# How to calculate the number of blades?

Inspired by building a new drone, I got lost in curiosity. What if I have a fixed length of rotor blades and a target amount of lift I want to, well, lift. How do you calculate the number of blades?

Searching around I did find the relation between the number of blades and decreasing efficiency/lift per blade when you add a blade. Also the fact that is would be easier to lengthen the blades is something I found. But if lengthening is not an option, how can you actually calculate this? And, to complete the question disturbing me, what effect does a second disk (like the chinook) have on these calculations?

Determining the number of blades is one of the steps in designing the main rotor of a helicopter, but not the first one. The main rotor design process is an iterative process with quite a few input parameters, which depend on the mission type:

• Max. TO weight
• Max. speed
• Installed engine power
• Autorotation performance
• Noise
• Manoeuvring g

Using these inputs, the design process determines:

2. Tip speed. As close as possible to the drag divergence speed in cruise, also a limiting factor in selecting rotor diameter.
3. Rotor solidity. How much disk area is actually occupied by blade area, determined by blade stall limits in manoeuvres. Increasing the rotor solidity requires more engine power, but increases the blade stall margin.
4. Number of blades. Mostly based on dynamic criteria, less on aerodynamic ones. The Bell 212 shows that rotor solidity can be increased by using wider blades, but using more blades minimises vibratory loads and the noise per blade passing.

The rotor design process is quite broad, and can best be found in the theory books which also contain statistical data to aid the initial stages. The plot below is from Leishman and shows that light helicopters have a blade length of about 4 meters, apparently a good length for this weight and mission type.

The overlapping twin rotor disks of the Chinook result in one rotor operating in the wake of the other, reducing efficiency and requiring more disk area. So total disk loading will reduce.

Two good books on helicopter stuff: Helicopter Aerodynamics, Stability and Control by Ray Prouty, and Principles of Helicopter Aerodynamics by J. Gordon Leishman.

• Thanks so far. What I try to understand is, what does an extra rotorblade actually add in lift, percentage wise? And what should be deducted for the overlapping disk area at e.g. the ch47? It surprised me that the diskloading/m2/blade for many helicopters is about 8.7, but for the ch47 it is 15,3. Do you have some gestures to help me understand? BTW: I start thinking from scratch, so the other parameters can be dependent variables. – Arie Jan 30 '20 at 10:32