The "Zero Lift Axis" of an aircraft is a function of it's
aerodynamics ....
Think of it as an imaginary line, but this time it is
"fixed" to the aircraft and runs through the centre of gravity
from nose to tail.
Imagine that an aircraft is flying a true vertical line in still
air, then the ZLA will be perpendicular to the ground. The left
sketch shows the aeroplane flying vertically downwards
with it's ZLA through the pitch axis.
Look at the the two plan view 'planes in the centre and
clearly the yaw axis is perfectly vertical in both cases. In the one on
the right however the pilot has inserted a little roll and yaw during
the preceding pull or push and carefully straightened it up before
vertical ZLA was achieved, and you can see that a considerable sideways
vector to the CGT is possible - this is a key element in the art of
keeping the aircraft in the box when it's windy!
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