What is the best flap setting for the shortest t/o roll for a heavy weight? At 150,000 gw I struggled to make it off the ground using a 12000 foot r/w, and had very poor climb performance using flaps 15.

Michael,Take off performance is dependant on a lot of factors:Airport altitudeAIrport ambient pressureAirport ambient temperatureWindsRunway slope (not modelled in FS)Derate/Selected temp thrust modeOne thing that many forget is the elevator trim. Below is for -700 but should be in the same ballpark as -800. For your weight (and a center of gravity of approx 27%MAC) and a flap setting of 15 or 25 gives a trim setting of between 4

Well it was definatly a heavy t/o, almost full fuel, and full pax. The first t/o was from katl using a 12000 ft r/w and full power I bearly made it off the ground at flaps 15 in the -800, I set trim to what the fmc said, 4.90. And the other t/o was with full fuel in the wings and 2000 lbs in the center with full thrust and full pax, it also came close to runing out of r/w, but this was at kslc which is slightly over 4000 ft.