Prop Balancing & Vib Spectrum plots

Did a couple of RV prop balances yesterday and thought the results were interesting. One engine came from Matituck and one from PennYan. The crankshaft 2.0x tell the story on internal engine balance (piston/rod matching weights primarily). There are no "limits" on this and we are talking small differences but interesting to see.

Both aircraft were out of limits on propeller vibs, both ended up with great final numbers of 0.010 and 0.011 IPS after just 3 runs

Both aircraft owners will definately notice smoother operation based on the large decrease in first order (prop rate) vibration.

RV3 plots below:





RV4 plots below

A few years ago a guy gave a presentation at our regional RVators club. Said he did balancing but with a newer 2D or 3D method, instead of the older lower-dimensional method. It was supposed to offer some advantages. Can you shed light on this?

Sorry I have no clue what he was referring to

New England and North East

Anyone know who does dynamic prop balancing in New England and the North East?

Jim Sharkey
RV-6 O360-A1A FP 72/85 Sensenich

Sensenich will do it at Lancaster PA (KLNS) usually while you wait. Set up an appointment before hand.

Walt said:

Did a couple of RV prop balances yesterday and thought the results were interesting. One engine came from Matituck and one from PennYan. The crankshaft 2.0x tell the story on internal engine balance (piston/rod matching weights primarily). There are no "limits" on this and we are talking small differences but interesting to see.

Click to expand...

Just trying to follow along here. Why is the 2.0x peak so critical, as opposed to say the 2.5x or 4.5x? Looks like there were substantial differences at those peaks as well. Also, not sure what limits you are talking about - Care to explain a little further?

thanks
erich

Securing the aircraft during balancing...

I would like to get the prop balanced on my -4. What is the recommended approach for securing the aircraft during testing? Tie down the tail only? Wings and tail?

Thanks!

erich weaver said:

Just trying to follow along here. Why is the 2.0x peak so critical, as opposed to say the 2.5x or 4.5x? Looks like there were substantial differences at those peaks as well. Also, not sure what limits you are talking about - Care to explain a little further?

thanks
erich

Click to expand...

For a 4 cyl engine the 2.0x second order vibration peaks (2x crankshaft speed) are directly related to internal engine balance. This vibration is caused by the reciprocating masses of the rods and pistons mainly. For a 6 cyl this would be reflected in the 3x vibs.

The 1.0x or first order vibrations are from the rotating masses of the propeller crankshaft and flywheel assemblies mainly.

The 2.5x and 3.5x spikes typically peak a bit higher and follow along with the 1.0 and 2.0 vibs for the most part. There is no specific cause for the 2.5/3.5 and they are the harmonics of combustion and balance.

Another one to note is the 1/2 order .5x vibs, a vib at this speed is the result of combustion cyclic variations, or variations from the combustion process. For example a fouled plug or low compression will show a high 1/2 order vibration.

The only "limits" are for the 1.0 vibs which are 0.2 IPS, there are no limits for any of the other vibs/harmonics.

Dean Pichon said:

I would like to get the prop balanced on my -4. What is the recommended approach for securing the aircraft during testing? Tie down the tail only? Wings and tail?

Thanks!

Click to expand...

I balanced quite a few RV's and the 'A' models are definately easier
On a 3 I will add some weight to tail, flaps up and stick full aft, 4, 6, 7's and 8's are not a problem using the same method. I will tie the tail if I (or the operator) does not trust the brakes.

Walt -

Thanks for the info. I have found a local shop to balance my prop. How do I know when they are done? It this minimum acceptable balance?

Walt said:

The only "limits" are for the 1.0 vibs which are 0.2 IPS, there are no limits for any of the other vibs/harmonics.

Click to expand...

Should we start with a more agressive "stretch goal"?

Thanks again,

Dean

Dean Pichon said:

Walt -

Thanks for the info. I have found a local shop to balance my prop. How do I know when they are done? It this minimum acceptable balance?



Should we start with a more agressive "stretch goal"?

Thanks again,

Dean

Click to expand...

Have a look at the prop balance page on my website, lots more info there, here is one of the charts I have posted there. You should expect the after balance vibration to be no more than 0.07 IPS, I like to see no more than 0.05 IPS personally.



DANGER ? 1.25 Inches Per Second (IPS) Peak Amplitude. The propeller should be removed and a static balance performed.

VERY ROUGH ? 1.00 Inches Per Second (IPS) Peak Amplitude. Propeller can be dynamically balanced; however a large amount of weight will be required. A propeller static balance is recommended. Operation at this vibration level could cause damage.

ROUGH --- 0.50 Inches Per Second (IPS) Peak Amplitude. Propeller definitely requires dynamic balance. Long term operation at this vibration level could cause excessive wear.

SLIGHTLY ROUGH -- 0.25 Inches Per Second (IPS) Peak Amplitude. Dynamic balance will improve passenger comfort.

FAIR---- 0.15 Inches Per Second (IPS) Peak Amplitude. This is the maximum acceptable vibration level after dynamic balancing.

GOOD ---- 0.07 Inches Per Second (IPS) Peak Amplitude. Vibration levels below 0.07 will not be detectable by pilots or passengers.