Browsing by Subject "Rotorcraft"
Now showing 1 - 5 of 5
- Results Per Page
- Sort Options
Item A comprehensive study of a coaxial, co-rotating rotor in hover(2023-04-18) Johnson, Chloe Marie; Sirohi, Jayant; Clemens, Noel; Goldstein, David; Hamilton, Mark; McDonald, RobRecent innovations in electric vertical takeoff and landing technologies have spurred research of coaxial, co-rotating (stacked) rotors. A stacked rotor employs two sets of two-bladed rotors offset axially and azimuthally. This thesis focuses on the experimental measurements of a 1.108 m stacked rotor in hover at a tip Mach number of 0.42. The goal of this work is to quantify the effects of rotor geometry and operating conditions on the performance and noise in hover. A coaxial stacked rotor test stand was designed that allows for variable axial and azimuthal spacings and differential collective, or unequal collective on the upper and lower rotor blades. Axial spacings ranged from 0 to 1.75 blade chords and azimuthal spacing ranged from -90° to 90°. A method to characterize the acoustic facility was developed and implemented to quantify the effects of reflections on acoustic measurements. Stacked rotor performance measurements revealed significant variations of individual rotor thrust (67% upper rotor thrust, 50% lower rotor thrust) and power with azimuthal spacing, leading to total rotor thrust and power variations. Variations were greatest at small axial spacings and unequal collective pitch angles on the upper and lower rotor (differential collective), achieving 16.4% thrust change with 15.3% power change at zero differential collective and 76.5% thrust change with 46% power change at negative differential collectives. Moreover, large thrust variations with small azimuthal spacing changes suggest a new method for rotor thrust control. A numerical model was developed that coupled blade element momentum theory with a vortex panel method, with which it was revealed that chordwise circulation along the rotor blades is the dominant factor resulting in measured thrust variations. Rotor noise measurements revealed that at the given operating conditions, tonal and broadband noise are of similar magnitude and are both sensitive to stacked rotor parameters. Tonal noise increased at nonorthogonal azimuthal spacings due to a larger 2/rev noise component. This phenomenon was most pronounced at zero differential collective and nonzero axial spacings. Broadband noise variations were examined through tip vortex trajectory measurements extracted from flow visualization images. It was found that broadband noise around 3 kHz was greatest when the lower rotor operated below the upper rotor tip vortex. Broadband noise decreased by 1.5 dB as the distance between the rotor blade and vortex increased to 0.6 blade chords. Furthermore, if the blade operated in the vortex core or passed above it, broadband noise was reduced by 4.5 dB. The distance between the lower rotor blade and upper rotor blade tip vortex was highly dependent on azimuthal spacing and blade tip clearance. From these results, a quiet stacked rotor was obtained by optimizing tonal and broadband noise through rotor spacing and differential collective. Tonal noise variations are achieved by changing the axial and azimuthal spacing, while broadband noise can be controlled by altering the vortex trajectories with differential collective. This resulted in a mode of operation with 20% more power but a 2.5 dB reduction of total noise.Item Comprehensive aeromechanical measurements of a model-scale, coaxial, counter-rotating rotor system(2016-12) Cameron, Christopher Glenn; Sirohi, Jayant; Bogard, David G; Goldstein, David B; Ravi-Chandar, K.; Singh, RajneeshWith the renewed interest in rigid, counter-rotating coaxial rotor designs, and the increased fidelity of fully coupled CFD/CSD simulations, there exists a lack of comprehensive experimental data for a rotor system with which to validate analyses. The goal of this dissertation is to generate a new set of measurements on a model-scale rigid coaxial rotor systems in hover and high speed forward flight. A counter-rotating transmission was built, incorporating 6-component upper and lower rotor load cells for individual hub load measurements. Upper and lower rotor control systems, as well as complementary instrumentation including pushrod load cells, root pitch measurement and blade tip clearance sensors were developed. Two sets of rotor blades were fabricated and characterized using stereoscopic digital image correlation in combination with static and dynamic loads. A novel rotating-frame operational modal analysis successfully identified the first blade flap frequency and aerodynamic damping. Hover testing focused on quantifying the effects of upper and lower coaxial rotor interference when compared to isolated rotors. Statistical analysis of the measured data revealed clear trends with a known confidence level. Due to mutual interference, the upper and lower rotors of the coaxial configuration consumed 18% and 49% more induced power than that of an isolated two-bladed rotor. The coaxial counter-rotating configuration was found to consume 6% less induced power than an isolated, four-bladed single rotor of equal solidity. While torque balanced, the upper rotor was found to produce 54% of the total system thrust regardless of blade loading. Significant four-per-revolution vibratory thrust was observed in the lower rotor, with primary and secondary peaks corresponding to bound vortex and blade thickness interactions respectively. Wind tunnel testing examined the effects of lift offset and rotor phasing at high forward flight speeds. Rotor effective lift-to-drag ratio was found to increase with increasing advance ratio and lift offset, resulting in a 50% peak efficiency gain. The lower coaxial rotor was found to operate at higher lift-to-drag ratio than the upper rotor, due to the reversal of differential upper and lower rotor thrust compared to hover. Lift offset resulted in a decrease in blade tip clearance with a corresponding rise in rotor side force. Vibratory loads increased with advance ratio, with the largest occurring at two and four-per-revolution harmonics. Lift offset decreased vibratory forces while increasing vibratory in-plane moments. The coaxial system experienced reduced vibratory in-plane forces and torque compared to the isolated rotors due to cancellation between upper and lower rotor loads. Adjusting the inter-rotor index angle modified vibratory forces and moments transmitted to the fixed frame, increasing some components while decreasing others.Item Electromechanical modeling and testing of a novel electrically-driven coaxial, co-rotating rotor system(2021-05-07) Asper, Matthew S.; Sirohi, JayantA novel thrust control method for an electrically-driven coaxial, co-rotating (stacked) rotor system by controlling rotor index angle, or azimuthal spacing is described. The stacked rotor comprises of two 2-bladed rotors spinning in the same direction at the same rotor speed, with a fixed axial spacing and variable azimuthal spacing. Changing the azimuthal spacing by approximately 22 degrees results in a 17% change in the total rotor system thrust. An electromechanical model of the rotor and drive system was developed with a blade element aerodynamic model and field oriented control of two phase-synchronized electric motors, each driving one rotor of the stacked system. The model was validated with measurements on a single, 2 m diameter rotor in hover driven by a single electric motor at constant rotor speed as well as during transient rotor speed changes. The validated model was used to explore the behavior of the system in response to a commanded change in rotor azimuthal spacing. At a blade loading of C [subscript T] / [sigma] = 0.08, and a rotor speed of 1200 RPM, computations indicated that a 5° change in azimuthal spacing could be achieved in less than 0.2 s, or less than five rotor revolutions, requiring a transient power increase of 12% the mean power. This may lead to total thrust variations of 9% at an axial spacing of 0.73 chord. These results indicate the feasibility of achieving small changes in thrust at a high bandwidth with a small increase in motor power output.Item Estimation of helicopter rotor loads from blade structural response(2020-03-27) Uehara, Daiju; Sirohi, Jayant; Goldstein, David B; Ravi-Chandar, Krishnaswa; Bennighof, Jeffrey K; Bhagwat, Mahendra JMeasuring the load distribution along a helicopter rotor blade has been one of the most challenging tasks in experimental aeromechanics. Conventional loads measurements with on-blade instrumentation, such as pressure transducers for airloads and strain gages for structural loads, require the experimentalist to overcome a large number of technical barriers; for example, sensor integration to the rotor blade structure, sensor failure due to strong centrifugal forces, and influence of sensor installation on rotor blade dynamics. The goal of this dissertation is to develop a new, combined experimental and theoretical methodology to estimate helicopter rotor loads without using these conventional on-blade sensors. The rotor loads estimation methodology begins with the measurement of blade structural deformation measurements using non-contact, optical, time-resolved Digital Image Correlation (DIC). The time-resolved DIC technique successfully showed its capability of measuring the three-dimensional deformation time history of a rotating blade for both a small- and a large-scale rotor in hover. The modal properties (natural frequencies, mode shapes, damping ratios, and modal coordinates) of the blade in the rotating-frame were then extracted from the deformation time history using Natural Excitation Technique - Eigensystem Realization Algorithm (NExT-ERA) and Complexity Pursuit (CP), which are operational modal analysis (OMA) algorithms. The first three modes were identified by the OMA algorithms and well correlated with a numerical model. Rotor loads were then finally estimated based on the measured deformations and blade modal characteristics. Having validated the present approach incrementally with measurements performed on rotors at different scales, configurations, and operating conditions, the current study estimated the spanwise lift distribution and integrated thrust at the hub for a 2 m-diameter, two-bladed, isolated single rotor in hover. Due to a lack of participating modes (only the first and second flap modes), the estimated sectional lift distribution did not capture the lift loss typically observed at regions of the blade tip and induced by trailing tip vortices. Nevertheless, the mean value of the estimated thrust at the rotor hub was within 5% of the measured value for all the operating conditionsItem Experimental investigation of the far-field rotorcraft wake structure(2012-05) Stephenson, James Harold; Tinney, Charles Edmund, 1975-; Sirohi, JayantThe tumbling tip vortex effect of a reduced-scale, 1 m diameter, four-bladed rotor during hover is studied using vortex methods, combined with a center of mass analysis approach. Measurements of all three components of the velocity field are acquired using a stereo PIV system synchronized to capture up to 500 degrees of vortex age, with 10 degree wake age offsets, during hover conditions. The nominal operating condition of the rotor is at a rotational rate of 1520RPM, corresponding to ReC = 248,000 with a chord length of 58.5mm. The rotor is operated with a pitch of 7.2± 0.5 degrees and a CT/sigma of 0.045. The far wake vortex tumbling phenomenon is captured and described. It is shown that tip vortices from two blades tumble through approximately 90 degrees of rotation before they coalesce. It is also seen that the constituent parent vortices do not combine to create a stronger daughter vortex as was previously thought to happen. Instead, the merged vortex has a lower large-radius circulation than either of its parent vortices. An accurate characterization and prediction of the trajectory of the far wake vortex tumbling can enhance the ability to predict and alleviate the resuspension of particles during brownout as well as provide a database for far wake validation of CFD codes.