NASA Research ABS Aircraft

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SOAkP1WFZGTS OF ADVERSEWEATHER CONDITIONS ON


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2. Government Accession No.- I. Report No.NASA TN D-82024. Title and SubtitleSOME EFFEC TS OF ADVERSE WEATHER CONDITIONS ON

PERFORMANCE OF AIRPLANE ANTISKID BRAKING SYSTEMS7. Authods)

Walter B. Horne, John L. McCarty, and John A. Tan ner~~~~

9. Performing Organization Name and AddressNASA Langley Research CenterHampton, Va. 23665

12 . Sponsoring Agency Name and AddressNational Aeronautics and Space AdministrationWashington, D.C. 20546

15. Supplementary Notes

3. Recipient's Catalog No

5. Report DateJuly 19766. Performing Organization Code

8. Performing Organization Report No.L- 10690- ~~10. Work Unit No.505-08-31-01

1 1 , Contract or Grant No.

13 . Type of Report and Period CoveredTechnical Note

14 . Sponsoring Agency Code

. ~~16. AbstractThe performance of c urre nt antiskid braking sys tem s operating under adv erse weather

conditions was analyzed in an effort to both identify the c au se s of locked-wheel skid s whichsometimes occur when the runway is slippery and to find possible solutions to this opera-tional prpblem. This an alysis was made possible by the quantitative test data provided byrecently completed landlng res ea rc h progr ams using fully instrumented flight test airplanesand w a s further supported by tests performed at the Langley a irc ra ft landing loads and tr ac -tion facility. The antiskid syst em logic for bra ke c ontrol and for both touchdown and locked-wheel protection is described and its response behavior in adverse weather is discussed indet ail with the aid of available data. The ana lysi s indicates that the operation al per form ance


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SOME EFFECTS OF ADVERSE WEATHER CONDITIONS ON PERFORMANCEOF AIRPLANE ANTISKID BRAKING SYSTEMS

Walter B. Horne, John L. McCarty, and John A. TannerLangley Res ear ch Center

SUMMARYThe perf orma nce of c ur re nt antiskid braking s ys te ms operating under adv ers e

weath er conditions wa s analyzed in an effort to both identify th e ca us es of locked-wheelski ds which so me tim es occur when the runway is slippery and to find possible solutionsto t his operational problem.data provided by recen tly completed landing r es ea rc h pro gr am s using fully instrumentedflight test airpla nes and was further supported by test s performed at the Langley air cra ftlanding load s and tra ctio n facility.both touchdown and locked-wheel protection is described and its resp onse behavior inadver se weather is di sc us sed in de ta il with the aid of available data.ca tes that the operational perfo rman ce of the antiskid logic ci rcu its is highly dependentupon wheel spin-up accel eratio n and can be ad vers ely affected by c erta in pilot brak inginputs when accelerations are low.runway traction is sufficient to provide high wheel spin-up acc ele rat ion s or if the systemis provided a continuous, a cc ur at e ground speed ref ere nce .is complicated by the necessi ty for trade-o ffs between ti re braking and c orne ring capa-

Thi s anal ysi s was made possib le by the quantitative test

The antiskid sy ste m logic fo r brak e control and forThe analy sis indi-

Normal antiskid performance is assured if the t i re-to-The design of antiskid s ys te ms


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Historically, airpla ne skidding accide nts under ad ve rs e conditions re su lt mo st oftenin hull damage to the airpla ne with occasional pass enge r and cre w injuries, but s eldom isthere los s of life. However, the potential do es exist fo r c atastr ophic skidding acc identsto occur, especially sinc e the advent of wide-body jet tr an sp or ts car ryi ng la rg e numbersof pas sen ger s. Thus, a need exi sts to uncover and to e limina te the c au se s fo r skidding.

Studies of air pla ne skidding incidents and accid ent s have reve ale d that th er e we re acons idera ble number of ins tan ces in which skid pat che s (flat spots ) wer e found on the tiresdur ing post incident/acciden t inspection; th is would indi cat e tha t the wheels had locked up.However, in many of the se instances, inspection al so showed tha t all ele me nt s of the land-ing gear system, including wheels, br akes, and skid-c ontrol units, wer e operating prope rlyand within their respe ctive toler ances.normally functioning antiskid systems suggested the possibility that inaccurate or e r r o -neous information was being supplied to the antiskid logic ci rc uit s when s lippe ry runwayconditions existed.

The fact that wheel skid s were occu rring with

It is, of c our se, highly des ira ble to avoid wheel lockups bec aus e they lead to tir eblowouts on high friction s urfac es, they red uce braki ng friction on wet su rfa ces , and theycause a complete l os s of ste eri ng control on all surfaces.ples.)dents or accidents that occur when the airpla ne dep ar ts the runway may st ill b e a problemdue to insufficient tire-runway traction.

(See refs. 2 and 3 for exam-However, it should be pointed out that, even without wheel lockups, landing in ci -

The purp ose of th is paper is to analyze the perform ance of antiskid braking s ys tem sunder a dv er se weather conditions in an effort to both identify the ca us es of locked-wheelskids that som etim es occur when the runway is slippery and to find possible solutions to


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system req uire s posi t ive wheel accelerat ion after touchdown and after brake release fol-lowing spin-down.esp eci ally und er ad ve rs e wea the r conditions, by examining the wheel equation of motionand by pres enting som e ti re hydroplaning considerations.

This sec tion of the paper di sc us se s wheel spin-up c harac teristic s,

Equation of MotionThe exter nal forc es and moments acting on an airplane wheel under braked rolling

conditions are identified in figure 1.or moments acting on the wheel may be written as follows:

The equation of motion which re la te s the net tor que s

whereWL

I-1

'1

F W

XC

fr ac tio n of a irpla ne weight supported by a wheel and tirefra ctio n of air pla ne lift acting on a wheel and ti ret i re -ground fric tion coefficient (includes unbraked rollin g resist anc e)ra di us of loaded tire,fluid dr ag acting on ti re (ze ro on dry surfa ce)fore -and- aft location of wheel ground react ion with res pe ct to axle center line

T i re free radius - Vertic al t ir e deflection


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The second term, Fw rl , ,re pr ese nts the spin-up moment act ing on the wheel and isdue to the fluid (water, slush, etc., on the runway) dr ag for ce. Th is moment, which tendsto produce a positive or wheel spin-up acceleration, inc re as es with inc reasing fluid depthand air pla ne ground spe ed up to the onset of dynamic hydroplaning. However, the accel-erat ions due to this moment in no way comp ensate fo r the lo ss in spin-up accele rationass oci ated with redu ced tire-g round fric tion under conditions of higher spe eds and fluiddepths.

The next ter m in the equation,The location of thi s force r elative to the axle ce nter line is a function of the tire

(W - L)xc, represents the moment which is devel-oped when the ground reac tion f or ce (W - L) does not pas s through the wheel axle cen terline.opera ting mode and of the sur fa ce wet nes s condition. The moment (W - L)xc tends tospin up the wheel during braking on high friction su rfa ce s as the react ion forc e movesbehind the axl e ce nt er line. However, as pointed out in refe re nc e 9, the react ion forcetends to move ahead of the axle, producing a spin-do6n moment under all unbraked rollingconditions and under so me high-speed, low friction, bra ked conditions.

The last t e r m in the equation of motion, Tb, is the torque on the wheel; this torqueresu lts f rom ei ther brak e application or res idua l torque in the system.

Hydroplaning ConsiderationsResearch has indicated that nonrotating, unbraked wheels, such as exist at air-

plane touchdown, will not spin up on a flooded runway surface due to dynamic hydroplaninguntil the airplane has de cre ase d it s ground speed to a value equal to or below a cri t icalground speed. ptThat critical ground speed in knots is approximately 0.093,&, where


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standing water o r slu sh on the runway f or dynamic hydroplaning to o ccu r, beca use of thepath-clearing action of the front wheels.

ANTISKID BRAKING SYSTEM OPERATING PRINCIPLESOriginally, antiskid braking systems were designed purely to prevent the deleterious

eff ect s of wheel lockups. Cur ren t sy ste ms are much more sophisticated, however, inthat they are req uir ed not only to preven t the tir e fr om skidding but als o to maintain max-imum braking e ffort under all weather conditions. Ideally, these sy st em s are designed toope rat e at wheel sli p rat ios (the ra tio of r ela tive s lip velocity between the tir e and thesur fac e to the airplan e ground spee d) up to and including that ra tio nec es sa ry fo r develop-ing the maximum available frictio n coefficient between the tir e and the surf ace . To meetthese design objectiv es, the antiskid contro l s ys te m must have knowledge of both the air-plane ground spee d and the instantaneous angular velocity of each braking wheel.the sy ste m must provide a means for reducing the hydraulic pre ssu re to a brake d wheelwhen an excessiv e skid is detected and a means f or reapplying the p re ss ur e when thewheel speed recovers.

Further ,

Each braking wheel of the airplane is equipped with a sen sor which produces a signalproportional to the angular velocity of the wheel. Most cur re nt antiskid sys te ms dependupon the se brak ed wheel-speed se ns or s to provide the information to the antiskid elec-tro nic logic c ir cu its needed to de termin e both the airp lan e ground speed and the wheelangul ar velocity. However, som e syst em s have been developed which rely upon se ns or slocated on eith er unbraked nose ge ar wheels o r on sma ll auxiliary wheels which providethe airplane ground speed to the skid-control logic.


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backup to the brake control circuit. These circuits are described in som e detail and areanalyzed in te rm s of th ei r behavior under ad ve rs e wea ther conditions in th e followingparagraphs.

Brake ControlGeneral description.- The pri ma ry function of the antiskid sys tem is to control the

braking effort by constantly readjusting the brake pr es su re so as to maintain a braketorque sufficient to balance the maximum friction f or ce available between the ti re and therunway. Bet ter and m o re efficient bra ke cont rol ha s been the object of engineering devel-opment sinc e antiskid sys tem s were first introduced. In the earlier syst ems, the antiskidlogic cir cu its monitored the ra te of change in the wheel ang ular velocity by mean s of abraked wheel-speed se ns or as hydraulic pre ssu re w as applied to the wheel brake. Whenthe tire-to-ground adhesion lim it was exceeded, the wheel began to skid and rapidly dece l-erat ed toward a locked-wheel condition. However, upon rea chi ng o r surpassing a prese twheel angular acceleration (velocity rate threshold), he antiskid syst em generated' anelec tric al signal that commanded the ser vo control valve to r educe hydraulic brake p res -sure to zero. Once the pressure was relieved, the wheel was free to spin up, providedthat the tire/runway friction torque exceeded any residual brake drag and bearing frictiontorques . The full brake pre ss ur e was then reapplied and the cycle was repea ted whenanother sk id was ente red.

La ter antiskid sy st em s modulated the reapplication of b rake p re ss ur e following entryinto a skid. In these sy ste ms , the hydraul ic pr es su re was modulated on the basis of thedepth of the previo us wheel skid; that is , the rate of change of the angu la r veloci ty of th e


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The latest antiskid s ys te ms have de parte d from the us e of velocity rate thresholdsas governing par ame ter s. Instead, these sy stem s rely upon either slip velocity or sl ipratio, either of which req uir es a knowledge of both the ai rpl ane ground sp eed ref er en ceand the braked wheel speed. For example, one of the latest sys tems tries to maintain asl ip velocity of app roximately 3 m/sec (10 ft/sec).logic continuously co mp ar es the instantaneous wheel angular velocity with the r ef er en ceground speed as determined from a brake d wheel-speed senso r. When the braked wheelsli p velocity excee ds the 3 m/s ec (10 ft/sec) threshold, a signal is generated to the servocontrol valve to red uce brak e pr es su re , allowing the wheel to spin up. When the wheelspeed approaches o r equals the ref ere nce ground speed, a signal is trans mitted to theser vo control valve to incr eas e brake press ure ; this pre ss ur e gener ates another wheelskid and the proce ss is repeated. Another antiskid syste m tr ie s to maintain a preselectedo r calculated s lip r atio by continuously comp aring the instantaneous wheel angular velocitywith the airpl ane ground spe ed as determined from a nose wheel senso r. In this model,brake pressur e is reliev ed when the bra ked wheel sl ip rati o excee ds the command valueand is reapplied when the wheel speed appro aches the re fer enc e ground speed. Note that,as opposed to the lat er sy ste ms , most ear ly antiskid system s did not re qui re a referencespeed or an actual mea su re of th e braked wheel speed in the ir skid detection logic.Inste ad, the information provided by the braked wheel-speed se ns or was used only todetermine wheel deceleration levels.sti ll retain the adaptive pr es su re modulation feature of the ea rly s yst ems ; that is , thelevel of applied brake pr es su re is a function of the runway traction as detected in theprevious skid.

During braking , the skid-control

However, most of the advanced antiskid systems

Behavior in ad ve rs e weath er.- The effectiveness of antiskid brak e contro l can be-. ___lost or , at least , severely reduced if the friction gener ated between the ti re and the run-


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Figure 2 illustrates the antiskid bra ke co ntrol res pon se of the wheels on a four-wheel bogie of a l a rge jet tran spo rt to brake applications that took place pri or to andafte r full wheel spin-up du ring a landing on a runway wetted to an average water depth ofapproximately 0.06 cm (0.024 in.). (See ref. 7.) Time histories pf the an gular velocityof the four wheels durin g and following brake application which occ urr ed p ri or to wheelspin-up are prese nted in figure 2(a). Based upon the wheel-speed information that wasreceived up until the time the bra kes we re applied, it is apparent that the airpla ne groundspeed assumed by the skid-control logic cir cui ts for the two front wheels immediatelyfollowing brake application is consi derab ly below the actua l speed of the airpla ne. Thelow wheel spin-up acc ele rat ion s following brake release during each braking cycle pr e-vented the two fron t wheels f ro m attaining synchronous spe ed (equivalent vehicle groundspeed) until approximately 30 sec after touchdown. By that time , the airpl ane had slowedto the point where the friction between the tires and the pavement had in cr ea se d sufficientlyso as to produce overpowering wheel spin-up accele'rations; thus, t he wheels are caused toexceed the assu med low ground speed ref erenc e and to acquir e a higher one.shows that, after se ve ra l cyc les of such behavior, the skid-control logic finally acqu iredthe pro per ground speed ref ere nce . At that point sat isf acto ry contr ol of braked wheelmotion was established and maintained down to a stop.

Figure 2(a)

The benefit path cl ea ri ng provided by tandem wheels is als o illustrated by the dataof figure 2(a). The data obtained from the antiskid logic circuits clearly indicate that therear tandem wheels recover synchronous speed at a much faster rate than the front tandemwheels. The highe r wheel spin-up rates assoc iated with the r e ar wheels sugge st that theydevelop bet ter tractio n than the fron t wheels of the landing ge ar .tandem ge ar rem ove s the bulk water and tends to pene trat e any thin fluid film on the pave-

The leading wheel in a


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Figure 3 i l lust rates brake control los s on a low frictio n surf ace due to rapid brakeapplication after ful l wheel spin-up.re co rd obtained during flight tests of an a irpla ne equipped with a dual wheel m ain landinggear i n the Joint FAA/USAF/NASA Runway R ese arc h pro gra m descr ibed in ref er en ce 1.For this test, the airplane first touched down on a dry surfa ce to as su re full spin-up ofall wheels and then trav er se d a sec tion of the runway th at had been wetted by wa ter trucks .Following entry in to the wet se ction, the pilot c alled fo r immed iate maximum braking.The outputs fr om se ns or s which monitored the applied brak e pr ess ur e and the angularvelocity of each of t he fou r wheels ove r a time period extending fo r roughly 7 sec afterbraking was initiated are prese nted in figure 3.of both the brake pr es sur e and the wheel angular velocity of the outboard wheels (wheels 1and 4) were approximately the same shape and magnitude; both wheels locked up after onlythree brake pre ssur e cycles. From the figure, i t is apparent that the antiskid control fo rthese two wheels dropped out at about 2.6 s e c, allowing full-system pressure applicationto the wheel brak es and thus preventing any possibility of wheel spin-up.brake control is typical f or velocity- rate dependent, skid-detection logic circui ts that donot r equire o r even use a ground speed ref ere nce . Newer antiskid sy ste ms supplied withan acc ura te ground speed refe ren ce would not have ex perien ced this lo ss of brak e co ntrol.The skid-control logic syste m for these two wheels a ssume d that the airplane was simplyrapidly decelerati ng on the runway r ath er than decele rating at a rate defined by the slopeof the dashed line noted on all four wheel angular velocity tra ce s.

This figure is a reproduction of an osc illograph

The figure shows that the time h isto ries

This lo ss of

(See fig. 3.)The test r esu l ts presented in figure 3 show that the right inboard wheel (wheel 3 ) was

the only wheel on the main ge ar of the airplane over which the antiskid sys tem exhibitedoptimum control.into deep skids.

The left inboard wheel (wheel 2) w a s allowed to overbrake and to enterAs w a s true for the outboard wheels (wheels 1 and 4), the behavior of


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diffe renc es can be attribute d to the runway crown. Both lig hte r loaded outboard wheels(wheels 1 and 4) api dly spun down to a locked-wheel skid, but the logic ci rc uit s fo r theinboa rd wheels (wheels 2 and 3) maintained skid control, although wheel 2 did encountersome deep skids.

Figure 4 pr es en ts the angu lar veloc ities of the outboard wheels on a four-wheel bogieof an air pla ne equipped with an ea rl y antiskid design. Th es e angu lar veloc ities we rerec ord ed during heavy brakin g on flooded grooved and ungrooved sur fac es. The longitu-dinal dece lerati on of the ai rpl ane is also presented in ord er to provide some me asure ofthe overa ll braking effort. The purp ose of this figure, which was taken fro m refe ren ce 5,is to illu str ate the effects of t i re tr ead and surf ace texture on antiskid braking response.As w a s done in the previou s exa mple , the air pla ne ente red the wet section of the runwayfollowing full wheel spin-up on a dry surface.tre ad t i re s, whereas figure 4(b) prese nts data from relat ively new, rib-tread t i r esequipped with five groo ves. In both te st s, the airpl ane tr av er se d fir st the grooved and thenthe ungrooved runway te st sec tion . Fig ure 4(a) shows that the leading wheel rapidly spundown as a re su lt of braking action. The 105-knot air pla ne ground spee d at entry into thetest sect ion is well above the hydroplaning sp eed fo r this tire pt = 7 58 kP a (110 psi));hence, the wheel spin-up acc ele rat ion following any brake re l ease is too low to permit thewheel to rea cqu ire synchronous speed. Actually, the ti re is in a hydroplaning mode as i tleave s the grooved section and it rem ain s in that mode ove r the en ti re length of theungrooved sec tion . The benefits of path cl ea ri ng are clear ly illustrate d in this figure bycomparing the angular vel oc itie s of the leading and tr ai lin g wheels. The depth of wa te rtra ver sed by the rear t i r e is apparently le ss than that encountered by the fron t ti re sincethe rear t i re is not hydroplaning on the grooved section. Th e good ski d control that wa s

Figure 4(a) pre sen ts data from smooth-

(


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r

f o r the leading wheel, which oc cu rr ed at roughly 2 sec , se ems to verify this est imate.Although the wheel is operating at below hydroplaning spee d beyond that point, t heungrooved surface is still sufficiently slip per y to re ta rd wheel spin-up acc ele rat ion s andthus to hinder antiskid performance.

To remedy the partial o r complete lo ss of brake control in pre sen t antiskid sys tems ,the pilot must release the bra ke s, allow the wheels to sp in up to the air pla ne synchronousspeed, and then rec ycl e the brak ing effort. Thi s action is neces sary whether l os s of brakecontrol is attributed to brake application prio r to o r subsequent to full wheel spin-up. Itwould ap pear f rom the data presen ted he re that brake control prob lems could be avoidedwith new antiskid sys te ms by providing th ei r logic circ uits with an ac cu ra te refe ren ceground speed that can be continuously re la ted to the spe ed of ea ch braked wheel. Brakecontrol pro blems with both cur ren t and future antiskid s ys te ms essentially could beavoided i f sufficient traction ex isted between the tire and the runway to provide rapidwheel spin-up following bra ke r el ea se .runway conditions, but i n adver se weather the keys to good tractio n include ri b-t rea dtir es and a runway su rfa ce which pro vides good drain age.obtained by grooving or by applying a porous overlay.)

Good tract ion is generally available under dry(Good runway dr ainage can be

Touchdown ProtectionGeneral description.- Most antiskid s ys tem s ar e provided with a touchdown pro tec -

tion cir cui t to preven t hydraulic p re ss ur e fr om being applied during the touchdown phaseof a la nd kg should the pilot inadvertently apply brak es. This circui t is gene rally .deacti-vated by squat sw itch es on each landing gea r , by the wheels spinning up to o r exceeding apred eterm ined angular velocity, or by a combination of the two.

. .~

Upon deactivation, normal


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l o s s is delayed wheel spin-up. Delayed wheel spin-up can re su lt from e ith er landing on aflooded runway when the touchdown speed is in excess of the tire dynamic hydroplaningspeed or fro m landing on a low tractio n runway under low tire loading which would result,fo r example, fro m a delayed spoiler deployment.

Figure 5 i l lus t ra tes a prolonged spin-up of a n unbraked wheel d ur ing touchdown on aflooded tes t su rfa ce with a n average water depth of 0.5 cm (0.2 in.) at the Langley aircraftlanding loads and tracti on facility. Touchdown occ urs when vert ica l load is first appliedto the tire. The figure indicates that the tire was e ssen tial ly fully loaded at 80 kN(18 000 lb) approximately 1 s e c afte r init ial touchdown; however, the wheel did not spinup to full vehicle (ca rri age ) speed until approximately 5 sec later. Although the wheeldid attempt to spin up 2 1 s e c after touchdown, p erh ap s due to shallow water conditionsin a runway sec tion , the wheel speed nev er actually approached synchronous s pee d untilthe car ri ag e had slowed to approximately 90 knots. It is int ere sti ng to note that with theti re tes t inflation pr es su re of 965 k Pa (140 psi), the cd cu lat ed wheel spin-up speed was91.1 knots.protection s ch eme s inco rpor ate delay periods of less than about 5 sec , the antiskid sy s-tem would not have been activated and the pilot would have been in a manual brakingmode following that period.

2

The significance of figure 5 is that, f or thos e a irpl anes whose touchdown

The test describe d in figure 6 was s imi lar to the tes t de scribed in figure 5, but

1remained above the tire hydroplaning spin-up speed throughout the initial 2- sec , the2t i r e did not spin up. Obviously, if the touchdown protec tion delay period had been withinthat time fr am e, the antiskid s yst em would not have been activated and brak e pre ss ur e

with brake application approximately 2-1 sec after touchdown. Since the c arri age speed2


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up to the car ri ag e synchronous speed. Although it is not shown he re , the application ofbrakes beyond that point re su lte d in good antiskid control. The information providedsug ges ts that the l os s of touchdown pr otectio n could be avoided i n new antiskid sy st em sby providing the logic circ uit fo r those s ys tem s with as accurate a refere nce groundspeed as could be obtained fr om , fo r examp le, an onboard navigational aid sys tem .an ac cu ra te knowledge of the ground s pee d, the logic cir cu it fo r touchdown protectioncould de ter min e when antiskid protection is needed and when the airplane is parked. Ofcourse, all cu rr en t touchdown protecti on sch em es would be sa tisfac tory i f the runway sur-face offered sufficient traction to avoid prolonged wheel spin-ups.

With

Locked-Wheel ProtectionGeneral~ description.- Locked-wheel protection circuits are provided in antiskid

logic syste ms to guard against a dragging brake o r an oth erwis e abnormally slow wheelrecovery from a deep braking skid. The se circuits prevent hydraulic pre ss ur e from beingapplied to the slow wheel until i t has rec ove red to a speed determ ined by the logic s yste m.In essen ce, the locked-wheel protection circui t is a backup to the skid- contr ol circ uit. Onsom e airc raf t , the rotat ional speed of all wheels is incorporated under a single locked-wheel protection ci rcu it; however, mos t antiskid sy ste ms provide this protection by eit herpairin g two wheels o r combining wheels fro m opposite si de s of t he airp lane in sep ar at ecircui ts.may be paired together.com par ed and, whenever the sp eed of one wheel is below a pre sel ect ed value of i ts com-par ed ma te, the br ake of that wheel is rele ase d to permi t the slow wheel to spin up.

For example, the front outboard wheels of the left and right main landing gearsIn this and other like pai rs , the two wheel spee ds ar e continuously

Behavior in adv ers e weather.- To be completely fai l-sa fe, the speed of the slowwheel should be co mpare d with the ai rpla ne synchronous speed si nce all the wheels on a


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already been pointed out by using figure 2; however, were it not fo r the "dialogue" betweenthe front and rear wheels in each locked-wheel protecti on cir cuit , the front wheels in thistest would have most assu redl y locked up because of thei r dee p sk ids and slow spin-upaccelerations.

Ineffective wheel pairi ng fo r locked-wheel protect ion is illustrate d in figure 3 wherethe inboard and outboard wheels a r e again on sep ara te c ir cu it s, only in this ca se the air-plane is equipped with a dual wheel gea r and the benefits of path c lea rin g cannot berealized. As pointed out in the e a rl ie r disc ussi on of th is fig ur e, the wheel loading whichres ult s when the ai rplane lands on or near the cen ter line of a crowned runway subjectsthe inboard wheels to high er spin-up acce le rat ion s than the outboard wheels. The lowtrac tion conditions ass oci ate d with this t est for ced both of the lig ht er loaded outboardwheels to lose br ake control and rapidly spin down at essentially the sa me rat e, althoughthis rate is higher than that of the airplane.pri se d one locked-wheel protection circ uit and the re was no othe r sour ce of a irpl aneground speed information fo r compar ison purpose s , protection was lost and both wheelslocked up. Fig ure 3 does show, however, that a pair ing of the inboard wheels appea redto have preven ted lockup of the number 2 wheel which, for some reason, lost brake con-tro l logic. It is inte rest ing to note that the runway used in the tes t des crib ed in figure 2was a tilted sl ab with no ce nter -lin e crown; hence, the loading and bra ked behavior of thecorresponding inboard and outboard wheels we re essentia lly the s ame .

Since the two outboard wheels together com-

To su mma rize , los s of locked-wheel protection can occur under adv erse weatherconditions as the result of both poor tire traction, which ca uses a lo ss of brak e controland pe rmi ts l ar ge wheel motion excu rsio ns, and ineffective wheel pairing.this loss is that pair ed wheels de celerate to a locked-wheel condition because, although

The effect of


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dat a obtained fro m a single braked wheel ov er a ran ge of yaw (ste eri ng) angles duri ngtra ck tes t pro gra ms can shed som e light on airpla ne behavior under antiskid controldurin g cross-wind operations.during the program described in reference 11, show the variation of d rag- and sid e-f orc efriction coefficients with slip r atio during a single bra ke cycle fr om fre e rolling to wheellockup. The test was performed for a 20 X 4.4 airplane t ire at nominal yaw ang les of Oo ,5O, and 16O. Data are included fo r a dr y sur fac e (fig. 7(a)) and a wet surface (fig. 7(b)),both at a nominal ground spe ed of 100 knots. The figure shows that, re ga rd le ss of thesu rf ac e condition, the maximum available co rne rin g capability occ urs when th er e is nobraking (Slip ra ti o = 0) and that there is a rapid decre ase in cor ner ing capability withincrea sing brak e effort (increasing slip ratio) until all cornering is lost at or befor e wheellockup (Slip ra ti o = 1.0). On the othe r hand, the fig ure show s that a certain amount ofwheel slip is required to develop a measurable braking force.

For example, the da ta of figure 7, which were generated

Similar braking and cornering char acteris t ics we re observed during tra ck tests oft i r e s undergoing the cyclic braking of antisk id control.the t ime history of the brake press ure recorded during a 75-knot test of a tire with anti-skid protection oper ating on a dry surface at a yaw angle of 6 O with corres pond ing dra g-and side-force friction coefficients as rel ate d to the wheel sli p ratio. Two s e ts of fric tioncoefficients a r e presented: one includes the data fro m all brake cycles during the test andthe oth er des cri bes the variatio n of the coefficients with s lip rat io during a selected brakecycle.ti re fr iction al behavior under antiskid contr ol can be approximated by it s behavior duringsingle cycle braking tests.

As an example, figure 8 presents

A com par iso n of the c ur ves of this fig ure with those of f igu re 7(a) does su gge st that

In adve rse weath er, the friction level s for both dr ag and corne rin g become quite lowas shown by the wet s ur fa ce condition of f igu re 7(b) and by the cyclic braking dat a unde r a


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re co rd s obtained during flight tests of a two-engine and a three-engine jet t ra nspo rt in theJoint FAA/USAF/NASA Runway Rese arc h prog ram desc ribed in r efe ren ce 1. The lowwheel slip ratio operation of a two-engine je t t rans port i l lustrated i n figure lO(a) res ul tsin extensive period s of no braking, thereby pr ese rvi ng t ire corn ering capability for direc-t ional control purpose s in the pre sen ce of a c r o s s wind, but reducing the air pla ne stoppingperformance.(fig. 10(b)), the wheel spend s much of the ti me (at least during the ti me per iod shown) in adeep skid, thereby al most totally de stroying its corn ering capability. The over all brakingeffectiveness in this example is al so undoubtedly diminished in view of the wide ra ng e ofsl ip rat ios apparent from a comp ariso n of th e air pla ne ground s peed (wheel synchronousspeed) and the instrumented wheel speed. Thus, antiskid sys tem design necessita testrade-offs between t ir e braking and corn ering capabilities.

For the high wheel sl ip ra tio op eratio n of a three-engine jet t ransport

Both good brakin g and cor ner ing are provided at low wheel sl ip rati os when the run -way friction level is high, as exemplified by figures P(a) and 8 , Hence, one means ofavoiding potential braking and/or ste eri ng problems in a dve rse weather , ncluding thepres ence of cr os s winds, is to provide su rf ac es with adequate textu re s o that t i r e t ract ionlo ss es during wet runway ope rations a r e minimal.

CONCLUDING REMARKS

The behavior of curre nt antiskid braking syste ms operat ing under adverse weatherconditions h as been analyzed.touchdown and locked-wheel prote ction , indicated that the oper ation al per for man ce ofthese systems is highly dependent upon wheel spin-up acc ele rat ion and can be ad vers ely

This a nalys is, which co nsidered brak e control and both

- -


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and path-clearing benefits. The design of antiskid sy st em s is complicated by the neces-sity for trade-o ffs between the tire brak ing and corn eri ng capabi liti es, both of which arenece ssar y to provide safe operations in the presen ce of cr os s winds, particularl y undersli pp ery runway conditions.Langley Research CenterNational Aeronautics and Space AdministrationHampton, Va. 23665May 24, 1976


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REFERENCES1. Mer ritt , Le sli e R.: Impact of Runway Traction on Possib le Approaches t o Certifica-tion and Operation of Jet Transport Aircraft. Paper 740497, SOC. Automot. Eng.,

Apr.-May 1974.2. Horne, Wal ter B.; Yager, Thomas J.; and Taylor , Glenn R.: Review of Causes and

Alleviation of Low Tire Traction on Wet Runways. NASA TN D-4406, 1968.3. McCarty, John L.; and Leland, T. J. W.: Recent Studies of Ti r e Braking Performanc e.

Tire Sci. & Technol., vol. 1, no. 2, May 1973, pp. 121-137.4. Pa vem ent Grooving and Trac tion Studies. NASA SP-5073, 1969.5. Yager, Thomas J.; Phi llips, W. Pelh am; Horne, Walter B.; and Sparks, Howard C.

(appendix D b y R W. Sugg): A Co mpar ison of Aircraft and Ground Vehicle StoppingPe rf or ma nc e on Dry, Wet, Flooded, Slush-, Snow-, and Ice-Covered Runways.NASA TN D-6098, 1970.

6. Model 737 Data - FAA Evaluation of Prop ose d Landing Certif ication Rules. Doc.No. D6-43078, Boeing Co., Dec. 1973.

7. Model L-1011 ( Base Aircraf t) Landing Perfor manc e Repo rt for FAA Evaluation ofConcorde SST Special Condition 25-43-EU-12.Ai rc ra ft Corp., Jan. 14, 1974.

Rep. No. LR 26267, Lockheed

8. Mer ritt , Le sli e R.: Concorde Landing Require ment Evaluation Tes ts. Rep.NO. FAA-FS-160-74-2, 1974.

9. Horne, Wal ter B.; and Leland, T raf fo rd J. W.: Influence of Ti re Tr ead Pa tte rn and


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D i r e c t i o nmot ion- f


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- - - - - ----.-I I - - - - - - - - - - - . I% 9'

I

A p p ro x . a i r p l a n e s y n c h r o n o u s sp ee d

O u t b o a r d f r o n t

C J60T i m e , s ec

(a) Brake application prior to full wheel spin-up.Figure 2. - Wheel-speed time histories of angular velocity of four-wheel bogie showing

antiskid brak e co ntrol respons e to brake application prio r to and aft er full wheel spin-upduring landing on wet s urface. Average water depth, 0.06 cm (0.024 in.). (See ref. 7.)


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P B ake app l i ca t i on1 n b o a r d f r o n t

IA pprox . a i r p l an e synch r on ous speed ( t yp i ca l )

Inboa r d r ea rW h e e la n g u l a r Ive I c i t y ,

I

O u t b o a r d f r o n t

O u t b o a r d r e a r

0 IO 20 30 40 50 60Time, sec

(b) Brake application after full wheel spin-up.Figure 2. - Concluded.


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, _ _ _ _ _ _ _ _ _ - - - _ -I I - - - - - - - - - I ! M a x . s y s t e m p r e s s u r e 7

=iF1 o u t b o a r dLEFT GEAR

, - A i r p l a n e s y n c h r o n o u s s pe edWhe el lock up / -Angu lar ve loc i ty , w

pb- - - - _ .. . - . -- -- - -- - - _ .2 i n b o a r d

pb# 3 i n b o a r dw -IGHT GEAR /pb M ax . s y s t em p r es s u re

. - - - - - .. - . . . .# 4 ou t boa rd

w

I I I I I I I I0 2 4 6 8Time, sec

Figure 3. - Wheel and brake time historie s from wet runway brakin g testsof air pla ne equipped with dual wheel gear. Average wate r depth,0.1 cm (0.04 in.).


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Maximum braking---Damp

Grooved Un g roovedDamp+-

Leading wheelangula r vel oci ty

Trai 1 i ng wheelangul ar vel o c i t y

Longitudinald e c e l e r a t i o n ,g u n i t s

Approx. a ir p l ane synchronous speed-------- ---

,-Speed = 10 5 knots Speed = 87 knots7

0 1 2 3Time, sec

(a) S m o o t h - t r e a d tires.

4 5 6

Figure 4 . - B r a k i n g b e h a v i o r of s m o o t h - a n d r i b - t r e a d tires o n o u t b o a r d w h e e l sof f o u r - w h e e l b o g i e g e a r w h i l e t r a v e r s i n g fl o o de d g r o o v e d a n d u n g r o o v e ds u r f a c e s ( r e f . 5).


26/33

Damp -+-Flooded

Leading wheelangul ar v e l o c i t y

Tra i l ing wheelangular vel oc i ty

-7- Grooved1-pngroovedMaximuin braking

ai rplan e synchronous speed

.4Longi tudina ld e c e l e r a t i o n ,g u n i t s

0

Speed = 9 3 . 5 knots

1 2 3 4 5 6Time, sec

(b) Rib-tread tires (five grooves).Figure 4.- Concluded.


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100500

V er t i ca ll o a d , kN

Wheelsp eedrPs

20

10

0

1 Ver t ica l1 l o a d , l bJ I I I IJ 0S y n ch ro n o u s sp eed ( ca r r i ag e )

Speed = 90 kno ts

12 4 6 8

Time secFigure 5.- Unbraked wheel response during touchdown on flooded runway test surfaceat Langley airc raf t landing loads and tract ion facility. Average water depth,

0.5 cn i (0.2 in.).

Ncn


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V e r t i c a lload, kN

Wh e e lspeed,rPS

Skids i g n a l ,volts

6 r a k e

10 -

1 x lo5 -

I I I I

Figure 6. - Effect of tire hydroplaning on touchdown protection .Average water depth, 0.5 c m (0.2 in.).

MPa I p s i-I I 0


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-

Yaw ang le , deg05

16~-- -- -

S i d e - f o r c ec o e f f i c i e n t


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Drag-forcecoe f f i c i ent(braki.ng)

Yaw angl e, deg05~


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p r e s s u r e ,ps i/

I I I I c0 1 2 3 4 5 6 7 8 9

Time, secA 1 1 brake cycles' r

Frag-f orcec o e f f i c i e n t . 4(b r ak i ng)

Single brake cycle[I ( 7 t o 8 . 5 s e c )

Si de-fo rce toef f i c i en t .4( c o r n e r i n g )0 . 2 .4 0 . 2 .4

S l i p r a t i o S l i p r a t i oFigure 8.- Variation of dra g- and s ide-force frict ion coefficients with s l ip rat io

during cycl ic braking at 75 knots on d ry surface. Yaw angle = 6


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w0

1 .0

Si de-fo rcec o e f f i c i e n t . 5

FLOODED

---

Drag-fo rcec o e f f i c i e n t . 5( b r a k i n g )

I I I I I I

0 . 2 .4 .6 0 . 2 .4 .6S l i p r a t i o S l i p r a t i o

Figure 9.- Comparison of friction coefficients developed during cyclic braking on d r yand flooded surfaces at 75 knots. Yaw angle = 6


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,-Brakes ap p l ie d (114.4 kno ts )

Wheel speed, rp s 10r --No b r a k i n g

1 I I I I 1 I I0 2 4 6 8 10 12 14

Time, sec(a) Low wheel slip ratio operation (two-engine jet transport).

2o I_ rB rakes app l i ed (103 k n o t s )Wheel speed,

0 2 4 6 8 10 1 2 1 4Time, sec

(b) High wheel slip ratio operation (three-engine jet transport).Figure 1-.- Typical airplane wheel behavior under antiskid-controlled braking at high sp eeds on slipper y surface,.WI

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