Propellers

[Javster]

Why do propellers have different numbers of blades? Same with helicopters rotors, how come you can have some with 2 blades some with 4, and probably more too?

I assume they generate more lift, but does the extra weight mean more work for the engines?

[Red Iron Crown]

More blades means fewer revolutions for the same lift/thrust, which reduces tip speed. That's important because some bad things can happen if the tips go supersonic. More blades can help reduce noise as well.

[magnemoe]

More blades means fewer revolutions for the same lift/thrust, which reduces tip speed. That's important because some bad things can happen if the tips go supersonic. More blades can help reduce noise as well.

Yes, you also want to keep the rotor diameter on a helicopter down to make it fit in smaller hangars and can land in narrower space.

Downside is that its heavier and probably less efficient.

[K^2]

Yeah, the obvious up side is that you get more lift. The downside is that you hit diminishing returns, because as you have more blades, each blade is working against already moving air. You can end up with more problems with tip vortices as well. That can lead to settling with power, and other nasty business. Both of these problems are reduced if you place a cowling/duct around the propeller. Hence the ducted fans.

[cpast]

Some engines are too powerful even for a single set of blades, and have two or more sets of blades on the same axis.

[PakledHostage]

Some engines are too powerful even for a single set of blades, and have two or more sets of blades on the same axis.

When you see multiple stages of propellers configured like that, it is usually because the two stages rotate in opposite directions to each other. This is done in an attempt to increase overall efficiency of a propeller assembly that is constrained from operating at peak blade efficiency by other design considerations. You see propellers like this on some boats too. As has already been mentioned above, a propeller blade works like a wing to create lift perpendicular to the propeller disk. Like with wings, they also create downwash and tip vortices in the process of creating lift. Because propellers rotate, the downwash has a component that swirls about the axis of rotation and the tip vortices form a helical pattern behind the propeller. The more blades you have, the more disturbed the airflow is and the less efficient the disk is overall. Higher blade counts are required at the expense of individual blade efficiency for some of the reasons already mentioned, however.

The counter-rotating second propellers shown in the image in cpast's link recover some of the energy in the swirling downwash (and upwash) of the propeller ahead/behind it. This same principle is used in axial compressors in jet engines. In jet engines, each stage of the axial compressor is followed by a stage of stationary stator blades that are oriented to harness maximum energy from the swirling flow within the engine.

For low speed propellers, higher efficiency is obtained by using fewer blades because the blades are less affected by the disturbed airflow created by the blades around them. This can be seen in in AeroVelo's human powered helicopter design. Each of the four propellers on their human powered quad copter has only two blades. Presumably they chose the quad copter configuration because it simplifies flight control problems, but efficiency was critical to their success so they used a two blade propeller at each rotor.

[Camacha]

Yeah, the obvious up side is that you get more lift. The downside is that you hit diminishing returns, because as you have more blades, each blade is working against already moving air.

That is one of the main arguments against adding too many blades - every blade will be operating in a very turbulent environment left by its predecessor. For pretty much the same reason wings and tails are often placed in different planes, so that the turbulence of one does not disturb the other.