Stall: Different angles of attack????

[Mr. Cole]

VNugget gave you the right explanation but as User997 mentioned you're thinking in the right direction. There are two pieces that must be connected in the scenario you just described: longitudinal stability and power. The stability curve will plot the moment about the lateral axis as a function of coefficient of lift. For a certain configuration of the elevator and trim there is a value of CL that results in a zero moment, this is what we typically call the trimmed condition. If the aircraft is distrurbed it will seek out that value of CL that results in the equilibrium condition. So if you lift the nose to increase the AOA then release the yoke, the increased lift behind the CG will create a nose down moment, restoring the original AOA. If you push the nose down and release the yoke, the decreased AOA will result in less lift behind the CG and a nose up moment, thus restoring the original AOA. Of course this assumes positive stability

But in straight and level flight a particular AOA corresponds to a particular airspeed. The other piece is the power available versus power required at a particular airspeed. At a particular airspeed you can either climb, descend, or fly straight and level depending on the power available. In your example you pull the power and suddenly there is insufficient thrust to overcome drag. The plane then slows, which reduces the lift. The nose then pitches over because of the reduced moment contributed by the lift. As the nose pitches over the airspeed will probably build but as the plane descends the air comes more from underneath as you mentioned, increasing the AOA. Just because the AOA increases due to the increased relative wind coming from under the plane doesn't mean it reaches critical AOA. If you haven't changed the trim the plane will want to maintain the same AOA as it had before you pulled the power and will oscillate until it does reach it.

The plane also wants to balance the forces on it and left to its own devices it will seek to make lift equal weight. Therefore it will assume the value of airspeed that corresponds to that particular value of CL for unaccelerated vertical flight. I used the term unaccelerated vertical flight instead of straight and level flight because a constant rate descent or climb is the equivalent of straight and level flight regarding the forces, in other words L = W. So now the plane has found an AOA and thus a CL to satisfy stability. The plane want equilibrium in the vertical plane so settles on a AOA/airspeed combination to provide that. This airspeed is the airspeed that you had before you pulled the power, at least ofter a few oscillations.

However, you don't have the power required to support that airspeed value so you will either have to descend at that airspeed or fly straight and level at some lower airspeed. Because stability wants to keep the old airspeed you have no choice but to descend. However, the other option is to pull the power and fly at a slower airspeed which is sufficient for that power setting, but to fly hands off you would need to retrim and change the stability equilibrium point.

Dave

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....

 

[DashTrash400]

Just to add to the confusion...to say that critical AoA *never* changes is incorrect. Changing the shape of a wing will change the critical AoA...of course, you do this every time you extend or retract the flaps. Interestingly enough, the critical angle of attack with flaps extended is several degrees less than normal...ie if your normal critical AoA is 18 degrees, with full flaps it may be 15 degrees. For any given AoA up to that critical AoA, however, coefficient of lift is greater with flaps extended.

 

[minitour]

Just to add to the confusion...to say that critical AoA *never* changes is incorrect. Changing the shape of a wing will change the critical AoA...of course, you do this every time you extend or retract the flaps. Interestingly enough, the critical angle of attack with flaps extended is several degrees less than normal...ie if your normal critical AoA is 18 degrees, with full flaps it may be 15 degrees. For any given AoA up to that critical AoA, however, coefficient of lift is greater with flaps extended.

Actualy flaps just change the angle of attack because they change the chord line

Remeber the AOA is the angular difference between the chord line and the relative wind.

When you lower flaps, you change the chord line of the wing, thus raising the angle of attack.

-mini

 

[Oakum_Boy]

When the engine starts sputtering, isn't that a stall? What about when flames are seen coming from the airplane just before a crash, is that a stall? Thanks in advance-

 

[DashTrash400]

Yes, but...

Actualy flaps just change the angle of attack because they change the chord line

Remeber the AOA is the angular difference between the chord line and the relative wind.

When you lower flaps, you change the chord line of the wing, thus raising the angle of attack.

-mini

You're correct that extending flaps changes the mean chord line; however, this isn't the only reason extending flaps lowers your angle of attack. The main reason is that lowering flaps increases coefficient of lift for a given AoA. Thus, for a given airspeed, less AoA is required to produce the same amount of lift.

Find a graph that compares coefficient of lift for a wing with flaps extended vs retracted. You'll find that the Cl increases with AoA more rapidly for flaps extended than retracted, BUT that the "peak" - aka critical angle of attack - is reached at a slightly lower AoA.

I'll scan in a graphic if I can.

--SW

 

[DashTrash400]

Ok here's a graphic to illustrate what I mean by flaps changing both a wing's CL and critical AoA:

http://home.comcast.net/~sdweigel/aoa.gif

 

[FracCapt]

The most important thing to remember in all this is that pitch attitude DOES NOT equal AOA. You can be climbing vertically(straight up) and have an AOA of 0 degrees. If the relative wind is coming from straight ahead(parrallel to the chord of the wing), the AOA is 0. Most wings have a critical AOA(the AOA at which it will stall) of, I believe, about 14 to 17 degrees(can somebody verify that, it's been a long time since I studied this stuff).

Assuming you don't damage the airplane first(from pulling too many G's), you can stall any airplane at any speed at or above minimum stall speed. The F-16 video somebody referenced is good proof of that. That was an accelerated stall - where the aircraft stalls faster than the minimum stall speed due to increased load factor.

 

[VNugget]

Most wings have a critical AOA(the AOA at which it will stall) of, I believe, about 14 to 17 degrees(can somebody verify that, it's been a long time since I studied this stuff).

True, if not a tad on the high side.

 

[Rez O. Lewshun]

The most important thing to remember in all this is that pitch attitude DOES NOT equal AOA.

Agreed it is not equal, but changing pitch angle also changes AOA. Does it not? How else does one recover from a stalled wing?

The problem is AOA indicators are rare and so most pilots have never seen one or even used one.

http://xflight.powerweb.de/original/parts/center_console/aoa/aoa_04.jpg

http://www.airplanedriver.net/acpix/citaoa.jpg

I recall when flying GA aircaft, the wing stalled at the same yoke or stick position. (e.g. the same AOA) The theory was to mark the yoke shaft coming out the panel. When that mark was visible stall was imminent. Yes/no? maybe?

A gent created a home made AOA indicator by putting two vacuum ports on the leading edge of the wing, with a converter a/s indicator velcro-ed on the dash. (to keep the feds away for an illegal installation). When the -/+ pressure difference moved along the leading edge towrds stall it would indicate on his (converted a/s indicator) AOA indicator.....

Thoughts?

 

[FracCapt]

Agreed it is not equal, but changing pitch angle also changes AOA. Does it not? How else does one recover from a stalled wing?

Well, of course it does...but pitch is not a reliable indication of AOA in all circumstances. If it was, no airplane would be capable of a loop.

The problem is AOA indicators are rare and so most pilots have never seen one or even used one.

Unfortunately, that's true. I've flown with pilots that have AOA indicators at their disposal but insist that they must fly a given airspeed and totally disregard the AOA. I, on the other hand, was taught that AOA is primary. When flying a jet that does not have an AOA indicator, I feel naked(which, trust me, isn't a pretty thing ). Most of us have absolutely no idea what the jet we fly will stall like...or what the indications(I'm referring to aerodynamically, not a shaker or pusher) prior to a stall are. Most of us will never find out. That's what test pilots are for.

I recall when flying GA aircaft, the wing stalled at the same yoke or stick position. (e.g. the same AOA) The theory was to mark the yoke shaft coming out the panel. When that mark was visible stall was imminent. Yes/no? maybe?

I've never heard that theory before, but I can't imagine that working for all given scenarios. I think it would vary based on weight, CG, airspeed, power, and trim(to a lesser extent). Interesting thought, though. I wonder if anybody has ever done any actual tests on this?

 

[Rez O. Lewshun]

Well, of course it does...but pitch is not a reliable indication of AOA in all circumstances. If it was, no airplane would be capable of a loop.

Agreed... just checking I guess. I flamed on another thread for suggesting that lowering pitch lowers AOA....

 

[Singlecoil]

Assuming you don't damage the airplane first(from pulling too many G's), you can stall any airplane at any speed at or above minimum stall speed.

Not to get technical on ya, but you can stall at ANY airspeed, i.e. 5 knots, 1 knot, etc. Minimum stall speed is straight and level 1g gross weight flight.

 

[91100 100 set]

Nobody has mentioned how ice accretion will change the AoA a wing will stall at.

 

[Goose Egg]

...........

 

[FracCapt]

Not to get technical on ya, but you can stall at ANY airspeed, i.e. 5 knots, 1 knot, etc. Minimum stall speed is straight and level 1g gross weight flight.

You're absolutely right. I didn't bring low G and negative G maneuvers into the equation because it would probably do nothing but confuse students. People that want to know more about this, and experience it, should do so under the supervision of an instructor(preferably in an aerobatic airplane). Negative G maneuvers bring a whole new set of problems.

 

[Goose Egg]

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???

Nope. Ever think of actually doing the reading? That might help. An aerodynamic stall is, by definition, whenever the critical angle of attack for that particular airfoil design has been exceeded. No critical angle exceeded, no stall. It's that easy. In your example, letting the nose go over is what reduces the angle of attack to prevent the stall from occuring.

And actually, if you turned off the ignition (which I don't recommend) without touching the yoke, the airplane would immediately pitch down to maintain the airspeed that it was trimmed for. The only way the airplane would be close to the published stall speed would be if the airplane was trimmed for it to begin with.

Just be aware that an airplane can stall at airspeeds other than the published reference stall speeds. Ask your CFI to demonstrate some accelerated stalls.

-Goose

 

[apcooper]

Here is the best way to think of it plain and simple. The difference in the direction of which the nose is pointed vs. where it is actually flying is the angle of attack. If this is over 17 degrees or so BINGO a stall will occur regardless of airspeed, power setting, pitch attitude or weight!!

 

[Tinstaafl]

Unanswerd, perhaps the following will help:

Some basics that you're probably already familiar with but it helps me to lay out my comments:

Angle of Attack is the angle between the relative airflow (relative wind, to some) and the chord line. The relative airflow is opposite in direction to the flight path through the air. The chord line, in turn, is defined as a straight line from the leading edge of the wing (ie the most forward bit...) and the trailing edge of the wing (its most rearward part). This is not the same as the pitch angle of the fuselage (what the pilot readily perceives). Think of an aircraft somehow descending vertically while in a normal flight attitude. The pitch angle is 'normal' but the relative airflow is from below.

Aerodynamically a stall is when the wing can no longer produce sufficient lift to counter weight as a result of exceeding its critical angle of attack (crit. 'AoA' or 'crit alpha'. Some texts refer to it as the stalling AoA. Note the wing doesn't suddenly stop producing all lift - there is still some amount but not sufficient for flight.

So, when the wing stalls, the angle between the chord line & the relative airflow has exceeded the point where increasing the angle of attack will increase lift. In fact the revers occurs: lift will reduce with increasing angle of attack. The corollary is that at the critical AoA the wing is producing its maximum amount of lift.

For a given shape an aerofoil stalls at a particular angle of attack. For all intents & purposes this doesn't change unless you change the wing somehow. Extending trailing edge or leading edge devices & ice accretion are all ways that change the shape of the wing. In effect it becomes a whole new wing. For the various aerofoils used on most aircraft the critical angle is about 15 or 16 deg.

If you were to look at the airspeed indicator just as the wing stalls you would see a particular speed on it. That speed isn't fixed. It varies depending on aircraft weight, load factor (think of it as apparent weight or 'g' force), configuration/damage/ice (the 'changed wing' thing) & power application (a small component of thrust helps support the a/c, reducing the load the wing has to support). What a pity we rely on airspeed indicators instead of AoA indicators!

In general trailing edge lift devices will slightly reduce the crit. AoA. The upside is that for any given speed the aerofoil can produce more lift than 'clean'(it has a higher Coefficient of Lift, CL) so the slightly reduced crit AoA is irrelevent. Leading edge devices tend to increase the crit. AoA while also increasing the CL). All these things also increase drag hence why we don't have them permanently extended...

If, while maintaining trimmed straight & level flight, you close the throttle the aircraft won't suddenly stall unless you make it. The aircraft is designed to be speed stable in normal flight conditions. It does this by the tailplane causing the aircraft to pitch nose up or down to maintain the trimmed speed, losing height to correct a speed reduction, and gaining height to correct a speed increase. Reduce power to less than what is needed for S&L flight and the aircraft will pitch ND to maintain the trimmed airspeed, losing height in the process. The AoA will be largely unchanged, except for bit of bobbing around while the aircraft stabilises in the new flight path.

If, on the other hand, you were to prevent the aircraft from pitching ND to maintain speed then you would have to cause the a/c to pitch NU. Do this enough and eventually the wing will reach its crit. AoA. Do it a bit more & the wing will stall. Depending on how much and the rate of NU pitch you cause to happen the flight path could be descending, level or even climbing.

In all of this, what's important is the angle of attack that the wing experiences, not whether the aircraft is climbing, descending, level or turning. Similarly, the pitch attitude is not what defines the stall. You can stall the wing in *any* a/c pitch attitude, if you make the wing exceed its crit. AoA.


Oakum Boy, the short answer is no, none of those things you mentioned is a stall. In aviation a stall is a specific thing that relates to an aerofoil as I discussed above. The term 'stall' is not used w.r.t. whether or not the engine is running. You can stop an engine, it can malfunction or fail, but it doesn't stall**

The engine sputtering or flames emanating from the aircraft are not necessarily anything to do with stalling - although a pilot could always cause one if s/he grossly mishandles things while trying to deal the with sputtering or fire.

Aircraft - of all sizes - glide without the engine(s) producing sufficient thrust to counter the drag resulting from moving through the air & as a byproduct of the wings producing lift. Some glide better than others eg purpose built sailplanes/gliders, some glide like well greased crowbars eg high wing loading fighter jets - but they all glide. Even passenger jets. BTW, a typical pax. jet has nearly double the glide performance of a typical light piston aircraft, using the glide ratio as the arbiter. Granted they travel forward & downwards at much greater rates than a lighty but they still travel further per unit of height. Of course the much, much greater speeds forwards & downwards in the jet means that the light aircraft is a rather better bet to be in when the earth eventually gets in the way....




**Caveat: Turbine engines work by having lots of compressor & turbine blades. These are aerofoils & therefore able to stall, so a jet engine can 'stall' - or at least it's aerofoil components can. It would usually be called a 'compressor stall' or similar to try to distinguish it from the common usage.