New prop: 2 vs 3 blades?

[gkey]

I know that within the next 2 years or so, my shop is going to recommend new prop blades. My plane has 2-blade props on the front and rear, and it is the rear one that needs attention soon.

I have a few questions, that I can't get answers for.
What are the pro's vs con's of 3-blade vs 2-blade props?
I know it's probably cheaper just to replace the props, but what about the hubs?
I think the 3-blade props are quieter due to less RPM, so does this mean I can get better perofrmance (speed, power) out of the engine with them?

[Roger]

I don't know this for sure, but I would be astounded if a 3 bladed prop would turn less RPM's, for the same HP output. There is no free lunch. I don't believe that's how it works. I would imagine if there is a sound or vibration decrease, it would be because more than likely the 3 blades would be shorter, and as such have less tip speed. But they do look pretty cool.

[tropical]

Performance difference between 2 or 3 blades is negligible. 3 Blades may be slightly quieter because of smaller diameter thus lower tip speed.

[WebMaster]

The prop diameter, for the MT props, is the same.
Les Brindle, who developed the STC, claimed that the cost for replacement is less, and that they were smoother, and the warranty was better. In addition, the two MT 3 bladed props weigh 60 pounds less than the steel props they replace.

NOTE: However, that you must replace both ends at the same time. The STC does not permit one two (2) bladed prop and one three (3) bladed prop.

Herb has reported in the past that the 3 bladed MT's are easy to repair, are smoother, and provide better climb performance. I have ridden in his plane, and the MT 3-bladed props are smoother, and it does climb very well. There is no change in engine RPM's due to the MT props.

[Roger]

It does make sense that you could run 3 blades and not change the RPM by virtue of the controllable pitch. On my boat, 5 bladed props of the same diameter are better for getting up on plane, and run a pit smoother than 4 blades, but you actually lose about 2% on the top end. You get the same forward motion (31 inches) x-slip, per rotation, yet there is some added "interference" casued by the extra blades in cruise. There is less fixed pitch on the 5 blades. Now if you could just get an Archemedic screw to run in the air or water on the end of a shaft, one could get almost a turbine's smoothness

[WebMaster]

You are right, in that there is generally some reduction in speed caused by having 3 blades instead of 2. When Mooney wanted a plane to go faster, they went from 3 blades to 2, for the same model. I think I remember Herb saying he lost 2 mph at cruise.

However, as in your boat, getting up and going with more blades happens faster. It's the common reason that some planes go from 3 blades to 4. Better acceleration and climb.