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temperature recovered very quickly. In the fire mature stage，thefirewasburningdepinsidethehouse，andwas notsupresedefectivelybywaterdrop.Inthefiredecline stage，thefI.ametemperaturegradualydeclined，andthe fire was extinguished. In the second experiment，the durationofthemock-uphousefirewasalsodividedinto thethrestages.Butinthethirdstagethefirethatseemed tohavebenextinguishedstartedburningagain.Themost efectivefiresupresionmethodistosprayalargeamount ofwateratonedropinthefireexpansionstage.Tomake thefire白ghtingmoreefectiveinthefirematurestage， whenfiresupresionefectsbysprayingwateraresmal，it isdesirablethatonewaterdropisfolowedbyanother beforetheserninglysupresedfireregainsstrength.It wasfoundthatinthefirematurestagethefirethatseemed tohavebensupresedonthesurfacebysprayingalarge amountofwaterbyonedropcouldbeginburningagain.
Basedonthedataobtainedbymeasuringthetempera- turesoftheburningmock-uphousesbyinfraredcameras instaledintheexperimentfield，thetemperaturechangesof a burning house resulting from drop water can be expresed by the thre parameters of time constant， temperaturedrop，andtemperaturerecoveryrate.Using theseparametersandtherelationsbetwencoveragearea and coverage level makes it pos sible to ca1culate the relationbetwenthenumberofwaterdropingsandfire extinguishing time. In our simulation，with the initial temperaturesetto 10oC，theextinguishingtimebya mid-sizehelicopterisabout2hat25%ofhitprobability， atairspedof5knots，andatreleaseintervalof3min. Usingalargehelicopter，theextinguishingtimeispredicted tobeabout3.5hatthehitprobabilityof25%，atairsped of20knots，andatreleaseintervalof6min.Theresultsof thesimulationareinclearagrementwiththeexperiment bythemid-sizehelicopters.However，inthefirefightingby thelargehelicopter，thefirewasnotextinguishedbyfive firefighting atacks，therefore，comparison betwen the simulation and the experiment could not be made. Our experimentandsimulationshowfrequentfI.ightsandpilot pro五ciencyisnecesarytoextinguishurbanfires.Helicop- teracidentrecordsshownumerousexamplesofhandling problems，suchasatemptingtohover. Pilottrainingis required to increase the hit probability and to avoid aci
Acknowledgments
The authors would like to thank to Ms. Kimiko WakayamaandMs.KarynZaayengaforasistancewith manuscnptpreparatlOn.
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