You might have heard the saying, 2 to go, 3 for show. But for light aircraft, what is really better: 2 blades or 3?
It goes without saying that choosing the right number of blades primarily depends on a variety of factors; how you want to use the aircraft, the aircrafts engine power, design limitations such as required ground clearance, operating RPM, the list goes on.
Generally speaking, 2 bladed propellers are more efficient, resulting in a higher cruise speed than a 3 blade due to the reduced parasite drag. Every additional blade will also cause disturbances which interfere with the airflow on the other blades. If all the above factors are held constant, adding more blades simply reduces the efficiency of the propeller. But we all know efficiency isn’t what propels an aeroplane - thrust does.
With an increase in engine power, the propeller disc area should also increase, but this increase is limited by the resulting speed of the blade tips. The drag at this section of the blades increases when the flow speed becomes supersonic, without a correlating increase in thrust. To overcome this, either the blade chord needs to be increased or more blades need to be added.
In relation to noise, 3 blade propellers are usually quieter than their 2 blade counterparts. This is because a 2 blade prop produces 2 pressure pulses per revolution, while a 3 blade produces 3 smaller pulses per revolution for the same amount of thrust, meaning a 3 blade will be somewhat smoother and quieter. 3 blade propellers often have a smaller diameter than 2 blade props which also reduces the blade tip speed and noise.
One final point to consider is performance. While 2 blade propellers generally reward an operator with a higher cruise speed, they usually see a reduction in takeoff and climb performance. While 3 blade propellers can perform better in this area, they can also be slightly heavier and reduce cruise speed.
At the end of the day, like everything, each propeller has its pros and cons. It ultimately comes down to operator preference and the aircrafts mission.
So, what are your thoughts? 2 blades or 3?