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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › 3 blade heads why the new thing ?
07-06-2014 06:51 PM  6 years ago
wjvail

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That is completely wrong.
When you balance blades . . . you are in fact balancing the moments.
What you have describe is "static equilibrium". The study of statics is the study of bodies not accelerating. Notice this doesn't say bodies not moving. It says bodies not changing velocity. They can be going 100mph and be referred to as static if that velocity is not changing. For a body to not be accelerating the sum of the forces in all three axis must equal zero and the sum of all the moments about all three axis must also be zero. This is what you have achieved with radius x mass = radius x mass. The use of a teeter balance will be effective if you concede the rotor head never changes rpm or plane of rotation.

Dynamics is the study of bodies that are accelerating and the sum of the forces and moments does not equal zero.

While a teeter balance works for a rotor head that isn't changing velocity (rpm or plane of rotation), it is not a perfect solution for a head that is.

Also... Please find a less confrontation way to address me than just announce I am "completely wrong". I am often wrong but I don't like the tone. I have found telling people they are *ucked up or announcing to a group they are wrong, creates a barrier to conversation and generally shows a lack of respect. I am a pilot by trade and have a degree in Mathematics with a minor in Physics. That is to say, while I have considerable formal training in these areas, it was many years ago and I don't use these skills daily in my job. I will accept correction but not insults.
"Well, nothing bad can happen now."
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07-06-2014 07:01 PM  6 years ago
AirWolfRC

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I have tried numerous times to present the right information but some people just don't get it. I don't have the patience of Job.

When I find a statement that is "completely wrong", I will call it what it is. If you don't like that, then quit reacting like a 10 year old with "sensitive feelings".

How's that for being "confrontational" ?
It's hard to have respect for false information . . .

As far as qualifications go . . . yea . . . me too but my degrees are ME & EE
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07-06-2014 10:18 PM  6 years ago
BobOD

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AirWolfRC, when a balanced pair of blades with different CG points goes through an angular acceleration, is the torque applied to the mast exactly the same for both blades? I'm curious if you feel they would be the same or just negligibly different.Team POP Secret
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07-06-2014 11:46 PM  6 years ago
AirWolfRC

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They would be exactly the same . . . and that's not a feeling but a fact.

That's what moment of inertia is all about.
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07-07-2014 12:47 AM  6 years ago
BobOD

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But, if the CG is in a different location, your moment of inertia is not the same. That is what moment of inertia is all about.

Torque = mass x distance squared x angular acceleration

The torque will not be the same for both blades.
Team POP Secret
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07-07-2014 12:58 AM  6 years ago
AirWolfRC

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? ? ? torque is only distance x force - - like ft lbs

moment of inertia is a mass at a distance from a pivot (like torque)

there's no "squared" in there anywhere.

You're thinking of centrifugal force . . . different animal.
. . . and then it's F = M x D X angular-velocity²
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07-07-2014 01:13 AM  6 years ago
BobOD

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I'm not talking centrifugal force. I am talking about moment of inertia...which is mass x r squared.
And the torque equation above is correct.

You are talking about a static moment...mass x r
Team POP Secret
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07-07-2014 01:15 AM  6 years ago
AirWolfRC

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Moment of Inertia does not "square" anything. I guess I shouldn't use the word "inertia".

Let's keep it simple and focused on the immediate question and simply call it Moment.

We're not talking about "with a given input torque, how long does it take for a rotor of a certain moment of inertia to come up to a certain speed?"

We're only talking about balance to eliminate vibration.
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07-07-2014 01:22 AM  6 years ago
wjvail

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Good bye."Well, nothing bad can happen now."
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07-07-2014 01:47 AM  6 years ago
AirWolfRC

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Ok, you got me digging deeper.
Let's say you are in a steady state flight condition, everything is smooth. You pull collective. The head speed slows slightly. Because the blades have a slightly different Polar Moment of Inertia, one blade will lead the other slightly limited only by centrifugal force. The offset to the line between the two blade CG's from the center of the main shaft will be the tangent of the angle difference between lead/lag angles of the blades and cause a slight temporary vibration.

As soon as the speed variation has steadied , everything will be back to normal.

How much is this perturbation ? Off hand I would call it splitting a gnat hair.
We don't have that kind of budget for equipment to find out.
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07-07-2014 01:53 AM  6 years ago
icanfly

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AirWolfRC, when a balanced pair of blades with different CG points goes through an angular acceleration, is the torque applied to the mast exactly the same for both blades? I'm curious if you feel they would be the same or just negligibly different.
it appears my masters degree in meta physics is in call to answering this, well, ok, maybe a its just a simple understanding of COG. If you have a single or multitude of weights extending 90deg from a rotating axis with counter weighting in any way to keep the vertical axis perfectly centered when rotating, the shape of the extended portions at 90deg can take on a airfoil blade shape to provide lift in a rotor disc. One or a hundred as long as each protruding span is balanced to maintain center at the mast.

This is fantastic you guys have taken this topic this far as it is absolutely information assisting yours truly in personal experimentation. With 2 blades you have two lift surfaces so no weight is added other than to perfectly center the total span at the mast axis. It is the discs COG regardless of number, chord, and length. The word EQUILIBRIUM comes to mind.

pretty simple huh.

you guys are great
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07-07-2014 02:02 AM  6 years ago
BobOD

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Well, I was talking about inertial forces on a disk which is rotating and accelerating on all 3 axis...as were a few others in this thread.
The math says balancing the moment without balancing the inertia can lead to vibration.
One can question the significance but first we need to realize they are not the same.
Team POP Secret
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07-07-2014 03:49 AM  6 years ago
turboomni

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Geeze ,,,on my 3 blade auto gyro I made my own blades. I added plumbers solder of equal lengths to the leading edge of each three blades near the tip. I then balanced them length wise to match them with tape so they have the same CG point. I then put pins in each end to find the the CG end to end and drew a line. The center of balance line at the root of the blade was the bolt hole point. I then balanced on a 2 blade balancer to find the heaviest blade of the three blades. Once found I added tape at the CG point of the light blades. It seemed to work fine ,,,is this the wrong approach?Setup is everything, All my heli's can fly far better than I can pilot them
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07-07-2014 04:03 AM  6 years ago
icanfly

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balancing the inertia
aren't the moment and inertial moment the same thing, polar cog? I do understand what your driving at, whether a blade if not perfectly matched in a set of three will create an imbalance, due to lead lag and blade cog. Something about the span wise hinge point on a blade has a lot to do with this. A small amount of tracking issue would be the result would it not? Would that make the gyro go crazy and the heli hard to fly? Probably not.

I'm not sure it would matter all that much ateotd.

I once mismatched a pair of blades cog wise and no noticeable affects arose, until I bent the feathering shaft some flights later with a blade touch on a tree.

To test the amount of offset at polar cog of a rotating assembly, rotor disc, you can put a dot at absolute center. A high speed camera overhead and a lazer pointed on the dot. The camera can detect any wandering as the dot will reduce in size as less light will be reflected back from dead center. You can also use a strobe and a alternating black and white 1/4 crescent at center and do pretty much the same thing. Bolt the heli down, drape a plumb over tdc, and see where the mast center moves if you have any doubts.

I try to maintain a sharp eye on the button when spooling up, what's nasty about vibes at the mast moment of inertia (sorry, AXIS) is that vibes will get extreme all of a sudden when rpm is increased and the mast turns into a wet noodle.

I think if you wanted to go crazy on this topic you will have to go to aircraft multi blade prop making and balancing to get a clear view of this matter. Make your own judgment after that on whether the tri blade is for you. I'm going for four as any two opposing blades with equal balance, weight , and cog are not going to affect any other opposing pair in a rotor. I can't tell you how far this concept could be stretched but hoo-wee on a 700 a second pair would add almost another lb to the heli, is that worth it, 200grams and all each?

One last thing, because the blades of a 700 in a disc will lift about 2lbs per square foot, the same area can be achieved with smaller blades, narrower chord (equaling the area of two wide ones), whose foil is more efficient than a wide chord pair due to stall, ain't it neat?
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07-07-2014 03:17 PM  6 years ago
BobOD

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aren't the moment and inertial moment the same thing,
As noted above, no.
Team POP Secret
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07-07-2014 03:48 PM  6 years ago
AirWolfRC

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Keep in mind that a difference in Inertial Moments only matters while the rpm is changing.

Tracking issues are aerodynamic . . . a different animal.
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07-07-2014 04:08 PM  6 years ago
BobOD

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Pitch and roll accelerations as well.Team POP Secret
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07-07-2014 05:05 PM  6 years ago
icanfly

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Pitch and roll accelerations as well.
is affected on a disc weighing 150% more, pitch remains on any number but momentum dictates response.

Regarding angular momentum, the motor will have to work harder to swing longer blades correct? I have a 450 that had Revolution 325s last year, super nice compared to hk 325 Align knock offs. I accidentally damaged the tips to about 70mm making them 280s. I cut them back, cleaned them up, did something extra to the tips and put them back on the heli. You may not think highly of this experiment as I did not re-install tip weights, the 450 is fbl. The result was that the heli flew great, the motor worked less, the heli more responsive, though it did not auto well due to lost angular momentum and blade area. They were touched down and broke again so I cut them to 250mms for my 300. A windy day sent my heli into a flower retainer and the blades completely split open to the root. Man they were the BEST.

Somewhere in all of this going on about technicalities and the art of BALANCE and what blade configuration performs best to satisfy a flyers taste is a point made best when a flyer actually TRIES things first hand, Tri, get it..

Again, 4 blades are a whole lot simpler to maintain balance wise, a no brainer really, that the smaller diameter rotor achieved with narrower chord blades nets an advantage in several ways, but it's possible to do this with better results when going to larger than 600 sizes.

You don't have a clue about the 1000 heli I'm considering but one with 4 700mm blades instead of expensive 900 or custom made blades. Even the 700 blade prototype (I have in a 600mm berp style) xs 4 will exceed the performance of just two 1000mm ones.

GO FLY SOME.

Is this topic about building a better mouse trap? See all the new traps that have been made since about 15 years ago? Don't get stuck.

You know, if more co's made a 4 blade heli way back when, the fb would never have gone far. What would you rather fix, a fb? or a set of blades? Ahh, but we're running into a packing and storage problem to take on/off a blade every time the heli gets put away for transport and or storage,,,,,,,,,,, ok, 3.

By the By, after giving this 4 blade non folding aspect some thought I came up with an idea that will permit 4 blades to fold without removal, one of two ways really, ny-eece.
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