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Old Aug 18, 2012, 08:49 PM
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United States, MI, Rochester Hills
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Originally Posted by Endlesslag View Post
I think I'm going to limit my throttle endpoint to 85%...above that it gets too squirrely, and I have a habit of maxing the throttle out in a trouble situation, and of course that causes a crash if I'm down low, and a mad scramble to get back under control if I'm up high.
Today I suddenly had pretty much exactly your issue.... massive instability on full throttle punches and less than usual stability on rapid descents.

First of all, the issue is definitely NOT the control board. The issue is that the four props do not produce identical thrust for identical power input. Assuming that all four motor channels reach 100% throttle when you command full throttle, in still air the mQX should climb straight up from a standing start on the ground OR a full throttle stab from level flight. Mine used to but today it did not. It had a PRONOUNCED forward tilt at or near full throttle.

So I started to attempt to separate out the possible causes (wiring, motor, prop, excess friction).

What I found was:

1. Swapping motors front to rear had next to no effect. So it did not appear to be a motor issue per se.

2. Replacing the motor on the weakest corner (the one the mQX tilted towards at full throttle) with a new one had next to no effect. So it still did not look like a motor issue.

3. The gear mesh on the right front (the corner that seemed the weakest) was tight enough to bind. There was gunk in the gears which I cleaned. But the mesh was still tight. I tried rotating the motor in the housing, and this helped a bit. I left it in the loosest orientation (which seems pretty much OK), but this needs further diagnosis.

4. The right front motor frame was VERY loose on the boom. The problem was the boom, which had just been replaced... the old one was nice and tight. Cracked, but tight. This allowed the motor assembly to vibrate at full throttle. Fixed with a drop of hardened CA on the boom, but didn't fix the unbalanced thrust.

5. So assuming the motors were delivering the same power to all four shafts, that left it up to props. I don't have an optical tach to check the props in the air, but by hovering at eye level, I could sight across the front/rear pair of props and see by the interference pattern whether they were turning at the same RPM. They were NOT. Since all four props were developing the same thrust (the heli was hovering level), the fact that the front and rear props on each side were not at the same speed indicated that they were not the same pitch. The props can get bent, and due to their shape, when they get bent, this changes the pitch. Since the rear props were developing more thrust than the front props (remember, the mQX tilted forward on full throttle climbs), I gently bent each rear prop to de-pitch it. After several iterations, I got to a point where the mQX climbed nearly vertically AND the props were turning at the same RPM in a level hover.

Once this was done, outdoor flight verified that the mQX was once again stable.

So the bottom line is that all four props must generate the same thrust at full throttle or the mQX will be unstable. This applies to partial throttle operation as well, since the gyro makes corrections based on the assumption that all four props will generate the same thrust at the same power setting. If they do not, it unbalances the system and it is less stable after a perturbation from turbulence or maneuvers. And there are several issues that can cause unequal thrust.

Long winded, I know.... sorry. But necessary if the mQX is suddenly unstable.

Good luck!
Paul
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Old Aug 18, 2012, 09:09 PM
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That's an impressive analysis and very thoroughly performed. It makes quite a bit of sense, as the motors may be more closely matched even after a few dozen flights, but any contact of a prop to a surface can easily change the pitch. It's also the least difficult item to "calibrate" in my opinion. Interesting idea to use an optical tach to balance rpm at full throttle, or perhaps even lower throttle settings if the pitch controls are kept neutral.
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Old Aug 18, 2012, 09:24 PM
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Originally Posted by fred_dot_u View Post
That's an impressive analysis and very thoroughly performed. It makes quite a bit of sense, as the motors may be more closely matched even after a few dozen flights, but any contact of a prop to a surface can easily change the pitch. It's also the least difficult item to "calibrate" in my opinion. Interesting idea to use an optical tach to balance rpm at full throttle, or perhaps even lower throttle settings if the pitch controls are kept neutral.
Yeah, the real issue is that most of us (including me) do not have the equipment to actually measure the various variables. So we are left to poking at it until something gets better

I also fly helis, and an optical tach might be in my future. Or maybe something that I can run in my Android phone? I have a guitar tuner for it that also reads out in Hz.... in this case, Hz/2 would be the RPM if I hold the phone near the prop in question in level hover.... assuming it can hear pitches that low.

Paul
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Old Aug 18, 2012, 09:51 PM
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The variety of optical tachometers on the market is mind-boggling. I have a PowerLog 6s that has a tach built-in, but it's forty two bucks and change. I think it reads ambient light and counts blocking of the blade, which is convenient, but pricey. I found one on Amazon:

that requires a reflective mark (included in the package) which might be applied to the hub for reading purposes and it's astonishingly low priced. Low enough that I might consider to pick one up to make my Q fly more smoothly.

$11.01 plus shipping is pretty darn inexpensive and the range is 2.5 rpm to 100,000 (99,999) with an accuracy of 1 rpm over the 9,999 mark and 0.1 rpm below that level. If we can tune our Q blades to that level of precision, I'll be further astonished.
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Old Aug 18, 2012, 10:09 PM
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Actually, I just remembered that I have an optical tach for propellers. If I can hover over it outdoors and shoot through it, I can get a good readout. Might take one more hand than I have, though

Fine tuning the props is easy. KEEPING them fine tuned might be harder, since they are pretty soft. But it really does help with stability.

Paul
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Old Aug 18, 2012, 11:58 PM
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United States, IL, Mascoutah
Joined Dec 2010
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Motors Not Working

Had something interesting happen tonight. Was flying late this afternoon in front of my house......had the neighborhood kids watching...had a hard landing and two opposing blades refused to spin up...oh no....fudged 4n1? Motor shot already? Took canopy off and check everything...tried again, two motors dead. Took inside and really went over it...seems that the two motors got push up and were not meshing with the white gears....Pushed them back down and seems to be ok...put a little dab of elmers glue to hold them in place.....whew!
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Old Aug 19, 2012, 12:34 AM
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Flying my mQX today for a public demo... and it just suddenly nose dived into the dirt. Upon inspection I found that the upper bearing on one of the motors had come apart. So, I grabbed my ball link pliers and snapped it back together... a little Triflow and she was good to go. Flew two more flights with no issues until they turned on the lights on the field and I got blinded and dumped it. All in all a pretty good day though... I flew planes and helis for seven hours and the kids and everyone really loved it. The mQX seems pretty durable too. I never crashed it twice in one day before... but it still works fine
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Old Aug 19, 2012, 06:20 AM
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Originally Posted by jasmine2501 View Post
Upon inspection I found that the upper bearing on one of the motors had come apart.
You mean the upper motor mount bearing? Don't have too much slack between it and the prop. 1/3mm or so. Not too tight either or they can bind.
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Old Aug 19, 2012, 06:26 AM
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Originally Posted by Brandigan View Post
You mean the upper motor mount bearing? Don't have too much slack between it and the prop. 1/3mm or so. Not too tight either or they can bind.
I think he's talking about the upper prop shaft bearing rather than the motor.
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Old Aug 19, 2012, 11:46 AM
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Originally Posted by Gerry__ View Post
I think he's talking about the upper prop shaft bearing rather than the motor.
That's what I meant too. To hit it from all directions: The bearing at the top of the prop shaft in the top of the motor mount on the end of the leg.

I can't quite see how it can come apart and if it did, you'd have teeny, tiny little ball bearings to find and put back in it.
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Old Aug 19, 2012, 01:01 PM
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Originally Posted by Gerry__ View Post
I think he's talking about the upper prop shaft bearing rather than the motor.
Yes that. It came partly apart, none of the balls fell out. It was easy to snap it back together with ball link pliers. Not sure if the crash caused it or the other way around.
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Old Aug 19, 2012, 08:05 PM
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Those bearings look like newer mCPx bearings, older one I had untill I broke cf main shaft had ball cage (he he) sealed bearings might not spin easy so these are easy to see/clean/lube. Too lazy to measure...for now
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Old Aug 19, 2012, 10:24 PM
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Originally Posted by Brandigan View Post
Your anecdotal experience isn't evidence of good practice. You've just been lucky.

Consider landing upside down in a tree, bush on rocks/uneven ground. This is different to landing on grass, or even concrete. Would you rather a rock/branch went through the board, or hit the canopy? If you landed upside down on water, the standard canopy may even float.

Note that a non-standard canopy, 'landing' feet first in a swimming pool does not help it float, as I found out.
I have crashed more than 1,000 times without a canopy, in a wide variety of ways. I am still flying without a canopy.

I would not suggest something this radical without testing it well first.

At what point do you think my experience might be statistically significant?
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Old Aug 19, 2012, 10:50 PM
Roters up... Skids DOWN!
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United States, UT, Kaysville
Joined Aug 2012
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My first post...
Ive been flying the MQX for two weeks now and love it. Need to pick up a DX6i soon and ditch the RTR. I've been reading a few pages back about the prop wash and the possibility of damaged booms or tweaked props that don't produce the proper thrust. This is good info for newbies... Thanks.

Now on to the recent talk about the bearing that came apart and it was just "snapped" back together. I had a upper bearing come apart and there was no popping it back together as there were only 3 tiny balls that were stuck to the motor. I replaced them with the recomended blade part number for this 3x6x2 bearing. I really don't like the fact there is only one steel dust shield on one of the two sides with the other side being exposed to the cage and balls.
There is a 3X6X2.5 blade bearing that has the metal dust shields on both sides of the bearing. So I was wondering if anyone has went and replaced all their bearings with this other piece?

I could weight each of the two bearing styles next Wednesday when a fresh shipment comes in, if that could help out.
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Old Aug 19, 2012, 11:45 PM
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NM this was unnecessary
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