In what case would one use a 3-blade prop instead of a 2-blade? What are the differences?Pros and Cons?

Here's one:
With a 3-blade you can use a smaller diameter and get the same performance. Good if you need more ground clearance for flying twins off grass etc.

When looking at engine specs, usually they list prop recommendations in terms of 2 blade props. Is there some conversion factor I can use to figure our what an equivlent 3 blade might be? For example, a 13x8 2-blade = ??x?? 3-blade...

Thanks!

I believe 2 blade props are supposed to be more efficient, though...

Lots of threads on here about 3-bladed props, if you do a search, you can read for quite a while.

To answer my own question, I found a reference to the following rule: "...if you're going up one blade go down one inch in either diameter or pitch..." However, the surrounding comments seemed to suggest this isn't entirely true. I guess this something that is just tried and discovered for one's self?

Three blade props are used in full scale aircraft for a number of reasons. The main one being you can have shorter blades as opposed to a two blade prop, which helps when you are swinging large props, like many WW2 warbird for example. In our modeling, however, that need is seldom the case unless we are working in scale models and such. At the small diameter and other factors in our models, the three blades loose efficiency over 2 blade props, so a 2 blade is usually preferred. Some ARF trainers come witha three bladed prop, because they can turn at a lower rpm, which makes the plane quieter, and also the big blades they use can act as an air brake a little better, which can be desireable in a training situation. So unless you are flying a scale airplane or need a smaller diameter blade for a unique configuration, it is best to use a 2 blade prop.

A single engine aircraft that has a three blade prop has more lateral stability compared to the one that has only two blades regardless of the diameter and the pitch. An experienced pilot feels it when he banks the aircraft to the right, which is the opposite of the torque effect. A two blade prop airplane looses altitude quickly in a turn. On the other hand, a three blade prop war bird can turn faster and can make a tight turn without loosing too much altitudes.

Alliot is right on the money with what he said

Single blade propellers have existed and have been used....
your pun has failed
volvete a tu asadito y tu mate

ps
when are you getting OUT of that wilderness

and REMEMBER :
L. M.S.A...................s !!!!
ja ja ja
R O F L

In the manual for my Hangar 9 Extra easy2, is says it uses a three blade prop for slower airspeeds, and noise reduction. Thier less efficient than 2 blades. Most 3 blade props are heavier than 2, so your losing power right there. The main reason is ground clearance.

There is no aerodynamic mystery here. The relative performance capabilities of two-bladed vs multi-bladed props was quickly established at the dawn of aviation, & the physics hasn't changed since. Two bladed props will always outperform multi-blade props, & single-blade props outperform 2-blade props (that's why the C/L speed planes use single-blade props). Valid uses for multi-blades on a model are chiely diametral constraints and scale appearance.

BTW, multi-blade props do not change the stall speed of an aircraft.

I agree with Britbrat

I also agree with Britbrat.


Unless the props in water.

I flew a SIG Four Star 60 with a Magnum .91RFS, prop'ed with a MA 13x8 three-blade. It was amazing! I swapped props for a 14x6 and the place flew like a sleepy elephant- no vertical, no straight-and-level speed. It actually went DOWN in performance, both vertical and speed. It would BARELY hover with the two-blade 14x6, and could climb out of a hover with the three-blade 13x8. I found that with the right three-blade prop performance was better than the recommended two-blade prop.

Lachlan

You obviously need more prop with the 2-blade -- you clearly had more power available

Interestingly enough, most of the top-end control line stunt fliers use three or four (even 5) bladed props. Most are running piped 60's, or 4-strokes. I don't think ground clearance is the whole story.

Jim

So you're trying to tell me that even though the SMALLER three-blade prop was faster and had more thrust (including a much quicker and shorter takeoff roll) a LARGER two-blade prop would have the same effect? That, to anyone with reasonable logic, would indicate that the smaller prop was indeed the more efficient propeller. Uh, how's that for efficiency? They were both spinning the same rpm on the ground, and one can only guess as to their "unloaded" rpm at flight speed. All of this theory about single-blade props vs. multi-blade props is just that when it comes to our models, since they turn their props at much higher rpm than their full-sized cousins, whose prop data is the source of this efficiency argument. Three-blades will pull all of my performance planes from now on, due to the greater thrust I have encountered with them.

Lachlan

Gringo Volador> Read carefully my post. You should be careful when you quote somebody's sentence. I wrote a two blade prop instead of a single one that doesn't exist.

You guys are rad. Such debates. Well, i tell you all, that once im proficient enough with general flight and such, i will make a single bladed prop fly. Hehehe

Alliot you should read my post carefully, all I did was agree with you.

I agree that a two blade prop aircraft is more efficient than the one that has three blades. By saying "efficient", you mean " performance". However, a three blade prop plane has more lateral stability or stability along longitudinal axis. To prove this, replace the two blade prop of your Full Scale Cessna 172 by a three one. Then, you will see the difference. Of course, we need to know aerodynamic to know how to "read" every airplane, even RC planes.

I will add that I have a 3 blade 11X7 Graupner prop on my P-40 and it seems to be a much smoother run,seems to be balanced better.
No problem with thrust either...plenty of it with the OS 70 4 stroke.
Just my 2 cents.

There seems to be some mixing of terms in this debate. Alliot talks of stability with a 3-blade vs. a 2 blade. The only reason the 3-blade would have "stability" would be more gyroscopic effect from the added weight of the 3rd blade. The downside to this is you will lose manueverability and some power.

Comparing a 14X6 to a 13X8 is a poor comparision. The greater pitch on the 13X8 is responsible for the increase in speed. Try comparing a 14X8 with the 13x8.

Now for efficiency. Don't confuse efficiency with speed. Efficiency is the measure of how well the propellor is converting the engines power into thrust. If you turn a 2-blade and a 3 blade the 2-blade will generally have more efficiency than the 3-blade because the blades are better able to get out of the disturbed air from the previous blade's passing. With that said, you'r 2-blade propellor may very well loose some of those gains because of the higher tip speeds due to its larger diameter. The tips are thrashing the air more and that does nothing for you.

Where the performance curves lie and the relative losses from multiple blades and the tip speed issue I don't know. It is generally accepted in full scale avaition that you add blades to reduce diameter. With that said, full scale engines turned props much slower than models. So the loses from increases in tip speed for models can come into play.

I know I haven't really answered this question, but I hope this clarifies some of the physics involved. The best way to select a prop for a model remains trial and error. If you find a multi-blade prop works better then go for it. Besides, they do look kind of cool.

jeepindog, you are telling us that all of the aeronautical engineering development on propellers since the Wright Bros. has been wrong? Both props had the same static RPM?

The 13-8 three blade has a greater power load than the 14-6 two-blade. If the engine was not constrained by valve train limits, breathing restrictions, or something else, it would have to turn faster with the 14-6. Maybe your engine is at its rev limits -- if that is the case, a steeper pitch 2-b will give more thrust.

For a given diameter & pitch, a three blade has 50% more blade area and drag than a two-blade & can transmit proportionately more power, but conversely, requires similarly more power to turn at the same rpm. However, since prop blades run in the wake turbulence of the preceding blade & since the blades of the 3-b prop are closer together, there is more loss of efficiency from wake turbulence. Additionally, there is more "mess" in the root transition zone, resulting in a shorter effective blade length -- again lower efficiency. If the 14-6 & 13-8 are turning at the same rpm, the discharge velocity of the 13-8 is 1/3 greater than for the 14-6 and has significantly greater slipstream shear losses -- another loss of efficiency, whereas the lager diameter prop is exerting the same work over a larger area at a lower discharge velocity -- more efficient (which is why helicopters have large disc areas). Finally, in the case of jeepindog's pitch & diameter change, the pitch increase more than offsets the reduction in diameter, requiring even more power --- so either there is a lot of stuff that isn't being revealed (like the engine at it's rpm limit), or the post is a troll.

BTW I've been in the modelling business for well over 50 yrs, flown thousands of hrs in FS aircraft & managed a fluid dynamics engineering research lab for 20 years. The physics hasn't changed.