bribrat is essentially correct , the reason two blades were used on early aircraft was that was what was practical and when more power became availible considerations like landing gear length&weight dictated going to 3 (or more) blades to absorb the power increases which is why they go faster in spite of some inefficiences. Corsair needed its gull wing to get enough clearance (& keep the gear length reasonable) which other wise would have been too short &wasted power. certainly in hover multi blades may have an advantage to pull better but the over all amount of power getting to the prop is important. there are a number of variables. that having been said the T-28A flies better with after market 3 blader than the original (& difficult to replace 2 blader) what missing here is that the prop needs to be matched to the plane &its mission for intended tasks.

From what I understand of aviation, you are 100% correct. Physics do not change, they are the same today as there were a thousand years ago.

2 Blades are more efficient then 3. A single blade prop would be the most efficient, however the vibration and balance would create massive problems.

Green Flyer,

I just browse this thread. It's a very general question and to give a exhaustive reply would be difficult. What type of flying are you looking for?, at what speed you need the prop to peak in efficiency ?, what engine are you using ( what's the torque graph ?)...
From my experience I discovered that commercial props are designed to cover a very big area of envelopes. Same diameter, same pitch, 3 different brands on my Saito 100 are pulling
from 9.4 to 10.8 lb ( huge gap in my opinion) on static thrust. Static thrust is not very relevant but I was looking for a prop to allow me to hang in it and get out vertical with some authority, which makes sense for this test.
There is a very nice site belonging to Martin Hepperle, with a lot of useful info about props and other stuff. It's a very interesting lecture. Take your time and dig in it. I remember that he has an explanation between 2 vs 3...n blades. It's related to max speed somehow ( the guy is in speed models) and there is a conversion formula: keeping the same pitch the diameter of 3 blades should be 90% of the one with 2 blades. Diam of 4 blades 85% of 2 and so on till you have no more room for blades. This is just in theory. I want to stress ouut: the pitch of a prop is just a little bit more than a convention. In all aerodynamic approaches of the prop, the pitch is considered at 70% of the blade, measured from hub. A well designed propeller will always have a variable pitch along the span to match the ideal flow at an estimated speed.
Ops ... here is the link : http://www.mh-aerotools.de/airfoils/index.htm
I was really impressed with this site and actually I was looking for something else when I found it. Martin Hepperle is a big shot in aerodynamics.
I know many people will say sometimes experience is better than theory, but in this case the experience is limited to just of few shots in the dark with few brands available and theory
hurt nobody.

Boby

Hello out there. I came to this thread seeking a good 3blade prop size for an OS 91. If you have a good combo please let me know.
In all the discussion one very important point has omitted. S -E - X !!!. NO kidding. I was very much involved in flight instruction and light aircraft sales in the 70's. When Piper came out with the Seminole, a light four place twin, 3blade props were offered as an option. The normally aspirated airplane was slower and less fuel efficient with the three blade. Not a bunch slower mind you but definitely not faster This is not a trial and error deduction. The data is in the performance charts for the aircraft. So why would someone order a new airplane with less efficient props ? As is the case with most situations involving humans that defy logic the answer is S-E-X. The plane looks like it is going 300MPH standing still with those 3blade props and chrome spinners !! You wanna impress the girls !! Walk em out to the plane with the three blades !!!!
There was one redeeming engineering plus. Vibration was reduced with the three blade. That is the primary reason you will see a three blade prop on a normally aspirated light aircraft. They are more expensive to buy, overhaul , maintain,and operate.... they weigh more, but they look cool and run noticeably smoother.
I know I will get better performance with a two blade on my P-38 but it looks S-ooo much better with the three blade. And the right three blade should only cost a small reduction in performance. Any suggestions ?

Hi!
General discussions on full scale prop efficiency is probably not leading anywhere..!
The guy just asked about what prop would be best for his 15cc fourstroke.
In the model world every experienced flier knows that a 2-bladed prop is better than a 3 or 4 bladed one.
For a 15cc (.91) foustroke a 13x6, 14x4-5 or 13x8, preferable APC would be good to try. (don't think there is a 3-blade 14x5 APC though ...but there should be).
A 13x8, 14x5N, 14x6 or 15x4 APC 2-blade prop would still better though!

So why choose a 3-blade then...like the CL aerobatic guys sometimes use and pattern pilots did in the eighties...!?
The reason CL guys use 3 or 4 blade props is because lack of ground clearance on their models and also..in only a minor part, some more inertia (heavier prop deliver a more steady engine run). Remember these guys are not after high performance (speed and rapid climbing ability) but steady engine behavior.
The pattern guys in the eighties also favored 3-bladed props! Why? Because there was strict noise regulations in F3A competition....and 2 bladed props with their slightly larger diameter was to noisy!! Propeller tip speed noise was reduced with 3-bladed props.
I have seen one bladed props in action to but these are a bit more difficult to manufacture and only made in small numbers by CL speed competitors. As you well know the one bladed props are preferred by CL-speed fliers.
For the rest of us seeking best performance for our models, a 2 blade model prop is always going to out perform a multi bladed one. Provided you seek out the best one....or modify one

Regards!
Jan K
Sweden