Low-speed Flight Research Articles

Optimal control of thrust-vectored VTOL UAV in high-manoeuvering transition flight

ABSTRACTIn this paper, a new type of fixed-wing vertical take-off and landing, unmanned aerial vehicle (UAV) has been designed. Thrust-vector direct force control has been introduced in three axes to make UAV exhibit superior manoeuverability in transition flight. Considering the characteristics of UAV's dynamic model, which are non-linear, non-affine, and have redundant input, a two-stage progressive optimal control allocation method is developed, which can optimise position and attitude control in synthetical, and motivate effectors to generates desired force and moments. A task-oriented weight selection scheme is proposed to make objective function suitable for different tasks and flight conditions. In addition, a general constraint strategy is designed to guarantee the feasibility of optimal allocation results, which can largely reduce the onboard computation time. Simulations show that UAV can adjust flight attitude and use control effectors in an optimal way, and demonstrating satisfactory tracking of low-speed high-manoeuver flight paths.

PARSIFAL Project: a Breakthrough Innovation in Air Transport

PARSIFAL is a Horizon 2020 project, started on May 2017, with the aim to design an innovative aircraft, based on the “PrandtlPlane” (PrP) configuration, for the civil aviation of the future. The PrP configuration allows us to reduce the fuel consumption and the external noise, especially during low speed flight. The high aerodynamic performances are used to limit the span to 36m, compliant with ICAO Aerodrome Reference Code C standard, and to extend the payload capacity to 250–350 passengers without significant penalties of aerodynamic efficiency. The paper presents the overall characteristics of this innovative aircraft and how the PrP configuration could allow to improve the civil air transport of the future, as far as aircraft manufacturers, airports, airlines and passengers are concerned; in particular, some aspects of the architectural solutions and the aerodynamic design and optimization are underlined in subsonic and transonic regimes. Some details are given here on the fuselage design and the lifting system.

Улучшение несущих свойств крыла на взлетно-посадочных режимах с помощью системы управления пограничным слоем с использованием актуаторов эжекторного типа

AbstractA device making it possible to control of the flow past a wing at low flight speeds is proposed. The device comprises an ejector-type actuator that simultaneous sucks the boundary layer on the upper surface and blows a gas jet in the vicinity of the trailing edge. Based on the Reynolds-averaged Navier–Stokes equations, a mathematical model of an ejector-type actuator is developed and experimental studies are carried out.

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.

Mechanical power curve measured in the wake of pied flycatchers indicates modulation of parasite power across flight speeds.

How aerodynamic power required for animal flight varies with flight speed determines optimal speeds during foraging and migratory flight. Despite its relevance, aerodynamic power provides an elusive quantity to measure directly in animal flight. Here, we determine the aerodynamic power from wake velocity fields, measured using tomographical particle image velocimetry, of pied flycatchers flying freely in a wind tunnel. We find a shallow U-shaped power curve, which is flatter than expected by theory. Based on how the birds vary body angle with speed, we speculate that the shallow curve results from increased body drag coefficient and body frontal area at lower flight speeds. Including modulation of body drag in the model results in a more reasonable fit with data than the traditional model. From the wake structure, we also find a single starting vortex generated from the two wings during the downstroke across flight speeds (1-9 m s-1). This is accomplished by the arm wings interacting at the beginning of the downstroke, generating a unified starting vortex above the body of the bird. We interpret this as a mechanism resulting in a rather uniform downwash and low induced power, which can help explain the higher aerodynamic performance in birds compared with bats.

Calculation Method of Stability Derivatives and Verification for Power-lift Aircraft

It is difficult to realize the STOL and high-speed cruise at one time when designing an aircraft, whereas the application of power-lift technology has successfully solved this technical problem. The research about the dynamic characteristics of power-lift aircraft in hover and low speed flight regime is key important for the selection of control strategy in transition process. However, if adopting the traditional aerodynamic calculation method based on thrust coefficient concept, the hover and low speed forward flight of power-lift aircraft cannot be studied as a unified process and the stability derivatives cannot be accurately calculated in low speed. In this paper, a method for calculating the stability derivatives and model the aircraft by using full dimensional aerodynamic data is proposed, which can not only solve the problems mentioned above, and it is very convenient for the design of the control law in the transient process.

Mapping of radiation anomalies using UAV mini-airborne gamma-ray spectrometry

Localization of size-limited gamma-ray anomalies plays a fundamental role in uranium prospecting and environmental studies. Possibilities of a newly developed mini-airborne gamma-ray spectrometric equipment were tested on a uranium anomaly near the village of Třebsko, Czech Republic. The measurement equipment was based on a scintillation gamma-ray spectrometer specially developed for unmanned aerial vehicles (UAV) mounted on powerful hexacopter. The gamma-ray spectrometer has two 103 cm3 BGO scintillation detectors of relatively high sensitivity. The tested anomaly, which is 80 m by 40 m in size, was investigated by ground gamma-ray spectrometric measurement in a detail rectangular measurement grid. Average uranium concentration is 25 mg/kg eU attaining 700 mg/kg eU locally. The mini-airborne measurement across the anomaly was carried out on three 100 m long parallel profiles at eight flight altitudes from 5 to 40 m above the ground. The resulting 1 s 1024 channel gamma-ray spectra, recorded in counts per second (cps), were processed to concentration units of K, U and Th, while total count (TC) was reported in cps. Increased gamma ray intensity of the anomaly was indicated by mini-airborne measurement at all profiles and altitudes, including the highest altitude of 40 m, at which the recorded intensity is close to the natural radiation background. The reported instrument is able to record data with comparable quality as standard airborne survey, due to relative sensitive detector, lower flight altitude and relatively low flight speed of 1 m/s. The presented experiment brings new experience with using unmanned semi-autonomous aerial vehicles and the latest mini-airborne radiometric instrument. The experiment has demonstrated the instrument's ability to localize size-limited uranium anomalies.

Effects of Stroke Deviation on Aerodynamic Force Production of a Flapping Wing

The effects of stroke deviation of a flapping model insect wing on its aerodynamic force production are studied by solving the Navier–Stokes equations. Stroke deviation of a flapping wing often takes the forms of various out-of-plane wing-tip trajectories that depend on the time variation of the stroke deviation angle. In the present paper, four typical wing-tip trajectories, referred to as cases A–D, were used to investigate the effect of stroke deviation by comparing the aerodynamic forces, flows, and power requirements of the flapping wings with and without stroke deviation. The stroke deviation has remarkable influences on the time course of the aerodynamic force of a flapping wing. The influences could be explained by two mechanisms: one is the added-rotation effect, which is introduced by the deviation angle of the wing; the other is the change in effective angle of attack of the wing, which is due to the angular speed of the deviation. In general, the stroke deviation only produces a small effect on the cycle-mean aerodynamic forces, because the changes due to stroke deviation could be roughly offset in a wingbeat cycle. The analysis of power requirements shows that stroke deviation will generally lead to an increase in energy consumption for insect flights, indicating that insects should have the tendency to maintain the flapping motion in a plane for hovering or low-speed flight, as is observed in nature.

Infrared Signature Suppression Systems in Modern Military Helicopters

Abstract Helicopters play an important role in air-to-ground fire covering and the short-distance air-to-air fights, as well as the anti-tank missions and battlefield force transferring. The detection and survivability of helicopters on a battlefield significantly depends on their infrared emissions level, as well as the methods, equipment and systems used by potential enemy. The automatic detection systems, recognition and identification of flying objects use among other the thermo-detection methods, which rely on detecting the infrared radiation emitted by the tracked object. Furthermore, due to low-altitude and relatively low flight speed, today’s combat assets like missile weapons equipped with infrared guidance systems are one of the most important threats to the helicopters performing combat missions. Especially meaningful in a helicopter aviation is infrared emission by exhaust gases, egressed to the surroundings. Due to high temperature, exhaust gases are a major factor in detectability of a helicopter performing air combat operations. In order to increase the combat effectiveness and survivability of military helicopters, several different types of the infrared suppressor (IRS) have been developed. This paper reviews contemporary developments in this discipline, with particular examples of the infrared signature suppression systems.

Procedure for Determining Operation Limits of High-Altitude Long-Endurance Aircraft Under Icing Conditions

To ensure flight safety and expand the availability of high-altitude long-endurance aircraft, the present study suggests a novel procedure for determining mission success or failure under icing conditions. First, the ice accretion shapes during the climbing stage are predicted under icing conditions using the one-shot method. Then, the total ice mass on the high-altitude long-endurance aircraft surface and the resulting aerodynamic performance is numerically calculated. Subsequently, metamodels are constructed to correlate meteorological parameters and aerodynamic performance. Finally, the total required power to reach the mission altitude is compared with the onboard battery capacity to make a quantitative judgment about mission success. The proposed procedure is applied to the , with a maximum takeoff weight of 20 kg, developed by Korea Aerospace Research Institute. The results show that, when the liquid water content is and median volumetric droplet diameter is below , the battery margin falls below 10%, implying that particular attention is required when operating the aircraft under those meteorological conditions. Second, when using the one-shot method to predict ice shapes, the flight speed of aircraft and air density should be taken at a midpoint altitude between the ground and the highest altitude at which droplets exist. Finally, due to the low flight speed of aircraft, only a small number of water droplets adhere to the surface, and there is little convective heat transfer, resulting in rime ice shapes even at high temperatures.

A novel yaw control method for flying-wing aircraft in low speed regime

The innovative control effectors (ICEs) such as split drag rudder (SDR) or spoiler slot deflector (SSD) have been proved useful for the directional control of flying-wing aircraft. As opposed to the traditional rudder, the yaw control moment produced by a SDR is non-linear with respect to its deflection. The control reversal phenomenon arises at higher angle-of-attack in the low speed regime, causing unwanted directional oscillations. Such a behavior may lead to flight accident while landing or take-off. The aerodynamic efficiency of the SDR is investigated for a miniature flying-wing aircraft (XQ-6B) using computational fluid dynamics (CFD) approach. The classical unilateral SDR operating mode and the biased-differential SDR operating mode are presented and discussed. A novel control strategy based on the angle-of-attack feedback for the biased-differential deflection is proposed and verified in nonlinear numerical simulation. The results show that the non-linearity and control reversal problems present in the behavior of SDR can be alleviated with this scheme. Finally, the results obtained by testing the proposed technique in real low speed flight are presented in order to validate the improvements in the yaw control capability of SDR.

Hybrid Reynolds-Averaged Navier–Stokes/Large-Eddy Simulations of F-16XL in Low-Speed High-Alpha Flight

This paper reports on the computational fluid dynamics study of flow around the F-16XL aircraft in low-speed high-alpha flight. Previous work established that the computed pressure for this case compared less favorably with those measured in flight tests than did similar comparisons for cases at lower angles of attack. One reason suggested for the discrepancy was that the flow over the outer-wing panel was unsteady. This paper presents time-accurate computations with physical modeling that can capture such unsteady flow phenomena, namely, so-called hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation modeling. The simulations obtained are compared with those measured during flight testing of the vehicle as well as with results computed with more conventional steady physical modeling. Over the outer wing panel, the unsteady simulations compare substantially better than the steady results with the flight-test data, confirming unsteady aerodynamic effects are at play. At the inner-wing locations, the correlations among the simulations (unsteady and steady) and with the flight test are good, confirming the suspicion that the flow is mostly steady there. Although differences are found among all the results compared, they are not exceedingly large; but, the conclusion that unsteady aerodynamic effects are at play over the outer-wing panel are undeniable.

Analysis of Helicopter Handling Quality in Turbulence with Recursive Von Kármán Model

A recursive von Kármán turbulence model is developed and integrated into the current distributed turbulence model for helicopter flight simulation and handling quality analysis in atmospheric turbulence. First, high-order filters are developed with rational expressions to approximate the von Kármán spectra. Recursive algorithms for modeling the von Kármán turbulence are derived by discretizing the filters with the Tustin transform. Then, the recursive von Kármán turbulence model is used to replace the Dryden model in the current distributed turbulence model for helicopter flight simulation in atmospheric turbulence. The results show that the spectra of the recursive von Kármán turbulence have an excellent agreement with the theoretical von Kármán spectra, and they are valid over a normalized frequency range up to 400 rad, which is suitable for helicopter handling quality analysis in atmospheric turbulence with low flight speed and large turbulence scale lengths. The present turbulence model significantly extends the capability of helicopter handling quality analysis to the higher-altitude and lower-velocity flight conditions compared with the current distributed turbulence model.

The defensive behavioral patterns of captive white-lipped and collared peccary (Mammalia, Tayassuidae): an approach for conservation of the species

Defensive behavioral patterns in response to human-induced rapid environmental change can affect animals’ fitness and may play a role in species conservation status. To test this hypothesis, we compared the risk assessment and defensive behavioral responses of captive white-lipped peccary (WLP; Tayassu pecari) and collared peccary (CP; Pecari tajacu), which retain different conservation status; WLP are considered vulnerable and CP of least concern. We used an adapted paradigm of the mouse defense test battery (MDTB) comprising four consecutive tests. Two of these tests simulated a novel environment, while the other two stimulated the expression of defensive behavioral patterns. Besides differences in risk assessment and defensive threat/attack behavioral patterns between species, we compared flight initiation distance, flight speed, and plasma glucocorticoid concentrations. When facing a novel environment and risk challenges from humans’ predator-like cues, the white-lipped peccary showed more exploratory and defensive threat/attack behavioral patterns, shorter flight initiation distances, and lower flight speeds, whereas the collared peccaries showed more cautious and retreat patterns, longer flight initiation distances, and higher flight speeds. There were also correlations between physiological and behavioral parameters. We confirmed our hypothesis that the collared peccary’s cautiousness may help to prevent a decrease in its population, while the white-lipped peccary’s exploratory and confrontational behavioral patterns in overhunted areas, together with other simultaneous factors as forest fragmentation, might contribute to placing this species in the vulnerable category.

Real-time solution of nonlinear potential flow equations for lifting rotors

Analysis of rotorcraft dynamics requires solution of the rotor induced flow field. Often, the appropriate model to be used for induced flow is nonlinear potential flow theory (which is the basis of vortex-lattice methods). These nonlinear potential flow equations sometimes must be solved in real time––such as for real-time flight simulation, when observers are needed for controllers, or in preliminary design computations. In this paper, the major effects of nonlinearities on induced flow are studied for lifting rotors in low-speed flight and hover. The approach is to use a nonlinear state-space model of the induced flow based on a Galerkin treatment of the potential flow equations.

Long-term effects of good handling practices during the pre-weaning period of crossbred dairy heifer calves.

The aim of this study was to determine whether applying good practices of handling during the pre-weaning period have long-term effects on behavioral and physiological indicators, health status, and average daily gain (ADG) of crossbred Bos taurus × Bos indicus heifer calves. During the pre-weaning period, 98 crossbred of Holstein × Gir heifer calves were allotted into three treatments: (1) good practices of handling + brushing (GPB; n = 25), (2) good practices of handling (GP; n = 25), and (3) control (n = 48). Every 2months, four evaluation periods (EV1 to EV4) were conducted to record data. Behavioral indicators comprised time to drive (TD), flight speed (FS), flight distance (FD), and composite reactivity score (CRS). Physiological indicators of acute stress during handling comprised respiratory and heart rates. Health status comprised data regarding occurrence of most common diseases (i.e., pneumonia and anaplasmosis). Collected data were analyzed by using a linear mixed model for repeated measures, Tukey's test, and chi-squared procedures. Treatment influenced (P < 0.05) TD, FS, and FD but not CRS (P = 0.78). From EV1 to EV3, the control calves had the lowest TD. The GPB group had lower FS than the control but did not differ from GP. The GPB group had lower FD means than the other two groups in EV2, EV3, and EV4. No differences (P > 0.05) due to treatment were observed on heart and respiratory rates, ADG, or occurrence of pneumonia and anaplasmosis. It was concluded that adoption of good practices of handling during pre-weaning period may lead to long-term positive effects.

Improving the noise stability of measuring the altitude and speed parameters aboard a helicopter

We consider the problem of measuring the altitude and speed parameters aboard a helicopter, as well as ways to improve the noise stability and accuracy of measuring at low and nearzero flight speeds at the expense of using the air flow data of the main rotor vortex column, the principles of complexation and filtering.

Frequent handling of grazing beef cattle maintained under the rotational stocking method improves temperament over time

Our aims were to assess the effects of handling frequency on temperament evolution in cattle kept under rotational stocking method (RSM), as well as to compare the temperament of animals maintained under rotational versus alternate stocking (ASM) methods. We evaluated 4256 males raised on pasture from four private properties (Farms 1–4), for a total of 9628 observations. The study was divided into two complementary approaches. In the first one, monthly visits were conducted at 35-day intervals to assess the cattle temperament (n = 1979) raised on grazing system using RSM, while, in the second approach, we evaluated the temperament of cattle (n = 3600) kept under two different stocking methods, namely, (1) RSM (n = 2007), characterised by a high frequency of animal handling and a 4-day grazing period, and (2) ASM (n = 1593), characterised by a low frequency of handling and a 20-day grazing period. Composite reactivity score (RS), and flight speed (FS) were considered as temperament indicators. Reductions of RS and FS means were observed during successive assessments under RSM. Significant effects of stocking method on FS (P &lt; 0.05) and RS (P &lt; 0.01) were found, with lower FS and RS means for animals under RSM than for those under ASM. In conclusion, the frequent handling in the rotational stocking method favoured the improvement of cattle temperament over time, with lower levels of reactivity among cattle raised under this stocking method than with animals raised under the alternate stocking method.

Optimisation of Ducted Propellers for Hybrid Air Vehicles Using High-Fidelity CFD

ABSTRACTThis paper presents performance analysis and design of ducted propellers for lighter-than-air vehicles. High-fidelity computational fluid dynamics simulations were first performed on a detailed model of the propulsor, and the results were in very good agreement with available experimental data. Additional simulations were performed using a simplified geometry, to quantify the effect of the duct and of the blade twist on the propeller performance. It was shown that the duct is particularly effective at low flight speed and that the blades with relatively high twist have better performance over the flight envelope. Design of the optimal twist distribution and of the duct shape was also attempted by coupling the flow solver with a quasi-Newton optimisation method. Flow gradients were computed by solving the discrete adjoint of the Reynolds-averaged Navier-Stokes equations using a fixed-point iteration scheme or a nested Krylov method with deflated restarting. The results show that the ducted propeller propulsive efficiency can be increased by 2%.

Effect of nose shape on the shock standoff distance at nearsonic flows

This paper describes a numerical solution of the bow shock shape ahead of some blunt and sharp axisymmetric noses containing sphere, blunt cone, and sharp cone at steady transonic flow in the Mach number range of 1.01 to 1.2. For validating the results, one sphere and three blunt cones are modeled, and their shock standoff distance is compared with other experimental and numerical studies. The flow over other noses with similar geometric parameters is then solved and compared with each other. In this study, the Reynolds-averaged Navier—Stokes equations are solved using the Spalart—Allmaras turbulence model. The purpose of this study is to determine the shock standoff distance for some blunt and sharp noses at low supersonic free flight speed. The shock standoff distance is determined from the Mach number curve on the symmetry line. The present numerical simulations reach down to M8=1.01 a range where it is almost very difficult to set in experimental studies. The shock wave locations were found to agree well with previous numerical and experimental studies. Our results are closer to the experimental results compared to other numerical studies. In addition, the results for shock standoff distances over paraboloids in these speed ranges have not been previously published as far as we know.