New prop: 2 vs 3 blades?

gkey · #1 08-08-09, 11:06 PM

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 · #2 08-10-09, 12:44 PM

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 · #3 08-10-09, 12:51 PM

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

WebMaster · #4 08-10-09, 02:49 PM

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 · #5 08-10-09, 03:05 PM

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 · #6 08-10-09, 03:16 PM

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.

gkey · #7 08-10-09, 10:42 PM

Thanks for the input. However, when one reads through Hartzell's web pages, one would swear only everything good is to come from installing 3-bladed props, from torque, power, speed, climb, vibration, noise, the whole caboodle. It should improve EVERY aspect of flight, which I take with a little salt.

It does seem though, that 3-blade props are more "powerful" on the climb, which makes me wonder if it is better to have in case of an engine-out (especially rear engine out).

I guess, my question still stands, if YOU had to choose between 2 and 3 bladed props, which one would you choose, and why?

tropical · #8 08-11-09, 10:11 AM

Quote:

Originally Posted by gkey

Thanks for the input. However, when one reads through Hartzell's web pages, one would swear only everything good is to come from installing 3-bladed props, from torque, power, speed, climb, vibration, noise, the whole caboodle. It should improve EVERY aspect of flight, which I take with a little salt.

It does seem though, that 3-blade props are more "powerful" on the climb, which makes me wonder if it is better to have in case of an engine-out (especially rear engine out).

I guess, my question still stands, if YOU had to choose between 2 and 3 bladed props, which one would you choose, and why?

2 blade. Cheaper, reliable.

Roger · #9 08-11-09, 02:06 PM

Three blades are not "more powerful" the power comes from the engine, not the appendage stuck on the front. Just as a side issue I had a Navajo panther with 4 blade Q tip props, with 350's. My friend had a standard Navajo with 310's and 3 blades. He liked mine better in cruise and on landing, but he was adamant that mine took longer to get off the ground. So I don't necessarily believe that the extra blades are an absolute performance booster.

hharney · #10 08-11-09, 10:46 PM

There are several threads on this website that explain the benefits of 3 blade MT props. The MT's are the only STC'd 3 blade for the IO-360. If you search MT Props you will be amazed at the information.

As for me, I have 6 years on my MT's and I wouldn't trade them back for McCauly's for nothing. Also, when I purchased my props from MT they were the same price as McCauly. Go with the MT's, move forward not backward.