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HelicopterMain Discussion › Blade balancing
04-18-2017 07:30 PM  6 months agoPost 21
MattJen

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airwolf
I have read your essays many times.
I have done loads of testing, loads of flying, you always seem to post your theories but never anything except copied and pasted from sites.

I have done, flying mega hours in all types of configurations and all types heads, multi blades, different head speeds and using vibe sensors.On all types of models from 30 size pod and boom up to large 20Kilo plus models, I have also done aerial camera work using 4K plus cameras in the early days where ANY vibe would blur the picture at that resolution..

On a turbine camera platform with perfectly balanced blades, using a very high resolution camera I put a couple of wraps of tape on tip deliberately to create an imbalance and change c of g, the picture picked up vibes and the pic was blurred.
I removed the tape and picture in same conditions was perfect.

Your adding to tip will be ok at a certain RPM, but as that RPM changes the moment of inertial (AFAIK) will change and blades will come out of track and thus add vibes, over a period of time you will cause premature wear on bearings and other parts..

I don't drink CoolAid maybe coke zero LOL

All The Best

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04-18-2017 07:45 PM  6 months agoPost 22
AirWolfRC

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I think you need to do more homework on rotary dynamics.

Balance (equal moments) will not change with rpm - that's a physical characteristic of the blade like weight and size.

The moment of inertia does not change with rpm - also a physical charisteristic.

Moment of inertia will affect the rotation dynamics only for instantaneous rpm changes.
- - - I'm talking about the instantaneous changes in speed when in forward flight and the advancing blade slows slightly and the retreating blade speeds up slightly. That is the only time Moment of Inertia will have any effect (if it can even be noticed).

By the way, if you want to fully drink the koolaid, you should also be making the fore/aft CG of the blades equal.
- - - if you don't you will change the polar moment about the blades aerodynamic center.
- - - but that is not that big a problem because all symetric blades on model helis are NACA00xx series airfoils that have a center of pressure that does not move for/aft with changes in angle of attack.

But I have never heard anyone talking about that last bit.

Any questions ?

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04-18-2017 07:46 PM  6 months agoPost 23
MattJen

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airwlf your clouding the issue.. WITH blurb which is what you always do..

Adding weight at the tip changes the c of g, a blade set with mismatched c of g will cause vibes END OF STORY....

Try it and go flying, having looked through your post history, its all theory taken from sites, I have never seen you talk about your flying, your testing on models, your testing in different conditions with different blades in different configurations...

Moment of inertia will affect the rotation dynamics only as instantaneous rpm changes.
- - - I'm talking about the instantaneous changes in speed when in forward flight and the advancing blade slows slightly and the retreating blade speeds up slightly. That is the only time Moment of Inertia will have any effect (if it can even be noticed).

thank you, for proving and finally agreeing my point.

All The Best

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04-18-2017 07:49 PM  6 months agoPost 24
AirWolfRC

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You want an end to blade balancing ?

Try this - -

This photo is from Model Aviation, Oct. 1987 pg. 102.

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04-18-2017 07:53 PM  6 months agoPost 25
MattJen

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yes I read that and did it on an eco 8

as said, at a certain RPM which that article accepts it will fly, but you change the rpm and you have issues.

The article also says flew QUITE well, not perfect.

Also what you have to realise is at a certain RPM it will balance, but you change the RPM which on a turbine you do, as it is inherent laggy.

That imbalance will become more pronounced which is what the one blade article said, whilst at a constant rpm it was fine...

I suppose a good illustration is if I tap you on the head with a brick it aint gonna hurt, but if I lob it at you from a high building it is gonna do some damage, why ? it still is the same brick? Answer because the mass has changed as a heavy blade would with faster rpm, there will be a sweet balance point between the weight and rpm but as battery power drops so does RPM, or in a turbine as you add load the ECU plays catch up, or doing extreme 3D manouvers the rpm will change, hence the mass will change with RPM, now in my Jetcat that rpm changed by 150RPM.. that's how I could feel the difference.

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04-18-2017 07:55 PM  6 months agoPost 26
AirWolfRC

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Come'on over some day and I'll prove otherwise.

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04-18-2017 07:58 PM  6 months agoPost 27
MattJen

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There's also the problem of thermal expansion which could offset the balance of the rotorhead,which is why full size don't use this method and you cannot auto

I think the BO-102 was a "hover training device"
I would be very doubtful it would be capable of achieving any speed.

As far as the construction would be:

flap : I can hardly see how a helicopter would be capable of forward flight without cyclic control which means flap...

lead-lag : is a consequence of flap. Since there is no symmetry as there is in a two bladed heli, Coriolis could be a problem depending on the detailed construction.

I think the BO-102 setup is a stiff rotor with one central flapping hinge. This forces symmetry in flapping which is beneficial for Coriolis, but at the expense of bending moments which are 10 times greater than Coriolis forces....

Looks a lot more simple to me to put two blades....

All The Best

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04-18-2017 08:03 PM  6 months agoPost 28
AirWolfRC

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A lot of full size helis have provisions for adding weights ONLY at the tip - - - and yes, they can and do auto.

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04-18-2017 08:05 PM  6 months agoPost 29
MattJen

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and yes, they can auto.

not with one blade which is what you were talking about,if they can Auto with one blade i'd love to see that.

At the end of the day YOU ACKNOWLEDGED What I was saying...
Moment of inertia will affect the rotation dynamics only as instantaneous rpm changes.
- - - I'm talking about the instantaneous changes in speed when in forward flight and the advancing blade slows slightly and the retreating blade speeds up slightly. That is the only time Moment of Inertia will have any effect (if it can even be noticed).

in a turbine where there is lag this wont be instantaneous

In regards to the one blade helicopter my concern is the total lack of symmetry at the hub.

A stiff setup leads to big bending forces between an aerodynamically active blade and a basically dead weight that only sees inertial forces. The magnitude of these bending force depend on the target forward speed and the result flap.

One of the nice things about rotor blades is that generally speaking tend to "compensate forces locally", that is aerodynamic and inertial forces, which reduces forces (at least the first order harmonic forces) drastically, and allows for light efficient blades. This also allows articulated setups

In order to let the blade go freely one could allow some elasticity between blade and counter weight, but the dynamics of the two are different.

One can easily setup the math model for this, but "ironing out" all the hurdles created by this set up makes me wonder why one would want to do this to begin with: what is wrong with putting two blades the only possible gain could be less blade vortex interaction.

All The Best

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04-18-2017 08:08 PM  6 months agoPost 30
AirWolfRC

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04-18-2017 08:10 PM  6 months agoPost 31
MattJen

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add half a tonne weight and see if auto then

in a full size that would need one heck of a rotor blade.. come on be serious.

but I reliterate again

you acknowledged my comment and that is all I was saying... if you fly precision or with sensitive equipment you WILL notice.

Moment of inertia will affect the rotation dynamics only as instantaneous rpm changes.
- - - I'm talking about the instantaneous changes in speed when in forward flight and the advancing blade slows slightly and the retreating blade speeds up slightly. That is the only time Moment of Inertia will have any effect (if it can even be noticed).

All The Best

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04-18-2017 08:28 PM  6 months agoPost 32
revmix

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tip weight & chordwise

rotor blades have static & dynamic balancing

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04-18-2017 08:28 PM  6 months agoPost 33
AirWolfRC

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add half a tonne weight and see if auto then
in a full size that would need one heck of a rotor blade.. come on be serious.
It all depends on wing loading.
you WILL notice
That's the part I do not agree with.

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04-18-2017 08:44 PM  6 months agoPost 34
MattJen

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you WILL notice
That's the part I do not agree with

Fair enough Airwolf..

I can only speak from my first hand experience and many tests over hundreds of hours of flying myself and a fellow flyer have done, these are not theories..

1g difference with the blades turning slowly you wont notice, but at 2000/3000rpm I guarantee you will notice it.

interesting debate as always.

As to checking the chord wise balance yes you can take it that far, but even I draw the line somewhere, LOL..
In fact I was once told if the chord wise balance is mismatched then use them for stirring paint. As making ones own blades is now really a thing of the past this has become almost non existent to do.

All The Best

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04-18-2017 09:36 PM  6 months agoPost 35
EEngineer

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"as that RPM changes the moment of inertial (AFAIK) will change"

The rotation rate is not contained in Newton's law of inertia.

Check here:

http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

Blade tracking changes might be due to something else.

Maybe change in momentum or change in angular acceleration(or both).

Logo 600SXs, 700XX, 800XX

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04-18-2017 09:51 PM  6 months agoPost 36
MattJen

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All The Best

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04-18-2017 09:56 PM  6 months agoPost 37
MattJen

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04-18-2017 11:55 PM  6 months agoPost 38
AirWolfRC

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A couple of points - -

DYNAMIC balance simply means "in motion" - - as opposed to static.
In the case of a car wheel, where there is no "plane" of rotation (like a pizza) but a portion of a drum. It is common for one end of the drum to have an offset one way while the other end of the drum has a weight offset at a different angle. This will cause the wheel to "wobble" when rotated. That's why you will have weights on both the inside of the wheel and the outside at different angular locations.

Rotor disks do not have this concern. They are more like a flat disk.

The only "dynamic" thing going on there is maybe a difference in Moment of Inertia. The key word here is Inertia. That is a resistance to a CHANGE in motion. A steady state motion, linear or rotary, does not concern itself with Inertia. By the way, Moment implies rotary.

An object possessing twice as much Inertia will require twice as much resisting force to Accelerate/decelerate.

Let's run some numbers, for typical 600 size blades

A 0.1 gram difference between two 140 gm blades is 0.07%
With the CG of both blades exactly 440mm out from the main shaft, the center of balance will shift only 0.15mm toward the heavier blade.
- - - notice that if you are checking down to 0.1gm then you must have your CG location within 0.15mm otherwise you're wasting your time.

Also vibration of the head will have an amplitude of 0.31mm
- - - there is way too much flex in the frame to let that be a problem.

Now if you do a simple see-saw balance, you will have no vibration in a hover and un-noticable effects in forward flight because there is simply not enough Moment of Inertia mass offset between the blades to notice.

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04-18-2017 11:59 PM  6 months agoPost 39
EEngineer

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TX

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"http://www.ewartwilson.co.uk/Helis/...20balancing.htm "

I'm going with Isaac Newton on this....not the above guy....sorry

Physics will win every time....

Logo 600SXs, 700XX, 800XX

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04-19-2017 12:06 AM  6 months agoPost 40
EEngineer

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TX

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"Answer because the mass has changed as a heavy blade would with faster rpm"

In Newtonian physics, mass does not change when in motion nor when accelerated or decelerated.....

But, I believe you when you say your blades are not tracking....it's just not that the blade's mass is changing.

Logo 600SXs, 700XX, 800XX

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HelicopterMain Discussion › Blade balancing
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