If you've ever had one of those balsa wood airplanes you wind up with a rubber band, you're familiar with the concept of the propeller. It's a twisted thing that corkscrews through the air to pull the airplane forward. The ones on the toys, and most training airplanes, have fixed pitch propellers, meaning that the twist is permanent, fixing the blades at one blade angle, such that if you advanced it by screwing it into a wall of jello it would always go the same distance forward (its pitch).
This is the one-speed bicycle of propellers. It has the advantage of being relatively cheap, lightweight and easy to operate. You have to push extra hard going up hills, and your feet have to go round and round really fast to power down hills, but you don't need to mess with shifters or worry about the chain falling off because you got it in the wrong gear. But once you've mastered the art of staying upright and can manage to take one hand off the handlebars, it's nice to have the ten-speed.
When I talked about the critical engine I mentioned that the amount of thrust increases with the angle of attack of the propeller to the relative airflow. It should also be understandable that the greater the angle of the propeller, the more torque that is required to push the propeller around. Small blade angles are called fine pitch and large blade angles are coarse pitch. Fine pitch is like a low, hill-climbing gear on a bicycle. You don't have to push as hard to turn the pedals, but you have to turn the pedals around more times to get the same distance. Substitute "propeller" for "pedals" and you have a propeller set to fine pitch. (The Americans call this low pitch, but we'll ignore them for now, so they don't confuse us). There are more influences, such as the fact that the propeller slips in the air, making its effective pitch less than its geometric pitch and that more energy is lost to drag at high rpm and high airspeed. Shake it all down together and we find that at take-off it's best to have a fine pitch propeller turning really fast and in cruise it's better to have a coarse pitch propeller turning at a more moderate rate. Due to the inconvenience of switching propellers midflight, we use constant speed propellers.
At the front of my propeller hub is a piston, attached through mechanical links to the propeller blades. As the piston moves, driven by oil pressure, the mechanical links twist the propeller blades, the way you might turn an oar in an oarlock to change the angle the oar blade met the water. If oil pressure is high, the piston moves full forward, twisting the blades past full fine (flat) to the mechanical stop at -15 degrees (yes, driving the airplane backwards). If oil pressure is lost, accidentally, or by feathering, springs force the piston back to the full aft position, twisting the blades to +87 degrees, the previously described feathered position. By the way, the angle is measured 30 inches out from the hub, about three-quarters of the way to the tips. Propeller diameter is eight feet, six inches.
The oil is the same oil circulating throughout the engine, but it's stepped up in pressure from 85 psi to 385 psi by a pump built into the base of the propeller governor. The oil is constantly circulating through the propeller hub, because it is allowed to leak out and be replenished. Here is the path the oil takes: through the pump, past the relief valve, through the beta back up solonoid valve, through the beta reverse valve, through the constant speed governor, past the overspeed governor, into the propeller hub, out through the transfer sleeve to the reduction gearbox, back to the rear of the engine for filtering and cooling.
I will start with the governor, because under normal circumstances in cruise flight, you can ignore all the beta and overspeed stuff and pretend it's just the governor and the hub. The pilots set the amount of torque the engines produce, based on the best compromise between fuel economy, how fast they want to get there, and not blowing up the engine. Then they set the desired propeller rpm based on altitude and company practices and personal whim. Notice that: they don't select the blade angle, they select the rpm. The governor sets blade angle in order to achieve the rpm the pilots asked for. As the torque is constant, the governor can decrease rpm by increasing the blade angle or increase the rpm by decreasing the blade angle. If the propeller is onspeed, the oil circulates just as described above, through a partly open pilot valve in the governor. The amount of oil allowed through the governor equals the amount allowed to escape from the hub, so the pressure and thus blade angle remains constant.
If the propeller slows down, the governor corrects it. The governor is driven off the propeller reduction gear, so it slows down, too, and as it does, flyweights on it swing inward. Those flyweights are levered such that when they move in, they open the pilot valve further, so the slower the propeller, the more oil pressure is allowed through the pilot valve, to drive the propeller to a finer pitch, so it speeds up. Likewise, if the propeller is overspeed, going too fast, the flyweights swing outward, closing the pilot valve and cutting off oil pressure to the hub so that the propeller moves to a coarser pitch, slowing it down. When the pilots set the selected rpm, they are adjusting a speeder spring on the governor that determines how fast the flyweights have to be going to close the pilot valve. There's a nice diagram of a propeller governor, and some excellent tips on propeller care in this AOPA brochure.
If you've managed to follow this, then you can tell me what the constant speed governor and the propeller hub piston would try to do to the blade angle if the torque selected were insufficient to turn the propeller at the selected rpm, even with the blades set to full fine.