For storage, especially on a carrier, 2 blades have some advantage as well. Else you need a folding blade - which has of course been done, but adds complexity and hence weight.
Rotor inertia is NOT about weight. It is about weight distribution. A solid disk that is 1 inch thick and 12 inches diameter will have a lower inertia about it's axis than a solid disk of the same weight, that is 1/2 inch thick. The diameter will be greater so the inertia will be higher. I do not know if a 2 blade rotor will end up with a higher inertia than a 4 blade rotor for the same lift - that depends too much on rotor aerodynamics for me to estimate. But I suspect it would.
There are a lot of factors to consider when choosing the amount of rotor blades. And as mentioned, easy storage is a great advantage for twobladed rotors. Could that be the reason for the Hueys being heavy and twobladed?
For larger helicopters the advantages of less wake turbulence are still there, but the disadvantage of a larger rotor becomes really large. The Mi-26 has a rotor diameter of 32 m and eight rotor blades, that's 128 m of rotor blade length. A rotor with 2 blades will probably be more efficient, but even with a 20% reduction in rotor blade length it would be about 100 metres in rotor diameter. A rotor that large will be hard to handle, but the big problem with large rotors is the speed of the blade tips, you don't want the tips to become supersonic.
Cost is a major factor, right? Two blade semi-rigid rotors likely are cheaper but still work very well. Of course they are susceptible to mast bumping, but then three blade systems are susceptible to ground resonance issues that, I think, two blade systems don't have.
Better late than never, thank to all of you for your answers!
Since then, I took the time to do a little comparison between two almost identical helicopters with the number of blades being the only difference: Bell 212 and Bell 412.
- both have the same engines (P&WC PT6T-3) producing the same power: 900x2=1800hp
- the rotor of the 212 is 2 blade with a diameter of 14,64 meters. The rotor of the 412 is 4 blade and only slightly smaller: 14,0 meters.
- the 212 is slightly (117kg) lighter than the 412: 2962kg vs 3079kg. I guess, that's the weight of the 2 extra blades.
- however the MTOW difference is 317kg (5080kg vs 5397kg) which means that the 412 can take 200kg more payload.
- both cruise speed and max speed of the 412 is 20 knots higher (120kn vs 100kn cruise and 140kn vs 120kn max).
- operational ceiling of the 412 is 2600 feet higher than the 212: 20 000 feet vs 17 400 feet.
- but the big difference is the range. The 412 has double the range: 529nm vs 237nm. This might be partly due to the 200kg extra payload, although I don't know how much bigger are the 412's fuel tanks. Some of it is probably due to the higher cruise speed. Is there also an advantage of the hourly fuel burn at cruise?
To conclude, it looks like the 4 blade rotor has some clear advantage from the efficiency point of view. The easiness of storage and the cost are probably the main drivers for operators who choose the 212 over the 412.