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2 Blade- vs - 3 Blade Props on Warbirds

Old 08-22-2014, 08:30 AM
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dasintex
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Default 2 Blade - vs - 3 Blade Props on Warbirds

In the quest to obtain a more scale prop appearance on Larger Scale Warbirds using 50-85cc Gas Engines, any noticable problems or what can I expect using 3 Blade versus 2 blade props?

Bear in mind, this is just a general question, not looking for a scientific explanation and using the properely 2 Blade equivalent sized/pitched 3 Blade Prop for the specific Engine.

Thank you.

Last edited by dasintex; 08-22-2014 at 08:50 AM.
Old 08-22-2014, 11:15 AM
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samparfitt
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My opinion is 2 blades are going to be more efficient than a 3 blade prop (at least on a model).
Having said that:
I'm partial to Mejzlik blades. I find them to be one of the most efficient props on the market. I find that I can usually sub a 3 blade mejzlik for most other 2 blade props (keeping the same diameter) and still have the same rpm's and usually still get the same performance.
I haven't flown them yet but I've got some 4 blade 20X8's Biela from TBM on a Bates B-26 marauder. 110" with two DLE-55's: way over power but can use near scale props.
Also, using TBM's 20" 3 blade variable blade props on a 120" ziroli B-25. Again lots of power with two DA-50's but getting near scale prop size, again.
My Ziroli P-61 (114") recommends G-38's: G-45's would have worked but I used G-62's (only about 30 bucks more per engine) so bigger props can be used.
If you want to go scale on props, need lots of power.
Also, the twins seem to handle 4 blades better than single engine planes. Their combined power seems to be greater than an individual engine. Also, twins usually just fly in a circle (so no big demand on power) versus a fighter that does aerobatics.
Of course, in the air, they all look alike, so you have to pick what you are partial to and enjoy.
I've found that keeping my large gas, single cylinder engines on fighters around 6600 rpm's gives me enough power to fly the plane: be it a 2 blade or 3 blade. Thus, if the 3 blade gets me lower than the 6600 then I switch back to the two blade. Usually best to start with a 2 blade for maiden and until you're comfortable with the plane and then you can switch over to the 3 blade (better too much thrust than just enough on a maiden).

Last edited by samparfitt; 08-22-2014 at 11:35 AM.
Old 08-22-2014, 01:22 PM
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da Rock
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amended to suit the size mentioned in the OP

lol.... almost nothing suited the size engine mentioned....

was hoping to keep the thread alive.......
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Last edited by da Rock; 08-23-2014 at 03:50 PM.
Old 08-22-2014, 05:23 PM
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Glad you chimed in here Rock. We see eye to eye on this issue but I don't have the energy to type all that out. Guess that's why you're a mod? My DLE 30s love the APC 15.75" four blades and I have clocked 110MPH with them. The 18 x 10 two blade clocked
the same speed. I think the efficiency myth is just that, a myth. To each his own though. Use what works for you and makes you comfortable.
Old 08-22-2014, 10:40 PM
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vasek
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From my observations, most 3 blades look too small on a model; so as a static "look" is not scale anyway and when the engine is running all you can see is the dia of the prop, not the no. of blades.
Old 08-23-2014, 01:16 AM
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da Rock
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Originally Posted by vasek
From my observations, most 3 blades look too small on a model; so as a static "look" is not scale anyway and when the engine is running all you can see is the dia of the prop, not the no. of blades.
We are always challenged by how our available model props look when resting on the ground. And have the problem of ground clearance as well. If you look closely at the pictures of my warbirds, the grass stains on the props might be visible. The white tip paint has to be refreshed often on my favorite Corsair for example. No matter how many blades, if you don't have the clearance, you don't have unlimited choice. I'm glad to have a way to have more blade area up front AND have a somewhat more scale appearance. In fact, I'm extremely pleased to have found the best performance from Master Airscrew's 3 blade props are from much larger diameter props than they recommend.

I have tested even larger diameter props but unfortunately, what I've got available to me don't have appropriate pitches available with the available diameters. Of course, those even larger diameters were short lived on my most used club fields as well. Even had I found something that worked in the air, the ground was always there.



I just noticed the pictures were taken with freshly painted props. The pictures were taken because 2 were brand new and one had just been freshened up, which is why the pictures were taken I guess.
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Last edited by da Rock; 08-23-2014 at 03:51 PM.
Old 08-23-2014, 02:30 AM
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chuckk2
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One of the reasons that model props don't look "scale' is simple.
The full size engines and props have a much lower RPM, requiring a large prop!
Old 08-23-2014, 02:49 AM
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Lifer
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I have been using MA 16x8x3 props on my 60-size warbirds for quite a while. All are equipped with 20cc gas engines. They perform well, look better and actually protect the cowl in case of a nose-over!

DLE 55cc warbirds such as my P-40's use the Xoar 20x8x3 props and they fly with a lot of enthusiasm. It also has used the Xoar 20x10x3 as well. The 8 inch pitch props turn around 7,200 on the ground and the 10 inch pitch props turn about 5,500. Beilas of the same size seem to load the engine down too much. I also tried a Beila 20x10x4 prop, but it loaded the engine down too much so it is now a $100 paperweight.
Old 08-23-2014, 02:59 AM
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I always used scale 3 blade props on my Twin Otters, although not scale diameter. They flew fine on the .28 FSR engines at weights between 11 and 13 pounds. I would not sweat the loss of efficiency too much on a gasser. I still use 3 blades on other ships and like the look.
Old 08-23-2014, 07:31 AM
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flycatch
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Originally Posted by dasintex
In the quest to obtain a more scale prop appearance on Larger Scale Warbirds using 50-85cc Gas Engines, any noticable problems or what can I expect using 3 Blade versus 2 blade props?

Bear in mind, this is just a general question, not looking for a scientific explanation and using the properely 2 Blade equivalent sized/pitched 3 Blade Prop for the specific Engine.

Thank you.
Just go to a company called Troy Built Models on the WEB and all your questions will be answered.
Old 08-23-2014, 10:54 AM
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da Rock
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What flycatch says........

Troy Built has a couple of charts that recommend 3 blade props for gas engines.

http://www.troybuiltmodels.com/categ...Balancers.html

They also have a couple of different mfgs listed and a good selection (most are in stock). Quite a luxurious availability.

Last edited by da Rock; 08-23-2014 at 03:51 PM.
Old 08-23-2014, 03:59 PM
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I use Master Airscrew 16-8 three blades on DLE 20s. I've noticed the newer model turns slower than the old one. They still do ok. Here is a VQ P-40 with a DLE 20 and MA 16-8. It is close to scale. Also a Koyosho Corsair with a DLE 20 and a MA 16-8. Works. Ok. The Tony (Ki 61) also has a DLE 20 with a MA 16-8. They give up a little bet of performance but not much.
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Old 08-26-2014, 07:41 AM
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vertical grimmace
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Interesting in that I do not see Scale models at scale competitions using multi blade props. Only the Warbird guys seem to be interested in them, not the scale guys. When I fly, I want the best prop I can have within reason. By that I do not use carbon Mesjlik stuff. When I want my plane to look scale, I throw on my static prop. You cannot see it when it is spinning anyway.

Also, why is it that the full scale warbirds had multi blade props?
Old 08-26-2014, 08:31 AM
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Lifer
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Large props at lower rpm's are more efficient. That's why they used gearboxes coming off the engines.
Old 08-26-2014, 09:07 AM
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da Rock
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Originally Posted by vertical grimmace
Interesting in that I do not see Scale models at scale competitions using multi blade props. Only the Warbird guys seem to be interested in them, not the scale guys. When I fly, I want the best prop I can have within reason. By that I do not use carbon Mesjlik stuff. When I want my plane to look scale, I throw on my static prop. You cannot see it when it is spinning anyway.

Also, why is it that the full scale warbirds had multi blade props?
Full scale has the luxury of building an appropriate prop. They also have variable pitch to work with so the problem of pitch is very close to being no problem at all for them. So with lots of full scale craft, they only had to match blade area to the mission. The missions don't change much for full scale. One exception is performance in combat. The mission of full scale warplanes has always been to remain competitive so.........

They not only needed adequate blade area for maximum performance, but needed more blade area when a specific design was fitted with more powerful engines. So many warplanes needed more blade area than two blades could provide from the gitgo, and many needed additional blades than originally fitted when engine power demanded it.

The Corsair needed so much blade area, they strapped on more blades and still had to get more ground clearance by bending the wings. The P47 used the same engine and solved the problem by gear with extending struts.

The original spit started out with two blades. Wound up with counter-rotating and a handful of blades.

Last edited by da Rock; 08-26-2014 at 09:14 AM.
Old 08-26-2014, 09:10 AM
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da Rock
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BTW, a comment I often get is that the spectator is surprised that they can tell my model has a 3 blade in the air. It is obvious on close fly-bys.
Old 08-26-2014, 09:20 AM
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da Rock
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Originally Posted by vertical grimmace
................. Only the Warbird guys seem to be interested in them, not the scale guys. ...............
Aren't most warbirds scale?

Of course, the vast majority of IMAC aerobatic modelers use 3 blade props. You seldom see 2-blades.

This thread really might get far better response on an IMAC forum. Of course, not much of a percentage of their planes nowadays are the "little 20-50 engine size".
Old 08-26-2014, 09:30 AM
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vertical grimmace
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I would say warbirds are "based" on a scale model. But even "Scale" models are only scale in so far as they match the documentation they were built too.

I always get a kick out of the warbird forums, where the builders are chasing scale, but in all actuality, it does not matter, other than for personal satisfaction.


Everything on full scale I have read and studied was the multi blade props were used to harness/utilize the power of the engine so the prop would not hit the ground. A two blade prop quite possibly would have been used otherwise.
Old 02-02-2015, 12:56 PM
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Hi all, this is my first post but I just wanted to comment since this is what I'm researching. I will say that from what I've been able to learn from my own builds and from a few aviator friends, the saying that "2 blades are always more efficient than 3" etc. is often wrong. You can say that propellers create drag via their tips and leading edges so adding more blades adds more drag. However, drag is just ONE force that acts on propellers and even through drag is the primary resistance force you need to combat to achieve higher effientcy, drag is also created via pitch and the thickness of each propeller both of which can be reduced when you add more blades at a given diameter and RPM range.

On full scale planes the bigger concern is making your thrust while keeping the propeller tips from spinning faster than the speed of sound via reducing the diameter of the propeller. In the old days you also had to worry about the strength of the propeller more as they had less advanced composites. They could simply add another blade and spread out the disc loading and keep the propellers from breaking or prevent having to use heavier materials to achieve the strength needed to support the higher loads. In many cases, adding an additional blade meant being able to construct the propeller with a lighter material and made it both stronger and lighter weight than a 2 blade propeller of the same output. Less weight and same power often meant additional blades made the design more efficient.

On electric scale models the materials aren't nearly as much of a limitation, so weight becomes less of an issue. Also, because they are much smaller and the models much slower, stalling due to hitting super sonic speeds isn't an issue either. But you still have dimensions to worry about on a scale model, that's the primary reason to use additional blades from my understanding. Sure, you might have better amp draw moving to a 7x4 from a 6x5 prop on a lower KV motor but if you don't have room, a 3 blade 6x4 will give you similar gains and still fit without having to extend landing gear or move your motor.

Also, more blades are quieter than 2 because they tend to be thinner and narrower. Remember the old Vietnam-era UH-1 "Huey" helicopter? They gave the name "chopper" to helicopters because of the extremely loud chopping sound the 2 blade design made. Those blades were long, wide and thick so they spun slowly but the outside tips moved quite fast and made for a huge pressure swing every time they came and went. Newer military helicopters use more blades, both to reduce noise but also to make them more stable and efficient. Reducing the disc loading means being able to use lighter weight materials and being able to operate in a wider RPM range while also reducing the rotor diameter and decreasing downwash speeds which make for more stable flight and less debris when flying close to the ground.

Anyway, there are way more factors in judging 'efficiency' between 2 and 3 blade prop designs so it is never safe to say 2 blades are "always" better than three.

Last edited by BrandonDrums; 02-02-2015 at 01:01 PM.
Old 02-02-2015, 01:14 PM
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vertical grimmace
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From a model airplane stand point (which is what we are dealing with here, not full scale. It may be that a multi blade prop could be better. The problem is, there are just not enough options. So being that there are many more 2 bladers available, you can choose a closer to optimum design than what is available with multi blades.
Old 02-02-2015, 04:00 PM
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Unfortunately, that is the root of the problem. There simply aren't enough options, at least affordable ones, for three blade or four blade props to accomodate every engine. The APC 15.75" four blade and the MAS 16 X 10 three blade are great on 30cc engines and the MAS 16 X 8 is a good match for the 20cc's, but when you move up to the 50cc class the props start getting very expensive and are often made of wood. Of course, that gets very expensive when you have a nose over.
Old 02-02-2015, 04:42 PM
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vertical grimmace
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Originally Posted by irocbsa
Unfortunately, that is the root of the problem. There simply aren't enough options, at least affordable ones, for three blade or four blade props to accomodate every engine. The APC 15.75" four blade and the MAS 16 X 10 three blade are great on 30cc engines and the MAS 16 X 8 is a good match for the 20cc's, but when you move up to the 50cc class the props start getting very expensive and are often made of wood. Of course, that gets very expensive when you have a nose over.

Not to mention, most guys wanting multi blade props want them for a more realistic appearance. Most of the reasonably priced options are far from realistic looking. I doubt highly that the scale shape would be optimum for the particular engine and airframe being used. Engines like a particular load, and airframes really only want 1 or 2 props. Duplicating a full scale situation on our models is just really not very realistic. Our fixed pitch props, really cannot compare with a variable pitch prop of the full scale.
Old 02-02-2015, 05:34 PM
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sidgates
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I have been under the impression that 3 blade props were not as efficient on engines that run in the 10,000 rpm range (2 stroke) and 3 blades were OK when running in the 6-7K range. No testing to prove this.
Old 02-03-2015, 12:13 AM
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Johnnie Red
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In my opinion there are only good companies or bad companies making 2 bladers or multibladers.
The myth that single or 2 bladers work better than the multibladers has its roots back in time, while model airplane industry was in the middle ages.
There are only good companies and bad companies.
In my experience I have had good and bad experience with 2 bladers that were stalling because of their bad airfoil or structure ( ballance problems etc.).
While I have had the same experience with multibladers.
A little reasearch before you buy is needed.
Keep always in mind that the 1:1 warbirds that we use as prototypes were having tremendous power from their powerplants so if you want to turn scalelike props on your models you have to pick up oversized engines.
A P47 could go civilian without all the armor and payload half the weight. It could fly with 1000hp like that. Nevertheless as a wartime instrument, had a powerplant of 2500hp in order to fly. The same applies to our models.
We want to put a scale prop? We pick up a bigger engine to turn that prop. Plus that we need less ballast to ballance. Of course we have to reenforce the firewall and the motor mounts but what the heck we are scalers aren't we?
Old 02-04-2015, 05:37 AM
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vasek
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There are 2 sayings which apply to a "T" to this debate:

1- "MORE is always better..."

2- "Less is more"

just my 2 cents

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