2 vs 3 blade props

If you use the same blade design, and spin the propeller at the same rpm, sure you get more thrust from the 3 blade propeller exactly the same way that three wing panels of identical design moving through the air at the same speed will generate more lift than two.

However..

What you lose, especially with smaller props and higher rpms, is efficiency. Therefore you may not get 1.5 times the thrust that you had with two blades. Each blade flies in the wake of the preceding blade, and with three blades they are closer together and thus the air is more "disturbed". Some c/l speed fliers and some free flight fliers spinning tiny props at high rpm use one blade props for that reason - to gain efficiency. For much the same reason biplanes generate more lift than monoplanes but the wings do not work as efficiently unless you have very large separation distances. You also get more parasitic drag due to additional wing/fuselage junctions etc.

Therefore in general, if you have the ground clearance to run a properly sized 2 blade propeller, you're better off with that. If you need ground clearance and thus can benefit from reduced diameter but need the load factor of a 3 blade prop, then go ahead and use one. Plus, they look cool.

Mike D.

Its generally accepted that two blades are better than three. And one blade is better than two. Efficiency wise. however balancing a one blader is non trivial, and at high RPM the thrust vector roattes round the shgft line causing vibration.

Resaon being the blades do not like travelling in the wake of the previous blade.

"Each blade flies in the wake of the preceding blade,"

While a three blade is slightly less efficient than a two blade prop, it has nothing to do with the above. This is because the blade doesn't fly in the wake of the preceding blade. By the time the next blade comes around the wake is behind the prop on its way to hit the rudder to cause P factor.

No, it isn't. Especially at slow forward speeds. Think of spiral tip vortices that don't run with teh main airflow for one thing. Plus stuff that bounces off cowls.

In static testing, the wake might possibly factor in. But at speed, one would think that the advancing blade will have far ourtrun the 'wake' of the preceding blade. Bill(oc)

More misconceptions. Each prop blade does indeed feel an effect from the previous blade. Disturbances over an airfoil are felt at considerable distance above that airfoil, and the higher the number of blades, the closer that interference gets. The effect is more pronounced at high RPM and low forward speed, such as in takeoff and climb. Disturbances caused by airfoils of any sort are spread outward from the airfoil at the speed of sound. While the propeller blast is to the rear, there is a disturbance to the sides and front, consisting of lowered air density and considerable turbulence. The same factors apply to helicopter rotors, particularly in hover or low forward speed, where power requirements rise dramatically as the rotor has to deal with the turbulence.

Dan

Thanks for backing me up on this one Dan... ;-)

My answer was a simple version of the explanation I have read over and over again in various references. To me it makes intuitive sense. If there is something amiss in what I wrote I don't know what it is, but I am relaying that which I have come to understand as the reasoning. I used the phrase "in the wake of the preceding blade" which perhaps is a narrow description of the situation but to me it covers the issue. And in a literal sense it has to be true to an extent as pointed out by some, in that the prop is generally flying somewhere below pitch speed except I guess in a dive. And especially statically or at low airspeeds. I don't think the original question was seeking a thesis on the situation but just a simple answer. Biplane wings, which I pointed out as a similar example, of course don't fly in each other's "wake" as in one behind the other, but each feels the effect of the pressure disturbances between the two planes, with low pressure above the lower wing and higher pressure beneath the upper creating some form of mayhem and corruption to the airflow that would increase as the gap is reduced. That's enough explanation for me anyhow. I suppose it also depends how you define "wake"... to me it's a big picture word describing the pressure disturbances around the blade/wing/whatever due to motion of one or the other.

Mike D.

Each blade tries to accelerate the same column (mass) of air, which is the column defined by prop diameter and column speed per second. According to Newtonian law, that is what generates thrust.

The three-bladed prop will produce a slightly higher acceleration (more column speed per second), but at a higher drag cost, which loads the engine quite a bit more. That is why efficiency suffers, but the prop generate more thrust on the available prop disk diameter.

Which is one of the reasons the IMAC guys are moving to 3 blades. Another (more important in some areas) is sound. More load, lower tip speed seems to be quiter.

I think most of the IMAC guys are moving to three blade props because they are going with larger and larger planes. This means larger and larger engines. As the planes approach 1/2 scale, the power requirements tend to be a lot greater to get the performance they require. Simply put, two blade props would have to be too large for the performance these engines and planes require. Just like full scale, they HAVE to go to multi blade props for ground clearance!

Agreed. And the losses are relatively lower with those large diameters and lower rpms.

Mike D.

We have some national IMAC competitors in my club and they fly some BIG birds. Most of them have changed to 3 bladed props in the last 2 or 3 years. I asked why and was told NOISE was a problem rather than ground clearance. I intend to do a maiden flight on a large one next month and probably will go to 3 blade for ground clearance since I am hanging a 4.4 where a 3.7 was supposed to live.

thrust

Nice newtonian explanation. But sometime Pe I suggest you take a prop and actually measure the volume of air blown at a given RPM. Then take exactly that same prop, install it backwards and again measure the amount of air blown at the same RPM as used in the forward orientation. You just may be surprised at the results. By the way, do the experiment so the prop is not stalled.

And to absorb power without the tips going supersonic.

Hello Mike,

Thanks for a very clear and concise answer to my question.

As I understand it; switching from a 2 blade to a comparable 3 blade will increase thrust but NOT 1.5 times more thrust. Got it.

It also seems 3 blades are more effective at moderate rpm's and larger diameter.

thanks, JK opinions will vary.

Precisely !!

Two prime examples are the Spitfire and P-47. Originally they had 3 blade props. Wasn't until the P-47 went to the 4 blade 'butter knife bladed' prop that it became an acceptable performer. The Spit needed the extra blade as HP went up on later version engines.

David

You won't get more thrust. At static, you'll likely get less. Smaller diameter props produce less static thrust because more of the horsepower is going into drag and turbulence. Fewer, longer blades are moe efficient. Two ways to get more thrust are to use a larger prop and gear it down, or add more horsepower. Three-and four-blade props are used for ground clearance and/or to absorb more horsepower in a given diameter. Basically the same thing. Sometimes they're used to reduce tip speeds and therefore noise. And sometimes to get a smooth, vibration-free ride. In turns, two-bladed props create vibration. Modern turboprop airliners have geared props, since the turbine turns way too fast to attach a prop directly. So they gear them way, way down and turn large, wide-bladed props running at very high pitch settings in cruise. Typical RPM on a commuterliner prop might be 800 or so. The WW2 Corsair turned its prop at 1300 for takeoff and 900 for cruise. Low RPMs were necessary to keep tip speeds low enough that the prop didn't go supersonic when forward speed was factored in.

Dan

Boy this one is too juicy to pass up..Kinda like being a liberal or a conservtive..Depends on which side of the fence that you're on.......Lets just blow past the smoke and mirrors about the p-factor and the drag or one blade running in the wake of the other blade or something like that ....Here's the deal....What do you like..maybe you like the looks of a three balder on you plane or maybe you like expermenting with different stuff or maybe you dont care about the drag or whatever that was that somebody else was using to talk down something probably they were too scared to try.. What it boils down to is that you have to be willing to try for yourself.... the thing about going down in length and up a pitch is just a lot of bull..... to start with there are not that many sizes in three balde props.. So sometimes you have to modify an existing one. I run the same size in a thee blade as I do a two blade and in MOST ALL cases it works.. If in fact you have a three blade that loads your motor too much try trimming off some of the backside until it works ,but knowing that a three blade prop wiil not turn up like a low pitch two blade prop and it's not necessary for it to begin with......Anyway a three blade prop will not make your plane crash....Bobby

Anyway a three blade prop will not make

ROFLOL! That is good because I sure don't need any more excuses!