Simviation Forum - What exactly is "stall"

Reply #15 - Jan 7^th, 2007 at 10:17am

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:
Posts: 3593

I have a genuine question. Do wing tips stall earlier on swept-wing aircraft because of the "sweep"; or because of the taper ?

Also (and this is pure speculation), I think that stalling (especially an accelerated stall) is rarer in higher-performance, bigger and faster aircraft because the gap between the top of MCA and the onset of stall is larger. Maybe not so much proportionally larger; but definately a bigger chunk of the ASI's arc. Not to mention that pilots are more experienced.

Back to top

IP Logged

Reply #16 - Jan 7^th, 2007 at 10:43am

Hagar Offline
Colonel
My Spitfire Girl
Costa Geriatrica

Posts: 33159

Brett_Henderson wrote on Jan 7^th, 2007 at 10:17am:

I have a genuine question. Do wing tips stall earlier on swept-wing aircraft because of the "sweep"; or because of the taper ?

Because of the sweep. This explains it better than I can. http://www.erau.edu/er/newsmedia/articles/wp3.html
Quote:

The most unfavorable stall characteristics are found with the swept wing, which is used in transonic aircraft to reduce wave and parasite drag at cruise speeds. The sweep causes high induced drag and stall conditions at low speed. The sweep creates an induced upwash angle of the relative wind at the leading edge that increases from wing root to tip. This means that the wingtip is flying at a higher angle of attack than the root. Thus, it will reach its critical angle of attack first. The wingtip is the worst possible location for an initial stall, since loss of aileron control is the result. The burbling airflow off the wingtips does not encounter the tail assembly and thus does not give the pilot a natural stall buffet warning.

Quote:

Also (and this is pure speculation), I think that stalling (especially an accelerated stall) is rarer in higher-performance, bigger and faster aircraft because the gap between the top of MCA and the onset of stall is larger. Maybe not so much proportionally larger; but definately a bigger chunk of the ASI's arc. Not to mention that pilots are more experienced.

I think this is due to the high power-to-weight ratio of high performance types like modern jet fighters. The "deep stall" occurs at low airspeed & high AoA just like a conventional stall. It develops into a "deep stall" due to the design of the aircraft with airflow over the high T-tail blanketed by the mainplane. I don't think pilot experience has much to do with it as even experienced pilots have been caught out by it.

I've just been reminded by a topic in the Screenshots forum that the Gloster Javelin suffered from this problem in the 1950s. In fact a test pilot was killed due to the "deep stall" phenomenon just over 10 years before the BAC One Eleven incident. The Javelin was a big & heavy "double-delta" with a high-mounted T-tail. Although it was jet fighter it was severely underpowered. http://www.thunder-and-lightnings.co.uk/javelin/history.html Quote:

However it was at this point the Javelin's high T-tail cost the life of pilot Peter Lawrence - such designs are susceptible to a condition known as a deep stall, where the wing blankets airflow over the tail assembly at high angles of attack, rendering the aircraft uncontrollable. At some point in the flight the nose had been pulled past 45 degrees and as the stall came on forward airspeed reduced to zero - the aircraft simply fell out of the sky.

Founder & Sole Member - Grumpy's Over the Hill Club for Veteran Virtual Aviators
Member of the Fox Four Group

Need help? Try Grumpy's Lair

My photo gallery

Back to top

IP Logged

Reply #17 - Jan 7^th, 2007 at 11:31am

C Offline
Colonel
Earth

Posts: 13144

Hagar wrote on Jan 7^th, 2007 at 6:20am:

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.

From a little reading around, it appears the aircraft was in a "stable" stalled state all the way to the ground, sadly with a fatal RoD...

Quote:

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

At last. I'm glad someone's mentioned that...

Back to top

IP Logged

Reply #18 - Jan 7^th, 2007 at 2:25pm

OTTOL Offline
Colonel
Fintas, Kuwait (OKBK)

Gender:
Posts: 918

Hagar wrote on Jan 7^th, 2007 at 6:20am:

Fatal TU154 crash August 2006[/url]
Three previous fatal accidents with the same type of aircraft have been attributed to the "deep stall".

.

Welp...let me rephrase that with a little more emphasis on some key words. Quote:

..... very few, if any modern jets fall victim to stall related accidents during normal operations and day-to-day flying........

You are correct. This is a recent "deep stall", jet accident. I think that a forty year old design that was "a bad knock-off of a mediocre design" (even back in 68', when it was new) is a weak example though.
Although I sympathize with and respect your personal experience regarding the early designs, these examples are dated and IMO don't have a real bearing on the bulk of the current airline fleet today.
The TU154 is severely underpowered and the wing is a flying aerodynamic history class. (Iran Air flies them here into Kuwait. They have wing fences the size of roadside billboards!!) The fact of the matter is; the only reason the aircraft is still flying is one of economics (they're cheap to buy and operate) not resilient design attributes.
I have to admit, most of my practical experience on T-Tail, swept-wing aircraft is with Learjets. I may be living in a bit of a vacuum by that distinction. Nonetheless, all of my stall-training has emphasized the violent roll which can occur as a result of tip stall and subsequent loss of aileron authority.
.....But.... I have this excerpt from a Captain of one of the most notorious "deep stallers", the DC-9. Jim Webb (the pilot...not the lawyer) wrote a book called "Fly the Wing", which is highly regarded by corporate and airline pilots. Quote:

...the DC-9-30 series may snap-roll idly, one way or the other or possibly hang in the tail-down attitude. The aerodynamics causing [this] aircraft to remain with the nose high and the elevator ineffective is very simply a stalled horizontal stabilizer and elevator. The ailerons are effective down to a speed about 40% less than the full stall speed; the aircraft may be rolled over vertically from the full stall with aileron, and the nose will very definitely fall through the horizon.

This is counter to what I have experienced in the Learjet (regarding control affectivity) and counter-intuitive to the very basic skills taught in early flight training. The second point is my key point. Poor training is usually the culprit, not poor design and for the overwhelming majority of civil airliners, deep stall is not an issue. Oddly enough, the best example that I could find of this was this one that chornedsnorkack eluded to. http://www.airdisaster.com/news/0106/22/news.shtml Quote:

The interim report contained a wealth of details from the plane's flight data recorder and cockpit voice recorder, showing that the crew was incorrect in saying that the plane had experienced a "dual engine flameout," and that the crew took the opposite action to recover from an aircraft stall than action that is taught to every beginner pilot. That suggests that the crew did not recognize that the plane was, in effect, stalling.

.....so I loaded up the plane and moved to Middle-EEEE..........OIL..that is......

Back to top

IP Logged

Reply #19 - Jan 7^th, 2007 at 2:50pm

Hagar Offline
Colonel
My Spitfire Girl
Costa Geriatrica

Posts: 33159

OTTOL wrote on Jan 7^th, 2007 at 2:25pm:

Hagar wrote on Jan 7^th, 2007 at 6:20am:

Fatal TU154 crash August 2006[/url]
Three previous fatal accidents with the same type of aircraft have been attributed to the "deep stall".

.

Welp...let me rephrase that with a little more emphasis on some key words. Quote:

..... very few, if any modern jets fall victim to stall related accidents during normal operations and day-to-day flying........

You are correct. This is a recent "deep stall", jet accident. I think that a forty year old design that was "a bad knock-off of a mediocre design" (even back in 68', when it was new) is a weak example though.
Although I sympathize with and respect your personal experience regarding the early designs, these examples are dated and IMO don't have a real bearing on the bulk of the current airline fleet today.

I respect your opinion OTTOL as you are the commercial pilot. I have no practical experience even with the early jet airliners. My comments are based on what I've learned or been told over the last 50 years or so.

Although it's an old design, production of the Tu 154 ended in 2006 according to Wikipedia so that makes it a current type in my book. The Tu 134 is still very popular & many are still in regular service. http://www.tupolev.ru/English/Show.asp?SectionID=144

I have little knowledge of, or interest in, modern jet airliners but note that the T-tailed swept-wing designs are still very popular today, especially the smaller bizjets & medium-range types. It's possible the winglets now in vogue have gone a long way to overcoming the airflow problems over swept wingtips. You would know better than me if there have been similar aerodynamic improvements to overcome the deep stall problem or whether they still rely on audible warnings & stick-shakers.

PS. Quote:

Oddly enough, the best example that I could find of this was this one that chornedsnorkack eluded to. http://www.airdisaster.com/news/0106/22/news.shtml Quote:
Quote:

The interim report contained a wealth of details from the plane's flight data recorder and cockpit voice recorder, showing that the crew was incorrect in saying that the plane had experienced a "dual engine flameout," and that the crew took the opposite action to recover from an aircraft stall than action that is taught to every beginner pilot. That suggests that the crew did not recognize that the plane was, in effect, stalling.

I understand that compressor stall can be experienced with this type of aircraft in a deep stall condition. This obviously makes a serious situation even worse & could explain why the loss of power was not immediately recognised for what it was.

Founder & Sole Member - Grumpy's Over the Hill Club for Veteran Virtual Aviators
Member of the Fox Four Group

Need help? Try Grumpy's Lair

My photo gallery

Back to top

IP Logged

Reply #20 - Jan 8^th, 2007 at 2:14am

OTTOL Offline
Colonel
Fintas, Kuwait (OKBK)

Gender:
Posts: 918

Hagar wrote on Jan 7^th, 2007 at 2:50pm:

Although it's an old design, production of the Tu 154 ended in 2006 according to Wikipedia so that makes it a current type in my book. The Tu 134 is still very popular & many are still in regular service. http://www.tupolev.ru/English/Show.asp?SectionID=144

.

They still manufacture the Ural motorcycle (a bad rip-off of a late 30's BMW design) over there. Sure, the Ural may still get you to work (most of the time) but it's far from a good example of a motorcycle by which current bikes can be measured. This is just another example of the previously mentioned point. Economics drives the production of the Ural (zero development, tooling and employee training costs) in the same manner that it drove the production of the Tupolev.
The bottom line; if you do a little research (even look at your own quoted article) pilot error is, overwhelmingly, the key cause of the majority of accidents today.

From your article.... Quote:

The latest data from the MAK (Interstate Aviation Committee) reports that Flight 612 had step-climbed to 12,400 m (41,000ft). Maximum allowed weight at 12,100 m is 85t for the TU154. Take-off weight must have been around 93.5t, given the distance from Anapa to St. Petersburg and with 160 passengers. This would mean a weight of about 88t at the time of the crash .

The thunderstorm was reported as having been a heavy one, reaching up to 12-15 km. It looked like the aircraft had stalled and entered a flat spin when it encountered turbulence at a low indicated airspeed (IAS), because it was way too high up for its weight. This was confirmed two weeks later in a statement by the Ukrainian Attorney General.

Crew error and compressibility (and the crew's lack of knowledge of its affects) are the culprits IMO.

Look, I'm not looking for an argument here. And although I appreciate the Kudos, I don't need my ego padded either. The reason I argue my point so vehemently is that I hate to see a false idea or rumor perpetuated. Airplanes don't just fall out of the sky. The crew usually has to do something terribly wrong to create these circumstances. And any modern, high-performance airplane can fall victim to crew complacency. The fact of the matter is; all too often an aircraft is given an undeserved "bad rap."

.....so I loaded up the plane and moved to Middle-EEEE..........OIL..that is......

Back to top

IP Logged

Reply #21 - Jan 8^th, 2007 at 4:00am

chornedsnorkack Offline
Colonel
I love YaBB 1G - SP1!

Posts: 363

beaky wrote on Jan 5^th, 2007 at 2:57pm:

chornedsnorkack wrote on Jan 4^th, 2007 at 4:15am:

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"?

Well, let´s compare and contrast an airliner and an ultralight.

An airliner which even when dirty stalls at over 100 knots at MLW and which routinely cruises at 0,85 M could move faster than this with ill luck. An object in free fall would accelerate from standstill to 300 m/s in 4500 m. So, a plane that flies at 0,85 M at 12 000 m might reach 1,0 M by diving as little as 1400 m... transonic drag would slow it down, but nevertheless it is perfectly realistic to exceed speed of sound somewhere on the way down.

Compare it with an ultralight, which by definition has "stall speed" of less than 24 knots, and which with maximum power cannot exceed 55 knots ahead. Chances are that even if it does climb to 5000 m and then enters in-flight upset, it cannot exceed the speed of sound - the drag would equal weight long before this.
beaky wrote on Jan 5^th, 2007 at 2:57pm:

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.

The real parachutes usually have a RoD between 4 m/s and 8 m/s. Which means you can touch down at 8 m/s and use the feet again. In free fall, 4 m/s would be achieved in 80 cm and 8 m/s in 320 cm.

Airliners like slightly lower rates of descent. Concorde was supposed to touch down at or below 3,0 m/s. When one did land at 4,3 m/s in Dakar, it crushed a tail bumper wheel and damaged rear fuselage. It was repaired and flew again, but was heavier than others due to repair, and eventually was scrapped for spares.
beaky wrote on Jan 5^th, 2007 at 2:57pm:

"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.

Quote:

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"?

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.

Then how do you go about finding the stall speed of a plane? Assuming you have plenty of altitude to recover?

Suppose that a plane is moving forward at 24 knots and descending at 5,1 m/s. Which means 10 knots down. By Pythagoras´ theorem, you have the vector airspeed 26 knots.

Now imagine the same plane can be brought further to move forward at 20 knots and descend at 7,7 m/s, which is 15 knots down. What is the relevant airspeed then? 25 knots, or 20?

How do you know if it is meeting 24 knot limit? And whether it is technically "stalled" in any steady combination of forward speed and RoD?

Back to top

IP Logged

Reply #22 - Jan 8^th, 2007 at 5:41am

Hagar Offline
Colonel
My Spitfire Girl
Costa Geriatrica

Posts: 33159

OTTOL wrote on Jan 8^th, 2007 at 2:14am:

Look, I'm not looking for an argument here. And although I appreciate the Kudos, I don't need my ego padded either. The reason I argue my point so vehemently is that I hate to see a false idea or rumor perpetuated. Airplanes don't just fall out of the sky. The crew usually has to do something terribly wrong to create these circumstances. And any modern, high-performance airplane can fall victim to crew complacency. The fact of the matter is; all too often an aircraft is given an undeserved "bad rap."

I have no wish to appear patronising or to cause offence. I happen to find this subject interesting & I'm simply expressing my opinion. If you call that argument so be it. I call it discussion.

I agree that a lot of these accidents are due to pilot error & most likely caused either by complacency or inadequate training. What worries me is that the stall doesn't seem to be fully understood by a large number of pilots flying regularly today. I think this is demonstrated by some of the comments in this thread & other similar topics. I'm not convinced that some instructors understand it themselves. This brings into question the basic training methods used by flying schools.

Founder & Sole Member - Grumpy's Over the Hill Club for Veteran Virtual Aviators
Member of the Fox Four Group

Need help? Try Grumpy's Lair

My photo gallery

Back to top

IP Logged

Reply #23 - Jan 8^th, 2007 at 7:34am

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:
Posts: 3593

Quote:

I agree that a lot of these accidents are due to pilot error & most likely caused either by complacency or inadequate training.

Unless there's an airframe or engine failure/malfunction..It's statistically certain that 100% of stall related accidents in certified airframes are pilot error.

Quote:

I have no wish to appear patronising or to cause offence. I happen to find this subject interesting & I'm simply expressing my opinion. If you call that argument so be it. I call it discussion.

Interesting.

Quote:

What worries me is that the stall doesn't seem to be fully understood by a large number of pilots flying regularly today. I think this is demonstrated by some of the comments in this thread & other similar topics. I'm not convinced that some instructors understand it themselves. This brings into question the basic training methods used by flying schools.

Being that, as far as I know, I'm the only CFI posting in this (and other) threads... and for my learning's sake, as well as the sake of challenging specifics ... I'm compelled to ask, "What exactly do you mean ?"

Back to top

IP Logged

Reply #24 - Jan 8^th, 2007 at 7:50am

Hagar Offline
Colonel
My Spitfire Girl
Costa Geriatrica

Posts: 33159

Brett_Henderson wrote on Jan 8^th, 2007 at 7:34am:

Being that, as far as I know, I'm the only CFI posting in this (and other) threads... and for my learning's sake, as well as the sake of challenging specifics ... I'm compelled to ask, "What exactly do you mean ?"

Please don't think my comments were aimed at you Brett. I could possibly have phrased it better but in previous discussions on the subject on this forum both you & Sean have confirmed the type of instruction to be found in a typical flight school today. It worries me that what used to be an honourable & respected profession is now seen in the majority of cases as a standby until something better comes along. There are obviously dedicated professional flight instructors still around but I get the impression these are now in the minority.

As you pointed out earlier, this topic has little to do with answers & is more like provocative thumb wrestling. I don't mind a sensible & informed discussion but a lot of chornedsnorkack's argument seems based on flawed theory.

Founder & Sole Member - Grumpy's Over the Hill Club for Veteran Virtual Aviators
Member of the Fox Four Group

Need help? Try Grumpy's Lair

My photo gallery

Back to top

IP Logged

Reply #25 - Jan 8^th, 2007 at 8:21am

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:
Posts: 3593

I didn't think it was directed at me specifically, but after a couple of reads, I wasn't quite sure. I could have better phrased my post as well, but I didn't want to over-think it and get patronising or offensive either

... I "had" to say something, though.

We can't even get an expert consensus on how lift is created (previous posts)... so I was ready to debate the merits of the text-book understanding of a stall vs the practical understanding of a stall. You could require a flying student's ground school to prepare him to pass a test on aerodynamic engineering, but that would just help put him into one of the "expert" camps on these matters.

The best, realistic way to go about this (as far as stalls go (but it can apply to any part of piloting)), is to make sure that the fundemtal, mental image is in place. "What is AoA and whay does the wing stop flying .. and what happens to AoA when the wings are other than level, bank-wise".

I learned to fly at a small FBO (Thorin Aviation, Escanaba, MI) and was taught by a crusty old (oft times hungover) pilot who was never in a hurry and had some pretty creative ways to drill these things home. After spending the last couple years at a large (200+ active members) flight school, I share your same concerns. CFIs come and go; sometimes by the day. Very few of them have any business sending new pilots off into the sky alone. Mostly, because they're just stopping off on the way to an airline job.. some because they just aren't good teachers, period. I've been a safety pilot (I've only taken two students and have since released them (I'm still not ready)) for many licensed pilots whom I wouldn't turn loose, alone, in an airplane, if it were up to me. The whole thing is dis-heartening Cry

Back to top

IP Logged

Reply #26 - Jan 8^th, 2007 at 8:22am

OTTOL Offline
Colonel
Fintas, Kuwait (OKBK)

Gender:
Posts: 918

My apologies Hagar. My comment wasn't directed at you but actually meant to be a clarification of my own stance in this.....uh....debate....

And I couldn't agree more about your comments regarding stalls and common misconceptions surrounding them.

If you want to get into a real mental wrestling mach; let's start a new thread about lift. You would be surprised (maybe not) at how misunderstood that one is. ....by the majority of the aviation community!!

.....so I loaded up the plane and moved to Middle-EEEE..........OIL..that is......

Back to top

IP Logged

Reply #27 - Jan 8^th, 2007 at 8:31am

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:
Posts: 3593

Quote:

If you want to get into a real mental wrestling mach; let's start a new thread about lift. You would be surprised (maybe not) at how misunderstood that one is. ....by the majority of the aviation community!!

Here, Ottol.. Read these five pages and if you like.. post it back into active discussion Wink

http://www.simviation.com/cgi-bin/yabb2/YaBB.pl?num=1148281582

Back to top

IP Logged

Reply #28 - Jan 8^th, 2007 at 8:39am

Hagar Offline
Colonel
My Spitfire Girl
Costa Geriatrica

Posts: 33159

Brett_Henderson wrote on Jan 8^th, 2007 at 8:21am:

I didn't think it was directed at me specifically, but after a couple of reads, I wasn't quite sure. I could have better phrased my post as well, but I didn't want to over-think it and get patronising or offensive either

... I "had" to say something, though.

OTTOL wrote on Jan 8^th, 2007 at 8:22am:

My apologies Hagar. My comment wasn't directed at you but actually meant to be a clarification of my own stance in this.....uh....debate....

No need for apologies. I'm sorry for any misunderstandings. We're all friends here & I'm delighted we cleared the air.

I know all too well that I can be a cantankerous old b*****d at times. Roll Eyes

Quote:

If you want to get into a real mental wrestling mach; let's start a new thread about lift. You would be surprised (maybe not) at how misunderstood that one is. ....by the majority of the aviation community!!

I'll sit this one out if you don't mind. Shocked

Founder & Sole Member - Grumpy's Over the Hill Club for Veteran Virtual Aviators
Member of the Fox Four Group

Need help? Try Grumpy's Lair

My photo gallery

Back to top

IP Logged

Reply #29 - Jan 8^th, 2007 at 8:44am

chornedsnorkack Offline
Colonel
I love YaBB 1G - SP1!

Posts: 363

And notice that while, at least, the pilots and instructors of "aircraft" have some certification, training, experience and knowledge requirements, those are not applicable to ultralights.

By FAR Part 103, a person does not need to meet ANY requirements of licencing, training, experience, knowledge, medical condition or age to fly an "ultralight".

By the same FAR Part 103, an ultralight does not need to be registered in any manner, nor bear any markings.

The airframe of an ultralight does not need to meet any certification requirements. Which implies that a person does not need to meet any requirements of knowledge or age to build an ultralight, or design an ultralight.

Verifying the empty weight is a simple matter of weighing. Verifying the fuel volume - ditto.

But how does one establish the exact stall speed? If you stall, and happily recover, at 25 knots, you should never have taken off to begin with...

Back to top

IP Logged

	Search the archive: