What exactly is "stall"

[chornedsnorkack]

What happens to an airfoil at "stall"? And how are "lift" and "drag" defined at high angles of attack?

Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust - no engines installed or engines shut down.

Absence of thrust means that in steady state, the plane is descending.

The plane might fly at "best glide" AoA. It would then have appreciable forward airspeed, modest rate of descent and good L/D ratio.

If the AoA is any lower, the total airspeed would be higher, L/D would be smaller and rate of descent would be higher.

Now, let

[Brett_Henderson]

As I've now seen and recognize your posts for what they are and where you're trying to go with them; I'll answer accordingly. But first I'm going to ask for clarification, because (as usual) you've already gone off on tangents that give windows for you to challenge the answers (that's what you're really after (and that's OK. I like debate.discussion too      :)

[flymo]

stalling is when the hamster take a break...or is that engine failure...hmmm

[Brett_Henderson]

stalling is when the hamster take a break...or is that engine failure...hmmm  

LOL  (not that kinda stall)

[Hagar]

Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust - no engines installed or engines shut down.

What happens if an airfoil is held at AoA of 80 or 70 or 60 degrees? It should still have a modest RoD - but it should also have a small but nonzero forward speed.

In a conventional aircraft this would be impossible, Once the wing stalled the nose could not be held up & would drop naturally putting the aircraft into a dive. This dive would be maintained until the airspeed built up enough to unstall the wing & also make the elevator effective. Then the whole process would be repeated ad infinitum in a series of swoops until it hit the ground or recovery action was taken.

A plane with zero forward airspeed still cannot drop out of the sky at any high speed. After all, as presumed above, it has low weight and large wing. Fast RoD at 90 degrees AoA would mean huge drag. The plane has to reach a steady state at a modest rate of sink and no forward speed. It would be "parachuting" vertically down.

This type of manoeuvre is possible only with high power-to-weight ratio aircraft like jet fighters or high-performance aerobatic types like the Sukhoi Su-26M. British aerobatic pilot Will Curtis features a "Parachute" in his display routine. Note that this is a power manoeuvre with the wing stalled & the engine providing the lift. Watch the video HERE.

[OTTOL]

Sounds like, in your zeal to cover the question in the greatest detail possible it ends up appearing vague and, at times, self contradictory (is that proper grammar?) "....i.e. ".....A plane with zero forward airspeed still cannot drop out of the sky at any high speed ....." I'll try my best though.... :-?

What happens to an airfoil at "stall"? And how are "lift" and "drag" defined at high angles of attack?"?

During a stall, the normal flow of air is disrupted and the normal flight or control of flight is affected. The reason I choose this wording, as opposed to the standard textbook answer, is that an airfoil refers to any airfoil and all airfoils (cannards, vertical stabilizers, horizontal stabilizers etc.) can experience a stalled condition.
At "high angles of attack" both lift and drag are high.

[quote]
Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust .... engines shut down.

Absence of thrust means that in steady state, the plane is descending.

The plane might fly at "best glide" AoA. It would then have appreciable forward airspeed, modest rate of descent and good L/D ratio."

[chornedsnorkack]

Thanks OTTOL for reminding that there are more variables and that covering all cases too shortly could tend to sound ambiguous and have contradicting implications...

Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust - no engines installed or engines shut down.

What happens if an airfoil is held at AoA of 80 or 70 or 60 degrees? It should still have a modest RoD - but it should also have a small but nonzero forward speed.

In a conventional aircraft this would be impossible, Once the wing stalled the nose could not be held up & would drop naturally putting the aircraft into a dive. This dive would be maintained until the airspeed built up enough to unstall the wing & also make the elevator effective. Then the whole process would be repeated ad infinitum in a series of swoops until it hit the ground or recovery action was taken.

That

[Hagar]

I mean, rearward CG stalling and stall recovery should be possible even if a plane is underpowered or actually unpowered?

You obviously have a good understanding of aerodynamics but this sort of discussion is in danger of becoming far too technical & you end up not seeing the wood for the trees. What seems logical in theory is often very different in practice. Let's go back to your original question & scenario.

What happens to an airfoil at "stall"? And how are "lift" and "drag" defined at high angles of attack?

Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust - no engines installed or engines shut down.

This is a very good description of a glider or sailplane. Even the most efficient modern sailplane is severely underpowered as it has no power at all, yet it can stilll fly under full control & even do aerobatic manoeuvres. The same basic rules apply to gliders as to all conventional aircraft. I've found that this confuses the general public terribly & when flying my R/C model sailplanes & aerobatic slope-soarers I've lost count of the number of times I've been asked "How can you control it without an engine?"

The point is that a stalled aerofoil (airfoil) produces no lift at all. Once the mainplane is stalled it would be impossible to hold the aircraft in your example at an AoA of 80 or 70 or 60 degrees for any length of time without the nose dropping. Therefore your "parachute" theory is also impossible. The aircraft is at the mercy of gravity until remedial action is taken. I'm talking about conventional aeroplanes here & it's very difficult (practically impossible) to stall pure canards & deltas. These tend to "mush" at low airspeeds & high angles of attack while steadily losing altitude which is more like your description of a "parachute".

However, there are a fair number of airplanes which have too small tailplanes or too powerful elevators or too far aft CG - and which are in danger of entering a stall. Some of them actually have the condition of "deep stall", which is stable so that once entered, it cannot be corrected by control applications. Accordingly, they have various stall warnings, stick shakers and pushers and flight envelope protection, so as to prevent them from stalling to begin with.

This is a long way from your original topic. All aircraft have to pass stringent testing by the appropriate authorities before they're certified for flight. This includes stalling & spinning characteristics in all normal configurations. The 'deep stall' phenomenon with high-set "T-tails" was first experienced with jet arliners back in the 1960s. It took the death of BAC test pilot Mike Lithgow in the prototype BAC One Eleven in 1963 before this condition was fully appreciated. Although he must have known he was doomed he talked it right down to the moment of impact over the radio. I've heard a recording of this & it's very moving. Investigations & research following this unfortunate accident helped to make the One Eleven one of the safest airliners of its time. The lessons learned were also passed to other aircraft manufacturers. http://oea.larc.nasa.gov/PAIS/Concept2Reality/deep_stall.html

Are there also aircraft where stalled state is neutrally stable - so that it is not a deep stall which cannot be recovered from by controls, nor is it impossible to sustain, but a state which can be recovered from at any time?

See my above comments on canards & deltas. There are also various high-lift devices to delay the stall or make it almost impossible in normal circumstances. One example of a light aircraft designed from the outset to be almost impossible to stall or spin is the Ercoupe. http://www.gatwick-aviation-museum.co.uk/ercoupe/ercoupe.html
I still have mixed feeling on this idea myself.

[chornedsnorkack]

Once the mainplane is stalled it would be impossible to hold the aircraft in your example at an AoA of 80 or 70 or 60 degrees for any length of time without the nose dropping. Therefore your "parachute" theory is also impossible. The aircraft is at the mercy of gravity until remedial action is taken. I'm talking about conventional aeroplanes here & it's very difficult (practically impossible) to stall pure canards & deltas. These tend to "mush" at low airspeeds & high angles of attack while steadily losing altitude which is more like your description of a "parachute".

However, there are a fair number of airplanes which have too small tailplanes or too powerful elevators or too far aft CG - and which are in danger of entering a stall. Some of them actually have the condition of "deep stall", which is stable so that once entered, it cannot be corrected by control applications. Accordingly, they have various stall warnings, stick shakers and pushers and flight envelope protection, so as to prevent them from stalling to begin with.

This is a long way from your original topic. All aircraft have to pass stringent testing by the appropriate authorities before they're certified for flight. This includes stalling & spinning characteristics in all normal configurations. The 'deep stall' phenomenon with high-set "T-tails" was first experienced with jet arliners back in the 1960s. It took the death of BAC test pilot Mike Lithgow in the prototype BAC One Eleven in 1963 before this condition was fully appreciated. Although he must have known he was doomed he talked it right down to the moment of impact over the radio. I've heard a recording of this & it's very moving. Investigations & research following this unfortunate accident helped to make the One Eleven one of the safest airliners of its time. The lessons learned were also passed to other aircraft manufacturers. http://oea.larc.nasa.gov/PAIS/Concept2Reality/deep_stall.html

Are there also aircraft where stalled state is neutrally stable - so that it is not a deep stall which cannot be recovered from by controls, nor is it impossible to sustain, but a state which can be recovered from at any time?

See my above comments on canards & deltas.

But if you have a look at the article, those T-tailed jets are conventional aeroplanes - high aspect ratio wing, stabilizer in rear. The problem with them is that they don

[Hagar]

[quote]But if you have a look at the article, those T-tailed jets are conventional aeroplanes - high aspect ratio wing, stabilizer in rear. The problem with them is that they don

[beaky]

A plane with zero forward airspeed still cannot drop out of the sky at any high speed.

What would you define as "any high speed"? assuming you mean a high rate of descent- for argument's sake, let's call "high" any VS above normal approach descent rate for the airplane in question, or perhaps Vbg.
If you stall any aircraft, any aircraft, and it remains stalled all the way to the ground, somehow miraculously not rolling over, spinning, tailsliding, etc.... it will come down at a much higher rate of speed, vertically, than one would like. High enough to kill you, in most cases.
"Parachuting" is not a very accurate term at all for describing how a flat plane descends when not flying... a round parachuting canopy collects air in a very particular way, and an airfoil-type canopy is actually a low-aspect-ratio type of wing.

Can someone explain what really changes about the airfoil behaviour if you compare non-vertical parachuting (in "stalled" AoA) with flying "at the back of the power curve", at AoA slightly below "stall"?

For what it's worth, "non-vertical parachuting" is really getting off-track... a stalled wing is not doing much of anything except falling, just as a barn door, desk chair, or goldfish bowl might fall. It may have a ballistic trajectory, having been moving forward prior to the stall, but again, that has nothing to do with parachutes.

Trying still to answer this last question: You've sort of answered your own question...

The airfoil behavior depends on airspeed and angle of attack. In the "back of the power curve" scenario you mention, the clue as to why the plane still won't climb is in the word "power".
Without sufficient thrust, once the airspeed gets low enough, even though the A of A is sufficient to prevent a stall by producing some lift, there is not enough lift for the plane to climb, or even keep from descending. It's all too easy to get into this pickle in any airplane: all you have to do is climb at full power while increasing your A of A almost to the point where it will stall the wing at any airspeed. i guarantee that no matter what airplane it is, no matter how much thrust it has, or what it's service ceiling is... if you climb at full power and hold the pitch at the edge of where it will stall as your airspeed decreases, the airplane will begin to descend before the stall occurs. If you do it just right, you will come down nose-high and quite rapidly, maybe showing airspeed on your indicator, maybe not. It will not be stalled, but because you already have max power, it will not climb unless you lower the nose to get some more airspeed.

Obviously, what normally happens in this sort of vertical-climb scenario is that the plane stalls... but my case in point is basically academic. A more common illustration of the "stalled but not stalled" or "death mush" thing would be any of a number of cases where stall-proof aircraft, like the ercoupe wirth its limited elevator travel, have carried careless pilots to their back-breaking doom when they let it get "behind the power curve".

In other words, in a "supermush" or whatever you want to call it, the wing behaves much as it would during a normal descent, only with A of A, airspeed and power in a different ratio than in a normal descent.

[Brett_Henderson]

These posts aren't, nor ever have been, about answers. It's mental/written thumb wrestling.

[Hagar]

These posts aren't, nor ever have been, about answers. It's mental/written thumb wrestling.

Indeed, I can see that now.

[OTTOL]

[quote]

But if you have a look at the article, those T-tailed jets are conventional aeroplanes - high aspect ratio wing, stabilizer in rear. [b]The problem with them is that they don

[Hagar]

I challenge you to show me a recent accident that occured as a result of failure to maintain proper flying speed (or even an accelerated stall incident). The Wellstone accident is the only one that I can recall and that one isn't even a "swept" aircraft.

Sorry OTTOL. I have to take up that challenge. Fatal TU154 crash August 2006
Three previous fatal accidents with the same type of aircraft have been attributed to the "deep stall".

While I agree with most of your comments I have to point out that the "deep stall" phenomenon is caused by several factors. The swept wing & rear-mounted jet engines might add to the problem but the phenomenon is mainly due to the high-set T-tail being blanketed by the mainplane at high AoA & low airspeed. (I'm not sure if T-tailed prop-driven aircraft with straight wings suffer from this problem to the same degree.)
Diagram courtesy of Wikipedia.


I can still vividly remember the BAC One Eleven incident in 1963 as it happened while I was working at Gatwick. Mike Lithgow was one of my schoolboy heroes & it shook me to hear his last words broadcast on the BBC radio news bulletin we were listening to. He carried on describing the situation right to the moment of impact in a calm & professional manner which served to increase my deep respect for him & his colleagues.

I've done a little research to refresh my memory of this phenomenon. As I recall, the BAC One Eleven was being tested at a rearward CoG. Once it had entered the "deep stall" condition it seems obvious that a highly skilled & experienced test pilot like Mike Lithgow & crew would have tried everything in their power to recover from it, including a wide variety of power settings & control inputs plus deploying the anti-spin parachute carried during this sort of testing. It seems likely that the aircraft ended up in a 'flat spin' from which recovery turned out to be impossible.

It's also worth considering that AoA is measured from the relative wind OTTOL mentioned & not necessarily from the horizontal as is shown in most textbooks. In this case with the wing & tail stalled the aircraft might appear to be in a straight & level attitude to an observer but as it's falling almost vertically the aerofoils are at a high AoA relative to the airflow. I hope this makes some sense. (Note the direction of the blue arrow in the diagram.)

I'd forgotten about all this & the fact that although the "deep stall" phenomenon is now widely known it's never really been overcome. Stick shakers & warnings are all very well but I'm not convinced that they've come up with an aerodynamic solution. This puts a whole new light on the Tu 154 incidents that have never really been resolved. The above article confirms that the Tu 134 & Tu 154 were never fitted with the safety devices used on their western counterparts.