MI-A FEDERAL. AVIATION ADMINISTRATION TECHN4ICAL CENTER ATL--ETC FAB I6 THE MRAKING PERFORMANCE OF AN AIRCRAFT TIRE ON MROVED PORTLAD-ET UNL0

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1 MI-A FEDERAL. AVIATION ADMINISTRATION TECHN4ICAL CENTER ATL--ETC FAB I6 THE MRAKING PERFORMANCE OF AN AIRCRAFT TIRE ON MROVED PORTLAD-ET UNCLASSIFIED UNL0 0 JAN 81 S K AGRAWAL, H DAZUTOLO FAA-CT FARO-80-78N

2 Report No. FAA-RD FAA-CT THE BRAKING PERFORMANCE OF AN AIRCRAFT TIRE ON GROOVED PORTLAND CEMENT CONCRETE SURFACES Satish K. Agrawal Bector Daiutolo FEDERAL AVIATION ADMINISTRATION TECHNICAL CENTER Atlantic City Airport, N.J. 084b5 INTERIM REPORT JANUARY C Lj Prepared for U.S. DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION Systems Research & Development Service Washington, C

3 NOTICE This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for the contents or use thereof. The United States Government does not endorse products or manufacturers. Trade or manufacturer's names appear herein solely because they are considered essential to the object of this report. I I IIl ll I I II II I g

4 1. Report*-.--.-' Technical Report Documentation Page S 2. Gover ment Accession No. 3. Recipent's Catalog No. D A,ROMNEOF ANbRCRFT f;ire ON GROOVED PORTLAND C EMFNT COITCRETE SURVFACES" a Lo.., Performing Orgcnizatidn Code., 8. Performing Organization Report No. 7. Author,$) ' P rnorn g Organization Name and Addreis" Federal Aviation Administration Technical Center D1. Contract or Grant No. Atlantic City Airport, New Jersey Intrducionof.ransers o grove of aod Rrpowa ai Pbi 12.Sponsor.ing Agency Nam.... d Address., ' Coerfmd U.. Department of Transportation in wet wah conitions Interimlepswt...t Federal Aviation Adminisraation (F) h rocteomume -n77 1Jul/-i Systems Research and Development Se rvic instl14..r w ycosh th otenti Washington, D.C H ARD Supplementary NotesI 16. Abstract.- Introduction e of tansverse grooves on runways improves braking and cornering performance of aircraft during operations in wet weather cortnd helps to alleviatenes hydroplaning. The Federal Aviation Administration (FAA) has recommended /4-inch square grooves spaced at -1/4 inches for installation on runways where the potential of hydroplaning exists. However, a large number of runways remain nongrooved. The major reasons are the high cost of groove installation and limited evidence as to the effectiveness of the grooved speeds of modern aircraft. The findings of the research described in this report indicate that by increasing the spacing of the conventional saw-cut grooves (in the portland cement concrete surfaces) p to 3 inches, groove installation cost can be redu cen copabl to tht compared to the installation cost of grooves spaced at inches. t-/4 The results further show that the friction levels available on these grooves under wet operating conditions are not significantly below those attained on grooves spaced at 1-1/4 inches. These results are valid for operating speeds of up to 150 knots. The results also show that a reflex-percussive cutting process is an alternativ e Ntonlecinstallation technique that produces V-grooves which provide braking performance comparable to that of conventional saw-cut grooves. The installation cost of these alternative grooves can be substanially less than that of saw-cut grooves.., 17. Key Words 18. Distribution Statement lunway Grooves, Portland Cement Concrete, Document is available to the U.S. public ;roove Spacing, Wet Runways, Braking through the National Technical Information?erformance, Hydroplaning, Groove Service, Springfield, Virginia nstallation Cost, Saw-Cut Grooves, eflex-percussive Groov of ag*s 22. tpr c 19. Security Classif. (of this report) 20. Security Classif. (of this pagel 21. No. Unclassified Unclassified 41 b Form DOT F (8-72) Reproduction of completed page authorized

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6 PREFACE The work described in this report was undertaken and accomplished in response to a request for research, development, and engineering effort from the Office of Airport Standards in the Federal Aviation Administration (FAA). The Airport Development Division of the Systems Research and Development Service provided program direction. The Naval Air Engineering Center, Lakehurst, N.J., provided test facility operation and data acquisition. L!4 iii

7 TABLE OF CONTENTS INTRODUCT ION 1 OBJECTIVES 2 SCOPE OF THE INVESTIGATION 2 EXPERIMENTAL PROGRAM Page Test Facility and Equipment 7 Test Sections 7 Test Parameters 13 Test Procedure 16 Data Collection and Analysis 16 DISCUSSION OF RESULTS 18 Conventional Saw-Cut Grooves 18 Reflex-Percussive V-Grooves 29 Cost and Performance Analysis 29 CONCLUSIONS 30 REFERENCES 3.1 APPENDIX V I1W=NUG PAGE a.azc-±0t F1lk"L

8 LIST OF ILLUSTRATIONS Figure Page 1 Grooving Machine Used for Cutting 1/4-In. Wide by 1/4-In. Deep 4 Grooves at Various Pitches in the Test Sections 2 Machine for Cutting Reflex-Percussive Grooves in the Test 5 Sect ions 3 Comparison of Grooves Produced by High-Speed Water Jet Technique, 6 by Saw-Cutting Technique, and by Reflex-Percussive Process 4 Jet-Powered Pusher Car for Providing Preselected Speeds to Test 8 Equipment 5 Dynamometer and Wheel Assembly Showing Vertical and Horizontal 9 Load Links 6 Hydraulic System for Applying Vertical Force on the Test Tire 10 7 Four Sections of the 200-Ft. Test Bed 11 8 Schematic of Grooved and Nongrooved Sections 12 9 Geometry and Dimensions of Various Grooves Tested A Typical Data Trace for a Braking Test 17 II Coefficient of Friction as a Function of Speed Under Various Test 19 Conditions on Saw-Cut Grooves and Nongrooved Concrete (4 Sheets) 12 Coefficient of Friction as a Function of Speed Under Various Test 23 Conditions on V-Grooves 13 Performance of Worn and New Tires on Saw-Cut Grooves and V-Grooves 24 Under Wet Conditions 14 Comparison of the Braking Performance of Worn and New Tires on All 25 Grooves Tested 15 Comparison of Attainable Speeds for a Constant Friction Level on 27 Saw-Cut Grooves 16 Braking Performance and Estimated Grooving Cost as a Function of 28 Groove Spacing vi

9 INTRODUCTION alternatively used to identify whether the density and viscous effects, respectively, are predominant. The braking and cornering performance of an aircraft during landing operations During dynamic hydroplaning, the tire depends upon the magnitude of the surface in the footprint is deformed horizontal forces developed at tire/ by the rapid buildup of hydrodynamic runway interface. This interface is forces; the space so created is filled commonly referred to as the footprint, with water which cannot escape. The The ratio of the horizontal forces in the rapid buildup of these forces can be footprint and the vertical force on the eliminated if bulk water is removed from tire is defined as the coefficient of the tire path. Runway grooves provide friction. The braking performance is escape paths for water during the tire controlled by the horizontal force in the passage over the runway. For viscous direction of tire travel, and the corner- hydroplaning to be alleviated, sharp ing performance is determined by the runway microtexture that can penetrate horizontal force in the direction through the thin viscous film of water perpendicular to the direction of tire is required. travel. Introduction of transverse grooves cut The magnitude of the coefficient of into the runway surface was first friction is influenced by many initiated by British researchers in parameters. The important parameters 1956 (reference 1). Isolated puddles are: speed of operation, runway surface that are likely to be formed on nontexture and drainage capacity, contam- grooved surfaces because of uneven inants, condition of tire tread, and the surface profile are generally not braking system characteristics. The visible when the same surface is grooved. presence of water on the runways This is particularly significant where adversely affects the level of friction large temperature variations may cause available for aircraft speed and direc- low magnitude undulations in the runway tional control (braking and cornering), surface. An extreme case of loss of control is hydroplaning. Grooves are identified by pitch, width, and depth. The pitch is the distance Hydroplaning is a peculiar tire-to-runway between groove centerlines and is often condition where the physical contact referred to as groove spacing. between the tires and the runway is lost, and the tires are supported on the Pavement grooves were extensively studied intervening layer of water. The separa- by the National Aeronautics and Space tion of the tire from the runway results Administration (NASA) in 1962 and 1964 when the sum of the forces from the (reference 2). In 1967, NASA conhydrodynamic and the viscous pressures in ducted another experimental investigation the footprint exceed the vertical force to determine the groove configuration on the tire. Hydrodynamic forces result that provided the best cornering and from fluid density effects and are braking performance under wet operating predominant where large water depths are conditions (reference 3). Three pitches present on the runway. Viscous forces were used: 1 inch, 1-1/2 inches, and 2 result from fluid viscosity effects and inches. Various groove widths and depths are predominant where a thin water film were included in the test program. NASA is present on a smooth runway. In all concluded that all groove configurations cases, however, both effects are pres- provided improved cornering and braking ent to some degree. Dynamic hydro- performances relative to nongrooved planing and viscous hydroplaning are surfaces; however, the 1-inch by 1/4-inch

10 by 1/4-inch groove configuration provided OBJECTIVES the greatest increase in available ftiction (reference 3). Based on these and further tests by NASA (references 4 There are three objectives of this test and 5), the Federal Aviation Administra- program. They are: tion (FAA) has recommended (reference 6) a standard 1-1/4-inch by 1/4-inch by 1. Determination of a cost-effective 1/4-inch groove configuration and has configuration for the conventional encouraged airport operators, managers, saw-cut grooves on pcc. and owners to groove runways where the potential of hydroplaning exists. 2. Evaluation of alternative groove installation techniques competitive to Regardless of the fact that runway saw-cutting; and grooves improve braking and cornering performance of aircraft and help 3. Evaluation of the performance of alleviate hydroplaning, a large number of alternative grooves relative to saw-cut runways have not been grooved. The major grooves on pcc. deterrents to the use and acceptability of runway grooves are the high installation cost at.d only limited evidence as to SCOPE OF THE INVESTIGATION the effectiveness of grooved surfaces at the touchdown speeds of air-carrier jet aircraft. Low grooving cost and acceptable braking performance are the two key factors in The cost of groove installation can be determining a cost-effective configurasubstantially reduced (reference 7) by tion for the saw-cut grooves. The term increasing the groove spacing beyond the "acceptable braking performance" is subcurrently recommended (reference 6) jective and is so treated in this study. value. However, the effectiveness of As such, the acceptable performance is these grooves in terms of braking per- defined as follows: formance of the aircraft/tire/runway system may also be affected. This The available friction levels on waterrelationship between cost and effective- covered surfaces which have grooves ness is not known. The research de- installed at pitches in excess of 1-1/4 scribed in this document attempts to inches are significantly higher than on provide information about the cost- the nongrooved surfaces and are not effectiveness of grooves of various significantly lower than on the 1-1,4- pitches on the portland cement concrete inch pitch grooves currently recommended (pcc) surfaces. The research effort is (reference 6). limited to evaluation of the braking performance on water-covered surfaces; The groove configurations which satisfy operation on dry surfaces is relatively the above definition are then evaluated safer even when the surfaces are not in terms of grooving costs. The congrooved. As an alternative to the con- figurations which provide acceptable ventional saw-cutting technique, other performance and low grooving cost will be groove installation methods were also identified as cost-effective. In this investigated as part of this research, evaluation process, it is possible that more than one configuration may be identified as cost-effective. 2

11 A new groove configuration can be mainly elastic, and it is almost immeobtained by changing the pitch, the diately given up in generating a rebound width, or the depth of the groove, and and causes the concrete to attempt the braking performance of each new to pass through its relaxed state into groove configuration could then be one of tension nearly equal to the evaluated. However, the cost of such a initial compression. However, being very vast program would be prohibitive, weak in tension, the concrete fractures Therefore, in order to rationalize the and elastic energy is given up as kinetic basis on which to choose the groove energy of the flying fragments. The dimension or dimensions that should great advantage of this method of cutting be varied in the experimental program, a is its ability of not loosening the construction cost consultant was con- aggregate particles within the matrix or tracted to examine the cost savings creating micro fractures in the undamaged available on different groove sizes surrounding concrete. This technique and pitches. Grooving costs were sampled provides a nonsymmetrical V-shaped groove in the Northeastern, Midwestern, and cross section. The grooving machine is Southwestern United States by the firm shown in figure 2. of Edward Sharf and Sons of Washington, D.C. (reference 7). Their investigation The inclusion of these grooves in the concluded that increasing the groove experimental program was based on two pitch has significantly more cost-saving factors. First, the cost-saving potenpotential than changing the groove size. tial of this process was high because Their results are valid for both the pcc of the high operating speed of the surfaces and the asphaltic concrete sur- machine and the longer life of replacefaces. The experimental program ment parts. Furthermore, the Canadian described in this document, therefore, manufacturer, who held the patent for the included the groove pitch as the major application of the percussive cutting test variable. Various groove pitches process for grooving, had demonstrated between 1-1/4 inches and 4 inches were the economics associated with the process included in the program. Nongrooved during resurfacing of part of an surfaces were also included for determin- operating runway. Second, the evaluation ing the relative improvement in per- of two other potential grooving techformance available as a result of niques showed them to be unsatisfactory grooving. A machine for installing saw- for further consideration either because cut grooves was developed by the U.S. of the nonuniformity of the grooves so Navy for the test program and is shown produced or because of chipping and in figure 1, which also shows the 3-inch spalling of the surface resulting from by 1/4-inch by 1/4-inch groove con- the process. Figure 3 shows the groove figuration. configuration obtained by one of these techniques - a high speed water jet Evaluation of alternative grooving tech- cutting technique (reference 8). For niques is limited to a reflex-percussive comparison, saw-cut grooves and V- cutting process. The reflex-percussive grooves are also shown in this figure. method )f controlled concrete removal was The other tecbnique, which uses the recognized by the Concrete Society of principle of vibration kerfing, was tound Great Britain in This method was not to be a viable cost competitive first developed to obtain a very rough method for grooving because of inherent finish on the pavement. When the cutting weaknesses in obtaining perfect alignment head strikes the surface of the concrete, of the kerfing tool wnich resulted in a it causes the material directly under the bush hammer effect (reference q). area of impact to deflect downward, thus creating a momentary and localized This program considered only the pcc compression. The compressive strain is surfaces. Testing on asphaltic concrete 3

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14 ,, *...- 1', :,--,- I - _ GROOVES PRODUCED GROOVES PRODUCED BY SAW CUTTING BY WATER-JET CUTTING -4r- o o,.. 4, '.,4....:. "J._ / GROOVES PRODUCED BY REFLEX-PERCUSSIVE PROCESS FIGURE 3. COMPARISON OF GROOVES PRODUCED BY HIGH-SPEED WATER JET TECHNIQUE, BY SAW-CUTTING TECHNIQUE, AND BY REFLEX-PERCUSSIVE PROCESS

15 surfaces will be accomplished in a (reference 10). Figure 5 shows the subsequent program. The terms "conven- dynamometer and wheel assembly and the tional saw-cut grooves" or "saw-cut details of the instrumentation for grooves" and "reflex-percussive measuring vertical and horizontal loads V-grooves" or "V-grooves" will frequently at the axle. The assembly is pivoted be used in the remainder of this report about an axis contained in the dead-load to identify the two distinctly different carriage. The carriage weighs 60,000 types of grooves investigated in this pounds. Figure 6 shows the hydraulic study. system for applying vertical load on the test tire. The hydraulic fluid in this system i3 forced into the cylinders EXPERIMENTAL PROGRAM by pressurized nitrogen; the braking system is also activated by pressurized nitrogen. In each system, pressurized TEST FACILITY AND EQUIPMENT. nitrogen is released by the action of a solenoid. The test program was conducted at the Naval Air Engineering Center, Lakehurst, The dynamometer is instrumented to N.J. Track No. 1 at this facility was measure the vertical load on the tire, jointly developed by the FAA and the U.S. the horizontal force developed at the Navy and has the capability of simulating tire/pavement interface, the angular a jet transport tire-wheel assembly under velocity of the test tire, and the touchdown and rollout conditions. Test vertical motion of the dynamometer speeds of up to 150 knots can be attained assembly relative to the dead-load on this test track. The track is I mile carriage. The water depth on the test long with guide rails spaced 52-1/2 bed was measured by the use of a NASA inches apart. Reinforced concrete strips water level depth gage. extend beyond the rails to a width of 28 feet. A four-wheeled jet car, powered TEST SECTIONS. with J48-P-8 aircraft engines (total thrust of 24,000 pounds), is used to The 200-foot test bed (figure 7) at the propel test equipment along the track recovery end of the mile-long test from the launch end at a preselected track was divided into five sections. speed. The jet car is disengaged when The first section was 20 feet long and the test speed is attained, and the test was intended for ensuring proper approa-h equipment is allowed to coast at this of the test wheel into the test section; speed into the test sections. The last the remaining 180 feet were divided into 200 feet of the track are used for in- four 45-foot sections. The dimensional stalling the test bed. The pcc test bed tolerance of the test surface was held is 30 inches wide and 5 inches thick, within ± 1/8 inch from horizontal An aircraft arresting system is located level throughout the test bed. The test beyond the test bed to recover the test sections were numbered from 0 to 4 with equipment at the completion of a test 0 representing the 20-foot section. The run. Figure 4 shows part of the jet car last section remained nongrooved and guide rails and components of the throughout the test program; sections I test equipment. through 3 were grooved. Thus, it was possible to compare the performance of The major components of the test equip- three differently-grooved surfaces and ment are: the dynamometer and wheel the nongrooved surface in one test run. assembly, the dead-load carriage which Figure 8 shows the schematic of grooved supports the dynamometer assembly, and nongrooved sections on each test bed. and the measuring system. The dynamometer Testing was completed on bed number I is similar in design to the one developed before proceeding to bed number 2 and by NASA for the Langley test facility to subsequent beds. Since the same 200 7

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21 feet of the track were used for all the 1-1/4 inches, 1-1/2 inches, 2 inches, test beds, the configuration shown for 2-1/2 inches, 3 inches, and 4 inches. bed number 4 was obtained after removing The cross-sectional dimensions of these the grooves shown for bed number 3. The grooves were 1/4 inch wide and 1/4 grooves were removed by the reflex- inch deep (figure 9). The geometry and percussive process. dimensions of the percussive V-grooves are also shown in figure 9. Nongrooved TEST PARAMETERS. concrete surfaces provided the baselines for performance comparison of each of the Four types of parameters were inves- two types of grooves. The texture depth tigated in the test program: tire, of the base surface for the saw-cut pavement, environmental, and opera- grooves and the V-grooves was inch tional. The magnitudes of these and inch, respectively. The parameters were carefully selected. increased texture depth for the latter The primary criterion was to choose a was a result of resurfacing by the value for a given parameter such that percussive process. it represented a value widely used or encountered by airlines or aircraft. Water depth was the only enviromental parameter applied in the study for Among the various tire parameters, the performance comparison. The average important ones are: the size, the ver- water depths on the test sections tical load, the inflation pressure, and ranged from the "wet" condition to the the tread design. All the test tires "flooded" condition. For the purposes of 2 were 49x17, 6 -ply rating, type VII. this study, a flooded condition indicates These tires are used on both the average water depths between 0.17 Boeing-727 and the Boeing-747 aircraft inch and 0.32 inch; average water and represent a large population of the depths between 0.02 inch and 0.16 tires used by the airline industry. inch are classified as puddled condition; To include the effects of tire tread anj average water depths below 0.02 inch design in terms of tread wear, two are referred to as the wet condition. extremes were selected - a completely worn tire and a fully treaded The operational parameters included the tire. Both the tires were recapped test speed and the mode of wheel operaexcept that the tread rubber was con- tion. The tests were run at speeds pletely worn out on one. The total between 70 and 150 knots. The wheel was vertical load on the tire in these tests braked for all the tests but generally was 35,000 pounds, a value representing held in the rotating mode. Where wheelthe average load on each landing wheel lock occurred, the data were not used. of a Boeing aircraft. The tire inflation pressure was maintained at 140 pounds/square inch, which represents the lower limit of the operational range of the Boeing-727 aircraft tires. With this lower inflation pressure, it was possible to initiate hydroplaning at relatively lower speeds. The pavement parameters included the type of surface, the groove spacings, and the microtexture of the surface. Only one type of surface - the pcc was employed in this study. Spacings for transverse, saw-cut grooves were 13

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23 The following is a summary of all the test parameters investigated in this research: Tire Parameters - Vertical Load Inflation Pressure Tread Design Tire Size/Type Static Tire Footprint : 35,000 lbs. 140 psi : Worn and fully treaded (six grooves) : 49x17, 26-Ply, type VII Worn 13 in. wide x 22 in. long Fully treaded 13 1/2 in. wide x 24 in. long Pavement Parameters - Type of Surface Microtexture (inches) Types of Grooves : pcc (broomed texture) (reflex-percussive texture) : Conventional saw-cut and percussive V-grooves Groove Spacings 1-1/4 in., 1-1/2 in., 2 in., (Saw-Cut only) 2-1/2 in., 3 in., 4 in. Environmental Parameters - Water Depths Less than 0.02 in. Wet in. Puddled in. Flooded Operational Parameters - Wheel Operation : Rolling to locked Brake Pressure : 300 psi - 1,800 psi Speeds : 70 knots knots 15

24 TEST PROCEDURE. Two persons were responsible for obtaining desired water depths in the test Since the test bed contained four sec- sections. At the launch end, two persons tionb (not counting the initial 20-foot were responsible for starting the engines section), four data points were collected and releasing the jet car. One more during each test run. The vertical load person was responsible for setting the on the tire and tire inflation pressure engine performance level, the vertical were held constant for all tests. load level, and the brake pressure level. On the recovery end, additional persons One complete braking test consisted of were responsible for the safe operation the following steps: of the arresting cable system to recover and return the test equipment to the 1. Test tire was selected and checked launch end for the next run. Data were for inflation pressure. transmitted from the dynamometer to the instrumentation room (located near the 2. Desired water depth was obtained on track) by the use of telemetry; three the test sections. technicians collected the data. The entire operation was under the control of 3. Jet engines were started at the the program manager and the site officer. launch end of the track and set at the performance level to provide the pre- In order to obtain the maximum friction selected speed in the test section. level available at each speed, multiple tests were conducted with gradually 4. Jet car was released to propel the increasing brake pressure. The speeds test equipment (dead load and dynamom- were kept constant in these multiple eter). The test tire remained in a tests, and the brake pressure was infree-rolling state during this maneuver, creased until wheel-lock occurred. 5. Jet car was braked and separated from DATA COLLECTION AND ANALYSIS. the test equipment several hundred feet ahead of the test section. This allowed The data were collected in the form of the dead load and dynamometer to enter analog traces. Typical data collected the first test section at the preselected in a test are shown in figure 10. This speed. The test speeds in the remaining figure shows two traces each for sections were within I to 2 knots of the horizontal force and vertical load on the speed in the first section as computed tire. The coefficient of friction from the analog traces, was computed from these four traces by dividing the horizontal force by vertical 6. Before entering the first test load. The coefficient of friction trace section, the hydraulic systems were is also shown in figure 10. Wheel revoluactivated to apply the vertical load and tions were measured at two sensitivities the brake pressure on the tire. (The to monitor the wheel spin-down and to magnitude of each was preselected.) measure wheel slip if desired. The test Thus, the wheel entered the sections at speed was computed from time/distance preselected test conditions, traces. 7. As the wheel left the test bed, The results on pcc surfaces with and unloading and brake release were without grooves are shown in tables A-i initiated and the test equipment was through A-3 in the appendix. The coeffirecovered by the use of arresting cients of friction in these tables repcables, resent the maximum available under each set of operating conditions; many more Various steps of the complete test run tests were conducted to obtain the maxwere accomplished by different personnel. imum. A least-square fit was obtained 16

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26 between speed and coefficient of fric- available friction levels. Broken-line tion. The equation ( =AV - B ) for the curves in figure 13 plots (a) and (c) fit was considered appropriate because a show the friction levels obtained under majority of the data showed a linear these conditions. relationship on the logarithmic scale on both the speed and the friction When a new tire is operated on wet, coefficient axes. Table A-4 in the nongrooved surfaces, a more complex appendix shows the functional relation- mechanics takes place under the tire. ships between the coefficient of friction The viscous pressure under the tire (M) and speed (V). groove is lower than under the rib. This results in a lower integrated pressure Figure 11 shows the coefficient of under the tire and provides more contact friction as a function of speed for the between the tire and the concrete surpuddles and the flooded conditions on face. The friction levels available are saw-cut grooves and on nongrooved significantly increased (over those surfaces, and figure 12 shows similar obtained with a worn tire) as shown by relationships for tests on V-grooves. broken-line curves in figure 13 plots (b) Figure 13 shows results on wet surfaces. and d). Data from figures 11 and 12 are replotted in figure 14 to show the relative per- When the concrete surfaces are grooved, formance of the saw-cut grooves and the the performance of a worn tire under V-grooves. Limited data on nongrooved wet conditions improves significantly surfaces are also included in this when compared with nongrooved surfaces, figure. as shown in figure 13 plots (a) and (c); the performance of a new tire is also improved under similar conditions. DISCUSSION OF RESULTS However, the introduction of grooves on the surfaces renders the performance of a worn tire comparable to that of a new CONVENTIONAL SAW-CUT GROOVES. tire as shown by the solid-line curves in figure 13 plots (a) and (b) or in The basic characteristics of the friction figure 13 plots (c) and d). speed relationships in figures 11 through 13 indicate a drop in friction The data scattered around the solid-line with increasing speeds - a trend curves in figure 13 are not indicative of which has been well documented in the the effect of groove pitch or shape on past (reference 3). These results the available friction levels. Rather, verify the validity of the experimental they show the sensitivity of the coeffiprocedures of this research and com- cient of friction to changes in the water plement the findings of the past depth. It is relatively more difficult research, to control small water depths (0.0 to 0.02 inch) on the surfaces precisely; Wet runway surfaces are normally en- therefore, it is likely that the water countered during or after a light or depths are varying slightly from these moderate rain. These surfaces may be limits. saturated with water but would not have measurable water depth present on An overall observation from figure 13 can them. A worn tire operating on a wet, be summarized as follows: nongrooved surface represents a situation where predominantly viscous hydroplaning Where predominantly wet surfaces are may occur. Even when hydroplaning does encountered during aircraft operations, not occur, the viscous pressures in the the introduction of grooves on the contact are high and remain high even at concrete surfaces will render the braking a relatively low speed. The result is low performance of a worn tire comparable to 18

27 WORN TIRE in. PITCH I,- inl. PITCH lo-0 x 0( 0- (azh FLO ODED FLOE ()SPEED, KNOQTS (di) NEFW TIRE -in. PITCH f~- n. PITCH 0-0 (g) SPUED, KNOTS (hi) sn FIGURE 11. COEFFICIENT OF FRICTION AS A FUNCTION OF SPEED UNDER VARIOUS TEST3 CONDITIONS ON SAW-CUT GROOVES AND NONGROOVED CONCRETE (Sheet 1 of 4) 19

28 WORN TIRE 0 +S ()SPEUD. KNOTS (p) FIGURE 11. COEFFICIENT OF FRICTION AS A FUNCTION OF SPEED UNDER VARIOUS TEST CONDITIONS ON SAW-CUT GROOVES AND NONGROOVED CONCRETE (Sheet 2 of 4) 20

29 30-3 in. PITCH WORN TIRE4 inpic (q) (s) SPEED, KNOTS () NEW TIRE 0 ws w u) (v) 30 LODDLODDFLOE 'w 0 (w) SPEED), KNO~TS (X) FIGURE 11. COEFFICIENT OF FRICTION AS A FUNCTION OF SPEED UNDER VARIOUS TEST CONDITIONS ON SAW-CUT GROOVES AND NONGROOVED CONCRETE (Sheet 3 of 4) 21

30 rc NONGROOVED SURFACES 30- z WORN TIRE "1 NEW ' IRE PUDDLED OR FLOODED PUDDLED TEST SU'RFACE 20TEST SURFACE REFLEX-PERCUSSIVE TEXTURE S2RROOMED CONCRETE CONCRETE 10-0 Il USPEED, KNOTS (v) (z FIGURE 11. COEFFICIENT OF FRICTION AS A FUNCTION OF SPEED UNDER VARIOUS TEST CONDITIONS ON SAW-CUT GROOVES AND NONGROOVED CONCRETE (Sheet 4 of 4) the braking performance attainable with a Tire wear is, thus, an important factor new tire. The available friction levels during low-speed operations (70-90 knots) with both a new and a worn tire are on grooved, flooded surfaces. insensitive to changes in the pitch of saw-cut grooves or the orientation of The braking performance on nongrooved V-grooves. surfaces is poor under puddled and flooded conditions, and the probability The puddled surfaces are representative that hydroplaning may occur is always of conditions that can be expected high. The results on nongrooved surfaces immediately after heavy rains of short under puddled and flooded conditions with duration. Puddles can also form on a worn tire are shown in figure 14 plots poorly drained runways or where large (a) and (b). A coefficient of friction temperature variations produce undula- of only 0.05 was available when operating tions in the runway surface. In any speeds were below 100 knots. Above event, the puddles are generally not 100-knot operating speeds, the wheel was continuous in either the longitudinal or locked at all the braking pressures. A the lateral direction. The flooded run- friction coefficient of 0.05 is generally way conditions can be expected as a accepted as a level representing hydroresult of continuous, heavy rainfall, planing. The friction force corres- Braking performance on grooved surfaces ponding to a coefficient of friction of when puddled and flooded conditions are 0.05 is 5 percent of the vertical load on encountered are shown in figure 14; the the tire. The introduction of grooves on results on V-grooves and on nongrooved the surface has increased the available surfaces are also shown, friction from 0.05 to a maximum of The smallest increase occurs for a worn Comparison of figure 14 plots (a) and (c) tire operating on a flooded surface; the shows that for all groove spacings, the largest increase occurs for a new tire braking performance on puddled surfaces operating on a puddled surface. is improved with a new tire over a worn tire. This improvement is available over. While the use of new tires and grooved the entire range of operating speeds. On runways will shift the onset of hydrothe other hand, when flooded conditions planing to a higher speed, they cannot, are present, the new tire provides in all cases, completely eliminate it. gradually improving braking performances As the operating speeds increase, the as the operating speeds are reduced. time available for the fluid particles to 22

31 ' W FF E- El <D 00 F-- F-- 0z 0 C0 E c D L!J C) > 44 C)z -4 w 0.4 C) C 0 0 C/ mf-cs o aolj -' 0n >~.. -~23

32 0C n (I) z 0 u~ CDi GF41 C AJ IN30- I C24

33 vi' 4 '-44 II.'' 0-4 Ai.ip C,4.4 e zi Iu 00 X~/ NOLOTJ IT 0 0A30 25k

34 escape from the tire/runway contact area inches cannot provide a p of 0.15 decreases. Any increase in the number with worn tires even at low operating of escape paths, either by providing speeds (below 80 knots). This can be patterns in the tire tread or grooves in expected as the only channels for water the runway surface, cannot totally to escape from the contact area are compensate for the reduction in avail- provided by the grooves, and there are able time brought about by higher not enough of them. In this situation, operating speeds. Closer spacings th- smaller the groove spacing the between the grooves, however, will better the braking performance. Howprovide more discharge outlets to the ever, the consequence that groove water entrapped in the contact area. spacings beyond 2 inches cannot sustain a Although the number of discharge outlets friction level of 0.15 is of little will thus be increased, total surface importance. The worn tire condition and area resisting the water flow will also the flooded runway surface represent be increased. The reduction in time two extremes of tire wear and runway available for a fluid particle to go contamination, respectively. The from one discharge outlet to another likelihood of this combination being when the groove spacing is reduced from, present at runways is very small because for example, 3 inches down to 1-1/4 the tire tread would have to be cominches will be second at 100 pletely worn, and the runway would have knots operating speed. The question, to be fully covered with standing water. therefore, arises as to whether the If the grooved surfaces were puddled, the entrapped mass of water, owing to same worn tire could attain a friction its inertia, can respond rapidly enough level of 0.15 and provide an operating to show any significant changes in speed range of 120 knots to 80 knots braking performance on the two spacings between the groove spacings of 1-1/4 used in the above example. Clearly, inches and 4 inches, respectively. much larger spacings will have adverse effects on the braking performance at In general, as the groove spacing is higher operating speeds and when runways increased, the operating speed needed to are flooded. Figure 14 plot (d) seems to maintain a constant friction level of verify the above argument. The figure 0.15 decreases. The rate of decrease is shows a small drop in coefficient of smaller with a new tire. At any groove friction for 3-inch groove spacing spacing, the operating speed can be over 1-1/4-inch groove spacing. However, increased by replacing the worn tire no definite trend is identifiable for the with a new tire. direction in which the friction force is changing when the entire spectrum of The effect of groove spacing on the groove spacings is considered. braking performance of a worn or a new tire on puddled and flooded surfaces can If a coefficient of friction ( ]j ) of also be evaluated from figure 16. This 0.15 is arbitrarily chosen as a perform- figure shows the data from figure 14 ance level that is expected from any of replotted in an alternative manner; the groove configurations included in the effect of groove spacing is compared the test program, it is possible to in terms of maximum available coefficompare the braking performance on these cient of friction under various test grooves in terms of attainable speeds conditions. under various operating conditions. The data from figure 14 are replotted in In all cases, the friction coefficient figure 15 for a constant friction le-el decreases as the speed increases for of Certain observations can be all the groove spacings. The friction made from figure 15. Under flooded levels attainable on nongrooved surfaces conditions, the grooves spaced beyond 2 with worn tires approach the 26

35 -. 4- z z 0 c Si i aaa 27-4/

36 x 75 SAW-CIII (;RoOVES (''ST 'OF C 1,0 V- G-ROOVES,, SO0 p p r Ip SI (OF U CR'V' Cu RFE CS C IR"'E (, - R 'E ')EE N1t)S 100Cl" 30-FFF LAELOO F-~ ED ST SURFACE. TIRE W FITO'O 'IXUS% C to- 30- FLOODED TEST SURFACE, NEW TIRE to ~ rgroove SPACING (INCHES) 8n FIGURE 16. BRAKING PERFORMANCE AND ESTIMATED GROOVING COST AS A FUNCTION OF GROOVE SPACING 28

37 hydroplaning level ( p = 0.05) at better on a grooved, puddled surface operating speeds of 100 knots or less, than a new tire on a nongrooved surface. while the introduction of grooves increases both the level of friction The braking performance of the V-grooves available and the attainable speeds - on wet surfaces is shown in figure 13, the lower the operating speeds, the which also shows the braking performance higher the available friction level. The of saw-cut grooves. In general, the use of new tires provides an additional braking performance on these two types increase in the available friction of grooves is comparable. In addition, levels, the braking performance of a worn tire is significantly improved by the intro- When the effects of increasing the duction of grooves (figure 13 plot (c)). groove spacings under constant operating In fact, the performance of a worn tire conditions are compared, the overall and a new tire are comparable on grooved effect is a decrease in friction level surfaces as shown by solid-line curves with increasing spacings; however, the in figure 13 plots (c) and d). decrease cannot be classified as significant. If the operation with new tires The friction-speed curves from figure 12 is considered, increasing the spacing were redrawn in figure 14 for direct from 2 inches to 3 inches does not change comparison of the braking performance on the friction force at 150 knots. In the saw-cut and the V-grooves. It fact, the decrease in friction coeffi- can be seen that the performance on cient when the groove spacing is V-grooves is comparable to the performincreased from the current recommended ance of saw-cut grooves in most cases value of 1-1/4 inches to 2 inches or 3 (figure 14 plots (a) through (c)). inches is a maximum of 0.06 with new The only exception is the performance of tires operating on puddled or flooded a new tire on a flooded surface (figure surfaces. A slightly greater decrease 14 plot (d)). occurs with worn tires under similar operating conditions. Figure 16 shows the relative position of the V-grooves in terms of the braking R EFLEX-PERCUSSIVE V-GROOVES. performance of the saw-cut grooves. Generally, the performance of the The test results on the V-grooves are V-grooves corresponds to that of the shown in figures 12, 13, 14, and 16. saw-cut grooves spaced at 2 inches or Since the V-grooves have an unsym- less. metrical cross section, the tire encounters different flow conditions on COST AND PERFORMANCE ANALYSIS. the leading edge of the groove depending upon the groove orientation (figure 9). The total cost of grooving is a function This, however, did not seem to affect of many variables; groove spacing the braking performance as shown in is one of them. The investigation by figure 12. the Washington, D.C., firm concluded that fixed and variable construction Figure 12 shows the results on puddled costs for grooving the runways are and flooded surfaces. The general 60 percent and 40 percent, respectively, characteristics of the friction-speed of total cost and that the variable cost relationships for the V-grooves follow savings increase with groove spacing the same trends as for the saw-cut nonlinearly. For example, by cutting grooves. The new tire shows a signif- grooves at 2-inch spacing, the cost icant improvement in braking performance savings over 1-1/4-inch groove spacing on a puddled, grooved surface (figure 12 are 15 percent (out of the total availplot (b)) compared to a nongrooved able of 40 percent). The cost savings surface. Also, a worn tire performs 29

38 for 3-inch and 4-inch spacings over CONCLUSIONS 1-1/4-inch spacing are 25 percent and 28 percent, respectively (figure 16). The following conclupions are drawn from As pointed out earlier in this report, the findings of this research and the cost-effective groove configura- are valid for portland cement concrete tion must meet certain criteria. It has surfaces: been shown (figure 14) that the overall effect of increasing the groove spacings 1. The conventional saw-cut grooves is a decrease in available friction, spaced at 3 inches or less will provide However, the decrease is not signif- acceptable braking performance to an airicant. In addition, the braking craft tire on water-covered surfaces. performance on all the grooved surfaces Installation cost of 3-inch spaced tested is significantly higher than on grooves is 25 percent less than that of nongrooved surfaces. Friction levels the grooves spaced at 1-1/4 inches. representing a hydroplaning condition were observed on both puddled and 2. The reflex-percussive cutting flooded nongrooved surfaces (figure 14 process is an alternative groove instalplots (a) and (b)). If performance lation technique competitive with the alone were a factor for selecting a conventional saw-cutting method. The groove configuration, 1-1/4-inch spaced reflex-percussive cutting process progrooved runways will provide maximum duces unsymmetrical, V-shaped grooves. friction levels under all operating Installation cost of these grooves can conditions included in this study. be significantly less than that of the However, in the majority of cases, both conventional saw-cut grooves. the cost and the performance are considered to be important when 3. The braking performance provided by installing grooves on runways. In these the V-grooves on water-covered surfaces cases, the groove spacings of 2 inches or is comparable to that provided by the 3 inches will provide sufficient braking 2-inch pitch, conventional saw-cit to allow a gradual reduction in the speed grooves. of an aircraft and thus develop further braking. In addition, savings of up to 4. The conventional saw-cut grooves 25 percent in groove installation cost spaced at 1-1/4 inches, although costing (compared to installation cost for the most, do provide maximum friction 1-1/4-inch spaced grooves) are available, levels under all operating conditions included in this study. The V-grooves installed by the reflexpercussive technique offer even higher cost savings than the savings offered by 3-inch or 4-inch spaced grooves. In fact, the V-grooves have a potential of costing even less than the fixed cost for saw-cut grooves (figure 16); however, realistic cost estimates and full savings potential can only be affirmed after application of these grooves on an operating airport. 30

39 REFERENCES 6. Method for the Design, Construction and Maintenance of Skid Resistant Airport Pavement Surfaces, Advisory I. Judge, R.F.A., A Note on Aquaplaning Circular No. 150/ , Department and Surface Treatments Used to Improve of Transportation, Federal Aviation the Skid Resistance of Airfield Pave- Administration, Washington, D.C., ments, 8556/PS Min. Public Bldg. Works June 30, (Britain), October Costs of Runway Grooving, Naval Air 2. Horne, Walter B.; Yeager, Thomas J.; Test Facilty, Report Job No , and Taylor, Glenn R.; Review of Causes Edward Galura Scharf and Sons, and Alleviation of Low Tire Traction on Washington, D.C., June 25, Wet Runways, National Aeronautics and Space Administration, Langley Station, 8. Labus, Thomas J. and Khan, Mohamed Hampton, Virginia, NASA TN D-4406, April S., Runway Grooving Using High Pressure Water Jets, IIT Research Institute, Chicago. Final Report No. D6112, January 3. Home, Walter B. and Brooks, George W., Runway Grooving for Increasing Traction - The Current Program and an 9. Schmid, W. E., Experiments for Assessment of Available Results, Vibratory Kerfing of Concrete Pavements, Paper Presented at the 20th Annual Final Report, Contract No International Air Safety Seminar, C-4408, Department of Civil Engineering, Williamsburg, Virginia, December 4-7, Princeton University, Princeton, New Jersey, December Yeager, Thomas J., Comparative 10. Joyner, Upshur T.; Horne, Walter B.; Braking Performance of Various Aircraft and Leland, Trafford, J.W.; Investigation on Grooved and Ungrooved Pavements at on the Ground Performance of Aircraft the Landing Research Runway, NASA Relating to Wet Runway Braking and Slush Wallops Station, Paper No. 3, Conference Drag, Report 429, Advisory Group for on Pavement Grooving and Traction Aeronautical Research and Development, Studies, Langley Research Center, Paris, January Hampton, Virginia, NASA SP-5073, November 18-19, Yeager, Thomas J.; Phillips, W. Pelham; Sparks, Howard C.; and Home, 5. Byrdsong, Thomas A.; McCarty, John Walter B.; A Comparison of Aircraft and Locke; and Yeager, Thomas J.; Investiga- Ground Vehicle Stopping Performance on tion of Aircraft Tire Damage Resulting Dry, Wet, Flooded, Slush-, and Icefrom Touchdown on Grooved Runway Covered Runways, National Aeronautics and Surfaces, National Aeronautics and Space Space Administration, Washington, D.C.; Administration, Washington, D.C., NASA TN NASA TN D-609g, November D-6690, March LA

40 APPENDIX A

41 LIST OF TABLES Table Page I Test Results on PCC Surface With Worn Tire A-2 2 Test Results on PCC Surface With New Tire A-3 3 Test Results on Reflex-Percussive Grooves A-4 4 Least Square Relationship Between Speed and A-5 Coefficient of Friction ( =AV - B) A-I

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43 C.) --- v'j r- M - 4 Q c1 N m VN1 I 0 C I C. ) C*N NQ C4 C4 C-4.- O--. z C r- 1 4 ID 00 ' C r-4 C14 N C4If N1 N C0 m 0. a, 4 V) N (1 r, -4 ' c-) 'T 'd 0, I a Na) 90 N N N N (n S. c4 '0 4n v ) C/ ) - 0 C C-4 N c. Q) To a4 0.T(1Ncn a, o 0c L, Q) > w -r o 0 >- CO~~~ 4'N a,4'4 N- c.)) NN - -. I.. oo -4 ~ - L 4 Lna '0 uiu 10 1 '4-4 (n "a 0n n -1 - C ao 0 M4 1 -' NQ - I N - 1~N 0N - -o a, NN I r- 0 Q C4 N DO S. a) c c np 0a o1 0o 00~ 10 = a) a- cn~ C1 C4C - C 4c4 4u L -n > N4 0 0 W) A00-

44 - - (14 N ~ C C,4-- - In fl (14 0 (0 c C) o C4 C- C C oc c X z '0 ' C)C (N C14 N C14 C- C1 za 0 u-e' cm(n- C 0 0 ao C),1.0 L. J L - w 00 0 En m 0 C > C (7,.. no r-. I- L) -(N Cl 0(N-r 0 U to~~~~c C*** 14~ C C* L ''T PQ r, ) CDc 2aO3 -UO e- I t A-4

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46 I mmmonto A T E I LM E