ORIGINAL: DarZeelon
Ernie,
Proximity to the ground also has an effect; a very profound one, in fact.
The prop-wash is in the form of a spiral, which with the plane 'out of ground effect', goes directly backward along the plane's thrust-line.
As a result, with the prop spinning in the normal direction; clock-wise, when viewed from the cockpit, the 'over' prop-wash will hit the vertical stabilizer and the rudder from the left; in turn causing the plane to veer to the right. The 'under' prop-wash hits nothing, thus causing no change in the plane's heading.
If the prop-wash "hits" the fin and rudder from the left it will it will cause the aircraft to yaw to the left.
But, what happens if the plane is a tail-dragger and is in the normal three-point attitude, on the runway, in the beginning of the take-off run? The prop-wash obviously cannot go into the runway... so, it deflected, becoming runway-hugging and parallel...
But where does the 'under' prop-wash go now???
It hits the vertical stabilizer and the rudder from the right, causing the plane to veer to the left; the exact opposite to what it does with the plane in level attitude (the 'over' prop-wash will hit nothing, now going over the vertical stabilizer and the rudder...
This effect reversal used to cause beginners in WWII fighters to do the notorious 'ground-loop'... It was usually blamed on 'Torque'...
The would-be pilot would have a bit of trouble explaining what he encountered, from the world he now resides in...
In practice there is no "effect reversal" for full sized warbirds such as the Mustang and the Merlin engined Spitfires. They both need right rudder trim during the take-off sequence (some Spitfire pilots preferred not to use the rudder trim, relying on rudder input only). As the aircraft starts to roll there is a swing towards the left that must be anticipated and corrected for using careful application of right rudder. The tendency to swing towards the left remains during rotation and take off, decreasing quickly as speed increases.