Originally Posted by Empty Cruise
Without derate, V2 is way below VL/D MAX. This is possible because the aircraft has so much power that flying at such a slow speed does not compromise performance.
When you derate, the performance edge is used to achieve a lower N1 & - most important - EGT during take-off. Now the aircraft needs a V2 that is a lot closer to VL/D MAX to meet the climb requirements. And since V2 needs to go up, up goes V1 and Vr as well.
Besides, I do remember something about the speed for max excess thrust increasing as thrust decreases - which just further adds to the requirement for increased speed.
Hope this helps - best regrads fm
Empty
I'm not sure that those are the reasons, EC.
The aim of the V2 climb is to achieve a minimum gradient. The consideration is V
L MAX or greatest coefficient of lift which occurs at greater AoA than V
L/D MAX. A V2 climb is less than V
L/D MAX because of this. If optimum
rate of climb was the primary target, you could discuss V
L/D MAX.
As to higher V1 in a derate/flex takeoff, remember that V1 must exceed Vmcg by a factor. If the aircraft is using less thrust, it will take longer to achieve Vmcg and require a longer field length in doing so, as mutt points out.