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914 vibrations


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Had a wheels up landing in my Sabre, mechanical failure, broken prop, gearbox checked out by Wal , all good, borrowed another prop from a mate, Kaspar, had to change the pitch as it was over reving, now have a bad vibration as is throttles back past 4000 revs , only noticed this as I was flying to deni to get some panel work done, could it be caused by a miss match in the pitch of the blades , cruise and idle it is good , got a mercury balance plate fitted. 

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54 minutes ago, rhtrudder said:

Had a wheels up landing in my Sabre, mechanical failure, broken prop, gearbox checked out by Wal , all good, borrowed another prop from a mate, Kaspar, had to change the pitch as it was over reving, now have a bad vibration as is throttles back past 4000 revs , only noticed this as I was flying to deni to get some panel work done, could it be caused by a miss match in the pitch of the blades , cruise and idle it is good , got a mercury balance plate fitted. 

One early check I would do is observe the movement of throttle levers / cables; any binding or hesitation in the movement from full to idle on one of the carbs compared to the other.

Edited by Blueadventures
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Hi Rhrudder- dont have a 914 (wish I had)but do have a 912 ULS so may be able to contribute a little or could be all fake news

 

In my application I took a great deal of time & trouble to static balance, pitch and then dynamic balance my 2 blade Fiti prop. Ran like a sowing machine from the static balance/pitch onward, so noticed little change with the dynamic balance. I guess I am going the long way about suggesting you may have been a tad hasty in your temporary re propping.

 

I would also second Blueadventures suggestion again you can only benefit from aa precise as possible pneumatic balance of the carburettors.

 

One other point - check out your engine mount & rubbers for nil damage/security

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1 hour ago, skippydiesel said:

Hi Rhrudder- dont have a 914 (wish I had)but do have a 912 ULS so may be able to contribute a little or could be all fake news

 

In my application I took a great deal of time & trouble to static balance, pitch and then dynamic balance my 2 blade Fiti prop. Ran like a sowing machine from the static balance/pitch onward, so noticed little change with the dynamic balance. I guess I am going the long way about suggesting you may have been a tad hasty in your temporary re propping.

 

I would also second Blueadventures suggestion again you can only benefit from aa precise as possible pneumatic balance of the carburettors.

 

One other point - check out your engine mount & rubbers for nil damage/security

If the pitch is not set the same for each blade could that make it vibrate when throttle is retarded, only set the pitch on each blade with a bubble type pitch gauge, took a few goes started off to fine then to course , struggled to reach 5000 on take off, so I split  the difference, I was surprised  how little movement made the difference , since found out you can use the inclinometer on a phone to check it, 

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Cause I'm anal, I use several different devices to check/adjust pitch:

 

Alan key, spanners, accurate torque wrench to loosen/tighten hub fittings

 

Tin of  Carnauba wax polish 

  • Lubricate blade hub socket (only need to be applied at first adjustment)

Masking tape

  • Wrapped around blade at approximate measuring radial (dont forget to remove befor engine start)

Steel metre rule

  • Used to make accurate measurements from, pre scribed,  centre of hub to recommended measuring location on each blade. I make two measurements/blade - hub centre to leading edge and to trailing edge.

Sharp pencil & short strait edge

  • For marking a strait measuring line/location, between above marked points,  on masking tap

Nice builders level

  • Set the hub/blades to exactly vertical for each measurement. I do all my measurements on the left side of aircraft so blades must be rotated 180 degrees

Digital protractor

  • Have fitted with straight edge that can be clamped exactly to blade measuring point/line. Protractors (both) are "zeroed" against vertical face of hub before each measurement/adjustment

Warprive bubble protractor

  • Rough check/adjustment befor using digital protractor

Home made wooden "clamp" (2 pieces of  wooden strip, hinge at one end)

  • Clamped (hand grip)around blade to give leverage & additional control in making fine rotational adjustments of blade

With all of the above I am able to make accurate adjustments down to better than  .5 of a degree

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Hell, at that rate mine will be out for sure , I did notice when I tightened the clamp bolts it would change the angle a bit , plane is at lame in deni , I am hoping this is my problem as it is bad enough to not fly it as I said good at cruise but bad when throttling back

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1 hour ago, rhtrudder said:

Thinking of replacing the prop studs and the flange bushes are they available from floods or the propellor mob

Dont know what your prop flange system looks like but mine has separate blade "root" /hub socket clamping to the flange securing bolts. This means that I can clamp the blades in the selected pitch without fear of making an inadvertent change when I tighten the 6 x 8 mm flange bolts. I always tighten progressively, using an accurate torque wrench, to minimise the chance of distortion. 

 

Your LAIME should be able to advise on what needs replacing. If your prop was a hollow composite, there may be little if any damage to the hub & "studs"/bolts.

 

My aircraft uses "off the shelf " Allan key " head, 8mm HT bolts and lock nuts - much cheaper than buying from BF

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Also check blade tracking. Getting the pitch the same on all blades is only half the job.
Chock the aircraft against movement.
Put a chair or a ladder near the prop, but where the prop can be turned without hitting the chair.
Turn the prop bringing each blad near the chair and measure the distance.
There are specs (RA Aus tech manual?) for prop blade runout, radially and axially.
The above is easier to do if you remove 1 spark plug from all cylinders.

But as Mike said, pneumatic synching of the carbs is a major cause of RPM related vibrations.

Edited by cscotthendry
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1 hour ago, cscotthendry said:

Also check blade tracking. Getting the pitch the same on all blades is only half the job.
Chock the aircraft against movement.
Put a chair or a ladder near the prop, but where the prop can be turned without hitting the chair.
Turn the prop bringing each blad near the chair and measure the distance.
There are specs (RA Aus tech manual?) for prop blade runout, radially and axially.
The above is easier to do if you remove 1 spark plug from all cylinders.

But as Mike said, pneumatic synching of the carbs is a major cause of RPM related vibrations.

Agreed - one one point I would differ slightly - I do my tracking check against a pointer (small piece of wood/plastic/even stiff cardboard), taped securely to the nose wheel fairing.

Rotate prop so tip of one blade nearest point to nose wheel.

Move pointer to within known distance from prop tip. Secure pointer with tape.

Rotate prop & observe each blade tip clearance to pointer.

By using a part of the airframe, there is little concern that aircraft movement might influence the test.

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