REVIEW OF CAUSES AND ALLEVIATION OF LOW TIRE TRACTION ON WET RUNWAYS

7/24/2019 REVIEW OF CAUSES AND ALLEVIATION OF LOW TIRE TRACTION ON WET RUNWAYS

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T E C H N I C L N O T E N A S A

TN

D-4406

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GPO PRICE

n

CSFTI

PRICE S1

Hard copy HC)

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2

z Microfiche MF)

ff

653

J u l y 65

REVIEW

OF

CAUSES A N D ALLEVIATION

OF LOW T IR E T R A C T IO N O N

WET

R U N W A Y S

by Walter

B.

Home

Thomas

J Yagr

and Glenn R.

Taylor

Langley Research

Center

Langley Station Humpton

Va

,

?

NAL AERONA UTICS AND SPACE ADM INISTRA TION WASHINGTON, D. C

A P R I L

1968


2/39

~~

NASA TN D-4406

REVIEW

OF

CAUSES AND ALLEVIATION OF LOW TIRE TRACTION

ON WET RUNWAYS

By Walter B. Horne, Thomas

J .

Yager,

and Glenn R. Taylor

Langley Re sea rch C enter

Langley Station, Hampton, Va.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

For sale b y the Clearing hou se for Feder al Scient i f ic and Techn ical Informot ion

Spr ingf ie ld , V i rg in ia

22151

-

C F S T I

pr ice

3.00


3/39

CONTENTS

Page

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

FACTORS AFFECTING TIRE BRAKING TRACTION ON DRY RUNWAYS . . . . . . . 3

SlipRatio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

Inflation Pr es su re and Vertical Load

. . . . . . . . . . . . . . . . . . . . . . . . .

5

Speed and Pavement Surface Textu re . . . . . . . . . . . . . . . . . . . . . . . . . 6

Tire

Braking Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

FACTORS AFFECTING TIRE BRAKING TRACTION ON WET

OR FLOODED RUNWAYS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

Dynamic and Viscous Hydroplaning . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Damp runways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

Flooded runways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Vertical load and

t ire

inflation pressure . . . . . . . . . . . . . . . . . . . . . . 10

Pavement surfac e texture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Measurement of pavement te xtur e depth . . . . . . . . . . . . . . . . . . . . . .

1 2

Tire-tread design - wet runways . . . . . . . . . . . . . . . . . . . . . . . . . 13

Tire-tread design - ice-covered runways

. . . . . . . . . . . . . . . . . . . . .

1 5

Fluid viscosity and slus h

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

Tread Reversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Evidence fro m skidding accidents . . . . . . . . . . . . . . . . . . . . . . . . . 16

Reverted rubber in skidding footprints . . . . . . . . . . . . . . . . . . . . . . .

18

Duplication of rev ert ed rubber during tr ac k

tests

18

Form ation of re ver ted rubber in skidding footprint

. . . . . . . . . . . . . . . .

20

Multiple rever ted rubber skid patches

23

23

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

Discussion of T i r e Braking Per formance on Wet

or

Flooded Runways

. . . . . . .

WAYS OF IMPROVING TIRE TRACTION ON WET SURFACES

. . . . . . . . . . . .

26

Pavement Surface Texture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Pavemen t Grooving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

A i r J e t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

CONCLUDING REMARKS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5

iii


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.

REVIEW OF CAUSES

AND

ALLEVIATION OF LOW TIRE TRACTION

ON WET RUN WAYS^

By Walter B. Horne, Thom as J. Yager,

and Glenn R. Taylor

Langley Rese arch Center

SUMMARY

Thr ee m ain fact ors which cause l oss of

tire

traction on wet runways, namely,

dynamic hydroplaning, viscous hydroplaning, and

tire

tread rubber reversion are reviewed.

Consideration is given to the interaction of c ert ain vari able s such as pavement surface

texture, runway water depth, tire tread design, vertical

tire

load, and tire inflation pre s-

s u r e with these facto rs. A method fo r measuring average texture depth of a runway sur-

face

is described and appe ars promising as a means of classifying runway su rf ac es

as

to

the ir sl ippe rine ss when wet. Finally, two promising methods fo r increasing

t ire

traction

on wet runways are discussed; namely,

air jets

placed in front of t i r e s and pavement

grooving.

INTRODUCTION

Extreme loss of tire braking and lateral traction some tim es occurs during landings

of aircraft on wet and flooded runways and is of utmost importance to safe operations.

The t er m hydroplaning,

or

aquaplaning, is usually used to describe

th is drastic

loss in

traction, especially when it occ urs under flooded runway conditions.

Although few statis-

tics are availabl e to indicate the frequency of occu rre nce of hydroplaning, personnel of

two large U.S. airl ines have estim ated that

5

percent of their landings each year are made

on

wet

runways.

Their experience al so indicates that in one of every 500 wet landings

hydroplaning was encountered to s ome degree and that the attendant los s of t ir e tractio n

contributed t o

a

tendency for t he ai rcr aft to d epart sideways (crosswind) fro m the runway

in some cases and to overrun the runway in other cases. Fortunately, most of the occur-

renc es involving lo ss of traction did not lead to accidents but wer e incidents in which the

pilot was able to keep the ai rc ra ft within the confines of th e runway during landing. It

should be noted, however, that the margi n between an accident where damage or injuries

lFor merl y presented

at

AIAA/RAeS/JSASS, Ai rc ra ft Design and Technology Meeting,

Los Angeles, California, November 1965

(AIAA

Paper 65-749).


5/39

a r e suffered and an incident where no aircr aft damage o r passenger injuries occur

is

very small. The possibility of the air cra ft tir es' losing traction

as

often

as

once every

500

wet landings demons trat es the urgent need for developing some method or device,

either on the runway

or

on the ai rcra ft, that

w i l l

alleviate the drastic loss in traction

during take-off and landing and thus

w i l l

increa se safety of flight.

Since 1960, the Langley landing-loads track

of

the NASA has been extensively used

in the study of t ir e traction (ref s.

1

o 19). This rese ar ch allowed two t y p e s of l oss of

trac tion on wet sur fac es to be separa ted and defined; namely, dynamic and viscous hydro-

planing.

It

w a s found that for complete dynamic hydroplaning (where the water lifts the

t i r e completely off the surfac e) to occur for a given airplane landing condition, the runway

must be flooded beyond

a

crit ical fluid depth, and the ai rcra ft must

be

traveling

at a

speed

which is in excess of

a

critical ground speed (referred to as the dynamic hydroplaning

speed) and which

is

dependent upon the square root of the t i r e inflation pr es su re used on

the aircraft.

It

w a s

found, on the other hand, that

a

viscous hydroplaning which re qu ire s

only a thin-fluid film to be present on

a

smooth runway may also occur. Thi s type of

hydroplaning can occur at much lower ground spe eds than dynamic hydroplaning. Fortu-

nately, the texture existing on most runway surfaces is sufficient to br eak up and dissi-

pate the thin viscous film which leads to thi s type of hydroplaning.

Studies of accidents and incidents of ai rc ra ft skidding sideways off the runway and

overrunning landing strips have disclosed a signif icant number of c as es that could not be

fully explained by either the dynamic

o r

the viscous types of hydroplaning. These stud ies

also disclosed that, i n most of these case s, white s tr ea ks wer e developed in the tir e paths

on the wet runway and that t he t i r e s showed evidences of

a

"reversion" of the tre ad rubber

to an uncured f or m in the skid patches developed on the tre ad surface.

In

a

recent re sear ch program

at

the Langley landing-loads track , a ser ie s of tes ts

we re made with the air craf t t ir e locked to prevent rotation so that the ti re was forced to

skid along the

wet

and flooded te st su rfa ces at different ground speeds. During these

tes ts, the ti re s developed tr ea d rubber reve rsion in the skidding footprint region, and

extremely low friction coefficients were meas ured down to spee ds

as

low

as 25

knots.

The purpose of this paper

is

to il lus tra te the effects of dynamic and viscous hydro-

planing on ti re trac tion performance, to descr ibe the effects of prolonged ti r e skids and

tread rubber reversion on aircraft braking and direction control, to suggest methods for

improving airc raf t tir e traction on wet runways, and to disc uss

a

method of measuring the

average depth of texture as

a

means of classifying runway sur fac es

as

to their slipperi-

ness when wet.

2


6/39

SYMBOLS

Measurements re fe rr ed to in thi s paper were taken in U.S. Customary Units and

'equivaient values a r e indicated here in in the International System of Units (SI). Deta ils

concerning the us e of SI, together with physical constants and conversion fact ors, a r e

given in reference 20.

FZ

P

s1

vP

P

PAV

PEFF

PMAX

P r

pSKID

IJ.

STATIC

ver ti cal load, lb (newtons)

inflation pr es su re , lb/in2 (newtons per centimeterz)

sl ip r at io (rat io of relat ive skidding velocity to horizontal velocity of

axle)

dynamic hydroplaning speed, knots

instantaneous

tire-

ground friction coefficient

average fric tion coefficient between sl ip rat io s of 0.10 and 0.50

effective frict ion coefficient (ave rage

p

developed by aircraft as modified

by pilot braking o r antiskid system

maximum friction coefficient

rolling r esi sta nce coefficient for unbraked wheel and tire

skidding fri cti on coefficient (friction coefficient

at

sl ip rat io of

1)

friction coefficient obtained at ground speeds

< 2

knots

FACTORS AFFECTING TIRE BRAKING TRACTION ON DRY RUNWAYS

An understanding of braking tract ion of t i r e s on

dry

surfaces

is

needed to provide

background information and also to serve

as

a

basis

of comparison for the tract ion re su lt s

obtained on wet surfaces.

Slip Ratio

When rolling tires are forced by braking action to slow rotation from

a

free-roll

condition to a locked o r full- skid condition, the fric tion coefficient developed between t i re

3


7/39

and ground va ri es with s lip rat io in the manner shown schematically in figure

1.

Slip

ra tio may be defined as follows: The difference between the peripheral velocity of the

wheel and the horizontal velocity

of

the

wheel axle is defined

as

the relative

FREE ROLL WHEEL

skidding velocity occurring between the

tire

and the ground. The ra tio of this

relativ e skidding velocity t o the horizon-

tal

velocity of the

axle is

defined

as

the

slip ratio

SI

With no braking (s lip rati o

ze ro), the lower limit of the fric tion coef-

ance of the rolling ti re , wheel bearings,

and unloaded brake . Under norm al

tire

operating conditions,

p

r

usually

falls

0

2

.4 6 .8 1.0 within the range 0.02 to 0.05.

A s

braking

torque

is

applied to the wheel,

the

friction

coefficient rises until it reaches a maxi-

SKID

)

ficient

p r is

determined by the resist-

S L I P R A T I O , S1

F i g u r e 1.- F r i c t i o n - c o e f f i c i e n t v a r i a t i o n w i t h s l i p r a t i o .

mum value ref erre d to as pMm. It

should be mentioned that up to t hi s point

practically no sliding

o r

slipping occ urs

between the t ir e footprint and the ground.

''1 ADAPTED FROM REF.

5)

0 20

40

60 80 100

GROUND SPEED, KNOTS

Fi gu re 2.- E f fec t o f speed on full s k id f r i c t i o n

c o e f f i c i e n t o n d r y

runway.

The apparent sl ip shown up to the

regionof

pMm

in figure

1

is

due to

elastic deformation of the ti re . After

is

attained, unless the brake torque

is

rapidly dec rea sed , the wheel quickly

locks

at

a friction coefficient pSmD usu-

ally considerably lower than ,urn

as

indicated in figure

1

and

as

is shown by

data in figure

2

(from

ref.

5). The locked

wheel skid

is a

highly undesirable condi-

I LMAX

tion for the

tire at

high speed s on dr y run-

ways. Shown in figure 3

is a

ti re which blew out

after

sliding only 60

feet

(18.3 meters)

at a ground speed of

100

knots on

a

dr y concrete runway under a vert ica l load of

10 000 pounds (44.5 kilonewtons). Calculations show that while the tire was sliding, the

contact patch of the t ir e , with an area of approximately 40 inches2 (258.0 cen timete rsa ),

was absorbing 460 horsepower (343.0 kilowatts).

tre ad rub ber and c arc as s cord was melted and eroded away in

less

than 0.36 second, the

dura tion of the locked wheel skid in th is case .

One way to avoid such situations of

During this time ,

1

inch (2.5 cm) of

4


8/39

L-2551-3

Figure 3.- Blowout with 32 X 8.8 aircraft tire after skidding 60 feet (18.3 meters)

on drv concrete at q round speed of 100 knots.

ssive t ir e wear

is

to provide an antiskid system which prohibits wheel lockup and

imultaneously fo rc es the t i r e to brake under conditions of favorable s li p rati o.

Inflation Pressure and Vertical Load

Data given in refere nce 19 indicate that the maximum tire-ground frict ion coefficient

at

very low ground speeds (hereinafter called ~ s T A ~ ~ ~ )ends to decreas e

with

e-ground bearing pr es su re on

a

dry runway. The

data

shown in figure 4

trend. The ti re inflation pre ssu re represents

a

reasonable approximation

tr ea d effects introduce some s ma ll differences.

These data taken from ref-

nce 19 wer e obtained at extremely low ground speeds ranging from 0.008 to 1.7 knots.

g many airplane ti re s ize s, a lso show that

STATIC

may be

cted for the range shown by the empiri cal equation:

of

vertical load changes on friction coefficients on dry surfaces

is

negligible

e the ti re acts as an elast ic body and the footprint ar ea i ncr eas es with the load with

nge in ti re pre ssure. The effect of the smal l ri se in t ir e inflation pre ss ur e which

as

the

tire

deflects under i ncreasing vertical load is small.

For

example, the

ssu re ri se due to loading a ti re from z er o to rated deflection (rated load) is

5


9/39

approximately 2 to

3

percen t of the initial inflation pre ssure.

s

can be seen from fig-

u r e 4, his pressure change will not modify the friction coefficient to any great extent.

Speed and Pavement Surface Texture

The influence of pavement surfa ce texture and speed on dr y runway braking effec-

tiveness is shown in figure 5. These data indicate that the pavement surf ace textu re and

the ma ter ial used affected the frict ion coefficient very littl e. However, the effect of

increas ing ground speed was to dec rea se the braking effectiveness.

It is

believed that the

trend of

pAv

to dec rea se with speed

is

primarily due to inertia effects acting on the

deflecting t ir e

as it rolls

through the ground contact zone. These ine rtia effects tend to

reduce the tire-ground contact

area

as speed inc reas es and thus cre ate

a

higher bearing

pres sure and hence, as shown in figure 4,a lower friction coefficient. Ti r e trea d tem-

peratur e and other

as

yet unknown

effects

may also contribute to this de crease .

It

should

be mentioned that somewhat lower dry friction values a r e obtained for very smooth su r-

fac es than for the textured surface s shown in figure 5. Also shown in figure 5, by the

horizontal

dashed

lines, are the

predicted

values

of STATIC

The data show that the calculated values ag ree

well

with the experimental ~ A V alues

obtained

at

low ground speed for these tests.

obtained fro m equation 1).

Figure

5

also presen ts rolling resistanc e values

(pr )

obtained from reference 17.

These d ata indicate that the aircraft t i r e free rolling resistanc e, in contrast to ~ A V ,

tends to incr eas e with increasing ground speed.

Tire Braking Performance

The braking performance of ai rc raf t tires on dry runways may be summariz ed as

follows:

(1) Maximum braking is achieved over

a

slip ratio range from about 0.1 to

0.3.

Operation at lesser sl ip ra ti os r educes the braking action but has the advantage of having

all the stopping energy being absorbed by the br ake and hence very little ti re tre ad wear

develops. Operation at greater sli p ratios than 0.1 to 0.3 also r esu lts in reduced braking

action, but increases

tire

tre ad wear since the energy absorbed in stopping

is

now divided

between the brake and the partially skidding tire. Operation at full skid condition or sl ip

rat io

1

re su lts in no energy being absorbed by the bra ke, reduced braking action, and

intolerable tire tread wear.

(2) Next to sli p ratio, the tire inflation pr es sur e ha s the lar ges t effect on braking

friction coefficient, with increasi ng pr ess ur e tending

t o

dec rea se the maximum attainable

values.

For

example, doubling the t ir e pre ss ure fro m 100 to 200 pounds per square inch

(69.0 to 137.9 newtons/centimeterZ) re su lt s in STATIC decrea sing fro m about 0.82 to

0.71 (fig. 4). Increas ing ground speed al so tends to reduce the d ry braking effectiveness.

6


10/39

1.0

.8

STATIC

.6

FR

I

C T l O N

COEFFICIENT,

,4

~ A T I C

.2 t

(ADAPTED FROM REF. 19)

0 80 160 240 320

0 55

110

165

220

F i g u r e 4.- St a t i c - f r i c t i o n - co e f f i c i e n t va r i a t i o n w i t h i n f l a t i o n p r e ssu r e . D r y co n c r e t e ;

G r o u n d sp e e d

=

0.008

to

1.7 knots.

I I

I

I

I

N/crn2

TIRE INFLATION PRESSURE, p

FOUR-GROOVE R IB TREAD

THREE-GROOVE R IB TREAD

Fz

=

12,000 LB (53.4 kN );

p =

140 LB/lN?(96.5 N/crn2) Fz

=

13,200 LB(58.7 kN); p

=

290 LB/IN2(200 N/cm2)

0 FLOAT FIN ISH CONCRETE

SMALL AGGREGATE ASPHALT

5 LARGE AGGREGATE ASPHALT A

-

1.(

AVERAGE

F R I C T I O N .(

COEFFICIENT,

AV

.4

L

0

F i g u r e 5.-

ROLLING RESISTANCE,

pr

(REF. 17)

I I I

20

40

60

80 100 120

GROUND SPEED, KNOTS

3 2 X 8.8 t ype V I I a i r c r a f t t i r e .

S u r f a c e t e x t u r e a n d s pe ed e f fe c ts o n d r y r u n w a y b r a k i n g e f fe c t iv e n e ss .

Increasing speed from 30 knots to

100

knots (fig.

5)

results in

IJ.AV

dropping from

0.77

to

0.68, a

13-percent reduction. Elevating the temperature of the tr ea d rubber al so has

a

pronounced effect on reducing braking effectiveness (fig. 2), but the fall-off in braking

friction with rubber t emperature

is

not known for tires operating under full-scale

conditions.

(3)

For

the conditions of load and pre ss ur e studied, the res ul ts indicate that pave-

ment sur face textur e and t i r e tr ea d design have little effect on dry runway braking

effectiveness.

7


11/39

FACTORS AFFECTING TIRE BRAKING TRACTION ON WET OR FLOODED RUNWAYS

The three main factors which, acting separate ly or in combination, ca n degrade

tire ,

braking traction t o extremely low values on wet surf aces

are

now discussed. These fac-

t o r s a r e dynamic hydroplaning, viscous hydroplaning, and phenomena associated with

tire

tread rubber reversion.

Dynamic and Viscous Hydroplaning

Considerable re se ar ch has been c arr ied out on both dynamic and viscous hydro-

planing in recent year s.

(See

refs. 1

to

18.)

Essentially, hydroplaning may be defined

as

the condition under which the

tire

footprint

is

progressively lifted off the runway surface

by the action of fluid pressure as speed increase s and then rides on a fluid film of some

finite thickness. Since fluids cannot develop shea r forces of a magnitude comparable

with the for ces developed during dr y tire-pavement contact,

tire

traction under th is con-

dition dro ps to negligible values. Water pr es su res developed on the surface of the tire

footprin t and on the ground surface beneath the footprint have been measured during a

recent investigation at the landing-loads track

(ref.

16). This re sea rch showed that

it

was possible for this wate r-pr essu re buildup under the tire footprint to originate fr om the

effects

of e ithe r fluid density or fluid viscosity, depending upon conditions; hence the

classificat ion of hydroplaning into two typ$s, namely dynamic

or

viscous.

Both types of hydroplaning

are

illustrated in figures

6

to 8. Also

shown

are

the effects of ve rti ca l load,

inflation pressure, fluid depth, and

pavement te xture on hydroplaning.

The data shown in these figur es were

obtained during

a

rece nt investigation

at

the landing-loads track where five

pavement surf aces w ere placed in line.

The test

tire

was braked in succession

from a free rol l to a locked wheel and

was then allowed to retu rn t o the free-

roll condition on each of these surfaces

as the test carriage proceeded down the

track. The first test surface encoun-

tered was

a

smooth, steel-troweled,

concrete s urf ace having an average tex-

ture depth of 0.04 millimeter. The next

A

n

WET ICE

SMOOTH CONCRETE

FLOAT FINISH CONCRETE


LARGE AGGREGATE ASPHALT

DRY SURFACE

(FIG.

5 )

.8t

+-+----

_

a,

VERAGE FRICTION

COEFFICIENT,

pAv

n

I

0

20

40

60

80 100

120

GROUND SPEED, KNOTS

F i g u re 6 .- Da m p ru n wa y b ra k i n g e f f e ct i ve n e s s .

3 2 X

8.8

smooth t read;

F = 12 000 Ib (53.4 kN); p = 140 I b / i n 2 (96 .5 N / c m 2 ) .

8


12/39

AVERAGE

F R I C T I O N

COEFFl C IENT,

'AV

F R I C T I O N

COEFFICIENT,

'.

AV

.4

.

.2 -

I I I I

FLOODED RUNM'AY DA MP RUNWAY

SMOOTH CONCRETE -

ARGE AGGREGATE ASPHALT

--+--

--+--

I

DRY SURFACE (FIG.

5)

kTC---

.4

-

.2 -

I

I

.2

:i

I

\

L O SS F R O M D YN AMI C

HYDROPLANING

ON

LARGE AGGREGATE ASPHALl

V = 9 p (REF. 12)

P S

20 40 60 80 100 120

GROUND SPEED, KNOTS

F i g u r e

7.-

C o m p a r i s o n of d a mp a n d fl o o de d r u n w a y b r a k in g e f f ec -

t iveness. 32 x

8.8

smooth t read; Fz = 12 000 Ib (53.4 kN);

p

=

1 4 0 I b / i n 2 ( 96 .5 N / cm2 ) .

F- = 12,000 LB (53.4 kN);

F_

=

22,000 LB (97.9 kN);

surface was a mor e textured

float-finish concrete having

an average texture depth of

0.19 mil lime ter. The next

two test surfaces were

asphalt. The

first of

these

used

a

small aggregate to

produce an average t exture

depth of

0.34

mm. The sec-

ond asphalt surface was

clas-

sified

as lar ge aggregate and

had an average tex ture depth

of 0.56 mm . The fifth and

final surfa ce was smooth wet

ice.

To obtain this surface,

a

200-foot (61 O-meter) length

= 290 LB/ lN .2 (200 N/cm2)

- -e--

140 LB/lN.2 (96.5 N/cm 2)

U

SMOOTH CONCRETE

[I FLOAT FIN ISH CONCRETE --.--

ARGE AGGREGATE ASPHALT

A

1.0

F R I C T I O N

COEFFICIENT,

'A V

of the tra ck was refrig erated and water was placed on this surfac e and frozen. At the

ti me of testing, t he ic e surface was lightly sprayed with water to produce

a

water film on

the

ice

surface.

Damp runways.- A series of smooth-tread braking tests wer e made on the five test

surface s just described under the following condition

of

wetness. All test surfaces

except

ice

we re flooded with water

to

a depth of 0.1 to 0.2 inch (0.2 to 0.5 cent imeter).

9


13/39

The water was then allowed to stand until the sur face s were thoroughly saturated. Jus t

before the test runs were made, the standing water w a s removed from each of the te st

surfaces by means of stiff bri stl e brooms. This action left each of the te st su rfa ces with-

out any standing water, but damp to the touch and discolored in appearance (from dry con- *

dition). The damp surface condition w a s selected to minimize dynamic fluid pr es su re

buildup. Any traction loss suffered by the test t i r e s under thi s damp condition would

therefore be primarily due to viscous type hydroplaning. The results from this study ar e

shown in figure 6.

Also shown in figure 6

is

the dry traction curve (from fig.

5)

obtained for the same

load and inflation pr es su re conditions. These data indicate that the water film presen t on

the damp smooth concrete su rface w a s sufficient to produce viscous hydroplaning, down to

very low ground speeds. It can be see n that the values obtained on damp smooth concrete

a r e essentially as low as those obtained on wet ice. The effect of pavement texture in

reducing viscous hydroplaning effects is strikingly illustrated by noting the d ec rea se in

trac tion los s encountered by the textured pavement sur fac es under damp conditions.

Flooded runways.- The effect of flooding the test surfaces on smooth ti re braking

effectiveness

is

noted and compared with damp surface data in figure 7. This figure shows

only the results obtained on the smooth concrete and large aggregate asphalt test sur-

face s. The data obtained for the other test surf aces were omitted for cla rity, but showed

sim ila r trends according to the degree of texture of the te st surface. In thi s instance

(fig. 7), the te st runways w er e flooded to a depth of 0.1 t o 0.2 inch (0.2 o 0.5 centimeter)

and the te st runs were performed immediately upon reaching this depth. The data show

that fo r the lar ge aggregate asphalt su rfac e, this flooding resulted i n

a

large

loss

in t rac-

tion as compared with the res ults for damp surfaces at the higher ground speeds. They

also indicate

a

total traction loss at about 106 knots which c orresponds t o the predicted

hydroplaning speed from dynamic hydroplaning theory (ref .

12).

The hatched ar ea in this

figure indicates the loss attributed to dynamic hydroplaning fo r the la rge aggregate

asphalt surface. The smooth concrete res ult s show that flooding this surfa ce resulted in

increasing the friction coefficients slightly over those obtained on the damp surface.

This

increase is attributed to the la rg er fluid drag created by the ti re displacing water f ro m

the ti r e path under the condition of gre ate r water depth. The data als o show that the

friction coefficients f o r smooth

tires

on damp smooth su rfac es

are

essentially equivalent

to the coefficients attained during com ple te dynamic hydroplaning.

Vertical load and ti re inflation pressur e.- Reference 16 states that viscous

hydroplaning is not g reatly affected by changes in ti r e verti cal load and inflation

pre ssu re. This conclusion

is

supported by the data shown in figure 8 for the flooded

smooth concrete surfac e. The data in this figure indicate that for this smooth sur -

face, approximately doubling the vertical load from 12 000 to 22 000 pounds

(53.4

o

10


14/39


15/39

Measurement of pavement text ure depth.- As can be inferr ed fro m the discussions

thus far, pavement sli pperiness is related directly to the pavement surfac e texture and its

ability or inability to alleviate dynamic and viscous fluid pressure buildup. It thus follows

that pavement slip peri ness can be possibly cor related with some c hara cter istic

of

the

physical makeup of the texture such as size , shape, and number of aspe ritie s per sq uare

inch. Rese arch along these lines, for example,

is

being conducted by

D.

F. Moore

(Cornel1 Aeronautical Laboratory), H. W . Kummer and W . E . Meyer (Pennsylvania State

University), and Robert

Sugg (Bri tish Minis try of

Aviation).

The Langley

Research Center (ref. 18)

has developed a simple

technique to determ ine the

average texture depth of a

pavement surface. This

method involves spreading

a known quantity of g re ase

on the pavement surfa ce

with

a

rubber squeegee to

fill

the runway pore s. The

average texture depth for

the surface

is

then obtained

by dividing the surface area

covered into the volume of

gre ase used. An illus tra-

tion of thi s procedure is

given in figure 10. The

corr elat ion of average tex-

ture depth with braking

traction for the test runway

surfaces

at

the track

is

given in figure

11.

It is

noted that, for a given

speed, increasing the sur -

face texture increas es

braking traction up to som e

level af ter which no further

increase in traction is

F ig u r e 1 0. - M e t h o d o f me a su r i n g r u n w a y su r f a ce t e x t u r e . L - 25 5 1- 1 0

FINISH CONCRETE


LARGE

AGGREGATE

ASPHALT

SMOOTH CONCRETE

. 0

OGROUND SPEED

=30 KNOTS

GROUND SPEED

= 60 KNOTS

VERAGE .

FRlCTlON

COE FFlC ENT,

-

GROUND SPEED

. 4

AV

YL

90

KNOTS

.2

-

.I

2 .3 .4

.5

6

AVERAGE TEXTURE DEPTH, m m

F i g u r e

11.-

C o r r e l a t i o n o f b r a k i n g f r i c t i o n c o e f f ic i e n t w i t h a v er ag e t e x t u r e d e p t h.

32

X 8.8

s m o o t h t r e a d a i r c r a f t t i r e ;

Fz =

12

000 Ib

(53.4 kN); p

=

140 Ib l in .2

(96 .5 N lcrn2 ) ; f looded run wa y; Wate r dep th = 0 .1 to 0.2 in c h (0 .2 to 0.5 cm).

12


16/39

obtained. The re su lt s indicate that a greater depth of texture

is

requi red t o develop maxi-

mum tire friction coefficient at the higher speeds. Photographs of the su rf aces are shown

in figure 12.

Additional corre lati ons of braking traction with average te xtu re depth on

actua l operational pavement surfac es are needed to clarify the usefulness of this technique.

Figure

12.-

Test runway surfaces at landi ng-load s track . L-2551- 12

The use of techniques, such

as

that previously described,

is

needed to define mini-

um sta nda rds of runway texture for

safe

operations on wet surface s. Such stand ards

a capability for quickly determin ing when a pavement surface needs to

be

or replaced as a result of its poor braking tract ion qualit ies when wet o r flooded.

Tire-tread design - wet runways.- Braking performance of smooth tr ead aircraft

on most wet

or

flooded runway s urf aces

is

great ly improved by cutting

or

molding a

of circu mferen tial grooves into the

tire

tread. The resulting traction improvement

wet surf aces with grooved tires over that with smooth tread tires is due to two effects.

irst,

the low pre ssu re channels formed in the

tire

footprint by the tr ead grooves and run-

facilitate water drainage from the footprint area even under

flooded runway conditions. This effect disappea rs, however,

as

the cr itic al dynamic

is neared

or

exceeded

as

shown in fig ure 13. Fo r shallow water depths

13


17/39

where the grooves do not become choked, total dynamic hydroplaning is delayed to a higher

speed as shown in figure 14.

A s a

res ult of the bett er drainage, traction performance of

the grooved tire

is

als o greatly improved over that of the smooth t ir e at sub-hydroplaning

speeds. The data presented in figures 1 3 and 14 show that t hi s benefit is lost as the tire

tread wears, or

as

the groove depth decre ases.

. 6

.5

.4

. 3

.2

.1

AVERAGE

F R I C T I O N

COEFFICIENT,

'AV

(I CLU DES

FLUID DRAG)

TREAD WEAR, PERCENT

OF

UNWORN TIRE SKID DEPTH

0 20-60

-

95-100 (SM OOTH TREAD)

-

-

-

-

(ADAPTED

F R O M

REF. 17)

0.6

.5

.4

.3

.2

.1

0 20 40

60

80 100 120

GROUND SPEED, KNOTS

F i g u r e 1 3.-

E f f ec t o f t r e a d w e a r o n f lo o d e d co n c r e t e r u n w a y . F i ve - g r o o ve r i b

tread; F

=

10 500 Ib (46.7 kN); p

=

90 I b / i n 2 ( 6 2 . 1 N / cm2 ) ;

W a t e r d e p t h = 1.0

in. (2.5

cm);

I n i t i a l t r e a d s k i d d e p t h = 0.25 in. (0.6 cm).

-

-

-

-

-

-

AVERAGE

F R I C T I O N

COEFFICIENT,

'A V

NT WORN

--

L1 50

PERCENT WORN

TREAD WEAR CONDITIONS

0 PERCENT WORN

L1 50 PERCENT WORN

75 PERCENT WORN

00 PERCENT WORN

--

vP

(ADAPTED FROM REF. 17)

\I

I

I I I I I l l

0 20 40 60

80

100 120

GROUND SPEED, KNOTS

Figu re 14 . -

E f f ec t o f t r e a d w e a r o n w e t co n c r e t e r u n w a y . F i ve - g r o o ve r i b

tread; F

=

10 500 Ib (46.7 kN); p

=

150 I b / i n 2 ( 103.4 N / c d ;

W a t e r d e p t h

= 0.1

to 0.3 in. (0.2 to

0.8 cm) .

14


18/39

The second effect to be discussed is the ability of the s ha rp edges or c or ne rs of the

tr ea d grooves to penetrate and displace the viscous fluid film separating the ti re fr om the

pavement. Adhesion between tire and ground is thus regained along the line of pavement

contact of the groove edges, which incr ea se s tract ion of grooved t i r e s over that of smooth

trea d ti re s on the smoother w e t pavements. Automobile t i r e manufact urers have found

that the addition of closely spaced si pes o r sma ll knife cu ts in the ri b ar ea s of c ircum-

ferentially grooved tires inc rea ses traction on wet smooth surfaces to a much greater

extent than sim ple grooving. Up to the present t ime , thi s feature has not been provided

in high performance a ir cr af t ti r e s because of tr ea d chunking or tread retention problems;

however, because of the substantial benefits which might accru e fro m t i r e siping, th is

procedure should be reexamined for application to aircr aft.

Tire-tread design - ice-cove red runways.- F or a number of ye ar s, ti r e manufac-

tu re rs have been requested to provide airc raft operators with "ice-grip" ti res . These

special t i re s have thousands of chopped up pieces of small di ame ter w ir e molded into the

tire tr ea d that become exposed as the trea d surface wears. This feature was designed to

inc rease t he gri p of the tire on icy pavements. Thr ee ice-grip ti re s having different wi re

content we re test ed on the i ce-covered runway of the Langley landing-loads tr ack , and the

results obtained

are

shown in figure 15.

0

- MOOTH RUBBER TREAD (NO WIRE)

0

THREE-GROOVE

R I B - 2-1/2

PERCENT CHOPPED WIRE I N TREAD (BY WEIGHT)

A

THREE-GROOVE R I B - 5 PERCENT CHOPPED WIRE

I N

TREAD (BY WEIGHT)

THREE-GROOVE R I B

-

10 PERCENT CHOPPED WIR E I N TREAD (B Y WEIGHT)

. lo

r

)'MAX

A

0 A 0 ag

I I

I

I

...

FR

l

C T l O N

COEFFICIENT,

I 0

0

20 40 60 80

100

120

GROUND SPEED, KNOTS

F i g u r e

15.-

I c e - g r i p t i r e - b r a k i n g e f fe c t i v en e s s on wet ice. 32 X 8.8 a i rc ra f t t i re ;

F z

=

13 200

Ib

(58.7

kN);

p

=

290

I b / i n 2 ( 2 00 N / cm2 ) .

N o

significant improvement in traction was apparent on the wet ice surface with the

ice-grip t ir es than with a smooth tr ea d ti re (without the wire).

It

is not known at this

ti me whether the minor improvements obtained we re the resul t of the ice-gr ip fe ature o r

of the th re e grooves in the tr ea d design of the ice -gri p tir e. In fact, the frictio n coeffi-

cients meas ured f or all the t i re s tested on ice differed only slightly from the values shown

in figure

5

for fr ee rolling. The four

tires

tested under both conditions were constructed

1 5


19/39

by using the s am e mold shape and rubbe r compounds, and only the wi re content and groove

design were varied.

Fluid viscosity and slush.- Data presented in refe rence 16 demonstrated that

increas ing fluid viscosity increa sed the fluid pre ss ur es developed between ti re and ground

at

a

given speed and, hence, enhanced the possibility of vi scous hydroplaning. The data in

figure 16 (from ref.

11)

show that tracti on

loss at

low speeds (where dynamic effects are

unimportant) is much grea te r on the mo re viscous slu sh than on the water-covered run-

way. These

tests

were conducted on

a

moderately textured concrete runway and it

is

believed, on the

basis

of data presented in reference 16, that these l oss es could be

dec rea sed by incre asing the textur e of the surfa ce. The effective friction coefficient used

in figure

16 is

the mean value of t he fr icti on coefficient developed at the tire-ground inter-

face using antiskid braking.

.5

. 4

A I RCRA FT

EFFECTIVE . 3

FRI CT l O N

COEFFICIENT,

"EFF .2

.1

I

I

I I

I l l

I I

0 20

40 60

80

100

120

140 160

GROUND SPEED, KNOTS

F i g u r e 16 .- B r a k i n g e f f e ct i ve n e s s o n

jet

t r a n s p o r t .

F ul l

a n t i s k i d b ra k i n g ;

p =

150 Ib / i n2 (103.4 N/cm21.

Tread Reversion

Evidence fro m skidding accidents.-

A

rec ent survey of conditions that prevai led

during landing accidents on wet runways showed

a

very interesting correlation. In numer-

ous documented cas es on wet runways involving air cr af t depar ture off t he sid e of the run-

way

o r

in an overr un (both types of acciden ts indicate d rasti c l oss of

tire

traction), the

runway surfa ce was found to have developed white st re ak s in the tire paths.

The

aircraft

t i r e s showed evidences of prolonged locked wheel skids with indications that the r ubber i n

the skid patches had reve rted t o an uncured

state.

Examples of the two types of acciden ts

made available to

NASA

a r e shown in figures 17 and

18.

In figure

18,

where effects of an

overrun accident ar e shown, the white str eak s pe rsisted down to the point that the air craf t

stopped.

16


20/39

W h i t e t i r e s tr ea ks o n r u n w a y

R e ve r t e d - r

u

b b e r sk id p a t ch

Figure 17.-

Hydroplaning accident with four- engi ne prop jet on flooded runway du rin g heavy crosswind.

L- 2551-17

Wh i t e s t r e a ks on r u n w a y

~ 7 0n o t s

Wh i t e s t r e a ks o n s i d e w a lk

2 5 k n o t s

* E

6

W h i t e s t r e a k s o n o v e r - r u n

- 5-10

k n o t s

. .

z ------+ .

9:

., --

r --?

-2

Ut

S k i d patch w i t h

r e v e r t e d r u b b e r

Figure 18.- Hydroplaning accident on flooded

r i

nway with twin-engine transport aircraft.

L-2551-18

17


21/39

In contrast, if the wheels lock on dr y runways

at

speed, black stre aks from molten

rubber eroding from the ti re

are

immediately deposited in the

tire

paths. Although fric-

tion dec rea ses under this condition,

at

lea st one-third of the maximum dry fric tion coef-

ficient is still available for stopping the

aircraft, as

was illustrated in

figure

2. It thus

appears that som e mechanism other than viscous or dynamic hydroplaning must be

at

at

very low ground spee ds as indicated in figu re 18.

.

work to prevent t he skidding tire fro m developing trac tion on wet textured pavements even

Reverted rubber in skidding footprints.- The tread condition known as reverted

rubber was pointed out as early as 1943 in a n unpublished memorandum by J . H. Hardman

and V. E. Gough of Dunlop Rubber Ltd., England. In thi s case, the revert ed rubber devel-

oped during locked wheel skids on wet g ra ss .

(See fig. 19.) Hardman and Gough stated

that their examination of the tread blisters revealed that the deterioration was of a fine

porosity in a thin layer pa rall el to and jus t below (about 0.02 inch; 0.05 centimeter) the

original trea d surf ace and that th is porosity resembled overheating of the rubber due to

attainment of local temp era tur es around 392O

F

(473.15O K). The trea d surface

itself

was,

however, unaltered and showed the original surface

cracks

and fine abrasion marks .

This

film, which was about 0.01 inch (0.03 centimeter) thick, showed no

trace at all

of any

effects

of high tem perat ure.

This surf ace film was, however, torn away in places having

a

ffscabby ft ppearance (as shown in fig. 19). The porosity extended under all adhering

pieces of surfa ce film and was noted to

be

quite tacky when the

latter

was t orn off.

Hardman and Gough further stated that puffs of steam o r white smoke were visible at

points along the whole length of the fl sl idef t about 200 or 300 yards; 182.9

or

274.3 meters)

after

touchdown.

Further examples of tread reversion made available to NASA

are

shown in fig-

u r e s 17, 18, 20, 21, and 22. The se examples show the tacky porous revert ed rubber men-

tioned by Hardman and Gough but do not show the undamaged surface layer that they men-

tioned.

It

perhaps was destroyed during the low-speed ranges of the skids.

Because the

examples cited cover such a large range of aircraft types,

it

is suspected that most air-

craf t can suffe r o r experience this phenomenon during prolonged wheel s kids on wet

or

flooded runways.

Duplication of re verte d rubber during tr ac k

tests.-

A

series

of

tests

was made

at

the Langley landing-loads tr ac k with

tires

locked to prevent rotation on dry, wet, and

flooded runways at spee ds ranging from about 25 to 100 knots. A typical footprint of

a

smooth-tread tire after a 467-foot (142.3-meter) skid on a flooded runway at 78 knots

ground speed is shown in figu re 23. In thi s particu lar test, the tire was allowed to skid

first along 62 feet (18.9 meters) of a dry, smooth concrete runway to produce molten or

reverted rubber in the tread; the

rest

of the sk id took place on the runway flooded to a

depth of

0.1

to 0.2 inch

(0.2

to 0.5 centimeter). Other tests were made where the skid

w a s initiated on either damp or flooded smooth conc rete , and al so for an alterna tely wet

18


22/39

(a1 General view.

Sm a l l p i e ce of

surface

pa rt

I

detached

-

P o r o s i t y

T r e a d su r f a ce

P o r o s i t y

be low sur face

L-2551-

19

b l

Photomicrograph.

Figure

19.-

Reverted-rubber skid patch obtained dur in g locked wheel skid at hi gh speed on wet grass. Photograph from

Hardman and Gough, Dunl op Rubber Ltd., 1943.

19


23/39

and puddled runway condition. Under

all

conditions of the

tests, if

reverted rubber devel-

oped in the footprint, the tract ion values fell to v ery low values in co mparison with those

obtained with the tire under norma l rubbe r tre ad conditions. (See figs. 24, 25, and 26.)

A s

shown in figur e 27, the reverted rubber condition tends to make all runway surfaces

smooth acting. Pavement sur fac e texture, which

(as

discussed previously) has such

a

large effect on traction losses from dynamic and viscous hydroplaning (normal rubber

curves), has but little effect for the rev ert ed rub ber condition fo r the texture-depth range

shown.

Formation of rev ert ed rubbe r in skidding footprint.- Data regardin g the formation of

rev ert ed ru bber in the skidding footprint on wet runways are very limited, and the effects

of pavement texture, wetness, and ground speed

are as

yet unknown. However, so me fact s

are known about the p rocess . A reverted-rubber state has been obtained at Langley on

dry pavement by skidding the tire a shor t distance, which suggests that the p rocess can be

initiated at touchdown on drained sec tio ns of wet runways pri or to wheel spin-up.

L- 255 1-20

Figure 20.- Reverted-rubber skid patch obtained with four-engine jet transport on wet runway.

20


24/39

Generation of st ea m in the skidding t i r e footprint,

first

discussed by Obertop

(ref. 22), offers an explanation for the rev erte d rub ber and the reduced tra ction that may

be developed b y tires at very low speeds on textured and untextured surfaces.

It may be

note'd that the s tea m p re ss ur e in the footprint, if developed, must closely equal the tire-

ground bearing pressure at all speeds. Moreover, if steam is formed, then it must be

superheated stea m, and for tire-inflation-p ressure ranges of airc raft, th is temperatu re

is

sufficiently high to melt the rubber tre ad surface. In

this

concept, once

the

reve rted con-

dition starts, it is possible t o have a steam pump established

in

the footprint, and the

Figu re 21.- Reverted- rubber skid patches. Hydroplan ing accident wi th superson ic figh ter on wet run way wi th isolated puddles. L-2551-21

2 1


25/39

L-2551722

Figure 22.- Reverted-rubber skid patch obtained wit h four -en gine

prop jet on wet runway.

reverte d rubber formed may

act

to

effect

an

efficient seal which p erm its high stea m p res-

su re s to persist right down to

a

stop on the run-

way. The stea m pr es su re would tend to lift the

ti re away fro m the pavement surfac e and thus

reduce traction on wet surfac es in

a

manner

sim ilar to the fluid pres sur e buildup from vis-

cous and dynamic hydroplaning effect s. The

ste am concept al so of fers an explanation for

white s tr ea ks developed in the tire paths in that

the paths would be cl eared of d ir t and other con-

taminants by high pressure superheated steam.

L-2551-23

Figure

23.-

Reverted-rubber skid patch fr om

t es t s

at

landing-loads track. 32

X

8.8 smooth tread air cra ft

tire; F = 16 000

Ib (71.2 kN ;

p = 250 I b / i n2

(172.4

N/crnZ .

With ref ere nce t o the question of t emp era tur e buildup in the footprint,

a

few prelim -

inary test s were made.

A 32 X

8.8 aircraft tire was equipped with thermocouples (fig. 28)

and tested

at

the Langley track. The peak temper atures experienced by this airc raf t

tire

at

the end of an approximately 65-foot (19.8-meter) skid at a ground speed of

77

knots on

a damp, smooth concrete surfac e are shown in figure

29.

The res ul ts show that the peak

temperature d ecreases

as

the

tire

inflation pressure decreases, which perhaps explains

the lower automobile tread te mpera tures reported in r eferen ce 16 where an attempt was

also made to investigate the st eam hypothesis

at

much lower tire pressu res. The data

show that the te mpe rat ure buildup on the tre ad surfa ce of a skidding

tire

on wet surfaces

is

related to the unit p re ss ur e the t ir e develops in the skid patch on the pavement,

is

higher near the center of the skid patch, and varies with pavement texture (fig.

30) .

In summ ary , the re su lts of the limited studies made

at

Langley indicate that

revert ed rubbe r may fo rm and possibly provide better sealing around the periphery of

2 2


26/39

the footprint (see fig. 23) than

norm al rubber, thus allowing

a very thin film of water t o

be trapped in the footprint,

heated up, and converted to

steam . The ste am cushion

generated could support the

tire off the surface and pro-

duce the low frict ion coeffi-

cients measured. It could

also provide the mechanics

for scour ing the runway and

producing the white s tr ea ks

which have been observed

after numerous aircraft

skidding incidents.

Multiple reverted

rubbe r skid patches.- Numer-

ous instances have been

observed where multiple

reverted rubber skid patches

such

as

those shown in fig-

u r e 21 develop around the ti re

circumference. For these

cases ,

it

is believed that

intermittent

wheel

lockups

fro m braking, actuated eit her

by the pilot or

by

antiskid

SMOOTH CONCRETE SURFACE

NOR MA L RUBBER REVERTED RUBBER

O

%KID

p~~~

%KID

----

32

x

8.8

SMOOTH TREAD

Fz = 22,000 LB (97.9 kN);

3 2 x

8.8

R I B T RE A D

F

= 16,000 LB (71.2 kN);

2

p

=

290 LB/IN? (200 N/crn2) p

=

250 LB/lN . (172.4 N/crn2)

B R A K I N G

6

FRI CT l ON

GROUND SPEED, KNOTS

F i g u r e 24.- Ef f e c t o f r e ve r t e d - r u b b e r f o o t p r i n t on b r a k i n g f r i c t i o n .

F looded runway.

B R A K I N G

FR lCT l ON

COEFFICIENT,

IJ

NOR MA L RUBBER REVERTED RUBBER

C ~ ~ ~

%KID

KID

- - - -

32 x

8.8

SMOOTH TREAD

32 x 8.8 R I B T R EAD

Fz = 22,000 LB (97.9 kN);

Fz = 16,000 LB (71.2 kN);

p

= 290 LB/IN.2(200 N/cm 2)

p =

250 LB/IN.(172.4 N/crn2)

GROUND SPEED, KNOTS

F i g u r e 25.- Ef f e c t o f r e ve r t e d - r u b b e r f o o t p r i n t o n b r a k in g f r i c t i o n .

F looded runway; float f i n i s h c o n c r e t e s u r f a c e .

devices, cause the multiple patches. This explanation

is

borne out by the film sequences

(ref. 23) of the action of an antiskid-equipped bogie landing gear of a supersonic bomber

during wet landings. The se sequences show wheel lockups fo r as long

as

21 seconds and

successive wheel lockups before the wheels have regained free-roll rotational velocity.

Thus, the antiskid syst em appears to permit the ti re s to operate (for this low tire-ground

fricti on coefficient regime) on the back side of the cu rve of fricti on coefficient

as

a func-

tion of sl ip rati o in figure

1.

Discussion of T i re Braking Perfo rmanc e on

W e t or

Flooded Runways

The preceding sec tions of the paper have attempted to show that viscous hydro-

planing, dynamic hydroplaning, and tire-tread reversion all can, depending upon conditions,

23


27/39

NO RM AL RUBBER REVERTED RUBBER

F

D =

290 LB/ lN.*(200 N/cm2)

P = 250

LB/IN.(172.4 N/cm2)

C l ~ ~ ~

%KID

O v~~~~

- - -

32

x

8.8 SMOOTH TREAD

32

x 8.8

R I B T RE AD

.8

Fz = 22,000 LB (97.9 kN);

F =

16,000 LB (71.2 k N);

2

COEFFR I C T I O NR A K I N GC

I

ENT,

1

1

IJ .4

I

*

-_

_

-

-

--

. 2

0 8

I I I

I I

I

0 20 40 60 80 100 120

GROUND SPEED, KNOTS

F ig u r e 2 6. - E f fe c t o f r e ve r t e d - r u b b e r f o o t p r i n t o n b r a k in g f r i c t i o n .

F looded runway; sma l l agg rega te aspha l t su r face .

_ _ - - 30

KNOTS, GROUND SPEED

60 KNOTS

FLOAT SMALL

FINISH AGGREGATE

CONCRETE ASPHALT

. I

/ /

/

F R I C T I O N

. 6 -

COEFFICIENT,

IJ

.4

-

.2

-

-_____ - - - - -

I

I I

I

0 05 10 .15 .20 .25 .30 .35

..

AVERAGE TEXTURE DEPTH,

m m

F i g u r e 27.- E f f ec t o f su r f a ce t e x t u r e o n b r a k in g f r i c t i o n . F lo o de d r u n w a y .

ground speed dependent upon infla-

tion pressure and fluid depth conditions.

Once this crit ical ground speed is exceeded, tota l

traction l oss occu rs until either the speed

or

the fluid depth is reduced.

The condition of tir e- tr ea d rev ers ion

is

potentially the gravest problem because its

occurrence requ ires only that the pavement be smooth and damp. The prevention of tr ea d

rev ers ion can be simply stated : Prev ent the wheel fro m locking during landing runout.

The allev iation of t rac tion los se s on wet runways not only by viscous and dynamic

hydroplaning but a lso by tire -tre ad reve rsio n has been the subject of recent and continuing

research at the landing-loads tra ck. Some of the more promising means of alleviation

that have been studied a r e discu ssed i n the following section of the paper.

create total loss of tire traction on

fluid-covered pavements. Fort u-

nately, the critical conditions

required of the tire, pavement,

or

airc raft to achieve almost total loss

of

trac tion for each of th ese phe-

nomena are different. This makes

possible cer ta in avenues of approach

to combat and alleviate each hydro-

planing type. Fo r example, the

critical parame ter for viscous

hydroplaning

is

pavement su rfac e

texture. If an adequate textur e is

provided on the runway su rface,

viscous los ses a r e greatly allevi-

ated over all ranges of operating

speed, inflation pr ess ure , and tire

tre ad condition ranges.

On the ot her hand, the crit i-

cal requirement for dynamic

hydroplaning to occu r

is

fluid

depth on the pavement. Removing

the fluid from the tire paths, by

air jets

or

otherwise, eliminates

dynamic hydroplaning as a prob-

lem. Fluid removal can be accom

plished by satisfactory tire t read

designs up to a certain critical

24


28/39

PEAK TREAD

SURFACE

TEMPERATURE

355

344

333

322-

311

T r e a d g r o o v e a l o n g t i r e c e n t e r l i n e

180

-

-

160

-

- 140 -

120

100 -

/

T

h e r rnocou

p

e s

Figur e 28.- Thermocouple insta llati on on rib-tread tire. L-2551-28

O K OF

3 6 6 ~00

r

DIRECTION

OF

MOTION

-THFRMnCOUPLE

LOCATIONS ~

-

I

I

I

I I

I I LB/IN.*

0 80 160 240 320

3WL

8 0 1

I I

I

I

I

N/cm2

0

55 110 165 220

T IRE

I

NFIATION-PRESSURE

Figur e 29.- Tread-surface temperatu re ri se after 65-fl (19.8-m) skid on damp smooth concrete.

Ambient te mperature

=

820

F

(301O K).

25


29/39

TREAD

SURFACE

TEMPERATURE

OK

366

322

D I R E C T I O N OF MOTION-

F

THERMOCOUPLE

LOCATl ONS

120

I

SMOOTH

CONCRETE

S K I

D D l N G

F R I C T I O N .1

COEFFICIENT.

SKID

0

.2 .4 .6

8

1.0 1.2 1.4 1.6

TIME, SEC

F i g u r e 3 0.- T i m e h i s t o r i e s o f t r e a d - s u r f a ce t e m p e r a t u r e a n d f u l l - s k i d f r i c t i o n c o e f f ic i en t .

L o c k e d wh e e l s k i d a t 77 k n o t s g ro u n d s p e e d ; Fz = 10 000 Ib (44.5 k N ) ; p

=

1 5 0 I b / i n 2

(103.4

N

/cm21.

WAYS OF IMPROVING TIRE TRACTION ON WET SURFACES

Pavement Surface Texture

The paramount importance of pavement s urfa ce textu re in alleviating viscous tra c-

tion los se s has been discussed in this paper.

It

is important, for definition and classifi-

cation of surfac e traction c har acte rist ics , to be able to describe quantitatively the relative

slipper iness of pavements in t er m s of some easily mea sur ed pavement su rface par am ete rs

such

as

the average texture depth described earlier.

Other res earc her s such as those

previously mentioned, Moore (Cornel1 Aeronautical Labora tory), Kum mer and Meyer

(Pennsylvania State University), and Sugg (Br iti sh Ministry of Aviation) have suggested

alterna te methods.

Additional data, obtained by these

or

other techniques, a r e needed to

cor rela te appropriately pavement texture with ti re t raction under wet conditions.

In the

accumulation of such data, particula r ca re should be exercise d to account for such fact ors

as t i re pres sure , test speed, and tread design to ass ure that the data obtained are rep re-

sentat ive of ai rc ra ft operational conditions on wet runways. Fo r example,

a

significant

point can be drawn fr om the photograph of figure 31 where the skidding aircraft tire left

white ti re s tr ea ks on the runway up to the point where the pavement wa s resurface d. The

new surface, having a rougher textu re, shows black skid mar ks , indicative of higher fr ic-

tion coefficients.

26


30/39

Note Depos ited rub be r in s k i d m a r k s o c c u r

d u r i n g c h a ng e i n r u n w a y s u r fa c e t e x t u r e

L-

2551-31

Figure 31.-

Hydroplani ng accident wit h four-e ngin e piston transport. Large areas

of

runway covered wit h water duri ng landing.

Pavement Grooving

Pavement grooving shows great promise as a means of allevia ting all for ms of ti re

tractio n los s on wet runways. It was initiated by Judge (ref. 24) in England and

first

applied ther e to military airfields in 1956. The following discussion attempts t o describ e

some beneficial aspects

as

well

as

to

pose

some possible problems associated with pave-

m ent grooving.

The work of Gray (ref . 2 1 and fig.

9)

demonstrated that tr ans ver se grooves can

greatly i ncre ase the c rit ical water depth on the runway requi red

for

dynamic hydroplaning

to occu r, and thus reduce (because of the higher rainfal l precipitation r at es required) th e

probability of ai rc ra ft encountering dynamic hydroplaning. The incr eased trac tio n due to

tr an sv er se grooving of a flooded runway is illust rated by the wheel spin-up time hi stor ies

measured at the Langley t ra ck and presented in figure

32.

In this figure a r e presented

wheel spin-ups from

a

locked-wheel skid condition on dry and flooded, grooved and

27


31/39

ungrooved, l arge aggregate asphalt su rfa ce s under conditions of constant verti cal load

and inflation pre ssu re for a ground speed of approximately

100

knots. The slope of the

angular -velocity-time curve during wheel spin-up is the angular acceleration and thus

indicates the amount of tract ion developed between the

tire

and ground. The re su lt s

showed that the tr ansver sel y grooved section of pavement, pictured in figure 33 , restored

the t ract ion developed by the

tire

on the flooded pavement to that obtained on the dry su r-

face. This resul t indicates, for example, that the installation of tra nsv ers e grooves in the

touchdown areas of runways might prevent prolonged skids from developing on

aircraft

tires

and thus prevent th e dangerous development of rev ert ed rubber i n the

t ire

skid

patch.

It

has been shown in the previous discussion that once rever ted rubbe r

is

established

in the t i r e footprint, extremely low fric tion coefficients develop on wet pavements down to

very low ground speeds. Because the frict ion coefficients that resul t

are

as low or even

lower, in som e

cases,

than the

tire

free-rolling resis tance coefficient the

tire

has

a

tendency to r ema in in the locked wheel condition for long distance s on the runway. Tests

at

the landing-loads

track show

that

tra nsv ers e pavement grooving can remove

the

reverted rubber in the tire footprint as the tir e slides acro ss the grooves and res tor e

tire

traction capability to normal rubber conditions. This result is shown in figure 33.

The res ul ts presented show that tra nsv ers e grooves on runways can effectively

rest ore normal braking action to tires operating in the revert ed rubber mode.

It

may not

be

nec essary f or the runway t o be completely grooved to obtain this beneficial effect.

Consider that only the middle third of a runway is trans verse ly grooved, and assum e that

an ai rc ra ft touches down off runway cent er l ine with one main landing wheel operating

on the grooved runway and the other wheel outside the grooved area. When wheel braking

commences, a higher braking force should

be

developed by the landing-gear ti re( s) ope r-

ating on the grooves than the

tire(s)

off the grooves. Thi s situation should produce

a

large moment about the aircraft center of gravity, so that the aircraft tends to return to

the cente r of the runway. This effect would al so be beneficial during crosswind landings

on wet runways.

The

effects

of pavement tran sver se grooving on the runway life, on airc ra ft vibra-

tions, and on tire -trea d wear ar e as yet unknown and further ass es sment of potential

problems is required. However, the very promising res ult s obtained from limited tests

made on grooved runways indicate that further full-scale re sear ch on actual runways is

needed to evolve both beneficial and adverse effects encountered during operational usage.

On the

basis

of present available information, grooving of complete runways cannot

be

recommended at this t ime but installing tra nsv ers e grooves in paved runway overrun

areas appear s justifiable. In ti me s of emergency, the grooves in overr un

areas

would

be

available to help s top the ai rcraft , yet the grooved su rface would not inte rfere with

28


32/39

WHEEL

UNGROOVED ASPHALT ASPHALT

\ ,

THIGH TRACTION

\

I

I

I

I

400

I /

ANGULAR

VELOCITY, 800-

REV/ MIN

LOW TRACTION

, 1 1

0

~ / c m 2 LB/I N

FLOODED RUNWAY

(0.1

-0.2 IN.; 0.2-0.5 cm)

DRY RUNWAY

BRAKE

PRESSURE

689

0

0 .I .2 .3 4

5 6

.7 .8 .9

TIME, SEC

Figure

32.-

Effect of tr ans ver se grooves on wheel spin-u p. Grou nd speed zz

100

knots; F, =

12 OOO

Ib

(53.4

kN);

p

= 140

Ib/ in2

(96.5 N/cm2).

DIRECTION OF MOTION

-

GROOVE DIMENSIONS

5/16 IN. WIDE BY

1/8 IN. DEEP ON

3/4-IN. CENTERS

(0.79 cm B Y 0.32 cm

ON 1 .90 -cm CENTERS)

t

-

\ y

1

I

I I

0

.I

.2

.3 4

TIME, SEC

L-2551-39

Figure 33.- Effect of transverse grooves on reverted rubber skid. 32 X 8.8 aircraft t i re;

Ground speed

= 77

knots;

Fz = 16 OOO

Ib

(71.2 k N ) ; p = 250

Ib / in2

(172.4

N/crnz);

Water depth

=

0.1 to 0.2 in. (0.2 to 0.5 cm).

29


33/39

operations in n ormal landing

areas.

Also, some experience on pavement deterioration

through weathering effects of the grooving could be established.

Some tra ck tests were also conducted with longitudinal runway grooves, but the.

res ul ts wer e inconclusive and ar e not presented.

Air

Jets

Air jet rese arch to improve tire traction was first performed

at

the Langley

Resea rch Center in 1958; the re sul ts of th is initial work, performed on a sma ll wheel and

belt arrangement, are reported in refe renc e 25. Air jet research at Langley was re sumed

at

the landing-loads t ra ck in 1964, and some of the re su lt s obtained during thi s investiga-

tion (refs. 15, 16, and 18) are summarized i n the following paragraphs.

Figure 34

shows

a

view of the test fixture and air jets used in the cur rent test pro-

gram. The test tire was a regu lar 6.50 - 13 automobile

tire

inflated to a pressure of

2 7

pounds per squ are inch (18.6 newtons/centimeter2). Both

a

smooth-tread and

a

4-groove ribbed-tread

tire

were tested.

was a tandem-nozzle arrangement with the trai ling nozzle located about 7.5 inches

(19.0 centimeter s) behind the front nozzle and about 10 inches (25.4 centimeters) in front

of the

tire.

The

tire

in this figure is resting on a gla ss plate which was located flush

with the concrete s urfa ce in the test runway. Fr om beneath thi s gla ss plate, photographs

of the tire footprint we re taken

as

the ti re traveled acro ss the plate. The water over the

Essentially, the air

jet

arrangement, as shown,

/

\

- LASS

PLATE

Figure

34.-

Arrangement

of

air jets. Airflow

E 2.7

Ib/sec (12 N/sec);

10 20-2u65

Nozzle pressure

= 390

I b / i n2

269

N/cmz).

30


34/39

glass plate was colored with a green sea -ma rke r dye to give better contrast. These

photographs showed the ve ry beneficial effect which can be obtained by completely cl eari ng

water fr om the footprint (fig. 35). The left portion of the figure , without

air

jets, shows

the tire in a completely hydroplaning condition when traveling at a speed of 54 knots.

right portion of the figure , with

air

jet s, shows the good contact of the tire with the run-

way when the

tire

is

traveling

at a

speed of 87 knots. In th is te st , hydroplaning wa s

alleviated.

The

Without air jets

(Ground speed

=

54 knots)

With air jets

(Ground

speed =

87 knots)

L-255

-33

Figure

35.-

Pat h-c lea rin g effectiveness of tandem ai r jets (adapted fr om ref. 18).

Water depth

= 0.3 in. (0.8

cm).

The surface of the test runway shown schematical ly in figure

36 w a s

very smooth

except for

a

%foot (15.8-meter ) sec tion in the middle which w a s sandblasted in ord er to

have a surf ace texture somewhat

mo re repre sent ativ e of highways and

runways in use today. The begin-

ning of

the

sandblasted concrete sur

face had a n average te xture depth

of

0.104 millimeter as meas ured by the

gre ase technique described ea rl ie r

(fig. 10). This value is about half

the text ure depth of the float finished

concrete surfa ce also described e ar-

li er . (See fig. 11.)

. I

Figure 36.- Schematic of

leve l

runway surfaces

(adapted

from ref. 18).

Figure 37 presen ts va lues of locked wheel fri ct ion coefficient and hydrodynamic

pr es su re plotted against runway distance for one run. Thes e values wer e obtained at a

car ria ge speed of 90 knots, almost twice the hydroplaning speed of 47 knots, with the

ribbed-tread t ir e and

a

runway water depth of

0.3

inch (0.7centimeter). The runway su r-

face condition

is

noted in figu re 37; here were th ree sections, a smooth concrete section,

the sandblasted textured concr ete section, and another smooth concrete section. The

31


35/39

TEXTURED

SMOOTH CONCRETE SMOOTH

CONCRETE +*- CONCRETE

28- 40

HYDRODYNAMIC 14

- 20

PRESSURE

0

-14- 20

REF.

18

-

- i

4

I

I

I

F T

I I

I m

0 12 24 36

49

RUNWAY DISTANCE

Figure

37.- Ef fec t

of air j e ts

o n b ra k i n g t ra c t i o n .

Water depth = 0.3

in.

(0.8 cm);

G ro u n d s p e e d

=

90 knots .

resu lts with the air jet off show that hydrodynamic p re ss ur es above 40 pounds per square

inch 2 7 . 6 newtons/centimeter2) we re developed on the ti r e footprint and that ver y low

values of @SK I D we re obtained. With the air jet on, hydrodynamic pres sur e was

reduced to near zer o on the

tire

surface and

p s a ~

as substantially increased, espe-

cially on the textured concrete section where the friction coefficient is greater than 0.4.

The gradual decre ase in

SKID shown as the

t ire

traveled over the sandblasted run-

way section is due to

a

nonuniform texture achieved during sandblasting as indicated in

figure 36 .

Figure 38 pres ents wet runway corne ring force data as

a

percent of dry runway

values plotted against the sa me runway distance as in figure 37.

The yaw angle was 5 O

the water depth was

0.3

inch

0.7

centimete r), and the speed was

77

knots, which was

gr ea te r than the cri tic al hydroplaning speed. With the air jet off, less than 5 percent of

the dry runway cornering force is achieved; but, with the air jet on, more than 50 percent

of the smooth ti re d ry runway cornering fo rce

is

obtained in the textured concrete section.

Figure 39 summa riz es r es ul ts obtained with the locked wheel, the 4-groove ribbed-

tread tire, and 0 .3 inch (0.7 centimeter ) of runway sur fac e wa te r. The values of IJ-SKID

on both smooth and textured runway surfaces are plotted against forward speed. On the

smooth concrete surface, th ere

is a

relatively sm all difference between the resu lts

obtained with the air

jet

off and the air jet on because of the inability of the air jet to

remove the ver y thin fluid film which adheres to the smooth surface . On the textured

32


36/39

PERCENT OF

FORCE AT 5

YAW ANGLE

DRY

C O R N E R I N G ~ ~

40

2ot

SMOOTH TEXTURED SMOOTH

CONCRETE

I I

I I

I

I

I

I r n

0 6 12 18 24 30 36 42 49

RUNWAY DISTANCE

~

F i g u r e

38.-

Ef fec t o f a i r je ts o n co r n e r i n g f o rce . W a t e r d e p t h =

0.3 in. (0.8

cm) ;

G r o u n d sp e e d = 77 kno ts .

TEXTURED CONCRETE

.6

.4i

LOCKED-WHEEL . 'F IR

FRI CTI ON COEFFICIENT,

I

I I I

I

0 30 60 90 I 2 0

GROUND SPEED, KNOTS

F i g u r e 39.- Ef f ec t of g r o u n d sp e ed o n a i r - j e t p e r f o r ma n ce in i m p r o v i n g b r a k i n g t r a c t i o n .

W a t e r d e p t h

= 0.3

in.

(0.8

cm).

surfa ce, however, the many surfa ce irregu larit ies puncture this thin surface film , and

much higher values of

,USKID

a r e obtained with t he

air

jet on than with the air jet off.

CONCLUDING REMARKS

The purpose

of

thi s review has been to enumerate and evaluate the principal facto rs

involved in

t ire

tract ion los ses that develop during air cra ft operation on wet or flooded

runways in the light of available res earc h result s. The resul ts have shown that the

33


37/39

haz ard s to ai rc ra ft ground operation because of dynamic and viscous hydroplaning and

rubber tread reversion

are

signifi cant and can lead to conditions of low tra ct ion mo re

frequently than was previously realized.

Future landing operations may introduce more difficulties than in the past because

the number of high performance

jet

airc raft in operation

is

increasing

at a

rapid

rate.

Also, many more a ir por ts with s hor te r runways a r e expected to accommodate these je t

airc raft . These contingencies emphasize the need for expanding research on pavement

slipperiness and ai rcraf t braking system s, especially in the promising ar ea s associated

with pavement surface texture, pavement grooving, and air jet development.

Langley Researc h Center,

National Aeronautics and Space Administration,

Langley Station, Hampton, Va., April 17, 1967,

126-61-05-01-23.

34


38/39

REFERENCES

1. Hqrne, Walter B.; Joyner, Upshur T.; and Leland, Trafford

J.

W.: Studies of the Retar-

dation For ce Developed on an Aircraft Ti re Rolling in Slush

or

Water. NASA

.

TN D-552, 1960.

2. Horne, Walt er B.; and Joy ner , Upshur T.: Some Effects of Runway Slush and Wate r on

the Operation of Airplanes, With Parti cula r Reference t o Je t Transpo rts. SAE

Trans., vol. 70, 1962, pp. 99-108.

3. Horne, Walte r B.; and Leland , Tra ffo rd

3.

W.: Predict ion of Slush Drag on Air craf t

Performa nce. Joint Technical Conference on Slush Drag and Braking Prob lems.

FAA and NASA, Dec. 1961, pp. 79-93.

4 .

Horne, Walter B.; and Leland, Trafford J. W.: Cor rela tions of Braking on Slippery

Surfaces.

Joint Technical Conference on Slush Drag and Braking Problems . FAA

and NASA, Dec. 1961, pp. 153-162.

5. Horne,

Walter

B.; and Leland, Trafford

J.

W.: Influence of Ti re Tr ea d Pat te rn and

Runway Surface Condition on Braking Friction and Rolling Resistance of a Modern

Ai rcra ft Ti re . NASA TN D-1376, 1962.

6. Horne,

Walter

B.:

Slush and Water Drag Effects on

a

Bogie Landing Gear. 15th

Annual Int er. F light Safety Foundation Seminar "Notes," Otto E. Kirchner, S r.,

repor te r , Airplane Div., The Boeing Co., 1962, pp. 99-101.

7. Joyner, Upshur

T.;

and Horne, Wal ter B.: NASA Tra cti on Test s.

Interceptor, vol. 4,

no.

1,

Jan. 1962, pp. 8-13.

8. Joy ner , Upshur T.; Horne, Wal ter B.; and Leland, Trafford

J.

W.: Investigations on

the Ground Per for mance of Airc raft Relating to Wet Runway Braking and Slush Drag.

AGARD Rept. 429, Jan. 1963.

9. Anon.:

Hazar ds of T i r e Hydroplaning to Air craft Operation. NASA Langley Resea rch

Cent er F ilm Seri al No. L-775, 1963.

10. Leland, Trafford

J.

W.; and Horne, Walter B.: Considerat ions on T i r e Hydroplaning

and Some Recent Experimen tal Results. NASA paper presented at the Paper Session

of Committee E-17, Am. SOC.Testing Mater., Atlantic City, New Jersey, June 26,

1963.

11. Horne, Walter B.; and Leland, Trafford J. W.:

Runway Slipperiness and Slush.

J .

Roy.

Aeron. SOC.,vol. 67, no. 633, Sept. 1963, pp. 559-571.

12 . Horne, Walter B.; and Dr eh er , Robert C.: Phenomena of Pneumatic T i r e Hydroplaning.

NASA TN D-2056, 1963.

35


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13. Horne, Walter B.: Haz ards of Tir e Hydroplaning. Ninth Annual Bus ine ss Aircra ft

Safety Seminar , Flight Safety Found., Inc., Apr. 1964, pp. 12-13.

14. Cobb, Jere B.; and Horne, Walte r B.: Per for mance on Slippery Runways in CrosS-

winds.

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Air Traffic Control, vol. 6, no. 6, May 1964, pp. 16-20.

15. Horne, Walter B.: Air J e t s

-

A Possible Solution to Hydroplaning and Other Associ-

ated Runway Wetness Problems. 17th Annual International Seminar. Clifford P.

Seitz, ed., Flight Safety Found., Inc., Oct. 1964, pp. 59-65.

16. Horne, Walter B.; and Joyner, Upshur T.: Pneumatic Ti re Hydroplaning and Some

Effects on Vehicle Performance.

p re pr in g 970C, SOC.Automotive Engrs., Jan.

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17. Leland, Trafford J . W.; and Taylor , Glenn R.: An Investigation of the Influence of

Ai rc ra ft Tir e- Tr ea d Wear on Wet-Runway Braking. NASA TN D-2770, 1965.

18. Horne, Walter B.; and Joyner, Upshur T.: Tra cti on of Pneumatic Ti res on Wet Run-

ways. Conference on Air cra ft Operat ing Prob lems , NASA SP-83, 1965, pp. 9-17.

19. Smiley, Robert F.; and Horne, Walter B.: Mechanical Proper tie s of Pneumatic Ti re s

With Special Reference to Modern Aircraft T ir es .

NASA TR R-64, 1960. (Super-

se des NACA TN 4110.)

20. Mechtly, E. A.: The Inte rnat iona l System of Units - Physical Constants and Conver-

sion Fa ct or s. NASA SP-7012, 1964.

21.

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Aquaplaning on Runways.

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22. Obertop, D. H.

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24. Judge, R. F. A.: A Note on Aquaplaning and Surface Treatm ent s Used To Improve the

Skid Res ist ance of Airfield Pavement s. 8556/PS Min. Public Bldg. Works (Brit.),

Oct. 1965.

25. Harrin, Eziaslav

N.:

Investigation of Tandem Wheel and Air -Je t Arrangement s for

Improving Braking Fricti on on Wet Surfaces . NASA TN D-405, 1960.

Documents

REVIEW OF CAUSES AND ALLEVIATION OF LOW TIRE TRACTION ON WET RUNWAYS