Browse Topic: Optics
Helicopter tail shake constitutes a significant limitation to both passenger comfort and aircraft stability. Under powered descent conditions, elevated Angle of Attack (AoA) cause flow separation around the rotor hub and engine cowling, leading to the development of an unsteady wake dominated by large-scale turbulent structures. To support the helicopter tail shake phenomenon investigation, a dedicated Particle Image Velocimetry (PIV) experimental setup was designed in this work, together with four aerodynamic devices aimed at mitigating tail shake. These components were then tested through a wind tunnel campaign with the PIV setup. The proposed aerodynamic components were conceived to either deflect the hub wake away from the tail empennages or to decrease the Turbulent Kinetic Energy (TKE) within the wake. To achieve these objectives, a dorsal fin, a horse-collar, and two spoiler configurations inspired by automotive applications were designed and experimentally evaluated. The devices were tested both as standalone solutions and in combined arrangements on a scaled helicopter wind tunnel model featuring a rotating hub and blade shanks. The vertical velocity component, was used as an indicator of wake deflection, and the Turbulent Kinetic Energy was used as an indicator of wake turbulence. The Horse Collar and the Large Spoiler showed a reduction in both indicators suggesting possible tail shake mitigating capabilities, and additional improvements were achieved when the two devices were deployed in combination.
An experimental investigation was conducted to characterize the effects of partial-ground on the aerodynamics of a hovering rotor. A model-scale rotor was tested at a range of heights above ground and under partial-ground coverage, and rotor hub forces and moments were measured using a six-axis force/torque transducer during constant-power operation. The measurements were used to develop a semi-empirical thrust ratio model that accurately captures trends from out-of-ground effect to full-ground effect conditions. This model predicts realistic thrust behavior at low ground-coverage conditions, exhibiting high adjusted R2 and minimal root mean square error. Time-resolved particle image velocimetry was conducted for selected cases to examine induced flow features and to qualitatively assess changes in the downwash and edge-driven crossflow associated with partial-ground interactions. A geometric rotor-ground interaction area based on a circular-segment formulation was correlated to the thrust coefficient over the interacting region. Results show that thrust increases with ground coverage and decreases with increasing out-of-ground height, whereas moment coefficients increase with decreasing height. The increases in thrust and moment can be attributed to a reduction in the induced velocity above ground, as observed in the measured flow field, thereby increasing the effective ground-induced pressure beneath the rotor.
Rotor-rotor and rotor-boundary aerodynamic interactions of a quadrotor system without a fuselage in ground effect and ceiling effect for varying rotor-boundary distances and hub spacings were investigated. A GPU-accelerated Lattice-Boltzmann Method (LBM) was coupled to new unsteady actuator disk method (ADM) and actuator slice method (ASM) based rotor models for this purpose. Validation was conducted against experiments for both performance and particle image velocimetry flow field data. The trends in thrust and power were accurately predicted by both actuator methods, with high computational efficiency. Interactional flow physics were resolved, causing the consistent performance benefits very close to the ground, the performance penalties caused by the fountain flow effect between rotors occurring over a limited range of ground distances, and the persistent performance augmentation in ceiling effect. The ASM rotor model, with its individual blade representation, was found to predict rotor-rotor interactions more accurately than the ADM. However, it generally overpredicted aerodynamic loads, which was attributed to the Gaussian regularization applied along the blades. The results indicate that the GPU-accelerated LBM coupled with the introduced actuator methods is capable of computationally efficient prediction of multirotor aerodynamic performance and flow fields in ground and ceiling effects.
This experimental study showcases the changes to tip vortex evolution caused by unsteady plunging and pitching motions. Three cases of motion are tested on a NACA 0012 airfoil using time-resolved stereoscopic particle image velocimetry (PIV). Tests are taken while the airfoil is subjected to sinusoidal plunging and sinusoidal pitching motions. A control test is also taken for a static airfoil for the baseline comparisons. Changes to the tangential velocity, axial velocity, turbulence intensity, and Reynolds shear stress are analyzed for the duration of each cycle of motion and compared to the static results. For the plunging and pitching cases, the swirl velocity magnitude during the downstroke is significantly greater than static values, exhibiting greater levels of turbulence intensity and large Reynolds shear stress peaks about the vortex core. In contrast, data from the upstroke phase yields minimal turbulence contents, due to the loss of lift. The vortex grows weaker during the upstroke plunge and nose-up pitch from the lift decrease for the plunging and pitching cases, even dispersing before reaching the test section at z/c = 5 for a period of the oscillation. While the tangential and axial velocity magnitude are similar for the plunging and pitching cases, the local velocity maxima come at different times during the cycle, with the plunge cycle reaching its maximum first earlier in the downstroke phase. The static, plunging, and pitching cases follow a similar Richardson number trend, where the outer region of the vortex falls below the stratification threshold to become turbulent. However, the unsteady motions introduce variations in the Richardson number’s stratification radius, and relaminarization is observed outside the core radius. The turbulence intensity exhibited by the plunging and pitching cases during the downstroke is greater than for the static case, but lesser during the upstroke.
The empennage of a helicopter is largely responsible for its stability in forward flight. Its performance is mainly determined by its aerodynamics. In this paper, the empennage of a CoAX 2D ultralight research helicopter is analyzed in detail. For this purpose, the helicopter was equipped with flow measurement devices and flight tests were performed, covering different flight conditions. Measurements from a nose boom as well as the pilot’s control inputs and helicopter's position are available for evaluation. For the empennage in particular, seven-hole flow probes were mounted on it and various cameras were used to record the movement of the surface tufts.
This paper presents findings from a joint computational-experimental venture that seeks to advance the physical understanding and validation-quality database for a model-scale generic tractor proprotor–wing system during the tiltrotor conversion maneuver. This study evaluates the interactions in a quasi-static manner for various proprotor tilt angles (θ) across the tiltrotor conversion maneuver. Independent experimental measurements of the wing and proprotor loads accompany synchronous wing surface pressure measurements along with stereoscopic particle image velocimetry flow field measurements at discrete spanwise locations. High-fidelity computational fluid dynamics simulations leverage the multi-disciplinary rotorcraft simulation tool CREATE™-AV Helios to assess the interactional aerodynamics of the proprotor–wing configuration across the tiltrotor conversion maneuver. Computational simulations use a newly implemented Helios module to trim to the experimental proprotor thrust. Validation of the computational model, along with a mesh refinement study, is shown via comparison of the integrated proprotor and wing loads as well as wing surface pressure distributions. The validated computational model is used to examine the proprotor-to-wing aerodynamic interactions further. For low proprotor tilt angles (0° ≤ θ ≤ 30°), interactions between the wing boundary layer and proprotor wake results in the breakdown of the wake along the wing chord. For moderate proprotor tilt angles (45° ≤ θ ≤ 60°), the interactions are dominated by the proprotor tip vortices curling around the wing leading edge. Approaching edgewise flight (75° ≤ θ ≤ 90°), the predominant proprotor–wing interactions stem from standing vortices on the wing upper surface, producing strong low-frequency responses on the wing. Overall, this work introduces new insights into the complex proprotor–wing aerodynamic interactions across the tiltrotor conversion.
This study investigates the interactional aerodynamics of multi-rotor systems with longitudinally canted rotors, focusing on force, moment, and wake behavior. Experiments using two 24-inch, two-bladed rotors in hover varied cant angle (0–20°) and hub spacing (1.1–1.5D). Increasing longtitudinal cant angle had the greatest effect on maximum longitudinal force, (| Fx |), yielding a reduction of up to -6.18% per 1°. Hub spacing had greater influence, especially on longitudinal force, | Fx |, and pitching moment, (| Mx |), which decreased by up to -16.00% and -31.07% per 0.1D increase, respectively. Time averaged flow results from Particle Image Velocimetry (PIV), showed that larger hub spacings and cant angles improved wake separation and flow symmetry. These results provide foundational data for minimizing parasitic loads and maximizing aerodynamic performance in advanced multi-rotor designs.
In this paper, we develop a new feature-based algorithm using stereo cameras to estimate stochastic ship-deck motion at high sea states. Unlike our previous algorithms, this algorithm is able to estimate the motion of arbitrary ship structures without prior information on the ship's visual appearance or geometry. The algorithm requires an initial pose and suffers from drift over time, which was resolved by fusing it with our previous 2D feature-based vision algorithm. The combined vision algorithm is validated using a simulated ship featuring 3D ship structures and 2D flight deck markings representative of a DDG-51 ship. The results indicate that the algorithm can accurately estimate the pose of a simulated ship undergoing Sea-State 6 motion. The vision algorithm was further validated in a simple free-flight test.
The performance and unsteady loads of a rotor operating in shipboard environments are highly sensitive to the influence of unsteady ship airwakes. In extreme cases, this interaction can significantly degrade rotorcraft handling qualities and constrain the safe launch and recovery flight envelope. This study presents wind tunnel measurements of azimuth-correlated rotor hub loads for a 1:100 scale single main rotor, modeled after the NATO Generic Rotorcraft, hovering above and around the landing deck of the NATO Generic Destroyer. These measurements were complemented with Particle Image Velocimetry (PIV) measurements. Unlike time-averaged data, azimuth-resolved measurements reveal detailed insights into the interactional aerodynamics between the rotor and ship airwake at specific rotor azimuth angles. By comparing phase-averaged rotor load responses to a trimmed reference condition measured up-and-away from the ship airwake, this study discovered both beneficial and detrimental load variations across different azimuth angles. These variations were strongly influenced by the hovering location of the rotor relative to the ship deck. While longitudinal shifts along the deck centerline resulted in gradual changes, lateral offsets induced significant changes in azimuthal thrust characteristics. Additionally, the wind-over-deck (WOD) angle had a considerable effect, with quartering wind conditions causing significant detrimental impacts on the upstream blades at certain azimuth angles, locally leading to a substantial loss of thrust. These findings were further supported by horizontal and vertical plane PIV measurements. Both the extracted downwash velocity data and local thrust estimates based on momentum conservation showed notable azimuthal and radial variations in downwash velocities and thrust production across the rotor disk, closely matching direct thrust measurements across different hovering conditions.
Accurate and quick-turnaround ship airwake simulations are essential for better understanding of shipboard helicopter aerodynamic interactions. However, for most realistically modeling a ship airwake, the interaction of the ship with the turbulent atmospheric boundary layer (ABL) must be resolved. In this study, an ABL was generated in the Lattice-Boltzmann simulation using the Synthetic Eddy Method (SEM), and the effects of the ABL inflow on the airwake of the Simple Frigate Shape 2 (SFS2) ship model were assessed. The Reynolds stress tensor components necessary for the SEM were obtained from particle image velocimetry (PIV) measurements. Mean velocity and turbulence intensity profiles obtained from experimental measurements and the Lattice-Boltzmann simulation were compared to profiles available in the literature. Results indicated that the profiles obtained from the PIV and simulations closely resembled the profiles in the literature. Ship airwake data from the LBM simulations were compared to the PIV data at several cross-planes on the ship, which showed good correlation. Frequency analysis revealed that a realistic ABL simulation that includes the effects of turbulence is necessary to accurately produce flow conditions in regions primarily affected by the inflow, but effects of the ABL become secondary near the ship. A new quantitative methodology to estimate pilot workload derived from the flow field was proposed to complement existing more qualitative workload ratings from flight test or piloted flight simulations. Analysis with this method estimated similar pilot workloads for steady and realistic (i.e., turbulent) ABL.
The Shake-The-Box technique was applied to experimentally quantify the time-resolved volumetric flow field around a free-flying quadcopter UAV with an overall span of about 0.5 m. State-of-the-art LED illumination and high-speed camera equipment was combined with modern Lagrangian tracer particle tracking and data assimilation techniques, facilitating a measurement volume larger than 1.5m3. The setup allowed for both hover and limited maneuvering of the quadcopter, while resolving even small details of the complex interactional aerodynamics. In hover out of ground effect, the four individual rotor wakes merged into a single jet within a few rotor radii below the rotor planes. Evaluating the mass and momentum fluxes over suitable control volumes yields accurate estimates for the quadcopter's total thrust, the asymmetric thrust distribution between front and back rotors, and the entrainment of external flow through turbulent mixing. Hover in ground effect decreases the power requirement and induces recirculating flow in the center of the four rotors. The outwash pattern is non-uniform with jets developing between the rotors and pointing in radially outward directions. Forward flight cases result in a skewed, rapidly merging wake flanked by the roll-up of two "super-vortices" similar to the wingtip vortices of fixed-wing vehicles.
As part of maintenance improvement on helicopters, Airbus Helicopters has made available a proactive analysis service based on Health and Usage Monitoring System data generated during the flight. The present paper describes the new approach used to detect and classify any changes in time series behavior thanks to A.I. (Artificial Intelligence) especially computer vision. This new approach is more efficient and relevant than the classical approach based one statistical law [Ref 1]; in fact, it is acting, as the human eye, which is able to identify easily any abrupt change on the time series, and classifies it, whether Machine learning or Deep Neural Networks both have shown excellent results in term of classification accuracy. First part of this paper highlights how the learning data were prepared, then the second and the third parts give more details about how the time series are transformed into image presentation and how the different Artificial Intelligence models were selected and feed, ultimately the obtained results.
ABSTRACT Small-scale rotorcraft exhibit degraded aerodynamic efficiency, which has been linked to non-ideal losses within the wake. Unique high-frequency, broad-band features have also been observed, without a physical verification of their origin. This work seeks to gather insight into the behavior of the rotor wake structures as a function of Reynolds number (Re), relate this to performance capabilities and the corresponding far-field acoustic signature. Two-component particle image velocimetry (PIV), performance, and acoustic measurements were performed using three small-scale, NACA 0012 rotors operated over a range of low-Reynolds number conditions. Rotor geometry and operational speed (Ω) were varied to obtain the desired Re variation. Span-wise PIV has demonstrated an absence of tip vortex formation as the operational thrust coefficient (CT) is increased, suggesting outboard tip stalling. Phase-locked, chordwise PIV has confirmed this hypothesis, showing the development of large-scale separation and a highly turbulent downstream wake. The wake characteristics at moderate collective angles (θ) show the presence of periodic counter-rotating structures attributed to laminar boundary layer vortex-shedding. Significant broad-band frequency content was measured, and found to correlate well with a physical quantification of the shedding phenomenon.
ABSTRACT Measurements of the flow field around a free-flying model helicopter in ground effect for both quasi-steady and unsteady maneuvering flights were performed using stereoscopic particle image velocimetry (PIV). The wake features for hover and forward flight at low advance ratios were characterized and changing flow patterns like recirculation and ground vortex flow were observed to be in good agreement with existing wind tunnel data. Parameters describing both general flow patterns and single blade tip vortices were extracted and showed significant dependence on the forward flight velocity. Landing approaches were performed and large vortical structures were observed close to the rotor disk, which contained high amounts of vorticity due to entrained blade tip vortices. The vortex structures developing for unsteady landing approaches contained distinctly higher velocities and momentum fluxes than expected from quasisteady conditions at the same advance ratios. For a vertical takeoff maneuver with a rapid increase of collective pitch, the bundling of blade tip vortices into a "starting vortex" with circulation values up to 6 times higher than for a single blade tip vortex was observed. These results show the significance of experimental data on free-flying helicopters in unsteady maneuvering flight because the resulting flow fields cannot be predicted using steady-state experiments or simulations.
Rotorcraft shipboard operations are risky and demand high piloting skills. Computational simulations are invaluable for pilot training and understanding of the complicated aerodynamic environment. Yet, simulations may not capture a realistic response of the rotorcraft due to simplified modeling of the interactional aerodynamics. Thus, improvements to modeling and simulation are required, and experimental data are needed that unveil the interactional aerodynamics and dynamics between the rotor and ship for computational validation efforts. In this study, an extensive experimental investigation of the ship-rotor dynamic interface problem was conducted to gain a general understanding of the interactional aerodynamics between a 1:100 wind-tunnel-scale NATO Generic Destroyer and a representative single main rotor, operating both stationary and dynamically in the vicinity of the landing deck. Data obtained through simultaneous measurements of rotor hub loads, ship deck surface pressures, and stereoscopic particle image velocimetry flow fields gave valuable insight into the highly coupled aerodynamic phenomena. Results showed that the rotor hub loads exhibited a high dependency on both the wind direction and also the position of the rotor relative to the landing deck. A bifurcation in the regions of high unsteady thrust was observed under certain wind conditions due to the ship airwake impacting different portions of the rotor disk. The inflow angles across the rotor disk were estimated and assessed through flow field measurement, revealing how the ship airwake altered the rotor inflow and wake structure that contributed to the unique change in rotor loads under various hovering conditions.
Launch, recovery, and deck handling operational performance on smaller ship platforms like Corvettes, Frigates and Destroyers are qualified as the most challenging tasks in the UAS ship-deployment of a VTOL Uncrewed Air System (UAS). One of the main hurdles is the random nature of seaway-created deck motions coupled with ship structure disturbed air wake patterns. The MoD has supported a range of work aimed at bringing Quiescent Period Prediction (QPP) technology to fruition. QPP firstly requires Wave Profiling RADAR to measure the sea wave system out to approximately 2km in the region around a vessel. Secondly these measurements are employed in a wave propagation model to predict the actual wave forces acting on a vessel. Using the wave predictions as inputs to a vessel model makes possible to predict the actual (deterministic as opposed to statistical) motions of a vessel. Wave systems naturally alternate groups of large waves with smaller waves, this property, combined with the predictive ability, allows to identify the quietest (most quiescent) periods in which to conduct wave limited naval operations. Naval mission planners in the Royal Navy, and elsewhere in the World, appreciate the need to maintain rapid, but safe, deck tempo. The fundamental concept is to measure remote sea surface profiles to predict the future wave forces acting upon a vessel. The objective is to expand ship operating deck limits to approximately Sea State 6+. The deck definitions generally empirically measured by using standard rating scales, are replaced by instrumented devices reporting the status of the deck prior to touch-down. In this paper, a thorough discussion describing the QPP deck measuring devices designed to replace piloted cueing is provided. Theory, previous simulation studies and current at-sea testing along with data results, are also discussed. To conclude, the interface of the deck measuring device into the next version of the UK UAS system, is provided. The results of the RADAR trial indicated that the RADAR data was reliable, with the RADAR images matching the physical map. The two-dimensional surface plot showed both the RADAR blocking fence along with an additional target. An additional observation concerning the operation over the deck whilst the ship is experiencing a quiescent ship motion period. The coupled secondary effect documents minimized air wake confusion. This is owing to fewer ship structure excursions into and out of the air flow. To better define deck airflow around the ship the integration of a Doppler LIDAR instrumented federate is proposed. This is meant to predict the future vessel air wake and look for quiescent periods in this paralleling the vessel motion QPP technique.
ABSTRACT Wind-tunnel tests of a heavy-class helicopter model were carried out to evaluate the effectiveness of passive flow control system in alleviating the fuselage parasite drag. An array of counter rotating vortex generators was selected to reduce/remove the flow separation occurring on the rear loading ramp responsible of the high pressure drag. Different technical solution for the VGs design and location were selected with respect to previous work. The basic fuselage geometrically scaled 1:7 of a heavy class helicopter was investigated with and without passive flow control system. The comprehensive experimental campaign involved the use of different measurement techniques. Indeed, pressure measurements and stereo particle image velocimetry surveys were performed to gain a physical insight about the results of load measurements. This paper addresses the promising results obtained during the wind-tunnel campaign, since significant drag reduction was achieved for a wide range of fuselage angles of attack and side slip angles without the detriment of the other aerodynamic characteristics.
ABSTRACT Phase-resolved particle image velocity measurements were taken to document the wake generated by a rotor operating in ground effect above inclined surfaces. In particular, the current work focused on the average wake structure and axial velocity distribution through the rotor. A two-bladed rotor was operated at a height of one rotor radius above a ground plane, and ground plane angles from 0° to 30° were investigated. Rotor performance measurements were also taken, using a six-axis load cell, to examine the effect ground plane angle had on the thrust produced and power required. The wake structure was found to be very sensitive to ground plane angle causing the radial distribution of axial velocity through the rotor to increase inboard and decrease outboard with increasing ground plane angle. The peak figure of merit of the rotor decreased with increasing ground plane angle.
ABSTRACT Reverse flow is the source of several unsteady effects that complicate load predictions for high advance ratio rotorcraft. As a way of improving load predictions, an ongoing series of experiments has been aimed at gaining a physical understanding of the unsteady aerodynamics of the reverse flow region. The current work contributes phase-averaged, three-component velocity field measurements collected on a rotor at high advance ratios. Stereoscopic particle image velocitmetry (PIV) was performed on a Mach-scale rotor across three advance ratios (0:6 ≤ μ ≤ 0:8), three radial stations (0:3 ≤ r/R ≤ 0:6), and one collective (θ0 = 10°). The present analysis focuses on how the reverse flow dynamic stall vortex, which results from flow separation about the sharp geometric trailing edge of a rotor blade in reverse flow, evolves over time in three dimensions. For a constant advance ratio, the size of the reverse flow dynamic stall vortex increases with decreasing radial station, creating a gradient in vorticity along the blade span. Tip-to-root radial flow, the amount of which increases for inboard radial stations, was also associated with the development of the reverse flow dynamic stall vortex. The current work postulates that due to an increase in radial flow, inboard radial stations are subject to a greater radial transport of vorticity than outboard sections and, in turn, produce less dimensional vorticity than outboard sections at the same convective time. Coupled with previous experimental models of reverse flow, the current work represents an important step in identifying the dominant unsteady characteristics of the reverse flow region and is designed to inform a low order model of rotors at high advance ratio.
ABSTRACT The tip vortex-system downstream of a four-bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached-flow and constant-pitch test cases for comparison reasons. The phase-locked PIV system and a rotation of the swashplate assembly allowed for an acquisition of the tip vortex system over the entire dynamic stall cycle and vortex ages up to at least 235°. The vortex structure and its relation to the blade shear layers were studied by means of both phase-averaged flow fields and the identification of vortex properties such as circulation and swirl velocity distributions. When approaching dynamic stall, a break-down of the vortex structure started at high vortex ages, accompanied by the entrainment of turbulent structures from the passing blade shear layers into the tip vortices. After the flow over the blade is fully separated and during large parts of the downstroke, the wake of the rotor tips appeared as a highly turbulent area with no individual tip vortices traceable, before reestablishing an ordered tip vortex structure shortly before the minimum blade pitch angle.
The rotorcraft community faces significantly higher accident rates compared to fixed-wing commercial aircraft, underscoring the critical need for enhanced safety measures. While Helicopter Flight Data Monitoring programs hold promise in improving safety, their widespread adoption remains limited, partly due to challenges associated with the acquisition and analysis of flight data. This paper proposes a Deep Learning (DL) solution to address safety concerns within the rotorcraft community by efficiently acquiring and analyzing flight data for a more automated and comprehensive safety assessment. Specifically, we leverage data obtained with cost-effective off-the-shelf cameras, and process it through Convolutional Neural Networks for automated detection and classification of gauges from several helicopters' cockpits. Our DL pipeline integrates a classifier for helicopter identification, an object detector for cockpit gauges detection and classification, and a network to infer the reading of each detected gauge. The contribution of this work is two-fold: (1) enhance rotorcraft safety by developing a DL framework capable of detecting, classifying, and inferring gauge readings for different helicopter types, and (2) boost research in the field by constructing a curated dataset valuable for aviation and machine learning communities.
A joint experimental-computational research campaign is underway to develop physical understanding and a validation-quality database for a model-scale tractor propeller-wing system. Separate load measurements on the wing and propeller accompany wing surface pressure distributions and flow field measurements via stereoscopic particle image velocimetry (SPIV) at discrete wing spanwise locations for a range of static propeller tilt angles. The physical wind tunnel test is modeled using a high-fidelity computational approach (Helios). Computational simulations aid in assessing the influence of the wind tunnel facility effects and test support structure wake interference, as well as in reducing uncertainties in the physical experiments for use in computational validation. The behavior of the induced thrust and lift at a zero-degree wing angle of attack in the axial flow regime (cruise configuration) is correlated with flow field measurements, showing distinct differences between upwash and downwash fields produced by the propeller. Recirculation regions near the wing upper surface are identified for high propeller tilt angles as the propeller wake interacts with the wing boundary layer. Increased propeller tilt angles result in a decreased wing lift and increased propeller thrust in forward flight, resembling a rotor in ground effect. Results indicate ways to improve computational modeling and refine the physical wind tunnel testing. These will be applied to refine the database to further elucidate physical understanding of the propeller-wing interactional aerodynamics.
Quadrotor performance and stereoscopic particle image velocimetry (PIV) flow field wind tunnel measurements presented in this work aim to quantify rotor-rotor interactions and their manifestations for various hub spacings, including vertical offset. Three quadrotor configurations were examined; the cross configuration and two plus (+) configurations. In the cross configuration, the fore rotors were lowered relative to the aft rotors. In the Plus 1 configuration the fore and aft rotors were lowered, whereas in the Plus 2 configuration the side rotors were lowered. In the cross configuration, increases in hub spacing led to decreases in the thrust coefficient (CT ) of the fore rotors for the same rotational speed. An increase in the rotor vertical separation resulted in an increase in CT of the aft rotors of up to 24%. Results showed that large vertical rotor separation and close hub spacings yielded best performance. The side rotors in the Plus 1 and Plus 2 configurations showed differing thrust coefficients based on relative rotor position and wind speed leading to aerodynamic asymmetries. In the Plus 1 configuration, the aft rotor experienced significant changes in performance based on both the hub and vertical spacings as it moved into the beneficial upwash regions from the side rotors, as evidenced by PIV flow field measurements. Performance results of the Plus 2 configuration showed positive effects of the fore rotor on the side rotors as they were lowered, exacerbating aerodynamic asymmetries. The aft rotor also experienced beneficial effects from the side rotors, which diminished as the side rotors were lowered. These results were further confirmed by measured rotor inflow velocities from the PIV. An examination of the thrust-to-torque ratios of the plus configurations indicated best performance for a rotor vertical separation of 0.5R.
In this paper, we develop and validate a 3D feature-based algorithm for tracking stochastic ship-deck motion at high sea states, specifically Sea-State 6 using data from the Navy SCONE dataset. The new vision algorithm was developed from the structure-from-motion technique, which recovers the 3D structure of an object from a series of 2D images, and was validated using a simulated 3D ship-deck attached to a moving Stewart platform. Algorithm performance with different feature detectors and image resolutions was compared. In hand-held tests, the vision algorithm was demonstrated to accurately estimate the pose of a moving ship-deck using a quadrotor. Visually degraded conditions were also evaluated; the algorithm is robust to occlusion and low illumination, but performance reduces in severe glare. The vision algorithm was then validated in a simple free-flight test. All results were compared with Vicon ground-truth data. Additionally, as the 3D algorithm is computationally demanding, we develop and validate a method to improve the computational speed of the vision algorithm.
AAM concepts use multiple distributed electric motors driving propellers and rotors to augment or directly generate lift and propulsive forces. Several current concepts incorporate separate drive systems for providing vertical lift, for takeoff and landing, and propulsive thrust for wing-borne cruising flight. Measurement of loads and performance on these rotating systems is very important in both the design and development stage, as well as for certification use and ultimately supporting HUMS monitoring. However, providing instrumentation in the rotating frame and extracting their associated measurements is often problematical, as it requires some means for both power and signals to bridge the rotating interface between the blade of the rotor/propeller and the fixed frame (fuselage) system. This paper describes work conducted to leverage prior CDI development of a novel optical telemetry/instrumentation system to create a prototype unit that can support ground and flight tests, allowing for multiple installations on the many rotors that constitute current AAM configurations. The resulting hardware was designed to expand the capabilities developed previously in types and rates of data collected, on-board processing, and user configuration options, supporting NASA and commercial organizations in their testing activities.
The parameters of a Pitt-Peters dynamic inflow model for a rotor undergoing collective inputs were extracted from experimental measurements on a hovering rotor. The four-bladed rotor of 2 m diameter featured straight, untwisted blades and a solidity of σ = 0.010. The nominal trim condition was CT /σ = 0.07 at a speed of 840 RPM. The rotor wake was measured using phase-resolved, 2D, 3-component particle image velocimetry (PIV) over a large region of interest (0.84 m x 0.77 m), and the integrated rotor aerodynamic forces were obtained from simultaneous hub loads measurements. The frequency response of rotor inflow to rotor thrust was found by measuring the system response to a stepped-sine collective input, which included frequencies of 0.2, 0.3, 0.4, 0.6, and 0.7/rev. The thrust amplitude increased with input frequency, reaching 27.4% of the steady thrust at the highest input frequency. The inflow amplitude was 4.3% of the steady inflow at 0.2/rev and decreased to 2.0% at 0.7/rev. A first-order transfer function was fit to the discrete frequency response to compute the parameters of the Pitt-Peters dynamic inflow model. The apparent mass term for a rotor undergoing collective inputs was found to be M11 = 0.0288±4.4% and the gain term was found to be ¯L11 = 0.118±6.3%. The results agreed well with examples in literature extracted from high-fidelity numerical models. However, there were differences in the steady-state gain and the frequency at which the inflow magnitude is attenuated. The methodology for extending the present approach to cyclic inflow components is presented.
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This SAE Standard applies to cranes which are equipped to adjust the boom angle by hoisting and lowering means through rope reeving.
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