I'm looking for a 3 blade propeller for a 91 4-stroke engine; my question is how big a propeller shall I mount on the engine?
I do not have any experience with 3-bladed propellers, but I have heard a lot of disadvantages about them. For instance people have told me that the plane will have a tendency to stall easier than a plane driven by a 2-bladed propeller is it true?.

Sorry for my English

Regards Mike G

Start with the recommended 2blade propellers.
Take the recommended diameter and reduce it by 1inch or by about 25mm. That will be a good diameter for a 3blade.
Take the recommended pitch and increase it by 1inch or 25mm. That is a good pitch.

The only real disadvantage is that there are very few of them available for sale. So very often the right size/pitch is not available for the engine. We try many 2blades to find the correct size/pitch. It's easy to do because there are many. But we have only a few 3blades to try and if they work they work. If they don't the next size is very different. The engine might like something in between but there isn't one made that size. It's harder to find one that suits, so the "experts" stroke their chins and proclaim that 3blades don't work as well. They would if there were enough to try.



No. Not true at all.

When you find the right 3blade for the engine (if it happens to be manufactured) the airplane flies just as good as with a 2blade.

I fly 3 different airplanes with 3bladed props. One flies just a good with a 3 as a 2. It's the P47 and I'm not looking for excellent performance, just good enough performance. The 3blade does better than just good enough. But that 3blade isn't better than the 2blades, unfortunately. But I'm fortunate that there is a suitable 3blade manufactured.

The Skybolt works extremely well with the 3blade shown. That 3blade is one of the top performing props for that engine/model combination. About 3 different props work equally well, and better than all the rest of the props. And the 3blade is one of those 3. The airplane is an excellent aerobatic model. And nothing about the 3blade prop causes any lessened performance. That 3blade shown in the picture is as good as the 2blade props that performed "best".

The Tiger60 with it's 75AX are standout performers. And the best performing prop that I've found for the combination is the 3blade shown in the picture of that red Tiger. The model is the one I choose to fly most often. And it's partly because of the extra performance that specific prop helps to provide.

I wish there was more selection available in 3bladed props. But they cost more and that reduces sales. And too many "expert" modelers, most of whom have little or no experience with 3's, repeat the "wisdom" they heard from somebody else about 3blades. And it's wrong.

There are two disadvantages that matter. There isn't enough selection, and they're more expensive.

There are a couple of advantages to 3blade props.

They are quieter.
The one that will suit your airplane (if you are lucky enough that there is one made the right size) will have less diameter by a slight amount. And that gives better ground clearance. One additional reason I like the 3blade on the Tiger60 is because our field grass is a problem. That 3blade reduces the amount of grass that is shredded and blown back on the wings. It sticks and is a pain to clean off.

These are not major advantages. But they are worthwhile advantages. Much more worthwhile than the advice of people who say things like "airplanes with 3bladed props stall sooner".

I do not have any experience choosing props for a 91FS. Either with 2bladed props or 3bladed ones.

But the way I went about choosing those 3blades was the same as I'd advise you to find a prop for your engine/model.

Try a couple of different 2bladed props that are recommended by people who have experience enough to trust. Pick which performs the best. Then see if there is a 3blade available that is one step shorter in diameter and one step greater in pitch. If there is another of the same diameter and two steps greater in pitch, get that one as well. Try them both. Try the best 2blade the same day. Then use whichever works for you. You'll probably find that two of the three work very well. I did. And found good 3blades for three airplanes of mine. I've not actually tried to find 3blades for my other airplanes. But I have complete confidence that it would be better than a 50/50 bet I would find one if I made the effort.

BTW, if you are like the majority of the people who ask what is the best prop to buy, and you hope to only buy that one and save yourself money, you are making a mistake. Our model/engine combinations are quite varied, and seldom does "one size fit all" combinations of model with the engine you are planning to prop. But if you do hope for someone to tell you the perfect 3blade for your engine, it would help to know what airplane the engine is going to pull.

But good luck. And by all means, try more than just the first one suggested. No expert in the world knows what is going to work best. But your airplane does. It just isn't talking. You have to get it to show you. That, it will gladly do.

So I have a new JBA .56A and have been testing diffrent props but i would like to try a 3 bladed prop. The recomended size for 2 bladed is 11 x 7 through 12 x 6 so would a 10 x 7 or 11 x 7 3 bladed be appropriate for this and who makes these?

Darock you posted just as I did so I will add that I am going to use this .56 on a Hangar 9 P-51 pts or a nexstar. I know the motor is a bit big for the nexstar so I thought a 3 bladed prop would slow it down possibly.

If it helps any, I have ran a 15x4, 14x4 and a 14x6 all APC props on my saito 91fs. So I would start with a 13x6 or 13x8 3 blade. I run my 91 on 15% and 30% nitro also.

Hi from Ireland.
It should let you know what props go well on your 91 if you take a look in the instructions that came with the engine.
I have a 91 Magnum 4st with a 3 blade 12x6 and it flys very well.

I did a litte looking and found a handy little conversion chart. http://www.zingerpropeller.com/_2_blade_.htm

what I have found is a 14/7 3blade is a good size for a 91 4stroke for warbirds,I can get 9,200rpm without bogging the engine down, when I went higher in pitch such as a 14/9 the engine was bogging down with the extra drag,if your looking for performance then a 13/7 is a good choice just a little less drag but still getting the bite if the 7pitch prop since your engine isn't a high reving one being a 4stroke it can handle the higher pitch props.
both of these planes have OS91 surpass engines in them

3 bladed props go great with some planes, especially the above P-40.

I think it depends on the airplane too.
I found a 14x7 2 blade gave me the best performance with my OS 91FS
which would equate to a 13X7 3 blade.
all this on a 1/4 scale RV-4
Now BigTim states best performance with a 14X7 3 blade prop on warbirds.
I have been a fan of Graupner props since I flew one last summer.
They have a 12.5X7 that might be a good match for your engine.
http://www.hobby-lobby.com/grprop.htm
This is a USA distributor, the Graupner props are made in Germany here is their website:
http://www.graupner.de/156.0.html?L=1

I've been a fan of them for about 17 years.

Since you're in Denmark, I would suggest seeing if you can get them!

BTW, I use a Graupner 12 x 8 on my 91 4-strokes

its just the size I felt went well with the particular plane not necessaly the best,probably the best for a 91 would be the 13/7 but with warbirds I try to get the largest dia. and best performance I can,for the looks as well.
I have quite a few duds in the hangar that really didn't fit what I was trying to do at all, its about matching the right prop with the plane,and what type of flying your doing the Mustang has a 14/6 and it flys just as well as the P-40, the OS 91 in that plane spins it up a little higher rpm's the main difference is the P-40 gets off the ground really fast and I only need about 1/2 throttle to fly at a scale like speed.

I CHANGED FROM A 14X8 TO A 13X7 MAS ON MY 90 FOR GROUND CLEARANCE. PLANE FLYS GREAT.

I have a 13x8 on an OS 91FS and it works great. I have flown 4*60, Lancair and now GP Ultimate with 3 bladers. I get plenty of performance and it looks cool as hell.

Ok Thanks for the help so far. The Airplane I’ve planning to but the 3 blade props on is a Grumman goose which is 250 cm in wingspan, and I think the weight on the plane is approximately 7 -9 kg. The props I use now are 14 x 7 2-bladed.

Another thing I been wondering about is the relation between the material the prop is made of and the performance it carries out. How big an impact on that performance does this have?

I think it has alot to do with performance the 3blade props I have been using so far are Master Airscrew and they really flex alotyou can really tell when you rev em up to full power they make alot of noise there probably not the best for sure but there not that expensive and there's the durability factor there pretty tough, in the photo of my P-40 you can see one of the blades has a chip of the white paint missing from a prop strike from that day, the paint chiped but the prop was no worse for the wear,I dip the tips for visability,I want to try one of the graupner props there suposed to be quite good,the weather has been nasty so there's not alot of flying going on at our field no need to buy a prop I don't need yet.

For an OS 91 and a scale plane try a 15x6. Anything else go 14x6.

Useful thread because 3 blades are a bit of a mystery to many of us less experienced barn stormers.

Years ago, I was recommended a 10x6 3-blade by a RC shop guy (which I bought; good prop). It was either for my 70 4-stroke (12x6 2-blade) OR for my 46 2-stroke (10x7 2-blade)... don't remember which. I never ended up using it before I side tracked from the hobby.

So that's either two steps down in diameter and same pitch OR one step down in pitch and same diameter. As this recommendation clearly deviates from the advise here, could it be that it was a compromise recommendation for both the 46 and the 70?

I'll need a 3-blade at some point for my 4 stroke for clearance... 2 steps down in diameter would certainly do the trick for clearance, but after reading this not sure if that's right. Any opinions welcome. But no biggy to get a new one...

edit: Oh but it's not like my 2-blade sizes are the only sizes for those engines, so... depends what you compare it to.

edit #2: Just realized from [link=http://www.rcuniverse.com/forum/m_7125594/tm.htm]this thread (contains a prop-chart)[/link] that my 10x6 3-blade must have been recommended for my 46 2-stroke (don't have the manual for the engine). I could get an 11x8 3-blade or larger for my 70 4-stroke.

Cheers