Head balancing issues

[MadBird]

I'm at a loss at the moment and need your help guys and gals. This is my first 700 size bird, I've been trying to maiden her and thus far have not had any success in anything more than half a dozen 5 - 15 second hovers.

My problem seems to be that the head appears out of balance some how.

Setup: (All brand new for this build)
VBar Silver (Swash Agility 82, Swash Gain 102, Flight Style 82, Tail Gain 85)
RJX Servos all round
YS 91-SRX w/ Hatori pipe
Rail 696's and Rail 106's

Details:
When I spool up, the head seems to me to have a slight wobble. As the rpm increases the heli visibly shakes on the ground and as it lifts off the shake is more noticeable. As you could imagine the collective and cyclic are very difficult to control while in the air. The tail has a bit of wag, but considering all that is going on with the shaking it is holding quite well.

I'm not running a gov just yet so I can't give you an indication of what rpm I am running, but even if I increase rpm the shake is still there. Once the throttle is cut and the blades slow down with no load you can see the heli rocking side to side slightly until the blades come to rest.

Thing that I have tried:
Made sure the blade bolts are not too tight.
Adjusted vbar gain settings on the tail down from 100 to 85.
Tried differing tunes on the engine (Very rich through to pretty lean)
Checked main blade CoG balance and weight
Removed main shaft and checked for bending
Checked head assembly to make sure shims, thrust bearings, dampers etc are installed correctly

Nothing seems to make any noticeable difference. The only thing at the moment that is drawing my attention is the main shaft. When i drop it down through the main bearings it slides through the first two with no problems. When it gets to the bottom bearing though, it slides through effortlessly in one instance, but when rotated 180 degrees it will hit on the inner race. With only a slight amount of weight it will drop through and there is no binding when the shaft is spun by hand. I've rolled the main shaft on the table with a light behind it to see if the light shines through indicating a bend however everything seems ok. Like I said earlier I am at a loss. I think I'll order a new main shaft and see how that goes. I'm building an E7SE at the moment and was hoping that shaft would fit, but no joy there.

Sorry for the long post, but hopefully someone might be able to decipher all of my ramblings and point me in the right direction.

Cheers
Matt 

[Ross Lawton]

Hi Matt,

It is a very strange problem you have there, but great report on your problem and your efforts to resolve it. Its good to have all the information on what you have tried so far. 

Usually with something like this it says to me there is something amiss in the head somewhere, have you tried another set of blades by any chance?

Also, do you grease your head dampers? If not i recommend this as it allows the head axle to float much easier as an ungreased axle can produce an effect like your experiencing. I use a silicone rubber friendly grease (Tri-Flow with teflon), and use a very small amount on the dampers and axle surface to help it to float.

Do the blade grips rotate freely when they have no blades or pitch control links attached to them?

Have you checked the head axle for true running?
Have you checked the main frame assembly for squareness?

Couple of things i would try, and although you have done some of them before it's worth double checking:

1) Check Main shaft for true running
2) Check Head Axle for true running
3) Completely dismatnle the head assembly and check the entire assembly, make sure everything is in it's correct orientation etc.. Thrust bearings largeID to inside, smallID to outside etc.. make sure there is no extra shims in there etc..
4) Grease the head axle if you don't already
5) Check the frames for Squareness
6) Change the main shaft & head axle

When you change the main shaft loosen the lower bearing block and allow it to float in the frame while you install the mainshaft in the two upper bearing blocks.

Hope this helps and let us know how you go.
Cheers,
Ross

[Kevin Dalrymple]

Have you changed the head shims? If they are 1 mm go to .750mm or even .500mm. You can get them from Synergy.   

[Rodney Kirstine]

To add to what Ross mentioned about grease on the dampers... Did you apply grease to the outside of them when you installed them?  If not, they may not be seated in properly, which could cause this as well.

[flyalan]

Lube the dampers. Try the thinner shims.
First thing I would do is lower your head gain and see if it improves. I did see you mention tail gain for your wag but have you tried lowering the head gain a bit. 
The dampening in the head is pretty hard which in turn usually requires lower head gains.
What head speed are you at on idle 1?
As the others mentioned do consider swapping out shafts etc also.  Still would try a lower head gain though. Maybe back it down to 85 or so.

Cheers
Alan

Sent from my SM-N900V using Tapatalk

[MadBird]

Thanks for all of the input fellas. I'm out of town for a couple of days, but when I get back I'll try out the suggestions.

I have stripped the head down and re-checked everything and lubed the dampers inside and out although I might try the head off the E7SE as well and see how that goes. In the little bag of extra bits 'n' peices I remember there being a couple of shims in there, are they 1mm shims also? I will be ordering a new main shaft etc, but don't want to bother buying extra shims if they're already supplied. I might grab some Trueblood dampers as well.

Cheers
Matt

[Roger]

You might have grips that are not equal in weight.  Try balancing the head and grips with a prop balancer.  ROG

[Clarence Creer]

As Alan said, you really need to back that head gain down. Take it down to 85. You will probably be able to get it up to 90-93, but for now, take it down. Also, what is your headspeed?

[Jon Mills]

Couple things come to my mind here. 
How tight are your links? I am assuming you have a botosizer because you mentioned your building an E7SE? The tail wag and hard to control cyclic and collective are all side effects of links not being properly sized.

How tight is not too tight on the blades bolts? I had a little problem with a wobble on my maiden caused by blades that I thought were really loose being too tight.

I found the N7 to fly best when the blades are loosey goosey.


Main shafts are not all perfect as we all have found I'm sure. If you don't have a dial indicator an easy way to tell if its bent is, when its rolling on glass if its bent your going to hear the whop whop whop whop....  if its not it will sound like, actually I can't describe the sound. It is almost mandatory to have a known straight shaft to compare the noise to when doing this test.

Good luck hope you get it sorted out, please let us know how it goes!

[MadBird]

Hey guys just a quick update to let you know where I'm at with this issue.

I only had a few minutes spare to have a play over the weekend. Problem still persists, but I have a few of your suggestions to try yet.

Things that I have tried since my initial post.
- Checked main shaft and head axle for trueness. Both ok.
- Dismantled head again and checked radial bearings and  thrust bearings, made sure races were in correct locations etc and lubed properly. Grips rotate freely.
- Removed and re-greased dampers inside and out.
- Checked frame for squareness
- Checked ball link tightness (originally sized with Boto-sizer and all have smooth free movement with no slop).
- Main blades nice and loose, just about too loose really. Have tried tighter grips also
- Lowered Vbar swash gain down to 80.

Hopefully I'll get some time after Easter to have another crack. I've still got the freshly built E7SE head to try and also some thinner shims for the N7 head. There were some 0.8 and 0.5mm shims supplied in the kit and I bought a couple of extras too. Even though the main shaft and head axle seemed alright I have some spares now that I can also try if the shims etc don't help. I have another set of main and tail blades to try as well.

I don't have the gov set up yet so I'm not sure what the head speed is during the hover atm, but I will get that set up in Stunt 1 for the next try so that I have a base measure. I will also try to get a live vibe analysis via blue tooth to post up for you all to have a squiz at.

Thanks again for your help, it is greatly appreciated.

Cheers
Matt
 

[Matt Botos]

Hello Matt,

Check your Vbar logs to see at what RPM the vibrations are coming from. If the vibrations are from 14,000-16,000 in your spectrum analyzer then your motor is producing the vibrations and you can start troubleshooting in the right place. Odds are your are looking to the left when the problem is to the right.

I would try setting up the governor and making sure the engine is running at 1950 rpm. Also make sure your engine is tuned properly. Many times a motor that is running rich will "4 Stroke" and create excessive vibrations.

Hope that helps! Let us know.

Thanks,

Matt

[Kevin Feil]

Matt,

 I'm going to be out of town this week in FLA. however, if you are still having the issue and feel it is in the Head Assy, let me know. I will send you one of My N7 Heads that I built up but haven't flown yet. 0 Time so to speak. If it comes to that I will have you send me yours and I can evaluate it with Matt B. but like he said, follow his advice and keep us posted. I can say it of all N7's produced to date, this is the first time I have heard of this, "Keeping in Mind" the head becomes the focus. Not common at all. The offer is there if you need it..

Cheers,

Kevin

[MadBird]

I'm back!  Sorry for not posting for the last week and a half with any updates, I've been away with work and also had a short trip away over the Easter period, so I haven't even looked at the N7 let alone touched it.

Today I finally made some time to have a look at this beast to try and sort her out and I can happily say that I have had great success! ...and surprise, surprise it was all my fault... Doh!

Basically, most of my problems can be attributed to over-speeding of the head. Mechanically the setup was fine and as I had posted earlier I've stripped the entire head assembly, main shaft and gear train down and checked everything for trueness, binding etc. After that all checked out and taking into consideration all of the advice that was given about setting up the governor, the first thing I did today was set up that bloody governor! I also reset all of the needles back to factory default on the engine and started from scratch there too.

With a little bit of tuning and the gov set at 1800 rpm I could tell instantly that the head speed was much lower than when I was just using the throttle curve. I managed to hover through a full tank in my back yard without any of the aforementioned behavior except for a bit of wagging in the tail. The vbar log didn't even throw out any elevated or high vibration warnings for the entire flight. Tomorrow I hope to make a trip out to the field and undertake some fine tuning to tighten up the tail and give her a thorough maiden.

Thanks to everyone who has offered advice. I can't wait to start enjoying this bird.         

Matt,

 I'm going to be out of town this week in FLA. however, if you are still having the issue and feel it is in the Head Assy, let me know. I will send you one of My N7 Heads that I built up but haven't flown yet. 0 Time so to speak. If it comes to that I will have you send me yours and I can evaluate it with Matt B. but like he said, follow his advice and keep us posted. I can say it of all N7's produced to date, this is the first time I have heard of this, "Keeping in Mind" the head becomes the focus. Not common at all. The offer is there if you need it..

Cheers,

Kevin

Thanks for the offer to lend me one of your spare head assemblies Kevin, it is quite humbling to see the amount of support you all provide for a product that you love so much.

Cheers
Matt

[Ross Lawton]

Great news! It's good to hear you have finally traced this and got it rectified. 

Happy flying and enjoy that N7, she truly is a beast!! 

Cheers,
Ross

[Kevin Feil]

Like Ross said, glad to hear you got it sorted out brother! And your more than Welcome the pleasure is all mine?.

Best Regards,
Kevin