Just going to quickly explain Centrifugal Twisting Moments (CTM - also known as centrifugal twisting force in some areas) and Aerodynamic Twisting Moments (ATM - also known as Aerodynamic Twisting Force) on a propellor any input/corrections (regarding the theory/terminology) would be appreciated. Oh obviously this is dealing with VP props. I'm trying to explain it without drawing a diagram (which would make it way too easy to explain).
As a prop rotates centrifugal force acts outwards through the center of mass trying to a) pull the prop out of the hub, b) turn the prop to a finer pitch about the pitch axis. To explain point b) further. Visualise a prop blade and divide it into half. Each section now has it's own center of mass. Once again centrifugal force is acting radially outwards from the hub center through the center of mass of both sections. Basic trigonometry tells us that a force canbe divided into component parts (triangle/rectangle of forces) thus centrifugal force can be broken down into one force that acts parallel to the plane of rotation of the prop and one parallel to a line drawn through the center of the prop. Now visualise a cross section of the prop, the two centres of mass each have a force acting out towards the leading/trailing edge of the prop and parallel to the plane of rotation. This force tries to pull the prop towards a finer pitch. This force is called Centrifugal Twisting Moment. Factors that affect the value of CTMS are 1) Blade mass, 2) Speed of rotation of the prop assembly, 3) Blade chord length (longer moment arm), 4) Blade aspect ratio (longer blades have longer moment arms).
A wing moving through air creates lift due to a pressure difference between the upper and lower surfaces. Air flowing over the top of the wing has a greater distance to travel than air beneath the wing and has to move faster in order to "catch up" to the air beneath. This creates an area of low pressure over the wing (think I should mention Bernoulli's here) which creates lift. The amount of lift and the direction that it acts (in regard to a point on the chord of the wing in the region of 1/3 of the chord's length from the leading edge) is a function of the centre of pressure (the center of this lower pressure area). Likewise as a prop blade moves through the air, an area of low pressure is created just in front of the prop. The forces generated by a prop as it rotates are a force for torque that is parallel and opposite to the direction of rotation and a force for thrust which acts in the direction that the aircraft is moving (which in most instances is forward). The resultant force (once again rectangle of forces comes into play here) and acts through the centre of pressure. This resultant force is termed the aerodynamic twisting moment and tends to turn the prop towards a coarser pitch.