I am quite amazed no-one has hit upon the obvious, its all to do with starting. Being both a pilot and totally arrogant of the unfortunate minority who are left handed. On a car with a starting handle, I would cup the handle in my right hand and with a mighty swing. Spin the handle from a low point 'upwards' and physically towards my center of gravity causing the engine to turn in a clockwise direction as viewed from the front.
Starting handles; what are they? Great lumps of bent iron which could be fed through a hole in the bumper and engage with a dog on the end of the crankshaft.(quite sure these keys and starter buttons will never catch on) And here is the reason; a petulant engine on a damp cold day will do anything to stay in the garage, include oiling its plugs and electrocute anyone's hand that is fiddling with the HT leads, and after half a dozen attempts at starting, the battery will go flat, hence out will come the handle.
In a politically correct world the majority would bow to the misfortunes of the minority but after swinging an engine for half an hour 'sod the left handed minority'.
So there you have it, its a throwback to the golden age of motoring and right handed people rule but what about aeroplanes.
There is no simple way of saying this but; stand in front of a PA28 with a 140 horse power engine with a metal prop and a flat battery, (probably the finest mincing machine known to mankind).
Facing the aircraft with mags and switches off, the propeller would be turned by hand to a point where the compression on the engine is just building then stand back. A command to make the engine live is given.
If the rotation of the engine is counterclockwise as viewed from the front of the aeroplane, a stance is made forward and clear of the arc of the propeller. Just the fingertips of the right hand grasp the trailing edge of the blade and in a single pull; spin the propeller over top dead center and instantly clear the arc of the propeller. This is a highly dangerous practice but can safely be accomplished with care.
If the rotation of the propeller were to be to the right or clockwise as viewed from the front, I would then grasp the propeller with the finger tips on my left hand. (as I am right handed) I would hold hands with another person standing behind me to prevent any possibility in over balancing.
----------There is absolutely no reason why an engine or propeller should have a preference to turn in either direction apart from commercial considerations, custom and practice and the desire for safety in hand starting the contraptions-------For the record I ran an airflow test laboratory for many years and I can assure anyone that the absorbed power of a fan, provided it is of a mirror airfoil section will be identical.