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My friend fly ultra light Savannah, 2 seats plane,wings dont have slats ,it has vortex generators at leading edge.

He says that he cant stall plane with agressive yoke pull (I talk about speeds above stall-speed).

My question is this really true,why airflow dont separate from wing at high AoA and cause reduction in lift and sudden drop in altitude when pull yoke as fast you could?

He says plane just starts climb an that is what happend.

enter image description here

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My question is this really true,why airflow dont separate from wing at high AoA and cause reduction in lift and sudden drop in altitude when pull yoke as fast you could?

I will try to answer this explicitely stated question in more generic terms.

As already mentioned, a wing will stall when AoA increases above particular value. This is true regardless airplane, wing profile or anything else. Particular angle and particular lift/drag curves in stalled region of course depends.

First think to keep on mind is: Angle of attack is measured relative to the airflow. Assuming no wind, no thermal lift or other airmass movement, it will be the angle between wing chord and direction of airplane movement. So in a climb the angle between horizon and wing chord will be higher (for same given AoA). During a rapid descent wing chord can point below horizon and it will be still positive AoA generating lift as usual.

So, there are generally two (three) things which can happen when you suddenly pull on a stick in horizontal flight:

  1. If you pull softly enough, the plane is light, does not have enough elevator authority and/or has powerful engine ... then ... as soon as AoA starts to increase, the lift will increase too above weight of the plane. This leads to extra vertical force accelerating you upwards. Gaining vertical velocity, the AoA (for same pitch) decreases somewhat.

    Initial horizontal flight. AoA as necessary to generate lift equal to weight.
    Horizontal flight

    First consequence of the stick input is pitching up. Pitch up But the increased AoA results immediately in excessive lift from main wings (higher then weight) and as a consequence the plane starts to accelerate upwards (any net resulting force = acceleration).

    As it builds up vertical speed relative wind starts to turn (it has always direction against the actual motion) and decrease AoA. Climbing Steady climb would be reached when increase in pitch equals increase in vertical velocity, so effective AoA is back at original 1g-level. (Note: for simplicity, I am omitting from images change in direction of lift force with pitch. It does not change anything principal here except adding a drag which needs to be overcome by propeller -- energy necessary for climbing.)

    So, if the nose pitch is increasing slower than you are gaining vertical speed, you will overgo into climb without stalling wings. Because this is exactly a standard way to initiate a climb, it has to be possible to perform with appropriate stick input without stalling.

    (Would you continue pulling on the stick, you will eventually lose airspeed because of engine power limit, get out of the elevator authority or eventually complete a looping. :) But this happens only later.)

  2. If you pull abruptly, but elevator is small, you can actually stall the elevator. That is, you will still get some nose-up input, but it will be limited not by the stick movement, but the maximal force/torque elevator can provide. In practice the result will be same as in the previous point.

  3. If the pull is sudden enough and elevator has enough authority to pitch up nose faster than airplane can gain vertical speed, you can exceed stall AoA before initiating a climb. Then wings will stall in horizontal flight.

    For reasonable speed and common airplane, it can be well impossible to do this and only result will be either #1 or #2. OTOH this is exactly how you fly snap (flick) roll in an aerobatic plane, so with big enough control surfaces you can definitively induce stall from horizontal flight in moderate speed range.

Note below: it is good to keep on mind that "stall" does not mean "airplane is going to crash". Airfoil stall (any airfoil can stall, not only main wing, but usually people implicitly means main wing) is an angle of attack where the lift (for given airspeed) starts to decrease with further increasing AoA.

Just this, nothing else. It does not mean zero lift. If does not mean crash. It even does not need to mean significantly less lift (depending on particular airfoil). The fact that AoA versus lift dependency is inverted makes airplane behavior quite contra-intuitive – for example aileron inputs will have practically opposite effect than usual (which can in turn easily lead to crash), or a stability is lost, but there is still lot airplanes which can fly seemingly quite "normally" even with main wing stalled.

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  • $\begingroup$ Gaining vertical velocity, the AoA (for same pitch) decreases somehowThis part I dont understand.Plane is not going up as whole like helicopter.Plane is climbing in straight line ,so where vertical airflow velocity comes from? When plane climb his pitch angle must be greater then when fly in horizontal flight,but plane still travel in straight line so dont feel vertical airflow velocity. $\endgroup$ – user53913 Dec 27 '20 at 19:54
  • $\begingroup$ @EBV821 somewhat not somehow, of course, pardon my English. Anyway, yes, it is not purely vertical movement, but there is still non-zero vertical component and therefore relative wind has to be oriented partially from above too. If pitch with respect to ground/horizon/inertial reference does not increase further, an increase in vertical speed means lower AoA. I have tried to sketch some illustration, not sure if it helps. :) $\endgroup$ – Martin Dec 27 '20 at 20:24
  • $\begingroup$ Actually, in the first approximation, for given airspeed, there is only one possible AoA (two if you count stalled region too) for steady balanced flight (straight, either horizontal or climbing/sinking, but not turn). Any different angle will result in excessive force in some direction (lift or gravity) resulting in imbalance. When the flight stabilizes again in new regime, the AoA has to be back at value for 1g-lift. $\endgroup$ – Martin Dec 27 '20 at 20:30
  • $\begingroup$ Indeed your second picture(pitch up when still going horizontaly) is my topic question:But from your answer I see that it is very hard to stall plane like that.That means my friend was right. $\endgroup$ – user53913 Dec 27 '20 at 20:56
  • $\begingroup$ If pitch with respect to ground/horizon/inertial reference does not increase further, an increase in vertical speed means lower AoA Again I dont understand what you want to say.There is no climb if you dont increase pitch angle compare to horizontal pitch angle.So you must pull joke ,lift your nose to climb.But then when you are climbing,thurst from propeler produce additional lift so plane fly with reduce AoA to keep gravity in balance.AoA is not reduced because of vertical induce flow ,because vertical flow dont exist if plane fly up in straight line... $\endgroup$ – user53913 Dec 27 '20 at 21:03
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I have 500 hours in one, more or less... This one:

enter image description here

In my experience, if –with the engine at cruise, say 2/3 power– you start pulling the stick slowly, the machine does obediently pitch up, with no stall, but one has to be careful, and fast with the pedals. At the extreme, the airplane is pointing more or less to the zenith, and starts falling tail first while rolling, in reaction to the prop's torque. But it doesn't stall...

If you try the same gliding with the engine at idle, you won't be able to avoid a wing dropping down, more or less when the pitch reaches 30--40º...

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  • $\begingroup$ Is this normal behavior for every plane that you cant stall plane with agressive yoke pull?So only way to force plane to stall is to fly below stall-speed? $\endgroup$ – user53913 Dec 27 '20 at 12:30
  • $\begingroup$ Stall speed is the airspeed at which, for a given load, you have to increase the AoA to the critical in order to get just enough lift as to stay in s/l flight. Speed, by itself, has nothing to do with the stall, this being related with and determined by the AoA. Concerning the 'aggressive yoke pull', you should start by defining that properly... $\endgroup$ – xxavier Dec 27 '20 at 13:13
  • $\begingroup$ marvelous photo and good information! I hope this behaviour is communicated to you by the designers/manufacturers. Are you saying it is difficult to stall it without a wing drop, or that it directly goes into an incipient spin? $\endgroup$ – skipper44 Dec 27 '20 at 13:29
  • $\begingroup$ @xxavier in video he say that stall can happend at any speed.How can initiate stall with pulling yoke at higher speeds (speeds above stall-speed) ? My firend say you cant force Savanah to stall like that,so why this man say that stall can happend at any speed?..youtube.com/watch?v=9vdnO-1I6Vs&ab_channel=JimAlsip $\endgroup$ – user53913 Dec 27 '20 at 14:20
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    $\begingroup$ @EBV821 And he's right... Stall can happen at any speed. You only need to have a big enough AoA. It depends on the airfoil, but it's around 18º for most airfoils. The Savannah (and many STOL planes) can stay aloft with stalled wings, but the aircraft itself stays under control. In most airplanes, a stalled wing means a loss of control and a crash may ensue... $\endgroup$ – xxavier Dec 27 '20 at 15:10

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