Bit of a digression here. Just thinking about this theoretical straight vertical climb...
Post #7
Or, for a more intuitive approach, analyse the extremes. Assume the climb angle to approach the vertical. Lift will go to zero. It thus makes sense to assume that lift will in fact decrease with increasing angle of climb.
In a straight vertical climb, wing lift will definitely not got to zero. If the aircraft is climbing vertically, nose pointing up, then the wings see a relative airspeed, therefore they produce a force perpendicular to the relative velocity vector. In this case the velocity vector is perpendicular to the ground, as the aircraft is moving straight up. So the force produced by the wings is acting to push the aircraft away from the vertical. It's kind of counterintuitive to call it "lift", since it's not acting to lift the aircraft, but it is wing lift nonetheless. So, in a straight vertical climb, lift is not zero.
Furthermore, if the climb velocity is not that great, then there is a good chance that the wings will actually be stalled, which means there will be airflow separation over the wing surface, and that means there will be lots of drag, not no drag. Therefore, the thrust from the engines not only has to act against the weight of the aircraft, but also against a good amount of drag.
Just a thought. Sorry for the digression.