Originally Posted by
FlightDetent
Vessbot I am sure we're on the same page, perhaps viewing from slightly different angles, like two people sharing a read would.
That graphic is maybe the first figure in all of the Principles of Flight books out there, and cannot be wrong. You will find a thread where I argued against several more experienced people that it is with thrust how I control sink rate and chase the G/S on approach, even on a swept-wing jet.
However, it describes a static state with constant speed and AoA (which is why the latter is missing in the depiction). An attempt to use it to explain what happens in different places of the envelope as you change from one to another would not work. There are full volumes of aerodynamic theory beyond this drawing for this very reason, the drag polar being just the first next one.
E.g. the graphics cannot explain the difference between Vx and Vy, and we both know how vitally important those are especially if someone finds himself well slow of both.
I had a bit of resilience accepting
"Climb gradient is a function of excess thrust, nothing to do with lift." Took it the wrong way, not realising against the missing
"For a constant speed and stable AoA,,.. " was there, just silent. Culpa maxima.
The climb gradient after takeoff has massive amounts to do with how much lift you have to begin with, and that depends on the speed (squared) at the beginning of the second segment. Sure, drag does too.
During the take-off performance analysis we can select the V2 speed higher above the low limit, closer up towards best L/D speed, which then yields a steeper climb with the same amount of thrust applied, over-ruling the drag increase.
Once airborne in the equilibrium state, all that ammo is gone and the basic principles take over - it is the thrust (excess or lack of) which defines the changes of the climb rate if we were keeping the airspeed steady. No disputing there.
I'll wait for your closing public remarks and then we really should continue elsewhere to avoid floggin'.
Sounds like we're in agreement... or at least if there's disagreement somewhere, I can't find it anymore.
If I'm reading you right, you agree that for steady states it's excess thrust that determines climb angle, and lift has no effect. And it's steady states that I was talking about, as I clarified in my last post. Also I was addressing your specific comment where you laid out a situation with a specific climb angle, pitch angle, and speed (V2) which is a steady state.
Actually, I think I found the remaining disagreement halfway through your post. At the risk of seeming flogging.... I hope you reconsider and come back, as why are we talking about flying on the internet to begin with?
Anyway, you seem to be treating the second segment climb, or flying at V2, as a transient condition and therefore not subject to the excess thrust rule. Now I'm treading into ground where I'm less sure than my previous posting in this thread, but I disagree with that too. It may not
feel "steady state" as, depending on our SOP's we may only be holding that speed for a few seconds, some or all of which may be taken up by overshooting, corrections, etc. However, any of that is merely slop in execution, and real-life imperfections away from the platonic perfectly executed maneuver where we rotate exactly into the speed, freeze the pitch, and the speed then doesn't waver. If the thrust and speed are steady, then it's a steady state condition, and lift is irrelevant. It
was relevant a few seconds ago in the pitch up (and vertical acceleration) off the runway and into the V2 attitude, but not once V2 is established.
Additionally, you bring up changes to V2, where it's selectable form a range. Then, it's the consideration of different steady state speed targets. You raise an example where we select a higher V2, closer to best L/D, that gives a steeper climb "overruling the drag increase," but actually the drag
decreases, not increases in that case so there's nothing to overrule. Remember that the lowest drag is at best L/D, so by flying closer to it, we're flying at less drag, and thus in exact accordance with excess thrust yielding the higher climb angle.
Again, I hope you do come back, I'm thoroughly enjoying this discussion.