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.