead to a reduction in overall energy consumption. For this, the preliminary sizing of a partial-turboelectric regional passenger aircraft is performed, and its performance metrics are compared to a conventional twin-turboprop reference for the 2035 timeframe. The changes in lift, drag, and propulsive efficiency due to the OTWDP system are estimated for a simplified unducted geometry using a lower-order numerical method, which is validated with experimental data. For a typical cruise condition and the baseline geometry evaluated in the experiment, the numerical method estimates a 45% increase in the local sectional lift-to-drag ratio of the wing, at the expense of a 12% reduction in propeller efficiency. For an aircraft with 53% of the wingspan covered by the OTWDP system, this aerodynamic coupling is found to increase the average aeropropulsive efficiency of the aircraft by 9% for a 1500 n mile mission. Approximately 4% of this benefit is required to offset the losses in the electrical drivetrain. The reduction in fuel weight compensates for the increase in powertrain weight, leading to a takeoff mass comparable to the reference aircraft. Overall, a 5% reduction in energy consumption is found, albeit with a uncertainty due to uncertainty in the aerodynamic modeling alone.