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Old 05-07-2012, 09:33 PM   #1
mfremont
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Unhappy vibe blues

My FBL Protos has been an awesome-flying heli from day one. But recently I've had some really odd shudders in flight (looked a bit like a static hit), and the heli has developed a tendency to pitch backwards on its own while spinning up on the pad.

The bearing closest to the tail rotor was shot, creating quite a wobble in the tail shaft. The tips of the blades were also burred up. I'm not sure if the tail strike toasted the bearing or the backwards lean led to the tail strike.

Replacing the two tail shaft bearings, tail shaft, and pulley calmed the worst of the mid-frequency vibrations, but now I have a huge spike when I mount the new tail blades. It's the light green peak right around 250Hz in the first attachment. The dark green is with the tail blades removed.

The blades balance out as best I can measure with my balancer.

Based on suggestions I read here, I've loosened up the tail case screws (it helped a little) and played with belt tension (tight is better than loose).

The faces of the new pulley do not track straight, so I started pulling the tail apart bit by bit to determine if that's the source of the problem. The next two attachments are with no tail rotor but with and without the pitch slider installed.

To my eyes, the vibes increase as I add the slider back, then the tail hub and grips.

Since the vibe levels are so ridiculously high with the tail rotor in place, I'm not sure what my next steps should be.

New tail hub? Maybe thrusted tail?
New tail pitch slider?
CNC tail pulley instead of plastic?
Something else?
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Old 05-07-2012, 10:24 PM   #2
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When you spool up the heli on the bench, check the rear tail pulley and make sure the belt is not going up and down. Most of the time, those plastic tail pulleys just have a bent out side. But that should not be a problem. Just make sure the tail shaft hole is centered on the pulley.

The other thing to check is the tail hub. You probably have the original one and not the thrusted tail one. Check to make sure with a digital caliper that the measurement from edge of the tail shaft hole to the respective end of the stub is the same as the other side. I've had a few tail hub where the tail shaft hole was not centered and caused a lot of vibes. That's the reason why I dumped the original tail hub and went with the thrusted tail hub as it had much better tolerances in that regard.
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Old 05-07-2012, 10:25 PM   #3
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I should have looked at the graphs first. It seems like your motor is not balanced in the last two graphs. Was the first graph during spool up when it had not reached final speed? I can't imagine that is your tail speed as it is too fast.

Spool up on the bench with the main blades off and the tail rod disconnected. Use your fingers on tail boom and tail boom block as well as near the motor and see if you can feel the vibes. I'm guessing by your graphs that your motor is way out of balance.
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Old 05-08-2012, 02:57 AM   #4
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I'm inclined to agree with alexf about the motor as the source. Unless your tail really is spinning 15krpm+, it has no business creating vibes at 250+ Hz. Stock Protos tail would create vibes in the 200 Hz range. Check your motor bearings, one of them may be damaged.
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Old 05-08-2012, 05:50 PM   #5
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Thanks for all of your suggestions.

Quote:
Originally Posted by alexf1852
When you spool up the heli on the bench, check the rear tail pulley and make sure the belt is not going up and down. Most of the time, those plastic tail pulleys just have a bent out side. But that should not be a problem. Just make sure the tail shaft hole is centered on the pulley.
The belt isn't going up and down perceptibly, but it is moving from side-to-side. It seems like the pulley is not at a right angle to shaft.

Quote:
Originally Posted by alexf1852
The other thing to check is the tail hub. You probably have the original one and not the thrusted tail one. Check to make sure with a digital caliper that the measurement from edge of the tail shaft hole to the respective end of the stub is the same as the other side. I've had a few tail hub where the tail shaft hole was not centered and caused a lot of vibes.
Indeed, I have the original. The distance from the inside of the tail shaft bore to the outer edge of the grip shaft is the same on both sides. The outer diameter of each shaft is the same, but looking down the center of each grip shaft, they appear different. If I measure the wall thickness of the grip shaft, one side is a tiny fraction of a mm thicker than the other, which doesn't make much sense to me given that they are threaded for the same size bolt.


Quote:
Originally Posted by alexf1852 View Post
I should have looked at the graphs first. It seems like your motor is not balanced in the last two graphs. Was the first graph during spool up when it had not reached final speed? I can't imagine that is your tail speed as it is too fast.
Quote:
Originally Posted by Dr. M View Post
I'm inclined to agree with alexf about the motor as the source. Unless your tail really is spinning 15krpm+, it has no business creating vibes at 250+ Hz. Stock Protos tail would create vibes in the 200 Hz range. Check your motor bearings, one of them may be damaged.
All of the graphs were captured after the motor had reached a stable governed RPM. I had the same thought initially that the 250Hz peak was too high for the tail. The heli tach app reports ~3100 RPM for the bench spool-up, so with the ratio of 4.65 tail:main, that's close to 15k RPM for the tail.

I thought I had previously ruled out the motor with a belt-off test, but your questions prompted me to run the test again. Attachments are for the motor with the pinion installed, motor with just the adapter installed, and the motor by itself.

The peak at 358Hz/21.5K RPM is a little higher than I'd expect for an 880KV motor on 6S (but perhaps if the motor is at the high end of the KV tolerance range, it's not that far off). It would seem that the motor is the source of some significant vibes.

In the disassembly I noticed that the upper radial bearing (between the clip and the one-way) was looser on the adapter shaft than the lower. The adapter shaft was also galled at the point where the radial and one-way bearings meet, enough so that I needed to smooth out the shaft a bit with 600 grit paper just to remove the lower radial bearing from the adapter shaft.

The motor has some play between the rotor and stator. The rotor spun easily on the bearings when I pulled it apart, and the bearings felt smooth. There was a little vibration, but that could have been because the of the play allowing the shaft to wobble in the bearing?

Motor will be on its way to Innov8tive tomorrow in order to get the bearings replaced.

The one way is a bit notchy when rotated on the adapter shaft. I can't recall how it felt when new, so maybe both a new pinion and adapter shaft for good measure?
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Old 05-08-2012, 06:38 PM   #6
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Vibrations are always hard to get rid of.
It does seem your motor is a little out of balance, but there are many other factors to consider.

I assume you have the black VBar version, more sensible to vibrations. Strapping it down helps a little (I use a strap, not very tight, and currently a Spartan gyro pad).

Tail blades must be dynamically balanced by trial and error. Balancing them on a blade balancer simply isn't enough, even with the entire rotor. You can take most of the vibrations out using the dynamic balance method.

I also chased the pulley, main gear, guide pulleys, tail bearings, one way (mine also drags the motor when I turn the rotor by hand - I bet the pinion adapter has some deep bearing marks) and what was needed was patience to balance the tail blades.

I should have around 400 vibration level on my FS black VBar in an hover.
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Old 05-08-2012, 08:50 PM   #7
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I stand corrected. Seems you do have a motor towards the high end of the kV range. It's not unheard of.

In my experience, the Protos tail is finicky. I've had a resonance vibe that I could only reduce by adding weight to BOTH tail blades. It took 5-6 pieces of tape on each blade and a gew packs of dynamic balancing on the bench to get rid of the pesky resonance.
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Old 05-08-2012, 08:56 PM   #8
mfremont
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Quote:
Originally Posted by whisperdancer View Post
I assume you have the black VBar version, more sensible to vibrations. Strapping it down helps a little (I use a strap, not very tight, and currently a Spartan gyro pad).
vBar is the blue line mini. IIRC, it is less susceptible to vibrations than the black sensor version.

Prior to the most recent problems, my Protos has always been a very smooth flying heli, something my flying buddies have often commented on. I only upgraded to the Pro firmware after encountering the strange behavior, mainly to get access to spectral analysis and log features. So, unfortunately, I don't know what the vibration levels were prior. However, I will say that the vibrations on my Protos in its current state are much higher than my Trex 600 with a black sensor vBar.

Quote:
Tail blades must be dynamically balanced by trial and error. Balancing them on a blade balancer simply isn't enough, even with the entire rotor. You can take most of the vibrations out using the dynamic balance method.
I'll need to read up on the technique for dynamic balancing.

Quote:
I also chased the pulley, main gear, guide pulleys, tail bearings, one way (mine also drags the motor when I turn the rotor by hand - I bet the pinion adapter has some deep bearing marks) and what was needed was patience to balance the tail blades.
Any pointers on the technique you used?
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Old 05-08-2012, 09:08 PM   #9
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Anyone ever tried one of these: DSS MicroVib II Helicopter Kit?
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