Vibrations at higher RPMs

[Anticept]

Also, quartering tailwind causes funny effects on props, and many planes vibrate when it gets like that.

[IrishAl]

I have had vibrations in all the modes mentioned here, including the mode which started this thread which was a vibration during run nup when I experienced a quartering tailwind. I originally checked and corrected uneven pitch on my prop blades and rebalanced my carbs. This helped vibrations during low speed flying but didn't help the run up vibration. All engine mount bolts on the engine and firewall were checked and found to be correctly torqued. Prop track was checked and found to be to spec. I then had the prop dynamically balanced. This improved my cruise vibrations but didn't help my ramp run up vibration. I then checked the "torque to turn" of the prop. This value can be determined by locking the crankshaft and then applying a fish scale at a known distance out from the center of the prop. The prop is turned approximately 30 degrees and the lb./ft. result is recorded. I found this to be on the absolute low side of the spec. Discussion with Roger Lee and others resulted in the conclusion that my gearbox needed to be inspected and probably re-shimmed. The box was inspected and re-shimmed and reinstalled. This reduced my vibration during run up to a level that I considered to be highly acceptable and compared to other CT's which my friends own. It also improved vibrations in other modes of operation, including steady cruise.

Dick,

How many hours were on your engine when you reshimmed the box?

[FlyingMonkey]

 Me too, but balancing the carbs sorted it out as Roger suggested.

 

My carbs have been recently balanced, and I had the prop dynamically balanced (to within 0.54) at last annual in May.  Before the prop balanced I had noticeable vibration at all RPM, enough that you could see the mushroom shaking a little.  As I said, everything is buttery smooth now except at that tiny RPM range, and only in flight.  I'm just going to chalk it up to my plane's personality I think.

[WmInce]

My carbs have been recently balanced, and I had the prop dynamically balanced (to within 0.54) at last annual in May.  Before the prop balanced I had noticeable vibration at all RPM, enough that you could see the mushroom shaking a little.  As I said, everything is buttery smooth now except at that tiny RPM range, and only in flight.  I'm just going to chalk it up to my plane's personality I think.

 

I just try to avoid any excessive vibrations in any mode (ground or flight), by slightly changing RPM.  Sometimes that means adding a little throttle at the lower end, during taxi.  That's okay.

That withstanding, I also try to avoid "riding the brakes."

[Jim Meade]

For those who have dynamically balanced the prop, does your prop manufacturer have a published procedure? Where do the weights go?

[Roger Lee]

Gearboxes get re-shimmed at 600 hrs. for 1500 hr TBO's and 1000 hrs for 2000 hr TBO's

[Anticept]

For those who have dynamically balanced the prop, does your prop manufacturer have a published procedure? Where do the weights go?

Weights go in the bulkhead, not the prop. On cts, you drill a small hole and put washers on both sides to spread out the forces.

[Jim Meade]

Weights go in the bulkhead, not the prop. On cts, you drill a small hole and put washers on both sides to spread out the forces.

And is that per the manufacturer's instructions? I was told Neuform had no procedure for dynamic balancing.

[Anticept]

And is that per the manufacturer's instructions? I was told Neuform had no procedure for dynamic balancing.

In the united states, dynamic balancing is an airframe alteration, not a prop alteration. That is why neuform does not have a procedure. It is flight design who needs to provide the instructions. I was told to use the chadwick helmuth or aces general documentation.

 

Side note: Static balancing IS a prop repair/alteration (std aircraft), and can only be done by prop shops or appropriately rated persons (which, in standard aircraft at least, there is no such rating for individuals that i know of).

 

See chapter 3 here: http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_20-37e.pdf

[Roger Lee]

I use an Aces Dynamic balancer. Instructions are in their documentation. The system cost around $4K

[Anticept]

I use an Aces Dynamic balancer. Instructions are in their documentation. The system cost around $4K

http://www.probalancersport.com

 

Those are about 1.5k. Haven't tried one yet, but seems like it is a good simple balancer for fixed wing.

[Doug G.]

Something that could be cost shared among repairman/experimental folks and shipped among us? Details would have to be worked out.

[Anticept]

I thought about buying the 4k model and letting people rent it, but without knowing anyone, it feels like it might get damaged and nobody take responsibility.

 

If you have a good rep with local shops, try asking them.

[Tom Baker]

Our EAA chapter just bought a Dynavibe balancer for our members to use, but I haven't used it yet. Last year we bought a set of aircraft scales.

[Runtoeat]

IrishAl, my CT had 620 hours when I had the gearbox re-shimmed. The previous owner of my CT had the gearbox inspected by Lockwood at 200 hours because he mistakenly thought this was required by FD. I do not know if Lockwood just inspected the box or re-shimmed it. I suspect, with 200 hours on it, re-shimming was not needed. The re-shimming resulted in noticeable changes for the good regarding vibrations in all modes but especially in the run up on the ramp with winds coming from the rear. As Anticept indicates, this vibration mode is noted on many aircraft but my CT was particularly sensitive to this. It is noted that I have also had one of the carb vent lines which run into the airbox on older CT's become dislodged. This will definitely get one's attention due to the pronounced engine roughness that results from this.

 

A comment about shared diagnostic equipment. When working, I shared vibration and sound and balancing equipment in labs and in the field. The WORST job was to be the one responsible for maintaining this equipment. Although there were no charges levied for broken co-ax cables, accelerometers, etc., very few who broke equipment would take the time to let the maintenance guy/gal know about the damage. I would highly recommend that rules be drawn up for any shared equipment and there be a chosen "baby sitter" who insures that missing or damaged equipment is dealt with at the time of return. This position can be rotated among the equipment users. This give everyone the experience of being responsible.

[cdarza]

These vibrations can be caused by a couple of things. At that RPM it can be carb sync is out. Then it could be one or more prop blades not the same pitch as the others. You may need to do a dynamic prop balance. Your gearbox may need to be re-shimmed.

 

 

Is there a "test' to check if gearbox may need to be re-shimmed?  

[Anticept]

Is there a "test' to check if gearbox may need to be re-shimmed?  

 

Most often, the friction torque in free rotation test will tell you. LMM 12-20-00 14.1 (page 67). Does your mech give you the completed checklist after each inspection? Have a look at it and see what was written down for the test.

[Roger Lee]

Sometimes it is not just getting the gearbox re-shimmed. You may even need to replace the gears. If re-shimming or a prop balance doesn't take care of the vibration then replace the two gears. 

 

p.s.

Just make sure the carbs are synced.

[cdarza]

Noted on the friction torque ( it is within normal limits)  and balanced carbs.  Thank you.   No Rotax mechanic in the Philippines so i do my on research and have a certified mechanic do the work that i guide him on.   Perhaps i am over thinking but best to be on safe side.   When i read about re shimming, i just assume perhaps mine might need it too.   I am not even sure how small a  vibration would qualify as a vibration that needs attention.  Big ones would be obvious

[Roger Lee]

There is a normal vibration as you throttle back around 4000-3600 rpm. If this is what you are talking about you're okay. If it is at take off then make sure all your wheels are balanced.

[cdarza]

YES Roger - thanks for reply.   The take off  'shudder' was thought to be unbalanced wheels and i had meant to inspect and attend to it - but delayed it way too long.    I temporarily just applied the brake after take off to stop the wheels spinning.   Just yesterday i decided to finally attend to it and as much as i hate to admit my lapses - here is what happens when something is neglected/ not inspected  (Sharing this to learn from my mistake)  

[FastEddieB]

Wow!

 

Nice catch.

 

Better late than never!

[FlyingMonkey]

YES Roger - thanks for reply.   The take off  'shudder' was thought to be unbalanced wheels and i had meant to inspect and attend to it - but delayed it way too long.    I temporarily just applied the brake after take off to stop the wheels spinning.   Just yesterday i decided to finally attend to it and as much as i hate to admit my lapses - here is what happens when something is neglected/ not inspected  (Sharing this to learn from my mistake)  

 

Yikes.  I don't think your tire wear is caused by balance issues.  You seem to have an alignment issue that needs to be checked.  Note the lack of wear on the outside and severe wear on he inside of the tire.  PM me if you want to talk about how to check the alignment.  You'd need to put Matco wheels and brakes on it to be able to adjust it.

 

How many hours are on those tires?

[Tom Baker]

Yikes.  I don't think your tire wear is caused by balance issues.  You seem to have an alignment issue that needs to be checked.  Note the lack of wear on the outside and severe wear on he inside of the tire.  PM me if you want to talk about how to check the alignment.  You'd need to put Matco wheels and brakes on it to be able to adjust it.

 

How many hours are on those tires?

 

The alignment can be fixed without having to install Matco wheels and brakes. However the Matcos do make it easier.

[Anticept]

No cords are broken on that tire. Needs replaced, but it's not like it was going to blow out on the next flight.