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HelicopterMain Discussion › Blade balancing
04-19-2017 12:18 AM  6 months agoPost 41
MattJen

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UK

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

SO a brick hitting you at 1mph will do no damage now the same brick hitting you at 70mph wont either ?

still same brick, same weight, all that has changed is its mass due to its speed.

the same could be said of a car, hitting you at 1mph nothing is gonna happen but the same car hitting you at 70mph is gonna kill you, same car same weight.. just different motion speed, so I would disagree with your above comment.

or I put a metal ball in a sock and spin it really really slowly above my head like a disc and it hits you aint gonna do much damage, but if I really spin it really fast and it hits you it will put a hole in your head, same ball same weight.. mass has changed..

All The Best

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04-19-2017 12:38 AM  6 months agoPost 42
MattJen

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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.

I was talking about 2 or 3 gram differences not 0.1

The whole thread was talking about adding tape to the tip of the blade, my argument is if you do you will change the C of G of the blade, surely when you change the c of g of the blade you alter its lift point, I have flown two blades where I deliberately put wraps of gaffer tape on the tip, the c of g moved a lot towards blade end, the blade when deliberately altering the rpm would come out of track and the machine would shake violently, I then removed the tape and re - checked the c of g they were back identical, took off and varied the head speeds all was smooth No vibes at all.

I can only report what we saw... so I will always always as it is the most common practice over many years of model flying

Find the C of G
Match the C of G
and Balance on the C of G

If you can demonstrate in real life you get no vibes whatsoever when the RPM changes on a 800 size model with mismatched weighted blades with different c of g I will accept your findings, at 700rpm you probably wont notice it but at 2500rpm you will.

Really interesting thread.

All The Best

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04-19-2017 01:07 AM  6 months agoPost 43
AirWolfRC

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Out of track is not a balance issue.
If you are out of balance, then you will vibrate - - to some degree.
You may also go into resonance at a certain speed which makes things a LOT worse.

If you have not seriously tried simply putting weight on the light blade tip with a see-saw balance and flying then you owe it to yourself to try it and find out first hand.

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04-19-2017 01:29 AM  6 months agoPost 44
MattJen

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Out of track is not a balance issue.

The purpose of Static Chord adjustment is to balance the blade in the chord wise direction. If the CofG is located aft of ideal, the rotor blade would tend to climb since the turning moment that the aft located CofG would produce would tend to make the nose climb.
If the CofG is forward of the ideal CofG, the turning force created by the blade mass distribution about the chord would be a nose down tendency, making the blade dive.
-----------------

I do get what your saying...

that by adding at the tip I wouldn't have to add as much weight to balance as I would if I balanced on the c of g...

If you have not seriously tried simply putting weight on the light blade tip with a see-saw balance and flying then you owe it to yourself to try it and find out first hand

I have as I have said many times,

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.

All The Best

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04-19-2017 01:48 AM  6 months agoPost 45
AirWolfRC

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The blade chord wise balance would have to be GROSSLY DIFFERENT for that to be an issue. A difference that simply isn't found in a set of blades.

If you want to put the Moment of Inertia question into perspective, run some numbers and see what the MoI difference would be if the CG was, say, 5mm different but the see-saw static moment was still the same.

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04-19-2017 04:04 AM  6 months agoPost 46
revmix

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NJ

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04-19-2017 06:34 AM  6 months agoPost 47
EEngineer

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TX

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"I put a metal ball in a sock and spin it really really slowly above my head like a disc and it hits you aint gonna do much damage, but if I really spin it really fast and it hits you it will put a hole in your head, same ball same weight.. mass has changed.. "

Just see what happens to you......

What you're referring to is "kinetic energy"....which is proportional to mass x velocity x velocity....

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

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04-19-2017 07:54 AM  6 months agoPost 48
MattJen

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maths isn't always correct.

if it was the humble bumble bee would not fly.

All The Best

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04-19-2017 10:35 AM  6 months agoPost 49
Richardmid1

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Leeds, England

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That gets them statically balanced which is useless really.
Don't knock it until you've tried it.
If you want to do it the hard way then knock yourself out.
There is nothing harder about adding the weight to the center of the blade instead of the tip. It's a no-brainer.

60% of the time, it works every time!

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04-19-2017 04:28 PM  6 months agoPost 50
EEngineer

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"maths isn't always correct."

Yeah?

Is that what you'd have everyone believe?

You're a funny guy.....

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04-19-2017 04:29 PM  6 months agoPost 51
EEngineer

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"if it was the humble bumble bee would not fly. "

Prove it....

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04-19-2017 05:19 PM  6 months agoPost 52
AirWolfRC

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There is nothing harder about adding the weight to the center of the blade instead of the tip. It's a no-brainer.
except that you need to add two to three times as much.
if it was the humble bumble bee would not fly.
But yet the humble bee keeps flying in spite of peoples efforts to prove it can't

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04-19-2017 05:31 PM  6 months agoPost 53
EEngineer

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And hummingbirds....

With ultra-high frame rate video, we can now see how they achieve such flight.

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04-19-2017 05:33 PM  6 months agoPost 54
EEngineer

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TX

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Why are bumble bees considered "humble".

The last one I encountered stung me....

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04-19-2017 05:37 PM  6 months agoPost 55
AirWolfRC

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Interesting article referenced by revmix,
http://www.avionavtask.com/img/pdf/..._or_Science.pdf
from the article,
The United States Army now employs dozens of Universal Static Balance Fixtures within
all levels of maintenance, depot, intermediate and unit. The United States Army has
saved millions of dollars in reduced rotor blade returns to the manufacturers and depot
repair facilities. The United States Army has saved millions of dollars in reduced
maintenance test flight expenses by cutting associated activity in half.
Note - - STATIC (see-saw) balance is the default procedure

They don't drink the koolaid either

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04-19-2017 05:43 PM  6 months agoPost 56
EEngineer

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TX

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What about the "tennis racket effect"?

Along with the "Chinese weight" solution....

(where do "they" come up with these terms?)

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04-19-2017 05:44 PM  6 months agoPost 57
EEngineer

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TX

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"They don't drink the koolaid either"

Well, if it had ethanol in it....they'd drink it....

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04-19-2017 06:03 PM  6 months agoPost 58
AirWolfRC

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What about the "tennis racket effect"?
Along with the "Chinese weight" solution....
They're the same thing. - - - What about it ?
Has little to do with rotor blades. - - - not nothing, just little.

From my home page,

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04-19-2017 06:06 PM  6 months agoPost 59
MattJen

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lol

I'm just pulling your leg, there has been so much debate about the humble ( little in the big scheme of things) bumble bee, cos it is it has tiny wings compared to its body size..so theoretically it should not fly..

there was some interesting debates... even dynamic stalling discussions which went into mega detail... was funny..

but the conclusion was
The reality is that bees and comparable insects fly in an incredibly complex way that utilises, get this, mini hurricanes! if you’re interesting in the nitty gritty physics, but in lay terms, bees fly by rotating their wings, which creates pockets of low air pressure, which in turn create small eddies above the bee’s wing which lift it into the air and, thus, grant it the ability to fly.

never heard of the tennis racket effect apart from when hitting ground after poor serve.and the effect is a knackered racket or dent in the grass

I still stand by my findings that by adding weight to the tip you change the c of g and alter the blade performance on a large heavy scale model...and at different rpm you will induce vibes, even if it is momentry, over time it will affect bearings and stress out parts of the machine..

All The Best

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04-19-2017 06:12 PM  6 months agoPost 60
EEngineer

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TX

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"From my home page"

That is an excellent diagram of the effect....

Remember that guy "Heim Joint", I think that was his name?

Used to make precision weights mechanically calibrated to a variety of tail rotors....had a computerized "jig" to empirically determine the exact weight needed....

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HelicopterMain Discussion › Blade balancing
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