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

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

Hogster
Key Veteran
Location: Surrey, UK

It's back :(

Hi all,

Well over the previous weeks/months that vibration problem has come back

I've tried the following:

- Checking the balance of the blades, they're both spot on
- Checking the main shaft - dead straight
- Checking the feathering spindle - dead straight (although has some wear marks on it ... seems it's been rubbing on the plastic inside the rotorhead ... looks a bit like this:

- Running the motor up with no main or tail blades - no vibration
- Running the motor up with only tail blades - very little vibration
- Swapping the main blades over ... the vibration seems to be worse in one configuration than in the other

- Tightening/loosening the blades and noting the effect ... running them a bit tighter seems to help a little ...
- Replacing the rubber O-rings with some older 'harder' ones (the ones I was using seemed to be quite soft ... thought this might have been causing it) ...

The strangest symptom of all is that the heli seems to have a time period for oscillation! One second it will be vibrating, then it will disappear, then it will come back again, with a regular period of a few seconds! I thought it may have had something to do with the frequencies of the main rotor and tail rotor 'beating' with each other ... ie. one being at a frequency not quite at a multiple of the other ... but why has that only become noticable now? I'm running 1800 on the head btw.

Another strange thing I've noticed, combined with the above effect, is that when the heli's vibrating, if I look down on the rotor disc at an angle, I can see the (normally-smooth) blurred disc flickering somewhat .... as if the blades are fluttering very slightly ... Also I've noticed that the blade tracking is good on the left side of the helicopter (looking from the rear), and not so good on the right!

Argh, so many different problems! Do you think something has simply worn out and needs replacing? If so, what?

Any thoughts guys? In my eyes it's not very professional seeing the heli shaking so much when flying

(as it was when I was flying a big AP shoot yesterday!)

Many thanks

David

02-18-2008 10:56 AM

HOMEPAGE

FCM
Elite Veteran
Location: Back in Blighty!

Yes, your flybar paddles are not aligned properly. This can cause split blades one side of the disc and in-track on the other. Just do a proper paddle alignment and fly and retrack the mains.

Hope this helps.

Paul.

02-18-2008 03:17 PM

CoastalTom
Senior Heliman
Location: Foley, AL (7 miles N of Gulf Shores/Orange Beach)

David,

Another thing to consider is the actual blade grips. I don't know if yours are metal or plastic. I recently had problems with some raptor plastic grips in that they were old, and the bearings had actually worn the inside of the grips enough that there was considerable wobble when mounted. Replacing bearings/dampners/spindle didn't help until I replaced the grips.

Good luck my friend.

Tommy Patterson - Gulf Coast Aerials

02-18-2008 03:28 PM

HOMEPAGE

Flyingeye
Senior Heliman
Location: San Diego, CA, USA

Fine Tuning CG

Here's my 2 cents on adjusting CG.

To get close, check this way:
With the blades 90 degrees to boom, pick up heli, one hand on each blade grip. Then tilt the heli a little (say 10 or 20 degrees) one side then the other holding the blade grips. If the tail swings, adjust the CG.

Now you're close.

Use a swash plate leveling tool to get the swash plate completely flat at your hovering pitch. Center your trims and use the subtrims to make the final adjustments. With swash plate completely flat, pick up into a hover and now move the CG around to trim the helicopter out. Do this on a windless day. If you have any stability augmentation devices, turn them off for this test. Do not adjust the trim settings once you have leveled the swash plate. Be sure to remove the leveling tool before flight; it's easy to forget!

Now you have a helicopter that is truly in trim as there is no mechanical cyclic input being used to maintain a level hover. Now you know that the cyclic is contributing the absolute minimum to your overall vibration level.

There is an example of a swash plate leveling tool on this page:
http://helihobby.com/html/rc_heli_tools.html

02-18-2008 03:40 PM

tabbytabb
Elite Veteran
Location: seattle

Loosen the blades,

Logos hate tight blades. I run mine so loose that I have to prespin by hand and then fire it up.

Give it a shot it might help.

Reason you might notice it sometimes and not others is the weather changing from warm to cold changes how tight the blades are in their grips.

Tabb

02-18-2008 04:56 PM

HOMEPAGE

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
Now you have a helicopter that is truly in trim as there is no mechanical cyclic input being used to maintain a level hover.

A single rotor helicopter with an opposing tail rotor cannot hover without cyclic because of the sideward’s force created by the tail. The heavier the helicopter the greater the force is needed to oppose the main rotors and that will result in more cyclic to keep the heli stationary. Counter rotating main rotors is the only way of eliminating cyclic in a stationary hover.

A stabilizer does not add cyclic to a rotor system. It is more apt to dampen its oscillations vs. being controlled by hand. That is why they are so useful for AP.

David, is there a lot of backlash in one of your servos, maybe just at the hover point? If the flybar paddle alignment doesn’t work is one blade weaker in torsion than the other, maybe a hairline crack?

Helis … when they work they are awesome, when they don’t, they can get irritating.

Ace
What could be more fun?

02-18-2008 05:35 PM

Hogster
Key Veteran
Location: Surrey, UK

Wow thanks so much guys!! You've come up trumps

I will try all your suggestions and let you know how things go!

Thanks again!

David

02-18-2008 05:42 PM

HOMEPAGE

eyeinsky
Senior Heliman
Location: Fall River, Nova Scotia, Canada

Dave

May be your blades have a twist or a soft spot. Try and put it out of track a little bit in one direction see if it gets better, if not try the other way. Some blades just like to run out of track. Not ideal for your application. You may have to live with the vibration till you retire those blades.

Good Luck

Hard job competing with gravity.

02-19-2008 12:56 AM

Hogster
Key Veteran
Location: Surrey, UK

Hi all,

Haven't had much time to do much to the heli, but have just straightened the flybar and paddles - the flybar was a little bent ...

I also moved the Rx pack to the front of the heli under the main battery pack as I've always suspected my heli of being slightly tail heavy ... and hence I've had to use a little bit of cyclic to keep it level in the hover = bad!

Once the crazy winds we have here die down a bit I'll give it a fly to see how things are now

Cheers,

David

03-01-2008 12:21 PM

HOMEPAGE

xfc3dcd
Heliman
Location: West Carrollton, Ohio usa

Quote

Hogster I feel your very rear heavy helicopter with cyclic compensation to keep it level may likely induce cyclic vibration. On a heli with a good C of G the main blades are primarily lifting with little or no cyclic involvement. Now when you trim it fore 90% like you said not only do you run into travel range limitations, but the rear of the rotor disk supports more load than the front, requiring the cyclic movements, leading to vibration. I also feel that excessive constant cyclic loading will likely wear out the rotorhead faster than a balaced setup that only uses the cyclic control to change the angle of the rotor disk and runs a relatively neutral mode when right stick commands are not put in.

Having a helicopter that is overly nose or tail or side heavy and the associated need to add cyclic to hold position is not, on its own, usually a cause of vibration. In essence dynamically balanced blades need to come out of track to vibrate. If they don't - it won't. Cyclic induced vibrations are the result of inconsistencies between one side of the rotor to the other, resulting in unequal blade lead-lag causing a change in each blades position relative to the other. Common causes for lead-lag variations could be unequal blade grip tightness, unequal blade chordwise CG, unequal blade torsional rigidity (one twists more than the other), and so on. A quick test to eliminate blade lead-lag as a potential problem is to lock both blade grip bolts down tight with each blade EXACTLY 180 degrees apart (and in-line with the blade grips) and hover.

Wendell

03-01-2008 03:43 PM

Hogster
Key Veteran
Location: Surrey, UK

Gave it a test today and there's a definite improvement

However, the tracking still seems to be worse on one side than on the other (looking at the heli from behind) ... and there's still a small vibration present .... which increases when I start climbing quickly.

So that can be caused by the flybar paddles being out of alignment? They're as in-line as I can get them at the moment ...

At some point soon I hope to upgrade to a flybarless head which should cut down the number of possible culprits down considerably ... wish I had the money to do that straight away

Is it worth trying new blade grips? They don't seem to be very loose/sloppy though .....

I tried increasing the headspeed and the vibration did seem to lessen ... at the cost of putting more strain on the motor ....

Any thoughts are most appreciated,

Thanks,

David

Edit: Hmmmm having said that, there does seem to be a fair amount of flex if I apply a lifting force midway along each rotor blade ... I would say the blades cone upward by about 20mm at the tips when lifted from a resting position .... could that point to the blade grips being worn?

03-02-2008 07:52 PM

HOMEPAGE

rroback
Key Veteran
Location: Irvine (UCI), Ca

make sure the linkages are all the same lengths.

Rhett... There's no power like E Power!

03-02-2008 07:58 PM

Hogster
Key Veteran
Location: Surrey, UK

Thanks Rhett

I've had a look and they all appear the same length. The thing is, this problem has just manifested itself over time ... I haven't done any modifications to the head in any way. That's what's making me think it's a problem caused by component wear ...

David

03-02-2008 08:05 PM

HOMEPAGE

Disciple4123
Veteran
Location: Waynesboro, VA USA

I think your issue is still located in the flybar system due to your symptom: tracking is different on different sides. One issue with straightening the flybar, it can still have curves. Fore example it can be curved off of center when looking downward on it, or can bow up and then down at the last minute. Was it straightened off the heli or on? Are the blade grips making a clicking/popping sound as if they have loose bores, or just flex? I would only consider them to be causing issue if their vertical test flex is accompanied by visible bearing movement in the bore, or popping sensations.

After getting the flybar straight, I'd zero out each paddle with a pitch guage to within 1/2degree. Do it in the same position, i.e. zero out with it aiming boomward, then do the other one at the same position, then check both and see that they are near zero at the front positon (ensuring your swash was level during adjustment).

Regarding the topic of CG: Our heli's do not function like an R22 or other 2 bladed fullscale, where the teetering axis is inches above the blade centerline, allowing the system to dynamically offset blade flex induced, blade CG changes. The elevated teeter cancells most of the tendency to generate vibrations by cyclic on fullscale. Our heads tend to be limited to a dampened teeter central with the blade centerline, losing that advantage.

Then again any rotorhead with 2 blades has moments when the blades are perpendicular to the given cyclic command, and they relieve and re-apply cyclic twice per revolution as they rotate to a polsition of maximum influence, and then minimum influence, leading to an opportunity for a vibe. CG issues are not the first place to look when experiencing a vibe, but Flyingeye has the right idea for getting in the ballpark.

Eric

03-02-2008 10:42 PM

HOMEPAGE

eyeinsky
Senior Heliman
Location: Fall River, Nova Scotia, Canada

David

These vibrations on rotating blades can be quite difficult to over come.
For instance I just spent the entire weekend working with squawk on my company’s corporate jet. We had noise coming from the left engine low frequency hum and slight vibration at 50 to 70 % Fan speed.

We start by performing a dynamic vibration analysis of the engine. Every thing was with in limits, but we continue to balance the blades reducing the vibration by 0.1IPS but there is little change to the noise.

Every one involved was scratching there head. At this point I must add, I have brought in the big guns, with all sophisticated test equipment.

Second round we decide to perform an acoustical frequency test. In a nut shell microphone connected to laptop with some software that graphs noise level at all frequencies, this yields a noticeable peek at fan speed frequency 95db's.

So we have noise and vibration and it is with in limits. Most people would walk away but this is a corporate jet and we have to get rid of this noise.

So we remove all 30 fan blades and take them to a special scale device. This allows weighing of each blade then entering the weights into a computer program that positions the set of 30 blades for best static balance order. Next we assemble the fan in the determined order. Dynamic vibration analysis carried out and balance weight installed reducing the vibration to as low as .04IPS

Only at this point we decide the noise level was acceptable even though all limit were acceptable in all previous test. It seemed like there was some harmonic resonance that the airframe was amplifying and untill we change the harmonic characteristic of the fan we could not reduce the noise.

David what I trying to say is change what you like, analyze the result then move on to the next item and next till you move to an acceptable level but do not give up. You will get it.
I had a vibration issue with my AP heli too. I found out that had accidently swapped out one of my buddy’s bent rotor shaft that I had on the work bench.

Good luck

Hard job competing with gravity.

03-03-2008 04:00 AM

Hogster
Key Veteran
Location: Surrey, UK

Gosh thanks so much guys!

I will do my level (pun intended

) best to get the flybar and paddles aligned as best as I can.

I've just ordered a new flybar too just in case the other one was still slightly bent (I hammered it as straight as I could get it by eye ... doesn't mean it's straight though

)

Will let you know how things go! Thanks once again!

David

03-03-2008 11:00 AM

HOMEPAGE

FCM
Elite Veteran
Location: Back in Blighty!

They need to be dead true to prevent the blades splitting. I use flybar paddle gauges and although I was sceptical about their necessity to start with, I have found them to be invaluable in setting my Benzine paddles. Before I used to 'eyeball' paddles, that is before I started AP

Paul.

03-03-2008 12:24 PM

Hogster
Key Veteran
Location: Surrey, UK

Hmmm interesting! I knew they had to be level but I didn't know that they had to be *so* level ...

Will do my best and report back

So when the swashplate is level, the flybar paddles should be perfectly parallel to the rotor blades at all times?

Thanks

David

03-03-2008 01:33 PM

HOMEPAGE

FCM
Elite Veteran
Location: Back in Blighty!

Yes, the swash should be level when you finally align the paddles.

Without going into too much detail, what I do is align the two paddles with each other using the gauges first with the flybar lightly gripped in the carrier. I then level the swash and align the paddles with gauges fitted, with the blade grips or blades if you have room to swing them. I then finally tighten the flybar in it's carrier. Then I recheck everything in case something was bumped when tightening.

Edit: Watch out for tight ball links giving you false readings i.e. sticking, when doing this.

Of course, none of this will help much if you do have worn head components.

Paul.

03-03-2008 01:59 PM

eyeinsky
Senior Heliman
Location: Fall River, Nova Scotia, Canada

Dave

There is some thing else you may give a try. If you take a video with a camera that operates at 30fps it will give the visibility that your blades are stationary. I Know this because my machine operates at the same rpm, you may be able see something if you review the video. An other option; take strobe light that you use to time a old school car engine then add a frequency generator to signal trigger pickup, you would have useful tool for trouble shooting. The strobe would give you the same effect when dial to the correct frequency.

http://chucker54.stores.yahoo.net/timinglight.html

Look at the video you will see what I mean. It appears like the blades are stopped.

http://www.youtube.com/watch?v=reKt5wh8f3o

Good luck

Hard job competing with gravity.

03-03-2008 10:07 PM

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

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