Low-speed Flight Research Articles

Effect of active Gurney flaps on overall helicopter flight envelope

ABSTRACTThis paper presents a study of the W3-Sokol main rotor equipped with Gurney flaps. The effect of the active Gurney is tested at low and high forward flight speeds to draw conclusions about the potential enhancement of the rotorcraft performance for the whole flight envelope. The effect of the flap on the trimming and handling of a full helicopter is also investigated. Fluid and structure dynamics were coupled in all cases, and the rotor was trimmed at different thrust coefficients. The Gurney proved to be efficient at medium to high advance ratios, where the power requirements of the rotor were decreased by up to 3.3%. However, the 1/rev actuation of the flap might be an issue for the trimming and handling of the helicopter. The current study builds on the idea that any active mechanism operating on a rotor could alter the dynamics and the handling of the helicopter. A closed loop actuation of the Gurney flap was put forward based on a pressure divergence criterion, and it led to further enhancement of the aerodynamic performance. Next, a generic light utility helicopter was built using 2D aerodynamics of the main aerofoil section of the W3 Sokol blade along with a robust controller, and the response of the rotorcraft to control inputs was tested. This analysis proved that the 1/Rev actuation of the Gurney did not alter the handling qualities of the helicopter, and as a result, it can be implemented as a flow control mechanism for aerodynamic enhancement and retreating blade stall alleviation.

Calculation method of blade element aerodynamic loads based on small perturbation theory

A calculation method of blade element aerodynamic loads based on small perturbation theory is established for helicopter flight simulation application. The method synthesizes flap-lag-torsion elastic rotor blade mode,couples rotor dynamic inflow,unsteady aerodynamics model with dynamic stall and rotor trimming model. The entire model is developed in a standard state-space form. To verify this model,the UH-60 A helicopter is taken as an example,and the effects of yaw flow on rotor aerodynamic performance are studied at low speed flight and high speed flight. It is shown that yaw flow has a greater effect on rotor aerodynamic performance at high speed flight. Blade induced velocity fields and aerodynamic loads are simulated,and the results are compared with flight test data. It is shown by the results that the method can accurately predict the unsteady aerodynamic rotor loads in steady flight by capturing its variational characteristics along blade azimuthal locations,and it is still applicable for high speed flight.

低速度領域におけるスタビレス産業用無人ヘリコプタの運動特性

Increasing total thrust of unmanned helicopter is highly demanded. Stabilizer-bar system is widely applied to improve stability and handling quality of unmanned helicopter. It, however, is a serious obstacle to increase the total thrust. Handling quality of stabi-less unmanned helicopter, which does not have stabilizer-bar system, is usually inferior to conventional unmanned helicopter because of large and very fast cross-coupled responses. For safe operation of stabi-less unmanned helicopter, development of flight control system is required. The purpose of this paper is to develop an accurate model of stabi-less unmanned helicopter. We focused on low-speed flight and induced velocity's change due to angular rate and forward speed. Following results were obtained based on analysis of the derived model; Uniform change of the induced velocity causes trim change due to flight speed and does not cause cross-coupled responses. Non-uniform distribution of the induced velocity causes large and fast cross-coupled responses. Numerical simulation data agree with flight experimental data using a specially developed stabi-less unmanned helicopter. Consequently, a dynamical model proposed in this paper is enough accurate to describe properties of stabi-less unmanned helicopter at low-speed flight.

A Coupled Helicopter Rotor/Fuselage Dynamics Model Using Finite Element Multi-body

To develop a coupled rotor/flexible fuselage model for vibration reduction studies, the equation of coupled rotor-fuselage is set up based on the theory of multi-body dynamics, and the dynamic analysis model is established with the software MSC.ADMAS and MSC.NASTRAN. The frequencies and vibration acceleration responses of the system are calculated with the model of coupled rotor-fuselage, and the results are compared with those of uncoupled modeling method. Analysis results showed that compared with uncoupled model, the dynamic characteristic obtained by the model of coupled rotor-fuselage are some different. The intrinsic frequency of rotor is increased with the increase of rotational velocities. The results also show that the flying speed has obvious influence on the vibration acceleration responses of the fuselage. The vibration acceleration response in the vertical direction is much higher at the low speed and high speed flight conditions.

1408 前進翼の低速空力特性について(流体工学VIII)

1408 前進翼の低速空力特性について(流体工学VIII)

Single propeller airplane minimal flight speed based upon the lateral maneuver condition

This paper presents the application of the previously presented general analysis method to determine the safe flight boundaries of the asymmetrically loaded airplane within the terminal flight phases as applied to the case of inherently asymmetric single propeller airplane. As the lateral control surface design is done for the flight conditions out of the terminal flight phases, the key objective is to improve the flight safety of the asymmetrically loaded airplane by introducing the lateral flight controls verification at the low flight speeds. The concept of the authority of control surfaces presenting their capability to generate the forces and moments needed by the airplane to perform required maneuvers is the basis of the analysis. Control surface authority is the function of the control surface aerodynamic properties, structurally available flight control displacements and dynamic pressure. The analysis method scope is based upon the requirement to supplement the safe flight boundaries of symmetrically loaded airplane within the terminal flight phases, with the lift coefficient observed as the function of the angle of attack being at the linear limit. Control surface demands are lateral maneuver execution and asymmetric load and lateral wind compensation, the method scope permitting them to be addictive. Thus defined, the method is based upon the comparison of the available control surface authority and demands. For the defined flight conditions, the analysis is reduced to the comparison of the demanded and structurally available flight control displacement. The method combines the simple roll dynamics model, the stationary equations of the airplane lateral-directional motion and several numeric analysis procedures to obtain the results. This new combination possesses synergy properties and is implemented as the computer program. The method is applicable for any combination of airplane asymmetric loads and can be used throughout entire airplane life cycle. The contemporary trend of downsizing training and light combat airplane types with the rising number of the introduced medium and high power single propeller airplane types increases the significance of the method application in the design procedure.

Particle-image velocimetry investigation of the fluid-structure interaction mechanisms of a natural owl wing

The increasing interest in the development of small flying air vehicles has given rise to a strong need to thoroughly understand low-speed aerodynamics. The barn owl is a well-known example of a biological system that possesses a high level of adaptation to its habitat and as such can inspire future small-scale air vehicle design. The combination of the owl-specific wing geometry and plumage adaptations with the flexibility of the wing structure yields a highly complex flow field, still enabling the owl to perform stable and at the same time silent low-speed gliding flight. To investigate the effects leading to such a characteristic flight, time-resolved stereoscopic particle-image velocimetry (TR-SPIV) measurements are performed on a prepared natural owl wing in a range of angles of attack and Reynolds numbers 40 000 based on the chord length at a position located at 30% of the halfspan from the owl’s body. The flow field does not show any flow separation on the suction side, whereas flow separation is found on the pressure side for all investigated cases. The flow field on the pressure side is characterized by large-scale vortices which interact with the flexible wing structure. The good agreement of the shedding frequency of the pressure side vortices with the frequency of the trailing-edge deflection indicates that the structural deformation is induced by the flow field on the pressure side. Additionally, the reduction of the time-averaged mean wing curvature at high Reynolds numbers indicates a passive lift-control mechanism that provides constant lift in the entire flight envelope of the owl.

Performance Comparison between Optimised Camber and Span for a Morphing Wing

Morphing technology offers a strategy to modify the wing geometry, and the wing planform and cross-sectional parameters can be optimised to the flight conditions. This paper presents an investigation into the effect of span and camber morphing on the mission performance of a 25-kg UAV, with a straight, rectangular, unswept wing. The wing is optimised over two velocities for various fixed wing and morphing wing strategies, where the objective is to maximise aerodynamic efficiency or range. The investigation analyses the effect of the low and high speed velocity selected, the weighting of the low and high velocity on the computation of the mission parameter, the maximum allowable span retraction and the weight penalty on the mission performance. Models that represent the adaptive aspect ratio (AdAR) span morphing concept and the fish bone active camber (FishBAC) camber morphing concept are used to investigate the effect on the wing parameters. The results indicate that generally morphing for both span and camber, the aerodynamic efficiency is maximised for a 30%–70% to 40%–60% weighting between the low and high speed flight conditions, respectively. The span morphing strategy with optimised fixed camber at the root can deliver up to 25% improvement in the aerodynamic efficiency over a fixed camber and span, for an allowable 50% retraction with a velocity range of 50–115 kph. Reducing the allowable retraction to 25% reduces the improvement to 8%–10% for a 50%–50% mission weighting. Camber morphing offers a maximum of 4.5% improvement approximately for a velocity range of 50–90 kph. Improvements in the efficiency achieved through camber morphing are more sensitive to the velocity range in the mission, generally decreasing rapidly by reducing or increasing the velocity range, where span morphing appears more robust for an increase in velocity range beyond the optimum. However, where span morphing requires considerable modification to the planform, the camber change required for optimum performance is only a 5% trailing edge tip deflection relative to cross-sectional chord length. Span morphing, at the optimal mission velocity range, with 25% allowable retraction, can allow up to a 12% increase in mass before no performance advantage is observed, where the camber morphing only allows up to 3%. This provides the designer with a mass budget that must be achieved for morphing to be viable to increase the mission performance.

Strong geographical variation in wing aspect ratio of a damselfly, Calopteryx maculata (Odonata: Zygoptera).

Geographical patterns in body size have been described across a wide range of species, leading to the development of a series of fundamental biological rules. However, shape variables are less well-described despite having substantial consequences for organism performance. Wing aspect ratio (AR) has been proposed as a key shape parameter that determines function in flying animals, with high AR corresponding to longer, thinner wings that promote high manoeuvrability, low speed flight, and low AR corresponding to shorter, broader wings that promote high efficiency long distance flight. From this principle it might be predicted that populations living in cooler areas would exhibit low AR wings to compensate for reduced muscle efficiency at lower temperatures. I test this hypothesis using the riverine damselfly, Calopteryx maculata, sampled from 34 sites across its range margin in North America. Nine hundred and seven male specimens were captured from across the 34 sites (mean = 26.7 ± 2.9 SE per site), dissected and measured to quantify the area and length of all four wings. Geometric morphometrics were employed to investigate geographical variation in wing shape. The majority of variation in wing shape involved changes in wing aspect ratio, confirmed independently by geometric morphometrics and wing measurements. There was a strong negative relationship between wing aspect ratio and the maximum temperature of the warmest month which varies from west-east in North America, creating a positive relationship with longitude. This pattern suggests that higher aspect ratio may be associated with areas in which greater flight efficiency is required: regions of lower temperatures during the flight season. I discuss my findings in light of research of the functional ecology of wing shape across vertebrate and invertebrate taxa.

Impact of the temperament of Nellore cows on the quality of handling and pregnancy rates in fixed-time artificial insemination

The aim of this study was to evaluate the effects of temperament on the quality and efficiency of handling and on the pregnancy rate of Nellore cows submitted to a fixed time artificial insemination (FTAI) protocol. Temperament and handling procedures were simultaneously assessed in 798 Nellore cows on the first (d0), ninth (d9) and 11th days (d11) of the FTAI protocol, using the flight speed (FS) and crush score (CS) tests. During the insemination process performed on d11 of the FTAI protocol, the following handling indicators were recorded: rough or overly aggressive handling of the cows by stockpersons (AGGRESSION), accidents (ACCIDENT), time to inseminate (TIME), and the degree of dirtiness on cows’ perineal region (DIRTINESS). There was a significant effect of CS on TIME (P=0.013) and DIRTINESS (P=0.004), while FS tended to affect TIME (P=0.06) and the likelihood of AGGRESSION (P=0.07). Cows subjected to aggressive handling and/or accidents had a greater FS mean than cows that did not face such aversive situations (2.07±1.18 vs. 1.74±0.75m/s, P=0.0003). The chance of pregnancy (expressed in odds ratio) was greater in cows with low FS (OR=1.48) than in cows with high FS (defined as the reference class, with OR=1) (χ2=3.73, P=0.05), and the low FS group had 10% more pregnant cows than the high FS group (with 52.59% vs. 42.62% pregnant cows, respectively). Pregnant cows had significantly lower FS means relative to non-pregnant cows on d0 (1.79 vs. 2.10m/s, P<0.001), d9 (1.67m/s vs. 1.79m/s, P=0.038) and d11 (1.70m/s vs. 1.90m/s, P=0.004). We conclude that excitable temperament has an effect on the quality and efficiency of handling during FTAI, by increasing the time required for insemination, the dirtiness on cows’ perineal region and the likelihood of aggressive actions by stockpersons. Also, cattle temperament measured by FS reduces the chance of pregnancy in Nellore beef cows.

Command-filtered backstepping control for a multi-vectored thrust stratospheric airship

A stratospheric airship is an airship flying at a high altitude of 20 km as a stratospheric platform. Due to the low atmospheric density and flight speed, the efficiency of a conventional actuator, such as an aerodynamic control surface, is decreased. Thus, a new multi-vectored thrust airship called a flat peach is discussed in this paper. This article describes the derivation, design and simulation implementation of a non-linear controller for an airship with multi-vector thrust. The controller is designed using a command-filtered, vector backstepping approach, and can set the airship track three Cartesian positions [Formula: see text] and yaw angle to their desired values and stabilize the pitch and roll angles. The controller is non-linear to address the kinematics and airship dynamics. The approach guarantees exponential stability of a compensated tracking error in the sense of Lyapunov. Both the stability analysis and simulation results are included.

Robustness investigation of a ducted-fan aerial vehicle control, using linear, adaptive, and model predictive controllers

A comparison of three common controllers for stabilising a vertical take-off and landing air vehicle is presented. RMIT is a small sized tail-sitter ducted fan air vehicle with a particular configuration layout, multiple control surfaces, low weight, and high-speed flight capability. The main problem here is control effectiveness at low flight speeds and transition manoeuvres because of the inherent instability. In the current study, a comprehensive nonlinear model is firstly developed for RMIT, followed by a validation process. Subsequently, linear, adaptive and model predictive controllers are designed in vertical flight. Based on the simulation results, it is shown that the linear controller is not able to eliminate the inherent instability of the vehicle in hover, while the adaptive and model predictive controllers are able to tune for stabilisation. However, simulation results show that adaptive controller cannot eliminate the large external disturbances from the system, while the MPC has a robust behaviour against them and provide an acceptable closed-loop performance over a wide range of external disturbances.

Helicopter Performance Improvement with Variable Rotor Radius and RPM

This paper examines rotor power reductions achievable through a combination of radius and RPM variation. The study is based on a utility helicopter similar to the UH-60A and considers +17% to –16% variation in radius and ±11% variation in RPM about the baseline, over a range of airspeed, gross weight, and altitude. Results show that decreasing RPM alone effectively reduced power at cruise velocities in low-and-light conditions, but the power reductions diminished at increasing altitude and/or gross weight, and in low-speed flight. Increasing radius alone, on the other hand, had greatest effectiveness in power reduction in high-and-heavy operating conditions and at lower flight speeds. When radius and RPM variation is used in combination, minimum RPM is always favored, along with radius increases at increasing altitude and gross weight, and in low-speed operation. At low-to-moderate gross weight, the significant power reductions seen in cruise and at low altitude with RPM variation alone are obtained even at higher altitude, and over the airspeed range, using radius and RPM variation in combination. In high-and-heavy conditions, the combination of RPM reduction and radius increase yields very large power reductions of over 20% and up to 30% over the baseline. Power reduction in low-and-light conditions comes almost entirely from profile power reduction due to RPM decrease. In cruise and high-speed flight, the profile power reductions progressively give way to induced power reductions at increasing gross weight and altitude. At low speeds, reduction in induced power due to increased radius and decreased disk loading dominates.

Prediction of Fuselage Surface Pressures in Rotor–Fuselage Interactions Using an Integral Solution of Poisson Equation

The fuselage of a helicopter is strongly affected by the rotor and its wake under hovering and low-speed forward flight conditions. Unsteady loading is observed on the fuselage surface owing to the blade passing effect and periodic collision of the wake vortices. Panel and vortex methods were combined for simulating rotor–fuselage aerodynamic interaction. Although the panel method involves the use of Bernoulli's equation to obtain the surface pressure, it is difficult to consider the increase in stagnation pressure in the vortical flow region under the rotor. In this study, an integral solution of the Poisson equation was adopted to calculate the pressure at the surface of the fuselage. The flow around the Georgia Institute of Technology's rotor–fuselage model was simulated to demonstrate the increase in stagnation pressure along the topline of the fuselage, during which unsteady loading caused by tip vortex impingement was observed. The ROBIN (rotor body interaction) model, which considers a generic helicopter fuselage, was also used to analyze the unsteady pressures at the top and the side of the fuselage. For the models, the Poisson equation was found to accurately predict both unsteady pressure fluctuation and stagnation pressure increase on the fuselage in the presence of the rotor wake without any special treatment.

A Differential Configuration of Split Drag-Rudders with Variable Bias for Directional Control of Flying-Wing

The directional stability and control is crucial for the low-speed flight of a flying-wing aircraft. The split drag-rudders are well known devices used to provide directional stability and control in a flying-wing aircraft. As opposed to conventional rudders, the control efficiency of split drag-rudders is typically low for small deflection-angles and the influence on yawing moment is nonlinear. Such characteristics limit the control capability of split drag-rudders at low speed flight with large angle-of-attack. In this paper, a simple method is presented to improve the control efficiency of split drag-rudders at low speed flight with large angle-of-attack. The method is based on a strictly differential configuration of split drag-rudders which operates around a certain variable bias. The bias can be varied according to different flight conditions in order to achieve desired performance. The CFD simulation results are presented in support of this concept. Results also show that the proposed configuration has linearizing effects on yawing moment vs. deflection curves, which may prove helpful in control system design process. The possible control reversal in yaw at large angle of attack can also be avoided with this method.

Design and Construction of a Quad Tilt-Rotor UAV using Servo Motor

Unmanned aerial vehicles (UAVs) that have been recently commercialized can largely be divided into fixed-wing aircraft and rotor aircraft by their styles and flight characteristics. Although the fixed-wing aircraft represents higher power efficiency, higher speed, longer flight distance and larger loading weight than the rotor aircraft, they have a disadvantage of requiring a space for take-off and landing. On the other hand, the rotor aircraft can implement vertical take-off and landing (VTOL) and represents various flight modes (hovering, steep bank turns and low-speed flights). But they require both precision take-off control and attitude control. In this study, we used a quad-tilt rotor UAV to combine advantages in both the fixed-wing aircraft and the rotor aircraft. The quad-tilt rotor (QTR) system was designed and constructed by adding a tilt device with a servo motor to a general quad-rotor vehicle.

Highly Efficient Unsteady Panel Time-Marching Free Wake for Aerodynamics of Rotorcraft

A highly efficient unsteady panel time-marching free wake is established to achieve fast and accurate prediction of the unsteady aerodynamics of a helicopter. The unsteady panel method is used to calculate the airload of rotor blades, and the free-wake method is applied to simulate the dynamics of rotor wake. The two methods are tightly coupled by convecting panels to vortex filaments, and the rotor wake is divided into three parts to reduce the computational cost. The first is shed wake indicated by doublet panels. The second is near-wake filaments, which are described by a series of vortex filaments attaching to the shed wake. The third is far-wake filaments connecting to the near-wake filaments. A velocity-field integration technique is adopted to overcome the singularity problem during the interaction of the rotor wake with blades. Helicopter rotors including the Caradonna–Tung and the AH-1G rotors are simulated in hover and forward flight to validate the accuracy and efficiency of the present approach. The predicted blade pressure and load distribution agree well with the measured data and computational-fluid-dynamics results in hover and low-speed forward flight. Compared to computational fluid dynamics and the traditional panel/vortex-rings free-wake method, the present method is more efficient.

Multibody analyses for performance and aeromechanics of a rotor in low-speed flight

Purpose – This paper aims to correlate the flexible multibody analysis for the performance, blade airloads, rotor pitch control angles, and blade structural loads of a full-scale utility helicopter rotor in low-speed forward flight with wind tunnel test and flight test data. Design/methodology/approach – A nonlinear flexible multibody dynamics analysis code, DYMORE, is used to analyze the performance and aeromechanics of a utility helicopter rotor in low-speed forward flight. The main rotor system is modeled using various multibody elements such as rigid bodies, nonlinear elastic beams, mechanical joints, and elastic springs/dampers. The freewake model is used to capture rotor wakes more elaborately in low-speed forward flight. Findings – Fair to good correlations of rotor performance such as figure of merit in hover, rotor power, propulsive force, and lift in low-speed forward flight are achieved with sweeps of the thrust, rotor shaft tilting angle, and advance ratio, against wind tunnel test data. The b...

Powering Out of Trouble

This article presents a study of Pratt &amp; Whitney’s J58, till date the best and high-powered engine for manufacturing lessons required for the development of F35 Joint Strike Fighter. The J58 Blackbird engine is a variable cycle engine, a turbojet/ramjet combined-cycle engine. It is a conventional afterburning turbojet for take-off and transonic flight, and it approximates a ramjet during high-speed supersonic cruise. The power plant for the Blackbirds is a marvelous development on the part of Pratt &amp; Whitney, as it is the only engine of its kind in the world. The noise and vibration from a J58 test was so great that it could rattle the side-view mirror off nearby cars. The engine was developed at an isolated research center in Florida. At take-off and low-speed flight, the J58 engine/afterburner provides most of the thrust. Both of the Blackbird’s twin nacelles contain an engine supersonic inlet, the J58 engine with its afterburner, and an exhaust ejector nozzle. All three components contribute to the Blackbird’s propulsive thrust in varying proportions, depending on flight speed.

High Performance Propeller System for a Multi-Mission Micro Aerial Vehicle

The new concept of a multi-mission micro air vehicle (MMMAV) was designed in order to enhance the efficiency, endurance and performance of this small vehicle which must be able to perform several capabilities: vertical take-off and landing (VTOL), hovering, low speed translation and high forward speed flight. The key factor of achievement is the propulsion system. Firstly, to ensure the capacity of the new propeller system concept, a high performance propeller system was designed and studied. Two testing conditions were investigated: hovering and a forward flight at a desired maximum speed. Initially, an analytical approach was adopted with preliminary calculations using XROTOR and CROTOR code to determine the 10 best propellers with optimal efficiency. Then, an experiment of both static and dynamic thrust was conducted. The rotation speed, thrust, torque, electric tension and current were carefully measured and compared with measurements taken for the in-line ‘APC-7×5’ propellers which are used by other MMMAVs. The results showed the new coaxial propeller system was highly efficient when combined with the ‘GWS9×4.7 Slow’ and the ‘APC6×4’ propellers. The power required was reduced by approximately 20% and 30% for hovering and forward flight, respectively.