AFFF,
Just to add to checkboard The VP operates from fine (thin bite angle) to coarse (thick bite angle)
With the engine at a given power setting the prop angle will decide the speed and the govenenor selects ( and maintains) this best speed.
When the engine starts ( in MOST cases) the propelleor is fully fine and as the engine speeds up the propellor goes towards coarse the stop the engine overspeeding.
In fact the maximum rpm adjustment is on the propellor not the engine.
During flight the propellor has a natural tendency to go to fine pitch but is held back by the govenor. When a prop overspeeds or runs away it is normally the govenor giving up the fight !
Now with the propellor in fully fine it its like a childs windmill which as you push it through the air it spins so the aircraft flying along the propellor is driven by the airflow. The faster the aircraft the faster the rpm.
On big 4 engine pistons we had a procedure for runaway props which will indicate how serious it is.
1. Reduce power and shut the engine down and cut off the fuel. ( Even in this conditon the forward speed of the aircraft is driving the propellor above the red-line.
2. Continue to try and feather the engine ( again the hi RPM will overpower the feathering pump.
3. Descend and reduce airpeed ( low and slow ). low speed and more dense air should reduce the RPM.
Of course there is a minimum speed that you can go to and in some cases even this is not slow enough and the drag from the propellor disc is very high.
We had a DC 3 overspeed on takeoff which the crew were quick enough to put back on the ground but not before the RPM not only went past the red line but off the clock !!
The engine case, normally air force green, was bright blue from the overheating!!
4.