Can someone explain to me the difference a two bladed prop has with a three bladed prop, other than having one more blade.

I had a theorey, but I amy be totally off base, so please tell me if I'm wrong. Does a three bladed prop provide more acceleration and a two bladed prop provide more speed?

Is one a lot different to fly than the other?


I would appreciate any feed back.

Thanks,

Wings,

From what I've read, a three-bladed prop is not as efficient as a two-blader. There is one more blade to cause drag, it is heavier and you should reduce either the diameter or the pitch compared to the recommended two-blade prop you are replacing. There's also one more blade to balance!
Unless it is for pure aesthetic reasons, (looks cool), or scale competition, I would use the two-bladed prop recommended by the engine manufacturer.

Jesse

Yep, 3 blades are all about looks and ground clearance

Are you sure? The guy at the hobby store recommended a three bladed prop for beginners, I flew with a three bladed prop no problem. I put a 2 bladed prop on and it was very unstable at lower speeds. There must be a reason.

Wings,

Nope, your LHS guy was probably jumping on the 'alpha trainer' boat, seeign that it uses a three bladed prop. This particular prop was designed for teh beginners, and does in fact have three blades.

In reality, three blads are less efficeint than two blades. The speed in which our model engines run put teh following blade in the wake (turbulence) of teh previous blade's path. So, it never really get 'clean air' to bite. Therefore less effecient

Most (if not all) three bladed props are the same pitch as a two bladed prop. And true, the rule is to generally remove 1" from the diameter, and usually 1 from the pitch when coverting frome a 2 to a 3 bladed prop.

And, as stated, the three blade props allow for more ground clearance.

The question you should be asking is about teh pitch of teh prop..

The Alpha three bladed prop specifically is designed for "trainer like" performace. Look at the blades, they are very wide and squared off, just for starters.

When you take an Alpha trainer and remove the 3-blade prop and put on an APC 10x7 or 11x5 or even 10x6, you'll find the plane has a lot more power, more speed and more climb. But it's not all because of the 3 vs 2 thing, it's because that particular 3-blade is designed that way.

Look in the warbird section for pictures and you will find plenty of pictures of both 3 and 4 blade props, even some two blades on trainers. The Northrop XB35 had contrarotating 4 blades on the same shaft. That being said, also drum up the rotation speed of these multiblade props. They are often not even one tenth of the speed of a model engine so it strengthens Dr.Wogz statement that the trailing blade of multiblade props has to overcome the turbulence of the preceeding blade
so the fewer blades you have the more clear air. There was guy in about 1946 or 7 that put a single blade on a 65hp continental on a Piper Cub. It had a counter balancing weight opposite the blade on the hub. He easily proved that it was more efficient. What he couldn't overcome was the fact that the uneven pull on one side of the crankshaft knocked out bearings in about 1/3 the time of a normal 2 blade.

Keep in mind that on full scale planes, it was easier to put a three, four or more bladed propeller to develop the necessary thrust, rather than th have a much longer two bladed prop, which would be impractical due to the length the landing gear struts would have to be. Look at a WW II Corsair. The gull wing and multi blade prop allowed a larger total area for the prop blades to swing through the air ( the gull wing elevated the nose to clear a longer prop). keep in mind the RPMs those engines delivered compared to ours, and the fact that these blades were huge, compared to our tiny ones, so there was ample time for the air stream to clear before the next blade swept past. That's why you seldom see more than a two blade on our models. The Alpha trainer's three blade actually slows the plane, and is quieter than a two blade prop, but it is not as efficient. That was what the design accomplished though, as these planes do fly on the slow side with these props and are quieter, especially at low RPMS.

perportionaly, our engines with 3 -4 blades are = to the wwII fighters, their engines turned faster thus the larger circumference was cleared at the same ratio as the smaler one on our slower models. i only use a 3 blade cuz i got a big engine on a little plane and need the clearance, dont apreciate grinding the extra inch of the 2 blade against the ground as the engin turns over.

if you feel im wrong explain why you feel this way.

this is my reasoning simplified to show the processes running through my head.

note also that the longer a blade is the faster the tips move.

oh and i forgot to / by 3 but that doesent matter since they both had to be / by 3 so well just compare c instead of c/3 ok?

and even if everything isnt nice neat doubles in real life the advantage or disadvantage is so slim it really doesent matter. i personaly feel 3 blades are more stable when spining, but do cause mroe drag and do lower rpm and total speed as a result of both and the are heavier but this all breaks down to personaly oppinions and the fact that for what we use them for its not going to matter if you use a 2-3-4- or 35 blade prop. (well 35 may be pushing it?) also real planes have no impact here if they did we could also compaire weight [sm=wink.gif] and i no we dont want to go there. (unless you want to fly an aluminum brick off the runway.) also keep in mind the great difference in HP and tourque. the corsair could turn a much steeper blade too. no to mention were compairing compleatly differnet types of drive systems, multi cylinder 4 stroke with out put put half burning gas spewers.

Multiblade props are not as effecienct as single blade props. CL racers use a single blade prop with a mass balance for a good reason.

The more blades you have, the more prop drag, wich means less power transfered into thrust. Additionally, more blades mean the following blade is closer to the wake left by the leading blade, reducing the effeciencey of the blade. In general, you will always be better off with a two bladed prop vs a three blade with respect to effeciencey.

3 bladed props are used in RC for three reason I can think of off the top of my head: sound reduction, increased braking, ground clearance. Lower tip speed with a 3-blade reduced sound. The extra drag and more dense prop arc increase braking. Smaller diameters increase ground clearance.

3 bladed props also put less stress on engine bearings due to gyroscopic forces, but that is mainly a non-issue for RC, i.e. it is very hard to break a crank in flight with RC engines.

I don't know what's up with the 3-bladed trainer. My guess is they selected a 3 bladed prop for three reasons: increased ground clearance, controlable (i.e.reduced) performance and it looks differnet setting thier trainer apartf from the zillion other RTFs.

Cheers

it depends on the pitch and weight

Is the difference between a 2 and 3 blade noticable to the sunday flier type?

I agree with Crash90....I don't think it really matters to the easy flier. I would choose a 2 blade over the 3 blade for cost and ease of use.

I'm not exactly a "Sunday Flier Type." I can't really say who would and who wouldn't notice the change. This is a similar question to asking if a Sunday flier notices the differance between high/low wing loadings on the same plane, or higher nitro fuel, or high precision servor, etc. Anyone that is paying attention to how thier plane behaves should notice all of these changes, be it Chip Hyde or the average Sunday Pilot. A better question would be "Is the performance between 2-blade vs. 3-blade props significant enough for a Sunday Flier to worry about." To that I'd say no. There are differances between the props, and you will (should) notice the change, but the differance isn't enough to make/break the flight capability of a typical Sunday RC plane.

Yep4: What depends on pitch & weight?

Also, when trying to compare full-scale prop efficiencies to our R/C, the full-scale warbirds that swung a Hamilton Standard, or Curtis Electric for that matter, were equipped with automatically adjustable-pitch props. The Hamilton's being hydraulic and the Curtis' being mostly electric. As the variables on flight changed, such as speed, angle of attack, etc, so did the pitch of the blades to keep the engine in its proper power band. Our little engines have to contend with fixed-pitch props which are a compromise between thrust and speed.

Jesse

I have a 1/3 sukhoi from H9. I am installing the BME 110 xtreme. The CF landing gear I got from graphtech is 1" shorter than the stock gear and I am afraid that I will need a 3 blade prop for ground clearance. I hope it doesn't hinder performance too much.

I am a beginner at glow engines. I can say that I flew it just fine a few times till I broke the three bladed prop. I replaced it with a 2 bladed prop and crashed a few seconds after taking off. I repaird he plane, put on a three bladed prop and and have flown it fine, no crashes. Could be purely coincidence, or maybe there is something to it. Who knows. I just know I'm sticking with a three bladed prop untill I become more confident.

Thanks for all the info. I will tend to probably agree that it don't make a hill a beans worth a difference realistically. I am just a little superstitious I guess, lol.

Take care,



Wings,

thanks for the info people.
Very helpful.

... and yeah, when are they going to produce those variable pitch props for the r/c world anyway... LOL

=D

Mr B.

Wings,
What you haven't told us is what other changes were made besides 2 vs 3 blades.

What was the diameter/pitch of the 2 bladed prop?
Diameter/pitch of the 3 bladed prop?
Did you get sufficient speed before pulling up elevator with the 2 blade prop?
Did the plane pull or roll to the left when you took off? (Did it drop the left wing before it hit the ground?)

There are a number of variables that could have caused you to crash with the 2 bladed prop. In your second post, this statement made me think that the pitch of the 2 bladed prop was quite a bit higher.

If you fly a high pitched prop too slowly, the blades of the prop will actually stall, just like a wing will stall if flown too slowly. If the blades of the prop stall, the prop doesn't pull the plane.

You may have been flying on the edge of a stalled prop while flying slowly.

In any case, trust the answers you've been getting. 2 blades are more efficient than 3.... All the other stuff.
Dennis-
Dennis-

Ya Dennis, I agree there are probably 100 variables.

It was rolling to the left. Since I am new my instinct kicked in since I was so low to the ground. Unfortunetely my instinct is geared for my little electric plane. Way less sensitive. So I was over correcting. Roll left , roll right. left. right .left. right. smash!

LOL, I think It lot had to do with operator error. But I never had this problem with the three bladed prop. The two bladed prop probably did have a different pitch. I am confident that all the info above is good info. I think it was a combo of having a different prop and lack of experience.



Wings

I think it bears saying again that there are 3-bladed props and then there is the specific prop that comes with the Evo trainer power system.

You can get APC or MAS or other brand 3 blade props, and they perform much like their two-blade counterparts, though they aren't quite as efficient. But the difference is small. The 3 bladed props you see on IMAC planes, for instance, are designed with the same goals in mind as a 2 blade prop, that is, to acheive the best possible performace. So guys use 3 bladed props to reduce noise and improve ground clearance and such, and get performace that is very close to a 2 bladed prop.

However, the Evo TPS 3-bladed prop was designed totally differently. It is NOT designed to be a direct replacement for a two bladed prop. It's not desined to maximize performace. It's designed to keep the top speed of the plane down and make the plane "beginner friendly". As far as I know, there are no 2 bladed props that are designed the way the Evo prop is designed, so there are NO props out there that would be a direct comparison to the Evo prop.

In other words, the Evo prop is different. It's not just the blade count, it's the whole prop design.

So, when talking about an Evo engine and "2 vs 3" blade props, it's important to make clear if we are talking about 3 bladers in general, or that one propeller design specifically.

I have tried several props and found that a 10 x 6 is pretty good.

Now, if I went to a 11 x 6 would I get more thrust same speed?
More thrust more speed?

What would change?

I understand larger pitch is faster speed and larger diameter is more thrust.

When you start changing diameter and pitch at the same time, I get confused.

Wings,

11x6 will turn slower than a 10x6. For example, the 10x6 will turn at 15krpm, and the 11x6 at 12krpm, Thus, theoretically, you loose 3k x 6 inches per minute in speed... which is 45k cm per minute, which is 27km/h. Thus, you will lose 27km/h top speed by going from a 10x6 to an 11x6 (theoretically).

You will get a more efficient prop though, so your thrust will probably be more with an 11x6. Your plane will be more responsive, accellerate faster, slow down faster, and be able to do better torque-rolls .... ;-)

At some point though, if your prop diameter gets too big, two things will happen:
1. Your prop will hit the ground....
2. Your engine will not be ale to turn it fast enough to be efficient.

So, play around, and find out what your flying style like to have n the nose. Typically, stick with the ange of props suggested in the manual. The Big Diameter, small pitch prop will give you agility, the small diameter large pitch one will give you speed.

gus

The biggest difference between a 3 blade to a 2 blade prop is ..............1

gus is pretty much right on. Moving to a bigger prop usually means less RPM, which might or might not fly "better" in the air. Also, a whole lot depends on the engine.

As an example, I was playing with props last weekend on my old Cap 21 that has an old OS .61FS in the nose. (that's the pre-surpass, 2nd generation 4-stroke).

I started with an APC 12x6. The prop tached at 9,600 on the ground and flew pretty well, but I had too much speed in horizontal lines and not enough pull in verticals.

I popped on a Zinger 13x5. The engine still tached at around 9,600. In the air, climb was a hair better, but the plane didn't have the performance I was looking for, it was too slow in level flight.

Switched to an APC 13x6. The engine still tached at 9,600. And I had more pull in the verticals than either previous prop, and the level flight speed was actually a bit less than the 12x6, but not much. Overall, it was the best prop so far. (The 12x6 was unloading more in the air apparently, leading to a slightly higher airspeed vs the 13x6, even though they tached the same on the ground).

Another guy at the field had an old 14x8 (we think, it was unlabeled). Put that on. Tached around 6,500 on the ground. In the air, the plane was ok, but climb and level speed were only so-so. Both the 12x6 and 13x6 were better.

So, I found out that this engine really doesn't want to run faster than 10k, no matter what prop is on there. The lack of RPM change from the 12x6 to 13x6 tells me the 12x6 is really too small for the engine. Not small enough to hurt anything, but I can do better. The 14x8 was too big. If I wanted to fo fast, a 12x8 wouldn't be out of the question, but that's not what I'm looking for. I gotta get my hands on a 14x6 to see how that runs. Based on the 13x5, I'm not going to bother with a 14x5.