FACTORS AFFECTING WHEEL BRAKING:A) TIRE BRAKING

1) BRAKING IS MAX WHEN :ACTUAL WHEEL SPEED = 10 % FREE ROLLING SPEED ( free to rotate speed)SLIP RATIO = ACTUAL SPEED / REFRENCE SPEED

TIRE GRD FRICTION

TIRE SLIP 0 10 % 100 % FREE ROLLING LOCKUP

2- VERTICAL LOAD ON A/C: MUST BE PLACED TO GENETAE BRAKING FORCE NLG : GROUND MAX VERTICAL LOAD ON A/C GRD SPOLIERS: UP ( MOST OF A/C WEIGHT ARE ON WHEELS)

~ 50% DRAG INCC

TOUCH DOWN NOSE GEAR DOWN GROUND SOPILERS

( NEGLIGIBLE (~ 30 % WEIGHT ON TIRES) (~ 100 % WEIGHT ON TIRES)WEHIT ON TIRE)

- PROTECTIONS OF ANTISKID SYSYTEM :

TOUCH DOWN PROTECTION- TO ENSURE WHEELS ARE FREE TO ROTATE DURING TOUCH DOWN BY USING AN ARTIFICIAL SKID RELEASE SIGNAL TO HOLD THE BRAKES RELEASED FOR A SHORT MOMENT AFTER T. DOWN

LOCKRD WHEEL PROTECTION- TO PREVENT ANY

WHEEL FROM BEING LOCKED TO MAINTAIN MAX GRD FRICTION

- THE LOCKED WHEEL WILL BE FULLY RELEASED BY A.SKID SYS

B) RUNWAY CONDITION

WHEN A WHEEL ROLLS ALONG A WET RUNWAY, IT SQUEEZING A WATER FILM WHICH GENERATES PRESURES .PRESSURE CAN LIFT THE TIRE OFF THE RUNWAY AND REDUCING FRICTION. THIS ACTION IS CALLED HYDROPLANING. FRICTION IS LOW AT HI SPEED AND CAN IMPROVE WITH SLOW SPEED

- HYDRO PLANING : HYDROPLANING = A TIRE MOVING ON A WET SURFACE

THIN FILM OF WATER SQUEEZED

NOTE: REFRENCE SPEED THE KEY FOR A/SKID OPERATION.WITOUT IT,SKIDDED OR LOKCED WHEEL CANT BE DETECTED

ICY RUNWAYS

-ICY ( & FROST, SNOW) RUNWAYS WITH TEMP NEAR FREEZING

VERY SLIPPERY AT ALL SPEEDS

- VERY COLD ICY & SNOW RUNWAYS CAN GENERATE HI ENOUGH FRICTION ( BECAUSE IT ACTS AS DRY RUNWAY)

WHEEL

1- FOR ROTATING WHEEL

1st) VISCUS HYDROPLANING : FRICTION IS REDUCEDON WET RUNWAYS BUT WHEEL STILL IN SPIN UP . LOW FRICTION IS NOT SO LOW TO ACTIVATE A/SKID SYS. THIN FILM OF WATER ACTS LIKE LUBRICANT.

(THAT OCCURS ON SLIPPERY RUNWAYS)2nd) DYNAMIC HYDROPLANING : FRICTION

IS ALMOST NILL SO THE TIRE IS OFF THE RUNWAY AND RIDES ON A WEDGE OF WATER. SO RARE CONDITION.

(THAT OCCURS ON FLOODED RUNWAYS)

2- FOR LOKEDUP WHEEL ( REVERTED RUBBER HYDROPLANING):

FRICTION HEAT CAUSES STEAM- STEAM PRESSURE - LIFTE OFF THE TIRE- STEAM HEAT - REVERTS THE RUBBER TO A BLACK GUMMY DEPOSITE

STOPPING FORCE

EFFECTIVE IN HI SPEED. 80 % TOTAL FORCE ON SLIPPERY RUNWAY AT HI SPEED EFFECTIVE IN LOW SPEED. 20 % TOTAL FORCE ON SLIPPERY RUNWAY AT HI

SPEEDEXAMPLE : F = M x ACCELERATION FOR B747 : W = MASS= 500,000 lb DECELRATION = .5 g F = 250,000 lb

- COMPARISON

SPEED DRY WET ICYHI 50 % DRAG + T/R

50 % BRAKING80 % DRAG + T/R20 % BRAKING

80 % DRAG + T/R20 % BRAKING

LOW 90 % BRAKING10 % DRAG + T/R

90 % BRAKING10 % DRAG + T/R

70 % BRAKING 30 % DRAG + T/R

- WHEN STOPPING FORCE , STOPPING DISTNACE

STOPPING DIST( % OF DRY RUNWAY)

(C) (B) (A)

- ICY WET DRY

(1st) : SPOILERS + T/R + BRAKING(2nd) : SPOILERS + BRAKING(3rd) : BRAKING

- WHEN A/C SPEED : TIRE BRAKING EFFECT

T/R EFFECT IS HIGHER - ON WET RUNWAY:

DRAG ( SPOLIERS)

THRUST REVERSER

WHEEL BRAKING

- TOTAL STPPING FORCE AVAILABLE = 70% DRY RUNWAY FORCE AVAILABLE ( DUE TO TIRE BRAKING LOWER EFFECT)