prop balance

[dennysleen]

Vibration readings should be taken off the engine perpendicular to the cylinders. this would be where the case splits. you need take readings at a consistent location. i use my breather, which is left of center, because I can reach it comfortably without fear of the prop and always set the phone in the same place. my breather is the orange block in the photo.

[Tom H]

Denny, I was familiar with you trial weight technique, and have practiced using it on a test set-up.  It works, but the problem I remember is that the technique produces a solution of "a certain amount of weight at a certain polar angle at a certain distance from the center of rotation".  On an engine, it may be difficult to find a location that meets the requirements?

[dennysleen]

I agree! balancing is not"simple". it is not always easy to get weight at exactly the right spot. A balancer from Dynavibe or Aces will take several runs to get it right as well.

[Dan_]

forgive my use of computers. I included a picture of the skf vibration display. prop balance is specified in inches per second and this gives you mm per sec so you have to convert but it is easier to work with mm anyway. my first run is my baseline starting point which is shown graphed with the black circle. the meter showed 10.1 mm per sec and I used 101 mm on my graph. i added an extra nut at the 0 position and recorded the result with purple @ 92 mm radius. the nut is moved to the next position and then to the last position with the results graphed. where those three circles intersect is the vector where the weight needs to be located. the balance weight needed is calculated by dividing the baseline radius by the intersection radius then multiplying that by the test weight. if the circles intersect outside of the baseline radius your balance weight will need to be less than the test weight.

Another question...

Your baseline run was 10.1 and you used 101 as your plot size.  Working your formula you came out with 25.5 grams @ 58 degrees, should that not be divided by ten giving you 2.55 grams..?

[dennysleen]

Good question. I was vague finishing the formula.  the ratio between the baseline radius and intersection radius gets multiplied buy the value of the test weight. using my numbers 101/28=3.6     3.6X7 grams=25.25  (different answer from rounding differences)   whether you use 10.1 and 2.8 or 101 and 28 it is the same ratio which is visually represented in the radius graph.  the 28 number is found using a ruler between the center and the intersection point.

[dennysleen]

Looking again @ what might be confusing. the added weight does not get added at the intersectin point on the graph. in my sample problem it is 28mm. the weight will be put at the same radius the test weight was at. At the bolt radius.

[Dan_]

Thank you sir.

And of course after you add the weight you run it up again for another base line... 


I did not punch in the 2 sets of #s,  I just blurted out the question when it occurred to me.

[dennysleen]

engine and prop manufacturers specify limits of vibration but here are a few found on the web. using the SKF app you have to convert from inches to mm. 1"= 25 mm  1/2"=12.5mm .25"= 6 mm.  (approximately)
Anything we do to reduce vibration will make our equipment last longer and more enjoyable to fly.

[Tom H]

Denny, you clarified something to me:
"the weight will be put at the same radius the test weight was at. At the bolt radius."
It was many years ago that I was messing with this, and was going from memory.  This makes it much more doable.

[flyover1974]

Hi guys,
very interesting thread but of my poor english im not sure if i understood all.
Anyway i have a question about this vibration test method:
Maybe that work also to balance the crank ?
When i cutted my stroker crank i did it  this way on this link here  and for the counterweight piece on the prop hub i had no numbers...only estimate.
My balancing machine of the other thread is not jet finished because of no free time and then i dont want disassemble the nice running engine again.
So...what you think, if i mount a (balanced) old unused flywheel on the prop hub and run the engine without the prop, that method work also to find the right counterweight on the prop hub for smooth engine running?
I think its important to have a smooth, well balanced engine BEFORE you want to balance a mounted prop, right?

[BobSeverance]

My current prop is from Performance Propellers and he does an admirable job of balance. I previously modified another prop to take some of the pitch out and rebalanced with a wire. I took a dowel that fit in the 1" center hole and drilled a 1/16" hole on the centerline and threaded the wire thru and hung the prop from a shelf. This will show which blade is heavy and which side of the hub is also heavy. Easy enough to put a screw or drill  out a spot to add weight or remove  on the hub.. I sanded the heavy blade on the FRONT side to remove weight from a blade.. after sanding, varnish raw wood and rebalance....Apply a little extra varnish to the light blade and check after it drys..

[flyover1974]

and how you balance your engine?
The best balanced prop on a bad/no balanced engine don`t work very well...

[BobSeverance]

I would HOPE that the engine is already balanced......at least to some degree... I KNOW Scott Casler spends time balancing his engines and Bob Demillo spends time balancing his crankshaft/rod/piston setups...

I got a balance plate from Balance Master that took the fine balancing out of my engine... AS&S sells it now: http://www.aircraftspruce.com/catalog/appages/engblncmstr2.php 
Works really well on MY engine!

[Mark Kramer]

Here it is, three years after Denny’s post. I am currently test running and trying to smooth out my Mosler cb40. I think I understand how the method he describes works. One question though is, are the mm measurements, taken with the phone app, circle radius’s, or diameters? I have an android phone and couldn’t get the skf app. Found and used a different, vibration measuring app that gives movement in mms. Multiplied by 10 to get larger circles. Wasn’t sure if the Multiplied numbers were radius or diameter numbers. Also not sure if I’m using the app correctly. Do I pick a particularly shaky rpm and take measurements each time at same rpm? The tiny tach goes nuts after 2000 rpms, guess that should be another topic. Also not sure where the egts should be. Another topic? Anybody out there know some of the answers to the initial set-up issues? I could surely use some assistance.

[dennysleen]

All test runs need to be done at the same RPM. The circles are drawn using the radius that corresponds to the mm/second on each test run.