Switching from a 2 blade prop to a 3 blade prop

[Mike Griffin]

I did not want to disrupt Ken's thread he started asking about a triblade prop but I do have a question.  Years ago, I built a Rayette with a LA.46 engine on it and had conferred with Allan Brickhaus about what size 3 blade prop to put on it but I cannot remember the size these many years later.

Is there a conversion or equivalent chart telling you what size triblade prop to use when you switch from a 2 blade prop?  For example lets say I am using a 11 x 5 two blade prop and want to switch it out for a tri-blade, would I use the same size in a tri-blade or go up or down a size and pitch?

Thank you
Mike

[Dave_Trible]

Mike there is an old rule of thumb that says when you add a blade you should drop 1” in diameter.  Modern engines produce more power so that may not be as nessessary.  Props vary in many other ways too so I might start with the same diameter and if the engines seems loaded then whittle off 1/4” segments until the engine doesn’t sound or act stressed with it.  Then you’ll have the best choice for that pitch and blade.  It’s been my experience that large diameters load the engine faster than number of blades or even pitch.  If you have tons of power the blade count doesn’t seem to matter much,  but diameter does.  More blades do provide more surface area in a given prop disc.  Whether that matters may be more about your airplane and your preferences.

Dave

[Howard Rush]

See Brett's comment on the other thread.