Ric[U said:
hard_SWave;1074341]Ok, so I understand it is hard to figure out the angle of attack and you can't predict it and there's no general rule to aproximate how it will behave in that situation.
Unfortunately I think you're looking for a simple answer to a complicated (and somewhat ill formed) question but perhaps we can clear things up a bit with some related basics. Ed is correct that the one thing that's most consistent with stalling is the wing's angle of attack. It's also true that AoA isn't necessarily easy to visualize or understand and IMO pilot training has been a bit too vague about the subject.
First let's consider things from a static perspective (your reference to pulling back abruptly on the yoke introduces dynamics that add considerable complexity). If you were to draw a curve of AoA vs lift coefficient (the amount of lift generated for a particular calibrated airspeed) you would see something that looks like a hill with a curved top and typically a steeper slope on the "back" (greater AoA) side of the hill. The top of the "hill" represents the maximum coefficient of lift the wing can achieve and the AoA where that occurs. This is called the "critical" AoA and is the AoA at which the wing stalls. Also typical but not necessarily always true is that the front side is fairly linear from below the origin ("zero lift AoA") up to 70-80% of the critical AoA point.
Another relevant curve plots CAS vs AoA for a constant
amount of lift (e.g. the weight of the airplane in unaccelerated flight). This one looks somewhat like an inverted copy of CoL but since lift is proportional to the square of CAS and therefore for the linear portion of the first curve CAS is
proportional to 1/SQRT(AoA), the left end goes to infinity (infinite CAS is needed to genearate lift at the zero lift AoA) and the curve looks more like an inverted parabola.
If you are in level flight in smooth air your AoA has a fixed linear relationship to the airplane's pitch attitude with the difference being the angle between whatever you consider "zero pitch" and the chord line of the wing (often called the "angle of incidence"). If you start at your maximum speed for level flight and slow the airplane very gradually to point where a stall begins your AoA and the corresponding pitch attitude will follow the second curve described above.
Finally, because the amount of lift (and hence the g-force) for a give AoA is directly proportional to the square of CAS, the change in the CAS at the critical AoA is equal to the square of the g force.
It's also generally true that the elevator and stick/yoke position has a fixed relationship to AoA under static conditions (i.e. the AoA that exists when the elevator position has been constant long enough for things to stabilize) although this is also affected by the airplane's gross weight and CG.
Now to try an answer your original questions:
A small airplane always stalls when you pull back on the yoke and your airspeed is below Va?
Assuming you're asking if this is true or false, the most succinct answer would be no. A stall will eventually be generated if you pull the yoke far enough back and hold it there long enough so that the airplane's wing reaches the critical AoA. If the yoke movement is applied slowly enoug.h that the critical AoA is reached while the g force is 1 then the stall will occur when the plane has slowed to the same CAS which generates a stall in level flight, i.e. when the power is reduced and altitude is maintained (note: this is ignoring the small contribution to lift from the propeller thrust). OTOH, if you go to the opposite extreme moving the elevator as rapidly as possible the wing will stall
almost immediately with virtually no change in CAS. I say almost because the airplane will still have to rotate about the lateral axis (i.e. the pitch angle must go up) and that always takes some time, during which the airspeed will drop slightly due to the increased drag. Also note that some airplanes don't have enough elevator authority to achieve the wing's critical AoA (e.g. Ercoupe) and any airplane lacks that ability if the CG is far enough forward (typically beyond the approved envelope).
Or is it possible that at some speeds to not have enough elevator authority to reach the G for the accelerated stall? Basically, can you pull back on the stick (especially the case you pull full back) without having an accelerated stall?
Off the top of my head I'd say that if there's enough authority to stall in unaccelerated flight then there will be enough authority to stall at Va. The ability of the elevator to generate downforce is (like most things related to lift) proportional to the square of the CAS and the force required to affect any change in AoA is likely also proportional to the square of the CAS. Note that the force you must apply to the yoke to alter the AoA also increases with CAS in most airplanes.
Please pay attention to my assumption here: for example, a Cessna stalls at 2G at 66 kts. A full back yoke will produce 2G at 66 kts?
Not necessarily. I'm assuming that you're talking about 66 Kts CAS not IAS and that this is at a constant (likely max GW) weight (CAS diverges from IAS at high AoA so we must use CAS here). As described already, the airplane's behavior is going to depend on your initial CAS, the CG, and how rapidly you move the yoke. Clearly if you move it gradually you will never get anywhere near 2G but even if you yank it as quickly as you can it's not likely you'll reach 2G before the wing stalls because the airplane will slow down some before the critical AoA is reached.
As an example, always a 80% yoke aft deflection produces 80% of the stall load factor?
Definitely not true. As mentioned earlier for a specific CG and weight the yoke position will have a relatively constant relationship with AoA under static conditions but while the yoke is moving and/or the airplane is changing airspeed that relationship goes out the window. In addition, while static elevator position does relate well to AoA, there's no such relationship between elevator position and G load unless you can somehow hold CAS constant.
I figured out that in some Cessnas at some speeds below Va you can pull back on the stick, you feel the G, but the aircraft doesn’t stall immediately but just seconds after that when the airspeed decreases enough. Is this right or I noticed it wrong?
Sounds correct for any airplane.
Also, at some speeds I think you can deflect let’s say 80% aft yoke without accelerated stall, but I figured out not at all.
I'm not sure what that is intended to mean.
What I know is that an aircraft can be stalled at any speed and attitude. All it matters is angle of attack, however, this doesn’t help me too much to figure out my problem.
If all airplanes had an AoA indicator (BTW one of mine does) and pilots were taught to reference that instead of IAS to predict stalls you'd probably find this a lot easier to understand. It's too bad someone decided a long time ago to mark a dynamic pressure gauge with numbers representing the airspeed required to achieve that dynamic pressure in a standard atmosphere at zero MSL. If instead, they had just used a pressure scale (e.g. PSI, KPa, etc) for that instrument and added an AoA indicator there'd probably be a lot less concern about stall "speed" and instead of Va, Vfe, and Vne we'd be taught about Pa, Pfe, and Pne and allowing pilots to ignore the complicated relationship bewteen stall behavior and dynamic pressure.
Perhaps that will change with more flight experience.
