I remember reading an article (Flight International I belive) about GE tests on the 90-115B when they used a 747, if my memory serves correctly, they used weights and fuel to balance out the weight of the test engine and reduced thrust (on the 90-115) to maintain control. Also the test pilot said that take-off was critical, as an engine failure on the non-test side would cause a huge yawing moment away from the test engine!
