Showing papers in "Journal of The American Helicopter Society in 2007"

Review of Rotor Loads Prediction with the Emergence of Rotorcraft CFD

Abstract: This paper reviews the state of the art in helicopter rotor loads prediction using Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) coupling. The application of CFD to rotorcraft problems has evolved, over the period 1990 to 2005, as a viable means to improve the aerodynamic modeling used in rotorcraft comprehensive analyses (CA). CFD/CSD coupling has the potential to meet the long term objective of a coupled rotor-fuselage analysis that can predict accurate loads and vibration at all critical flight conditions without empirical or semi-empirical inputs. This paper reviews the capabilities and limitations of current CFD analyses to capture the key aerodynamic phenomena driving critical rotor loads: wake, dynamic stall and tip transonic effects. Recent initiatives in CFD/CSD coupling, targeted to improve predictions of rotor loads, are discussed. The high speed (155 kts) forward flight condition of the UH-60A BlackHawk is studied as a test case. The flight condition falls in a critical high vibration regime, where the mechanisms of rotor vibratory loads were not clearly understood. Significant improvements in fundamental understanding and prediction of vibratory loads at this flight condition has been recently demonstrated by researchers using CFD/CSD coupling. The physical mechanisms involved, and the role of CFD in improving predictions is explained.

Hover performance of a cycloidal rotor for a micro air vehicle

Abstract: In recent years, interest has been growing in a new class of very small flight vehicles called micro air vehicles (MAVs). Hover capability is highly desirable with respect to the mission requirements of these vehicles. Due to the small size of MAVs and the low Reynolds number regime in which they operate, scaling down conventional rotorcraft configurations to the MAV scale may not yield optimum performance. Unconventional vehicle configurations can be explored to realize high endurance hover capable MAVs. This paper investigates the hover performance of a small-scale cycloidal rotor to determine its viability for use in a micro air vehicle. A 6 inch diameter prototype rotor was constructed and tested to determine the effects of number of blades, blade pitch angle, and rotational speed on thrust output and power requirements. Pressure distribution was measured to obtain insight into the downwash and flow through the rotor. An analytical model, using a combination of vertical axis wind turbine theory and an indicial solution for the aerodynamic response was developed to predict rotor performance, and was validated with the experiments. The performance of the cycloidal rotor was compared to that of a conventional rotor of the same diameter in terms of power loading. Based on the analytical model and the experimental results, a conceptual design of an MAV utilizing cycloidal propulsion was developed. The conceptual cyclo-MAV utilizes two cycloidal rotors, providing thrust, propulsion, and control. Complete vehicle weight is envisaged to be 240 g, with two three-bladed rotors of six inches diameter.

How long do pilots look forward? Prospective visual guidance in terrain hugging flight

Abstract: The title of this paper reflects the notion that pilots need to predict their future flight trajectory, and hence exercise prospective control, to ensure safe passage through and over cluttered and undulating terrain. Of critical importance is how long, in time, a pilot needs to be able to see into the future to maintain an adequate safety margin for guidance. The emphasis here is on how long in time, rather than how far in space. Understanding the nature of the temporal mental ‘models’ required to support this prediction is considered key to understanding the workings of the human motion perception system, and how they might be exploited in the design of aids to flight guidance in degraded visual conditions. A related question is how the safety margin might connect to the handling qualities construct, the Usable Cue Environment, which was developed to help determine the optimum control augmentation requirements for flight in poor visibility. Answers to these questions can be used to inform the design of pilot vision aids on the one hand and to provide a more quantitative basis for the UCE construct on the other. The paper reports results from research underway in a collaborative project involving The University of Liverpool and the French Research Laboratory, ONERA, aimed at developing a more quantitative basis for the UCE, based on fundamental perception principles. In the present paper the visual requirements for vertical manoeuvring over undulating terrain are addressed, and how these lead to natural relationships between the selected speed and height. The theoretical framework is supported by results from piloted simulation trials, with a detailed examination of the temporal variables associated with vertical and horizontal flight path control in degraded visual conditions.

High Resolution Wake Capturing Methodology for Hovering Rotors

Abstract: A high-resolution Reynolds-averaged Navier‐Stokes (RANS) solver is applied to study the evolution of tip vortices from rotary blades. The numerical error is reduced by using high-order accurate schemes on appropriately refined meshes. To better resolve the vortex evolution, the equations were solved on multiple overset grids that ensured adequate resolution in an efficient manner. For the RANS closure, a one equation wall-based turbulence model was used with a correction to the production term to account for the stabilizing effects of rotation in the core of the tip vortex. While experimental comparison of the computed vortex structure beyond a few chord lengths downstream of the trailing edge is lacking in the literature, reasonable validation of the vortex velocity profiles is demonstrated up to a distance of 50 chord lengths of evolution for a single-bladed rotor. For the two-bladed rotor case, the tip vortex could be tracked up to two rotor revolutions with minimal diffusion. The accuracy of the computed blade pressures and vortex trajectories confirm that the inflow distribution and blade-vortex interaction are represented correctly. The accuracy achieved in the validation studies establishes the viability of the methodology as a reliable tool that can be used to predict vortex evolution and the aerodynamic performance of hovering rotors.

Stability and Control Analysis of the Slowed-Rotor Compound Helicopter Configuration

Abstract: : Stability and control of rotors at high advance ratio are considered. Stability of teetering, articulated, and gimbaled hub types is considered with a simple flapping blade analysis. Rotor control in autorotation for teetering and articulated hub types is examined in more detail for a compound helicopter (rotor and fixed wing) using the comprehensive analysis CAMRAD II. Autorotation is found to be possible at two distinct trim conditions with different sharing of lift between the rotor and wing. Stability predictions obtained using the analytical rigid flapping blade analysis and a rigid blade CAMRAD II model compare favorably. For the flapping blade analysis, the teetering rotor is found to be the most stable hub type, showing no instabilities up to an advance ratio of 3 and a Lock number of 18. Analysis of the trim controls, lift, power, and blade flapping shows that for small positive collective pitch, trim can be maintained without excessive control input or flapping angles for both teetering and articulated rotors.

Design and testing of an autorotative payload delivery system

Abstract: The design, testing, and analysis of an autonomous autorotative payload delivery system called the Autobody is presented. The Autobody must be capable of passively deploying a payload from a conventional aircraft, by means of an autorotative rotor. Operational requirements specify the Autobody to have a four bladed rotor with a diameter of four feet, a total mass of 2.27 kg (5 lb) and a maximum steady state descent velocity of 4.57 m/s (15 ft/s). A novel rotor hub design incorporating negative pitch–flap coupling in conjunction with negative blade pitch and a negative precone is implemented to passively achieve the transition to steady autorotation. An analysis is developed to predict the steady state behavior of the Autobody. Only vertical autorotation is considered as it will result in a conservative design and is the simplest state to analyze. Wind tunnel tests were performed on a scaled model rotor to validate the analysis and to investigate the effect of different rotor parameters. The analysis was then used to perform a parametric study of the effect of several rotor variables on the system performance, from which an optimum full scale configuration is identified. An instrumented full scale prototype was flight tested by dropping it from a hot air balloon. For an Autobody of mass 2.27 kg, with a −41◦ pitch-flap coupling angle, a −10◦ fixed collective pitch, and a −4◦ precone, a steady state descent velocity of 4.11 m/s (13.5 ft/s) was observed. Based on the predictions and the flight tests, it was concluded that the proposed Autobody design satisfactorily meets all operational requirements.

Toward noise abatement flight procedure design: DLR rotorcraft noise ground footprints model

Abstract: As a first step towards noise abatement flight procedure design, this paper presents techniques for rotorcraft noise ground footprint prediction developed at DLR, using either purely numerical computations or measured sound fields. When based on purely numerical computations or first-principles, a combination of 3D free wake unsteady panel method and a Ffowcs Williams-Hawkings (FW-H) method is used to provide noise source characteristics on a hemisphere. The noise ground footprints for a BO105 helicopter at 6° descent flight is simulated and compared with flight test data. The noise sources considered are only from a main rotor and tail rotor. When based on measured sound fields from the flight test, a depropagation procedure is used to define the noise source characteristics on the hemisphere. The verification of the depropagation procedure using purely numerical computations is introduced. Two hypothetic noise abatement procedures are simulated based on the RONAP test data.

Design optimization for improved tiltrotor whirl flutter stability

Abstract: The results of a formal design optimization study to improve tiltrotor whirl flutter stability are reported. The analysis used in this investigation considers some design parameters which have not been explicitly examined in the literature, such as the distribution of blade flexibility inboard and outboard of the pitch bearing. While previous studies have investigated the individual influence of various design parameters, the present investigation uses formal optimization techniques to determine a unique combination of parameters that maximizes whirl flutter stability. Constraints on the optimization are selected that prevent unrealistically large changes in the design parameters. The influence of rotor and wing design parameters are first considered separately, after which concurrent optimization studies are conducted. Emphasis is placed on a physical interpretation of the optimization results, to better understand the means by which certain combinations of design variables improve stability. The rotor parameters with the greatest influence on flutter speed are pitch-flap and pitch-lag couplings in the rotor blade and the distribution of flap flexibility inboard of the pitch bearing. The important wing parameters are wing vertical bending and torsion stiffness and vertical bending-torsion coupling. Changes in the rotor design parameters provide a greater stabilizing influence than changes in the wing parameters. Optimized designs are presented which require only modest changes in design parameters, while substantially improving whirl flutter stability. For the XV-15 rotor used as a baseline for this study, an optimized configuration obtained while imposing tight constraints on the design parameters increased flutter speed from 310 knots to 450 knots. If the constraints on the design parameters are relaxed, flutter speed may be increased beyond the speed range considered in this investigation. Graduate Research Assistant, student member AHS †Associate Professor, member AHS Proceedings of the 29th European Rotorcraft Forum, 16–18 September 2003, Friedrichshafen, Germany

Actuator Disk for Helicopter Rotors in an Unstructured Flow Solver

Abstract: Motivated by the demand for a fast flow simulation tool that takes the interaction between a rotor and a helicopter fuselage into account, an actuator disk boundary condition suited for helicopter rotors in forward flight has been implemented in the unstructured grid DLR TAU code. The time-averaged effect of the rotor, which accelerates the flow and adds energy to the fluid, is imposed using source terms in the Navier-Stokes equations. The actuator disk is located in the grid. The transfer of this approach, previously implemented in the structured grid DLR FLOWer code, involved adapting the strategy to the unstructured framework. It is shown that propeller simulation results are in agreement with FLOWer results and simple one-dimensional theory predictions. Moreover, the rotor in forward flight cases prove the robustness of the implementation and resemble FLOWer results. Further development involved testing the implementation in parallel mode, and a more sophisticated rotor force distribution is applied instead of a constant pressure jump. Finally, a comparison of the viscous flow field around the EC145 helicopter computed by TAU and FLOWer is performed. It shows that there is good agreement between the two codes in predicting the effect of the actuator disk on the fuselage pressure distribution.

Buckling interaction in regular arrays of rigidly supported composite laminated plates with orthogrid, isogrid, and anisogrid planform

Abstract: The initial buckling strength of composite plate arrays with novel planform configurations are evaluated for combinations of uniform in-plane compression and shear loading and are presented in the form of dimensionless buckling interaction curves, for direct comparison with isotropic panels of equal mass. Results illustrate the buckling strength sensitivity with respect to changes in the stacking sequence of symmetric, specially orthotropic angle-ply composite laminates including the consideration of both cross-symmetric and non-symmetric

Technical note gyroscopic feathering moments and the 'Bell stabilizer bar' on helicopter rotors

Abstract: Rotor blade feathering moments caused by gyroscopic forces acting during helicopter pitching and rolling motions are identified The consequent blade elastic feathering gives rise to blade flapping, which reduces the pitch/roll cross-coupling due to the aerodynamic effects imposed by pitch and roll rates For blades of low feathering stiffness this reduction is considerable Inclusion of this effect into rotor analyses accounts for some of the difference between calculated cross-coupling and observed flight behaviour A review of the Bell rotor system shows that the stabilizer bar acts as a blade chordwise extension, and that gyroscopic feathering moments are fundamental to its operation