[ QUOTE ]
Here's another one - why is the RPM limited from 2100 - 2350? I was told it is because of vibration problems between the engine and airframe - the POH only limits it for 'extended operation' though.
[/ QUOTE ]
Most everything has a frequency that it naturally wants to resonate or vibrate at. Usually when you design a piece of machinery you make sure that this does not occur in normal operation. If this happens it can range from annoying (having some piece of interior trim vibrate like a tuning fork) to very, very bad (having a critical part break due to fatigue).
The parts of an engine are also subject to this. When you look at a crankshaft it looks massive and you think that it must be very strong. In reality it is constantly being twisted in torsion as each cylinder fires and that crank throw is pushed slightly ahead of where it should be while others are pushed slightly behind during the compression stroke. To counter this it is common to attach movable counter weights to the crankshaft. The swinging weight helps damp vibration and their mass helps smooth out the power pulses in the engine. The weights are roughly half moon shaped and hang on the crankshaft by two pins that pass through holes in the crankcheeks. There is a certain amount of play between the mounting pins and the holes they ride in, as it is necessary for the weights to move back and forth to function, but under certain combinations of engine load and rpm, these weights can start to 'chatter' or vibrate, causing damage. This can sometimes happen if rapid throttle movements are made. If you remember back to the private and commercial written, there are some questions that deal with 'detuning' an engine and this is what they are referring to. It can also happen during operations outside the normal range of RPM.
It is common now to develop a new propeller to best match the performance characteristics of an engine/airframe combination when certifying new aircraft. In the past what was usually done was to just find something that was servicable and call it 'good enough'. It was cheaper to put a limitation in the POH than do some more Research and Development. Now it is possible to replace the prop on the Arrow and remove the limitation.
As far as 'extended ops', the goal here is not to spend an extended time in the prohibited range. It is not possible to avoid the prohibited range all together, as you will go throught this range during initial throttle application for takeoff or when pulling the throttle to idle during landing. Just don't stay there.