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

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

• In three sentences or less, please state your question. – Ryan Mortensen Nov 24 '16 at 18:12
• Sorry @Ryan it's stated in a very beginning, am I too imprecise wi this short formulation? "In short (details below) the question sounds: what is the factor causing different behaviour, resulting in either increasing or reducing angle of climb, after the pilot pulls the stick back (raising AoA)." – agronskiy Nov 24 '16 at 18:21
• I'm not sure someone will take time to read your post, with the question is buried in the middle of a long post. My suggestion would be to create two identifiable sections in your post: one very crisp and synthetic with the question (s) stated very straightly. If this really adds something, a second section with your current ideas, but no additional request/question. The first section should be sufficient for the person able to provide an answer, the second would just help them to confirm the question is correctly understood. Just a suggestion. – mins Nov 24 '16 at 19:27
• Sorry, there is too much here to understand what exactly it is you are seeking. I think you need to remove at least 50% of the text to fit the user-friendly format of this site. In the meantime, if you have no excess power to climb then you are at the absolute ceiling, which is not really related to angle of attack. To help you, I think you should read about slow flight, induced drag, and power/thrust/drag curves. – Ben Nov 25 '16 at 2:54
• I think I understand what you're asking. The reason descent angle steepens with an increase in AOA at slow speeds is because the airplane is already below the best glide speed meaning that any reduction in speed (and increase in AOA) will cause the distance gained horizontally to decrease in relation to the distance descended. It is because the induced drag is increasing, hingering the plane's forward/horizontal movement. At least, that's the simplest explanation I can give you to assist you in understanding the concept although it may not be 100% scientifically accurate. – Ryan Mortensen Nov 25 '16 at 4:26

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 Vx

• Many thanks! It makes more clear for me! 2. I meant the same effect: when pulling stick decreases speed vector angle instead of increasing it. In normal speed descent (like 65 knots) when I pull lightly — I descend less steeply (please read carefully what I wrote: "angle between the horizon and the speed vector" goes up). But at very low speed descent when I pull the stick it starts to descent steeper (NOT stall). I was asking whether the cause is the same as in Paragraph 1. – agronskiy Nov 25 '16 at 12:06
• With 3 again, I was not meaning going to stall (not "stalling AoA"). I was asking whether there is a special AoA (I called it "critical", sorry for a clumsy term) at which pulling stick will result not in speed vector going up, but going down. My guess it is $V_X$. I know that $V_X$ is higher than $V_{stall}$. Thanks for your help! – agronskiy Nov 25 '16 at 12:09

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):

1. 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:

1. As explained in 1 and 2, the points on the power curve at which this happens are $V_X$ and $V_{L/D}$.
• "Pulling back on the yoke will always make you slow down." Unless your name is Patty Wagstaff. – Ryan Mortensen Nov 25 '16 at 16:41
• @RyanMortensen not getting it even after reading about her. Explain pls? Sorry for dumb questions, you know, I'm yet only a small beginner student trying to think over something before I convert it to the experience and the wisdom of flight :) – agronskiy Nov 25 '16 at 17:01
• Sorry, that was meant to be a joke... She is a world-famous aerobatic pilot, and if you're flying upside down and pull back on the yoke, you will speed up, not slow down. – Ryan Mortensen Nov 25 '16 at 17:04
• @RyanMortensen :)) I thought of that but you know needed to check whether there MIGHT BE any other regimes where I miss something. Thanks for nice guidance through my first question here! – agronskiy Nov 25 '16 at 17:06