Vibrations

As a courtesy to other owners of RV aircraft, I have been offering to balance their propellers free of charge for the past 3 months. For over 12 years, I worked as a consultant with a company manufacturing vibration monitoring and analysis equipment. I recently acquired a dynavibe classic with the intent of balancing the prop on my 7 when I finish building but in the meantime I decided to help others and get to know the experimental community since I have been building in my garage. I have balanced 6 lycoming engines, 5 with CS props. What I am astonished by is the very small response I have gotten with the free offer including all necessary hardware. Are builders just unaware of what damage vibration causes or do they just not care?

Typical response: During a conversation with someone I know at the airport with a RV-10. Asked if he had balanced his prop. "Nope." Would you like me to balance it for you? "Nope." Maybe people think I am just a "rube" that fell off a turnip turnip truck. I'll have to self analyze that one.

Interesting note is that all of the lycomings with CS props had a vibration level of over .3 IPS. That is on the borderline between slightly and moderately rough. That would definitely be a noticeable vibration but maybe we have just gotten used to that level of vibration and consider it normal. As for CS props, maybe they just have too many movable parts to obtain a good static balance. Not sure whats going on with that but they definitely vibrate.

So the question is why are owners ignoring the cancer eating away at their aircraft? I don't care if it is free or costs $300, it is cost effective. I know enough about vibration to know it causes damage.

I?m flying back to SoCal next week. It?s been just over 100 hours since I had my balanced last. I might come see u and see how much mine has changed. I?ll contact u when I?m there.

Ever hear the addage "something for nothing is only worth as much as you paid for it"?

Maybe you should offer to 'experiment' on someone's prop balance & if they like the result, they could buy you lunch & a couple brews? Someone who starts at 0.3 IPS and end up with 0.05 or less IPS will definitely feel the difference / benefits, and word would get around. You might start getting too many 'free lunches'.

I?ll buy him lunch anyway.

More comment on dynamic prop balancing

I have been doing dynamic balancing for many years (although not for free), and could add any number of "stubborn pilot stories"! Just one here -


There are a number of common reasons that people don't get their props balanced.

1. Human beings are generally not very sensitive to vibrations in the 40? Hz (2400 RPM) range. We can feel such vibrations, but cannot easily differentiate magnitude. I have taken planes down from >.50 to <.04 and had the pilot later report that he didn't feel any difference on the way home.
2. The whole purpose of a DynaFocal engine mount is to *isolate* the engine shaking from the airframe. This definitely is better for the firewall-aft structure, but further reduces the pilot's ability to feel engine vibration.
3. Certified shops charge a *lot* for the service, often ~$400, because officially the addition of mass to rotating components is considered an engine modification and requires a 337!
4. We call it "dynamic propeller balancing" even though we actually are minimizing the synchronous vibration of the engine/prop combination as measured near the front main bearing. Very rarely do the balancing weights go directly aligned with one or the other blade. When someone has paid $$$ for a propeller and the company says they balanced it (statically) it is hard to convince people it needs another round.

Note that I just installed a new Whirl Wind 74HRT prop on my RV-8, and Whirl Wind (smarter than your average bear!) has it in bold print right there in their installation instructions:


Anyway, most RV guys that get it done become lifelong converts. I give a $25 discount to EAA members.

Andy

3.Certified shops charge a *lot* for the service, often ~$400, because officially the addition of mass to rotating components is considered an engine modification and requires a 337!

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I agree with most of your points except for the above.
For the record, propeller balancing is not considered a major alteration and does not require a 337 (ref AC 20-37) and "certified" shops/persons of which I am one, don't always charge as much as you mention.

One thing that "certfied" shops do is maintain a record of all balances as well as annual calibration for their generally higher priced test equipment compared to the "hobby" balancers, this stuff does increase our cost of doing business.

Jim, I?d come by if I was closer. Had mine balanced in KY. Not a big change, but glad to confirm proper balance. Small price to pay to reduce vibration damage IMHO.

Well heck, if you are still around in 5 years or so (that's my estimated timeline for the build), I will definitely take you up on the offer. It might take longer.
We shall see.

In the mean time, if you need someone to assist in any tasks with your 7, I would love to help. I am building a 7 too, and a complete noob to both building AND flying, so not much in the advice dept. But I don't mind working. Lord knows I could use some experience to help build the confidence level to do this.

I am a licensed powerplant mechanic, So I am not a complete noob to airplanes, but my expertise was with turbines. Although I do know my way around Lycoming a little too, so it is not really a problem.

Good Luck

jask said:

For over 12 years, I worked as a consultant with a company manufacturing vibration monitoring and analysis equipment...... Maybe people think I am just a "rube" that fell off a turnip turnip truck.

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Jim, you have a very large audience here, so why not write a good tutorial about the physics of engine and propeller balance? Not the balance process, but the mechanics and forces. For example, I'd bet only a small percentage understand the difference between static and dynamic balance, or how much force a tiny mass can generate at 2700 RPM.

I agree with Dan. I have a sense of the differences but a nice tutorial would be a useful addition to the digital library.

Explanation of Dynamic Balancing

I have a PowerPoint presentation I use to explain dynamic balancing at EAA meetings, etc.
I have made this available for anyone to download by clicking HERE.
Andy

vibrations

I had not really calculated the centrifugal force that would be involved with an unbalanced prop but below is an example.

The average I have found so far for a CS prop is about .35 IPS.

That would entail a rough weight of about 28 grams placed on the lycoming flywheel at 5.125 inches from center. That comes out to about 67 foot lbs of force applied every revolution of the engine at 2700 RPM. That is a hefty amount of force but it is only about 1 ounce of weight.

Also, that is a really good Power Point presentation. There are some highlights that I consider very important in selling prop balancing.

"Average value for unbalanced aircraft > 0.40 (ips RMS).
Typical value after balancing = 0.04 ips!
10X reduction in fatigue load to airframe and engine "

That is really important when you consider the life expectancy of our aluminum air-frames. The other highlight of the PPT is the frequency of vibration.

"Operating speeds for aircraft engines (35-45 Hz) are outside the range that human beings sense well. You cannot reasonably differentiate levels of vibration at those frequencies."

35 to 45 Hz is 2100 to 2700 RPM. That's where we take off and cruise. With our inability to sense those vibrations well, we can just sit on our foam cushioned seats with carpet on the floor and be none the wiser to the vibrations eating away on our aircraft.

The most difficult job in vibration work in persuading customers that reducing vibrations will save them money.

Interesting, thanks. So what causes a system to go unbalanced between annuals (other than the obvious things like prop nick)? Do you typically see changes over 100 hours (again, neglecting the obvious)?

Greg, I would not see the need to balance unless you mess with the spinner or prop. We are only balancing in the vertical. A more expensive balancer could also monitor the horizontal and give you some clues on engine health.

Why does balance change over time?

Couple of notes here:

1. Balances are done to the 1/10 gram (.004 oz ) level for the weights on the flywheel.
2. On RV's, we typically shoot for very low final levels.
3. The difference of 1/2 gram (depending on prop.engine combo) can change vibration levels by up to 0.05 IPS.
4. Moving the weights by 30? (one hole in the flywheel) can easily change vibration levels by 0.1 IPS or more. Sometime we have to split the weights between two holes.
5. Differential wear on the blade retention mechanism of just a mil or two (hub bushings, bearing, blade cuffs, grease migration) can easily cause the equivalent change. Remember the blades weigh ~10# or more each and the equivalent radius is ~2 feet.
6. Normal wear and tear on the engine over 100 hours can also cause equivalent changes. Remember we are balancing the system, not just the prop!

Additionally, Jim's statement that "We are only balancing in the vertical." is not correct. While it is true that for Lycomings/clones we usually attach the accelerometer to the case oriented vertically, this is absolutely not a requirement, is not true for many other engine/prop combos that I do, and it does not imply we are only reducing vibration along one axis. Of course, the balancer software must know how the accelerometer is oriented and how the optical tach pickup is relatively clocked.

What we are doing (as explained in the PPT I posted HERE.) is reducing the synchronous (occurs at a frequency of once/revolution of the engine) vibration of the highly coupled engine/prop system.

FWIW,
Andy

Prop balance

Most of the discussion of prop balancing seems to talk about imbalances in the prop plane of rotation. An imbalance is then corrected with a weight that compensates for that imbalance. But suppose vibration is resulting from a blade pitch differential (or airfoil difference) between 2 blades? That would result in longitudinal components of vibration that cannot be corrected with weights. How is this problem diagnosed and corrected?

One of the publications on prop balancing describes how to measure propeller track and the limitations. The spark plugs are removed, engine rotated and blade track is noted. I'm sure you could do the same for blade pitch. That is corrected by work on the propeller or prop mounting, not balancing.

Let's say we have a plenum that takes a couple of hours to remove the top side in 2 halves with a T spine down the center just above the case spine where the sensor is often mounted.

1) Are there any other locations on the engine that come to mind where the sensor could effectively be bolted to avoid the hours of plenum access?

Possibly some of the un used alternator mount holes threaded into the case? or other threaded holes on the bottom of the engine?

2) Can the sensor be mounted on the accessory case over 12" aft of the propellor mass disc?

3) So lets's say we do one balance session to .04IPS or less with the sensor mounted on the top case spine near the flywheel/ crank nose.

Then with no other changes we relocate the sensor to the back end of the case closer to the accessory case, but still on the case spline, are we likely to see a higher IPS of vibration value ?

4) Are there systems that allow multiple sensors in various locations at the same time so to give a better dynamic balance result?

For dynamic prop balance, all you need is the top cowl off. The accelerometer should be mounted on centerline close to the front of the engine. Multiple sensors can be used with the more expensive analysers to give a total vibration analysis. The "hobby" balancers as mechanics sometimes call them, just measure in one axis. If you move the sensor to the rear of the engine, the ips will be reduced and it will be more difficult to balance the prop. The weight added is usually not much more than an ounce or less.

It is just this simple. If you take your aircraft to some one with a balancer with multiple sensors, you an get a more extensive analysis of vibrations. Most of the vibrations come from propeller imbalance. The less expensive balancers like the dynavibe classic and the aces pro sport just use one sensor. We orient that sensor in the plane of rotation of the prop close to the front of the engine to maximize the ips readings. For conventional engines, we mount the sensor perpendicular to the piston movement to mostly reduce any vibrations caused by the combustion process.

What a great offer, I lived in Ramona 20 years ago, if I ever get down that way I'll come call on you. Thanks for your service.

no one should say 'no' to a prop balance

My Whirl Wind 200RV prop was initially about 0.35 ips....much better than average for a C/S prop. When Kregg Victory was done ( I think he used Dynavibe, but I forget) it was less then 0.015 ips.

I could convince myself that if I put my hand on the canopy rail, I could feel the difference. I don't really know for sure if I could feel it.

But why on earth would you ever say 'no'?

So you have a plenum. I think we can find an alternative mount point for the accelerometer, but might not need to. Your plenum must attach firmly at the front of the engine somewhere to the case. Where ever that attachment is, put the accelerometer there.

I would say anyone within an hour's flight of Ramona that has not had a dynamic prop balance should call 'jask' and make an appointment.

How much is prop and how much is spinner?

The whole thread calls this prop balance, but during a Hartzell factor tour I watched them balance check each prop at the end of the line. Many needed nothing some, only needed a #10 washer or half washer.

But - the back plate using a fiberglass nose cone seems like it could generate quite the opportunity for imbalance, not to mention uneven painting.

What is your experience with metal spinners vs glass ones for initial imbalance?

jask said:

One of the publications on prop balancing describes how to measure propeller track and the limitations. The spark plugs are removed, engine rotated and blade track is noted. I'm sure you could do the same for blade pitch. That is corrected by work on the propeller or prop mounting, not balancing.

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At least on helicopters, the track was tested dynamically. I wouldn't expect a static prop track to do more than verify the prop mounting.

Dave

My DSS balancer is certainly capable of Dynamic blade tracking (normal stuff on Helos) , but because tracking is not generally performed on aircraft I didn't purchase this option.

BillL said:

The whole thread calls this prop balance, but during a Hartzell factor tour I watched them balance check each prop at the end of the line. Many needed nothing some, only needed a #10 washer or half washer.

But - the back plate using a fiberglass nose cone seems like it could generate quite the opportunity for imbalance, not to mention uneven painting.

What is your experience with metal spinners vs glass ones for initial imbalance?

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Hartzell static balances their props, not at all the same thing as dynamic balance after you add all the other stuff like flywheel, spinner bulkhead, spinner etc..
(Hartzell recommends dynamic balancing in their owners manual)
I've seen quite a few cracked metal spinners but never a cracked fiberglass one.
For initial "imbalance" not much different between metal and glass, seen quite a few bad ones in both camps.

BillL said:

The whole thread calls this prop balance, but during a Hartzell factor tour I watched them balance check each prop at the end of the line. Many needed nothing some, only needed a #10 washer or half washer.

But - the back plate using a fiberglass nose cone seems like it could generate quite the opportunity for imbalance, not to mention uneven painting.

What is your experience with metal spinners vs glass ones for initial imbalance?

Click to expand...

You are really balancing the engine/prop system, with all its attachments. At least in my case, the balance weights (bolt and washers) were attached to the ring gear.

I did have a friend with a poorly fitting spinner -- the attach screws were continually working loose and the spinner screw holes were wearing oval. He could not get a good dynamic balance until that was fixed. So it does show that there is some participation in the balance from the spinner, even though it is pretty light, and not very far from the rotation axis.

I've been balancing props for a few years now. One difference I notice is that there seems to be a lot less cracking of baffling and other metal parts on the airplanes that have their props balanced.

Vic