Stall: Different angles of attack????

[UnAnswerd]

I was asked if the airplane always stalls at the same AOA. I thought about it, and said "no". I was then corrected and told that the plane always stalls once the "critical" AOA is exceeded.

But if I am straight and level, and pull the throttle all the way out, and then try to maintain altitude, won't the airplane stall at an AOA far below the critcal angle????

 

[minitour]

Nope...the plane will only (only) stall once you exceed the critical angle of attack...

if I had a model to use I would show you how it works, but...I got nothing...

but rest assured, you will ONLY stall if you exceed the critical angle of attack...

-mini

 

[seethru]

UnAnswered,

In your example, as your airspeed decreases after closing the throttle you will have to continually increase your pitch (AOA) in order to maintain straight and level flight (i.e. hold altitude).

Eventually the aircraft will pitch up enough to reach the "critical AOA", and stall.

Greg

 

[KeroseneSnorter]

The AOA is always the same for stall. Doesn't matter if you are going 60 kts or 600 kts, the wing will ALWAYS stall at the same AOA.

Speed does not matter, an airplane can be stalled at ANY speed. (well not any speed, get too fast and the wings or tail will fail before the critical AOA is reached.) But assuming an indestructable airframe, any speed can be stall speed.

Ask your instructor to (Safely) demonstrate accelerated stalls to you. That should make it more clear for you then.

In light aircraft, the books show stall speed as a constant for a given condition (ie. flaps up or full etc.), this is a simplified way of looking at it for light aircraft purposes. In a large aircraft you have to adjust landing speed for your current weight. Your typical airliner will have a 10 to 20 knot landing speed (And stall speed) difference between gross weight and empty weight. However it will always stall at the same AOA, doesn't matter if its empty at 100,000 pounds or full at 170,000 pounds. Stall speed actually does vary for weight in a 172 just like the big ones, but it is such a small change as to be unmeasurable by the airspeed indicator so it is not a factor.

 

[minitour]

UnAnswered,

First of all, seethru is right on.

I'll try to expand a bit...if I do a sh*tty job, someone will let us know so just disregard...but I'll give her a try...

Lift is a product of two things: Airflow (speed) and Angle of Attack...

Take a 30,000' runway with no obstacles and it's completely flat...

Start at the very end (no wind) and line up with the centerline...apply takeoff power and release the brakes...

Do you have to "rotate" or will the plane just takeoff on its own?

It will go on its own eventually. Why? You have a positive angle of attack (angle of incidence helps with this one) and now you have enough airflow over the airfoil (wing) to produce enough lift to suck (or push depending on how you see it) the airplane into the air...

If you take away one or the other (airflow or AOA) you will reduce lift...

When you stall an airplane, you are basically taking away the airflow....the wing is pitched up so high that the air actually "separates" and you stop producing lift...

If you pull that power to idle and decide to maintain altitude, you are constantly increasing the back pressure...remember AOA and Airflow...you're losing airflow so you need more AOA to keep the lift or you will descend...

So really what happens in a stall?
1. You exceed the critical angle of attack (this has to happen)
2. You take the airflow away. The airfoil simply cannot produce lift at this AOA

How'd I do?

I'm still a rookie in training re: the whole teaching and explaining thing...

-mini

 

[VNugget]

You guys are gonna hate me for this (and those who know the answer will laugh), but the critical AOA does in fact change ever so slightly with airspeed.

Can anyone say why?

Would you like the red pill or the blue pill?

 

[KeroseneSnorter]

Aw come on nugget, don't confuse the guy any more than we probably already have!!

 

[minitour]

You guys are gonna hate me for this (and those who know the answer will laugh), but the critical AOA does in fact change ever so slightly with airspeed.

Can anyone say why?

Would you like the red pill or the blue pill?

I'll bite

why?

 

[Vik]

Based on what others have written to you .. Let your mind chew on this for awhile ...

You can stall ANY airplane at ANY airspeed.

I was asked if the airplane always stalls at the same AOA. I thought about it, and said "no". I was then corrected and told that the plane always stalls once the "critical" AOA is exceeded.

But if I am straight and level, and pull the throttle all the way out, and then try to maintain altitude, won't the airplane stall at an AOA far below the critcal angle????

 

[minitour]

Based on what others have written to you .. Let your mind chew on this for awhile ...

You can stall ANY airplane at ANY airspeed.

...below Va...

then the airplane is supposed to break up...so they say...

Personally, I'm not going to test out that one..

-mini

 

[KeroseneSnorter]

Looking over the posts, I see that everyone is correct in what they have told you.....which means that you are probably a pretty confused student pilot right now!!

Get your flight training manual out and look up these items, maybe it will help you decipher all our responses. Until you can get a basic grasp of the forces at work, all this is probably turning your brain inside out.

1. The four forces, Lift, weight, thrust and drag.
2. Basic airfoil- Chord, relative wind, Newton, and Bernoulli (sp? it's late) and Angle of attack.
3. Load Factor- And if you just giggled......Lay off the porn!!!!
4. Accelerated stall
5. Boundry layer
6. Washout or twist
7. Stall strips

That shoud give you a decent start....Now I don't want to hear that your dog ate your homework!!

 

[User546]

There's a term that no one has used yet that might make things make a little more sense for our inquiring friend: Relative Wind.

The direct angular relation between the aircrafts Angle of Attack (AOA) and the aircraft's Relative Wind determines when the aircraft stalls. That's why an airplane can stall at any airspeed, attitude, or configuration.

You can be in 500 ft/min descent and still stall the aircraft. If the angular difference between the Relative Wind and the AOA exceed the critical AOA, then you got yourself a stall.

A lot of pilots confuse stalling with "The wings have to be at a high pitch attitude, or atleast above the horizon" to stall. Not the case at all. Relative Wind and how it is striking your aircraft is what determines it.

 

[UnstableAviator]

http://www.afsoapbox.com/downloads-file-28.htmlhttp://www.afsoapbox.com/downloads-file-28.htmlhttp://www.afsoapbox.com/downloads-file-28.html

^Thunderbird accident video in Idaho.

That is a great example of how you can stall at any attitude, at any airspeed. I don't think I would be too far off if I said he was doing well over 250 knots indicated when you can see the stall occur 27 seconds into the footage. Notice how his flight path stays basically the same even though the aircraft attitude is pitching nose up.


Kind of mind boggling to realize an F-16 wing is basically producing 200,000 pounds of lift during those 8-9g continuous turns. Lot of induced drag, a lot of power, and most importantly, a lot of afterburning noise.

 

[minitour]

There's a term that no one has used yet that might make things make a little more sense for our inquiring friend: Relative Wind.

The direct angular relation between the aircrafts Angle of Attack (AOA) and the aircraft's Relative Wind determines when the aircraft stalls. That's why an airplane can stall at any airspeed, attitude, or configuration.

You can be in 500 ft/min descent and still stall the aircraft. If the angular difference between the Relative Wind and the AOA exceed the critical AOA, then you got yourself a stall.

A lot of pilots confuse stalling with "The wings have to be at a high pitch attitude, or atleast above the horizon" to stall. Not the case at all. Relative Wind and how it is striking your aircraft is what determines it.

While true, isn't the definition for angle of attack something along the lines of:
The angular difference between the chord line of the wing and the relative wind...

so that would include Rel. Wind in with AOA

but I think I see where you're coming from...from a student pilot standpoint its important to realize you can be nose down and stall the aircraft...if you are at or below Va...

-mini

*edit*
With the G's thing...if you take the square root of the Gs you get the percentage of stall speed increase

If you had a 4 g maneuver (we'll assume an aerobatic plane) you'd doube the stall speed.

square root of 4 is 2...2 * Vs = ___

ouch...

 

[UnAnswerd]

Thank you all for the informative replies. Let me just try this scenario:

In staight and level flight, I reach over and turn the ignition off. The airspeed starts to drop to stall speed, but I do not touch the yoke. Instead, I just let the nose go over. Is this not a stall? I never touched the yoke, so I assume the angle between the wing chord-line and the relative wind never exceeded the citical limit? Is this not a stall at a low AOA???

EDIT: Or maybe I am wrong. Maybe when the nose goes over and the airplane starts to sink, relative wind hits the airplane from more-less underneith it, and certainly that would exceed the critical AOA....

 

[VNugget]

Thank you all for the informative replies. Let me just try this scenario:

In staight and level flight, I reach over and turn the ignition off. The airspeed starts to drop to stall speed, but I do not touch the yoke. Instead, I just let the nose go over. Is this not a stall? I never touched the yoke, so I assume the angle between the wing chord-line and the relative wind never exceeded the citical limit? Is this not a stall at a low AOA???

EDIT: Or maybe I am wrong. Maybe when the nose goes over and the airplane starts to sink, relative wind hits the airplane from more-less underneith it, and certainly that would exceed the critical AOA....

Go ahead and do it in real life. The airplane will more or less maintain the same AOA and airspeed, but will nose down and begin a descent due to a lack of power. That's not a stall. Just a descent.

 

[VNugget]

As far as critical AOA (all student pilots not interested in the finer details of fluid dynamics: just ignore this, critical AOA never changes!)

Remember the good ol' graph that relates Coefficient of Lift (Cl) to AOA? You were shown the peak Cl, and how it's dependent solely on the AOA and nothing else, so speed, altitude, and all other factors had no effect?

Well what you weren't told is that the graph itself changes based on the Reynolds numbr (Re). Generally, at a higher Re, the straight part of the graph extends further and the peak is at a higher AOA and Cl. (This is not theoretical calculus derivation mumbo jumbo, but has been shown experimentally.)

Remember what defines the Re?

Re = (density * airspeed * length) / dynamic viscosity

Well if you change any of those values you cange the Re, which change the location of the peak in the lift curve, which means the critical AOA is now different.

A little bit.

 

[KeroseneSnorter]

Thank you all for the informative replies. Let me just try this scenario:

In staight and level flight, I reach over and turn the ignition off. The airspeed starts to drop to stall speed, but I do not touch the yoke. Instead, I just let the nose go over. Is this not a stall? I never touched the yoke, so I assume the angle between the wing chord-line and the relative wind never exceeded the citical limit? Is this not a stall at a low AOA???

EDIT: Or maybe I am wrong. Maybe when the nose goes over and the airplane starts to sink, relative wind hits the airplane from more-less underneith it, and certainly that would exceed the critical AOA....

The airplane can be stalled at any attitude as well as any airspeed. In a spin the wing has exceeded the critical angle of attack, BUT the nose is pointed almost straight down.

IF you are stalled, you HAVE exceeded the critical angle of attack. There is no in between. Some confusion you have may be coming from the stall warning horn going off while you are in the condition you describe above. The horn will start sounding BEFORE you are actually in a stall(It gives you a buffer to correct the situation in an unintentional stall). It is possible to fly a 152 or 172 at near zero indicated airspeed with the stall horn sounding continously and NOT be stalled. However even though the A/S reads near zero, you have NOT exceeded the critical angle of attack.


As stated above, read carefully the explaination of "Relative wind" (Nope, it's not your uncles oderferous output after the chili cookoff!!)

 

[minitour]

The airplane can be stalled at any attitude as well as any airspeed...

...at or below Va

If you're above Va and put enough load factor on the plane to stall it, usually the tail falls off first...then the wings...bad stuff...

but yes, any airspeed at or below Va, the plane will stall before structural damage will occur...that's why we have Va

-mini

 

[User546]

Maybe when the nose goes over and the airplane starts to sink, relative wind hits the airplane from more-less underneith it, and certainly that would exceed the critical AOA....

Good... your getting the hang of it!

The FAA has a great Advisory Circular on it you might like to download from their site: AC 61-67C "Stall And Spin Awareness Training" Goes into more detail about how airplanes stall in various situations, as well as defines some key terms that might help you understand things a little better.