What do 3 blades do in 3D that 2 blades dont?

[CommandingSloth]

I was just wondering, as it's not the most frequent sight, why do some heli's have 3 blades and some have 2? I'm referring to the 3D heli's in this case, not the scale, but you scale guys would know too. What difference would it make in 3D if you had 3 blades instead of 2? Just wondering, thanks!

[ahahn]

Usually the reason to go to more than 2 blades is due to clearance issues.

Obviously if you have the power, then a 3 blade hell will generate more thrust than a two blade hell, everything else being equal. True three blades will be somewhat (slightly?) less efficient than one blade, but efficiency sometimes just doesn't matter.

A secondary reason perhaps is that to generate the same thrust, the 3 blade hell will have a lower diameter rotor disk and/or rotate with a lower head speed. Both will reduce the gyroscopic effect of the rotor head which goes as the square of the rotor diameter and linear in the mass and rpm. The square factor in length should end up beating the 3 vs 2 mass factor I think. Anyhow if that is true, then it would make flips and rolls faster

I am not sure of the "real reason" 3D fliers do this, but since no one else has answered, I thought I'd start the ball rolling!

[extrapilot]

OP

There are a lot of caveats in a question like this. Are they the same blades? What about RPM? Etc

If you assume same blades and RPM, this breaks down just like wing area on an aircraft. If you have more wing area, you have the potential to put more energy into the air as lift. For 3D- if you add a blade, and keep the same headspeed, you have 50% more wing area, so you have the potential to put more thrust into the air. It is not that simple though, because it assumes you have enough power to generate this added thrust. Almost no stock machine has the power to do this in hover. It happens in hard acro maneuvers, but it will typically bog the motor. There is no performance benefit to stalling the rotor regardless of how much horsepower you have-you lose lift/thrust, which means, your acro performance drops.

Some people argue that 3 blades vs 2 means you have less control lag, because a blade passes any given point (i.e. the boom) in a given period of time. But at these headspeeds, we are talking about 2-5ms difference. It is not as if these rotors are spinning 100RPM where it takes it half a second to move to the position you command. Im sure the better pilots in the world can detect subtleties like this- but there is so much else in the way (radio framerate, FBL control loop time, servo reaction time, head stiffness, blade stiffness, etc) that this really needs proper logging to find out- on a case by case basis- how much an impact it has- good or bad.


Ahahn-

Some bad assumptions here mate. For example, if you do the math on say a 2-bladed 62” rotor vs a 3-bladed 50” rotor, for the same (acro level) thrust, assuming 2” chord, and 192g vs 150g blades, you have to run the 3-bladed head at about 2600rpm for the same collective pitch as the 62” runs at 2000RPM. When you do the math, the 2-bladed head sees effectively identical gyro torque as the 3-bladed head (within the noise margin- very dependent on blade mass/distribution). But for those rotor diameters/chord/RPMs, the 2-bladed system can generate more thrust, and do it with less horsepower for any typical thrust level.

Likewise, if you keep the same blades (go from 2 to 3 192g blades, 2” chord, 62” diameter), you can drop the headspeed from 2000 to 1730RPM for the same thrust. But gyro torque is substantially higher with the 3-bladed system. You also lose performance in FF/FFF with the lower tip speed.

[ahahn]

Originally Posted by extrapilot

OP

There are a lot of caveats in a question like this. Are they the same blades? What about RPM? Etc

...<snip>....

Ahahn-

Some bad assumptions here mate. For example, if you do the math on say a 2-bladed 62” rotor vs a 3-bladed 50” rotor, for the same (acro level) thrust, assuming 2” chord, and 192g vs 150g blades, you have to run the 3-bladed head at about 2600rpm for the same collective pitch as the 62” runs at 2000RPM. When you do the math, the 2-bladed head sees effectively identical gyro torque as the 3-bladed head (within the noise margin- very dependent on blade mass/distribution). But for those rotor diameters/chord/RPMs, the 2-bladed system can generate more thrust, and do it with less horsepower for any typical thrust level.

Likewise, if you keep the same blades (go from 2 to 3 192g blades, 2” chord, 62” diameter), you can drop the headspeed from 2000 to 1730RPM for the same thrust. But gyro torque is substantially higher with the 3-bladed system. You also lose performance in FF/FFF with the lower tip speed.

Ok, good points.

[CR22]

I spoke with Nick Maxwell briefly at the Snohomish flying regarding his tri-blade raptor he was flying that weekend. Told me the biggest difference with 3 blades is the speed of cyclic response and cyclic authority.

[Varc]

More blades more lifting power. I bet in some 3D maneuvers you would want more lifting power.

[CommandingSloth]

Originally Posted by Varc

More blades more lifting power. I bet in some 3D maneuvers you would want more lifting power.

Very basic but makes a lot of sense.

[desertstalker]

Originally Posted by Varc

More blades more lifting power. I bet in some 3D maneuvers you would want more lifting power.

Wider chord blades, more lifting power as well, without the expensive head.

[Jmann841]

Will a wider chord blade stall sooner?

I asked a few giys about running wider chord blades and they said when they tried it the blade stalled sooner and it did t not work well. Is this true or was something else possibly at play?

I saw the 3 blade SAB Goblin 700c fly today and was very impressed. It sounds awesome.

I asked Kyle Stacy about it and he said the only difference he notices is it feels more stable to him. I'm not exactly sure what that means or why it would feel that way, but, I do believe him as he can probably "feel" differences much better than I can.

[extrapilot]

On the stall thing, you have to control a lot of variables. The general rules say that for our Reynolds number range, stall angle is not changed much if at all- which is to say- chord length and AOA at stall are considered independent for this range of chord/flow speeds.

This is not quite true, in a perfect test environment with laminar flow, etc. You will see lots of chatter about laminar separation bubbles with low-speed glider wings etc. But we don’t live in that world- the rotor blades see all kinds of weird flow/turbulence by virtue of their operation.

The bigger factor in this question is similitude. That is, putting (2) 30” blades with 2.5” chord is not the same rotor solidity as (3) 30” blades of 2” chord- the later has 20% more total foil area, which means it has the potential to deliver more thrust just prior to stall (or the same thrust at lower AOA). But much depends on the blades- their foils, their mechanical properties, etc.

Likewise on stability- the rotor has substantial centrifugal component that requires cyclic to overcome. You would have to know the mass properties of the blades in question to know how this plays out- but if you just add a 3rd blade of the same (installed) type and run at unchanged headspeed, the centrifugal component will be higher for the 3-bladed setup. Doesn’t mean it is more stable- nothing about the head has been defined, etc.

[Jmann841]

Very cool, in speaking with the designer it was interesting to hear his take on the 3 blade head. He kept elluding to the benefit of it in F3C where lower headspeeds and additional stability is favored on a 3 blade head. I'm not sure why or how, but, it seems many F3C machines are a 3 blade setup.

He told me that for 3D flying he does not think 3 blades offers any great advantage over running 2 blades at a bit high HS. Kyle Stacy was running his 3 blade at 1950 rpm and runs his 2 blade at around 2150-2200. The designer stated the additional cost of the blades and rotor head is not really practical for most 3D pilots as it is substantially more expensive.

But, it does sound cool haha, and, in a hobby where "different" and "cool sounding" sell, I'm sure SAB will sell tons of their 3 blade heads on 3D machines as well as their F3C machine

[extrapilot]

A problem with this type of discussion is that that people are almost never talking about ‘like’ scenarios. That is- they may throw a 3rd blade on- but it is the same blade type as before- adding 50% more blade area (dropping blade loading by that same amount), and perhaps dropping RPM, etc.

And pretty much by definition, the 3-bladed head has no teeter on the grip, where the 2-bladed head generally has some (or a lot). Even if the blade area is controlled, you still run into questions about airfoils, mass distribution/gyro forces, etc. Not to say most of this cannot be controlled- it can. But you typically are not going to find 3-bladed heads running blades with 2/3 the chord of the 2-bladed variant of the same length, just because those blades are not all that useful to 2-bladed acro machines, etc. People end up talking about what amounts to very different machines- and attributing that to some inherent benefit of some blade count they find wonderful.

[Jmann841]

Haha yup, the blades they were running were considerably smaller I chord than the 2 blade setup. I forget the actual sizes, but, they were not a full size 700 blade on the SAB 3 blade head.

[FeriGGGG]

3 blades with only little to no leads in the tips are more agile solution for larger helis:
-better agility ...roll rates
-worse autorotation,
-worse smack,
all due to less weight in the tips = less inertia against fliping the rotating disc, this starts to be a limiting agility factor at size of 600 and above.
to my knowledge those kind of blades are only made by Nick Maxwell Helix Blades for the 3 bladed E700 rotor head.
Since standard FBL blades usually have leaded tips, there is no point in coverting to a 3bladed head without using special blades. probably there are some light tipped nonFBL blades out there.
also there is no need for more than 2 blades for 3D on smaller helis since they dont have much overall inertia, and mostly they dont have leads in the blade tips.

[brokenheli666]

I see it as "resolution" .. example.. say your at full forward elevator. each time the blade crosses the swash angle/servo it provides the pitch for that thrust direction... more blades crossing that "lets say, per second" more thrust "pulses" more thrust pulses more accurate the response..

fart fart fart per sec...... vs ... fart fart fart fart fart fart fart fart fart per sec.. lol

typically 2 blade to 3 blade conversions require smaller blades because the power system isnt being changed to handle the extra load of adding another of the same size blade..

i like big slow farts... not turbines

is that right? did it get it right? i hope so or this is making me sound like a goof

[Jmann841]

Not quite, typically for equal power you will spin the two blade head at a higher speed so than the 3 blade head.

The noise offset shouldn't be much if the 2 blade head is running faster than the 3 blade head.

[Icanfly]

pretty interesting, usually you go with a third blade to add lift or slow rpm because of increased blade area without larger disc diameter. If you consider you could go with two 3 inch chord blades or 3 two inch chord blades they have the same area, never is this the practice but always a third blade of the same chord as the original two.

[eric_burcham]

I realize I'm super late to the party here, but here is my suggestion:

Purchase a helicopter that has both 2 and 3-blade heads available, and run the manufacturer's recommended configurations on both, with regards to blades and head speeds. Swap them periodically. I suggest the GAUI X3L. Just feel it out. If you can't tell a difference, then practically speaking, there isn't one. You can always sell your 3-blade head and blades, and find out for yourself relatively cheaply if you like it our not.

Here is another practical thought: unless you are very good, you probably won't notice the difference.

Cheers.

[timewerx]

3 blade gives a bit of advantage when the headspeed is already maxed out (nearing structural limitation)

However, a 2 blade + lightweight wider chord blades (with suitable airfoil shape) would be better even against 3 blade.

The wider chord will give you higher Reynolds # at the same headspeed - this means the maximum lift coefficient and stall angle of the blade will be higher -- in other words, you'll be pulling more G's -- more G's = more spectacular 3D moves.