EASA / GCAA Part-66 · Module 17 · Propeller · 17.1
Objective: five separate forces act on a turning blade, and they do not act in the same direction or at the same size. Two of them try to twist the blade in opposite senses, and which one wins decides how the whole pitch-change system has to be designed.
On the numbers: centrifugal force, thrust bending and torque bending are calculated directly from the settings above and are real. The two twisting moments are shown at magnitudes representative of a light-aircraft blade, because their exact values depend on the internal mass and pressure distribution of a specific blade. What is examinable, and what this shows correctly, is the direction each one acts and the fact that CTM is the larger of the two.
Centrifugal force dwarfs everything else. With CF = mω²r, and ω squared, a blade of only a few kilograms pulls on its hub with a force measured in tonnes. This is the force the hub, blade retention and bearings are designed around, it is why a blade root is such a substantial piece of metal, and it is why any crack found in a blade root or hub is a ground-the-aircraft finding rather than something to monitor.
Two moments fight over the blade angle. The centrifugal twisting moment (CTM) arises because every part of the blade wants to swing into the plane of rotation, and since the blade is set at an angle to that plane, the effect is to twist it towards fine pitch. The aerodynamic twisting moment (ATM) arises because the centre of pressure sits ahead of the pitch-change axis, so the aerodynamic load twists the blade towards coarse. In practice CTM is the stronger, so a blade left to itself runs to fine pitch. That single fact drives the design of the whole pitch-change system: it is why many propellers use counterweights to oppose it, why feathering springs and accumulators exist, and why a loss of governor oil pressure on a single-acting propeller usually results in the blades running towards fine.
The last two are bending loads. Thrust bending tries to bend the blade tips forward, and torque bending tries to bend them back against the direction of rotation. Both are resisted largely because centrifugal force keeps the blade in tension and stiffens it, which is one reason a propeller is far more robust turning than it is standing still, and why blades are handled carefully on the ground.