eper! He. NA-1-31[IS61-1JFINAL REPORT

Preject No. 516-103-DIX

FRICTION EFFECTS OF IUNWAY GROOVES, RUNWAY 18-36

WASHINGTON NATIONAL AIRPORT

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DEIPARTHENT OF TRANSPORTATIONFEDERAL AVIATION ADMINISTRATION

National Aviation Facilities Experimental CenterAtlantic City, New Jersey 08405

Reproduced by theCLEARINGHOUSE

for Federal Scientific & TechnicalInformation Springfield Va. 22151

_ _ _ _ _ __ __ __ __ __ __.

A

The Federal Aviation Administration is responsible for the promotion, regulation

and safety of civil aviation and for the development and operation of a commonsystem of air navigation and air traffic control facilities which provides for the

safe and efficient use of airspace by both civil and military aircraft.

The National Aviation Facilities Experimental Center maintains laboratories,

f a cil'Aities, skills and services to support FAA research, development and imple-

mentation programs through analysis, experimentation and evaluation of aviation

con cepts, procedures, systems and equipment.

FINAL REPORT

FRICTION EFFECTS OF RUNWAY GROOVES, RUNWAY 18-36WASHINGTON NATIONAL AIRPORT

PROJECT NO. 510-003-07X

REPORT NO. NA-68-31(DS-68-21)

Prepared by:

WILLIAM A. HIERINGand

CHARLES R. GRISEL

for

AIRCRAFT DEVELOPMENT SERVICE

DECEHBER 1968

This report is approved for unlimited distribution. It doesnot necessarily reflect Federal Aviation Administrationpolicy in all respects, and it does not, in itself,constitute a ttandard, specification, or regulation.

DEPARTMENT OF TRANSPORTATIONFederal Aviation Administration

National, Aviation Facilities Experimental CenterAtlantic City, New Jersey 08405

ABSTRACT

Wet and dry runway friction tests were conducted on bituminous concreteRunway 18-36 at Wishington National Airport using a Fixed Slip RunwayFriction Tester. These tests were conducted to determine if significantfriction changes were generated as a result of grooving the runway -surface with 1/8- by 1/8-inch-transverse grooves spaced on 1-inch centers.Data analysis indicates that at test speeds of 10 to 60 mph, no appreciableincrease or decrease in overall runway friction values was obtained for Jthis series of tests. The treatment of the runway surface, however, by thecutting of uniformly sp3ced grooves markedly smoothed the resultant wet frunway friction values. It is hypothesized that these smoother wet runwayfriction values result in a surface that affords more efficient operationof aircraft antiskid braking devices and more eff ective manual braking.(')

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TABLE OF CONTENTS

Page

ABSTRACT iii

INrROINTION1

Purpose 1Background 1

Description of Equipment 1Friction Tester 1Instrumentation 4Test Tire 4Towing Vehicle 6

Test Methods and Procedures 6Calibration 6Runway Pattern and Nomenclature 6Test Runs 6

DISCUSSION 9

Friction Tests 9Dry Runray Surface Conditions - December 10, 1966, 9and May 4, 1967, Tests

Damp Runway Surface Conditions - March 16, 1967, 11and May 23, 1967, Tests

Runway Contamination Comparative Tests - 12May 10, 1967, and September 14, 1967, Tests

Lots Tire Pressure Tests 12Calibration 13

SUMMARY OF RESULTS 13

CONCLUSION 18

RZCNMNDATIONS 18

WERENCES 19

ARPWDIX I Ptiction Data - Dry Runway Surface Conditions - 1-1December 10, 1966, and May 4, 1967, Tests(16 pages)

APPENDIX II Friction Data - Damp Runway Surface Conditions - 2-1March 16, 1967, and May 23, 1967, Tests

(16 pages)

APPENDIX III Friction Data - Runway Contamination 3-1Comparative Tests - Dry Surface ConditionsMay 10, 1967, and September 14, 1967, Tests(16 pages)

APPENDIX IV Low Tire Pressure Friction Data (8 pages) 4-1

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LIST OF ILLUSTRATIONS

Figure Page

1 FAA's Fixed Slip Runway Friction Tester 2

and Towing Vehicle

2 Water Dispensing System 3

3 Instrumentation Cabinet and Recorder 5

4 Calibration Equipment 7

5 Runway 18-36 Test Track Configuration 8

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LIST OF TABLES

Table Page

I Friction Measuring Tests 10

II Percent Changes in Measured Friction 14Values Due to Grooving - December 10, 1966,-nd May 4, 1967, Dry Runway Tests

III Percent Changes in Measured Friction 15Values Due 1o Grooving - March 16, 1967,

and May 23, 1967, Damp Runway Tests

IV Percent Changes in Measured Friction 16Values Between Post-Grooving Tests -Mlay 10, 1967, and September 14, 1967,Dry Runway Tests

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INTRODUCTION

Purpose

The primary purpose of this phase of the-project was to measure thebrake slip friction value of Runway 16-36 at Washington National Airport,before and after runway grooving, using the Federal AviationAdministration (FAA) Fixed Slip Runway Friction Tester (FSRFT) MeasuringSystem. The secondary purpose was to investigate hydroplaning effectswithin the limitations of the test equipment and test condit-ons.

Backaround

Commercial jet transport aircraft have generally experienced moredifficulty in stopping on wet runways than propeller-driven aircraft.This is due mainly to the hJgher landing speeds and low aerodynamic draginherent in the sleek jet transportsz

In April 1966, jet transports began operating at Washington NationalAirport which is owned and operated by the FAA. The longest runway(18-36) is 6870 feet long, 200 feet wide, and accommodates most of thejet traffic.

To enhance safety operations on this runway, the FAA awarded acontract to groove the entire runway length and 150 feet of the runway

width, omitting touchdown lights, and subsurface wiring areas. Theprimary purpose of grooving is to forestall hydroplaning by improving

surface water drainage properties (nrerence 1). The contract effort

to cut transverse grooves 1/8-inch wide, 1/8-inch deep, on 1 inch

spacing was begun in March 1967, and completed in April 1967.

Description of Equionent:

Friction Tester - The equipment used to measure runway frictionwas the FAA's FSRFT, Figure 1, which is a modified Swedish Skiddometer,Model DV-6. The Skiddometer operates in a fixed slip mode and wasoriginally conceived by the Swedish Road Institute for the purpose ofmeasuring the friction values of snow and ice-covered runways. Thisfriction tester utilizes the standard automotive test tire and loading,specified by the American Society of Testing Material (ASTM) for frictiontesting,

This tester was used in a different manner than thatprescribed by the Swedish Road Institute; namely, to measure frictionof wet and dry runway surfaces. To accomplish this, the projectengineering personnel designed and installed a special water dispensingsystem, Figure 2. This design incorporated a belt-driven constantdisplacement water pump coupled to the axle of the FSRFT. The output ofthe pump varies directly with speed, thereby providing a constant waterthickness independent of vehicle speed. A water thickness ofapproximately .020 inch was obtained, meeting the ASTM Specification E-274

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which states that a water depth of .020 + .005 inch be used when measuring

we't pavement friction. The punp is operated by means of a magnetic

clutth, powered and controlled from the tow vehicle. During friction

measuring operations, the magnetic clutches of the three-wheel axle are

engaged (locked) thus forcing the test wheel to rotate with the same

angular speed as the two outer wheels. Since the diameter of the test

tire is smaller than the diameter of the outer tires, the peripheral

speed of the test tire becoes less than that of the outer tires. Thus,

the design causes the test tire to be retarded, generating a tire/pavement

slipping action. This action produces a constant slip ratio of

approximately 13 percent. The torque of the test wheel, generated by

friction forces between the test tire and pavement, is measured by a

strain gage force trannducer.

The water dispensing system and friction recording system

are separately and remotely controlled by the operator in the tow vehicle

enabling, first, dry testing, followed by wet testing over the same path.

then the water dispen-iag system is activated, a water film approximately

.020-inch thick is deposited cn the pavement surface 18 inches ahead of

the zest tire, thus providing conditions for vet testing.

Instrumentation - The friction forces exerted on the test wheel

are registered by a pen recorder, Figure 3. The recorder and associated

electrical equipments. are located in an instrtment cabinet motnted on the

frame of the trailer. The reado on the chart paper is traced in analog

iorwat and the displacement of the pen provides a numerical value known

as "Brake Friction -uber" (Bl 1 1). This value is the zeasured

"oefVicient of friction times 100, obtained by testing in the brake slip

mode at 13 percent slip. The recorder has a combination electric chart

paper drive whIch is used during calibration, and a mechanical external

chart paper drive for recording friction.

The mechanical drive consists cf a flexible cable

connected to the left outer trailer wheel. This arrangement produces

chart paper lengths proportional to the distance tested - independent of

test speed. Each distance subdivision of chart paper length is equal to

approximately 77 2/3 feet of runay distance and each friction mubdivision

represents an uncorrected BF 13 value of 1, full scale representing a

BFN 13 value of 20. The recorder is also equipped with an electric timer

which proviaez a pip at 1-second intervals. This pip is recorded by a

pen on the margin of the chart paper. The distance between pips facilitates

verifying the speed of the tester.

Tes. Tire - The friction measuring tire used in these tests

was developed by the ASTM to provide a standard test tire which is

manufactmred to closely held specifications. The tire, designated by

ASTM zs E-249, was specifically designed for pavement friction measuring.

This four-ply tire is a standard automotive size (7.50/14) which is

inflated to a specified pressure of 24 psi and vertically loaded to

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1085 pounds. A smooth tread ccnfiguration (no circumferential grooves)was used to eliminate variances in friction due to wearing tire treadand groove depths.

Towing Vehicle - The vehicle provided for towing theSkiddometer was a late model station wagon. This automobile is equippedj with a two-way radio (for airport communications) and a specially built150-g11cn capacity water tank. This amount of water is sufficient to wet20,000 linear feet of pavement. The gross weight of the vehicle, withtwo operators and a full tank of water, totaled approximately 4500 pounds.When towing the 3400-pound FSRFT with the test wheel in the braking mode,the top speeA of the system was limited to slightly over 60 mph.Acceleration -as also affected, and 50 mph was the maximum speed within1000 feet.

Test Methods and Procedures:

Calibration - Prior to each series of test runs, the FSRFTwas calibrated, Figure 4. This calibration was accomplished byapplying known horizontal loads to the platform of the calibrationstand. The horizontal forces are transmitted to the contact area ofthe tire resting on the plztform. The dynamometer reading was relatedto the displacement of the recorder pen which recorded the uncorrectedcoefficient of friction (WFN). Repeated calibrations providedinformation from which system accuracy and/or deterioration would beobserved.

Runway Pattern and Nomenclature - The test pattern used onthe 6870-foot runway consisted of four test tracks or paths shown inFigure 5. One thousand feet at each end of the runway were reservedfor accelerating and stopping; the remaining 4870 feet were friction-tested. Track No. I is located 3 feet east of the runway centerline,to clear centerline paint marks, and all runs on Track No. 1 were madein the 36 or north direction. Track No. 2 is located 35 feet from theeast edge of the runway, and tests on this track were conducted in the18 or southerly direction. Test runs on Track No. 3 were conducted inthe 36 direction, 75 feet from the west edge of the runway (25 feetwest of the centerline), while tests on Track No. 4 were made in the18 direction and 75 feet from the east edge of the runway (25 feet eastof centerline). Tracks Nos. 1, 3, and 4 provided data of the mostcontaminated portion of the runway, while Track No. 2 (runway edge)provided data on the least contaminated portion of the runway. Thistest design allows a comparison to be made between the rubber-contaminated(touchdown and rollout area) portions of the runway and the relativelyuncontaminated portion along the runway edge.

Test Runs - Twenty-four "standard" test runs were made on eachof the four tracks. Of these runs, 12 were made in a dry condition(without use of the water dispensing system) at 10, 30, and 50 mph,followed by 2 wet runs at the same speeds. At the completion of the

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24th run, 4 additional higher speed wet runs were made, 1 on each track,at approximately 60 mph. These runs, however, required an additional500 feet for acceleration, thereby reducing the test portion of therunway by an equal amount. The higher speed runs were conducted in anattempt to approach speeds at which dynamic hydroplaning could occur.Dynamic hydroplaning has been calculated to occur at approximately65 mph in accordance tith the four-ply automotive tire formula of 13.2times the square root of the tire pressure (Reference 2).

Test speeds over 60 mph were unobtainable due tohorsepower limitations; consequently, other efforts to induce hydro-planing or the onset of hydroplaning were attempted by lowering thetest tire pressure. The pressure was lowered from an initial 24 psito 17.4 psi, then to 14.3 psi, and finally to 11.6 psi, at which, ifthe proper conditions are present, dynamic hydroplaning should occurat 55, 50, and 45 mph, respectively.

DISCUSSION

Friction Tests

Friction measurement tests were made on 12 occasions, of which 6were conducted before the runway was grooved and 6 after, (Table I).Sim lar tests, matched for eivironmental conditions, were compared;i.e., before-grooving tests were compared with after-grooving testsconducted in similar environmental conditions. Visual inspection ofthe four test tracks disclosed deposits of tire rubber predominately atthe 1000-foot ends of Tracks 1, 3, anj 4. Track 2 was found to berelatively uncontaminated by rubber deposits.

Dry Runway Surface Conditions - December 10, 1966, and May 4, 1967Tests: In comparing the pre-grooved tests conducted December 10, 1966,rtith the post-grooved te3ts conducted May 4, 1967, the following resultsare noted from the test data shown in Appendix I:

Track 1 (centerline) - There is no significant change eitherin BFN1 3 or curve shape for the dry tests, pages 1-1, and 1-2. In thewet test configuration, however, pages 1-3 and 1-4, there is noticeableand measurable differences in the BFN1 3 values, favoring the after-grooving condition. The BFN1 3 values are generally increased and theoverall analog traces are markedly smoother.

Track 2 (runway edge) - Since this track is near the edge ofthe runway and relatively free of rubber deposits and the polishingaction of traffic, it would be expected that any friction value changeswould be caused mainly by the grojoves. The rough trace of the 10 mphafter-grooving dry test is unexplainable; otherwise, the dry tests arealmost identical before and after grooving, pages 1-5 and 1-6. In

the wet tests, pages 1-7 and 1-8, the before- and after-grooving tracesof the 10- and 60-mph tests are very similar, whereas a definite

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TA=L I

FRICTION( 1CASRD TEST

ASHIIMCTK XATIOIW. AIRI RUWAY 18-36TEST RUN LG

Pfl-GROOVL'G TES

Test Test No. of Ambient Tact Water Runway SurfaceF4, Date Teat Rums Temp, OF Tmp OF Conditions Remarka

1 12/10/66 28 5- 55 50 - 50.9 Dry Standard test runs at st..ndardtire pressure.

2 12/22/66 7 32 - 33 Not used Dry Test curtailed by airvortauthoritiea due to freezingteaperatures.

3 2/2/67 28 41 - 46 44.6 Damp Very light rains at start oftest runs. Standard teat rumsat standard tire pressure.

4 2/3/67 36 35 - 40 42.6 - 52.6 Dcp Rain ended . hours prior to testsat variable tire pressures. Virst

20 runs standard vater floiv. Last16 runs high water flov rate.

5 2/21/67 42 39 - 40 41 - 50.9 Wet Light rains first 26 runs atstandard 24 psi tire pressure,

all tests high water flow rate.

6 3/16/67 40 34 - 38 42.8 Damp Rain ended 3 hours prior to tests.Pirst 28 runs standard tire

pressure, last 12 tests conductedat variable tire pressures.

.G'T- n0WVi_ TESTS

Test Test No. of Ambient Teat Rater Runway SurfaceYO, Date Test Runs Temp. 07 Temp, Of Conditions Remarks

7 4/6/67 24 55 54.5 Damp Tested partially-grooved section2000 feet south end of runway.Standard tests at standard tirepressure.

8 4/20/67 24 45 - 47 56.8 Dimp Tested partially-grooved section4000 feet south end of runvay.Standard teats at standard tirepressure,

9 5/4167 40 45 - 48 59.9 - 64.4 Dry First 28 runs standard test atstandard tire pressure, last 12tests conducted at variable tirepressures.

10 5/10/67 28 49 59 Dry Standard test runs at standardpressure.

11 5/23/67 28 52 - 5% 59 - 59.9 DVp Rain and fog prior to tests.Standard test runs at standardtire presoure.

12 9/14/67 28 56 - 57 74 - 75 Dry Standard test runs at standardtire pressure.

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smoothing of the traces is noted in the 30- and 50-mph post-groovingtests. The BFN1 3 values at 30 and 50 mph wet are also slightly higherafter grooving, while the 60-mph values are slightly lower.

Track 3 (25 feet west of centerline)- This track, like Track 1,is also in a highly rubber-contaminated area. The post-grooving tests,pages 1-9 through 1-12, show an unexplainable decrease in BFN1 3 valuesfor the 30- and 50-mph dry test runs and also for the 30-cph wet testrun. The remainder of the wet and dry test runs, however, indicates nosignificant decrease or increase in friction value, but, again, adefinite smoothing of the analog traces is evident in the post-groovefriction data.

Track 4 (25 feet east of centerline) - Track 4 is thecounterpart of Track 3 and, consequently, produces similar frictiondata as those from Track 3. This is borne out by an analysis of thedata, pages 1-13 through 1-16.

Damp Runway Surface Conditions - March 16. 1967. and May 23, 1967Tests: Comparing the pre-grooved tests conducted on March 16, 1967, withthe post-grooved tests conducted on May 23, 1967, both for similar damprunway environmental conditions, the following results are noted fromthe test data shown in Appendix II:

Track 1 (centerline) - The BFN1 3 values for the post-groovingdry tests for the 10- and 30-mph runs are higher than the pre-groovingvalues and all the traces are smoother, pages 2-1 and 2-2. The valuesfor the wet tests, pages 2-3 and 2-4, indicated that, although thecu.rve shapes are similar, there was no major change in BFN1 3 valuese'ccept that a post-grooving decrease is noted in the 60-mph tests.Again, the post-grooving traces are smoother.

Track 2 (runway edge) - These test data, pages 2-5 through2-8, indicate that the BFN1 3 values and curve shapes are almostidentical for the pre- and post-grooving wet and dry tests and, again,a definite smoothing of the analog friction traces is noted, especiallyfor the wet tests. A slight post-grooving decrease in friction valuesis noted for the 60-mph wet tests and the 50-mph dry tests.

Track 3 (25 feet west of centerline) - The test data for thistrack, pages 2-9 through 2-12, show that the after-grooving BFN13 valuesfor the wet and dry tests were generally lower than the pre-grooved valuesexcept for the 10-mph dry tests. Again, the curve shapes are similar andthe post-grooving traces smoother.

Track 4 (25 feet east of centerline) - Track 4 is the

counterpart of Track 3 and, consequently, has produced data similar tothose found in analyzing Track 3, with the exception that the 10-mphdry tests produced no significant change of friction values. Again, theanalog traces are similar and the post-groove traces exhibit moresmoothness.

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Runway Contamination Comparative Tests - Dry Surface ConditionsMay 10, 1967, and September 14, 1967 Tests: It was noted while conductingthe immediate post-grooving tests that a dust-like residue was present.These dust deposits, resulting from the grooving operation, were foundprimarily along the edges of the runway. To determine if the post-groovingfriction values were influenced by this contaminant, another series offriction tests was conducted on Sertember 14, 1967, approximately 4months after grooving. These test results were compared to the post-grooving friction values obtained on May 10, 1967, which were conductedunder similar runway environmental conditions, shown in Appendix III.No significant changes of friction values or trace shape were observed,

thus indicating that groove dust did not influence test results to any

appreciable degree. These September trace6 were also compared to thepost-grooving analog traces obtained May 4, 1967, contained in Appendix I,pages 1-2, 1-4, 1-6, 1-8, 1-10, 1-12, 1-14, and 1-16, with similar results.

Low Tire Pressure Tests

Tests were run with low tire pressures in an attempt to assess theeffects of grooves with respect to the phenomenon of hydroplaning. Theresults indicated that lowering the tire pressure had little or noeffect on the wet runway friction values obtained. This agrees withthe results obtained 2t NASA Langley that slip frictional forces are

only slightly affected by inflation pressures (Reference 3).

It will be noted from the oscillograph records iN Appendix IV

that the friction force trace3 did not indicate low friction valuesassociated with hydroplaning either before or after grooving. Theanalog curves are very similar both in shape and magnitude for thevarious tire pressures, and if hydroplaning or partial hydroplaning hadoccurred, lower BFN1 3 values would have been recorded. Decreasing tirepressure or increasing speed are the only two factors governinghydroplaning that could be readily controlled during the t jsts. Thereare other important factors, however, which govern hydroplaning andwhich have to be taken into consideration; such as, ample water depthand surface drainage. After the first attempt to achieve hydroplaningby lowering tire pressura failed (Fnbrupzy 3, 19G7), it vies irdicated bythe data that sufficient water depth was not present. The output of thewater dispensing system was then approximately doubled by changing pump

drive pulleys. This small increase in water depth, however, still didnot provide any evidence of hydroplaning.

During the rain environment tests conducted February 21, 1967,hydroplaning tests with low tire preosures in conjunction with the highcapacity water dispensing system provided the deepest water testcondition of this series. These teats also proved futile !n producing

evidence of hydroplaning. It must be noted that in the above tests,the standard 24-psi tire was used, and at the lower inflation pressures(for which it was not designed) an elongated footprint resulted whichcould have reduced the onet of hydroplaning.

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The lowering of tire pressure affected both the fixed slip ratioof ;he braked test tire as well as the unit-bearing pressure, the netresult being to increase slip ratio and, correspondingly, decrease theaverage unit-bearing pressure. The slip ratio and average bearingpressure for the 24-psi tire was 12.8 percent and 39 psi; for the17.4-psi tire, 13.5 percent and 33 psi; for the 14.3-psi tire,14.1 percent and 31 psi; and for the 11.6-psi tire, 15.0 percent and28 psi, respectively.

Calibration

The analysis of the calibration records made prior to ecch series of Itests and past records indicate that this tester produces calibrationresults of less than + 3 percent deviation.

SUMMARY OF RESULTS

In summary, these friction tests indicate that at test speeds from10 to 60 mph and with the test tire loading and pressure inherent inthis friction measuring system, no major increaso in wet runway frictionvalues, due to these grooves, were observed. Markedly smoother wet runway friction traces were obtained, however, in the post-grooved data *acompared to the pre-grooved data. These post-grooved data resembledthe pre-grooved data of the unu3ed runway edge, Track 2, thus indicatingthat grooving may _:nd to restore the runway surface to friction valuesapproaching its original clean and homogenized state. It is evidentfrom the data that uniform spacing of grooves has created a homogenizingeffect which, in turn, produced a more uniform friction surface. Bygrooving a runway and reducing the magnitude and the amount of thefluctuations in friction coefficient, it is hypothesized that a moreeffective braking surface is produced. The smaller changes in frictionshould generate smaller fluctuations in braking forces, ultimatelyresulting in shorter stopping distances, Most aircraft antiskidsystems operate on the principle of modulating brake presstire uponsensing incipient skid conditions. Constant application of brakepressure as opposed to intermittent brake application will stop anaircraft in shorter distances.

For comparative purposes, the test portion of the runway wasdivided into three lengthwise sections. These'three sections were the1000-foot sections at the 36 and the 18 ends, and the 2870-foot centersection. The 1000-foot end sections were located primarily in the heaviestrubber-contaminated portion of the tested runway where definite changes offriction usually occur. The approximate percentage increase and decreasein measured friction values have been calculated, and the results arecontained in Tables II, III, and IV. The random increases, decreases, andzero changes present no set pattern from test to test and also indicateno overall significant change of friction values due to grooving.

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TABLE II

APPROXIMATE PERCFXT CHANGES IN MEASUReD FRICTION VALUES DUE TO GROOVINGDECEMBER 10, 1966, AND MAY 4, 1967 TESTS, DRY RUNWAY SURFACE CONDITIONS

DRY TEST WET TESTTest 1000' Test 2870' Test 1000' Test 1000' Test 2870' Test 1000' TestSpeed Sec.36 End Center Sec. Sec.18 End Sec.36 End Center Sec. Sec.18 End(mph) (percent) (percent) (percent) (percent) (percent) (percent)

TRACK 1 - CENTERLINE

10 +7 +5 +6 +5 +3 +11

30 -3 +3 +3 +14 +11 +42

50 0 +2 0 +88 +35 +76

60 -28 +3 +50

TRACK 2 - EAST RUNWAY EDGE

10 +7 +6 +5 +4 0 +3

30 -7 -8 -8 +8 +3 0

50 +9 +13 0 +23 +14 +15

60 -7 -6 -8

TRACK 3 - 25 FEET WEST OF CEUERLINE

10 +7 +5 0 -6 -6 -6

30 -7 -26 -32 -13 -26 -36

.50 -21 -14 -18 0 -8 +5

60 0 +3 -9

TRACK 4 - 25 FEET EAST OF CENTERLINE

10 +3 +3 -9 0 -5 -8

30 -21 -20 -22 -49 -37 -47

50 -10 -12 -8 0 -10 -11

60 0 0 -7

Note: (+) Denotes an after-grooving increase(-) Denotes an after-grooving decreace(0) Denotes no significant increase or decrease

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TABLE III

APPROXIMATE PERCENT CHANGES IN MEASURED FRICTION VALMS DUE TO GROOVINGMARCH 16, 1967, AND MAY 23, 1967 TESTS, DAMP &ZAWAY SURFACE CONDITIONS

DRY TEST WET TESTTest 1000' Test 2870' Test 1000' Test 1000''Test 2870' Test 1000' TestSpeed Sec.36 End Center Sec. Sec.18 End Sec.36 End Center Sec. Sec.18 End

(mph) (percent) (perent) (percent) (percent) (percent) (percent)

TRACK 1 - CENTERLINE

10 +33 +24 +26 +9 0

30 -9 +14 -5 -5 -8 +15

50 +8 0 0 +5 -4 +29

60 +4 -14 +17

TRACK 2 - EAST RUNWAY EWGE

10 +5 +5 +5 -6 -7 -10

30 0 0 0 0 0 -12

50 -9 -12 -9 +9 +4 0

60 -18 -11 -17

TRACK 3 - 25 FECT WEST OF CXNTZRLINE

10 0 +17 +15 +10 -22 -17

30 -10 0 .- 6 -11 -21 -9

50 -13 -12 -10 +11 -19 0

60 -23 -20 0

TRACK 4 - 25 FEET EAST OF CENTERLINE

10 --5 0 0 -15 -13 -10

30 -26 -22 -29 -18 -18 -20

50 -6 ".8 -9 +7 -12 -15

60 +24 0 -15

Note: (+) Denotes an after-grooving increase(-) Denotes an after-grooving decrease(0) Denotes no hignif.ctnt increase or decrease

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TABU IV

APPS a"E PCM CF.UIGES 3 YEASUPD -RIciOe VALLES BEI:S'.-GRiOMM TESTS MAY 10, 197, AM SEPTEXBM 14, 1967 TMST'

DWY RWWAY SVYE CMITIMS

DuY TEST WET TESTTest 1000" Test 2870' Test 1000' Test 1000' Test 2870' Test 1O0C' Test

Speed Sec.36 End Center Sec. Sec.18 End Sec.36 End Center Sec. Sec.18 End( ph) (percent) (pe-cntz) (percent) (percent) (percent) (percent)

TRAC I - CLMMUE

10 -13 -10 -13 -5 -7 -7

30 -5 +3 +3 0 0 0

50 -8 +8 +15 -60 -13 0

60 -70 -3 0

TRACK 2 - EAST RUNAY EWE

10 +3 0 -2 +2 -3 -9

30 +4 +5 +2 +5 +3 -8

50 +27 +20 0 +54 +24 +7

60 +15 +17 +33

TRACK 3 - 25 FEET WEST OF CENTERLINE

10 0 0 +3 +4 +3 +2

30 0 +6 +9 -20 -t3 -6

50 -25 +10 +16 -56 0 0

60 -50 0 0

TRACK 4 - 25 FEET EAST OF CENTERLIzE

10 -6 -6 +6 +9 +7 +4

30 +9 0 -27 +10 +5 -26

50 +21 +8 -21 +11 0 -48

60 0 +9 0

Note: (+) Denotes a September 14 increase(-) Denotes a September 14 decrease(0) Denotes no significant increase or decrease

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The friction values of Runway 18-36, prior to grooving, were inthe high range; i.e., above 50 BlNI13 . The cutting of grooves did not +materially improve this already high friction condition. Moreover,grooving is accomplished primarily to substantially aid in theprevention of hydroplaning, Reference 1, and not necessarily as ameans to improve friction.

A study of the comparison of the data for the same track for wetand dry, and for before and after grooving contained in Appendices I,II, and III, revealed a striking similarity in the analog trace shapeand, to a lesser degree, magnitude. Variance in magnitude can beattributed to deviation of test path, tire and pavement temperatures,environmental effects, and other variables. Since these testsencompassed a period of 9 months, the results attest to the reliability,dependability, and repeatability of the data obtained with thisfriction measuring system.

17I _ _ _ _ _ _ _ _ _ _,-

CONCLUSIONS

Based upon analysis of the results of these tests, it is concludedthat:

1. There is no appreciable increase or decrease in the overallfriction values of Runway 18-36 at Warshington National Airport beforeand after runway grooving (1/8-inch wide by 1/8-inch deep at 1 inchspacing) based on the wet and dry friction data obtained at test speedsof 10 to 60 mph; the test conditions and the limitations of the equipmentused in these tests did not produce any evidences of hydroplaning effects.

2. A more homogeneous friction surface due to grooving isindicated by the wet post-grooved analog friction traces being smootherthan the pre-grooved values; i.e., fewer oscillations and of lesseramplitude.

3. The braking effectiveness on wet runway surfaces islikely to be improved by the homogeneous friction surfaces created byrunway grooving.

4. The FAA's Fixed Slip Runway Friction Tester is a reliablefriction-measuring system which has the capability of producingrepeatable friction data under similar test conditions.

RECOMENDATIONS

It is recommended that:

1. Friction tests be conducted at speeds up to 80 mph andhigher, if obtainable.

2. Aircraft braking tests be conducted to determine theeffectiveness of transverse grooves in reducing stopping distances.

-* 18

REFERENCES

1. Home, W. B. and Lelend, T. J. W., "Influence of Tire Tread Patternand Runway Surfaice Condition on Braking Friction and Rolling Resistanceof a Modern Aircraft Tire," Langley Research Center, NASA, September 1962,Technical Note D-1376, Page 29.

2. Home, W. B., Yager, T. J., and Taylor, G. R., "Recent Research onWays tu Improve Tire Traction on Water, Slush or Ice," AIAA Meeting,Los Angeles, November 1965, Page 28.

3. Kumer, H. W., "Unified Theory of Rubber and Tire Friction,"Pennsylvania State University, College of Engineering, EngineeringResearch Bulletin, B-94, July 1966, Page 119.

19

- -- --

APPENDIX I

FRICTION DATA DRY1 RUNWAY SURFACE CONDITIONS-DkECGtR 10,. 1966, AND MAY 4, 1967 TESTS

B G V a T 1

RNolTC .1 MDRY TS

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APPENDIX Il

PRICrION DATA -DAMP RUNWAk SURFACE CONDITIONSMRCH 16, 1967, AND) MAY 23, 1967 TESTS

7X

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iI

I,APPENDIX III

FRICTION DATA - RUNWAY CQONAMINATION COMPARATIVE TESTS - jMRy co FAcE CON'DITIONS - MAY 10, 1967, .AIM SET 14, 1967 TESTS

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APPENDIX rv

LO)W TIRE PRESSURE FRICTION DATA

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