dasintex

My Feedback: (10)

Need to run a 3 bladed prop because of ground clearance issue and retention of scale; any difference other than sizing, in the way the plane will perform/handle, etc between 2 blades or 3 blades; if engine calls for 13x6-14x6, any problem running a 3 bladed: 11-11.5x6 -8 or 12x6-8

GPitts

My Feedback: (6)

I asked the same question to one guy that I know in the field the other day and he said that 3 bladed props are not efficient but not really sure what he meant. Several guys at where I fly at have YS1.10 on their 40 size mustangs which swings 12x8 to 13x10 props. YS 1.10's swings props of sizes from 14-16 inches just like what YS reccomended. So I thought I might ask the question because 4strokes have alot of torque and because of the prop clearance issue. I also thought that using a 3 bladed prop might pull even more without really sacrificing and rpm because of the torque which 4 strokes have. I have also thought about using a 3 bladed prop on my 25 size plane with a 52 size engine because of the prop clearance issue. Don't really know why they dont use 3-bladed props, probably because not many hobby shops around that sells them or because they just cost a little more or why they are not efficient to some people.

Hobbsy

My Feedback: (102)

I use three blade props almost exclusively, but run mostly Diesels and fourstrokes, at diesel and fourstroke rpm the effeciency issue is a non issue. You will have no trouble running a 12x8 three blade and if you get a MasterAirscrew three blade it will be factory balanced.

TLH101

My Feedback: (90)

Here is a link to thread where I compared several 3-blade props, and it has some rather surprising results.
http://www.rcuniverse.com/forum/m_21...tm.htm#2124815

dasintex

My Feedback: (10)

Hobbsy; I plan on using a 90 4stroke, operating rpm 9-11,000; the ground clearance is a result of the plane being overweight; originally the plane was a Kysho AgWagon crop duster; I made some scale enhancements which include actually water sprayng spray arms under the wings, full cockpit, etc; now it weighs 12lbs and the original YS53 will not do the job. with all this in mind that 3 bladed prop you suggested should work?

Hobbsy

My Feedback: (102)

A Saito 100 would turn that prop with authority, an example. Graupner has 12x8 three blade with fairly wide blades that pulls hard and looks good too. Hobby-Lobby has the props and spinners to match.

smokingcrater

3 blades props aren't as efficient for two reasons:

1)smaller reynolds numbers, that can cause quite a bit of problems, but the more diameter and lower rpm, the better

2)more of your thrust is being wasted by hitting the engine/plane. Think of where the majority of your thrust comes from, 50-80% out on the prop, with a 3 blade you are running less diameter when compared to the equivilant 2 blade, so more is lost to drag off the airframe.

GPitts

My Feedback: (6)

but dont 3-bladed props pull much harder with the extra blade?

TLH101

My Feedback: (90)

You don't loose any thrust from it hitting the plane. The prop is pulling through the air like a screw. Adrian Page says that he can fly his Gee Bee design with a prop that is nearly as small as the diameter of the fuse. I have flown a large scale Gee Bee with a 15" diameter fuse on a 20" prop with no loss of effeciency.
A 3-blade prop is less effecient if you dont have the power to use a large enough prop to do the job. In the case of putting a 91 into a plane designed for a 52-60, you will never feel or notice the loss. Quite often it is done on scale type birds for that reason.

Richard L.

My Feedback: (24)

3-bladed props aren't efficient for one reason: prop wash

Take a look at the real P-40 for example. It spins an 11-foot 3-bladed prop at a maximum of 3000 rpm and flies at 362 mph. This translates to 31856 feet per minute. By the time the same prop blade comes around, the plane would have gone 10.62 feet from its previous position so that blade is biting into fresh air.

On an RC model, the engine is spinning at around 9500 rpm and the model flies at around 90 mph. This is 7920 feet per minute. By the time the same blade comes around, the plane would have only gone .83 feet or 10 inches from its previous location. As a result, that prop blade is flying in the wake of the blade ahead of it, biting into turbulent air which might have only half the amount of air molecules. Well, you can't push air if there is no air to push.

RCV "SP" four strokes, which spin at around 5000 rpm max, still lose efficiency when trying to spin multi-bladed props. Someone has tested this and concluded that a 3-bladed prop can lose as much as 33% efficiency over a 2-bladed prop.

Hobbsy

My Feedback: (102)

If it has moved forward 10 inches since the last blade was there, it is also biting clean undisturbed air.

Richard L.

My Feedback: (24)

Yeah, but that's with a single blade. Throw one or two more blades in, and you won't be biting into undisturbed air.

By the way, the most efficient prop to use in RC application is a single-bladed prop with a counter balance on the other end.

Richard L.

My Feedback: (24)

Also, keep in mind that turbulent air occupies a volume of space and not just a point, so 10 inches might be partially or completely filled with turbulent air. Throwing three or more prop blades into this volume of space is like putting three or more fishes into a small tank. They might not like it very much.

dasintex

My Feedback: (10)

Hey Guys; this is all great info, and I appreciate it; there seems to be a difference in opions whichs is always good to see as there is always more than one way to look at things; my main concern in using a 3 blade is for ground clearance and to retain scale appearnace; my plane is a 1/5 scale CropDuster which was powered by a YS53; but due to extensive scale additions which includes actual working spray arms, 2 smoke pumps that pumps/sprays water has made the plane weigh 12+ pounds; a 90 size 4stroke will be needed; I need an engine that will get the plane in the air and fly half decent; if I lose some power according to some, that may be alright because I doubt that I will fly this plane at full speed/throttle probably never reaching max rpm's; what do think?

Richard L.

My Feedback: (24)

You will lose some efficiency, but you will be fine using a 3-bladed prop on that 90 4-stroke. Go for it.

dustbin

One GOOD point that has not been mentioned is that your aircraft WILL BE noticebly QUIETER with the three-blader.

tango28charlie

Contrary to popular opinion, the reason why multi bladed props are less efficient than 2 bladed has nothing to do with the blades flying in disturbed air. It's because you need to use a smaller diameter 3-blade prop to load the engine the same as a larger diameter 2-blade. A larger diameter prop translates into a larger mass of air being moved by the prop for the same amount of engine power. If you used a 1-blade prop you could have an even bigger diameter and more efficient prop disc, but the plane would shake a lot besides looking ugly.
Charlie

Richard L.

My Feedback: (24)

Yes, it does has something to do with the blades flying in disturbed air. It's like trying to paddle a canoe up stream or in turbulent water.

You can move a larger mass of air by increasing the pitch even though the diameter has been reduced.

The plane would not shake if a couter balance is mounted on the opposite side of the blade. This has been done before.

tango28charlie

I disagree. I have yet to see a convincing argument that a blade interacts with shed vorticity from the previous blade unless the prop is near zero pitch or the airflow over the blade is stalled and the plane is not moving.

True, but when you increase the pitch you also increase the change in velocity of the air going through the prop disc at a given rpm and airspeed. Since the power dissipated goes up as the square of delta-V you're wasting power compared to a larger diameter prop with less pitch.

You missed my point. I was not saying that a single blade prop would not have a counterbalance. Without a counterbalance, not only would the plane shake, it would shake itself to pieces within a couple of engine revolutions. A 1-blade prop would still have more vibration than a 2-blade due to the fact that the thrust from the one blade is never applied along the centerline of the crankshaft, and induces a moment that rotates around the centerline of the crankshaft at engine RPM.
Charlie