Its hard to notice when they're spinning but have you ever looked at a helicopter's main rotor and wondered why there are different numbers of blades?
Depending on the helicopter the number of main rotor blades will vary between 2 - 8.
R44 with two blades
H125 with three blades
A109 with four blades
AW139 with five blades
SA321 with six blades
CH-53E with seven blades
Mi-26 with eight blades
Did you notice a correlation between the number of blades and the size of the helicopter? Thats not a coincidence. Generally speaking a smaller helicopter will have fewer blades and a larger helicopter will have more blades. I say 'generally speaking' because there are a few exceptions.
The MD500 is a small helicopter with a four, and in later variants, five blade main rotor.
While the Bell 214ST is a large helicopter with a two blade main rotor (the largest with two blades, in fact).
The reason for the general trend has to do with cost, complexity, and aerodynamics.
Cost and complexity is pretty straightforward. Fewer blades have a more simple rotor head with less material. More blades requires increasingly more complex rotor heads with more material and complexity. Production and maintenance becomes more intensive with more complex rotors which will add to the cost.
Then theres aerodynamics which is a bit more tricky. A two blade rotor is the most efficient design from this perspective. This is because as a rotor blade moves through the air it will leave behind a turbulent wake. The following rotor passes through that wake which reduces its efficiency. Two blades allows the most distance between the rotors for the wake to dissipate and each additional blade incrementally reduces this distance.
And with fewer physical blades there is less drag experienced by each blade as they generate lift.
So why don't all helicopters just use two blades if it has the least cost and most aerodynamic efficiency? Mostly because of structural and material limits.
As helicopters get larger and heavier they require more lift to fly. A main rotor with two blades for a very large helicopter would be physically impractical due to the length of the blades required. With such a large rotor diameter the size of the helicopter itself would need to be extended to provide clearance for the tail rotor, which adds more weight, which requires more lift, and so on. This creates a detrimental cycle that needs to be managed.
Another reason is that materials strength only goes so far. Blades need to be both strong and lightweight and an excessively long blade isn't practical for either. Adding strenght often comes with more weight, which needs more strength, and another detrimental cycle appears.
So the best solution is to add more blades. This is why larger helicopters will have more blades on their rotors compared to a smaller, lighter one. There is a penalty to aerodynamic efficiency, cost, complexity, weight, and drag, but each additional blade contributes to lift and keeping the rotor diameter down to a reasonable size. Although adding too many blades will have diminishing returns so its up to the engineers to come up with a design that balances all of these factors.
There are a few other considerations such as the type of rotor head system used but I'll save that for a future post.
