Angle of Attack & Stalls

Theotokos

Theotokos

New Member
I do understand that speed and attitude do not cause a stall and that an airplane can stall at any speed or at any attitude. On the Sporty's video, Dick Collins, said at first this seemed illogical to him and he was slow to understand why this was. I think I am begining to understand this concept but can someone tell me if I am correct in my understanding.

1) When there is too great (or is it to small) an angle of attack, a stall is caused.

I theorize that the reason the speed of the aircraft is many times slower during a stall is because when there is a greater angle of attack, the aircraft is pitched up. Therfore since the aircraft is pitched up, there is a decrease in speed--as we all know nose up attitude causes a decrease in airspeed. The speed and attitude do not cause the stall, but the degree of the angle of attack is due to a greater nose up attitude and therefore causes a decrease in speed.

I know the plane can stall with nose down attitude also. But first, is my understanding above correct. This is just somthing that I have trouble in understanding. Perhaps when I start flying an instructor can explain better but you guys are always a help! :-) Thanks.
 
You need a higher AoA to maintain the same amount of lift as you slow down, but eventually there is no more AoA left to add to maintain the lift, and you stall.
 
There are so many ways to explain this. Here's a mathematical way to think of it:

Start by understanding the fact that a stall, regardless of airspeed or pitch attitude, is caused by exceeding the critical angle of attack. The critical AoA varies with wing designs, but is usually about 16-20 degrees.

Then, consider the basic equation for lift.

Lift = (airspeed) x (coefficient of lift) x (air density) x (angle of attack)

Coefficient of lift is pretty much a fixed number associated with the wing design. It could change for something like flaps being extended, but I don't want to get into that. I also believe some of those elements are squared, or divided by 2, or something, but for the sake of this discussion, that doesn't matter.

Next, understand that lift from the wings must always equal the load. In straight and level flight you could say that lift must equal the weight of the plane (1 G load factor).

Ok, so here's how you put it all together. In the equation above, "lift" has to remain constant, because the weight of the aircraft isn't changing. If lift remains constant, the four elements on the other side must change in relation to each other.

If your air density goes up (thicker air), something else, or a combination of more than one element, must go down in order to produce the same amount of lift. You could go slower, fly at a lower angle of attack, or a combination of the two.

So what will happen if airspeed is lowered? Something else has to go up, right? Well, air density and CoL are pretty much fixed numbers. So what else is there? AoA. AoA has to go up.

AoA has to go up and up and up as airspeed goes lower and lower. But what did we say happens when the critical AoA is exceeded? A stall.

Now to relate this to being able to stall at any airspeed or attitude--that is because you can put any size of a load on the plane at any airspeed or attitude.

Let's say you're in a dive. You yank back on the yoke. You feel yourself get pushed hard into your seat. That's because the load factor increased. So now, to the wings, the plane "feels" heavier than it actually is.

If the plane feels heavier, the wings need to produce a larger amount of lift. The "lift" side of the equation gets bigger.

What on the other side gets larger? Airspeed is already going as fast as it can. CoL and air density are fixed. All that is left is AoA. So AoA gets larger to produce more lift. However, it can reach the critical AoA and the wing becomes stalled. So there you are...stalled, going 100 knots, with a nose low attitude, pulling out of a dive.

Does this make sense?

Stalls are one of those things you really have to go play with in an actual aircraft to fully understand.
 
First of all do you understand what AoA is fully? AoA is the distance between the relative wind and cordline of the wing.....And a stall is caused when the airplane exceeds the critical angle of attack. This varies from airplane to weight and such but that's basically it.
 
So the angle of attack is like a triangle because you have:

Side 1: Relative wind
Side2: Wing Chord
Side 3: An imaginery line connecting Side 1 and 2 which forms the trinagle

Hence the angle of attack is determined by the angle of the triangle, which is determined by other variables.
 
Theotokos said:
So the angle of attack is like a triangle because you have:

Side 1: Relative wind
Side2: Wing Chord
Side 3: An imaginery line connecting Side 1 and 2 which forms the trinagle
Click to expand...
Correct.

Theotokos said:
Hence the angle of attack is determined by the angle of the triangle, which is determined by other variables.
Click to expand...

Pretty much. Just remember that the other variables I talked about above are what you need for a particular, stabilized situation, not necessarily what you have.

So just because you have an airspeed of A, coefficient of lift of B, and air density of C, you may or may not have an AoA of X. You need an AoA of X to be in stabilized flight, but if you're not in stabilized flight, it could be any value. When you change the pitch of the plane, the AoA changes instantly, but the other variables (airspeed, mainly) take a second to catch up.

My point with this is to stress that AoA is related to, but not dependent on the other variables. It is only determined by the other variables in stabilized flight.

You want to know a really simple way to control the AoA? Think of how much you're pushing or pulling on the yoke. The more you push, the lower you are making your angle of attack. The more you pull, the larger your AoA.
 
Yes, and by pulling back you decrease speed. By pushing you increase speed. Speed helps determine AoA. The slower you go, the greater AoA you need. So you need a greater AoA at 180 kts than at say 280 kts. And if you have to great of an AoA you can stall?
 
Theotokos said:
Yes, and by pulling back you decrease speed. By pushing you increase speed. Speed helps determine AoA. The slower you go, the greater AoA you need. So you need a greater AoA at 180 kts than at say 280 kts. And if you have to great of an AoA you can stall?
Click to expand...

That's all correct. However, just because a change in airspeed and change in AoA happen simultaneously does not mean that one caused the other.

I want to stress that AoA and airspeed directly correspond to each other during stabilized flight, but do not correspond during the brief moments while pitch is changing.

For instance, if you're flying straight and level at 100 knots, then pull back and raise the nose, you will change the AoA at a faster rate than your airspeed. For a few seconds you will actually produce more lift than you need and accelerate upward because the lift and weight vectors are unequal.

In the example you gave, you could fly straight and level at 180 knots with one AoA, then fly straight and level at 280 knots with a lower AoA and everything would be predictable and constant. That's all good. But it doesn't work that way if pitch or airspeed are changing.

I'm trying to make it clear that you can't make a graph that says at 120 knots AoA=X, at 150 knots AoA=Y, and at 180 knots AoA=Z. You could do that if you're talking strictly about stabilized, unaccelerated flight, but not for other times.

If you're flying at 150 knots and start shoving the control yoke forward and backward rapidly, the pitch attitude and AoA will change quite a lot, but because of momentum, the aircraft will remain at close to 150 knots.

Another way to look at the issue of stalling is to think of air "rounding the corner" of the wing. If the angle is fairly small, air has no problem flowing over the gentle corner of the wing. If you jerk back on the controls, the plane's momentum will want to keep it going in the same direction at the same airspeed. The flight path (read: relative wind) will remain constant. But jerking back on the controls will shove the tail down, rapidly increase AoA, and air will no longer be able to smoothly flow over the corner because it is too sharp of a turn.

In other words, AoA and airspeed are connected with a rubber band, not a steel rod. They are closely linked, but do not directly correspond to each other throughout all phases of flight.
 
jrh is explaining things great but this helped me understand it. Imagine your aircraft is flying holding 3,000 ft but has a 15 degree pitch up (whoch is similar to what you will do in slow flight), this aircraft has a high angle of attack. If you increase the angle of attack much more you will enter a stall because you will have exceeded the the critical angle of attack disrupting the airflow which allows aircraft to fly.
Not that you are....but again I used to confuse pitch and angle of attack. An airplane can have the hightest possible pitch and little or no angle of attack. Think of an F-18 with an afterburning on going straight up, and think about where the relative wind compared to the chordline of the wing is:)
 
Good job so far on working through the understanding of AOA.

Once you get the basic idea down, it sometimes helps to start thinking of extreme examples, like alphaone's F-18 going vertical, to really cement the concept in your mind.

When you think of extreme examples you will begin to realize that some of the other "absolutes" that you thought were true, are sometimes oversimplifications. For example, your statement that "Yes, and by pulling back you decrease speed. By pushing you increase speed". While true in most conventional situations, think about being inverted. When upside down pulling back will increase speed, and pushing will decrease speed.

Another example: "Speed helps determine AoA. The slower you go, the greater AoA you need. So you need a greater AoA at 180 kts than at say 280 kts." I think that you need to add some more words to the end of that sentence: "you need a greater AoA at slower speeds in order to maintain level flight."

Remember that a plane can stall at any airspeed. You can be flying at a very high air speed, and pull enough g's to create a large enough AoA to stall. Conversely you can fly at less than Vs by unloading the wings, reducing g's and flying at a reduced AoA (kind of hard to do this for very long, though :) ).

Don't fall into the trap of thinking that Angle of Attack is always related to airspeed.
 
SteveC said:
Another example: "Speed helps determine AoA. The slower you go, the greater AoA you need. So you need a greater AoA at 180 kts than at say 280 kts." I think that you need to add some more words to the end of that sentence: "you need a greater AoA at slower speeds in order to maintain level flight."
Click to expand...

I would say those extra words slightly differently.

"You need a greater AoA at slower speeds when in stabilized, unaccelerated flight."

If you're in a constant airspeed climb or descent, AoA works the same as in level flight. If you're in a stabilized descent at 70 knots the AoA is the same as it would be if you were straight and level at 70 knots, or in a steady climb at 70 knots.

Look back at my math equation example above. Since the load on the wings is the same in a steady climb or descent, the amount of lift needed is the same. Since the airspeed is the same, the AoA must be the same also. The only thing making the plane climb or descend is the amount of thrust produced...but that's a whole other discussion.

Come to think of it, this is another example of why pitch doesn't matter when talking about AoA. In a steady climb, descent, or straight and level segment of flight, the pitch is different, but the AoAs are all the same.
 
When the critical AOA is excedded the wing will stall, always and forever no exceptions.

The "stall speed" is the speed at which the critical AOA is excedded when maintaining altitude.

I've stalled airplanes well above Vs, and I've flown airplanes as slow as 20 Kts without stalling (durring a loop).
 
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