ironj21

My Feedback: (1)

Hey, can anyone tell me the differences between 2 and 3 blade props. My evolution engine came with a 3 blade prop, but thier kind of expensive and i am thinking of just using the wooden two blades. Also, is there much of a difference between wood and plastic props? thanks
-Jamie

--={Luft}=--

Ok if igot it right i think a three blad plopellers will be able to improve the speed of your airplane, but they would be harder for the motor to turn them then a two plopeller blade, therefore a more powerfull motor would be ideal for three blade plopellers. If i am wrong sombody plz correct me.

siclick33

I read somewhere that a three blade prop is the equivalant of an inch wider diameter or one inch of pitch coarser. I don't know if that is true but it gives you an idea.

i.e. three blade 10x6
two blade equivalent 11x6 or 10x7

I think the evolution uses the three blade prop to improve ground clearance.

Geistware

My Feedback: (16)

Basically a three blade propeller the same size as a two blade will have a highter loading and more thrust that its two blade counterpart.

im_a_rcav8r

The general rule for 2 blade prop vs. 3 blade prop is to take 1 inch away on one side of the equation. If you are using a 10X6 2 blade prop and want to go to a three blade you will need a 9X6 or a 10X5.

As for the 2nd part of your question, the diference in plastic and wood is quite small with a smaller engine. The more powerful the engine the more difference it will make. The more stiff your prop is the better the response will be. If you have a flexible prop like a master air screw the prop will flex and not deliver as high performance as would a wood or APC prop. However for learning, the master air screw is a good prop. It is forgiving if you bounce it off the runway on a hard landing or nose over. The wood and APC will chip and or break and be unusable with only a slight prop strike...

I hope this information is helpful to you...

Good luck

smokingcrater

also, a 3 bladed prop sized to the equivilent 2 bladed prop will NOT have the same power. 3 blades are always going to be less efficient, both due to reynolds numbers but also the fact that more of your prop blast is now directly hitting the fuselage, causing increased drag. so in other words, only run 3 blades if you have a little extra power to spare. if you want the absolute most out of your engine, run 2.

RCaillouet3

Well the answer to this is quite confusing, but can be very simple! In simple terms, a 3 blade prop for RC use generally run more effeiciently than a 2 blade prop. Before everyone starts jumping on me let me explain!

1. 3 blade props will generally require less energy from the engine to turn it do to a smaller thinner blade, thus less fuel burn. Now the down side is that a 3 blade prop doesn't hold it's rpm very well in aerobatic manuevers, thus not allowing the airplane to retain energy through the manuever.

2. A 3 blade prop is generally designed to run at a constant rpm for cruise flight. Do to aerodynamic energy, everytime you make a turn, the lighter weight prop will either bog down or speed up slightly. For most RC flight this is not that important, except for aerobatics.

3. 3 Blade props can genarlly be made to make an airplane run a little faster do to slightly less drag on the prop and slightly lighter weight.

4. A 2 blade prop is generally thicker in blade size, longer in blade length, and heavier for the same engine size requirements. Therefore, if you plan on doing an aerobatics, a 2 blade prop will carry a more constant rpm giving you airplane more torque through manuvers. This extra torque generally pulls an airplane through a manuver with less energy loss.

5. 2 blade props require more energy from the engine to maintain thus a higher fuel burn rate than a 3 Blade.

Now all of this being said, for RC use a 3 blade prop will make the engine run easier, faster, and with less fuel burn, but with a loss of avialable torque than a 2 blade prop. A 2 blade prop will generally give you better climb, stronger manuver performance, and more drag than a 3 blade prop.

Now if you want an airplane engine to run for longer, faster flights go with an appropriate 3 blade prop. If you want stronger, slower more aerobatic flight go with an appropriate 2 blade.

This is all assuming that you are using the appropriate 3 blade or 2 blade prop for your engine!

Reg

smokingcrater

The recent record setting flight across the atlantic used a two blade, and it is common in very high speed events (pylon) to use a SINGLE blade prop!! (with a counterweight)

RCaillouet3

Never said that there weren't exceptions. Was just giving some bascis is all.

Reg

forestroke

I agree with Kramer, it is general knowledge that given the same engine RPM, 2 blade props are more efficient and that's why racers sometime have one blade props.

The only difference is in the ground clearance. For the 2 blade props, the diameter is higher and thus the ground clearance required is higher. 3-4-5 blade props all allow more ground clearance while still allowing the engine to operate at optimum RPM.

Thus, for beginners, 3 blade prop is fine as long as it's not that much more expensive. Just imagine how much more prop you would be grinding off after each landing if the prop was half inch-one inch longer :-)

forestroke

Sorry don't mean that 2 blades are more efficient but that fewer props the better in terms of overall efficiency. For that matter, higher pitch as well... but at the end of the day, it just depends on what you want it for.

small prop, high pitch = racer

large prop, low pitch = glider towing or 3D

3-5 blade prop = looks nice :-) and could be better for planes that have a big engine and want a little more prop clearance and don't want all that torquing that comes from big pitches

just my two cents! (which is probably worth less than that!)

DBCherry

My Feedback: (4)

Problem with your theory; you forgot that it has an extra blade, so thinner or not, the three bladed will be heavier, so will require more energy to turn. (The 3 blades should also carry about the same amount of "flywheel" effect as two slightly larger blades, so I have to dispute that claim too.
"generally designed to run at constant speed and constant rpm for cruise flight." ?? Didn't know that RC prop manufacturers designed props specifically for anything other than plain flying. "Aerodynamic energy" ??
Absolutely NOT true. It's been well proven that a single blade prop is more efficient (less drag) than a two bladed, so how could a three bladed prop have less drag than a two bladed? Again, three blades are heavier than two.
Again I disagree about weight. You MIGHT get a bit more flywheel effect, but I'm not convinced.

I think you'll find that the only reason a two bladed prop is better for aerobatics is because a two bladed prop is just more efficient than a 3 bladed prop.

It might take a miniscule amount more energy to get a two bladed prop up to speed, but you just said that a two bladed prop holds more energy, therefore it will take LESS energy to keep it turning. So using your previous argument, it should burn less fuel.

The bottom line? A 3 bladed prop is less efficient due to the blades being closer together and "cavitating" in the disturbed airflow caused by the preceding blade. The ONLY reason to use a 3 bladed prop in RC is to gain ground clearance.
Dennis-

quint-rcu

My Feedback: (24)

Let's not forget that another reason for running a thee-bladed prop is scale looks without changing props and the ever popular 'I just want to...

quint

ironj21

My Feedback: (1)

I am a little confused...if a two blade prop is more efficient than a three blade, then why do higher performance full scale aircraft that require more power and higher cruising speeds use 3 blades? If you look, you will see that basic Cessna's and Pipers usually have 2 blade props and have lower crusing speeds, where as a higher performance single engine aircraft almost always have 3 blades and are capable of much higher airspeeds. If two blades were more efficient, why would they use 3+ blade props on real aircraft?

Farmbuyer

In my limited experience, three bladed props seem to be quieter in operation. Not exactly sure why unless the tip velocity is lower. Noise kind of matters where I fly.

RCaillouet3

Well I must admit that some of the things I said were a little more confusing than I meant, sorry was half asleep.

I guess a little more clarification is in order. My info might be a litlle old here (chart is 5 years old,) but Master Airscrew reccomended a 2 blade 10X6 or a 3 blade 9X4 for a standard 40 size motor. Now this is where some of my basis was formed on. Also, if you used an exact copy (10X6) from 2 blade to 3 blade, your engine would not run as well due to the now same (10X6) prop WOULD be heavier in 3 blade config due to the extra blade.

A 9X4 3 blade Master Airscrew has very thin and short blades. Now what I was meaning by my "constant rpm" comment is that if you hook a engine to a test stand and run it at a constant throttle setting to roughly simulate unaccelerated, steady level flight the prop becomes very effecient with how it lets then engine run from a fuel and power output case. As soon as you change ANY variable, (pitch, bank angle, airspeed, air density, altitude, etc. etc.) the engine will gain or loose rpm at a greater rate than 2 blade prop that is correct for that size engine.

I didn't mean that the prop could change it's pitch to maintain a steady rpm. I was just saying that due to any change of variables, the aerodynamic effect is to raise or lower a props rpm, and a heavier prop would resist that change more is all. This resistance to change rpm is what gives a larger prop more torque for manuevers (among other factors as well!!!!! This just happens to be one factor.)

An easy way to test this is to take an airplane, check with the engine manufacturer and get a 3 blade prop they reccomend. Fly the motor at a constant throttle setting. Let the airplane fly in level flight and watch the speed, then pull the airplane into the vertical and watch the rate that it looses energy. Then switch to the 2 blade prop and do the same. Now this must be done at a throttle setting that will allow the airplane to stall (as some of us tend to have an airplane that will continue to go vertical until out of sight!)

Now as too a single blade prop, theoretically I guess it should work, but have never seen a single blade prop (except after a Pilot Error Encounter!)

But I do have one question about it though, would the couner weight that would be used have MORE effective drag due to the less effecient aerofoil as it passes thru the air while spinning?

The recent Transatlantic flight from what I have been leaed to believe used a 2 blade wooden prop. Now I can understand this for the fact that because of the prop is heaveir it resists change more than a lighter prop. This would give the airplane a more consistent rpm and therefore make it easier to track progress to do one less variable. I am not sure if they had control of the airplane once out of transmitter range (via a gps linkup???) so if the rpm changed even 400 rpm, with no way to correct it, this could have generated some severe problems in the form of planning.

As for why Cessna's come with 3 blades, it is for the reasons above. As for all of them having 3 blades, no sorry. I did my primary and instrument training in a Cessna 152 and 172 that had 2 blade props!

I now fly a Fairchild Metro III as a career. I have now logged over 1500 hours in the Metro, and nearly 4000 total. Now I am just starting along the path! But along the way i have noticed some rather interesting things that are relavent to this discussion in some ways.
The Metro has 2 prop config. Dowty manufactured and McCauley manufactured are both used on the Metro III. Both are 4 blade configurations. The difference being that the McCauley has a thinner, lighter blade. This being said, the Mcauley props are a BEAR to keep in sync with one another. EVERYTIME you make a pitch, airspeed, bank, alititude change the props get slightly out of synch due to the bladxes gaining or losing rpm slightly. The Dowty props are wider and heavier, and once you get them in synch with one another, they stay for the most part.

Now I don't know all that there is to the different things that go on with a prop. But for RC use, a 3 blade prop gives you slight speed advantage, with a slight torque loss to an APPROPRIRATE 2 blade prop for that engine. This loss is NOT much at all! Just need to look at what your needs are. Doing heavy aerobatics? Go with a large 2 blade prop. Need speed? Go with an appropriate 3 blade!

One of the things I love about this hobby, is the experimentation should NEVER end! I know that if I ever get to the point that I feel I have figured it all out, I am taking up BOWLING!!!!


Reg

FHHuber

this one is full of fallacies.

The further explainations by the same author just make it worse... they don't improve anything.

**********

The reason to go to more than 2 blades is to be able to use a larger engine and not have the prop blade tips hit the ground without putting the airplane on landing gear that look like stilts.

A perfect example is the WWII Corsair. They used the inverted gull wing "dihedral" to help minimize the length of the landing gear. They used the 4 blade prop because a 2 balde prop appropriate for the engine would have had 2 feet longer main wheel struts needed. They wanted SHORTER landing gear to take the stresses of carrier landings, so the only answer that worked was the 4 blade prop. (and the plane was still a dog for attempting carrier landings... so it was almost exclusively operated from land bases.)

The Cessnas with 3 blade props simply use them because the correct size 2 blade prop would require longer landing gear. That's more weight and more drag.

The turbo-prop aircraft with 4 to 7 blade props would need double to triple the landing gear height to have a 2 blade prop clear the ground. the engineering of such a landing gear system is a nightmare.

The mose efficeint prop is one-bladed. However the dynamic balancing problems of such a propeller are too much of a problem. The 2 blade prop is the best answer to the dynamic balancing needs. Some aircraft have used single blade props and had good results in flight, but the stresses on the prop shaft caused premature bearing failures or cracked shafts (or both).

DBCherry

My Feedback: (4)

Thanks FH. You backed up what I said, and did it so eloquently.
Dennis-

Montague

I agree with Dennis and FH, the fewer blades, the more efficient, and the better the power output.

3 bladed props do help with sound as someone mentioned due to lower prop tip speeds. When the tip gets near (if I recall correctly) .6 mach, it starts making a LOT of noise, so going to a smaller diameter, either with higher pitch or more blades, or both, helps. This is why you see them on IMAC planes, they now get scored on noise in competition.

3 bladed props also help with braking in down lines. The more prop area you have, the more braking effect.

This also accounts for the Alpha's prop. The Alpha's prop is a very special case. Next time you see one, note that the blade shape looks nothing like other props you see. They are closer to oars than propellers. The Alpha's prop is specifially set up to PREVENT high speed flight, and reduce the thrust of the engine a bit while still allowing enough power for a decent rate of climb. A more "normal" 3-bladed prop will do some of the same things, but not to the degree that the Alpha's prop does.

Btw, if you want proof, just take the Alpha's 3-blader off, and put on a 10x7APC, and hold on. (A better choice for training would be an 11x5. The 10x7 makes the Alpha go like a rocket)

FHHuber

Also... look for my thread on "Choosing a Propeller" (its scrolled way down to never-never land somewhere in this forum... maybe page 16 now)

Montague

There are a TON of prop threads on here. doesn't mean anyone is going to scroll down to read them .