I've noticed that on Boeing aircraft, during the take-off preparations, at least on the 757/767 and the 777, V2 is dialed in the MCP speed window, and VNAV is simultaneously armed.
After rotation, the F/D pitch bar commands a target speed of V2+10 (or V2+XX). At some point during the initial climb (which I believe is thrust reduction altitude), the speed window goes blank, and speed is adjusted as planned according to thrust reduction altitude, acceleration altitude, initial climb speed, etc.
First of all, is this correct? And my question is, what is the purpose of selecting V2 on the speed window prior to departure? Why not V2+XX, or the initial climb speed after acceleration altitude (250KIAS, for example)?
Thanks in advance for your replies.