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.
I have 500 hours in one, more or less... This one:
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...
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:
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.
First consequence of the stick input is pitching 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.
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.)
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.
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.
