Well I must admit that some of the things I said were a little more confusing than I meant, sorry was half asleep.
I guess a little more clarification is in order. My info might be a litlle old here (chart is 5 years old,) but Master Airscrew reccomended a 2 blade 10X6 or a 3 blade 9X4 for a standard 40 size motor. Now this is where some of my basis was formed on. Also, if you used an exact copy (10X6) from 2 blade to 3 blade, your engine would not run as well due to the now same (10X6) prop WOULD be heavier in 3 blade config due to the extra blade.
A 9X4 3 blade Master Airscrew has very thin and short blades. Now what I was meaning by my "constant rpm" comment is that if you hook a engine to a test stand and run it at a constant throttle setting to roughly simulate unaccelerated, steady level flight the prop becomes very effecient with how it lets then engine run from a fuel and power output case. As soon as you change ANY variable, (pitch, bank angle, airspeed, air density, altitude, etc. etc.) the engine will gain or loose rpm at a greater rate than 2 blade prop that is correct for that size engine.
I didn't mean that the prop could change it's pitch to maintain a steady rpm. I was just saying that due to any change of variables, the aerodynamic effect is to raise or lower a props rpm, and a heavier prop would resist that change more is all. This resistance to change rpm is what gives a larger prop more torque for manuevers (among other factors as well!!!!! This just happens to be one factor.)
An easy way to test this is to take an airplane, check with the engine manufacturer and get a 3 blade prop they reccomend. Fly the motor at a constant throttle setting. Let the airplane fly in level flight and watch the speed, then pull the airplane into the vertical and watch the rate that it looses energy. Then switch to the 2 blade prop and do the same. Now this must be done at a throttle setting that will allow the airplane to stall (as some of us tend to have an airplane that will continue to go vertical until out of sight!)
Now as too a single blade prop, theoretically I guess it should work, but have never seen a single blade prop (except after a Pilot Error Encounter!)
But I do have one question about it though, would the couner weight that would be used have MORE effective drag due to the less effecient aerofoil as it passes thru the air while spinning?
The recent Transatlantic flight from what I have been leaed to believe used a 2 blade wooden prop. Now I can understand this for the fact that because of the prop is heaveir it resists change more than a lighter prop. This would give the airplane a more consistent rpm and therefore make it easier to track progress to do one less variable. I am not sure if they had control of the airplane once out of transmitter range (via a gps linkup???) so if the rpm changed even 400 rpm, with no way to correct it, this could have generated some severe problems in the form of planning.
As for why Cessna's come with 3 blades, it is for the reasons above. As for all of them having 3 blades, no sorry. I did my primary and instrument training in a Cessna 152 and 172 that had 2 blade props!
I now fly a Fairchild Metro III as a career. I have now logged over 1500 hours in the Metro, and nearly 4000 total. Now I am just starting along the path! But along the way i have noticed some rather interesting things that are relavent to this discussion in some ways.
The Metro has 2 prop config. Dowty manufactured and McCauley manufactured are both used on the Metro III. Both are 4 blade configurations. The difference being that the McCauley has a thinner, lighter blade. This being said, the Mcauley props are a BEAR to keep in sync with one another. EVERYTIME you make a pitch, airspeed, bank, alititude change the props get slightly out of synch due to the bladxes gaining or losing rpm slightly. The Dowty props are wider and heavier, and once you get them in synch with one another, they stay for the most part.
Now I don't know all that there is to the different things that go on with a prop. But for RC use, a 3 blade prop gives you slight speed advantage, with a slight torque loss to an APPROPRIRATE 2 blade prop for that engine. This loss is NOT much at all! Just need to look at what your needs are. Doing heavy aerobatics? Go with a large 2 blade prop. Need speed? Go with an appropriate 3 blade!
One of the things I love about this hobby, is the experimentation should NEVER end! I know that if I ever get to the point that I feel I have figured it all out, I am taking up BOWLING!!!!
Reg