Why are three-bladed helicopters relatively rare?

Question

I admit that this observation is entirely subjective, but it seems to me that there three-bladed rotors are significantly less popular than two or four.

Factors that I'm guessing contribute -

• Storage - two-bladed helicopters take up a lot less space. So you're probably going to stick with two blades until you're compelled to move to four
• Simplicity - while an odd number of blades can be perfectly balanced, it may be easier to manufacture systems where the forces counter-balance in opposition rather than having to balance the entire system.
• Cost - two slightly longer blades are presumably a lot less expensive to maintain / purchase than three shorter ones.

These are guesses though. My question is - is my perception correct, and if so, why?

Answer 1

Three bladed helicopters aren't rare. Many European small helicopters are three bladed:

• Eurocopter Fennec

• Eurocopter AS355

• Eurocopter Écureuil

• PZL SW-4

• Kamov Ka-226- it's coaxial, but still... Kamov has mostly used three blades for its coaxial helicopters.

• Most of the early small helicopters by the Mil Design Bureau also had three blades, rather than two or four.

The Eurocopter helicopters show a pattern. They are developed from one another and trace their lineage from the Aérospatiale Alouette II, III and Aérospatiale Gazelle- all with three blades.

Most American smaller helicopters have teetering rotors, which default to two blades. This has been the case from Bell 47 to Bell UH-1 Iroquois. Here again, the UH-1's transmission system was a development of the Bell 47. Again, if you're in US (or for training generally) , you'll be seeing Robinsons, so it appears that there are almost no three bladed helicopters (with Schweizer 300 being an exception).

There are advantages and disadvantages in having different number of blades. In general, the fewer, the better. You pay for more lift, performance and (usually) less noise with mechanical complexity and less efficiency. It's a trade off.

Three versus four blades shows a clear geographical difference, due to the availability of engineers and their differing experience.

Answer 2

Like with propellers, the blade count on a helicopter rotor is a matter of disk loading. Light helicopters get away with fewer blades. Complexity and losses increase with blade count, so helicopter designers try to fit as few blades as possible. However, this would result in huge rotor diameters for heavy helicopters. There are reasons to keep the diameter small:

• Blade root loads due to centrifugal forces
• Tip speed: The advancing blade will experience transsonic effects earlier if the helicopter flies in cruise the larger the rotor is. This will limit top flight speed.
• Blade bending on the ground. The blade needs to be stiff enough to clear the tail when the rotor spins up.

Now let's compare a few helicopters. I used the technical data on the linked Wikipedia page and divided the maximum take-off mass by the area of the rotor disk:

1. Sud Aviation Aloutette II: 2-bladed rotor. Disk loading 19.6 kg/m². Disk loading per blade: 9.8 kg/m².
2. Bell 205 / UH-1D. 2-bladed rotor. Disk loading 25.6 kg/m². Disk loading per blade: 12.8 kg/m².
3. Aérospatiale AS-350 Écureuil: 3-bladed rotor. Disk loading 25.07 kg/m². Disk loading per blade: 8.36 kg/m².
4. Bell 412: 4-bladed rotor. Disk loading 34.6 kg/m². Disk loading per blade: 8.65 kg/m².
5. Sikorsky S-65 / CH-53: 6-bladed rotor. Disk loading 50.2 kg/m². Disk loading per blade: 8.37 kg/m².
6. Mil Mi-26: 8-bladed rotor. Disk loading 69.6 kg/m². Disk loading per blade: 8.7 kg/m².

I guess you see the trend by now. The UH-1D is the odd man out, but it also stands apart due to its unusually high blade chord. Normally, the ideal loading per blade seems to be between 8 and 9 kg/m², and a little more is deemed acceptable if it allows the designer to get away with only 2 blades.

There is no conspiracy to suppress three-bladed rotors. It's the overall compromise between performance and efficiency which determines the number of blades on a helicopter rotor, and the main driver is mass.

Answer 3

While the above answers have some correct points, they miss the fundamental points of the structure and function of the rotor hub differences in the two and four blade rotors versus the three blade rotors. Both have safety issues that are nuanced, and the difference can change the mission for a given helicopter.

Two and four blade rotors are most likely semi-rigid rotor hubs that are cantilevered off a fixed pin. That allows them move free up and down through the rotation that occurs from lead and lag forces. The limitation for these rotor hubs is that it has almost no tolerance to negative G-forces because negative Gs lead to mast bumping and airframe damage in air. Pushovers have to be very subtle. This requires special training specific to SFAR 73.

Three bladed rotors have a fully articulated rotor head, which prevents mast bumping, so it has greater agility and can handle external cargo. These rotors handle lead-lag forces through hinges at the root of the rotor blade allowing the blade to move forward and aft relative to the direction of rotation throughout the revolution. The primary safety concern in these helicopters is touching down too hard or off balance jarring blades out of balance. If they do, gyroscopic forces react through the airframe, and if the helicopter is still touching the ground, that part touching the ground becomes the pivot point around which the airframe gyroscopic forces act flipping the helicopter over that pivot point. This is more likely on the three-bladed helicopters with skids instead of landing gear. The skids have shocks essentially to prevent the jarring, but if they're bottomed out or flat, the forces go through them unevenly. Helicopters with wheeled landing gear have less issue with that. Here is a good resource about fully articulated rotor heads.

Above 4 blades is when helicopters are looking to reduce noise, balance performance (both lifting capability and agility) and stability; and keeping the frequencies out of the pilot PIO and human factors frequencies become bigger issues.

Answer 4

In addition to the reasons stated in the other answers, there is still a bit more on the subject. Often, the number of blades incorporated into a rotor design is about blade/disk loading, load carrying, speed-performance. Many times the number of blades is driven by vibration / ride quality and noise.

In the case of 3 blade systems, rotor vibration and noise have multiples of 3/rev, 6/rev, 9/rev. 2 AND 4 bladed systems have multiples of 2/rev, 4,rev, 8/rev, 12/rev. Odd numbered systems rarely exhibit submultiples great enough to be of concern, while even numbered systems can cover the spectrum. Think 5 vs 6 blades. The 5 bladed MD500s are almost on par with fixed wing ride quality when properly tuned.

On the other hand, take the Bell 214/214ST. A big 2 bladed example. Even with nodal beam suspension to mitigate 2/rev, these beasts ride very harsh and can only be tuned for hover and one nominal cruise airspeed.

Regarding the subject of noise. In the late 80s Los Angeles County Fire Dept, against the better judgement of the pilots, were mandated to change from the 2 bladed, Bell 205A-1 merely for the reason of noise. They subsequently bought 4 bladed, Bell 412s who's performance numbers, on paper, were stated to be equal if not marginally better. The pilots always preferred the performance characteristics of the 2 blades systems. Been on many max gross takeoffs with LACO Fire from Barton heliport, in 2 and 4 blade ships. The pre-takeoff briefing in the 4 blade 412 was not without reference to the possibility of having to pickle the load during translational lift.