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Old Feb 04, 2013, 06:28 AM
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Vienna, Austria
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Quote:
Originally Posted by Odysis View Post
Any chance you can work out what 3ms^-2 comes to in G (units of unbalance)?
maybe I misunderstand but 1g is simply the gravitational acceleration here on Earth, and that is approx, 9.81 m/s^2 - so 3 m/s^2 is just 3/9.81 = 0.3g
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Old Feb 04, 2013, 06:33 AM
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load comparison 10-blade vs. 12-blade

I am a bit confused: PeterVRCs measurements gave almost identical results for the 10-blade and the 12-blade, and that is puzzling both because 12 blades (even with slightly different profile) are liked to be a heavier load than 10 blades - and because my own measurements suggest otherwise - on low power level (400-450W) I seem to get almost 10% higher current.

Anyone else who did a comparo using same motors and lipos?

PS: To me this seems to mean that certain combos which had a bit low amps before, like the 2300kV on 4s, are more interesting on the 12-blade, whereas others which were taxing the motors to the limit might be too much now for a certain motor when using 12-blade instead of 10-blade rotors...

thx
Clemens
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Old Feb 04, 2013, 07:00 AM
Life begins at transition
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sorry, capital G is balance grade, units of velocity [mm s^-1], lower case g is 9.8 [m s^-2]
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Old Feb 04, 2013, 07:15 AM
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Originally Posted by WhalleyB0Y View Post
Did you have to balance any of the parts?
None actually. I used the seismograph app for Android and the lines barely moved.
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Old Feb 04, 2013, 09:47 AM
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Quote:
Originally Posted by mopetista View Post
I am a bit confused: PeterVRCs measurements gave almost identical results for the 10-blade and the 12-blade, and that is puzzling both because 12 blades (even with slightly different profile) are liked to be a heavier load than 10 blades - and because my own measurements suggest otherwise - on low power level (400-450W) I seem to get almost 10% higher current.

Anyone else who did a comparo using same motors and lipos?

PS: To me this seems to mean that certain combos which had a bit low amps before, like the 2300kV on 4s, are more interesting on the 12-blade, whereas others which were taxing the motors to the limit might be too much now for a certain motor when using 12-blade instead of 10-blade rotors...

thx
Clemens
The 12 blade draws 7% more than the 10 blade from my experience.
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Old Feb 04, 2013, 10:08 AM
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thanks, that squares fine with my crude observations - how do the efficiencies of the fans compare, i.e. thrust at 500W with similar motor efficiency (in most cases motor efficiency will be a bit lower at higher load...)
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Old Feb 04, 2013, 10:33 AM
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from acceleration to balance grade (maybe)

Quote:
Originally Posted by Odysis View Post
sorry, capital G is balance grade, units of velocity [mm s^-1], lower case g is 9.8 [m s^-2]
ok, now I owed you some "research" :-)

if we can take the vibrating movements to be pretty similar to those of a harmonic oscillator, we can use simple relations between the max acceleration and the max velocity (they are of course phase shifted, when the speed is max the acceleration is momentariy zero, and in the return points, we have max acceleration), both vary like the sine (or cosine :-) function.
Here's where I got it from, long live Wikipedia: http://en.wikipedia.org/wiki/Simple_harmonic_motion

In the formulae you often have "omega", that is the angular velocity, and that's just 2*pi*f (f...frequency), I will only use frequency.

A is the amplitude, or max. displacement from zero position. I was worried we would need to know A, but it turns out we do not. But we need the rpm value, and revs per minute are divided by 60 to get the revs per second which is identical to the frequency f = rpm / 60

So the two formulae for acceleration a and velocity v are:

a (max) = A * (2*pi*f)^2 (actual acceleration varying like cosine)

v (max) = A * (2*pi*f) (varying like cosine, but phase shifted relative to a)

Now let's make a calculation using the mentioned 3 m/s^2 for acceleration:

If we got a = 3 m / s^2 and that was mearsured at, e.g., 6000 rpm,
then f = 6000/60 = 100, and we can write:

3 = A * (2 * pi * 100)^2 and so the amplitude A will be approximately A = 3 / 400,000 m = 0.0075 mm

Now we have everything to calculate v (max):

v (max) = 0.0075 * 628 = 47 mm/s (rounded)

- does that make sense?

cheers
Clemens
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Old Feb 04, 2013, 10:44 AM
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Wow! These forums get cool and confusing and very interesting at times!
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Old Feb 04, 2013, 01:25 PM
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Quote:
Originally Posted by mopetista View Post
ok, now I owed you some "research" :-)

if we can take the vibrating movements to be pretty similar to those of a harmonic oscillator, we can use simple relations between the max acceleration and the max velocity (they are of course phase shifted, when the speed is max the acceleration is momentariy zero, and in the return points, we have max acceleration), both vary like the sine (or cosine :-) function.
Here's where I got it from, long live Wikipedia: http://en.wikipedia.org/wiki/Simple_harmonic_motion

In the formulae you often have "omega", that is the angular velocity, and that's just 2*pi*f (f...frequency), I will only use frequency.

A is the amplitude, or max. displacement from zero position. I was worried we would need to know A, but it turns out we do not. But we need the rpm value, and revs per minute are divided by 60 to get the revs per second which is identical to the frequency f = rpm / 60

So the two formulae for acceleration a and velocity v are:

a (max) = A * (2*pi*f)^2 (actual acceleration varying like cosine)

v (max) = A * (2*pi*f) (varying like cosine, but phase shifted relative to a)

Now let's make a calculation using the mentioned 3 m/s^2 for acceleration:

If we got a = 3 m / s^2 and that was mearsured at, e.g., 6000 rpm,
then f = 6000/60 = 100, and we can write:

3 = A * (2 * pi * 100)^2 and so the amplitude A will be approximately A = 3 / 400,000 m = 0.0075 mm

Now we have everything to calculate v (max):

v (max) = 0.0075 * 628 = 47 mm/s (rounded)

- does that make sense?

cheers
Clemens
Clemens everything looks good so far except this.

v (max) = 0.0075 * 628 = 47 mm/s (rounded)

I belive you forgot a decimal point . Should it not read 6.28 (2*3.14) ????
v (max) = A * (2*pi*f)
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Old Feb 04, 2013, 02:05 PM
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thanks for checking - and you are right, I fudged one decimal:

v (max) = 0.0075 * 2 * 3.1415 * 100 = 4.7 mm/s (rounded)
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Old Feb 04, 2013, 03:53 PM
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I thought it was quite interesting that a few of the Apps let you 'remove the force of gravity'.
I meant to test it, but haven't so far.... eg what does it really mean? And for any axis? What happens if the phone is at some angle? LOL
It seems they apply it, or something(!), just to one axis. Not a vector if the phone was, say, at 45deg.
It just seems wishy washy to me.... but I need to investigate that more.

But the readouts (graphs) are still a waveform on top of any offset anyway, so you still get to see an absolute value of the variation anyway.

I don't worry too much about one session to another, I just aim at reducing the value seen. eg memorise or record what it was and just keep striving for less.
But I did find an issue with their 'detection' of harmonics.... you can have the fan reverberate some amount, at some RPM, and that is not shown in the graph much at all, if at all! Or not as plain as the large value it would/should be - seeing you can plainly see/tell it is a notable issue.
You need to go through the RPM range and find those (they will repeat at harmonics from what I can work out), and also find the largest graphed areas, and use them all as factors you are aiming to reduce!

I am still working on it to have a more fixed/repeatable way to do it as simply and quickly as possible. eg So far finding the first reverberation point (could be 7000rpm for eg) and fixing that range does a pretty good job of overall balance.

But I have had to also do a visual shaft check also, as I could get it to low value "X" on the graph, yet still have the same area of values (it wavers a bit) for rotor rotation adjustments over a fair angle range, but you can see shaft motion (motor shaft flex) change as you go across that angle range. And picking the angle of least flex of course makes those 'graph doesn't seem to show well' reverberations reduce.

That seems to be tied in with the phone sensor, or software polling, etc somehow missing out on certain cases of frequency or something. Thus not very well indicated in the graph at all for those cases.
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Old Feb 04, 2013, 08:25 PM
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OK so how do you use the phone app to measure the vibes ?Is the phone duct taped to the shroud ?Or do you hold it against the shroud while running it up ? I would suppose then just look for the fan position that shows the smallest oscillations ?
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Old Feb 04, 2013, 10:05 PM
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I have the fan mounted in a 'very lightly sprung cradle', so that it can still vibrate very well. If you bolted it to some rigid mount it would 'erase' some vibrations.
I tried laying the phone across the top, on a small piece of double sided tape.... plus then angled, from housing to the cradle plate, again with the bit of double sided tape to keep it there.
Both gave the same results.... just in different X,Y, Z waveforms. So both ways were useful to get readings anyway.
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Old Feb 05, 2013, 02:54 AM
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Quote:
Originally Posted by mopetista View Post
ok, now I owed you some "research" :-)

[snip]

- does that make sense?

cheers
Clemens
Nice! It's been too long since I looked at it from 1st principles. ISO 1940/1 gives the international standard for balancing grades (and "silky" doesn't appear anywhere )

Here's a quick rundown It'd be nice to get to G3 or so, but even G10 is pretty good for these little things.

We have the tools, we have the knowledge, there is a Standard way to do things, why must the EDF fraternity ignore the lessons of those who have been doing this since 1940?
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Old Feb 05, 2013, 07:23 AM
4 blades! Scale blades!
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Quote:
Originally Posted by pencon View Post
OK so how do you use the phone app to measure the vibes ?Is the phone duct taped to the shroud ?Or do you hold it against the shroud while running it up ? I would suppose then just look for the fan position that shows the smallest oscillations ?
I put the phone flat in a wooden table and the EDF sitting in a test stand as near as possible but without touching the phone. There must be a good coupling to the table surface to transmit the vibrations. I don't put it too far to avoid the table to act as an amplifier.

I got it from this video.

Balancing Brushless RC Motor (1 min 55 sec)
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