Why does a stalled wing stay stalled?

Discussion in 'Flight Following' started by SkyHog, Sep 20, 2005.

  1. SkyHog

    SkyHog Touchdown! Greaser!

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    Everything Offends Me
    So I'm trying to grasp the idea of a stall right now. I can't wrap my mind around it.

    Forgive my poor drawings, this was real quick. In a straight and level flight (figure 1), the air flows smoothly over the wings the way it should.

    When the AOA exceeds the critical range, the relative wind no longer flows smoothly over the wings (figure 2), and then the wing stalls.

    Here's where I don't get it. You continue to pull back, and the wings are stalled, and therefore fall. When they fall, I picture it looks like figure 3. Why is it that unless you push the yoke forward, the wing will stay stalled? It seems to me that as the wing stalls, it automatically would fall in line with the relative wind, and the stall would end. What am I missing?
     
  2. Areeda

    Areeda Pattern Altitude

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    Nick,

    I think what you're missing is that everything doesn't stall completely at the same time. What we pilots call a stall usually meens the wing roots have lost lift but not the tips otherwise the ailerons would work backwards.

    If you do an oscillating stall where you pull back the yoke, keep the plane straight and level with the rudders then what happens is pretty close to what you describe. As the stall deepens the plane pitches down enough to gain some speed, when the tail gains enough authority it pitches up and stalls again. It feels like you are in a continuous stall. I've always been too busy to look at the airspeed indicator but I assume it's varying +/- 10 kts.

    Joe
     
  3. gismo

    gismo Touchdown! Greaser!

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    The part you are missing is that as soon as the airflow over the wing exceeds the critical AOA and the wing's lift decreases (it doesn't go to zero, it just decreases), the wing accelerates towards the ground (if you started in straight/level flight). As the wing descends, the relative wind which is a function of your forward and vertical speed, shifts from a front to rear direction to one that's more bottom to top. IOW the AOA increases as a result of the reduction in lift, and that increase in AOA causes a further decrease in lift (repeat as necessary) and down you go. I'm sure you've noticed that the pitch attitude of the plane becomes more nose down, but that change is more than cancelled by the upward shift in the relative wind as long as you hold the stick back.
     
  4. mdreger

    mdreger Pre-takeoff checklist

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    It appears that you are neglecting the effect of the horizontal stabilizer/elevators. The way I remember it, the wing will act exactly the way you drew, if pressure on the elevator is released. If the elevator is not released it will hold the wing at the same angle to the relative wind no matter what the attitude of the aircraft relative to the earth.
     
  5. Henning

    Henning Ejection Handle Pulled

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    What you are missing is the change in the Relative Wind, as you stall and start to drop, the RW will shift further below the wing (rotate the arrows fom pointing at the leading edge to pointing at the bottom of the wing. BTW, you don't have to drop the nose, adding power works as well, anything that will rotate the RW arrow towards the leading edge.
     
  6. Eamon

    Eamon Line Up and Wait

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    As an addition to the many great answers.................

    You don't really (Maybe) need to push the yoke forward. ...

    As your speed decreases the rear "Upside down flying wing" (Elevator) will loose "Reverse" lift & cause the nose to drop automatically. As long as you are not pulling back hard on the yoke, most planes will accelerate past stalling speed quickly & the wind will "restick" to the wing and you are flying in no time.

    Did you ever hold the yoke straight or slightly pulled back during a stall in a 172? If you pull back the plane "mushes" toward the ground just below stall nose high. If you let go of the yoke the plane is flying in 1 second. If you push forward, the plane is flying in 1/2 second.
     
  7. Areeda

    Areeda Pattern Altitude

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    IOW if the plane stalls at 49 it flies at 50
     
  8. Maverick

    Maverick Line Up and Wait

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    In addition to all the other great answers I would also add that the wing will stall before the horizontal stabilizer. By keeping the yoke pulled back you are in essence maintaining the offending AOA and thereby keeping the wing stalled. This is easier in some airplanes than in others. The C182 that I used to own had a stol kit and it was so effective that I could hold the airplane in a stall as long as I wanted to or until the ground rose up to smite me. Not all airplanes are that controllable.

    The early model Cardinals had a flaw in that the stabilator would stall before the wing and that caused a few deaths prior to the AD requiring the slotted stabilator.

    Jeannie
     
  9. Bill

    Bill Touchdown! Greaser! PoA Supporter

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    Why the deaths? As I understand it, the horizontal stab provides downforce, or negative lift. So, if the the horizontal stab stalled before the wing, the plane should pitch forward, ie, the main wing would never stall. Now, if someone were idjit enough to keep the yoke back, the plane would then dive, yes?

    OK, so I can see that now, a pilot goes to stall, and the plane dives. He thinks its not stalled, wants to go up, and keeps backpressure on the yoke. Plane dives until you by the farm.
     
  10. Maverick

    Maverick Line Up and Wait

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    Just imagine trying to flair if the stabilator stalls!
     
  11. poadeleted20

    poadeleted20 Deleted

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    Not quite. When the tailplane (which is effectively an upside down wing) stalls, you lose the downforce that keeps the nose up. This causes the nose to drop in what looks like a wing stall. However, the wing is still flying. The pilot responds to what he thinks is a wing stall by pushing forward on the yoke to recover. However, this only lowers wing AOA (reducing lift) while causing the angle of attack on the tailplane to increase (forward yoke, elevator goes trailing edge down, higher AOA -- draw a little picture to see this if you need it), deepening the tail stall. Only by pulling back on the yoke (contrary to the usual respose to what appears to be a stall) can you lower the AOA on the tailplane (yoke back, trailing edge up, AOA reduced) and regain the downforce needed to bring the nose up.

    BTW, this is one of the things that kills people in icing conditions -- the tailplane being thinner ices up first, and stalls as you lower the flaps whose altered downwash raises the AOA on the contaminated tailplane, causing a tailplane stall and pitchdown. The pilot has to fight the trained response of reducing back pressure to lower wing AOA and instead pull back hard to unstall the tail and get the tail downforce needed to avoid diving straight into the ground from the low altitude where flaps are usually deployed. This is also why everyone tells you not to use flaps if you think you're iced up.
     
  12. Bill

    Bill Touchdown! Greaser! PoA Supporter

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    Oh boy, I didn't think about that....*splat*
     
  13. Bill

    Bill Touchdown! Greaser! PoA Supporter

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    Thanks for the explanation, but I WILL have to draw this one out to be fully convinced. Homework for tonight!

    Bill
     
  14. Eamon

    Eamon Line Up and Wait

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    This is VERY VERY common on the Caravan. I sat for an extra training class for this very subject. If it is cold enought for ice & you "feel" a stall condition, check the airspeed. If the airspeed is high, PULL..... If the airspeed is low, PUSH.

    Even if your Pitot is frozen over, I bet you can guess speed of your plane with your eyes closed.
     
  15. n741dm

    n741dm Pre-Flight

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    Wow, Ron - This is one of those pearls of wisdom that we have all been taught during our training and think we understand... Until the SHTF!

    Your explanation above could very well save a life or two. Wish the original explanation in the text and by my primary instructor had been that succinct and direct!

    {Note to self: Self, move that thought right back into the forefront of the memory stack!}
     
  16. Maverick

    Maverick Line Up and Wait

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    Great explanation Ron. :) Thanks.

    BTW I tried to give you positive rep points on that but the system told me I would have to spread some around before I could give you any more. To me knowledge I've only given you one once.

    Jeannie
     
  17. Henning

    Henning Ejection Handle Pulled

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    ehh....yes,but no, stall across the wing is progressive, so the plane progresses into and out of stall across a few knots, some more than others, but it's at least a couple.
     
  18. poadeleted20

    poadeleted20 Deleted

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    To elucidate a bit...

    Certification rules for light planes require that the wing does not entirely stall at once, and that the ailerons remain effective for roll control even after the stall break. What does this mean? That the wing is designed so the inboard portion stalls first, and then the stall progresses outward, and further, under normal g-loads, you will never have enough elevator authority to stall the outer portions of the wing, i.e., the loss of lift from the inboard portions will cause a pitchdown ("break") before the outer portions exceed their critical AOA.

    How do they do this? Cessna uses "twist" -- the outer portions of the wing are twisted leading edge down compared to the inboard portion so the AOA is lower out there than it is at the root. Some planes uses a different airfoil (cross-sectional shape of the wing) so the critical AOA is higher on the outboard portion than the inboard portion. Grumman uses an untwisted, constant airfoil wing, but puts "stall strips" on the leading edges of the inboard portions to disrupt airflow there before the wing reaches its "naked" critical AOA.

    Therefore (assuming full gross load, 1g loading, etc), the inboard portion of the wing stalls before book stall speed and as you slow, more and more of the wing stalls. That's why you feel all that buffet before stall speed -- you've already disrupted the airflow over the inboard portions and you can feel that disrupted flow both through the structure and as it goes over the horizontal stab/elevator. When enough of the wing stalls so the lift generated drops below the point at which full aft stick/yoke can hold it up, the nose falls, but the outboard portions are still unstalled, generating lift, and providing roll control. Short of a "whip stall" (essentally standing the plane on its tail so airspeed drop so fast it doesn't fall through before it all stalls -- a prohibited maneuver in most all light planes), you can't stall the whole wing of a light plane, and you will have part of the wing stalled well above book stall speed even without pulling g's.