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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 *

CD drag coefficient

CT thrust coefficient

D drag

D particle diameter

Nb number of blades

R rotor radius

Re Reynolds Number

T rotor thrust

U free stream velocity

Γ circulation

ν kinematic viscosity

µ advance ratio

ρ particle density

ρa air density

σ rotor solidity

Ω rotor rotation rate

Presented at the International Powered Lift Conference, October 5-7, 2010, Philadelphia, PA. Copyright © 2010 by the American Helicopter Society International, Inc. All rights reserved.

INTRODUCTION

Brownout - the entrainment and circulation of particles, dust and debris from the ground during rotorcraft take-off and landing over unprepared fields - represents a serious problem for rotorcraft operations (see Figure 1). Recent experience in Iraq and Afghanistan has heightened the awareness of the impact of reduced visibility on safe rotorcraft operations in brownout conditions, in addition to sand erosion of engine components and rotor blades. As of 2004, three out of every four helicopter accidents in Iraq and Afghanistan had been attributed to brownout [1], and on a DOD-wide basis, brownout related costs were estimated to be over $100 million per year [2]. Consequently, this problem has received considerable attention with recent 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 improved sensor systems [5]. Indeed, recent work by the Department of Defense under the SANDBLASTER program and others has successfully demonstrated an integrated sensor, cueing and synthetic vision based system [6, 7]. Still, sensor suites add weight and expense and do nothing to relieve blade erosion and damage to engine components. Also, it is likely that, given the choice, pilots would always prefer maintaining visual contact with the ground. These circumstances have sparked continued interest in exploring 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 validated analyses at Continuum Dynamics, Inc. (CDI) [10-13], indicates that aerodynamic modifications to the aircraft

may offer a complementary brownout solution to the sensor-based system successfully demonstrated by the Department of Defense [6, 7].

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, thereby reducing component life†. An aerodynamically derived brownout mitigation strategy would directly address both issues; offering the potential to significantly reduce brownout obscuration and airframe component erosion. For optimal pilot visibility, however, an aerodynamic solution requires that the rotor be able to ingest “clean air”, though this may not be the case for multiple helicopter operations (i.e. formation flight). In dusty environments, where the helicopter is already engulfed in a dust-cloud – i.e. from an aircraft landing nearby – a sensor-based system would be required to “see the ground”, though an aerodynamic mitigation strategy would be complementary, reducing the concentration of the dust cloud near to the airframe, and hence component erosion/ingestion.

An initial analysis would suggest that since brownout is a function of the rotorwash – the flow field induced by the helicopter rotor wake as it impinges on the ground – it should scale with the rotor downwash velocity, and hence the square root of disk loading (thrust/rotor area). Consequently, one could conclude that a reduction in brownout requires a reduction in aircraft weight or an increase in rotor radius. However, given that the disk loading is defined by the mission specifications of the aircraft, it seems unlikely that an aerodynamic solution to brownout based on reduced disk loading that does not degrade performance is feasible. Moreover, recent numerical and experimental work at CDI [10], as well as simulations by the University of Glasgow [14, 15] and micro-rotor tests by the University of Maryland [16], have indicated that conventional changes to the basic scaling parameters (number of blades, chord length, twist etc) of the rotor system have little impact on the brownout

†It should be noted that the SANDBLASTER program was initiated to eliminate catastrophic landing mishaps.

characteristics. Nevertheless, the work of Whitehouse et al [17] (which demonstrates a 10% improvement in 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 2. Sea King brownout landing with new Carson

blades (left) [8]; AW101/EH101/US101 promotional material (right) [9].

However, there is as yet no published conclusive test data to explain why these airframes may exhibit reduced brownout. The research described herein, and indeed to some extent the preliminary work presented in [10], aims to provide insight into aerodynamic factors affecting brownout at a sufficient level that changes can be made at the design stage to mitigate dust entrainment.

In the effort documented herein, a fast, high-fidelity brownout analysis tool has been developed, validated and used in preliminary investigations. A model-scale test facility has also been designed and fabricated. After a brief review of the analysis and its validation, as well as preliminary testing in a model-scale “long track” brownout testing facility, observations will be made about the underlying fluid dynamics structures and their impact on brownout. Practical methods to implement retrofittable modifications, that do not degrade aerodynamic performance, will be presented.

BROWNOUT ANALYSIS AND SIMULATION

To support the development of advanced rotorcraft concepts, flight control and sensing technology development, pilot training, and operational planning for mitigating the effects of rotorcraft brownout, advanced simulation tools are required. Recently, a physics-based high fidelity brownout analysis has been developed and validated based on modular rotorcraft simulation physics-based technologies [10-13] (see Figure 3).

A central element of CDI’s brownout model is the advanced CHARM rotor wake solution that uses full-span free vortex wake methods to represent the complex flow field of

maneuvering rotorcraft in the proximity of the ground. The CHARM free-wake model is ideal for brownout analysis because it is the fastest modeling approach that can still capture the relevant flow field features [19].

Figure 3. Brownout Model Components and Structure.

The CHARM model has been combined with CDI’s well-validated LDTRAN (Lagrangian Deposition and TRajectory Analysis) particle entrainment and transport model. LDTRAN is a key part of the U.S. Army’s suite of tools for simulating the effects of chemical/biological clouds on the passage of rotorcraft [20]. The output from the CHARM/LDTRAN coupled analysis is processed by a single-scattering visual obscuration model appropriate for brownout cloud modeling to determine the brownout cloud characteristics on a physical basis. The brownout cloud is then processed and displayed in an OpenSceneGraph visual viewer using the method of dynamic imposter generation [21]. While competing brownout analyses have been developed at the University of Glasgow [14, 15], AgustaWestland [22, 23], University of Maryland [24] and Flow Analysis [25], the key advantage of the CHARM/LDTRAN approach is the ability to predict brownout in timeframes commensurate with design, and even training, cycles.

Validation In 1967 Rodgers [26, 27] documented a series of series of brownout flight tests using an H-21 tandem rotor helicopter to establish the concentration of dust picked up by the helicopter’s downwash as a function of position and hover height. A total of 98 flight tests were performed, including several with another helicopter operating nearby, at three different test sites. Dust samples were taken at 25 locations near to the aircraft with vacuum style samplers placed on booms attached to the helicopter fuselage, and were turned on for 4 minutes once the pilot had attained a steady hover at one of three altitudes (wheels at 1ft, 10ft or 75ft above the ground). The dust samples were then analyzed and concentrations determined.

To verify the performance of CDI’s brownout analysis, a series of calculations were performed to predict Rodgers’ [27] experimental observations. For these calculations the baseline brownout analysis was employed (i.e. aeromechanic model for the H-21 consisting of two articulated rotors with

rigid blades, linear local aerodynamics and CHARM full-span wake model without fuselage), where both rotors were trimmed to match half the aircraft weight and to ensure that 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 of brownout predictions, performed with grid of 81 x 81 particle clusters covering an area spanning 100ft in each direction away from the center of the front rotor of the aircraft (each cluster represents approximately ~1014 particles), with Rodgers’ measurements that illustrate the effectiveness of the model in accurately predicting the dust concentrations near a hovering helicopter. The prediction shows that this relatively low resolution calculation accurately predicts the dust concentration on both the starboard side of the helicopter, (Figure 4) and the port side (Figure 5).

Figure 4. Brownout model prediction of dust

concentrations along the starboard side of an H-21 tandem helicopter compared with test data from [27]– sampler location 0.0 corresponds to the aircraft nose.

Figure 5. Brownout model prediction of dust

concentrations along the port side of an H-21 tandem helicopter compared with test data from [27]– sampler

location 0.0 corresponds to the aircraft nose.

These correlations are very encouraging and demonstrate that the model can accurately and quickly predict brownout cloud concentrations near a hovering tandem rotor configuration. The calculation time for the solution shown above was <150 seconds on the single-core of a 2GHz Intel Core Duo processor.

INVESTIGATION OF BROWNOUT FLOW PHENOMENA

Prior numerical investigations of brownout have indicated that the fine-scale brownout characteristics, i.e. local small scale structures in the cloud, are sensitive to the flight maneuver, ground cover and entrainment model settings [10, 13, 17]. Macro scale characteristics, particularly the visual obscuration, however, appear to be fairly insensitive to changes in the rotor system [10]. Figure 6 shows the minor effect of reducing the number of blades from five to four and changing the twist on brownout behavior during a flare-type maneuver. Indeed, recent numerical simulations by Phillips and co-workers at the University of Glasgow [14, 15] and micro-rotor tests by Leishman and co-workers at the University of Maryland [16], have also confirmed that conventional changes to the basic scaling parameters (i.e. number of blades, chord length, twist etc) of the rotor system have little impact on the brownout characteristics. Nevertheless, such observations, while consistent, appear to contradict the observations of pilots of the AW101/EH101/US101 and of the more recently upgraded Sea King.

Figure 6. 3D visualization of brownout conditions for a helicopter undergoing a flare maneuver during landing with five blades (top), four blades (middle) and with five

blades with twist increased from -8° to -18°(bottom).

Such observations and conclusions place the rotorcraft community in the difficult position of trying to reconcile the apparent contradictions between operational observations on

the one hand and numerical predictions and sub-scale testing on the other. Are the numerical models neglecting something? Does sub-scale testing neglect or suppress certain brownout phenomena? While none of the developers of numerical methods would claim to fully simulate the entire rotorcraft brownout problem, and indeed experimentalists are only too aware of the limitations of subscale testing, we do not believe that these limitations are the source of this paradox. After a review of the fundamental flow phenomena, an explanation for the difference between full scale and simulated predictions will be given, along with a postulated method for exploiting this phenomenon for reducing brownout obscuration. Finally, we will present results from a proof-of-concept simulation and experiment.

As has been well documented, the rotor wake is dominated by strong tip vortices associated with the rolling up of vorticity trailed and shed by the rotor blades. The resulting vortex wake is inherently unstable and, in ground effect, will often propagate back up through the center of the rotor as well as down and out toward the ground exacerbating the unsteadiness [28, 29]. Moreover, recent investigations have observed a periodic bundling of these tip vortices into large coherent structures [28, 30], see Figure 7, that induce significant velocities parallel and perpendicular to the ground. Moreover, the tendency of tip vortices trailed by rotors in ground effect to bundle in the vicinity of the ground produces a “pulsation” phenomenon in the wake, typically occurring at a fraction of the rotor rotation frequency (e.g., with a period of roughly 1 sec. for representative rotorcraft) [12]. Indeed, it is likely that these strong unsteady vortical structures play a significant role in lofting the fine dust particles associated with brownout. Impeding the formation of such structures would likely have a significant effect on brownout since the resultant flow would resemble a jet-flow, which is known to have more benign brownout characteristics than a rotor wake. Since, traditional changes to the rotor system (twist, chord and simple tip modification) do little to alter the fundamental nature of the tip vortex formation and the bundling phenomenon; it is consistent that recent numerical and experimental work has concluded that brownout is insensitive to rotor scaling parameters.

Mitigation Strategy Given that the bundling process is dominated by the coalescence of the tip vortices into larger coherent vortical structures, a practical approach to disrupt this behavior would be to prematurely diffuse the local tip vortices. In this vein, work at CDI has focused on assessing retrofittable methods to diffuse the tip vortex flow field more rapidly than is the case for current-generation helicopters. As noted above, the tendency of tip vortices trailed by rotors in ground effect to bundle in the vicinity of the surface produces a “pulsation” phenomenon in the wake; this is depicted schematically in Figure 8. The importance of this behavior from the point of view of brownout is that such

bundles (or “super vortices”) have a tendency to induce significantly larger vertical transport rates for material lofted from the surface than would be the case for more “jet like” vortex wakes without such large structures.

Figure 7. Flow visualization of a hovering scale rotor in

ground effect (Retip=O(104)) [30].

rotor blade circulationdistribution

concentrated vorticesundergo periodicbundling in far wake

ground plane


concentrated vorticesundergo periodicbundling in far wake

ground plane Figure 8. Illustration of baseline lofting/brownout

mechanisms.

To disrupt both the periodic bundling of the tip vortices and the recirculation of the lofted material through the rotor, one general approach is to produce more diffuse individual vortices; this is depicted schematically in Figure 9. A variety of physical mechanisms (e.g., the introduction of increased turbulence into the roll-up process, the presence of higher ambient turbulence levels, strains exerted by companion vortices) can lead to more rapid core diffusion.

Figure 9 suggests one hypothetical system, in which deployable vortex generators [31] are used to produce increased turbulence in the boundary layer near the tip of the blade; such devices could possibly be used in combination with the deflection of on-blade flaps inboard, which would have the effect of shifting load inboard from the tip region and producing a weaker, less strongly rolled-up tip vortex.

Figure 10 indicates a second possible scenario that can lead to enhanced diffusion, e.g., the existence of a tapered planform that can, in a manner somewhat analogous to the BERP tip, though the notch produces two vortices with opposite sign [32], produce multi-vortex system, though likely with a less discrete inboard vortex; such a wake configuration can cause the primary tip vortex to be

considerably less concentrated downstream than would be the case with a rectangular tip. (Clearly, however, other factors such as the twist distribution across the tip region would also play an important role in determining the initial wake roll-up process).

local popup VGs

on-blade tabs/flaps

more rapid diffusion mitigatesbundling, lofting, brownout


ground plane

local popup VGs

on-blade tabs/flaps

more rapid diffusion mitigatesbundling, lofting, brownout


ground plane Figure 9. Illustration of the possible disruption of

baseline lofting/brownout mechanisms via on-blade devices.

Secondaryvortex from

swept/taperedtip a possible

source of more rapid diffusion

Circ

ulat

ion

Planform

Tip vortex and possible inboard

vortex

Secondaryvortex from

swept/taperedtip a possible

source of more rapid diffusion

Circ

ulat

ion

Planform

Tip vortex and possible inboard

vortex

Figure 10. Multi-vortex system trailing from a nonrectangular planform – a candidate method for

inducing a more diffuse vortex.

INVESTIGATION OF S-61

Information has recently become available regarding the brownout behavior of an existing helicopter that provides significant support for the strategy outlined in the previous section. The S-61 Sea King helicopter (operated as the SH/VH-3 by the U.S. Navy and Marine Corps and as the Sea King by the U.K. Royal Navy and Royal Air Force) has been in service since the mid-1960s. The original aircraft employed constant-chord blades with a linear (-8 deg.) twist rate and a NACA 0012 airfoil section. Work led by Carson Helicopters, supported by modeling studies at CDI, in the period 1993-2002 [33, 34] saw the development of an improved rotor blade set for the S-61, featuring a swept/tapered blade tip, advanced airfoil sections, and a higher-twist design (-12 deg. linear twist). This redesign was undertaken to improve the heavy lift and high altitude

performance capability of the aircraft in support of Carson’s commercial operations; however, the performance improvements were so dramatic that several military services have adopted the Carson blades for use in reblading their Sea King aircraft.

One such service was the Royal Air Force; RAF Sea Kings with Carson blades (Figure 11 left) have been operating in Afghanistan since mid-2007 (Figure 11 right). During operations, it was noted by several RAF pilots that the aircraft with the new Carson blades displayed markedly more benign brownout behavior than the baseline vehicle; the rebladed helicopters displayed a “donut” flow pattern similar to the AW101/EH101/US101 [8]. While evidence of this remains anecdotal, this case represents a potentially ideal opportunity to investigate the effectiveness of aerodynamic mitigation techniques, since here the S-61 variants involved are identical except for their rotor blades.

Figure 11. Rebladed RAF Sea King (left), and

photograph of brownout showing the dust being pushed away radially from the aircraft (right).

Since CDI was quite active in supporting the design activities of Carson during the development effort, design data for these rotor blades was readily available. It was judged useful to compare the predicted load characteristics for the S-61 with and without the Carson blades to attempt to identify flow features that could contribute to the observed brownout behavior. Figure 12 and Figure 13 show the computed rotor blade circulation distribution for the baseline S-61, the modified variant with Carson blades, and, for reference, the UH-60 in hover. All three aircraft operate at approximately the same gross weight (~20,000 lbs.), while the UH-60 has approximately 30% higher disk loading, one fewer blade (four vs. five for the S-61), and a nonlinear twist rate, including a distinctive “up twist” near the blade tip. The bound circulation distributions, computed with CDI’s CHARM analysis [35], are broadly similar. Focusing on the comparison between the base and modified S-61s, the higher twist in the modified blades shifts the loading inboard and reduces the slope of the bound circulation distribution near the tip; both of these circumstances tend to slow and weaken the roll-up of vortices in the rotor wake. Moreover, in the slight “notch” in the circulation distribution around 95% radius is evidence of the effect of the sweep in the planform that may indicate the initiation of a secondary vortex system that could enhance the tip vortex diffusion rate. While the causal link is not definitive, such differences in blade loading (and thus in vortex wake roll-up) could produce a

significant difference in far wake behavior, driving the reported mitigation of brownout.

0

0.005

0.01

0.015

0.02

0 0.2 0.4 0.6 0.8 1

Base S-61Mod S-61UH-60

Bou

nd C

ircul

atio

n/(

ΩΩ ΩΩ*R

2 )

r/R

Figure 12. Predictions of baseline and modified S-61 normalized circulation at constant thrust in hover. UH-

60 also plotted for comparison.

0

0.005

0.01

0.015

0.02

0.94 0.95 0.96 0.97 0.98 0.99 1

Base S-61, Mod S-61, UH-60

Base S-61Mod S-61UH-60

Bou

nd C

ircul

atio

n/(

ΩΩ ΩΩ*R

2 )

r/R

Figure 13. Close-up of tip region of baseline and modified S-61 normalized circulation at constant thrust

in hover.

To further explore this issue, a supporting analysis was employed – the CDI UNIWAKE parabolized Navier Stokes solver [36]. While this solver was designed for and is restricted to modeling viscous flows in the crossflow plane with uniform onset flows (vs. the more general onset flow with rotation in rotor cases) it is well suited for modeling differences in vortex roll-up. Using the circulation distribution for the modified blade in Figure 12 as input, UNIWAKE was used to track the early roll-up behavior of the tip vortex in this case. Figure 14 shows the early phases of the process, suggesting how the secondary vortex is entrained in the primary vortex roll-up, contributing to its diffusion.

“blade tip”

secondary vortex

primaryvortex

Figure 14. UNIWAKE predictions of primary tip-vortex and secondary vortex interaction over the first five rotor

chord lengths of the roll-up process.

The long-term consequences of this process become evident when the time history of the peak vorticity is computed for cases with and without the presence of a secondary vortex (corresponding to the modified and base S-61 cases, respectively) (see Figure 15). As is evident, the diffusion rate is markedly faster. To quantify the importance of this difference, a third curve is plotted in Figure 15, the time history of imposed vortex diffusion required to produce the brownout mitigation shown in Figure 16; which effectively shows the difference in behavior anticipated for vortex roll-up processes that follows the red curve rather than the blue curve in Figure 15. Since it is expected the modified S-61 would track with the green curve (which exhibits faster tip vortex diffusion than the successful mitigation case shown in Figure 16), there appears to be significant evidence supporting the proposition that the new Carson S-61/Sea King aircraft would exhibit relatively benign brownout characteristics.

0.5

0.6

0.7

0.8

0.9

1

1.1

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Parameterized Diffusion

NS Computed Diffusion (no secondary vortex)CHARM Brownout Diffusion Model

NS Computed Diffusion (with secondary vortex)

Frac

tion

of In

itial

Pea

k V

ortic

ity

Time (sec)


0.5

0.6

0.7

0.8

0.9

1

1.1

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Parameterized Diffusion


NS Computed Diffusion (with secondary vortex)

Frac

tion

of In

itial

Pea

k V

ortic

ity

Time (sec)


Figure 15. Comparison of UNIWAKE predicted vortex core diffusion with parameterized diffusion in the CDI

brownout model.

Retrofittable Actuation Concepts In terms of effecting one or more of these scenarios, a variety of mechanisms can be adopted. Figure 17 shows two variants of a combined system of tabs/flaps [37] and VGs [31], one with a relatively “low-footprint” installation with small-chord tabs inboard and one assuming larger-chord flaps. An important element of this effort was assessment of

actuation technology suitable for implementing such concepts; a key requirement was that the devices could be integrated directly onto the rotor blades. Figure 18 shows candidate devices and how they are sized relative to a representative blade segment. Prior work at CDI has demonstrated the use of Shape Memory Alloy (SMA) devices to provide compact actuation to deploy such surfaces, see for example [31, 37], and the following discussion will present appropriate configurations, scaling and energy requirements.

Figure 16. Brownout predictions of a UH-60 during a hover-descent. Baseline UH-60 (upper) and Modified

UH-60 (lower) employing devices to adjust wake properties to mitigate brownout.

The prior work at CDI on SMA applications has produced a large data base of information on device performance and power requirements. To assess likely power needs for candidate brownout mitigation installations, baseline assumptions for the devices described above were made. Figure 19 below shows the force requirement for deflection of the SMA-actuated flaps shown in Figure 18; these requirements were determined by scaling up results from prior wind tunnel tests of smaller devices. Since the flaps are intended for use on the inboard portions of full-scale rotors, the relevant onset flow speed range is up to roughly 400-450 ft/s. Clearly, the force requirement for deflection

increases substantially over this range, but this is well within the capability of current-generation SMA actuator wire.

Conceptual Active Rotor – Flap Variant

Part span tab array(~ 0.4R to 0.6R-0.8R)

Schematic (top view) (side view)

Rotor hub

Deployable VG array

Chordwise extent of tabs (~ 0.05-0.07c)

Conceptual Active Rotor – Tab Variant

Rotor hub

Deployable VG array

Spanwise extent of flap(~ 0.2R to 0.7R)

Chordwise extent of flap (~ 0.1-0.2c)

Conceptual Active Rotor – Flap Variant



Rotor hub

Deployable VG array



Rotor hub

Deployable VG array





Rotor hub

Deployable VG array



Rotor hub

Deployable VG array




Rotor hub

Deployable VG array



Rotor hub

Deployable VG array



Figure 17. Conceptual active rotor solutions.

Given the ability of .020” dia. actuator wire (a commonly used size in prior applications) to generate approximately 15 lbs. of actuation force, this requirement can be met with two wires per flap element. Conservatively assuming the most extensive active flap installation (see Figure 17) would entail the use of 60 2.5” span flap segments to cover the spanwise region from 20% to ~70% of radius for the UH-60, total actuator wire length for this application would be roughly 1 ft. per flap; using a typical estimate of 2 W/ft for transition of 70 deg. C SMA, yields an estimate of ~120 W/blade for actuation. This estimate, however, neglects cooling losses which could as much as double this requirement. A conservative estimate of power required, therefore, for a four-bladed UH-60 would be approximately 1 kW per blade, which is small compared to the 3 kW per blade installed on the UH-60 for rotor deicing applications.

Full size UH-60 blade section

Small scale VG/spoiler

Baseline PUVG design

Scaled-up “flap” attaches to underside of blade section

Closeup of small scale & large scale spoiler designs

Figure 18. Size of conceptual active surfaces compared to typical rotor blade; these surfaces can be actuated via

compact SMA devices.

Additional preliminary design considerations include the likely weight addition required for this system. The total mass of the flap installation would be in the range of 3-4 lbs. per blade. While this is not large compared to the total mass of roughly 180 lbs. per blade, however, the UH-60 is sensitive to center of gravity considerations and thus placement of these devices would need to be carefully considered. Should the “tab variant” of the overall concept (Figure 17, bottom) be found to be effective, the overall

impact on power and weight would be substantially less. SMA tab devices [37] weigh less than half of the flap mechanisms shown in Figure 18. Moreover, the self-locking nature of their design means that continuous power is not required to maintain deployment; thus, the peak power of 2-3W per device is maintained only briefly during actuation, reducing peak power requirements substantially. The power requirements of the deployable PUVG devices [31] are comparable, and their mass is considerably less than 100 grams per installed device.

0

5

10

15

20

25

30

0 100 200 300 400 500

For

ce (

lbs)

Flow Speed (fps)

Figure 19. Actuation force requirement for an SMA-actuated trailing edge flap deflected 10 deg. as a function

of onset flow speed.

EXPERIMENTAL FACILITY DEVELOPMENT AND TESTING

To support the investigation of aerodynamic solutions to rotorcraft brownout, a subscale test facility has been developed to perform model testing of isolated rotors in brownout conditions.

A central aspect of any experimental study of ground effect aerodynamics is proper representation of the interface between the rotorwash and the surface. This interaction is ideally captured by use of either a moving belt in a wind tunnel or a moving model rig over a fixed surface so that boundary layer effects on rotor induced flow are properly captured. One of the premier examples of this latter category of facility was the Princeton University’s Dynamic Model Track (Longtrack), first constructed in 1960, which was designed to evaluate rotorcraft flight dynamics in a semi-free flight operating mode, where a gimbaled traversing mechanism would allow for restricted degree-of-freedom motion as a model was “flown” down the length of the building. An advantage of the Longtrack relative to other dynamic model facilities was the fact that the model operated in a “still air” environment (save for its own wake),

and thus the “boundary conditions” for low altitude flight were correctly captured. This was also a “disadvantage”, in that the top speed capable for a model depended upon the available facility length and acceleration capability of the combined model carriage and model support. Thus, only low-speed and hover testing of rotorcraft models was possible in that environment. Although the Dynamic Model Track is no longer in service, its design principles have been adopted in a smaller scale version [10]. This facility has benefited from lessons learned, in addition to technology advancements in hardware and instrumentation.

Rotor Scaling Adaptation of this type of facility for testing brownout scenarios is highly advantageous since the use of a moving rig prevents problems associated with particle lofting, motion, wall impact, recirculation and abrasion associated with operating in a conventional wind tunnel, as well as particle injection and belt contamination associated with moving floor tunnels.

Use of subscale rotor models in this context is affected by Reynolds number effects on the aerodynamics of the model rotor as well as on the development of wake and surface boundary layer characteristics. While the use of small scale rotors with lower vortex Reynolds numbers can affect velocity profiles in the wake as well as modify wake-on-wake interactions, prior success in small scale experimental studies of rotor and wing wake development at small scales (see [32] for a comprehensive review) indicate that appropriate wake dynamics can be captured at reduced scale. Moreover, recent experiments at the University of Maryland [30] and the University of Glasgow [38] studying scale rotors in ground effect have been able to reproduce full-scale fluid dynamics phenomena (see Figure 7).

Given the proximity of the rotor to the ground during brownout it is critical to verify that the aerodynamic phenomena associated with ground effect – i.e. flow recirculation, ground vortex shedding, bundling of tip vortices etc – will also scale. The rotor tip vortex, which governs the evolution of the aforementioned phenomena, typically has a core radius on the order of the thickness of the rotor blade, and is relatively insensitive to Reynolds Number, provided that the Re>104 [32]. A more detailed scaling analysis reveals that to scale the rotor wake, the full size and scale Reynolds Number associated with the tip vortex (Rev=Γ/ν) should be matched. However matching Rev imposes significant constraints on the scale rotor apparatus since Γ is proportional to the thrust coefficient multiplied by the rotor area and the rotor rotation rate divided by the number of blades. Thus, for a scaled rotor with tip speed of 200 ft/s (i.e., Retip=O(105), assuming a blade chord of 1 inch), the formation, size and evolution of the vortex core should be the same as for a full-scale rotor.

Prior work at CDI in developing micro rotorcraft for NASA Ames [39], developed a set of 20 inch diameter rotor blades suitable for the testing in the preliminary testing presented here. These rectangular blades feature -20 degrees of twist, a 1 inch chord and custom low Reynolds number airfoil section. Based on the properties of this rotor and the scaling sumarized above (see [10] for more details), a rotational rate of 2300 RPM is required to match full-scale flow phenomena. For the tests presented here with a rotational rate of 2300RPM, Retip=O(105). Moreover, to match the CT/σ for a full-scale UH-60, the scale rotor should be trimmed to 2.48lbs of thrust.

A dynamically scaled brownout test facility, shown schematically in Figure 20, was designed and fabricated. The 30ft x 12ft x 12ft facility was enclosed in clear plastic sheeting, with two viewing windows and double doors for dust containment. The rotor and fuselage model is mounted on a dual track with variable shaft tilt that is supported on a custom composite I-beam mounted on adjustable floor stands. The plywood sub-floor has alternating red and silver optical targets installed 18 inches apart along the 28ft maximum working length of the facility.

Cinderblocks

~2ft

~26ft

Open truss structure

Main support

Fuselage/camerasupport

~10ft

Open truss structure

3-20”

Wheel-track conceptson next slide

Figure 20. Schematic of the brownout test facility.

The test facility is controlled externally from a shielded control station consisting of a laptop computer for controlling the servo motor, an external variable power supply for the rotor, tachometer to measure rotor rpm and a monitor (connected to the camera in the fuselage, see Figure 21) to view the test.

Scaled particles were used for testing in the brownout facility based upon the detailed analysis described in [10] and summarized by

DffR

DR

C

C

D

D

a

aff

sa

asss

s

f

s

f

Df

Ds

f

S

ρρρ

ρρρ

ρρ

−−Ω

−−Ω

ΩΩ

=

(1)

where the subscript s corresponds to the sub-scale particles and f represents full scale. Eq. (1) takes into account the physical mechanisms underlying particle pickup and entrainment described in [40, 41] to establish a relationship between full and sub-scale dust particles. For the rotor model described above, 10 micron silicon carbide particles were selected to satisfy scaling for both pickup and entrainment based upon estimates of typical full-scale particle size distributions from [40].

Pitman motor with output

Sealed dataLogger & fuselage

power supply

Sealed fuselage RotorCamera, laser & sensor Figure 21. Close-up of the rotor and fuselage model

including laser-based sensor package.

Obscuration Sensor Development A critical component of the experimental brownout test facility is measuring the visual obscuration of the dust particles picked-up by the rotor wake. Qualitative measurements can be made by taking photographs of the cloud and of pre-defined targets through the cloud, however, interpretation of these results can be subjective. On the other hand, quantitative measurements can be made by following the approach used by researchers in fire sciences, aerosols and optical communications by measuring turbidity in the air around the helicopter test rig [42, 43].

The principle behind this phenomenon is fairly simple, namely that as light passes through a cloud of suspended particulates, some of it is absorbed and/or scattered. The amount of absorbed energy is a function of the color of the suspended matter, whereas the amount of light that passes through the sample or is scattered is a function of the amount of suspended matter. If we measure the amount of scattered

light, we can determine the amount of suspended matter, and thus the obscuration.

The development of a large-scale unsteady obscuration/turbidity measurement system is described in detail in [10], and is shown schematically in Figure 22. Here a 650µm laser is shone through a long focus lens at a reflective optical target, and a collocated photo-diode measures the laser intensity of reflected light. The returning laser light passes through an interference filter to eliminate the influence of ambient light, and when test measurements are combined with a rotor-off calibration run prior to each test (to eliminate any atmospheric and target fouling effects) the obscuration associated by with the dust cloud can be determined. For the experiments presented here, alternating silver and red optical targets were placed on the ground (i.e. Figure 23) and the reflected light signal was sampled with 10bit resolution at 100Hz.

Laser

Photo-diode

Dust cloud

Lens, splitter & filter

Reflector

KeyOutgoing laser lightIncoming laser lightOutgoing and incoming laser light

KeyOutgoing laser lightIncoming laser lightOutgoing and incoming laser light

Figure 22. Schematic of laser-based obscuration sensor.

Scaled Brownout Testing The shakedown testing and sensor calibration is described in detail in [10]. In summary, for each test, two runs are executed, one with the rotor turned off to establish a case by case calibration for the obscuration measuring system that takes into account any atmospheric effects from prior tests, as well as fouling effects associated with particles adhering to the optical targets or the front of the fuselage model. The second run is a duplicate of the first run but with the rotor rotating. For all the runs presented here, the model was accelerated at 10ft/s2 up to a speed 10ft/s and then decelerated at 10ft/s2 over the 28 ft working length of the facility (µ≈ 0.05). Sample photographs of a test run are shown in Figure 23 from outside the facility and from inside the fuselage in Figure 24.

An initial test was performed to illustrate the effect of rotation rate on obscuration, since that reducing the rotor

rotation rate would be a practical, though somewhat unrealistic, demonstration of wake strength, and hence obscuration, reduction. Results are plotted in Figure 25, quantifying the expected increase in obscuration as rotation rate, and hence tip-vortex strength and mean downwash velocity is increased. Of course, current helicopters have very tight tolerances on rotor rotation rate (~5% of design speed) due to gearbox constraints associated with turboshaft engines, and in fact, once the engine is up to speed, the pilot has no, or little, direct control over engine torque and rotor rotation rate. Consequently, changes in the rotation rate are not practical solutions to the brownout problem of contemporary rotorcraft; however, future concepts, such as Joint Heavy Lift and various high speed concepts, are investigating the possibility of multi-speed gearboxes that could be exploited to alter the brownout characteristics.

Figure 23. Snap-shots of brownout testing (external

view).

Figure 24. Snap-shot of the view from within the fuselage. Prior to the test, the laser can be seen

illuminating the edge of the second target (top left). Prior to reaching the particle bed (top right), partial

obscuration (bottom left) and almost total obscuration (bottom right).

In light of the discussion of the practicality of varying rotor rotation rate in conjunction with the phenomenological observations from above, a test was performed with a simple

spoiler-type tip modification installed on the upper surface of the outer 5% of the rotor blade. While the drag, and hence the torque on the rotor increased noticeably (not a problem for a full-scale aircraft during landing, since the goal is to burn-off power), the tip modification was observed to reduce the brownout obscuration by approximately 10%, see Figure 26. In fact, by comparing the plots in Figure 25 and Figure 26, it is apparent that this small tip modification is slightly more effective at reducing brownout as reducing the rotor rotation rate by 25%.

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

1.2

Target

Nor

mal

ized

Vis

ibili

ty

100% RPM75% RPM50% RPM0% RPM

Figure 25. Visibility measurements for varying rotor

rotation rate. Silicon carbide placed between targets 8 and 10.

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

1.2

Target

Nor

mal

ized

Vis

ibili

ty

Mod1Baseline RotorNo Rotor

Figure 26. Visibility measurements for varying rotor

rotation rate. Silicon carbide placed between targets 8 and 10.

While overly simple, the tip modification tested here, confirms the feasibility of the vortex-dynamics-based brownout mitigation concept described above where altering the rollup and bundling of the tip vortices can be used to reduce brownout obscuration. Given the, albeit preliminary, concept confirmation presented here, ongoing and future work will continue to investigate the feasibility of aerodynamics-based dust/sand abatement solutions to enable visual approach and landing by helicopters in desert environments.

CONCLUDING REMARKS

Rotorcraft brownout, which is caused by the entrainment of dust and ground debris by the rotorwash during take-off and landing, is a critical operational problem, and has caused a significant number of military helicopter accidents in theater. Brownout affects safe operations due to the reduction of visibility, in addition to damaging engine components and rotor blades. Recent experience gained from international inter-service operations in brownout with a variety of rotorcraft configurations indicates that aerodynamic modifications to the aircraft may offer a true solution to the brownout problem, at least in terms of pilot visibility, without degrading performance. However, this observation is at odds with recent analytical and experimental findings, which indicates that rotor system modifications do little to alter brownout, yet conventional rotorcraft operating in brownout demonstrate fundamentally different characteristics.

This paper builds upon prior work in brownout simulation, prediction and experimental test facility development, as well as rotor system design and analysis, to present a vortex-dynamics-based explanation for the apparent obscuration paradox between scaled-brownout testing, numerical predictions and full-scale observations. Based on this explanation, a broad strategy for mitigating brownout, by inhibiting the tip vortex bundling process near to the ground, has been presented, along with several retrofittable concepts. When developing retrofittable concepts, aerodynamic performance degradation is a key consideration for aircraft in combat operations in high/hot/dusty environments, and the concepts presented here do not require the addition of significant onboard equipment or consumables. Finally, preliminary numerical simulations and scale-model proof-of-concept tests have demonstrated the potential of the postulated concepts.

FUTURE WORK

Current work has focused on developing a unique dynamically scaled brownout test facility for investigating the influence of fundamental rotor properties on brownout obscuration as well as concepts for aerodynamically mitigating brownout. To date, only rudimentary proof-of-concept tests have been performed to demonstrate the capabilities of such concepts. Ongoing and future work will continue to investigate the feasibility of aerodynamics-based dust/sand abatement solutions to enable visual approach and landing by helicopters in desert environments through both analysis and testing. Key emphasis will be placed on investigating solutions that fit into the current design and operational paradigms, as well as looking at retrofittable solutions.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the COTR of this effort Robert Pearson at AFRL/RYZC, and his coworkers Walter Harrington, Kenneth McNulty and James Ramage at Wright Patterson Air Force Base, whose input, guidance and support were of great value during this effort. We would like to thank William Warmbrodt of NASA Ames, Thomas Maier, Art Ragosta and Barry Lakinsmith of the U.S. Army at Ames for their ongoing input and support on requirements for full-scale blade modifications and full-scale testing. This research was primarily funded by the U.S. Air Force Research Laboratory at Wright Patterson Air Force Base, OH within the Small Business Innovative Research (SBIR) Program.

REFERENCES

1. Brower, M., "Preventing Brownout." Special Operations Technology, 2004.

2. Anderson, L., P. Doty, M. Griego, K. Timko, B. Hermann, and J. Colombi. "Solutions Analysis for Helicopter Brownout." 9th Annual NDIA Systems Engineering Conference. 2006. San Diego, CA

3. MacIsaac, M., L. Stiles, and J. Judge. "Flight Symbology to Aid in Approach and Landing in Degraded Visual Environment." 61st Annual Forum of the American Helicopter Society 2005. Grapevine, TX.

4. Walls, C.S., S.M. Paris, J.A. Wood, and N. Scoggin. "Brownout Situational Awareness Upgrade (BSAU) Development and Integration on the CH-47D and UH-60L Helicopters." 61st Annual Forum of the American Helicopter Society 2005. Grapevine, TX.

5. Colucci, F., "'Sandblaster' Gives Helicopter Pilots Hope for Safer Landings," National Defense. 2007.

6. Harrington, W.W., S.R. Braddom, J.C. Savage, Z.P. Szoboszlay, R.A. McKinley, and H.N. Burns. "3D-LZ Brownout Landing Solution." 66th Annual Forum of the American Helicopter Society. 2010. Phoenix, AZ.

7. Szoboszlay, Z.P., R.A. McKinley, S.R. Braddom, W.W. Harrington, H.N. Burns, and J.C. Savage. "Landing an H-60 Helicopter in Brownout Conditions Using 3D-LZ Displays." 66th Annual Forum of the American Helicopter Society. 2010. Phoenix, AZ.

8. Slade, K., "Sharp New Blades Improve Sea Kings," Desider. 2008, UK Ministry of Defense, http://www.mod.uk/NR/rdonlyres/C5EA5D7B-D39C-49EE-ADE0-F721525ECA1D/0/desider_04_aug08.pdf.

9. Anon., "The Clear Choice." 2006, Team US101: http://www.teamus101.com/media/ad-the-clear-choice.pdf.

10. Whitehouse, G.R., D.A. Wachspress, T.R. Quackenbush, and J.D. Keller. "Exploring Aerodynamic Methods for Mitigating Brownout." 65th Annual Forum of the American Helicopter Society. 2009. Grapevine, TX.

11. Keller, J.D., G.R. Whitehouse, D.A. Wachspress, M.E. Teske, and T.R. Quackenbush. "A Physics-Based Model of Rotorcraft Brownout for Flight Simulation Applications." 62nd Annual Forum of the American Helicopter Society. 2006. Phoenix, AZ.

12. Wachspress, D.A. "Advanced Rotorcraft Brownout Aerodynamic Analysis and Flight Simulation." 2nd Annual Joint Search and Rescue Conference. 2007. Washington, DC.

13. Wachspress, D.A., G.R. Whitehouse, J.D. Keller, and K. Yu. "A High Fidelity Brownout Model for Flight Simulations and Trainers." 65th Annual Forum of the American Helicopter Society. 2009. Grapevine, TX.

14. Phillips, C., H.W. Kim, and R.E. Brown. "The Effect of Rotor Design on the Fluid Dynamics of Helicopter Brownout." 35th European Rotorcraft Forum. 2009. Hamburg, Germany.

15. Phillips, C., H.W. Kim, and R.E. Brown. "The Flow Physics of Helicopter Brownout " 66th Annual Forum of the American Helicopter Society. 2010. Phoenix, AZ.

16. Milluzzo, J., A. Sydney, J. Rauleder, and J.G. Leishman. "In-Ground-Effect Aerodynamics of Rotors with Different Blade Tips." 66th Annual Forum of the American Helicopter Society. 2010. Phoenix, AZ.

17. Whitehouse, G.R., D.A. Wachspress, and T.R. Quackenbush, "Innovative Lightweight Onboard Aerodynamic Palliatives for Helicopter Brownout Dust Abatement." 2009, CDI Report 09-05, SBIR Phase I final report under FA8650-08-M-1387.

18. Staff, A.H.S., "British Merlin in Combat Operations: Lessons Learned," Vertiflite. 2008, American Helicopter Society.

19. Wachspress, D.A., J.D. Keller, T.R. Quackenbush, G.R. Whitehouse, and K. Yu. "High Fidelity Rotor Aerodynamic Module for Real-Time Rotorcraft Flight Simulation." 64th Annual Forum of the American Helicopter Society. 2008. Montreal, Canada.

20. Quackenbush, T.R., R.M. McKillip, T.B. Curbishley, and A.Z. MacNichol. "LDTRAN/CB: A Model for the Assessment of the Impact of the Chemical/Biological Environment on Military Rotorcraft Operations." American Helicopter Society 53rd Annual Forum. 1997. Virginia Beach, Virginia.

21. Harris, M.J. and A. Lastra, "Real-Time Cloud Rendering." EUROGRAPHICS 2001, 2001. 20(3).

22. D'Andrea, A. "Numerical Analysis of Unsteady Vortical Flows Generated by a Rotorcraft Operating on Ground: A First Assessment of Helicopter Brownout." 65th Annual Forum of the American Helicopter Society. 2009. Grapevine, TX.

23. D'Andrea, A. and F. Scorcelletti. "Enhanced Numerical Simulations of Helicopter Landing Maneuvers in Brownout Conditions " 66th Annual Forum of the American Helicopter Society. 2010. Phoenix, AZ.

24. Tritschler, J.K., M. Syal, R. Celi, and J.G. Leishman. "A Methodology for Rotorcraft Brownout Mitigation Using

Rotor Design Optimization " 66th Annual Forum of the American Helicopter Society. 2010. Phoenix, AZ.

25. Haehnel, R.B., M.A. Moulton, Y. Wenren, and J. Steinhoff. "A Model to Simulate Rotorcraft-Induced Brownout." 64th Annual Forum of the American Helicopter Society. 2008. Montreal, Canada.

26. Rodgers, S.J. "Evaluation of the Dust Cloud Generated by Helicopter Rotor Blade Downwash." 7th Annual Conference on Environmental Effects on Aircraft and Propulsion Systems. 1967. Princeton, NJ.

27. Rodgers, S.J., "Evaluation of the Dust Cloud Generated by Helicopter Rotor Downwash." 1968, USAAVLABS Technical Report 67-81.

28. Brown, R.E. and G.R. Whitehouse, "Modelling Rotor Wakes in Ground Effect." Journal of the American Helicopter Society, 2004. Vol. 49(No. 3): p. 238-249.

29. Curtiss, H.C., Jr., W. Erdman, and M. Sun, "Ground Effect Aerodynamics." Vertica, 1987. Vol. 11(No. 1): p. 29-42.

30. Lee, T.E., J.G. Leishman, and M. Ramasamy. "Fluid Dynamics of Interacting Blade Tip Vortices With a Ground Plane." 64th Annual Forum of the American Helicopter Society. 2008. Montreal, Canada.

31. Quackenbush, T.R., R.M. McKillip Jr., and G.R. Whitehouse. "Development and Testing of Deployable Vortex Generators Using SMA Actuation " 28th AIAA Applied Aerodynamics Conference. 2010. Chicago, IL: AIAA-2010-4686

32. Leishman, J.G., Principles of Helicopter Aerodynamics. 2nd ed. 2006, Cambridge, UK: Cambridge University Press.

33. Curtiss, H.C., Jr., F. Carson, and J. Hill. "Performance of a Sikorsky S-61 with a New Main Rotor." 28th European Rotorcraft Forum. 2002. Bristol, UK.

34. Curtiss, H.C., Jr., F. Carson, J. Hill, and T.R. Quackenbush, "Performance of a Sikorsky S-61 with a New Main Rotor." Journal of the American Helicopter Society, 2003. Vol. 48(No. 3): p. 211-215.

35. Wachspress, D.A., T.R. Quackenbush, and A.H. Boschitsch, "Rotorcraft Interactional Aerodynamics with Fast Vortex/Fast Panel Methods." Journal of the American Helicopter Society, 2003. Vol. 48(No. 4).

36. Quackenbush, T.R., M.E. Teske, and A.J. Bilanin. "Dynamics of Exhaust Plume Entrainment in Aircraft Vortex Wakes." 34th Aerospace Sciences Meeting & Exhibit. 1996. Reno, NV: AIAA-1996-0747.

37. McKillip Jr., R.M. "“Digital” SMA-Based Tracking Tabs for One-Per-Rev Vibration Reduction." 59th Annual Forum of the American Helicopter Society. 2003. Phoenix, AZ.

38. Nathan, N.D. and R.B. Green. "Measurements of a Rotor Flow in Ground Effect and Visualisation of the Brown-out Phenomenon." 64th Annual Forum of the American Helicopter Society. 2008. Montreal, Canada.

39. Quackenbush, T.R., D.A. Wachspress, A.H. Boschitsch, and C.L. Solomon. "Analytical and Experimental Micro

Rotorcraft Design Studies." AHS International Specialists Meeting on Unmanned Rotorcraft. 2005. Chandler, AZ.

40. Bagnold, R.A., The Physics of Blown Sand and Desert Dunes. 1965: Methuen & Co. Ltd.

41. Baas, A.C.W., "Complex Systems in Aeolian Geomorphology." Geomorphology, 2007. Vol.91(No. 3-4): p. pp. 311-331.

42. Hirschler, M.M., "How to Measure Smoke Obscuration in a Manner Relevant to Fire Hazard Assessment: Use of Heat Release Calorimetry Test Equipment." Journal of Fire Sciences, 1991. Vol. 9.

43. Willeke, K. and P.A. Baron, eds. Aerosol Measurement: Principles Techniques and Applications. 1993, Van Nostrand Reinhold: New York, NY.

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