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912 with low RPM--- new vibration problem

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Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Mon Mar 10, 2008 9:35 am Post subject: 912 with low RPM--- new vibration problem

At 12:04 AM 3/10/2008, you wrote:

 	  Quote: 	
		  I spoke to a Rotax 912/914 mechanic today. He says he has seen Warpdrive

props that will vibrate once the pitch goes below some level.


	


         This is very suspect. I use the same Warp as a 912 on a 582, 

just with 3 degrees less pitch. No problem.

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

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Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Mon Mar 10, 2008 9:35 am Post subject: 912 with low RPM--- new vibration problem

At 03:21 PM 3/9/2008, you wrote:

 	  Quote: 	
		  Although I very carefully changed the pitch of the 3-blade warp drive prop

about 3/4ths of a degree, and the maximum RPM is now within the range you

suggested, the plane now vibrated a lot more than it did at all but idle

setting. Yes I double checked the uniformity of prop pitch and made certain

that the blades were seating right..


	


         Where, on the blade, did you check the pitch? My Warp has 

3/4 degree variation at the tips so what I do is equalize them at 75% 

span after getting the RPM close. I use 75% because that's 

approximately the "max force" position on the blade.

         The other thing to check is that one blade didn't accidently 

recede slightly when you were changing pitch. Even a tiny difference 

in radius will result in a huge imbalance. Make sure they're seated.

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

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Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Mon Mar 10, 2008 12:41 pm Post subject: Re: 912 with low RPM--- new vibration problem

So long as each blade was measured and checked at exactly the same point it should not matter. You aren't actually setting it for the first time to get close for a first time run, so the actual degrees at this time should not make a difference. You are only fine tuning it. So long as each blade was checked at the same point and the same amount + or - was done to each then they should be equal. You could put the prop protractor at the square tip if you wanted. You are only trying to get them all equal. I agree with guy just double check to make sure they are all the same.

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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
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Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Mon Mar 10, 2008 8:17 pm Post subject: 912 with low RPM--- new vibration problem

At 01:41 PM 3/10/2008, you wrote:

 	  Quote: 	
		  So long as each blade was measured and checked at exactly the same 

point it should not matter.


	


         I'm not sure I understand you, Roger. Are you saying that 

you'll get the right pitch later using the dynamic tracking method 

with the colored tape? Or are you saying that the blades have exactly 

the same twist? What I found on mine was that they had different 

twist, by as much as 3/4 degree. Thus if you set equal at the tip, 

they were 3/4 degree off at 75%. This makes a difference, since the 

bulk of the force is generated around 75% span. Thus I set mine equal 

at 75% and let the tips fall where they would.

         I'm not positive the dynamic tracking method is more 

accurate than the geometric method. These blades are solid carbon 

laminates and subject to some variation in flexural properties. Also, 

the tips are HAND GROUND, yes you heard that correctly. (If you want 

a shock, measure the thickness / chord ratio throughout your blades. 

Very scary.) This means the tip flexure will probably be somewhat 

different, blade to blade.

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

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Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Mon Mar 10, 2008 9:21 pm Post subject: Re: 912 with low RPM--- new vibration problem

Hi Guy,



Tracking the blades are different than a balance. Tracking is done with the engine off and just rotating the blades to make sure each crosses the line you have made at the same place. One blade should not be off the line you marked on the box. Each blade should cross the line at the exact same point and angle.

A dynamic balance is done for vibration only looking for an off set of a weight imbalance and it is done at cruise rpm while tied down and under a cruise rpm load. Any one of the three, tracking, static or dynamic balance can cause a vibration if out very far. On the setting of the blade pitch it won't matter where you take a measurement so long as you do it at the exact same spot on each blade. I do mine at the tip of my squared off Warp Drive prop. Some of the other props I do I can't do it that way so I measure in 3" on each blade to mount the protractor. I can set any prop dead on for a 2 blade or a three and I always triple check my work. Your blades should be exact matches as far as twist. If they are really off 3/4 degree then you have found the vibration problem. If they are off 3/4 degree then they can not be set to for each blade to perform equally at the same rpm under a load. They will all be different at different rpms. 3/4 degree is a lot. My blades are equal from the hub to the tips. The warp drive blades should be identical and you should be able to measure back 2"-3" and set the protractor there. When you set the pitch on the ground and not under a load you can make the fine tuning adjustment anywhere on the blade because they are or should be all equal at that point and not flexing. We aren't trying to set up a course pitch for the first time. Just trying to add or subtract a small amount for a fine tuning. 

A dynamic balance can only be done when the tracking, blade pitch and static balance are all done or you may never get a decent dynamic balance. A dynamic balance measures vibration in the engine, gearbox and prop and then instructs you to as how much weight to add and where to add it so as to offset the weight imbalance in the prop hub assembly with all the hardware attached.

The tips can be used even if they are hand ground because you are going to put the flat side of the protractor against the flat side of the prop.



I hope this helps. You can call me if you have any further questions. Sometimes that's easier than long emails.  520-574-1080 Tucson, Az.

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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Tue Mar 11, 2008 5:48 am Post subject: 912 with low RPM--- new vibration problem

The hand grinding probably is the reason Warp requires the blades be set

using the tip.



As for dynamic balancing... It is really the fine tuning of the balancing

procedure.  Just like you can get a pretty decent balance on your car using

a counterbalance beam, if you really want it done right so you don't get a

shimmy at 70 mph you will want to have your tires dynamically balanced.

There are some places that even dynamically balance the wheels while still

on the car.



How critical can balancing be?  Well on Bell 206 helicopters the final

dynamic balancing doesn't occur with the main rotor turning.. It doesn't

occur in a hover... It must be done when the chopper is moving at least 60

kt.  The adjustments are very fine and even the weight of flies crushed on

the rotor can send the balance out.  BTW keep your blades clean they turn a

lot faster than rotors.



Noel



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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

lgold(at)quantum-associat
Guest

Posted: Tue Mar 11, 2008 8:21 am Post subject: 912 with low RPM--- new vibration problem

Guy,

I reset the pitch to where it was before the vibration started... No change;

it still vibrates throughout the rpm range (even seems a little worse). 



I set the pitch at the end of the blades, per Warpdrive's instructions. The

blades all track exactly the same and are the same length (yes, I pulled

them out as far as they would go before uniformly tightening the bolts), and

Warpdrive tells me they all weigh the same. 



As you suggest, I will see if the pitch differs at the 75% point, but since

there was no vibration a few days ago and since I have put the pitch back to

where it was before the problem started, and since I have no reason to

believe the blades warped in the last few days, I don't think that pitch

variation is the problem... so the trouble is probably in the engine!##?(at)!!



So far I have set the pitch really carefully, sync'd the carbs, checked the

motor mounts, changed the plugs, made certain the fuel is flowing properly,

and know that the engine can reach or get close to its maximum RPM. The

vibration is felt throughout the RPM range. The 912 ULS engine (SR 5643233)

only has 20 total hours on it 



I don't know where to go from here. Any ideas ??? 

Les    



[quote] --

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Tue Mar 11, 2008 10:35 am Post subject: Re: 912 with low RPM--- new vibration problem

Is your gearbox within the group for recall? Probably not, but I had to ask.

If it were me I would do a dynamic balance. Anything else at this point is guessing and chasing your tail. The balance guy can probably help you on finding the vibration if it's not the prop. Remember the prop blades may weigh very close to the same, but all the hardware to fasten it down doesn't.



Do you have a mag rpm drop when you do a mag check?



Check your gearbox magnetic plug and look you debris. If something has started to wear badly or chipped inside you will start to see it at the plug.



I know this is very frustrating.

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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

rmcmill(at)zoominternet.n
Guest

Posted: Tue Mar 11, 2008 5:16 pm Post subject: 912 with low RPM--- new vibration problem

what type of instrument did you check carb sync with vac. or mercury typ? 

what level is the circlip on needle jet? check for any debris in carb bowls.

---

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