Browsing by Subject "Helicopter"

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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, Rajneesh
With 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.
Extraction of blade-vortex interactions from helicopter transient maneuvering noise
(2014-05) Stephenson, James Harold; Tinney, Charles Edmund, 1975-
Time-frequency analysis techniques are proposed as a necessary tool for the analysis of acoustics generated by helicopter transient maneuvering flight. Such techniques are necessary as the acoustic signals related to transient maneuvers are inherently unsteady. The wavelet transform is proposed as an appropriate tool, and it is compared to the more standard short-time Fourier transform technique through an investigation using several appropriately sized interrogation windows. It is shown that the wavelet transform provides a consistent spectral representation, regardless of employed window size. The short-time Fourier transform, however, provides spectral amplitudes that are highly dependent on the size of the interrogation window, and so is not an appropriate tool for this situation. An extraction method is also proposed to investigate blade-vortex interaction noise emitted during helicopter transient maneuvering flight. The extraction method allows for the investigation of blade-vortex interactions independent of other sound sources. The method is based on filtering the spectral data calculated through the wavelet transform technique. The filter identifies blade-vortex interactions through their high amplitude, high frequency impulsive content. The filtered wavelet coefficients are then inverse transformed to create a pressure signature solely related to blade-vortex interactions. This extraction technique, along with a prescribed wake model, is applied to experimental data extracted from three separate flight maneuvers performed by a Bell 430 helicopter. The maneuvers investigated include a steady level flight, fast- and medium-speed advancing side roll maneuvers. A sensitivity analysis is performed in order to determine the optimal tuning parameters employed by the filtering technique. For the cases studied, the optimized tuning parameters were shown to be frequencies above 7 main rotor harmonics, and amplitudes stronger than 25% (−6 dB) of the energy in the main rotor harmonic. Further, it is shown that blade-vortex interactions can be accurately extracted so long as the blade-vortex interaction peak energy signal is greater or equal to the energy in the main rotor harmonic. An in-depth investigation of the changes in the blade-vortex interaction signal during transient advancing side roll maneuvers is then conducted. It is shown that the sound pressure level related to blade-vortex interactions, shifts from the advancing side, to the retreating side of the vehicle during roll entry. This shift is predicted adequately by the prescribed wake model. However, the prescribed wake model is shown to be inadequate for the prediction of blade-vortex interaction miss distance, as it does not respond to the roll rate of the vehicle. It is further shown that the sound pressure levels are positively linked to the roll rate of the vehicle. Similar sound pressure level directivities and amplitudes can be seen when vehicle roll rates are comparable. The extraction method is shown to perform admirably throughout each maneuver. One limitation with the technique is identified, and a proposal to mitigate its effects is made. The limitation occurs when the main rotor harmonic energy drops below an arbitrary threshold. When this happens, a decreased spectral amplitude is required for filtering; which leads to the extraction of high frequency noise unrelated to blade-vortex interactions. It is shown, however, that this occurs only when there are no blade-vortex interactions present. Further, the resulting sound pressure level is identifiable as it is significantly less than the peak blade-vortex interaction sound pressure level. Thus the effects of this limitation are shown to be negligible.
Study of compressible flow through a rotating duct
(2015-08) Karpatne, Anand; Sirohi, Jayant; Goldstein, David B; Raja, Laxminarayan L; Tinney, Charles E; Shannon, Daniel W
Several rotorcraft applications such as circulation control and tip jet driven rotors involve internal spanwise flow along the interior of a rotor blade. This dissertation describes a quasi 1-D numerical model of unsteady flow through a duct rotating about one end along with experimental validations. The numerical model is suitable for inclusion in the conceptual design stage for helicopter rotor blades with internal spanwise flow. To this end, centrifugal as well as coriolis effects, frictional losses, duct sweep and time-dependent duct boundary conditions are modeled, and a spanwise flow control valve can be included. One dimensional Euler equations are solved inside the duct using a finite volume formulation in which the advective fluxes are approximated using the Advective Upwind Splitting Method (AUSM). The model is used to explore the behavior of flow inside a 2 m long duct with a circular crosssection, rotating at tip speeds of up to 260 m/s. In the inviscid limit, at a rotor tip speed of 213 m/s, the model predicted the evolution of a shock which showed periodic oscillations with a time period of approximately 17.5 rotor revolutions. However, when friction was included, a shock did not form until the rotor tip speed was ~ 260 m/s. The effects of suddenly opening a flow control valve at different spanwise stations, x [subscript valve] = 0.0R, x [subscript valve] = 0.5R and x [subscript valve] = R, were also studied numerically. Predictions of both steady and transient flow properties from this model are validated with experiments conducted on a 1.32 m long cylindrical duct, with a cross-sectional diameter of 52 mm, rotating at speeds of upto 1050 RPM (Tip Speed = 145 m/s). Spanwise pressure distribution, duct velocity, temperature, hub forces and moments results from the numerical model showed good correlation with experiments. Considerable internal mass flow rate (~ 0.3 kg/s) was also observed. In the presence of a time-varying valve at the inlet, transient spanwise pressure variations showed periodic fluctuations in pressure which diminished once the valve was fully open. The quasi 1-D model was found to be a much faster computational tool than any conventional 3-D CFD solver to study spanwise flow inside rotor blades. The experiments revealed key information about pressure at the duct's outlet. It was observed that when the duct's inlet is closed, the duct's outlet pressure is less than its ambient value. The knowledge of these boundary conditions is essential in modeling flow through rotating ducts. For more accuracy, the current internal flow solver could be coupled with an external flow code to iteratively obtain boundary conditions at their interface.