Tom,
This is how I understand the phenomenon:
ORIGINAL: tomfiorentino
1. How is the angle of attack across the semispan different (particularly hard to understand on the rectangular wing)?
The wing forces its path through the air, creating a disturbance with its shape.
That disturbance is no accident or chaos, it is meant to create an acceleration and deceleration in the flow of air, which produces pressure imbalances, as we all know.
That pressure imbalance sucks the air that is some distance in front of the leading edge upwards (upwash), and also pushes the air that is left behind the trailing edge downwards (downwash).
The stronger the disturbance, the higher the pressure differential, the lower level respect to the LE and TE the upwash and downwash reach.
Upwash makes the wing hit the air that is moving upwards; hence, the real AOA (or induced AOA) is higher than if it is measured respect to the geometrical direction in which the wing is moving.
The high pressure under the bottom is pushing the air into the lower pressure above the top of the wing.
The easiest path is around the wing tip, and the second easiest path is around the LE.
However, the stream of the downwash makes that path around the TE difficult for a vigorous stream, and no so difficult for a weak stream.
Now, let’s take the top surface of a right half wing for example.
Let’s divide the semi-span in several narrow strips running between LE and TE.
Along the chord of each strip the air is accelerated and its pressure is reduced as it should.
The left side of each strip sees lower backpressure from the strip on the left than from the strip on the right.
This happens because there are infiltrations of air coming from the bottom of the wing and around the wing right tip.
This infiltration kills the “vacuum” or low pressure above the top surface as we move toward the wing tip, and it does it in a non-linear pattern.
In terms of lift, the strip that is closer to the tip is unable of developing as low pressure (or “vacuum”) as the strip closer to the center of the wing (I don’t say fuselage because this also happens for flying wings).
As explained above, the upwash for each strip is a function of the low pressure above the top surface; hence, the strip closer to the center produces an upwash more pronounced than the strip closer to the wing tip.
Then, the strip closer to the center sees an AOA that is higher than the AOA that strip closer to the wing tip sees.
For a rectangular wing shape, what strip sees the critical AOA, the one that leads to the stall, first?
The strip that is closer to the center of the wing.
ORIGINAL: tomfiorentino
2. If the rectangular wing has the highest AOA at the root and the elliptical AOA is constant across the whole span, then why is the first region of stall for both the same....at the trailing edge root?
What happens to the air stream above the top surface just before the stall happens?
The air stream becomes slower and slower, using its dynamic energy to create more and more “vacuum” and adhesion to the top surface.
Because of this reduction, air from the bottom infiltrates around the TE, bursting the low pressure bubble that was sustaining lift.