Aerodynamic Design of Helicopter Rotors for Reduced Brownout Glen R

Aerodynamic Design of Helicopter Rotors for Reduced Brownout Glen R. Whitehouse, Daniel A. Wachspress and Todd R. Quackenbush [email protected] , [email protected] , [email protected] Continuum Dynamics, Inc. Ewing, New Jersey, 08618 ABSTRACT Rotorcraft brownout is caused by the entrainment of dust and ground debris by the rotorwash during take-off and landing, and is a critical operational problem. Brownout affects safe operations due to the reduction of visibility, in addition to damaging engine components and rotor blades. Recent experience gained from operations in brownout with a variety of rotorcraft configurations, dynamically scaled test facilities as well as ongoing work with validated analyses at Continuum Dynamics, Inc. (CDI), indicates that aerodynamic modifications to the aircraft may offer a complementary approach to sensor-based brownout solutions. This paper reviews the underlying brownout flow-physics, with particular emphasis on the critical connection between wake dynamics and tip vortex properties, and the development of a strategy to best exploit fluid- dynamic mechanisms to reduce brownout without adversely affecting the aerodynamic performance of the rotor system. Particular emphasis will be placed on explaining why basic rotor scaling parameters have little influence on brownout and why the AW101/EH101/US101, and the Sea King with advanced blades, might offer improved brownout performance. Practical methods are presented for implementing modifications in a retrofittable manner that do not degrade the aerodynamic performance of the helicopter and are commensurate with current procurement systems. NOMENCLATURE * INTRODUCTION CD drag coefficient Brownout - the entrainment and circulation of particles, dust and debris from the ground during rotorcraft take-off and CT thrust coefficient landing over unprepared fields - represents a serious D drag problem for rotorcraft operations (see Figure 1). Recent experience in Iraq and Afghanistan has heightened the D particle diameter awareness of the impact of reduced visibility on safe N number of blades rotorcraft operations in brownout conditions, in addition to b sand erosion of engine components and rotor blades. As of R rotor radius 2004, three out of every four helicopter accidents in Iraq and Afghanistan had been attributed to brownout [1], and on a Re Reynolds Number DOD-wide basis, brownout related costs were estimated to T rotor thrust be over $100 million per year [2]. Consequently, this problem has received considerable attention with recent U free stream velocity work focusing on the use of pilot cueing systems to augment situational awareness in the degraded visual environment associated with brownout [3, 4] as well as the use of Γ circulation improved sensor systems [5]. Indeed, recent work by the ν kinematic viscosity Department of Defense under the SANDBLASTER program and others has successfully demonstrated an integrated µ advance ratio sensor, cueing and synthetic vision based system [6, 7]. ρ particle density Still, sensor suites add weight and expense and do nothing to relieve blade erosion and damage to engine components. ρa air density Also, it is likely that, given the choice, pilots would always σ prefer maintaining visual contact with the ground. These rotor solidity circumstances have sparked continued interest in exploring Ω rotor rotation rate the flow physics of brownout with the goal of possibly developing aerodynamic methods for mitigating dust entrainment. Recent experience gained from operations in brownout with a variety of rotorcraft configurations [8, 9], dynamically scaled test facilities [10] as well as ongoing work with Presented at the International Powered Lift Conference, October 5- validated analyses at Continuum Dynamics, Inc. (CDI) [10- 7, 2010, Philadelphia, PA. Copyright © 2010 by the American Helicopter Society International, Inc. All rights reserved. 13], indicates that aerodynamic modifications to the aircraft may offer a complementary brownout solution to the sensor- characteristics. Nevertheless, the work of Whitehouse et al based system successfully demonstrated by the Department [17] (which demonstrates a 10% improvement in of Defense [6, 7]. obscuration for a minor tip-modification and will be described below), and recently Milluzzo et al [16], suggest that a more unconventional approach to blade shape design, such-as diffusing the tip vortex, may be required. Indeed, this may be just the phenomenon that leads to improved brownout characteristics for the rebladed-Sea King [8] (Figure 2 left) and the AW101/EH101/US101 [18] (Figure 2 right). Figure 1. External and interior views of helicopter operations during brownout conditions. From an operational standpoint, sensor-based solutions offer improved safety by enabling the pilot to see virtually “all the way to the ground”, however such approaches require expensive glass-cockpits, in addition to sensor suites and other cueing systems to accurately indicate rotorcraft motion in low speed low altitude flight. Such an approach makes no attempt to address the logistical problem of mitigating the severity of the brownout dust cloud as it impacts the aircraft, † Figure 2. Sea King brownout landing with new Carson thereby reducing component life . An aerodynamically blades (left) [8]; AW101/EH101/US101 promotional derived brownout mitigation strategy would directly address material (right) [9]. both issues; offering the potential to significantly reduce brownout obscuration and airframe component erosion. For However, there is as yet no published conclusive test data to optimal pilot visibility, however, an aerodynamic solution explain why these airframes may exhibit reduced brownout. requires that the rotor be able to ingest “clean air”, though The research described herein, and indeed to some extent the this may not be the case for multiple helicopter operations preliminary work presented in [10], aims to provide insight (i.e. formation flight). In dusty environments, where the into aerodynamic factors affecting brownout at a sufficient helicopter is already engulfed in a dust-cloud – i.e. from an level that changes can be made at the design stage to aircraft landing nearby – a sensor-based system would be mitigate dust entrainment. required to “see the ground”, though an aerodynamic In the effort documented herein, a fast, high-fidelity mitigation strategy would be complementary, reducing the brownout analysis tool has been developed, validated and concentration of the dust cloud near to the airframe, and used in preliminary investigations. A model-scale test hence component erosion/ingestion. facility has also been designed and fabricated. After a brief An initial analysis would suggest that since brownout is a review of the analysis and its validation, as well as function of the rotorwash – the flow field induced by the preliminary testing in a model-scale “long track” brownout helicopter rotor wake as it impinges on the ground – it testing facility, observations will be made about the should scale with the rotor downwash velocity, and hence underlying fluid dynamics structures and their impact on the square root of disk loading (thrust/rotor area). brownout. Practical methods to implement retrofittable Consequently, one could conclude that a reduction in modifications, that do not degrade aerodynamic brownout requires a reduction in aircraft weight or an performance, will be presented. increase in rotor radius. However, given that the disk BROWNOUT ANALYSIS AND SIMULATION loading is defined by the mission specifications of the aircraft, it seems unlikely that an aerodynamic solution to To support the development of advanced rotorcraft concepts, brownout based on reduced disk loading that does not flight control and sensing technology development, pilot degrade performance is feasible. Moreover, recent training, and operational planning for mitigating the effects numerical and experimental work at CDI [10], as well as of rotorcraft brownout, advanced simulation tools are simulations by the University of Glasgow [14, 15] and required. Recently, a physics-based high fidelity brownout micro-rotor tests by the University of Maryland [16], have analysis has been developed and validated based on modular indicated that conventional changes to the basic scaling rotorcraft simulation physics-based technologies [10-13] parameters (number of blades, chord length, twist etc) of the (see Figure 3). rotor system have little impact on the brownout A central element of CDI’s brownout model is the advanced CHARM rotor wake solution that uses full-span free vortex †It should be noted that the SANDBLASTER program was initiated wake methods to represent the complex flow field of to eliminate catastrophic landing mishaps. maneuvering rotorcraft in the proximity of the ground. The rigid blades, linear local aerodynamics and CHARM full- CHARM free-wake model is ideal for brownout analysis span wake model without fuselage), where both rotors were because it is the fastest modeling approach that can still trimmed to match half the aircraft weight and to ensure that capture the relevant flow field features [19]. there were no rolling or pitching moments on the aircraft at each time step. At the end of the calculation, for a bed of quartz particles six inches thick, as tilled in the experiments, with a mean diameter of 10 microns, the output was processed to determine the particle concentration at each sampler location. Figure 4 and Figure 5 show correlations

Load more