Does take off thrust steadily reduce for fixed N1 during take off roll
In a simulator on the Instructor panel, several engine parameters are recorded such as N1, N2, EGT and actual thrust delivered. One would expect the power (thrust reading in LBS) would be approximately as advertised for engine type throughout the take off roll. For example 20,000 lbs thrust per engine at 91.5%N1 rated power at start of take off roll.
Was rather surprised to see the thrust reading steadily rolling back on both engines from 20,000 lbs at start of take off roll to 18,500 lbs at 80 knots and 17,000 lbs thrust at VR of 135 knots, thereafter further steady thrust reducing at take off N1 during climb to 3000 ft agl in ISA conditions. N1 constant at 91.5N1 throughout. Is that a normal expectation? Figures are approximate except N1 which was steady throughout until N1 reduction at 3000 ft agl (sea level take off)