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Jain R.,HyPerComp, Inc. | Yeo H.,Ames Research Center | Yeo H.,U.S. Army | Chopra I.,University of Maryland University College | Chopra I.,Alfred Gessow Rotorcraft Center
Journal of Aircraft | Year: 2013

Effects of trailing-edge flap gaps on rotor performance are investigated using a high-fidelity coupled computational fluid dynamics computational structural dynamics analysis. Both integral flap (the flap is an integral part of the blade such that there are no physical gaps at the flap ends) and discrete flap (the flap is a separate entity with physical gaps in the span and chord directions) are examined on an UH-60A rotor at high-speed forward-flight conditions. A novel grid deformation scheme based on the Delaunay graph mapping is developed and implemented to allow the computational fluid dynamics modeling of the gaps with minimal distortion of mesh around the flap gap regions. This method offers an alternative to the traditional approach of modeling such configurations using overset meshes. The simulation results show that the effectiveness of the flap is minimally affected with span gaps; the penalty on rotor performance is of the order of 1% compared to the integral flap. On the other hand, the chord gaps significantly degrade the benefits of active flap on rotor performance due to the flow penetration between the upper and lower surfaces of the flap. Copyright © 2012 Clearance Center, Inc. Source


Hrishikeshavan V.,University of Maryland University College | Chopra I.,University of Maryland University College | Chopra I.,Alfred Gessow Rotorcraft Center
Journal of Aircraft | Year: 2012

Experimental studies were conducted to study the response of a shrouded rotor micro air vehicle to edgewise gusts. In edgewise flow, the thrust, drag, and pitching moment produced by three platforms were compared: elliptic inlet, circular inlet shrouded rotor, and an unshrouded rotor. The elliptic inlet shrouded rotor was more efficient in hover but had a higher penalty in drag and pitching moment in edgewise flow. Cyclic pitch variation of a hingeless rotor was used to counter these adverse pitching moments. The control authority of the shrouded rotors was at least 80-100% higher than the unshrouded rotor, with the elliptic inlet shrouded rotor producing the highest control moments. By optimizing rotor collective settings, it was possible to reduce the deteriorating effect of edgewise flow on control moments. To increase the control authority and gust tolerance of the shrouded rotor, the cyclic pitch travel and blade planform modifications were made. With a careful selection of rotor solidity, planform, operating revolutions per minute, and cyclic pitch travel, it was possible to achieve a gust tolerance of about 3 m=s for the circular inlet shrouded rotor. Free flight tests were then conducted to study the ability of the vehicle to hover in a given position in the presence of gusts generated from pedestal fans with honeycomb flow straighteners. A combination of VICON ™ and an onboard sensor were used for feedback control. The vehicle was satisfactorily able to maintain hover position in edgewise gusts of up to 3 m/s. Copyright © 2011 by Luis Delgado. Source


Benedict M.,University of Maryland University College | Benedict M.,Alfred Gessow Rotorcraft Center | Mattaboni M.,Polytechnic of Milan | Mattaboni M.,Alfred Gessow Rotorcraft Center | And 4 more authors.
AIAA Journal | Year: 2011

This paper describes the aeroelastic model to predict the blade loads and the average thrust of a micro-air-vehiclescale cycloidal rotor. The analysis was performed using two approaches: one using a second-order nonlinear beam finite element method analysis for moderately flexible blades and a second using a multibody-based largedeformation analysis (especially applicable for extremely flexible blades) incorporating a geometrically exact beam model. An unsteady aerodynamic model is included in the analysis with two different inflow models: single streamtube and double-multiple streamtube inflow models. For the cycloidal rotors using moderately flexible blades, the aeroelastic analysis was able to predict the average thrust with sufficient accuracy over a wide range of rotational speeds, pitching amplitudes, and number of blades. However, for the extremely flexible blades, the thrust was underpredicted at higher rotational speeds, and this may be because of the overprediction of blade deformations. The analysis clearly showed that the reason for the reduction in the thrust-producing capability of the cycloidal rotor with blade flexibility may be attributed to the large nosedown elastic twisting of the blades in the upper half cylindrical section, which is not compensated by a noseup pitching in the lower half-section. The inclusion of the actual blade pitch kinematics, unsteady aerodynamics, and flow curvature effects was found crucial in the accurate lateral force prediction. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source


Mayo D.B.,University of Maryland University College | Lankford J.L.,University of Maryland University College | Benedict M.,University of Maryland University College | Chopra I.,University of Maryland University College | Chopra I.,Alfred Gessow Rotorcraft Center
Journal of Aircraft | Year: 2015

Targeted experiments in parallel with a systematic computational-fluid-dynamics analysis were performed for a micro-air-vehicle-scale rigid flapping wing in forward flight. Two-component time-resolved particle-imagevelocimetry measurements were performed in an open-circuit wind tunnel on a wing undergoing pure flap-wing kinematics at a fixed wing-pitch angle. Chordwise velocity fields were obtained at equally spaced spanwise sections along the wing (30 to 90% span) at two instants during the flap cycle (middownstroke and midupstroke) for the reference Reynolds numbers of 15,000. The flowfield measurements were used for the validation of the threedimensional computational-fluid-dynamics model. The computational-fluid-dynamics analysis used a compressible Reynolds-averaged Navier-Stokes solver to resolve the complex, highly vortical, three-dimensional flow. The objectives of the combined efforts were to understand the unsteady aerodynamic mechanisms and their relation to force production on a rigid wing undergoing an avian-type flapping motion. Overall, the computational-fluiddynamics results showed good agreement with the experimental data for resolution of the overall highly unsteady and vortical flowfield. A control-volume approach used to calculate the strength of the leading-edge vortex from the particle-image-velocimetry measurements and from the computational-fluid-dynamics-generated flowfields showed comparable results. A hybrid momentum-based method was used to estimate the sectional vertical force coefficient from the particle-image-velocimetry-measured flowfield, which agreed well with the computational-fluid-dynamics force prediction over a range of flapping frequencies and wing-pitch angles. In general, it was observed that the flow over the wing was highly susceptible to changes in induced angle of attack resulting from the flapping motion and variations in reduced frequency, which manifested in the predicted airloads. Based on the computational analysis, the spanwise flow component was not significant, except near the wing tip, and therefore most of the vertical force and propulsive thrust produced could be explained using the magnitude and direction of the sectional lift and drag forces acting on the wing. For the present wing kinematics, most of the upward vertical force was produced during the downstroke and positive propulsive thrust during the upstroke, which shows the need for appropriate temporal and spanwise pitch modulation of the wing along with flapping to produce positive vertical force and propulsive thrust during the entire flap cycle. © 2014 by David Mayo. Published by the American Institute of Aeronautics and Astronautics, Inc. Source


Benedict M.,University of Maryland University College | Shrestha E.,University of Maryland University College | Hrishikeshavan V.,University of Maryland University College | Chopra I.,University of Maryland University College | Chopra I.,Alfred Gessow Rotorcraft Center
Journal of Aircraft | Year: 2014

A study was conducted to demonstrate the development of a micro twin-rotor cyclocopter capable of autonomous hover. The blades used on the twin-cyclocopter were fabricated mostly out of foam with carbon-fiber reinforcement inside and a single-layer 0/90 degree carbon composite prepreg skin wrapped around the foam core at the blade tips. Foam helped in maintaining the required airfoil shape for the blades, while most of the bending and torsion stiffness was provided by the carbon-fiber structure embedded inside the foam. One of the key requirements for the success of a cyclocopter was a simplified lightweight blade-pitching mechanism. A feedback control system was required to provide sufficient attitude damping and stiffness to achieve stable hover. Source

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