dnvgl-rp-0171 testing of rotor blade erosion protection systems · 2018. 2. 21. · recommended...

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RECOMMENDED PRACTICE

DNVGL-RP-0171 Edition February 2018

Testing of rotor blade erosion protectionsystems

FOREWORD

DNV GL recommended practices contain sound engineering practice and guidance.

© DNV GL AS February 2018

Any comments may be sent by e-mail to [email protected]

This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of thisdocument. The use of this document by others than DNV GL is at the user's sole risk. DNV GL does not accept any liability or responsibilityfor loss or damages resulting from any use of this document.

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Recommended practice — DNVGL-RP-0171. Edition February 2018 Testing of rotor blade erosion protection systems

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CONTENTS

Changes – current.................................................................................................. 3

Section 1 General.................................................................................................. 71.1 Introduction.................................................................................... 71.2 Objective...........................................................................................71.3 Scope................................................................................................ 71.4 Application........................................................................................71.5 References........................................................................................ 81.6 Definitions and abbreviations........................................................... 9

Section 2 Test procedure.....................................................................................132.1 Test procedure.............................................................................. 13

Section 3 Rotating arm test rig........................................................................... 143.1 Outline............................................................................................ 143.2 Rotating carrier arm....................................................................... 153.3 Number of carrier arms.................................................................. 153.4 Radial position of specimen............................................................153.5 Distance from origin of droplet to centre of specimen in rotorplane.....................................................................................................163.6 Angle of incidence.......................................................................... 163.7 Distance of test specimen to side wall........................................... 17

Section 4 Specimens............................................................................................184.1 Geometry........................................................................................ 184.2 Material...........................................................................................184.3 Specimen preparation.....................................................................184.4 Accelerated ageing......................................................................... 194.5 Tapes as erosion protection............................................................20

Section 5 Test parameters...................................................................................215.1 Test condition parameters.............................................................. 215.2 Derived test parameters................................................................. 22

Section 6 Calibration............................................................................................. 236.1 General........................................................................................... 236.2 Calibration intervals........................................................................236.3 Calibration specimens.....................................................................236.4 Evaluation of calibration results..................................................... 24

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Section 7 Inspection parameters.......................................................................... 257.1 Overview of inspection parameters................................................ 257.2 Inspection interval and time...........................................................257.3 Cleaning method.............................................................................257.4 Inspection method..........................................................................25

Section 8 Result parameters................................................................................. 268.1 Overview of result parameters....................................................... 268.2 Mass loss........................................................................................ 268.3 Failure modes................................................................................. 268.4 Stages of erosion progress............................................................. 268.5 End of incubation period.................................................................268.6 Breakthrough.................................................................................. 28

Section 9 Displaying results................................................................................ 309.1 Displaying results..........................................................................30

Section 10 Test report.......................................................................................... 3210.1 Test report....................................................................................32

Section 11 Summary............................................................................................3611.1 Summary..................................................................................... 36

Appendix A Specimen geometry.......................................................................... 37A.1 Specimen geometry...................................................................... 37

Appendix B Derived test parameters.................................................................... 38B.1 Droplet velocity.............................................................................. 38B.2 Rain intensity................................................................................. 39B.3 Droplet impact velocity...................................................................39B.4 Specific impact frequency...............................................................39

Appendix C Results from round robin tests.......................................................... 43C.1 Parameter overview........................................................................43C.2 Reference curve for calibration specimens..................................... 47C.3 Test results on coating systems..................................................... 53

Appendix D Influences to be considered...............................................................61D.1 Overview........................................................................................ 61D.2 Shadowing effect............................................................................61

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SECTION 1 GENERAL

1.1 IntroductionThis recommended practice (RP) provides technical recommendations to support the execution of rain erosiontests on rotating arm test rigs. The intention of the RP is to reach a position where results from differentrotating arm test rigs are comparable.

1.2 ObjectiveThe objective of this recommended practice is to:

— specify a detailed test procedure for rain erosion tests (RET) performed with a rotating arm test rig toensure comparable results when using different test rigs

— specify the main influencing parameters for the assessment of the rain erosion. These include:

— mechanical properties of the tested system (protection system + laminate)— substrate preparation— method of application for leading edge protection system— curing conditions of the coating— testing temperature— accumulated number of droplet impacts to reach a pre-defined erosion stage— test rig parameters, e.g.:

— droplet size— droplet distribution— impact speed.

— specify the geometry and material of a calibration specimen

— provide guidance for defining a calibration reference band

— provide guidance for the testing of coating systems and tapes and how to document the results

— provide anonymised test results of the round robin tests.

1.3 ScopeThe rain erosion performance of the test specimens is dependent on many parameters which are notdirectly connected to the erosion protection system itself such as, the substrate below the protection system(laminate and filler) and the test rig parameters. This recommended practice provides guidance as to whichparameters will influence the test results, and therefore shall be monitored and controlled during erosiontesting, to ensure comparable results when using different test rigs. As far as applicable, the parametersare set to represent the environmental conditions that a leading edge of a rotor blade on a wind turbine isexposed to.In addition, guidance is provided for the selection of a calibration specimen.The results from a round robin test on calibration specimens and on three coated specimens are anonymisedand provided in App.C. The designs of all three test rigs used for these tests were very similar.An evaluation of the erosion test results with regards to the erosion performance, lifetime, outliers or therequired number of specimens, is not within the scope of this RP.This RP was developed as an extension to the requirements specified in the ASTM G73-10 standard.

1.4 ApplicationIn the following paragraphs the application of this RP compared to other publications connected to rainerosion at DNV GL is clarified.


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1.4.1 Testing of rotor blade erosion protection systemsThis RP supports the execution of rain erosion tests on rotating arm test rigs. It specifies the boundaryconditions to ensure comparable test results on different test rigs. In addition to that, guidance forrepresentative test parameters is provided.This RP does not specify requirements, such as minimum survival times, for the certification of an erosionprotection system.The objectives of this RP are especially important for blade manufacturers who aim to improve theperformance of their erosion protection system based on different test campaigns. Also for comparisons oftest results with the erosion performance on the turbines, it is essential to have a basis of reliable and wellaligned test results.

1.4.2 Coatings for protection of fibre reinforced plastic structures withheavy rain erosion loadsClass programme DNVGL-CP-0424 defines a test matrix to acquire a type approval for a coating system.The class programme specifies a minimum quality level and in this way helps filter out unsuitable andlow performing materials when considering loads and ageing effects such as temperatures and climaticinfluences.The objective of the class programme and the certification of materials, is to ensure that the coating systemwill be produced with a constant quality and ensures that changes in formulation and properties are correctlydocumented. The class programme DNVGL-CP-0424 is not limited to erosion tests, but also covers tensileand gloss tests at different temperatures, with and without UV exposure.It must be emphasized that material qualifications do not consider the materials survivability underoperational loads for the turbine life.

1.4.3 Rotor blades for wind turbinesRotor blade component certification is based on DNVGL-ST-0376 in combination with DNVGL-SE-0441.When considering an erosion protection system within the context of a blade certification, the followingadditional considerations shall be made:

— it shall be shown that the specimens are representative for the specific blade production considering thefollowing items:

— leading edge lay-up— materials— substrate production method— application method and quality of leading edge protection system.

— appropriate maintenance intervals and maintenance measures shall be defined.

1.5 ReferencesTable 1-1 Normative DNV GL documents

Document code Title

DNVGL-CP-0424 Coatings for protection of FRP structures with heavy rain erosion loads

DNVGL-SE-0441 Type and component certification of wind turbines

DNVGL-ST-0376 Rotor blades for wind turbines


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Table 1-2 Normative external documents

Document code Title

ASTM G73-10 Standard Test Method for Liquid Impingement Erosion Using Rotating Apparatus

ISO 2808 Paints and varnishes - Determination of film thickness

ISO 4618:2014 Paints and varnishes - Terms and definition

ISO 6507-1:2005 Metallic materials - Vickers hardness test - Part 1: Test method

ISO/IEC 17025 General requirements for the competence of testing and calibration laboratories

1.6 Definitions and abbreviationsFor the purposes of this document, the terms and definitions given in ISO 4618 and the following apply.

1.6.1 Definition of verbal formsTable 1-3 Definition of verbal forms

Term Definition

shall verbal form used to indicate requirements strictly to be followed in order to conform with thedocument

shouldverbal form used to indicate that among several possibilities one is recommended asparticularly suitable, without mentioning or excluding others, or that a certain course ofaction is preferred but not necessarily required

may verbal form used to indicate a course of action permissible within the limits of the document

1.6.2 Definition of termsTable 1-4 Definition of terms

Term Definition

angle of incidence impact angle of the rain drop on the specimen surface

breakthrough point in time when the erosion progress breaks through the protective layer tothe underlying substrate

droplet concentration number of droplets per cubic meter

end of incubation period exposure time until the first mass loss or damage is visually detectable

exposure zone the area the rain is distributed on

failure mode e.g. cracking, peeling, abrasion

gauge zone length the area on the specimen where the erosion performance will be evaluated

number of specific impacts number of impacts per projected unit area perpendicular to the impact velocity

rain intensity height of raining water accumulated per unit of time


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Term Definition

specific impact frequency number of specific impacts per unit of time

stage of erosion progress reference point in time: end of incubation period or breakthrough

terminal velocity highest droplet falling velocity due to air resistance

water volume concentration cumulated volume of water when considering all droplets contained in a unitvolume of space

1.6.3 Definitions of symbols and equationsTable 1-5 Symbols

Symbol Unit Definition

A [m2] area covered with rain

b [m] distance of test specimen to side wall

COV [-] coefficient of variation

d [mm] mean diameter of a droplet

g [m/s²] gravitational acceleration

I [m/s] rain intensity

k [-] constant value for power law equation

K [1/s] constant

l [m] length

L [m] length

m [-] exponent for power law equation

[#Impacts/m2] specific number of impacts

[#Impacts/m2] specific number of impacts N following the best fitreference line for the data points of sample j

[#Impacts/(s·m2)]specific number of impacts per unit timespecific impact frequency

P [m3 /s] water volumetric flow rate

q [#Droplets/m3] droplet concentration

r [m] radius

Ra [ μm] average surface roughness

s [#Impacts/m2]alternatively [m/s]

standard deviation for specific number of impacts N(alternatively standard deviation for vs)

t [s] exposure time

v [m/s] velocity


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[m3] volume of a droplet

x [m] distance from origin of droplet to centre of specimen inrotor plane

[#Impacts/m2]

alternatively [m/s]mean value for specific number of impacts N(alternatively mean value for vs)

[°] angle of incidence

[°] half angle between rows of rain dispensers

[rad] coverage angle

[-] ratio of coverage angle and 2

[-] water volume concentration

[rad/s] angular velocity

centre point of specimen

outer point of specimen

inner point of specimen

index for number of test sample

gauge zone

rotor plane

(impact velocity of) the sample with the drops

(impact velocity of) the sample with the drops at thecentre position of specimen

reference (impact velocity of) the sample with the drops

maximum specimen (impact velocity of) droplets at theouter position of specimen

minimum specimen (impact velocity of) droplets at theinner position of specimen

droplet falling (velocity)

terminal droplet falling (velocity)

droplet falling (velocity) when reaching the rotor planewhere impacts with the specimen occur

(distance of) influence


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specimen distance

1.6.4 AbbreviationsTable 1-6 Abbreviations

Abbreviation Description

FRP fibre reinforced plastics

RP recommended practice

RET rain erosion test


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SECTION 2 TEST PROCEDURE

2.1 Test procedureTesting laboratories should comply with the requirements of ISO 17025.The erosion damage is reproduced on specimens mounted on an arm which rotates horizontally, throughan artificial rain field. The rain impacts the surface of the test specimen and erodes the surface, which isprotected with the coating or tape, to be tested.The degree of erosion damage caused by the droplet impacts shall be inspected and documented. Thisshall be performed by visual inspection and picture documentation at defined intervals. Detailed picturedocumentation enables the investigation of the initial damage at the end of the incubation period, as well asthe damage progress.The time needed to erode the surface to a specified limit, is the measure which is used to compare theperformance of the protections systems with each other. There are two erosion stages which are commonlyused to specify the survival time of the specimens:

1) end of incubation period2) breakthrough to the underlying substrate.

It is essential to monitor and control all parameters which influence the test result. The test apparatus,test procedures and the substrates are not fully standardized, the parameters listed in Sec.10 shall as aminimum, be controlled and monitored.The relationship between accelerated erosion tests to real-life erosion is part of current research and, cannotyet be quantified. It is currently state of art to use accelerated erosion tests with high impact speeds toassess the performance of rain erosion protection systems.


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SECTION 3 ROTATING ARM TEST RIG

3.1 OutlineAn outline of the rotating arm test rig is shown in Figure 3-1. An artificial rain field may be generated overthe entire swept area of the specimen, or a part of it.

Figure 3-1 Rotating arm test rig

The test parameters relating to the rig design are shown in Table 3-1.

Table 3-1 Test rig parameters

Test parameter Unit Nominal condition

rotating carrier arm [-] aerofoil shaped with an integrated specimen


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number of specimen carrier arms [-] max. 3

radial position for the centre of the specimen, rc [m] min. 1.0

vertical distance from origin of droplet (needle) tocentre of specimen in rotor plane, x [m] min. 0.2

angle of incidence, α [°] 90

distance of test specimen to side wall, b [m] to be documented

3.2 Rotating carrier armThe aerofoil contour of the carrier arm reduces the influence of the support structure on the test result. Theinfluence of any uneven air flow within the test chamber, on the test results is not fully established, thereforethis influence is one of the main design drivers for the test rig. As an aerofoil contour for the carrier arm, thespecimen geometry may be used. The specimen geometry is specified in App.A.

3.3 Number of carrier armsA maximum of three carrier arms should be used to avoid an influence of the turbulence of one specimen onthe preceding to avoid any shadowing effect (see [D.2]).

3.4 Radial position of specimenThe radial distance from the rotor centre to the centre of the test specimen shall be at least 1.0 m in order toreduce the aerodynamic influence of the support structure (considering a constant rotational speed).The influence of the centrifugal forces and the resulting longitudinal stresses on the test results is unknown.Thus, a minimum radius of 1.0 m shall be specified to limit the centrifugal forces on the specimen comparedto a set impact velocity.


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3.5 Distance from origin of droplet to centre of specimen in rotorplane

Figure 3-2 Distance from origin of droplet (needle) to centre of specimen in rotor plane

The falling distance, x, from the needle to the specimen centre plane should be at least 200 mm as shownin Figure 3-2. One reason for specifying a falling distance above 200mm is the decreasing risk of influencesfrom shadowing effect when the droplet falling speed is increased (see [D.2]).

3.6 Angle of incidenceThe angle of incidence α is defined as shown in Figure 3-3:

Figure 3-3 Angle of incidence


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3.7 Distance of test specimen to side wallThe minimum distance between the test specimen and the side wall shall be determined based on theindividual test rig and rain field. An influence of the side wall onto the test result shall be avoided.


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SECTION 4 SPECIMENS

4.1 GeometryThe relevant geometry parameters for the specimens are listed in Table 4-1:

Table 4-1 Parameters related to the specimen geometry

Specimen geometry parameters Unit Nominal condition

cross-sectional shape of specimen [-] U-shaped and integrated in the aerofoil design of thecarrier arm. Leading edge curvature shall be measured

exposure zone [m] length of exposure zone shall be larger than gaugezone

gauge zone length of specimen lgz [m] min. 0.2 m

4.1.1 Cross-sectional shapeA U-shaped cross section is considered most representative for rotor blade leading edges. A standardspecimen geometry is described in App.A.

4.1.2 Exposure zoneThe exposure zone is the area the rain is distributed on. It may be smaller or larger than the specimenlength. To avoid edge effects, the exposure zone shall be larger than the gauge zone. An illustration of theexposure zone is shown in App.A.

4.1.3 Gauge zone lengthThe gauge zone is the area on the specimen where the erosion performance is evaluated. To avoid edgeeffects, the gauge zone shall be smaller than the exposure zone. In App.A a sketch of the gauge zone isshown.

4.2 MaterialThe specimens typically consist of two main components, the substrate and the protection system which shallbe tested.If the test is referencing a particular blade or blade family, the test specimen substrate should be built withthe same materials as the leading edge in the blade production. The same is valid for the protection system,e.g. coating or tape. Any deviation from the rotor blade production shall be documented and evaluated.

4.3 Specimen preparationProduction methods, manufacturing tolerances and materials have a large influence on the test results.The test specimens should be built with the same production methods as the leading edges in the bladeproduction. Any deviation from the rotor blade production process shall be documented and the influence ofthe deviations on the test results shall be evaluated.The following parameters shall be documented:


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Table 4-2 Specimen parameters

Specimen parameter Unit Nominal condition

identification number of specimen [-] to be documented

materials (fibres, resins, filler, coatingetc.) and material suppliers [-] as in blade production, to be documented

lay-up [-] as in blade production, to be documented

surface preparation [-] as in blade production, to be documented

curing cycles, temperatures andduration for substrate and coating [-] as in blade production, to be documented

all layer thicknesses (filler, primer,coating etc.) [μm] minimum values of blade production, to be documented

coating / tape application method [-] as in blade production, to be documented

coating application quality [-] as in blade production, to be documented

4.3.1 Production method and coating/tape applicationThe influences of the blade production method and manufacturing tolerances on the blade leading edge areashould be considered during testing. For these considerations, all materials and layers at the leading edgeshall be considered.For generic specimens, appropriate assumptions for the items listed above shall be made.

4.3.2 Layer thicknessesAll materials at the leading edge shall be applied with the minimum thicknesses compared to the real bladeproduction. The thicknesses of all layers shall be specified and measured.The thickness of the dried leading edge protection coating shall be measured in micrometres by one of theprocedures specified in ISO 2808.

4.4 Accelerated ageingThe reference baseline testing should be performed on virgin specimens.If climate influences are part of the test campaign, accelerated ageing of the test specimens shall be carriedout in the same manner, to the highest possible extent, as the conditions the blades are subjected to.The following climatic parameters should be considered:

Table 4-3 Parameters for accelerated ageing

Parameter for accelerated ageing Nominal condition

extreme temperatures to be documented

UV exposure to be documented

humidity to be documented

salt spray to be documented


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To reduce the list of climatic influences for the tests, it shall be shown that the neglected climatic parameterhas no influence on the test result, or that the parameter is not relevant for the application purpose.It must be emphasized that there is currently no approach available to reliably relate accelerated ageing oftest specimens to wind turbine site conditions.

4.5 Tapes as erosion protectionThe transition area of the tape edges with the blade surface shall be investigated during testing. In additionto that, tape edges, start and end positions, overlaps, as well as transitions between two tapes shall besubject to erosion testing.It shall be ensured that the differences in failure modes are appropriately covered. Tape peeling is a criticalfailure mode.Since the failure modes for coatings and tapes may be different, any comparison of test results for coatingsand tapes shall be performed very carefully.


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SECTION 5 TEST PARAMETERS

5.1 Test condition parameters

5.1.1 Overview of test condition parameterThe test condition parameters that shall be specified and or monitored during test are listed in Table 5-1.

Table 5-1 Test condition parameters


duration of test [min] to be specified

normal impact velocity at centre of specimen,vs,c

[m/s] to be calculated

water temperature [°C] to be monitored

water quality [μS/cm] to be documented

test specimen temperature [°C]to be monitored during test

or alternatively during inspection

test chamber temperature [°C] to be monitored during test

test chamber pressure [Pa] to be documented if room pressure or vacuumis present

mean droplet size, diameter, d [mm] ~2.0

droplet size standard deviation [mm] to be monitored prior to test

5.1.2 Duration of testThe test duration shall be defined depending on the individual incubation period and breakthrough time of theprotection system. Since this is not known at the onset of testing new protection systems, careful monitoringof the first samples is needed to establish the test durations for subsequent test samples to establish abaseline. The test is completed when the required level of information on the erosion progress is reached.

5.1.3 Water temperatureThe influence of water temperature on erosion is not clearly understood. As a result, it is important tomeasure the water temperature as close to the needle as possible for each test performed. A possible effectof water temperature should then be evaluated during post processing of the results.

5.1.4 Water qualityThe selected water quality shall be documented by measuring the conductivity or composition. Deionizedwater, de-mineralized water, tap water, chloride-containing water or artificial sea water may be selected.


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5.1.5 Test chamber pressureThe chamber pressure should be monitored. The erosion test should be performed at normal atmosphericpressure.

5.1.6 Mean droplet sizeThe droplet size has a direct impact on the erosion damage. Therefore, test results should only be comparedfor similar sized droplets.The mean droplet size and standard deviation shall be determined and reported with a reasonable accuracy.Furthermore, the droplet size distribution should be determined to give a better understanding of the impacton the specimen.Droplet sizes are dependent on many parameters and have a large influence on the erosion behaviour. Thedroplet size shall be regularly measured using a laser disdrometer or appropriate methods.

5.2 Derived test parameters

5.2.1 Overview of derived test parametersThe following test parameters shall be derived from the test condition parameters specified in [5.1].

Table 5-2 Derived test parameters

Derived test parameter Unit Nominal condition

rain intensity, I [m/s] to be measured or computed from rig design(which needs to be defined)

max impact velocity, vs,max [m/s] to be computed

min impact velocity, vs,min [m/s] to be computed

droplet velocity when entering rotor plane,vdrop,rp

[m/s] to be computed

specific impact frequency per unit time atcentre of gauge zone, Ṅc (based on meandrop diameter, d)

[Impacts /(m2*s)] to be computed

The impact frequency should be selected in a way that the sample surface is able to recover after eachimpact, and no water film is generated on the sample surface. It is believed that, on an operating windturbine, the impact frequency is not high enough for any point of the protection system to simultaneouslyexperience stresses resulting from separate impacts.Further details on the calculation of these parameters are provided in App.B.


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SECTION 6 CALIBRATION

6.1 GeneralTo ensure the accuracy of the test equipment and to quantify the variation between different tests or testrigs a standardised calibration scheme is required. Further information on calibration test results and possiblereference bands are provided in [C.2].

6.2 Calibration intervalsA calibration is mandatory when a new test rig is set up. In addition to that, calibrations shall be performedas a minimum every second month and after any change of the test parameters.

Table 6-1 Parameters calibration intervals

Parameter Unit Nominal condition

date and time of calibration [YYYYMMDD] to be documented

Any modification of test parameters during a test campaign shall be explicitly listed.

6.3 Calibration specimens

6.3.1 GeometryFor the calibration specimens, the geometry, which is specified in App.A, may be applied.

6.3.2 Aluminium calibration specimensFor calibration specimens, the material defined in Table 6-2 and Table 6-3 may be used.

Table 6-2 Parameters for aluminium calibration material


specimen composition [-] EN-AW-3003, aluminium alloy

temper code [-] H112

average hardness

ISO 6507-1:2005[HV 2] 33

density [kg/m3] ~2700

Young’s modulus [GPa] ~70

Table 6-3 Parameters related to manufacture and preparation of aluminium calibration specimen


manufacturing process [-] extruded from blocks and polished

annealing [-] none


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surface roughness, Ra [μm] <1

6.4 Evaluation of calibration resultsThe evaluation of the calibration results is based on the end of the incubation period which is determinedfrom visual inspections. Alternatively, the calibration with aluminium specimens may be based on mass loss.

6.4.1 RepeatabilityFrom the results of the round robin tests, it is assumed that the repeatability of the calibration tests usingaluminium specimens, as specified in [6.3], should lead to a coefficient of variation COV of less than 20%.

with

s =standard deviation for specific number of impacts N

=mean value for specific number of impacts N

vs,ref = reference impact speed of droplet with the sample

Nfit,j = specific number of impacts N following the best fit reference line for the data points of sample “j”

This limit for the coefficient of variation should be used for calibration tests on one test rig. As a minimum,one set of calibration specimens, in this case 3 specimens, shall be used for the regular standard calibrations.

6.4.2 ReproducibilityFor the reproducibility of the calibration test results between different test rigs, it shall be shown thatthe results are similar. The test results of calibration testing should be compared with a reference curve,preferably in a similar way as shown in Figure C-3 through Figure C-6.The reproducibility of the calibration test results may be shown using the calibration reference curvedescribed in [C.2].It is recommended that a comparison between aluminium reference specimens test results and the referencecurve is included in each test report.


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SECTION 7 INSPECTION PARAMETERSCurrently, no unambiguous evaluation method of the rain erosion test results exists. The specimens arecommonly inspected visually and subsequently compared to other test results in terms of damage severityversus test execution time.

7.1 Overview of inspection parametersThe relevant inspection parameters are listed in Table 7-1:

Table 7-1 Parameters related to inspections

Inspection parameter Unit Nominal condition

inspection interval [min] to be specified

cleaning method before inspection [-] to be specified

time of inspection in relation to exposuretime [min] to be documented

picture at every inspection [-] high resolution pictures including a scale tobe documented

7.2 Inspection interval and timeBefore the erosion test is started, an initial visual inspection of the test specimens shall be performed anddocumented with pictures. As additional information, the specimen mass may be recorded.The inspection interval is individually determined. It is recommended to adjust the inspection intervals andthe test time to cover both stages of erosion progress, the end of the incubation period and breakthrough,with sufficient accuracy.The time of inspection in relation to the execution time shall be recorded for every inspection. The inspectioninterval has an influence on the accuracy of the test result. For calibration purposes, the inspection intervalshould therefore be kept constant for each run.Loss of gloss is not used to evaluate the performance of the erosion protection system.

7.3 Cleaning methodThe cleaning method, which is used before each inspection, shall be specified.The cleaning is mainly used for drying the specimens to avoid an influence of the water on the result of thevisual inspection or mass measurement.

7.4 Inspection methodThe test specimens shall be inspected visually. The results shall be documented with high resolution pictures,including a reference scale, for every inspection. It shall be ensured that the quality of the pictures is goodenough to derive the end of the incubation period. More elaborate inspection methods using microscopesmay be applied.Each location on the specimen shall be correlated with an impact speed. Depending on the rig configuration,the impact frequency might change along the length of the sample.


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SECTION 8 RESULT PARAMETERS

8.1 Overview of result parametersThe relevant result parameters are listed in Table 8-1:

Table 8-1 Result parameters

Result parameter Unit Nominal condition

mass loss [g] optional

failure modes [-] optional

stages of erosion progress [-] reference point in time: end of incubation period andbreakthrough

end of incubation period [min] document time of initial surface damage for eachlocation

breakthrough [min] document time of breakthrough for each location

8.2 Mass lossThe mass loss of the calibration specimens may be measured and monitored at the inspections. The massloss is used to monitor the erosion damage development on the complete specimen and independent of theeroded layers.

8.3 Failure modesThe different failure modes of the leading edge protection systems, such as cracking, peeling, and abrasion,may be documented as additional information. This information might be important to evaluate the reasonsfor varying performance levels of the leading edge protection systems. It shall be ensured that the failuremodes, which are triggered during testing, are comparable to the failure modes seen on the turbines.

8.4 Stages of erosion progressThe stages of erosion progress are reference points in time which may be used to assess the remainingprotective efficacy of the erosion protection system and to compare the performance of different systemswith each other. In context of this recommended practice, the end of the incubation period and breakthroughare specified as they are the most commonly used stages of erosion progress.

8.5 End of incubation periodThe incubation period is defined as the exposure time until the first damage is visually detectable on theouter surface of the test specimen. The incubation period depends on the impact speed and thus, for rotatingarm test rigs, on the position on the specimen.An illustration of an initial surface damage for a protected specimen is shown in Figure 8-1:


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Figure 8-1 Illustration of initial surface damage at the end of the incubation period

The first visual surface damage can be caused by different failure modes (e.g. cracking, peeling, abrasion).In some cases, the initial damage is not located on the outer surface, but in the underlying layers, and isthus not visible until a piece of the material is suddenly removed. Methods to measure damage below anundamaged surface are not yet common in rain erosion testing.Loss of gloss is not considered to be damage.For determining the end of the incubation period on rotating arm test rigs, as described in [3.1], measuringmass loss is not an appropriate parameter, since it is providing information about the status of the completespecimen, independent of the rotational speed and the affected layer.Thus, for visualization purposes only, Figure 8-2 uses mass loss to describe the meaning of incubation period.

Figure 8-2 Visualization of the incubation period based on mass loss for one specific section, e.g.section A-A of Figure 8-1


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Generally, it shall be clearly specified how the end of the incubation period is defined, how it is detected andwhich approximate resolution the detection method has. Especially for tapes, a detailed definition of the endof the incubation period and the differentiation to breakthrough (see [8.6]) is important.

8.6 BreakthroughBreakthrough is defined as the point in time when the erosion breaks through the protective layer to theunderlying substrate. It shall be clearly defined which layers belong to the substrate (e.g. laminate, topcoatetc.). The time of breakthrough depends on the impact velocity and thus, for rotating arm test rigs (asdescribed in [3.1]), it also depends on the location on the specimen.The end of the incubation period and breakthrough may be equal in some cases. This might apply when theinitial damage is caused on the underlying layers and develops without visible damage on the surface, until apiece of protection layer is suddenly removed.An illustration of the breakthrough erosion stage is shown in Figure 8-3.

Figure 8-3 Illustration of breakthrough erosion stage in B-B compared to the initial surfacedamage at the end of the incubation period in A-A

As described in [8.5], loss of mass is not an appropriate parameter to define erosion progress stages for thechosen test rig configuration (see [3.1]). However, for visualization purposes only, Figure 8-4 uses mass lossto provide further information on the breakthrough erosion stage.


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Figure 8-4 Visualization of breakthrough based on mass loss for homogeneous specimen material

Looking at the theoretical graph of mass loss vs number of droplet impacts, the point of breakthrough willbe shown as a change of slope, since the underlying substrate has different erosion properties than theprotective layer.Breakthrough times should be determined conservatively, by using the time-step before breakthrough isdetected on the pictures.


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SECTION 9 DISPLAYING RESULTS

9.1 Displaying resultsAn incubation curve may be expressed in terms of the recorded ends of incubation periods at different impactvelocities and specific numbers of impacts, see Figure 9-1.

Figure 9-1 Result data for end incubation period shown as droplet impact velocity vs specificnumber of droplet impacts (axes on a logarithmic scale)

The vs versus N diagrams, as shown in Figure 9-1 may be developed assuming that the data cloud isdescribed by a power law:

As often used for traditional fatigue S/N curves, the equation may be specified with N as the dependentparameter:

As a next step, the data is transformed into a log-log scale and the parameters k and m are determinedusing a least square fit:

If the end of the incubation period is plotted for droplet impact velocity versus specific number of impacts,the diagram resembles traditional fatigue S/N curves where the induced stresses are displayed versus thenumber of cycles.Since, for the test rig configuration specified in [3.1], the impact speed increases with the radial positionon the specimen, one test specimen provides information for several impact velocities. However, theestablishment of an incubation curve requires visual detection of initial damages at the individual specimencross-sections.


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Breakthrough data may be expressed in the same way, as shown in Figure 9-2:

Figure 9-2 Breakthrough data shown as droplet impact velocity versus specific number of dropletimpacts (axes on a logarithmic scale)


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SECTION 10 TEST REPORT

10.1 Test reportThe test report should comply with ASTM G73-10. Further a test parameter overview, as shown in Table10-1, shall be summarized by collecting the parameters specified in Table 3-1 to Table 8-1 in this document.

Table 10-1 Summary of parameters to be documented in the test report.

Test parameter Unit Nominal condition Deviations fromnominal condition

specimen carrier arm [-] aerofoil shaped with anintegrated specimen

number of specimencarrier arms [-] max. 3

radius position of centreof specimen attachment,rc

[m] min. 1.0

distance from originof droplet to centre ofspecimen in rotor plane,x

[m] min. 0.2

Test

rig

angle of incidence [°] 90

cross-sectional shape ofspecimen [-]

U-shaped and integratedin the aerofoil design ofthe carrier arm. Leadingedge curvature to bemeasured.

gauge zone length ofspecimen, lgz (zonewhere erosion isevaluated)

[m] min. 0.2

Spe

cim

en g

eom

etry

exposure zone [m] larger than gauge zone


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identification number ofspecimen [-] to be documented

materials (fibres, resins,filler, coating etc.) andmaterial suppliers

[-]as in blade production

to be documented

layup [-]as in blade production

to be documented

surface preparation [-]as in blade production

to be documented

curing cycles,temperatures andduration for baselaminate and coating

[-]as in blade production

to be documented

all layer thicknesses(filler, primer, coatingetc.)

[μm]minimum values fromblade production

to be documented

coating/tape applicationmethod [-]

as in blade production

to be documented

Spe

cim

en p

repa

ratio

n

coating application quality [-]as in blade production

to be documented

extreme temperatures [-] to be documented

UV exposure [-] to be documented

humidity [-] to be documented

Acc

eler

ated

age

ing

salt spray [-] to be documented

duration of test [min] to be specified

normal impact velocity atcentre of specimen, vs,c

[m/s] to be calculated

water temperature [°C] to be monitored

water quality [μS/cm] to be documented

test specimentemperature [°C] to be monitored during

test

Test

con

ditio

ns

test chambertemperature [°C] to be monitored during

test


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test chamber pressure [Pa] to be monitored duringtest

mean droplet size,diameter [mm] ~2.0

droplet size standarddeviation [mm] to be monitored prior to

test

rain intensity in exposurezone (exposure zoneneeds to be defined)

[m/s]

to be measured orcomputed from rig design(which needs to bedefined)

max impact velocity,vs,max


min impact velocity, vs,min [m/s] to be computed

droplet velocity whenentering rotor plane,vdrop,rp


Der

ived

tes

t pa

ram

eter

s

specific impact frequencyper unit time in exposurezone, Ṅc (based on meandrop diameter)

[Impacts /(m2*s)] to be estimated

Cal

ibra

tion

date and time ofcalibration [YYYYMMDD] to be documented

specimen composition [-] EN-AW-3003

temper code [-] H112

average hardness,

ISO 6507-1:2005[HV 2] 33

density [kg/m3] ~2700

Cal

ibra

tion

mat

eria

l

Young’s modulus [GPa] ~70


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manufacturing process [-] extruded from blocks andpolished

annealing [-] none

Cal

ibra

tion

proc

ess

surface roughness, Ra [µm] <1

inspection interval [min] to be specified

cleaning method beforeinspection [-] to be specified

time of inspection inrelation to exposure time [min] to be documented

Insp

ectio

ns

picture at everyinspection [-]

high resolution picturesincluding a scale to bedocumented

mass loss [g] optional

failure modes [-] optional

end of incubation period [min]document time of initialsurface damage for eachlocation

breakthrough [min]document time ofbreakthrough for eachlocation

Resu

lts

stage of erosion progress [-]

define if data point isrepresenting end ofincubation period orbreakthrough


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SECTION 11 SUMMARY

11.1 SummaryIn this recommended practice, the influencing parameters for erosion tests on rotating arm test rigs werespecified and in some cases nominal values were recommended. Furthermore, an aluminium calibrationspecimen is introduced. The performed round robin tests show comparable erosion performances for thethree rotating arm test rigs. The round robin test results are listed in App.C.


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APPENDIX A SPECIMEN GEOMETRY

A.1 Specimen geometry

Figure A-1 Gauge length explanation

Figure A-2 Specimen cross-section based on NACA 634-021


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APPENDIX B DERIVED TEST PARAMETERS

B.1 Droplet velocityIf the initial speed of the droplet at the needle is zero, the velocity of the drop is a function of how far thedrop falls from the needle to the sample, x, and the droplet diameter, d.

Figure B-1 Illustration of specimen impact velocity and droplet falling velocity

vdrop,rp = droplet falling velocity when reaching the centre of specimen in the rotor planex = distance from origin of droplet (needle) to centre of specimen in the rotor plane

For droplet sizes of 0.1 mm to 3 mm, the terminal velocity of the droplet may be defined by using thefollowing empirical relation in ASTM G73-10:

where d is the mean droplet diameter in [mm].If the travel distance is not high enough for the droplets to reach terminal velocity when reaching the rotorplane (vdrop,rp = vdrop,max), the droplet velocity may be derived from the two following relations:

using x(t = 0) = 0


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The velocity as function of time is established from the equilibrium equation in terms of an object fallingfreely in air. Using this equation, the velocity of the droplet when reaching the rotor plane, v(t@rp) =vdrop,rp, may be calculated.Alternatively, the droplet velocity may be determined experimentally by using a laser disdrometer.

B.2 Rain intensityIn line with ASTM G73-10, the rain intensity, I , may be directly derived from the water flow, P , and the areathe water is distributed over, A . Thus, the rain intensity is calculated as:

The area is dependent on how the rain field is generated. In cases where the rain is only generated directlyover the specimen gauge zone, the area may be computed as:

where:ro =

ri =

φ =

In this case, θ is the angle coverage where the rain is generated, and φ the distribution ratio.

B.3 Droplet impact velocityThe droplet falling velocity is very small compared to the sample travelling velocity. Thus, the resultingimpact velocity is assumed to be equal to the sample speed.

For the test rig configuration specified in this document, the impact velocity distribution, vs(r), across thegauge zone is linearly related to the radial position on the specimen carrier arm and the angular velocity:

B.4 Specific impact frequencyThe number of specific impacts, in terms of number of droplet impacting on a unit area during the exposuretime, t is computed from the droplet concentration, q, and the impact speed, vs:

Thus, the specific impact frequency per unit time at the gauge zone centre is quantified as:


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The droplet concentration, q, is the number of droplets per cubic meter. It is estimated from the watervolume concentration, ψ, and the volume of a single droplet, Vdrop, which is based on the mean dropletdiameter:

The water volume concentration, ψ, may either be experimentally characterized or estimated from the rainintensity, I, and the droplet falling velocity, vdrop,rp, when entering the rotor plane:

The leads to the following equation for the droplet concentration, q:

The specific impact frequency, Ṅc shall be reported to give an indication of the impact rate of the test setup.The number of specific impacts N should be used for expressing results.On some machine setups, in order to have a constant specific impact frequency along the sample's length,the rain intensity is intentionally inhomogeneous. The evaluation of the specific impact frequency for oneexample of such a rain intensity repartition is shown below.

In order to keep the specific impact frequency, Ṅ, constant along the samples being tested, one solution is togenerate the rain field through dispensers organized radially, in a spider web shape.


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Figure B-2 Dispensers set following a spider web pattern

For most machines, the above drawing is not exactly accurate as dispensers would, by design, not haveexactly the same radial position from one row to the other, in order to homogenize the rain flow along thesample.It is also common, as shown in the drawing, to have a certain angular section without dispensers, requiredfor example, to fit the needs of automatic inspection equipment. The complementary angle is called the angleof coverage θ.

With such a distribution of dispensers, if we consider an area delimited by 2 circles of arbitrary radius r and r+ Δr, we can see that the number of dispensers do not depend on r (provided that the discreet distribution ofdispensers is approximated by an equivalent continuous distribution). We can therefore write that:

with θ being the angle of coverage in radians.

In order to keep the specific impact frequency, Ṅc constant, it has to be independent of the radius r and therain intensity I(r) has to be proportional to l/r:

We find the constant K through the total flow of water P which us poured over the covered area:


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Replacing constant K , the rain intensity in the covered area can then be expressed as:

As shown earlier, q can be expressed through the water volume concentration and the volume of a drop:

Replacing q and vs , the specific impact frequency in the covered area, can be expressed as:

And by replacing the rain intensity I :

Deducing from this, we see that the specific impact frequency is independent of the radial position, henceconstant along the sample.By multiplying that impact frequency with the time the sample spent in the rain covered area, we can get thespecific number of drop impacts after a certain test time t:

with the rotational speed: .

Thus, the specific number of impacts is independent of the angle of coverage:


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APPENDIX C RESULTS FROM ROUND ROBIN TESTS

C.1 Parameter overviewThe test parameters for the three rotating arm test rigs, which were used for the calibration tests onaluminium specimens, are listed in the tables below.

C.1.1 Parameters constant for all testsTable C-1 Test rig parameters for round robin tests

Test parameter Unit Round robin value

specimen carrier arm [-] aerofoil shaped with an integrated specimen.NACA 634-021

number of specimen carrier arms [-] 3

radial position of centre of specimenattachment, rc

[m] 1.0

Table C-2 Specimen design parameters for round robin tests

Test parameter Unit Round robin value Specific valuefor test rig A

Specific valuefor test rig B

Specific valuefor test rig C

cross-sectionalshape of specimen [-]

U-shaped andintegrated in theaerofoil design ofthe carrier arm

as specifiedin App.A



gauge zone lengthof specimen, lgz

[m] 0.4 0.4 0.4 0.4

distance fromorigin of droplet

to centre ofspecimen in

rotor plane, x

[m] ~0.4 0.38 0.4 0.4

Table C-3 Test condition parameters for round robin tests




water quality [μS/cm] - 2.5 +/-0.5 16 0.3

test specimentemperature [°C] NA - - -

test chamberpressure [Pa] NA - - -

mean dropletsize, diameter [mm] ~2.0 2.3 2.34 2.36


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droplet sizestandard deviation [mm] - 0.21 0.12 0.18

angle of incidence [°] 90 90 90 90

Table C-4 Derived test condition parameters for round robin tests

Test parameter Unit Specific valuefor test rig A



rain intensity inexposure zone. [m/s] 9.38E-06 9.05E-06 9.38E-06

droplet velocitywhen entering

rotor plane, vdrop,rp

[m/s] 2.4 2.52 2.4

Table C-5 Inspection parameters for round robin tests

Test parameter Unit Round robin value

inspection interval [min] varies

cleaning method before inspection [-]machine rotation without rain

or gently dry with a clothor with compressed air

record time of inspection inrelation to execution time [min] varies

record picture at every inspection [-] done

Table C-6 Reporting parameters for round robin tests

Parameter Round robin value

failure modes not documented

stage of erosion progress end of incubation period

C.1.2 Parameters for calibrationTable C-7 Calibration parameters for round robin tests

Test parameter Unit Round robin values

specimen composition [-] EN AW-Al Mn1Cu 3003

tempering class [-] H112

density [kg/m3] 2730

Young’s modulus [GPa] 69


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Test parameter Unit Round robin values

average hardness

ISO 6507-1:2005[HV 2] 33

manufacturing process [-] extruded from blocks and polished

annealing [-] none

surface roughness, Ra [µm] <1

Table C-8 Test condition parameters for round robin calibration tests




duration of test [min] 600 600 600

normal impactvelocity at centreof specimen, vsc

[m/s] 131 131 131

water temperature [°C] 13 +/-2 15 +/-1 8 +/-2

test chambertemperature [°C] 12 +/-2 16 +/-1 17 +/-2

Table C-9 Derived test condition parameters for round robin calibration tests




max impactvelocity, vs,max

[m/s] 157 157 157

min impactvelocity, vs,min

[m/s] 106 105 106

specific impactfrequency per unittime in exposure zone,Ṅc (based on meandrop diameter)

[Impacts /(m2*s)] 81849 71341 67864

C.1.3 Parameters for tests of coated specimensThree coatings were selected. The coatings were expected to have similar erosion performances.

C.1.3.1 All test rigs

Table C-10 Parameters for accelerated ageing for round robin tests on coated specimens

Test parameter Round robin value

extreme temperatures none

UV exposure none


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Test parameter Round robin value

humidity none

salt spray none

C.1.3.2 Test rig A

Table C-11 Test condition parameters for round robin tests on coated specimens – rig A

Test parameter Unit Coating 1 Coating 2 Coating 3



[m/s] 115 115 105



Table C-12 Derived test condition parameters for round robin tests on coated specimens – rig A



[m/s] 138 138 125


[m/s] 93 93 84

specific impactfrequency per unittime in exposure

zone, Ṅc (based onmean drop diameter)

[Impacts /(m2*s)] 71969 71969 65427

C.1.3.3 Test rig B

Table C-13 Test condition parameters for round robin tests on coated specimens – rig B


duration of test [min] 240 150-240 130


[m/s] 115 115 105




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Table C-14 Derived test condition parameters for round robin tests on coated specimens- rig B



[m/s] 138 138 126


[m/s] 92 92 84



[Impacts /(m2*s)] 62731 62731 57028

C.1.3.4 Test rig C

Table C-15 Test condition parameters for round robin tests on coated specimens – rig C




[m/s] 115 115 105



Table C-16 Derived test condition parameters for round robin tests on coated specimens- rig C



[m/s] 138 138 125


[m/s] 93 93 84



[Impacts /(m2*s)] 59672 59672 54248

C.2 Reference curve for calibration specimensIn the context of the present round robin test campaign, calibration specimens according to [6.3] have beentested on all three test rigs. From the test results, a reference curve as shown in Figure C-1 is derived. Theindividual test results are shown in Figure C-3 through Figure C-6.


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Figure C-1 Reference curve (with approximate power law equations for upper and lower limits)for end of incubation period of aluminium calibration specimens; the limits correspond to avariation in lifetime of approximately ±50%


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Figure C-2 Reference curve (with approximate power law equations for upper and lower limits)for end of incubation period of aluminium calibration specimens; the limits correspond to avariation in lifetime of approximately ±50%, the axes are on a logarithmic scale

Based on the performed round robin tests on calibration specimens and coated specimens ([C.3]), it isappropriate to assume that a test rig may produce comparable results, on other leading edge protectionsystems, if the test results of the new or re-calibrated test rig, lie reasonably well within the prescribedreference band.However, it has to be emphasized that the three test rigs of the round robin test, which lead to the referencecurve in Figure C-1, were very similar. For a comparison of different test rig configurations, which exceed the+/-50% band, it shall be shown that the test results on the coated specimens remain comparable.It is recommended to conduct tests on calibration specimens as precisely as possible under the conditionslaid out in the present document, for the purpose of:

— setting up new test rigs or new test laboratories; or— re-calibrating existing test rigs in regular intervals.

The test results of such calibration testing should be compared with the reference curve, preferably in asimilar way as shown in Figure C-3 through Figure C-6.


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Figure C-3 Aluminium calibration test results (end of incubation period) from all three test rigs ascompared to the reference curve; approximately 95% of all individual results lie within the limits


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Figure C-4 Aluminium calibration test results (end of incubation period) from test rig A ascompared to the reference curve


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Figure C-5 Aluminium calibration test results (end of incubation period) from test rig B ascompared to the reference curve


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Figure C-6 Aluminium calibration test results (end of incubation period) from test rig C ascompared to the reference curve

C.3 Test results on coating systemsIn order to evaluate the reproducibility of test results on different test rigs, the present round robin testcampaign included tests on specimens with three different erosion protection coatings.The coatings were selected expecting a similar erosion performance. The selection was not connected tocertification purposes. Thus, the results may not and shall not be used as a performance reference forcertifications.As shown in Table C-17, all three coatings have been tested on all three test rigs.

Table C-17 Round robin test matrix for coatings

coating 1 coating 2 coating 3

test rig A • • •

test rig B • • •

test rig C • • •

The individual test results are shown in Figure C-7 through Figure C-10. The numbers indicate that the testresults from the three test rigs are quite similar:

— the performance ranking of the three coatings is the same for all three test rigs (coating 1 and 2 verysimilar and slightly superior to coating 3).

— the relative difference in performance between these is approximately the same in all three test places.


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— for each given coating, the absolute performance level is very similar between the three different test rigs.

Thus, this coating round robin test campaign, in connection with the tests on aluminium specimens asdescribed above, and with the recommended standard conditions for testing, as laid out in the presentdocument, seems to support the basic assumption that standardized and comparable testing is achievable.

Figure C-7 Round robin test results of coated specimens (end of incubation period) from all threetest rigs; the axes are on a logarithmic scale, with absolute values removed to protect proprietaryinformation


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Figure C-8 Round robin test results of coated material (end of incubation period) from test rigA; the axes are on a logarithmic scale, with absolute values removed to protect proprietaryinformation


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Figure C-9 Round robin test results of coated material (end of incubation period) from test rigB; the axes are on a logarithmic scale, with absolute values removed to protect proprietaryinformation


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Figure C-10 Round robin test results of coated material (end of incubation period) from testrig C; the axes are on a logarithmic scale, with absolute values removed to protect proprietaryinformation


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Figure C-11 Round robin test results for coating 1 (end of incubation period) for all three testrigs; the axes are on a logarithmic scale, with absolute values removed to protect proprietaryinformation


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APPENDIX D INFLUENCES TO BE CONSIDERED

D.1 OverviewThe following influences on the erosion performance cannot yet be measured or quantified, however, theyshould be considered for this kind of testing:

— aerodynamic influences, e.g. turbulences due to misalignments or aerodynamic interferences ofspecimens with each other, might occur. One aerodynamic effect, which has to be considered, is theshadowing effect. For further details on the shadowing effect please refer to App.A.

— the maximum impact frequency should be limited in order to reduce the risk of reaching a situationwhere the materials would not recover fully between raindrop impacts. For most leading edge protectionsystems, the field impact frequency will probably not influence material recovery period.

— for small droplet sizes the aerodynamic influences are larger, and thus might lead to erroneousestimations of the impact frequency.

— possible rain mist or fog influences may occur.— it should be kept in mind that several erosion test results are extracted from one test specimen. The test

results are therefore not independent from each other.

D.2 Shadowing effectParticularly, the unintended disturbance of the aerodynamics and the rain field from one specimen to thenext, commonly known as the shadowing effect and illustrated on Figure D-1, should be accounted for.


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Figure D-1 Shadowing effect

To avoid the shadowing effect the distance of influence Linf , defined by specimen and wake length, shall besufficiently small to allow undisturbed passage of the falling droplets which will impact the next specimen. Linfis difficult to calculate or determine accurately. The risk of shadowing effect may be reduced by:

— increased specimen distance— decreased rotational speed— increased vertical droplet velocity— increased droplet size— smooth aerodynamic specimen profile (reduced wake).

Cha

nges

– h

isto

ric


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CHANGES – HISTORICThere are currently no historical changes for this document.

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