I am buying a new magnum .91 four stroke and They recomend a 14/6 prop for it. I am putting the engine on a P-51 and want the three blade prop but since there is three vs two should I change to a 12/6 or something like that? Any advice welcomed. Dan

The three blade will definately be less efficient than a two blader is. A rough rule of thumb when converting from two to three blade props is to drop down an inch in pitch or diameter, maybe slightly more for the larger sized engines. You can only be certain by trial and error.

So the inch way would be to go to a 13/6 as opposed to a 14/5? and then spin it and look for performance?

Normally, when I go to a 3-blade prop, I drop an inch and keep the same pitch, so I would say a 13-6 or 13-7 3-blade.

Sounds like a winner to me. I'll start with the 13/6 and I'll also get the 14/6 two blade to see the difference. I take it that the preformance goes down with the three blade? The plane is a .60 size, so I don't think that it will be much of a problem. Thanks. Dan

Most of the time going down an inch in pitch is the better way to go. More thrust, it may go a bit slower but if you are not trying to break speed records you may get better performance that way.

Part of the problem with going to 3 blade is there is a very limited selection - it's not like 2 blade that you can get whatever you want. Three blades do not give the same performance as 2 blades but with being over powered you can make it work. I would still start off with a 14 x 6 APC first -

Three blade selection not so limited.

Here's a Three-Blade Prop Chart.
Remember, as you go up in Pitch (6 to 8 to 10) you also increase speed


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Schpankme

....OK, Hobbsy....'yer busted. Now we see where all the 3-bladers went.

FBD.

Hobbsy, I hope that it doesn't take all those three blade props for Me to find the optimum one for the magnum .91???!!! [X(]. Top end performance isn't as important to me as loww end as in getting her up in the air faster. The runway is short with trees at each end so up and down quick are important.

I suspect a 12x8 three blade would be about right, Graupner and MA and Zinger all make then. I use three bladers almost exclusively, so I have them from .15 siezed engines to a SuperTigre 4500, I'm converting the 4500 to Diesel operation so I'll need a bigger one yet. In the neighborhood of a 20x10 three blade. Something to keep it in the 7,500 rpm range.

Sounds good, I'll give it a try. Is there any chance of overrevving the engine with too small of prop. I hear a lot about trying to increase rpm's and always wondered about going past "red" line!? It's a brand new motor and I don't want to take any chances . Dan

A MAS 13-8 Three blade worked very nicely with my Enya 90 FS on a 4*60. It would even hang on the prop.

Efficiency of a 3 blade will go down with rpm so its better to give it a bit more of a load than a 13-6 as far as I'm concerned.