Stalls 101

Dan Thomas said:
The AoA does shift forward as angle of attack increases, as the boundary layer breaks up toward the trailing edge and lift starts to disappear over the aft area of the wing. But in the stall itself, the whole top surface becomes turbulent and the CP moves aft again.

Dan
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~~~~ Thanks, I thought as much but thought I'd better confirm it. And since the CP shifts aft it assures that the aircraft "breaks" nose down instead of into a deep stall/ tail stall. Aft CG is an important limit to never go beyond.
 
zaitcev said:
Apropos a "proper" stall, my FBO has 2 Cherokees, which are almost identical (one has a slightly more powerful engine). The w/b charts are very similar. One performs stalls easily, noses over (also tends to drop left wing, which needs to be caught early). The other one I can never get to do proper stalls, either power on or power off. It just starts sinking, and will sink as long as back pressure is maintained. The only way to make a semblance of crisp stall is to dive and zoom. I don't think examiner is going to appreciate the technique if I do that. I'm kinda starting to suspect that it's somehow tail-heavy or something. But on the other hand, it's quite simmetric and does not bank a lot in a stall. This used to bother me quite a bit, but my instructor rated my stalls as acceptabe, and I decided to explain it all to examiner if I get the "reluctant" plane for checkride.
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One could, in theory, give the yoke a good yank at low speed instead of smoothly pulling back to use some of the dynamic effects to get you a good break. But - the DE would likely be significantly less than impressed.

The way the airplane stalls is the way the airpalne stalls. And that is what will expected during the ride (as others have pointed out). No need to do something stupid.
 
Hmmmmm....when I'm trying to stall a C-172, I'm pulling, the airspeed is dropping, but no stall break until I give it a yank. Is it me or the airplane?
 
Jaybird180 said:
Hmmmmm....when I'm trying to stall a C-172, I'm pulling, the airspeed is dropping, but no stall break until I give it a yank. Is it me or the airplane?
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Based on my experience flying 3 different models of C-172, it's probably the plane. I fly a C-172C now that has a very gentle break. The C-172N and C-172R had a much more pronounced break. Actually all of the C-172N models I've flown had a more pronounced break.

John
 
Jaybird180 said:
Hmmmmm....when I'm trying to stall a C-172, I'm pulling, the airspeed is dropping, but no stall break until I give it a yank. Is it me or the airplane?
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If you can still yank it, there's still elevator authority remaining. Recommendation: Arm-strengthening exercises so you can get full elevator travel without yanking. I've run into this on many occasions, where the trainee says "It won't stall!" and I grab the yoke and pull that last inch out of the elevator and it stalls just fine. One of the things driving that in Cessnas like the 172 is that the last inch or so of travel requires pulling the yoke slightly up as well as back, and folks don't always do that.
 
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jsstevens said:
Based on my experience flying 3 different models of C-172, it's probably the plane. I fly a C-172C now that has a very gentle break. The C-172N and C-172R had a much more pronounced break. Actually all of the C-172N models I've flown had a more pronounced break.

John
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John -- I flew a C172D with the Continental O-300 (145 HP) and Johnson bar 40 degree flaps -- not a rocket but floated and would simply mush - no break. Hold the yoke all the way aft and keep the wings level with rudder as you descend straight down.

:D
 
Humbly, arm strength is not an issue. Instead of a break, I usually get a mush. All the while altitude isn't holding steady as it should.
 
Jaybird180 said:
Humbly, arm strength is not an issue. Instead of a break, I usually get a mush. All the while altitude isn't holding steady as it should.
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I know of no FAA requirement to hold altitude steady while entering a stall. In any event, once altitude starts to drop and further elevator travel won't stop the drop in altitude, the stall has occurred whether the nose drops ("break") or not, because CL has maxed out and dropped off. The stall itself occurs when CL goes past the peak in the curve above. The "break" occurs somewhat past the AoA for peak CL (i.e., past "critical" AoA) when the curve drops off almost vertically.
 
Ron Levy said:
I know of no FAA requirement to hold altitude steady while entering a stall. In any event, once altitude starts to drop and further elevator travel won't stop the drop in altitude, the stall has occurred whether the nose drops ("break") or not, because CL has maxed out and dropped off. The stall itself occurs when CL goes past the peak in the curve above. The "break" occurs somewhat past the AoA for peak CL (i.e., past "critical" AoA) when the curve drops off almost vertically.
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I was thinking the same thing. If you're pulling it all the way back and the plane won't hold altitude either you've stalled it or you've run out of elevator authority.
I guess if you are loaded near the front CG limit you might not be able to drive the plane into a deep stall because of a lack of elevator.
Also, you may be expecting something more abrupt than Cessna designed for. To spin proof/protect the plane they want a gradual stall the creeps from the wing root out to the tip, which will neccessarily make the stall more gradual since the whole wing doesn't stall at once. Usually the wing has a twist (washout) or an airfoil that gradually changes as you move toward the tip in order to ensure that the wintips stall last. Effectively the wingtips will be at a lower AoA than the root.
 
Actually, it is from the loss of lift from the wing. If the tail stalled first, you really wouldn't like what happened next.
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Thanks for the correction, Ron. I didn't get out the books, but was thinking (which is always dangerous for me) that the horizontal stabilizer stalled to let the nose drop and the plane start flying again. I guess I had better stay out of the discussion! Bad memory!
 
somorris said:
Thanks for the correction, Ron. I didn't get out the books, but was thinking (which is always dangerous for me) that the horizontal stabilizer stalled to let the nose drop and the plane start flying again.
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If the horizontal stab stalls, the nose will indeed drop in any normal airplane (the downforce from the h-stab being what balances the pitch-down moment caused by the cg being forward of the wing center of lift), but the way you recover is to pull back on the yoke/stick, not forward, and to retract the flaps. But since this isn't going to occur in a light trainer without ice having formed and the flaps extended, it's not a big issue.

I guess I had better stay out of the discussion!
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No sweat -- you don't learn any other way than getting involved.
 
So how do you detect a stalled tailplane? It would seem to me the designers would have thought of that and designed it so the wing stalls LONG before the tail.
 
Jaybird180 said:
So how do you detect a stalled tailplane? It would seem to me the designers would have thought of that and designed it so the wing stalls LONG before the tail.
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Yes, they design airplanes so that the tailplane lifts down.
The CG is in front of the wings center of lift, so the horiz stab must push down to counter act the CG being in front of the lift. This provides "static stability".
I suppose if the plane is statically stable and you manage to stall the tail plane you will just pitch down and gain speed so that the tail is no longer stalled (just like a wing stall).

If you load the plane aft of the CG limits it is possible that the plane is no longer statically stable and the tail is carrying some of the weight (ie. it is now lifting up). In this case if you stall the tailplane you will flip over backward.

There was a nine year old girl who died in Cheyenne because of a tailplane stall and an aft CG. She, her Dad and her instructor were on one of those publicity tours and were flying way over weight, out of CG and into bad weather.
 
In addition to what CFB said above, icing can change the flows over the various surfaces, and with flaps extended, that can lead to a tailplane stall -- and it's probably the most common cause of tailplane stall. Since the designers assume you're not going to fly the plane with ice on it, and the effects of icing are largely unpredictable anyway, they don't design with that aspect in mind.
 
Thanks for the responses w.r.t. mush in Cherokees. I know Jaybird flies high wing, but hoped it were relevant to his problem.
-- Pete
 
COFlyBoy said:
There was a nine year old girl who died in Cheyenne because of a tailplane stall and an aft CG. She, her Dad and her instructor were on one of those publicity tours and were flying way over weight, out of CG and into bad weather.
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I remember that crash well, and the investigation, and I don't recall anything in the official NTSB report or the scuttlebutt at the time mentioning a tail stall. None.

Do you have a reference on that, or is it one of those stories that has grown out of the years?

Jessica Dubroff was her name, by the way. And IMHO she was killed by her CFI. But that's a story that went all the way to the cover of Time Magazine...

Anyway...

Flightpath: http://en.wikipedia.org/wiki/File:Dubroff_crash_site.jpg
Radar: http://en.wikipedia.org/wiki/File:Dubroff-Radar.jpg

When you take off in an overloaded Cardinal in a thunderstorm in Cheyenne... you're probably living on borrowed time already.

NTSB's webserver seems to be having issues... but here's the link... (not coming up right now... "file not found")...

http://www.ntsb.gov/publictn/1997/aar9702.pdf

No mention of tailplane stall anywhere in the cached versions of the document I've found anywhere online. Reference?
 
Jeanie said:
Think of it as Center of Lift... all the "liftness" of the wing coalescing into one area -
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Key concept - the airplane "surfs" ON the Center of Pressure and manuevers AROUND the Center of Gravity is how I think of it (cue multiple people to tell me how wrong I am). But they are NOT necessarily the same point.

If you are within CG limits then when you stall I believe the CP will be aft of the CG, so the airplane will tend to pitch forward around the CG. Get that CG too far aft and you stall "flat". Do a spin in that configuration and you may not be able to get the nose down. This is a bad thing.
 
jsstevens said:
Based on my experience flying 3 different models of C-172, it's probably the plane. I fly a C-172C now that has a very gentle break. The C-172N and C-172R had a much more pronounced break. Actually all of the C-172N models I've flown had a more pronounced break.

John
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And have been known to drop a wing at the stall. I used this to my advantage on my private check ride 10 years ago. DPE asked for a stall while in a 20 degree banked turn. She didn't say which way. So I turned right, stalled, plane dropped the left wing, I caught it with rudder at wings level and then recovered from the stall. Worked quite well, especially for something we hadn't practiced. :D
 
Ghery said:
And have been known to drop a wing at the stall.
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All production planes are equally likely to "drop a wing" at the stall. If they're in factory-perfect condition, and there's no yaw (centered ball) or bank (wings absolutely level), it won't happen. Any other condition, and they may roll off depending on what's worn/bent and how. Now, generally the condition is repeatable and predictable for any given plane, but from plane to plane? No way to predict.
 
somorris said:
Thanks for the correction, Ron. I didn't get out the books, but was thinking (which is always dangerous for me) that the horizontal stabilizer stalled to let the nose drop and the plane start flying again. I guess I had better stay out of the discussion! Bad memory!
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What you may be remembering here is how a canard-wing airplane works. In that case, they *are* designed so the canard ("front tail"?) stalls first, which drops the nose, which ensures that the wing will never stall. Very cool!
 
flyingcheesehead said:
What you may be remembering here is how a canard-wing airplane works. In that case, they *are* designed so the canard ("front tail"?) stalls first, which drops the nose, which ensures that the wing will never stall. Very cool!
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Unless you're flying something that has both a canard *and* a normal tail...

http://www.katmai-260se.com/

Then you can make up all sorts of stuff about where all the centers of lift are or aren't, etc... heh heh... :dunno:

That bird does some pretty impressive stuff with that setup. I want to fly in one just to see it happen from the inside.

"Might as well face it, you're addicted to slow..." :yesnod:
 
Jaybird180 said:
Instead of a break, I usually get a mush. All the while altitude isn't holding steady as it should.
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Sounds like you are using an an old outdated technique of throttling back while holding altitude until the stall. If so, that was a while ago, and mostly all you will get is a mush.

The current PTS description of entering a power-off stall is exactly as you would do a power off landing:

3) Establishes a stabilized descent in the approach or landing configuration..
4) Transitions smoothly from the approach or landing attitude to a pitch attitude that will induce a stall.

So, if you have the nose down in a power off descending glide, or partial power descent, then pulling power to idle, as in a landing, then smoothly pitching from the nose-low power off attitude to a nose high attitude that will induce a stall.

Practicing the rate at which you pull back to get the right attitude that will cause a slight break just as the elevator runs out of authority, is what you are practicing for; the elevator feel, like in a landing.
 
What you may be remembering here is how a canard-wing airplane works. In that case, they *are* designed so the canard ("front tail"?) stalls first, which drops the nose, which ensures that the wing will never stall. Very cool!
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Thanks Kent. I think that is exactly what I was confused about!
 
nosehair said:
Sounds like you are using an an old outdated technique of throttling back while holding altitude until the stall. If so, that was a while ago, and mostly all you will get is a mush.

The current PTS description of entering a power-off stall is exactly as you would do a power off landing:

3) Establishes a stabilized descent in the approach or landing configuration..
4) Transitions smoothly from the approach or landing attitude to a pitch attitude that will induce a stall.

So, if you have the nose down in a power off descending glide, or partial power descent, then pulling power to idle, as in a landing, then smoothly pitching from the nose-low power off attitude to a nose high attitude that will induce a stall.

Practicing the rate at which you pull back to get the right attitude that will cause a slight break just as the elevator runs out of authority, is what you are practicing for; the elevator feel, like in a landing.
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WOW - primacy in action - I learned to do power-off stalls from a constant altitude, and even though I've read and re-read the PTS for my upcoming CFI ride - I still think of them being done that way. Tomorrow I'm going to practice them with the PTS on my lap, so that I can reinforce the proper habit.
 
steingar said:
And, for what it's worth, if you are at a speed anywhere in the vicinity of a stall, your eyeballs should be where they should always be, outside! My aircraft has VGs and once had gap strips (and will again, dagnabbit!). I had no idea at what speed it stalled. I really didn't know until I had a CFI do an arrival stall and I got to watch the airspeed. 35 mph indicated on that occasion!

The idea (as I understand it) is to recognize the onset of a stall by the aircraft's behavior, not through the pitot static system.
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Unless you're practicing them on instruments...
 
While it may not be a good way to get a "full stall" in some types, doing stalls from constant altitude is still useful for approaching the stall very slowly so the trainee gets a feel for all the cues and sensations as the stall is approached.
 
Just as a follow up - I dug out my ASA illustrated manuevers guide, and it shows the power-off stall the "old" way. This was a recent purchase for the CFI training. So be careful what you show/sell your students.
 
TMetzinger said:
Just as a follow up - I dug out my ASA illustrated manuevers guide, and it shows the power-off stall the "old" way. This was a recent purchase for the CFI training. So be careful what you show/sell your students.
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As a CFI applicant, be ready to discuss the old way and demonstrate the revised task to standard.

Otherwise when you do your stall demo from a descent and the examiner says, "You lost 200...'" you don't get into a pee-pee contest.
 
dmccormack said:
As a CFI applicant, be ready to discuss the old way and demonstrate the revised task to standard.

Otherwise when you do your stall demo from a descent and the examiner says, "You lost 200...'" you don't get into a pee-pee contest.
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Fortunately - the ground lesson I'll be presenting to the inspector covers slow flight and stalls, so I'll be sure and highlight the differences between the way things were and the current PTS.

I'm not worried about a pee-pee contest on this particular topic - the PTS is pretty explicit - failures are related to headings or bank angles, the only altitude bust is completing the manuever below 1500 AGL.
 
TMetzinger said:
I'm not worried about a pee-pee contest on this particular topic - the PTS is pretty explicit - failures are related to headings or bank angles, the only altitude bust is completing the manuever below 1500 AGL.
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Tim -- I hope it works out as you say.

On my CFI ride I had an FAA Ops inspector as an examiner and he insisted that Bonanzas were death traps, all non-fixed blade propellers were driven by oil pressure (not really, see -35 electric prop), and that flying a C172 in the pattern with a thunderstorm 5 miles south of the field was "fine."

I obliged and kept my opinions to myself.
 
denverpilot said:
No mention of tailplane stall anywhere in the cached versions of the document I've found anywhere online. Reference?
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Speculation from 15 years ago (I was about halfway done with primary training at the time). I stand corrected.

The impact attitude was very nose low which indicates a normal stall. The AC was 96 lbs overweight and would have been within CG if it wasn't overweight. No telling what the handling is like when you are near the aft limit and overweight.
 
dmccormack said:
Tim -- I hope it works out as you say.

On my CFI ride I had an FAA Ops inspector as an examiner and he insisted that Bonanzas were death traps, all non-fixed blade propellers were driven by oil pressure (not really, see -35 electric prop), and that flying a C172 in the pattern with a thunderstorm 5 miles south of the field was "fine."

I obliged and kept my opinions to myself.
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I will keep my opinions to myself - on things that are matters of opinion.

I'd be surprised if we had differences on standards though since they are spelled out pretty well. But I'm gonna start another thread on one thing that is frustrating me.
 
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