Two or three blade propellers and performance.

[frangarcia]

I think I read something about it in the manual. But now I can’t find it again. Can someone tell me what the difference is? I remember something like this “a three blade propeller has more drag…”

[AKar]

Well, in general:

• • two-bladed props have higher aspect ratio and larger diameter - thereby they are more inherently more efficient;
  
  • three-bladed props tend to have more blade area, and therefore greater thrust for given blade angle and rpm - note you can't directly affect the first one but it shows up maybe in better low speed thrust;
  
  • three-bladed props - or, any multi-bladed propeller! - suffers from blades interfering with each other, which reduces the efficiency a bit;
  
  • three-bladed props are of smaller diameter, have slower tip velocity and thereby are quieter - much quieter in some cases. They also have better tip clearances.

So, to summarize, two-bladed props are generally a bit more efficient and three-bladed ones are quieter and may have a bit better slow speed performance. It is primarily the noise aspect why you see some relatively high-powered airplanes, such as Cessna A185s, routinely retrofitted with three-blade props.

Of note is that the other aspects of the design matter too, albeit rather little, but still so that a new and modern three-blade prop can be equal or better thorough the operating range than an old two-blade one.

-Esa