Angle of Attack
What happens to the angle of attack of an unpowered aircraft when it
encounters lift and sink?
If I understand you correctly, the answer is "nothing." The angle
of attack only changes when the horizontal wind velocity does.
Let's assume the unpowered aircraft is a sailplane gliding through still air. It's
flight path downward may be, say, 3 degrees. Let's also assume the angle of attack
(aoa) is approximately + 0.2 degrees. If the sailplane suddenly encounters lift (a
rising air current), the aoa will momentarily increase due to the sudden change in
relative wind, or "shear," then return to it's original value. The wing in still air
has a certain vertical component to it's flight path, as well as a horizontal
component. Let's say, for every 30 feet of forward travel, it sinks 1 foot down.
Upon entering a rising column of air, the vertical motion of the air will produce
the effect of increasing the "downward" motion of the wing through the rising air
(MOMENTARILY). You can now visualize how the aoa will increase as this happens. Once
the forces stabilize, the aoa and the flightpath (through the air) will will return.
Of course, the flight path relative to the ground will now have changed. The opposite
will occurr when encountering sinking air.
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Update: June 2012