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Rob, I'd be careful about drawing conclusions from the Hartzell plots. I've spoken with Les Doud too and got the impression that the three blade Hartzell is more efficient not because it's three blade, but because the design is newer and thus benefits from improved analysis, manufacturing, etc. Performance also depends on the power and speed of the airplane. Constant speed props are better over a range of speeds and powers than fixed pitch, but they're still optimized around a design point. I may be all wet, but thought the three blade scimitar was designed for the Legacy (speed and power) while the two blade is an older design not specifically optimized for the Legacy/IO-550. Where MT's various offerings fit within this range is a whole separate set of variables. Les, please correct me if I'm wrong on any of this. - Kyrilian
Sent from my iPad I am interested in this subject, because I purchased a partly built kit some years ago, (which I am still building!) which came with an MT 3 bladed constant speed prop. In my conversation with a builder in South Australia recently, he mentioned that he had swapped out his 3 Blade MT prop for a 2 blade prop, and increase his cruise speed by 7 knots. (Can’t recall if that was 7 Kts indicated, or 7 Kts TAS.) Not sure how this fits with the graphs which indicate the 3 blades are more efficient! It was my understanding that because a 3 bladed prop generates 3 tip vortices against only two on a 2 bladed prop, it is less efficient. Rob Stevens Perth, Western Australia. From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Charles Brown
Sent: Monday, 30 June 2014 7:38 PM
To: Lancair Mailing List
Subject: [LML] Re: Two-blade or three-blade prop for I-550 I've enclosed the spreadsheet provided to me by Les Doud of Hartzell. In 2009 his phone number was Phone: 937-778-4262 . He believes that the 3-blade is more effiicient than the two blade even in cruise. I have a hard time believing that, and my airplane with 3 blades has not lived up to the prototype Legacy's performance with the 2-blade as documented in the CAFE report; and the 3-blade is heavier and more expensive to buy and overhaul. Of course, my airplane is probably not as clean as the prototype. But the 3-blade prop sure looks cool.
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