View
216
Download
3
Category
Preview:
DESCRIPTION
A/skid
Citation preview
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)
Recommended