skyboltone
Well-Known Member
- Joined
- Jul 14, 2011
- Messages
- 459
- Reaction score
- 82
I've had some training on propellers; i.e. measuring pitch angle at several points along it's length and calculating what portion of the prop is doing how much of the work at any given speed etc. If one does that, on say a McCauley on the nose of a C150 one finds that those folks at Mac and Cessna really know what they're doing. On the other hand I got ahold of the good people at Catto about a new propeller for my SA-100 and told them 125hp more or less etc. Craig came back with an off the cuff recommendation of a 78X37. Mr. Baxter says maybe something like a 74X55 or so. I know that Catto knows what he's doing and I know that Dave knows what he's doing so therein lies the dilemma. Using common formulas and assuming that the pitch numbers in both cases are at 75% of the way out from the hub I come out with something like 90mph running wide-ass open with the Catto. That just can't be right. I love the idea of the robust climb generally afforded by a longer prop but what I don't grok is the relationship between length and pitch. For instance a normal prop on an old 172 is what? 76 by 60" ballpark? The Borer seaplane prop is 80 by 40. So what's the deal? There are some very smart folks here. Can you help me wrap my head around this?
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