RunRyder - Main-blade-induced vibration ....?

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Aerial Photography and Video > Main-blade-induced vibration ....?

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
Perhaps before the major vibration which bent the shaft

Vibration doesn't permanently bend the spindle. It takes a force greater than the yield strength of the steel shaft and it only needs to occur once.

I am only familiar with the head design of the Predator but if the Logo is of the teeter type it is important that the teeter axis occurs at the ball. When head dampeners get unevenly warn or change in durometer because of vibration the teeter point can change.

You balance the blades because you want the feathering spindle to rotate on the teeter ball. Unbalanced blades will cause the rotors to rotate in another position (a different CG point). Assuming the rotors are rotating at the teeter point the main shaft must also rotate about that same point. If it isn't the spindle will be thrown side to side on each revolution. That is your vibration. so you can see how important it is to have the main shaft perfectly straight. If it is not it will beat the hell out of your dampeners. Most likely one side more than the other.

To recap:
The rotor system includes the blades, grips, bearings and hardware, and the the feathering shaft. If your blades are perfect and the rest is not your rotors are NOT. Any dimensional variation in the grips, feathering shaft and head block is a problem.

One of the things I do is indicate the head block.

Note: your finger is not an indicator but you could use a stationary pointer and observe the gap.

Ace
What could be more fun?

05-03-2008 01:06 PM

Hogster
Key Veteran
Location: Surrey, UK

Hmmm I think I'm using a hard set of O-rings now, although I'm not absolutely certain ...

Ace - It was the vibration that bent the main shaft (not the spindle) - it was a terribly harsh vibration just after take-off that did it....

Oh and regarding putting my finger on the main shaft, I also held a set square next to the shaft as it was rotating and the shaft remained the same distance from the square at all times ...

I've just installed a new main shaft and the vibration is very minimal now. I think some tweaking with the tracking is still needed, but it's pretty good now

David

05-03-2008 03:10 PM

HOMEPAGE

rerazor
Elite Veteran
Location: Mich.

I know you must get tired of me posting this but if you bent a main shaft I would be pretty sure the main shaft bearings took a nice lateral load.

05-03-2008 03:26 PM

Hogster
Key Veteran
Location: Surrey, UK

I checked both of them and there's no slop or grittyness when they rotate

... it's one of the first things I checked after I removed the old shaft

David

05-03-2008 03:30 PM

HOMEPAGE

xfc3dcd
Senior Heliman
Location: West Carrollton, Ohio usa

Plastic certainly has its place for some parts of a helicopter, but blade grips is not one of them. On some areas of the rotor system 1/16" or more slop/play doesn't cause much of a problem, like side to side play (horizontal plane) in the flybar for example. Conversely, having one blade extend out radially .001" more than the other will cause noticeable vibratiom on video. If you drop a Logo blade bolt through the grip and note the amount of play in the bottom grip bolt, it has to be .020" on a brand new grip. This is all well and good as long as the other grip is dimensionally identical.

Here is an interesting experiment to perform. Hang the rotor system upside down and hang a weight from the tip of each blade and measure the deflection. Is it the same? Does it change with blade bolt tightness? This is why I don't care for plastic blade grips.

Proof of this problem is if you swap blades and anything changes, this is due to inconsistent tolerances in either the grip, blade or both.

Wendell

05-03-2008 04:09 PM

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
having one blade extend out radially .001" more than the other will cause noticeable vibratiom on video.

I am sure there is more variation then that in all helicopters whether it is plastic grips or not. A common bolt used to hold the blades to the grips are not that accurate. Take into consideration machining tolerances, washers, bearing race location etc. and run out, no way are the blades rotating to that accuracy.

Ace
What could be more fun?

05-04-2008 01:20 PM

xfc3dcd
Senior Heliman
Location: West Carrollton, Ohio usa

I just measured the shank section of 6 blade bolts with my Starrett metal dial calipers and they were all within .001", +/- .0005" when measured at the same distance from the head. (The shank was thicker just below the head for a short distance but equally for all measured)

Paying attention to the fitment of all of these parts through measurement, then mixing/matching or modification/tweaking is why some helicopters always vibrate and others fly like world champions.

All other things being equal, metal blade grips hold tracking and blade position better than plastic grips.

Wendell

05-04-2008 02:28 PM

Hogster
Key Veteran
Location: Surrey, UK

Just to keep you posted where I'm at with this problem ...

For those who have been following my ball-mount thread, you will know I've been trying to track down some vibration problems on the heli.

After making another modification to the front mount to make it more rigid, I had another test flight today .... and there was another intense vibration/explosion which bust/bent other parts of the mount

It was so violent it shook the SD-card door on the A620 open

So once again I've stripped the heli down and have been trying to identify the cause. With the main blades removed, but the rotorhead still on, to BEGIN WITH there is no vibration at all. Then, as the motor approaches full speed, a strong buzzing starts to come down the tail boom from the tail rotor. I have tried adding little bits of tape to one of the blades and that has made it better (not perfect) ..... but the most worrying part, is that after a while at full-throttle, the tail rotor will suddenly start making a MUCH stronger vibration

I instinctively turn the motor off immediately as I'm afraid something is about to break! Hmmm.

How about with the tail rotor blades removed? Mmm better, although there's still a small buzz on the tail boom ... Ok how about with the tail pitch slider and other associated linkages removed so it's just the shaft sticking out of boom? Boom is now TOTALLY free from vibration ....

I'm starting to wonder whether that tail rotor mechanism is due for a replacement ... it's seen its fair share of gentle bumps/knocks in its time. I'm not sure what is causing that sudden increase in vibration, but it's certainly not right.

If I'm going to replace some parts of the tail rotor mechanism, should I just replace all of it? Just a few parts?

Any thoughts are most appreciated, gents!

David

05-04-2008 05:10 PM

HOMEPAGE

AceBird
Elite Veteran
Location: Utica, NY USA

David,

When you are making these tests is the HeliCommand active for the Flybarless head? Make sure you are not confusing something wrong with balance with something wrong with control.

Ace
What could be more fun?

05-04-2008 05:39 PM

Hogster
Key Veteran
Location: Surrey, UK

The Helicommand is disabled, other than providing a degree of flybarless stabilisation. Without the rotor blades on, the Helicommand should have very little effect on the results of these static vibration tests ....

Cheers

David

05-04-2008 05:46 PM

HOMEPAGE

AceBird
Elite Veteran
Location: Utica, NY USA

I am confused. Your spinning up the head without any load on them? How do you know how fast they are turning? Backlash in gears under no load could cause huge vibrations. I guess I am not sure what you are doing.

Ace
What could be more fun?

05-04-2008 05:59 PM

Hogster
Key Veteran
Location: Surrey, UK

Yes I have removed the main rotor blades and am just investigating the tail rotor at the moment. Obviously something is going on there as without the tail blades or pitch slider there is NO vibration at all.

I'm sorry, I don't see how backlash can cause vibrations ....

Cheers,

David

05-04-2008 06:02 PM

HOMEPAGE

Highviz
Senior Heliman
Location: Newbury, Berks, UK

David

Are you running a carbon tailboom or aluminium? I have had similar problems in the past when running carbon booms. At a particular rpm you can hit the natural frequency where the boom will twist in torsion and resonate violently. The severity will vary with load, such that with no tailblades there will just be a buzz, but with even perfectly balanced blades on the thing will try and tear itself apart. I've got some video somewhere which shows this rather well

Various solutions include adding mass to the boom to change the critical frequency (not very effective without adding silly amounts),
changing the gearing to the tail so you never hit the critical frequency (worked on my MJ), or changing the boom to one with a different natural frequency. (Going to aluminium from carbon fixed the problem on another heli)

Malcolm

05-04-2008 11:23 PM

HOMEPAGE

Hogster
Key Veteran
Location: Surrey, UK

Hi Malcolm,

Thanks for that thought. Unfortunately I am already running an aluminium boom ... I know because I had to scrape off the anodising to attach my static-grounding cable

The curious thing is that this problem has never manifested itself before .... that's what's making me suspect it's a wear-and-tear problem, possibly made worse by the recent severe vibrations I've been having (the heli, not me

), and that part of the heli is trying to tell me it needs replacing!

Cheers

David

PS. Would be interested to see those vids if you can find them!

05-04-2008 11:28 PM

HOMEPAGE

Highviz
Senior Heliman
Location: Newbury, Berks, UK

Quote
... I know because I had to scrape off the anodising to attach my static-grounding cable

Ah yes ......tricky trying to anodise carbon

Sounds like you are on the right track with a wear induced problem then if nothing else has changed since it was ok. Just a thought though - you haven't changed anything like the dampers on the camera mount have you?

Malcolm

05-04-2008 11:35 PM

HOMEPAGE

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
I'm sorry, I don't see how backlash can cause vibrations ....

Admittedly, there are a lot of variables to consider but if there is no load at all on a gear train the output shaft can ricochet back and forth between the clearance of the gear teeth. This doesn't usually happen if the gears are in oil or heavy grease but a dry mesh like we have in our helis makes it possible. I am not saying this is happening I just saying it is possible to get vibration under a no load condition.

Ace
What could be more fun?

05-04-2008 11:41 PM

Hogster
Key Veteran
Location: Surrey, UK

Well it's been during the revamp of my camera mount that this problem has come to my attention .... when the heli used to look like this:

(note: with flybar)

the vibration was very very minimal ... to the extent that it didn't both me, or the video footage.

When I upgraded to flybarless:

I started to notice more vibration (which really baffled me as there are fewer rotating mechanical parts

). I did experience some violent shakes during spool up shortly after converting to flybarless, but I found that swapping the rotor blades over made things much better.

Then when I modified the heli to look like this:

(which is the sweetest it's ever looked

)

... I've been plagued by severe vibration problems

I'm not sure how long that random-increase-in-tail-resonance has been occurring, but my guess would be not for very long ....

As you see it in that last image, this is what happened when I first flew it again

http://www.vimeo.com/962375

Thanks for all your input chaps, I'm at a loss here

Cheers,

David

05-04-2008 11:44 PM

HOMEPAGE

talk the torque
Senior Heliman
Location: SA

David

Not saying that this is your problem but earlier you asked about blade balancing. You said the same weight and same balance point from root. They should also be the same distance from LE. I'm sure you know this but if they are out then you will get the blades with different lead/lag positions which will cause balance problems. You could also check that the C of G is not to far back on the blades, very unlikely but if so this could cause the blades to aggresively seperate from tracking at a certain RPM.

As far as tail vibrations, I see you dont have tail supports before your mod?? That change might be causing it to be more drastic

05-05-2008 12:02 AM

Highviz
Senior Heliman
Location: Newbury, Berks, UK

OK - Lots of changes there.

~I would not be suprised if the combination of flybarless and mount modifications have led to severe resonance during spool up.

-Again from my own experience I originally attached my retracting cam mount to the heli using quick release anti-vibration mounts and had a really bad resonance during spool up. Changing to bungee fixing to the standard undercarriage totally transformed it.

Try changing a few simple things like taking off the Orange noodles and removing the rubber fixings on the camera mount... it might help.

For info here's the vid I mentioned:-

http://www.highvizphotography.co.uk/media/tailshake.wmv

Malcolm

05-05-2008 12:08 AM

HOMEPAGE

nooobs
Key Veteran
Location: Toronto, Canada

David

Try a spool up without the blades.

05-05-2008 12:08 AM

HOMEPAGE

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Aerial Photography and Video > Main-blade-induced vibration ....?

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