Durability of hybrid laminar flow control (HLFC) surfaces

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Aerospace Science and Technology 7 (2003) 181–190www.elsevier.com/locate/aesc

Durability of hybrid laminar flow control (HLFC) surfaces

Trevor Younga,∗, Brian Mahonya, Bryan Humphreysb, Ernst Totlandc,Alan McClaffertya, Julie Corisha

a Department of Mechanical and Aeronautical Engineering, University of Limerick, Limerick, Irelandb Aerospace Systems and Technology (AS&T), Consett, Durham, UK

c SAAB Aerospace, Linköping, Sweden

Received 15 July 2002; received in revised form 14 November 2002; accepted 13 December 2002

Abstract

As a part of the European Commission sponsored HYLTEC (Hybrid Laminar Flow Technology) project, a SAAB 2000 aircraftwith a number of small laser drilled panels on the wing leading edge – completed 20 months of routine service; the objectiveinvestigate contamination and durability aspects of Hybrid Laminar Flow Control (HLFC) suction surfaces. A post-flight test investigthese panels, manufactured from Nd-YAG laser drilled titanium, aluminium and carbon fibre polyetheretherketone (PEEK) compbeen conducted. Using Scanning Electron Microscopy (SEM), evidence of corrosion and damage was investigated. An opticaltechnique was used to measure hole geometries and the results were compared to pressure loss measurements through the pawas found to be the most robust material, displaying no adverse affect from this exposure, whilst aluminium was found to be subless durable. The PEEK carbon fibre composite showed signs of surface degradation after only two months of flight trials. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: Hybrid laminar flow control; Suction surfaces; Durability; Laser drilling

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1. Background: project HYLTEC

The HYLTEC (Hybrid Laminar Flow Technologyproject focused on the technologies required for the fscale development of HLFC (Hybrid Laminar Flow Cotrol); in particular focusing on manufacturing and contamnation related issues. The project was coordinated by AiGermany and had 15 industrial, research and academicners. The programme had three tasks, described by Bet al. [2]. Task 1 considered manufacturing, systemsoperational issues, seen as critical in terms of spoilingperformance of HLFC. Task 2 was devoted to retrofitquirements for HLFC to in-service aircraft, while task 3 wconcerned with the generation of new experimental dneeded for the validation of numerical flow predictions adesign strategies.

Two flight test campaigns took place to investigspecific elements of concern regarding HLFC. Humphr

* Corresponding author.E-mail address: [email protected] (T. Young).

1270-9638/03/$ – see front matter 2003 Éditions scientifiques et médicalesdoi:10.1016/S1270-9638(03)00013-0

-r

and Totland [3] describe the test programme involvingin-service SAAB 2000 commuter aircraft (Fig. 1). Ttests commenced in August 1999 with the aircraft flystandard passenger service routes in Northern Europeperiod of 20 months. The objective was to gain experieon the contamination and durability aspects of perforaHLFC surfaces. Two small specimen holders were insta

Fig. 1. HYLTEC SAAB 2000 aircraft, showing test specimens and thlocation [3].

Elsevier SAS. All rights reserved.

182 T. Young et al. / Aerospace Science and Technology 7 (2003) 181–190

Nomenclature

Notation and abbreviations

σ Standard deviationA,B Hole pitch in span-wise and length-wise

directions respectivelyAl AluminiumANOVA Analysis of varianceCu Copperd1, d2 Hole diameter on the laser exit and entry sides

respectivelyde Equivalent hole diameterHLFC Hybrid Laminar Flow ControlHYLTEC Hybrid Laminar Flow Technology

MFC Mass Flow ControllerNASA National Aeronautics and Space AdministrationNd-YAG Neodymium doped–Yttrium Aluminium

GarnetPEEK PolyetheretherketoneS Measured hole areaSEM Scanning Electron MicroscopeSi SiliconSLM Standard Litres per MinuteTi TitaniumVw Mean panel flow velocity

thet airtrol

wasd-

lderreinheAABndst

post

any;

ysen ifcraft

har-lita-d in

micon-

bre1).afttest

h aas

hapeing

in the leading edge; although no suction was applied,natural pressure differential at the wing nose ensured thapassed through the panels. A Liquid Contamination ConSystem was installed in one wing, while the other panelpassive, having no cleaning system. The durability of NYAG laser perforated test segments installed in the hoon the passive panel is the subject of the evaluation hereported. AS&T (Consett, UK) were responsible for tdesign and manufacture of the test panels and holder, SAerospace (Linköping, Sweden) for the flight testing athe University of Limerick (Ireland) for the post flight-teinspection and analysis of the panels.

2. Objectives of study

Three sets of objectives were established for theflight-test inspection of the panels. These were:

(1) To examine the panels for cracking, corrosion andother material defects (reported in Section 4 herein)

(2) To study the hole geometries, to statistically analthe data for the different materials and to ascertaithe hole sizes changed due to exposure on the air(reported in Section 5);

(3) To determine the pressure loss versus flow rate cacteristics of the different panels, and to relate quatively the results to the measured hole sizes (reporteSection 6).

3. Test materials

The materials that were evaluated were titanium, chroacid anodised non-clad aluminium, hard anodised nclad aluminium and APC-2, a thermoplastic carbon ficomposite of polyetheretherketone (PEEK) matrix (TableAn aluminium panel that was not installed on the aircrwas used as a reference against which the aluminiumpanels could be compared. All panels were drilled witNd-YAG laser using the single pulse method and Argonthe shielding gas. The panels were pressed, rolled to sand installed in a holder in a manner that prevented w

Table 1Materials evaluated

No. Description Material Notes Exposureon aircraft

1 Hard anodised Non-clad L166, Sulphuric acid anodised 18 months*

aluminium ∼ 0.9 mm thick2 Anodised aluminium Non-clad L166, Chromic acid anodised 20 months

∼ 0.9 mm thick3 Titanium – normal Commercially pure, Laser exit (small hole) 20 months

taper ∼ 0.9 mm thick on the outer face4 Titanium – reverse Commercially pure, Laser entry (large hole) 20 months

taper ∼ 0.9 mm thick on the outer face5 APC-2 Carbon fibre PEEK, 2 months

∼ 0.9 mm thick6 Aluminium reference Non-clad L166, Drilled at the same time nil

panel ∼ 0.9 mm thick as panel 2.

* Panel removed after 18 months and replaced by APC-2 panel.

T. Young et al. / Aerospace Science and Technology 7 (2003) 181–190 183

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an-ndTheysisents

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tingyedingsiveher

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Fig. 2. (a) Test panels installed in holder; (b) Detail showing normalreverse taper.

bending loads being transmitted to the test panels (FigThe holder was then attached to the leading edge of thewing.

t

4. Corrosion, cracking and surface damage

4.1. Apparatus and specimen preparation

An optical microscope (magnification 1660X) and Scning Electron Microscope (SEM) were used to identify arecord visual signs of cracking and surface corrosion.surface analysis method EDAX (energy dispersive analX-ray) was used to identify the presence of selected elemon the surface.

Sectioning of the aluminium and titanium samples wdone with a guillotine, and the composite material panelcut using a diamond-edgedsaw. The aluminium and titanspecimens were mounted both in an epoxy and a pheresin, while the composite specimens were mounteda softer thermoplastic resin, to aid with polishing. Aspecimens were ground and polished individually. Isoproalcohol was used as a lubricant and for cleaning; all wabased products were avoided in the process.

4.2. Results – hard anodised aluminium

The hard anodised aluminium panel had a very poorface finish with a significant number of surface blemishwhich could be seen by the naked eye. (It was for thisson that the panel was removed prematurely from theaircraft.) Under the optical microscope large areas of pitcorrosion extending to∼1.5 mm in diameter were clearlvisible (Fig. 3(a)). The EDAX surface analysis identifisignificant amount of sulphur, indicative of corrosion. Usthe SEM, an oblique cut through a hole displayed extenmicro-cracking (Fig. 3(b)). This feature was evident on otspecimens cut from the same panel.

4.3. Results – anodised aluminium panel

To the naked eye the surface of this panel was in abetter condition than the hard anodised panel. Sectiospecimens cut through the panel showed numerous scracks in the vicinity of the holes when viewed with toptical microscope (Fig. 4(a)). Higher magnification imag

ut through
Fig. 3. Hard anodised aluminium panel: (a) Optical microscope image showing corrosion due to anodising process; (b) SEM image of oblique chole.


and pit

idence of

Fig. 4. Anodised aluminium panel: (a) Optical microscope image of cross section through holes in test panel; (b) SEM image showing micro-cracktingcorrosion.

Fig. 5. (a) Optical microscope image of aluminium reference panel; (b) SEM image of cross-section of hole in titanium panel displaying no evcracking.

thence

astherossµm.

ionthel iningthe

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astals.romed

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inedare.

axishis

of the holes indicated many cracks on either side ofhole radiating outwards, which were absent on the referepanel (Fig. 5(a)).

Using the SEM pitting corrosion on the test panel wclearly noticeable and a micro-crack was identified onsurface emanating from one hole (Fig. 4(b)). Images of csection specimens revealed crack lengths of up to 50As with the hard anodised panel the EDAX inspectmethod confirmed significant amounts of sulphur onsurface of the panel. The microstructure of the materiathe immediate vicinity of the holes was studied by etchthe specimens and then viewing the sections undermicroscope, and also by micro-hardness tests. This waattempt to ascertain if the cracking was confined toheat-affected zone around the holes. The cracks appeto extend outside of this zone, however the results wconsidered inconclusive due to poor image quality.

4.4. Results – titanium panel

The visual inspection of the surface of the titanium parevealed no indication of pitting corrosion. Cross sectionthe holes were examined using the optical microscope

d

the SEM; in contrast to the aluminium panel, there wasevidence of any micro-cracking (example seen in Fig. 5(

4.5. Results – APC-2 panel

The image quality from the optical microscope wsubstantially poorer for this material compared to the meThe SEM was thus relied upon to evaluate the material. FFig. 6(a) it is evident that individual fibres had been strippfrom the surface between the holes. Focusing on theitself (Fig. 6(b)) it may be seen that a substantial amounthe matrix material had been removed in the vicinity ofhole, exposing the carbon fibres.

5. Hole geometries

5.1. Method for hole measurement

The hole area and effective diameter was determusing an optical microscope and image processing softwSections from each panel of approximately 60 mm× 30 mmwere scanned by the microscope fixed on a three-(two horizontal and one vertical) displacement system. T


hole.

pixel

Fig. 6. Images of APC-2 panel: (a) SEM images showing fibre removal between holes; (b) SEM image showing “burnout” of matrix around the

Fig. 7. Example of hole measurement process: (a) Image from optical microscope; (b) Initial processed digital image (black area represents redareas);(c) Image after removing pixels not associated with the hole.

to besing

then asizeelsitheread

ate”this

e toach

h

pitch

ity,ws

sede

s toe hasthededsizesingA)etersntwere

enabled images, such as the one illustrated in Fig. 7(a),captured and transmitted to a computer. Image processoftware was then used to identify the hole fromsurrounding area (Fig. 7(b)). Groups of pixels less thapredefined size were then removed (Fig. 7(c)). Theof the hole was calculated from the number of pixof the particular colour (red in this case) associated wthe hole. To expedite the process, computer routines wdeveloped. This allowed the software to automatically lopre-defined settings and eliminated the need to “re-calibrthe software settings for each hole. It was possible to dofor the aluminium and titanium specimens, however duthe surface irregularity of the composite panel, the approwas not successful with the APC-2 panel.

5.2. Hole size, pitch and porosity

An equivalent hole diameter (de) was determined for eachole measured, using the equation:de = √

4S/π whereS

was the measured hole area. Referring to Fig. 8 the holeis given as the distanceA (in the span-wise direction) andB(in the length-wise direction). The panel geometric porosoften expressed as a percentage, was calculated as follo

Porosity = π

4

(d̄2e

�A �B)

,

whered̄e is the mean panel equivalent hole diameter (baon the laser exit diametersd1) and �A, �B are the mean holpitch values.

Fig. 8. HLFC perforated panel.

5.3. Statistical methods for data analysis

Random sampling was used in selecting the holebe measured; this ensured that each possible samplan equal probability of being picked and each item inentire population has an equal chance of being incluin the sample. To analyse the data obtained for holeand diameter, confidence intervals were formulated ut-distribution values. The analysis of variance (ANOVtechnique was used to examine the means (of hole diamor pitch distance) of different data sets (from differesamples) and to test the hypothesis that these meansstatistically equivalent.


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5.4. Results – hole pitch

Table 2 summaries the hole pitch and standard deviadata determined in the span-wise and length-wise directfor the three perforated panels tested. One hundred meaments where made for both the aluminium and titaniumterials and 50 for the composite material.

5.5. Results – aluminium panel hole size

A random sample of 100 holes on each side ofanodised aluminium test panel and the reference panetaken as the data set for the statistical analysis. The mhole sizes, effective diameters and standard deviation(σ )

results are given in Table 3. The means were subjectean ANOVA analysis to determine if there was a changeeffective hole diameter due to the environmental exposon the aircraft. From the results obtained it was nothat there is a 99% probability that the laser entry hoon the reference panel were statistically equivalent into the laser entry holes on the test panel. Conductinsimilar analysis on the samples of laser exit holes, it wconcluded that statistically the two nominal hole sizes wsignificantly different.

5.6. Results – titanium panel hole size

One hundred holes on each side of the titanium pawere examined (Table 3). When evaluating this data the lexit holes on the reverse taper panel (i.e. where the smholes on the inner surface were not exposed on the aircwere used as a reference for assessing the normalpanel laser exit holes (which were on the outside). Similathe laser entry holes on the normal taper panel actedstandard for the laser entry holes on the reverse taper p

To compare the sample means and determine whethey can be considered equal, an ANOVA test was car

Table 2Mean values and standard deviation for hole pitch

Anodised aluminium Titanium APC-2

�A (µm) 700 722 708

σ(�A) (µm) 8.3 28.7 9.4�B (µm) 699 696 694

σ(�B) (µm) 49.3 29.7 24.9

-

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.r

out. It was concluded that there is a 99% probability thatlaser entry holes on the reverse and normal-taper panelsthe same nominal hole diameter. Similarly it was determithat there was also no significant difference in effective hdiameters between the laser exit holes on the two panel

5.7. Results – APC-2 panel hole size

As previously mentioned the carbon fibre composite wmuch more difficult to analyse than the other panels duthe fibres protruding through the surface of the material. Twas particularly evident when optical measurements of hareas were attempted. The results for this material are nreliable as those obtained for the other materials and shbe regarded as indicative of the hole characteristics. Ofive laser entry and exit holes were measured (Table 3).

5.8. Suction hole uniformity

Suction hole uniformity can have an impact on tboundary layer stability [6]. As seen from the resultsTable 3, there is a moderate scatter of hole size within epanel. The probability of a hole exceeding a particular s(and disrupting the flow) can be ascertained from Fig. 9,the titanium panel.

A second issue considered was the uniformity of the hshape and the departure of the hole from a circular shIt was not possible to analyse this numerically; Fig.provides examples of non-uniform holes identified ontitanium panel.

6. Panel flow measurements

6.1. Experimental apparatus

A customised rig (shown schematically in Fig. 11) wassembled to measure the flow characteristics of specimcut from the laser drilled panels described in Sectioabove; enabling plots of pressure drop against net pvelocity (Vw) to be produced. A compressor suppliair at one bar through a filter and a honeycomb fl“straightener” to the test section. The test section comprof two stainless steel flanges, one male and one femwhich clamped the test specimen between two gaskets

Table 3Geometric hole properties of aluminium, titanium and composite

Reference Anodised Titanium Titanium APC-2aluminium aluminium normal reverse

taper taper

d̄e laser entry (µm) 203.2 208.4 136.9 133.3 170.4σ(d̄e) laser entry (µm) 16.3 15.3 11.2 8.9 11.9d̄e laser exit (µm) 63.1 76.5 71.2 72.0 80.0σ(d̄e) laser exit (µm) 9.5 8.6 5.4 7.6 8.9Porosity 0.94% 0.79% 0.81% 1.02%


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sedthe

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nts.

theandnot

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allys inthens,sureten

theum13.en

over

heionersethelts.forrate

Fig. 9. Plot showing probability of a nominal hole size being below vastated on x-axis for laser exit holes on titanium panel.

small reductions in cross sectional area – resulting fdeformation of the gasket – would lead to large errorsrecorded data, hard nylon gaskets (1 mm thick) were uThe external diameter of the gasket was 35 mm andinternal diameter was 25 mm, matching the internal diamof the test section.

A Mass Flow Controller (MFC) was located down-streaof the test section, which enabled the flow in standlitres per minute (SLM) to be ascertained. The MFC wcalibrated for a full-scale flow rate of 750 SLM witaccuracy and linearity of one per cent, and repeatabilitless than 0.1 per cent deviation (full scale), being quoby the manufacturer. A digital hand-held manometer wused to measure the pressure drop across the test specA thermocouple was used to measure the internaltemperature and a rotometer was installed in series withMFC to assess the repeatability of the MFC measureme

.

n.

6.2. Test specimens and method

Three 35 mm diameter specimens were cut fromtest panels – one anodised aluminium, one titaniumone APC-2 specimen. The hard anodised panel wasevaluated. A fourth specimen was cut from the aluminireference panel. For initial tests the specimens werein the as-received condition and were later ultra-soniccleaned, to ascertain the affect on the airflow, of debrithe holes. Airflow measurements were performed onfour specimens in both normal and reversed flow directioand also before and after being cleaned. Mean presloss values were calculated from data recorded forexperimental runs for each specimen.

6.3. Results – pressure drop

The results for the anodised aluminium panel andreference panel are shown in Fig. 12; for the titaniand APC-2 specimens the results are given in Fig.It was noted that prior to cleaning the APC-2 specimwas almost completely blocked and a pressure loss of70 kPa was observed for a flow velocityVw of less than0.1 m s−1. This result was not included in Fig. 13. For tsake of clarity the results for the reversed flow directare not indicated in the figures. In all cases the revtaper flow exhibited a larger pressure drop, but followedsame trend with velocity, as the normal taper flow resuThe magnitude of this change varied between 19–30%the materials evaluated, as measured for a panel flowof 0.5 m s−1.

Fig. 10. Examples in titanium panel of: (a) Non-uniform laser entry hole; (b) Non-uniform laser exit hole.

Fig. 11. Schematic of flow rig (not to scale).


per).

theiumium.

tionsofded

n inaneameion,

er.ldedore in

talaft.thisnalcens),em.iredrial

besonseen

-, isg orl byfaced byast in).sul-n anrec-andnot

nelardthe

untlledandkstheythed/orils.

traryedolecksinsr of

Fig. 12. Flow measurements for aluminium panels (normal taper).

Fig. 13. Flow measurements for titanium and APC-2 panels (normal ta

Fig. 14. Porosity results taken during the HYLTEC programme on(passive) lower panel surface L9 – anodised aluminium; L10 – titanreverse taper; L11 – titanium normal taper; L12 – hard anodised alumin

6.4. HYLTEC results – pressure drop

Humphreys and Totland [3] describe the data collecof flow measurements takenin-situ on the test panelinstalled on the SAAB 2000 aircraft. Fig. 14 is a samplethe corrected data obtained during the programme, incluherein to compare against the laboratory results giveSection 6.3 above. The ordinate variable indicates the pporosity, which was measured each month at the slocation on the lower surface of the panel (i.e. in the reg

l

where in-flow occurred), using a custom built flow metMeasurements taken on the panel upper surface yiesimilar results, with little or no change of porosity fthe titanium panel being observed, however an increasporosity for the aluminium panel was noted.

7. Discussion

7.1. Corrosion, cracking and surface damage

HLFC is being considered for the wing, horizontailplane, fin and engine nacelles of jet transport aircrTitanium is regarded as the benchmark material forapplication, however for an engine nacelle, the additioweight that results from the use of titanium (in plaof the composite material used on most current desigreduces the net performance gain from the HLFC systFurthermore the perforated suction area is not requon the leading edge of the nacelle, making the matedurability less critical for this component, than wouldthe case for the wing or empennage. For these reaaluminium and carbon fibre reinforced PEEK has bconsidered [8].

The major difficulty in using aluminium, with its relatively poor corrosion resistance, for HLFC applicationsthat the base material cannot be protected by claddinpainting. This lead to the attempt to protect the materiaanodisation. On inspection it was observed that the surof the hard anodised aluminium panel was characteriselarge areas of pitting, known commonly as “burning”. It wnoted after the flight trails that the major alloying elementhis type of aluminium alloy is copper (typically 3.5–5%When anodised in this way, the copper reacts with thephuric acid (hard anodise) process in a way that results iuneven anodic film. For this reason the process is onlyommended for use on alloys containing less than 3% Culess than 7% Si [1]. The anodisation process was thussuitable for the grade of aluminium selected.

Initial inspection of the chromic acid anodised paindicated that it was in a much better condition than the hanodised panel, however a serious concern regardingdurability of the material arose when a significant amoof cracking was observed emanating from the laser driholes. The mechanism by which these cracks formed“grew” could not be precisely identified. As no cracwere seen on the reference panel, it was deduced thatwere the result of post drilling process(es), relating toforming of the material to the leading edge shape anthe environmental exposure it endured during the traThe absence of cracks in the reference panel is conto information presented by Yeo et al. [7], who maintainthat micro-cracking within the recast layer around the hoccurs due to the laser drilling process. Whether the crawere initiated by the drilling process or not, the fact remathat after exposure on the aircraft, a substantial numbe


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highly visible cracks were evident on the test specimewhich were not evident on the reference panel, wobserved under identical conditions.

The titanium panel appeared to be entirely unaffectedthe environmental exposure during the tests. Opticalscanning electron microscopy revealed no cracks at athe material. The observation regarding durability is csistent with the results of rain erosion tests conductedHumphreys on similar laser drilled coupons [8]. In thetests the titanium was noted to be “largely unaffectedthe erosion impacts and displayed essentially no damafter 36 hours of testing” [8]. These findings would crelate with those of the NASA Simulated Airline Serviprogramme conducted using a Jetstar aircraft modifieevaluate Laminar Flow Control. Maddalon and Braslowreport that there was “no measurable degradation of theforated titanium suction surface after four years of flight teing”.

The rain erosion tests mentioned above also evalulaser drilled carbon fibre reinforced epoxy material andthermoplastic material APC-2. Young et al. [8] report th“After about 30 minutes, fibres were removed betweenjacent holes. The laser drilled thermoplastic material asuffered some fibre removal, but this was considerablythan that of the carbon fibre epoxy specimens. Theforated thermoplastic material after 11 hours was apprmately similar in damage to the carbon fibre epoxy spmens after 30 seconds under the same conditions”. Theface damage mechanism, whereby fibres were strippedbetween the holes, described in Section 4.5, is entirelysistent with the damage observed on both the thermopland epoxy rain erosion coupons reported in [8]. It wouldpear the absence of resin around the holes, resulting fromdrilling process, initiates this process.

7.2. Hole pitch

The optical technique used to measure the hole afollowed a similar process to that outlined by Priest aPaluch [6]. It was found to be very effective for metals, bnot very good for the composite specimens, where surirregularities made it difficult to accurately identify the hoperimeter.

The claim of Preist and Paluch [6] that the inter-hdistance may be considered constant was investigatedsults in Table 2). For the aluminium panel, measurementhe span-wise direction indicated a relatively small scain hole pitch; the data was of a normal distribution, incating a homogeneous drilling process. Although the mpitch was almost identical in the length-wise directionthat measured for the span-wise direction, a bi-modaltribution with a higher standard deviation was observedthe length-wise direction, indicative of inhomogeneity in tdrilling process.

For the titanium panel the hole pitch in the span-wdirection was observed to follow a normal dispersion.

-

-

-

though there was greater scatter than was seen in theminium panel, the standard deviation was still relativsmall in comparison to the distances measured. Pitcthe length-wise direction was∼3.6% smaller than in thespan-wise direction, with a relatively small scatter ofsults, that displayed a minor departure from a normaltribution.

For the composite material fewer holes were measudue to difficulties with the imaging process. Hole pitchthis material is seen to be statistically equivalent in the spwise and length-wise directions, with both directions hava normal distribution with relatively small scatter.

7.3. Hole size

An important result from this investigation concernthe mean hole sizes in the test panels, as concernsbeen voiced regarding the effects on panel porosity of lterm in-flight exposure on the leading edge of a wing [No comparative assessment of hole size was possiblethe composite material, however for both the anodialuminium and titanium panels, this was possible. It wfound that the laser exit holes (which were on the outsidethe anodised aluminium test panel were significantly lar(∼21%) than those on the reference panel (i.e. 76.5 µmcomparison to 63.1 µm). The laser entry holes were allittle larger for the test panel (208.4 µm versus 203.2 µhowever statistically (based on the ANOVA method)mean hole diameters could be considered equivalenis suggested that the increase in hole diameter is dua depletion of the recast layer (formed during the ladrilling process) due to a combination of corrosion aerosion under the impact of airborne particles, rain and hassisted by the extensive cracking within the holes.consequence of a steady increase in hole size for a Hinstallation is that the pressure drop across the perforskin will change with time and may ultimately result in tsuction system being unable to provide the design massacross through the skin necessary to stabilise the extflow.

The measured hole sizes and scatter of results fortitanium specimens were compared to the results of Pand Paluch [6], although it should be noted that the drillprocess was carried out with different drilling parameterthe two cases. The hole diameter (laser exit) quoted inwas about∼23% smaller that that given in Table 3; howevthe standard deviations were observed to be relatisimilar. The ANOVA test revealed that statistically the laentry holes on the reverse and normal taper panels hasame nominal hole diameter. There was also no signifidifference in effective hole diameters between the laserholes on the two panels, in spite of the fact that one surwas exposed to the external environment and the othernot. It was thus concluded that any erosion affects onexternal hole diameter of the titanium panel during themonths of testing, was negligible.


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7.4. Flow characteristics

Four important aspects arise from the flow measuremresults reported in Sections 6.3 and 6.4. Firstly the impacpartial clogging of the holes is seen to be very significaAt 0.5 m s−1 panel velocity, this effect was responsible fincreasing the magnitude of the pressure drop by 71% foaluminium panel and by 23% for the titanium panel. Alsowas observed that the APC-2 panel was almost compleblocked on receipt. However this effect is likely to haveconsequence for an aircraft flown regularly, as the impacrain and ice on the perforated surface during flight is knoto have a substantial cleaning effect. After removal frthe aircraft the panels were “handled” a lot and specimprepared. It is likely that the most significant degreecontamination occurred during this time.

The second observation concerns the change of pordue to the environmental exposure of the panels onaircraft. For the anodised aluminium panel, the magnitudthe pressure drop across the test panel was∼32% lower at0.5 m s−1 than that measured for the reference panel (norflow direction). This trend is consistent with the increain measured hole size (Table 3) and also follows the tridentified in Fig. 14. A hard anodised aluminium surfagenerally has good wear resistance and in Fig. 14 it is evithat the surface treatment resulted in a smaller changpressure drop than the chromic acid anodised panelthe 18 months of simultaneous testing. The titanium nortaper and reverse taper panels appeared to be unaffectthe flight testing.

The pressure drop versus panel velocity characterifollow the identical trend previously shown by Poll et al. [and Priest and Paluch [6]. The pressure drop of the notaper titanium panels was observed to be a little lower tthe values measured by Priest and Paluch [6]; considethe fact that the holes in the current study were larger,would be consistent with expectation. In all cases airflthrough a converging tapered hole (i.e. a reverse taper inconvention adopted herein) was associated with a largerin pressure across the panel than a diverging hole (normper).

A limitation of the current investigation is that only onspecimen of each material type was evaluated and it wbe expected that some scatter in the flow measuremwould result from different specimens, as observed in [6

8. Conclusions

(1) Titanium is the most durable material inspectedthe panel surface showed no evidence of degradaafter the flight trials. The mean hole size appearedbe unchanged.

(2) Both the anodised (chromic acid) and hard anod(sulphuric acid) aluminium panels displayed evidenof corrosion and micro-cracking. As the reference pawas free of cracks, the cracks in the aluminium t

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y

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s

panels were formed possibly as a consequence of spost-drilling process associated with the forming ofpanel, but more likely as a result of a process associwith the flight tests.

(3) On the basis of the ANOVA statistical analysis pformed, it was observed that the holes on the exteriothe aluminium panel (laser exit side) were significanlarger on the test panel than on the reference panelthough the holes on the laser entry side were also laon the test panel, the relative amount was less andmean hole sizes could be considered as not beingnificantly different. It was concluded that the in-fligexposure was responsible for this increase in holeon the external surface.

(4) The laser drilled APC-2 composite material has retively poor durability with noticeable amounts of fibremoved on the surface between holes, after two moexposure to operational conditions.

(5) There was a relatively small scatter in hole pitch ahole size on the perforated panels; however hole shwas seen to vary considerably.

Acknowledgements

The authors wish to acknowledge the contribution ofEuropean Commission for financial support under the Indtrial and Materials Technologies Project HYLTEC within tFourth Framework (Contract BRPR-CT797-0606). Thesistance provided by Wayne Leahy, Catherine JohnsonJeff Punch of the University of Limerick is gratefully aknowledged.

References

[1] ASM Metals Handbook, Vol. 2, Heat Treating, Cleaning and FinishiEighth Edition, American Society for Metals (ASM), Metals Park, O1980.

[2] H. Bieler, P. Swan, B. Humphreys, The HYLTEC project – a hyblaminar flow technology investigation, in: CEAS Aerospace Aeronamics Research Conference, Cambridge, 2002.

[3] B.E. Humphreys, E.J. Totland, SAAB 2000 in-service test of porsurfaces for HLFC, in: CEAS/DragNet European Drag Reduction Cference, Potsdam, Germany, in: Notes on Numerical Fluid MechaVol. 76, 2000, pp. 89–98.

[4] D.V. Maddalon, A.L. Braslow, Simulated Airline Service Flight TestsLaminar Flow Control with Perforated Surface Suction System, NATP-2966, March 1990.

[5] D.I.A. Poll, M. Danks, B.E. Humphreys, The aerodynamic performaof laser drilled sheets, in: First European Forum on Laminar FTechnology, Hamburg, 1992, pp. 274–277, DGLR-Bericht, 92-06.

[6] J. Preist, B. Paluch, Design specification and inspection of perforpanels for HLF suction systems, in: CEAS Second European ForuLaminar Flow Technology, Bordeaux, 1996, pp. 6.20–6.35.

[7] C.Y. Yeo, S.C. Tam, S. Jana, M.W.S. Lau, A technical review of the ladrilling of aerospace materials, J. Mater. Process. Technol. 42 (115–49.

[8] T.M. Young, B. Humphreys, J.P. Fielding, Investigation of HybLaminar Flow Control (HLFC) surfaces, J. Aircraft Design 4 (200127–146.

Durability of hybrid laminar flow control (HLFC) surfaces

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