Why does a banked aircraft create sideslip?
Incorrect explanations abound for why an aircraft tends to sideslip when banked, in the absence of any &quot;coordinating&quot; rudder input.  Often the fact that the weight or gravity vector has a component pointing towards one wingtip is invoked.  See for example the diagram in Martin Simons' &quot;Model Airplane Design&quot;.
The truth is that this situation exists in a coordinated turn as well as in a sideslip, and so this cannot be the reason for the sideslip. According to Newton's principles, a steady sideways force tends to drive a curvature in a trajectory, not a linear sideways motion.  It is not correct to say that the weight or gravity vector tends to cause the aircraft to slide toward the low wingtip whenever the aircraft is banked, just because the weight or gravity vector contains a component that points toward the low wingtip.
Here is one reason that an aircraft tends to sideslip when banked-- banking tends to cause a turn (a curvature in the flight path), and the resulting curvature in the flight path &quot;bends&quot; the airflow or relative wind.  In other words, as the aircraft starts to yaw in response to the changing direction of travel, different parts of the aircraft are moving through the airmass in different directions at different speeds at any given instant in time, and so different parts of the aircraft experience different velocities (speed and direction) in the local relative wind, which can be described as a &quot;curve&quot; or &quot;bend&quot; in the direction of the relative wind.  In the absence of any rudder input, the vertical tail experiences a relative wind component that tends to oppose the turn and slow the yaw rotation rate.  This tends to cause a sideslip.
If this is unclear, consider a pinwheel, or an aircraft in a flat spin.  As the body rotates, clearly, different parts of the body are moving through the airmass in different directions at any instant in time.
This phenomenon can also be described as &quot;yaw damping&quot;.
This is also the phenomenon that is described as the &quot;long-tail slip&quot; effect in Section 8.10 of John S. Denker's &quot;See How It Flies&quot; website.
If the aircraft's fuselage could bend like a banana to conform to the curving path of the turn, then every part of the aircraft from the nose to the tail could be streamlined to the local direction of the curving relative wind, and this phenomenon would not occur.
Rotational inertia in the yaw axis also plays some small role in driving sideslip as the bank angle and turn rate increase.
Explanations that attempt to account for the relationship between banking and and sideslip, without discussing the curvature in the flight path that generally results from banking, are invariably incorrect.