It all derives from Newton's third law.
You know about basic propwash - the prop acts as a fan and blows the
air backwards, so that the relative airspeed behind the prop is greater
than the plane's actual speed through the air. It's easy to compute
this - the propwash delta is simply the prop thrust divided by the mass
flow rate through the prop. (Just another form of F = M*A.)
But... the prop is also exerting torque on the air, to be precise, the
engine torque. This force also has to be accounted for by a
corresponding acceleration of the air passing through the prop. So the
cylinder of air flowing through the prop doesn't go straight back, but
rotates as well. Again, the rotation speed comes straight from Newton's
third law, and is torque divided by mass flow rate and prop diameter.
An informal way of looking at it is that the prop is dragging the air
around with its rotation.
There's been some debate over whether there's really a "rotating
cylinder" of air behind the prop, or whether there are just little
ribbons of moving air behind the prop blades. It terms of working out
the overall effect on the plane, the distinction is unimportant -
Newton's third law says there is a certain average
spiral component to the
airflow. The easiest way to model it mathematically is as a rotating
cylinder, with a considerable degree of turbulence added by the prop
blades. The spiral component does eventually dissipate into the
surrounding air, but keep in mind that for a plane in flight, it takes
less than a second for the air to travel from the prop to the tail, so
the rotating cylinder model holds up pretty well.
Spiral propwash has a significant effect on single engine planes,
notably during takeoff and climbout, where you're fying at high power
and low airspeed. The reason is that the vertical stabilizer (in most
single engine configurations) sits in the upper half of the propwash.
So with your typical CW prop rotation, the spiral propwash component
hits the vstab from the left, pushing it to the right, causing the
plane to yaw left. This
is the primary reason you need right
rudder during takeoff and climbout. (There is also P-factor
, caused by
the prop shaft not being lined up with the airflow, which works in the
same direction. However, with most light planes, spiral propwash is the
X-Plane does a pretty decent job of modelling this as long as you have
enough elements in the wing and stabilizer areas that are exposed to
propwash. Turn on the graphical flight model display (hit the "/" key a
couple times) and you'll see different length lift vectors in the wing
surfaces in propwash.