1. I understand that props have problems with wave drag at the tips as the tips approach the speed of sound. Why is it that the fan inside a turbofan does not experience the same problem? Is it because it has so many blades?
R1no,
I think you are on the right track with the many blades, ie the prop blade tips are out there on their own, whereas the fan blades can help each other out with their shared passages between blades.
Fan blades (as well as the transonic compressor stages which you cannot see inside the engine) operate with blade relative tip speeds well beyond what propellers see so the same basic supersonic flow phenomena are there.
The wave drag equivalent term in transonic compressors is shock losses.
The reasons why fans can operate with rotor tip inlet relative mach numbers up around 1.5 these days is due in part to thin blade sections but also being able to control the supersonic diffusion within the blade passages in ways that minimise the losses, eg with multiple shock systems.
2. Carrying from the above question, I've also read that adding more blades to a prop makes it harder to balance, such that we don't usually see more than 8 blades on a single prop. Why is it that again, fans are granted some kind of exception from the rule?
I don't know anything about balancing props.
There is no max blade number rule as regards balancing a fan. The fan is what it is based on many difficult design decisions and being able to balance it is a foregone conclusion, as long as you haven't forgotten to design in a rim to put the balance weights on.
Having said that, balancing it is not necessarily easy because it isn't balanced until it meets a required specification which could be very demanding. The effects of small amounts of rotor unbalance can easily find their way into passenger cabins (noise) especially from fuselage mounted engines.
I'll try and compose something for Q3.
PK