Dynamic Balancing Question

[mlwynn]

Hi all,

I used our EAA chapter's Dynavibe Classic to balance my new Hartzell prop today. The readings were interesting, but I am not sure I quite understand what is going on.

The initial readings were:
2300 RPM .21 ips @317 degrees
2300 RPM .23 ips @313 degrees

2500 RPM .24 ips @316 degrees
2500 RPM .22 ips @313 degrees
All pretty consistent.

I added a bolt and nut, 11 gms at 120 degrees (my closest option) and got the following:

2300 RPM .12 ips @344
2300 RPM .14 ips @338
2500 RPM .14 ips @347
2500 RPM .11 ips @338
Still pretty consistent

I added a bolt and nut, 11 gms at 150 degrees and got:
2300 RPM .03 @81
2300 RPM .04@123
2300 RPM .05@198
2500 RPM .04@124
2500 RPM .07@115
A little less consistent

I used a smaller nut, removing 2 gms of weight at 150 degrees
2300 RPM .04@67
2300 RPM .04@61
2500 RPM .05@20
2500 RPM .03@39
The ips is pretty close, but the angle moves some.

I similarly removed 2 gms from 120 degrees and got:
2300 RPM .07@58
2300 RPM .01@95
2500 RPM .06@50
2500 RPM .03@39
Wider variations in both the ips and the angle

I did a second run with no changes:
2300 RPM .05@259
2300 RPM .05@211
2500 RPM .04@295
2500 RPM .05@250
Somewhat more consistent.

I puzzling through understanding the data. When the balance was relatively off, the numbers are consistent both in angle and ips. The closer I got to balanced, the more they seemed to scatter. I did notice some changes in the wind as we were doing the test and occasional slight fluctuations in the RPM, but mostly things were pretty steady.

Am I correct to assume that the closer you get to perfectly balanced, the more difficult it is to get that last couple of hundredths? 0.04 to 0.05 is not bad, but we obsessive/compulsive types are always looking for perfection....

Anyone have any insights on where to go next? Leave it and fly is probably the right answer.

[akarmy]

I've done about 20 planes now with the same Dynavibe unit for our EAA Chapter. I've found that .05 is usually about where the stability stops and it starts going random as you have described. You had some patience as I've never done that many runs on anyones prop! In general I've found that anything below .05 is good to go. I know some can balance finer with the newer units or the more expensive units, but it's really pretty smooth at that point anyways.

So ya, go fly!

[mlwynn]

I am running weight and balance tomorrow and will give it a go.

Thanks, that is pretty much what I had come to as well

[WA85]

If you plot the IPS and phase points on a polar chart, you can plot each adjustment and determine the "move" line for each adjustment. Looking at your plots, its apparent that you are moving somewhat through the center of the polar chart from one side to the other. Example, if you are at 0.05 ips at 090 and add weight with a resultant move to 270 at 0.05 ips, it may appear that you did not improve you balance, but you moved through the center of the polar chart (0.0 ips). Try cutting the weight adjustment in half.
Most automated prop balance equipment come with some type of script file that is already configured for 2-3-4 blade props, but not all props will react to the canned move lines and weight coefficients. Typically, I create individual polar charts and move line / weight coefficients for each prop I balance. The type of prop and where you can add weight has a big impact on this, as well as clocking of the accelerometer and optical tach sensor. With a two blade prop, one is target and the other is blank. I make one run with no weight, then add weight in the form of washers or lead stick weight to one blade. Plot both measurements on a polar chart and correct it through the center (0.0 ips). Now you will have a polar with a correct move line and balance weight coefficient for your prop. Depending on the equipment, any balance measurement below 0.07 ips, it becomes difficult for the equipment to get a good synchronous measurement (ips / phase) to provide accurate balance adjustments. Most prop MFG's list the balance limit as being OK below 0.2 IPS. If you are at 0.05, consider it more better and fly it.

[MikeS]

Next time you use the Dynavibe or any other balancer, do it early in the morning when there's no wind. Unless there is no choice in the matter, I always do balancing when there's no wind unless there's simply no choice in the matter.

If there IS any wind, it needs to be right on the nose. Worst is from behind, from the sides not much better.

When you're down below the .10 range you're not going to get consistent readings if there's any wind at all, although you seemed to have gotten pretty consistent readings even though you mention wind. You're dealing with extremely small corrections at that level and the slightest breeze will generally make your readings inconsistent.

If you got to .04 - .05 consistently, with wind, however slight - you did very well. If you want to get it to zero, do it with absolutely no wind. Also, buy some lead tape. They sell it to balance golf clubs. Has an adhesive back and you can cut a small piece off and tape inside the ledge (if it has one - most do) of your spinner backplate. This is for very fine adjustments, just so you can say you got it to .00. Only the machine can detect any vibration below .07 but it can be fun and a challenge to get to zero.

You did great.

Also, while on the subject of the Dynavibe - there's no need to mount the accelerometer pointing down, subtracting 180?, then adding or subtracting weight at the point you have computed. Mount the accelerometer with the cord pointing up and it will directly tell you exactly where to add the weight. That point is found by starting at the 0? position (reflector tape lined up with optical sensor) and countring forward in the direction of rotation. Doing that 180? business is a holdover from the Chadwick-Helmuth dark ages. Both ways work, but the way I suggest (and which Dynavibe mentions on their website somewhere) simplifies the process. Less opportunity for confusion.

[DHeal]

A little trick I used to establish the balance weight's location when balancing my engine was to calculate the circumference (pi x D) of the spinner 's rear bulkhead and use "inches along the circumference from the zero reference point" in lieu of "degrees". I "borrowed" my wife's cloth tape measure for the ultimate in radial accuracy. Using a protractor seemed unwieldy and difficult to implement. Worked great for me - got my prop down to 0.02 IPS!

[MikeS]

That lead tape I mentioned . . . goes without saying but just to say it anyway, clean the surface of your backplate really well (alcohol/acetone/Goof Off/MEK/Etc. where you'll be putting/moving/lengthening/shortening your little piece of tape.

[bret]

One time....at band camp.....I was trying to balance the celling fan with tape temporarily holding washers till I could get the weight correct and CRACK! WHAT THE? , yip, washer flew off right into the plasma TV at a high rate of speed.

[Ralph Inkster]

A few things I don't think have been mentioned above.

-Engine run ups should all be done at the same rpm if possible,
-preferably at cruise rpm
-& at best leaned out condition.

I have found that readings can be drastically affected if run-ups are performed near buildings (in my case hangar rows are fairly close together), I'm assuming effects from resonant feed back, so do all testing in an open area.

My usual target is .03 ips, have got better on some good installations. I find I usually get better results on injected engines (Gami even better) compared to carb'd ones. Results better than .02 are probably influenced by a host of things like mounts, exhaust hangars rubbing, baffle buffeting, even mechanical fuel pumps... etc.

[ColoRv]

I keep reading to do it at cruise RPM. Are you guys running a taildragger up to 2500rpm on the ground? Is that safe? Seems like my tailwheel starts hopping around before I wind up that far. Am I just being a sissy?