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betwenthehouses，whichwecaled“woodtrai1s".A1so，in ordertoimitatethehousesstruckbyanearthquake，the fl.orsofthehouseswerecoveredwithpilesofscrapwood. The fire 10ad of the center house and the sur rounding houseswere150and25kg/m2，respectively.Thefire10adof the“woodtrai1s"was25and50kg/m2.
Fig.1(b)showsaschematicilustrationofthemock-up houses used for the second experiment. The second experimentfie1d，whichwaspreparedforaerialfirefighting bylargehe1icopters，consistedofsevenwoodenhouses. Thewalsandc01umnsofthehousesweremadeofwood. Theywereofthesamesize，eachmeasuring7.2m 10ng， 7.2m wideand1.8m high.Thehouseswerebui1t3m apart fromeachother，andscrapwoodwas1aidbetwenthe houses，making“woodtrai1s".Althehouseshadarof andtheirfl.orswerecoveredwithpi1esofscrapwood.The fire10adofthehouseswas150kg/m2 andthatofthe “woodtrails"were25and50kg/m2.
Inthefirstexperiment，thecenterhousethatwasmarked bytheilustrationoffirewasignitedasshowninFig.l(a). Inthesecondexperiment，fourhouseswereignitedatthe same time as shown in Fig.1(b).For igniting them， methan01-soakedcotonrags1aidonthescrapwoodinthe houseswereused.
Astotheweatherconditionsoftheexperimentdays，the windspedofthefirstexperimentdaywas1.5-2.5m/sfrom northtonorthwestandthepeakgustwasabout4m/s.The atmospheric presure was 1013hPa， the atmospheric temperature was 18 0 C and humidity was 79%. The wind spedofthesecondexperimentdaywas8.5-11.0m/sfrom north-northwest and the peak gust was about 15.8m/s. Theatmosphericpresurewas1016.5hPa，theatmospheric temperature was 14 0 C and the humidity was 52%.
2.2. Helicoptersandhelibuckets
Severa1factorscanafectthegrounddistributionpatern ofwaterdispersionbyaerialwaterdrops.Theseinclude dropheight，dropsped，volumeandfl.owrateofthe1iquid re1eased，windspedanddirectionsandpi10tproficiency. Asdropheightanddropspedincreases，watercoverage areaincreasesandcoverage1eveldecreases.A1thoughthe acuracyrateofwaterhitsagainstfiretendtoincrease，the firesupresionefectdecreasesifthecoverage1eve1isnot suficient to retard the spread of fire. Knowing the characteristicsofthegroundpaternobtainedfromspecific he1icoptersisanimportantcomponentofproperaplica- tion. Much useful data about water dispersal for aerial e q u i p m e n t is c o l l a t e d o n t h e U S D A F o r e s t S e r v i c e ， Wild1andFireChemica1Systemshomepage[1].
For the first experiment，thre types ofhe1icopters， BK117C1，BK117B2andSA365N1，belongingtothefire department authorities of the 10ca1 governments that coperatedinthisexperiment，wereused.Theformertwo he1icopters，BK117C1，BK117B2cancarya60-1heli- bucket and the 1ast one，SA365N1 can cary a 540-1 helibucket. The fl.ight height ofthese thre he1icopters
rangedfrom86to14丘 (26.2-43.9m)andtheairsped rangedfrom0knots(hovering)to24knots(4.4km/h).
For the second experiment，thre 1arge he1icopters， CH47J，whichareasignedtoJapanMinistryofDefense， were used. The CH47J can car ry a 60 0 0-1 Grif fith Big Diperhe1ibucket[1].Weusedthisbucketwith5301of water.Thefl.ightheightofCH47Jrangedfrom147to160ft (4.8-48.8m) and the airsped ranged from 0 knots (hovering)to25knots(46.3km/h).
Ourexperimentinv01vesnotonlyquantifyingground paternsofdropwaterfrommid-sizeandlargehe1icopters， buta1sodeterminingfiresupresionefectsbydroping water over burning mock-up houses，thus making it posib1etoprovideinformationrequiredinmakingurban firesupresionp1ansusinghe1icopters.
Thecoverageareawasdeterminedbythephotographs takenbysixdigita1videocameras(SonyDSR-300F)that wereinstal ledonp1atformsat2m highabovethegroundat the 10cations shown in Fig. 2. The coverage 1eve1 was ca1cu1atedbydividingthev01umeofdropingwaterbythe coveragearea.
2.3. Temperaturemeasurements
Thefiresupresionefectbyaerialfire白ghtingcanbe detectedbytemperaturechangesintheburninghouses. Two temperature measurement methods were taken; thermocoup1es(point-by-pointmeasurement)fordetecting penetrationofdropwaterintothehouses，andinfrared cameras(areameasurement)fordetectingoveraltempera- turechangesofthehouses.Thethermocouplesusedwere fl.exible-sheathK-type(10m long，heatproftemperature 8700C).ThecenterhouseofthefirstexperimentA3，as showninFig.l(a)andthatofthesecondexperimentB5，as showninFig.1(b)，wereinstaledwiththrethermocoup1es at height of 50，10 0，and 170cm from the ground. The otherfourhousesinthefirstexperimentA1，A2，A4，and A5，asshowninFig.1(a)，andfourhousesinthesecond experimentB2，B4，B6，andB7，asshowninFig.1(b)were instal led with two thermocoup1es at height of 50 and 10cmfromtheground.Theconfigurationsofthermo- couplessetupareshowninFigs.7and8.
Topreventthewiresfromdisconectingbycolapsing scrapwoodpiledupinthehousesandtopreventshort circuitsbydropwaterpenetratingintothethermocouples， the thermocouples were instal led in iron poles with an outsidediameterof40m mwhichwerefirm1yfixedatthe centerofeachhouse.Thesheathsectionsofthethermo- couplesofover10m wereprotectedbyaluminumtape fromdropwater.
Twoinfraredcameras(AvionicsInc.TVS600)witha detection wavelength of 7-13μm，a minimum detection temperatureof0.1K andascaningrateof1/60swere used.Theywereinstaledattwolocations，80m awayfrom the center house both in the first and the second experiments，as shown in Fig.2. Therma1 emis sivity of infrared cameras was set to 0.90. The amount of
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