Originally Posted by R1no
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?
What this really comes down to is a relationship around a concept known as the solidity ratio.
The solidity of a propeller system is effectively a ratio of the frontal area of the propeller blades compared to the total propeller disc area. For example, if the propeller was simply a solid circle (and thus completely ‘blocked’ any airflow), it would have a solidity ratio of 1.
In general, as you increase the solidity ratio of a propeller, you can absorb more power from the engine (increasing the thrust you can produce). The trade-off is that you loose efficiency as the solidity ratio is increased beyond a certain point.
Anecdotally, as the tips of a propeller reach ~Mach 0.91, a large decrease in efficiency is observed. This can be remedied by increasing the width of the blades, but this can then have repercussions in other operating regimes. I believe the Open Rotor and UDF blades spin quite quickly (the tips therefore have a high mach number), but they’re also short and stubby (regaining efficiency).
All in all, propeller design is a tricky art where large compromises are made in the design – Typically you can optimise it for a specific condition (i.e. cruise flight) at the expense of other regimes.