About to finish my parts gathering and I need to purchases a custom 4.75 spinner . So before I get it cut for a 2 blade vs 3 can you please tell me what to expect . I am purchasing this for a saito 180 for a 73" p40 .

Just wondering the range of changes to expect from torque , overall performance , cost , ease of replacement &/or availability of 3 blades vs 2 .

Thanks for the help

G'day mate,
Go for 2 blades, 3 blades are less efficient & make more noise.

L, I've run three blade props almost exclusively for about 11 years, at fourstroke and Diesel rpm there is no noticeable difference in performance. A Graupner 15x8 3b would be a great choice for the 1.80 and looks good too.

thanks guys , yep agree that especially on warbird a three blade looks good . I also think that my 180 is on the high range of power for the CMP P40 . Is there any additional vibration of a three balde vs two and is cost signifignatly more ?

The extra weight of the 3b will dampen the power pulses of the big Saito better than a lighter 2b. You can check out the Graupners at www.hobby-lobby.com under Super Nylon props.

Yes, it is true, a 3-blade is less efficiant than a 2 blade, however, if you have a significant amount of excess power, (& it appears you will have) a 3-blade will perform quite well on your P-40.

thanks great info !!!!

Does anyone have real data that 3 blades are less effiecent or are they just saying it
could it really be that the engine is less effiecent meaning it can't reach it's peack power turning a 3 blade of the same dia and pitch?
if your engine can't turn the same size 3 balde as two blade why is it the props fault also is a 3 blade reallty 1 1/2 props because of the extra blade
On thrust calc if you put a 2 blade and 3 blade in to it side by side the 3 blade always needs more hp to reach the same rpm and it always makes more thrust.
I may have to do some real world testing and see what I get
example if you can turn a 10x6 2 blade 11000 rpm and a 10x6 3 blade 10000 rpm it's seems the 3 blade is more effiecent and is grabbing more air the the engine can keep up with
so it really doesn't mean the prop is less effiecent it just means it too big for that engine
wouldn't the true test of effiecency be turn the same diameter and pitch the same speed and then see which one pulls the plane better
food for thought

and if anyone has any websites about the topic please post them
think I'll put this to the test and see what happens

so if your theory holds true it would make sence that the three blade prop should be smaller then the two blade suggested for the same engine ?

Ski, when I convert engines to Diesel I use a three blade prop with the same pitch and diameter as the recommended two blade for glow operation. An example, my Fox .74 turns a Graupner 12x8 two blade at 11,200 rpm as a glow on 5% nitro, converted to a Diesel it turns a Graupner 12x8 three blade at 9,400, I've not tried the 12x8 three blade on the .74 on glow fuel.

Yes and that's what the basic accepted rule seems to be when it comes to 3 vs 2 blades
But when you do that it would make sense the 3 blade would not be the same performance wise even though it is now turning faster
I feel it's because for the same engine can't turn three blades the same speed as two and once you drop the size of the 3 blade it's no longer the same dia and pitch so it really can't be compared to the 2 blade.
I'm not trying to say I'm right because 2,3 and 4 blade props really aren't the same animals
I just enjoy learning new things about the hobby

Ok stupid question , why did the real warbirds use three bladed props ?

I believe it was to reduce the blade loading.
3 blades 1/3 for each blade instead of 50% for each blade
when you talking over 2000 hp
here's something I found on Hartzells website they make props for real planes has some good info

If two-blade propellers are the most efficient, then why don’t all propellers have two blades?
The short answer is because efficiency doesn’t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.
If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will be highest. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.

Like they said, you will lose quite a bit of performance with a 3 blade.

true if you don't increase the power to get the 3 blade to the same effiecency as the 2 blade
who's they?

Full scale aircraft use multi-blade propellers to absorb, and make use of the power produced by very powerful engines. They also turn the multi-blade props at much lower RPM,(using gear reductions) with much higher pitch, than a model engine will be able to do.
If you can reduce the prop speed on a model engine with a gear reduction, then you can turn a multi-blade with much more efficiency. Such as with a RCV engines that use a 2:1 gear ratio and recommens higher pitch. When you slow a prop, you need more pitch to get comparable speed. Pitch produces speed, and diameter produces thrust.

I have compared 2 & 3 blade props directly in a model several times. Always need to reduce either diameter, or pitch to get the same RPMs with a 3-blade. As result you will loose either speed or thrust, for a given situation. By increaseing engine power you then run a comparable 3-blade with a as much effeciency loss. That the "real world".

Hi!
There is also the problem with a full size 2 blade prop that the tips might reach super sonic speed...ever heard a AT -6 with it's direct drive 450hp Pratt & Whitney rew up [:@]
But for most models a 2 blade prop will out run and out perform a 3-or 4 blade prop. We don't risk our prop tips to reach super sonic speeds.

Regards!
Jan K

Great answers , I am getting a great education [8D]

super sonic on a full scale or model
a 10 inch prop would need to turn 26080rpm at 80degree F. for the tip to reach mach 1
so we never get close to those kind of numbers in fact we can't even turn or little 2 blades to the max
max blade effeiciency of .88-.92 mach
example at 80degrees F.
10 inch 23500 is mach .901 no regular ole 46 will to that
10 inch at say 11000 rpm is mach .42

here's a link to a prop mach calc if anyone wants to play around

[link]http://www.pponk.com/HTML%20PAGES/propcalc.html[/link]

They guys! They guys that wote buv me I have also seen it first hand on large gassers and a few glows

Another reason for three-bladed props on full scale warbirds was to increase ground clearance, especialy when off the tail wheel. Smaller diameter three-bladed versus same effective power with larger two-bladed.

Three blade props are less effecient, but not by much. The loss is from the tip and hub drag of the extra blade. Also more frontal area and drag for the same thrust. But they are also smaller and have less tip speed, even well below Mach I this helps to reduce the losses.

Gear reduced props are only used on the turboprop engines, modern piston engines do not use gear reduction, that was only the huge radial engine airliners of the past.

There are two major reasons to use multi blade props. The first and most common is for ground clearance with a larger engine. The second is for better high speed performance at altitude. For example a large prop will not do as well at high speeds and altitude because it turns a larger column at lower speeds than a smaller multi blade prop pushing a smaller column of air at higher speed. That is why on many GA aircraft you will find that the same plane made for high altitude and presurized cabin will have a turbocharger and three bladed prop, vs the two blade prop for the non pressurized version.

Here's some good reading from a prop maker that started with making props for the Wright Bros
they should know something

[link]http://www.propbits.com/2.htm[/link]

From Bollyprops


1, 3 AND 4 BLADE PROPELLERS

The rules vary when dealing with single blade and racing propellers. A small 2 blade 5.8 x 5.8 with 12mm chord is equivalent to a single blade of 6.2 x 5.7 with 17mm chord. The single blader is more efficient due to the extra diameter and chord (much higher Reynolds number). Large diameter single blade propellers are not practical due to the high weight of the counterbalance required.

For some racing applications a high aspect ratio blade is not possible due to the extreme loads imposed at high RPM, especially if the tip is expected to exceed mach 0.7.

Contrary to popular belief, multiple-blade propellers do not operate in severely disturbed air from the previous blade (when in forward flight). The reason multi blade propellers often appear inefficient is the need to use considerable lower diameter propellers (in comparison to 2 blades), for the same horsepower available. Diameter for diameter a well designed 4 blade prop will in some circumstances perform better than the equivalent 2 blade propeller.

It is rare to find an efficient 3 or 4 blade propeller manufactured from nylon based materials. The reason for this is the most efficient shape (thin, narrow blades) for these propellers is difficult to produce in anything but a carbon or glass composite construction. For this reason Bolly have one of the best and most efficient ranges of 3 and 4 bladed propellers in the world.



[link]http://1, 3 AND 4 BLADE PROPELLERS[/link]

You can and should reduce hub drag with a spinner. You may not get more drag from the tips, depending on prop speed and design. You could end up with less in most situations with the right prop.

You can basically prop any engine for whatever RPM you desire, be it 2 bladed or more. You have prop design, pitch, and diameter to choose from.

Aerodynamically, you do have more frontal drag with more props unless you reduce prop diameter when going from one to the other. That is one reason pattern guys like them for the down lines at idle. Likewise, they like the pull for up lines. A 3 bladed prop may assist in a stable idle. I'm not sure what gyroscopic effects a 3 bladed prop will have over 2 blades. P factor may be more evenly distributed along the fuselage and fin with 3 blades.

The statement that all 2 bladed props are more efficient than 3 or 4 blades is sort of ignoring what a person wishes to do. A 2 bladed prop is not as efficient in the climbs or for descent braking as a prop with more blades. I suppose that a case could be made for a racer needing only 2 bladed props....