That is the common folklore, where you are trying to keep the load similar. If you just go by blade area, an 11 inch prop sweeps out 20% more circular area than a 2 blade. However this ignores the fact that most 11" props actually have wider blades making the difference in loading more like 30-40% (a guess on my part)

A three blade prop has 50% more area than the same diameter 2 blade (3 vs 2). So going from a 2 blade 11" to a 3 blade 10" will present a larger engine load than the original 11" prop (even accounting for the somewhat narrower blades of the 10" prop). I guess that the 3 blade 10" is  approximately a 10-20% bigger load----trying to account for the narrower blades of the 10" prop.

But you are probably wanting a bigger prop load anyway, otherwise why do it?

A good reason to go to the multiblade props is when you have a powerplant that can handle more prop load, but don't have the clearance in the landing gear for the larger diameter prop (which is more efficient).

I've tried a few CF 3 blade props, and asked Expert fliers about the effect it will have on the plane. All the guys I talked to said that it would reduce the 'corner' somewhat. I did notice that, and that wasn't what I wanted. So far, I have not gotten anything in return for the extra blades (performance ~ line tension). If anything, I'd just say that I learned not to mess with 3 blade props without a really good reason...

Two blades works better, IMO.  Personally, I'd make longer LG legs. For one thing, a 3 blade is relatively tough to hand crank. I never know which blade to whack. If you use a 3 blade, I'd suggest also using the 'electric finger', but that's not a bad idea with CF props anyway. Remember also, that your odds of damaging a blade on Landing goes up considerably. Seems like +50%  to me. I don't see much to gain. They maybe do run smoother, but are also a real PITA to balance. You'll like longer LG legs better!  Steve

How much extra clearance do you need?  Would bigger wheels do it?

How much extra clearance do you need?  Would bigger wheels do it?

That's the easy thing to try, and is the first thing I do (and I have the problem in spades using electric motors with 12" props on a Nobler!). Of course if your have wheel pants, it is going to be problematic.

One thing to consider if you lengthen the gear, is that is you will need slightly thicker wire--because a longer length is springier and in grass might provoke more tip overs. Of course if you silver-solder an extension, then that is basically doing the same thing as using thicker wire.

Like I said earlier, the 3 blade prop is a compromise when you have the guts in the engine, but no easy way to harness it by using a larger diameter prop. If your engine is happy with the biggest 2 blade you can put on, it isn't obvious that a 3 blade of lesser diameter buys you anything.

I am not sure about the cornering, unless it is coming from a 3 blade having less moment of inertia (aka flywheel) than a larger 2 blade. But I am not sure that is even true if you only drop and inch, but add an extra blade. Maybe it is something else??

No one has mentioned the pain and suffering involved in cutting the notches in a metal spinner for 3 blades, vs the normal 2-slots provided in virtually all the commercial spinners.  I guess you could special-order a 3-notch spinner, but it seems so much easier to go with an off-the-shelf unit.

Floyd

No one has mentioned the pain and suffering involved in cutting the notches in a metal spinner for 3 blades, vs the normal 2-slots provided in virtually all the commercial spinners.  I guess you could special-order a 3-notch spinner, but it seems so much easier to go with an off-the-shelf unit.

Floyd

Dubro, Brodak and am sure all the others offer 3 blade spinners, this really is a non-issue.

I have a question to throw into this mess on top of the what if's and all. When you change from a 2 blade to a 3 blade you reduce diameter but do you have to up the rpm any to get the same lap time. I also wonder if you start with a bigger 3 blade prop so that the blades are wider and reduce the tips a bit to get down to the smaller diameter how does that work ?

On my Panther, I made the gear wires intially 3/4 inch longer, because I knew the electric power would require a bigger prop. The down side of this is that the plane is much harder to land smoothly because the longer wire allows more flex and consequently stores more energy to cause the plane to bounce. You really have to grease it on, particularly on hard surfaces. I average about one good landing in five --  almost anyone else will do better than that, still it's a factor to consider.

I'm building a Vector 40 right now that will at least start with the stock gear, so it will have a 3-blade Master Airscrew 10-7 for the first flights. The desired 12-6 hits the deck in level flight attitude.

John