Why doesn't the flight path angle increase beyond a certain angle even if the angle of attack is increased?

Question

1. From level flight, when the angle of attack is increased the flight path angle increases (the aircraft starts to climb). What causes the aircraft to stop to increase its angle of climb (angle between the horizon and the speed vector) as I continue increasing AoA at full thrust?

2. Is the effect described in Paragaraph 1 the same as with steepening the descent at idle thrust when I pull the stick?

3. Is it right that the critical AoA, above which continuing to pull the stick causes decreasing speed vector's angle (i.e. "going down instead of up"), is the same for climb and descent and corresponds to $V_X$ (best angle of climb speed) up to negligible corrections?

Thanks
Alex

Answer 1

1. If you keep increasing your angle of attack, your drag will increase and combined with the increasing along trajectory component (your path angle will increase) of the weight vector will slow you down (assuming your thrust does not exceed your weight + drag). Because you slow down, your lift will decrease. The lift will now be less than the across trajectory component of the weight vector and the flight path will become less steep.

2. I don't understand your second point.

3. The critical angle of attack is where the maximum lift coefficient occurs. If you increase the angle of attack further, the wing will stall and the lift will decrease. This does not occur at V_x

Answer 2

Thanks to all the commentators for pointing out partial answers.

I dare say I found the reference which answers why at the same power setting there exists a point (at some AoA) at which the vector of speed angle (= angle of climb/descent) starts reacting opposite to stick movements (pull stick => less climb angle / steeper descent).

It is power curve.

Power curve is best explained here from where I bring a citation (Paragraph 7.8):

The airplane is trimmed for a definite angle of attack, and hence a definite airspeed at 1 G. The yoke is part of the angle-of-attack control system. Pulling back on the yoke will always make you slow down.

If you are on the front side of the power curve and if you don’t mind airspeed excursions, you can use the yoke as a convenient, sneaky way to control altitude. This is because airspeed is linked to altitude via the law of the roller-coaster and via the power curve.

Warning: just because this works OK 99% of the time, don’t get the idea that it works all of the time. Bad habits are easy to learn and hard to unlearn. Do not get the idea that pulling back on the yoke always makes the airplane go up. On the back side of the power curve, it doesn’t work — and might kill you. In critical situations (including approach and departure), you simply must control the airspeed using the yoke and trim.

Power curve contains an implicit dependence of AoA (since AoA is roughly airspeed):

1. So when I am at full thrust climb and I start pulling, I increase AoA and move right to left on the power curve — increasing agle of climb. At some point for the reasons of fastly increasing drag coefficient and slowly increasing lift coefficient (see lift coeffcient and dreag coefficient curves) it stops increasing angle of climb. I'm in this point (which is $V_X$ by the way, see the link):

2. Exactly the same occurs in descent. In idle thrust regime, I start pulling stick back more and more and I move right to left on the power curve, descending less steeply.But after passing some speed (which is not $V_X$ but rather close $V_{L/D}$) I start descending steeper:

3. As explained in 1 and 2, the points on the power curve at which this happens are $V_X$ and $V_{L/D}$.