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Methods for Real-Time Rotor Stall Detection

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Methods for Real-Time Rotor Stall Detection Methods for Real-Time Rotor Stall Detection Ivan Grill Manuj Dhingra J.V.R. Prasad Graduate Research Assistant Research Engineer Professor School of Aerospace Engineering School of Aerospace Engineering School of Aerospace Engineering Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Atlanta, GA 30332 Atlanta, GA 30332 Atlanta, GA 30332 Abstract Main rotor stall in rotorcraft is characterized by the loss of lift, increased vibrations and significant structural loads on the rotor control system. The main focus of the current study is to examine an effective method for real time detection of rotor stall. The discussion related to rotor stall specifically pertains to an increase in control system pitch link loads. Observations related to various aerodynamic phenomena that influence the pitch link loads are discussed as well. Data from the UH-60A Airloads Project obtained through the TRENDS database are used in this study. An existing method based on the Equivalent Retreating Indicated Tip Speed (ERITS) measure is evaluated using the UH-60A data, and is shown to be overly conservative for rotor stall detection, both during steady and maneuvering flight conditions. A new method based on a previously developed Real-Time Nonlinear Observer algorithm is evaluated for its applicability to real time rotor stall detection and is shown to be a viable alternative. Nomenclature pilot’s ability to maneuver the aircraft. The term “rotor stall” is used in the current study to designate 1 c blade chord the rotorcraft flight condition in which a significant CL blade lift coefficient portion of the rotor disc has stalled, such that the CW aircraft weight coefficient rotor control loads have increased beyond a D total aircraft drag prescribed limit. Further, the term “control loads” h altitude refers specifically to the loads on the main rotor pitch L blade lift links which, as the name suggests, are responsible for Nb number of rotor blades the blade pitch control. The main rotor control system Nz load factor along the body z-axis contains pitch links in both the fixed as well as in the R blade radius rotating frame. It will be shown that the effects of S blade surface area rotor stall are different for the fixed and rotating pitch T main rotor thrust links, and hence it is important to distinguish them V aircraft true airspeed clearly. A detailed description of a generic main rotor Vi aircraft indicated airspeed control system along with an accompanying Vrel blade relative airspeed schematic is available in Ref. [1]. It is also important W aircraft current weight to distinguish between rotor stall and blade stall, W0 aircraft nominal weight (arbitrary) = 73396 N because the presence of stall at a few sections only of ρ air density a rotor blade does not necessarily imply significant ρSL sea level standard air density increase in pitch link loads. However, in order to σ main rotor solidity (Nbc/πR) understand rotor stall, we need to understand the ω pitch link load frequency individual blade stall as well. The mechanisms Ω main rotor angular velocity responsible for rotor blade dynamic stall and its effects are presented in significant detail by Introduction McCroskey [2] and Young [3]. In forward flight, rotor blade experiences airflow from vehicle motion Main rotor stall is one of the major factors that as well as its own rotation. The effective flow limit the high speed forward flight and impact the becomes asymmetric, with higher velocities on the maneuverability of a rotorcraft. The vibrations advancing side and lower velocities on the retreating produced by a stalling rotor may significantly side of the rotor disc. In order to compensate for the damage the rotorcraft’s control system. This lower relative velocity on the retreating side, the structural damage typically requires costly and time blade goes through a cyclic pitch (feathering) motion. consuming repairs but in extreme cases can impede That means that the blade pitch and therefore blade angle of attack must be higher on the retreating side. Presented at the 34th European Rotorcraft Forum, If the angle of attack is too high, the blade will Liverpool, UK, Sep. 16 – 19, 2008. exhibit dynamic stall. As the blade stalls, the top surface pressure in the front part of the blade No detailed explanation of the origin of k is given, increases due to loss of suction. This, in turn causes except for the fact that it is a function of both blade the center of pressure to move towards the trailing geometry and loading. However, another parameter edge, creating a short-term but large magnitude directly analogous to k is Equivalent Retreating negative pitching moment, which is the root cause of Indicated Tip Speed (ERITS), which is proportional oscillating pitch link loads as the rotor enters stall. to , where C is the equivalent lift coefficient 1/ CL L Although computational techniques have been used of the retreating blade near the tip at 270° azimuth to understand and predict the effect of dynamic stall with a simplifying assumption of uniform lift on pitch link loads, e.g. Datta and Chopra [4], a real- distribution. ERITS itself is not obtained empirically time oriented model has not been developed so far. but the limit that needs to be set on ERITS is an empirical value. Therefore ERITS itself will be An overview of various approaches to the problem considered an empirical rotor stall detection of rotor stall is presented in the following paragraph. parameter. There does not seem to be any written Since rotor stall is a consequence of blade stall references related to the origins of ERITS, for it is a occurring over a significant portion of a rotor disc, parameter that was originally developed at Sikorsky one way of avoiding rotor stall is to prevent blade as an “in-house” rotor stall detection method. Some stall itself. Considerable amount of research has been studies have considered ERITS as a viable rotor stall done related to the retreating blade stall control and detection method, particularly the works of Jeram [9] some of it is presented by Nguyen [5] and Lorber et and Yavrucuk et al. [10]. Considering that ERITS is al. [6]. Blade stall control requires that the blade be the only method specifically formulated as a rotor instrumented with actuators in case of an open loop stall detection parameter, its applications and control, and with sensors and actuators in case of a derivation will be discussed in further detail. closed loop control. Additional instrumentation however brings up an issue of complexity associated The current study has a dual focus: a deeper with blade stall control system integration, which is understanding of the fundamental aerodynamic quite significant [6]. In addition to the individual phenomena that affect the pitch link loads as well as blade stall control, some work has also been done in a feasibility investigation of real-time rotor stall the area of pitch link loads reduction using control detection. The analysis of the ERITS parameter techniques by Voskuijl et al. [7]. Unfortunately, due presented here illustrates its limitations as a rotor stall to the lack of experimental data available to Voskuijl detection method. Consequently, a new method for and others, no clear conclusions were made about the rotor stall detection that utilizes the fixed frame pitch obtained results. In the absence of physics based link load measurements has been proposed and models, neural networks could be applied for pitch evaluated using flight test data. The proposed method link load prediction, if the experimental data required tracks the frequency content of the pitch link load for training of the neural network was available. signal using a real-time algorithm previously Kottapalli [8] has shown that the neural network developed at the Georgia Institute of Technology. approach can accurately model the pitch link loads in This signal processing algorithm, called the Real- level flights and maneuvers. However, the neural Time Nonlinear Observer (RTNO), has the ability to network approach suffers from the disadvantage that extract the magnitude and frequency of the dominant it requires large amounts of training and validation component in a signal. Data used throughout this data covering the entire range of flight conditions. study are obtained through the TRENDS database [11], which contains flight test data generated as part Current methods utilized for rotor stall detection of the UH-60A Airloads Project. use empirical parameters obtained from numerous flight tests and experience in order to set a limit on The paper presented is organized as follows: First, the aircraft’s flight envelope. One of the empirical a brief description of the database which is the source parameters that set the limit on the rotorcraft airspeed of all the data used in the current study is presented. for an allowable level of control loads is shown in Next, the various aerodynamic phenomena that Ref. [1] and is represented by k. Expression for the influence the pitch link loads are discussed, in order aircraft velocity limit as a function of k is shown in to gain a better understanding of the problem at hand. Equation 1. This is followed by the discussion of the evaluation k of the ERITS method and the proposed RTNO Vlim ≈ ΩR − (1) method for real time stall detection. Finally, ρW concluding remarks are provided in order to summarize the main points of the study along with suggestions for future work. 2 hand, performance at high weight coefficients is UH-60A Airloads Data limited by the dynamic stall cycles which seem to be the main cause of the increased pitch link loads. Data used in the current study are obtained from Coleman and Bousman [13] claim that the increase in the TRENDS database, which consists of numerous pitching moments at high advance ratios is caused by flight test data generated in 1993-1994, as part of the unsteady, three dimensional flow at the blade tip UH-60A Airloads Project at NASA Ames Research which is unrelated to stall.
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