I accept your statement and respect your process, however, I disagree that it's "almost impossible"; I've been successful at obtaining values between 0.00 to 0.02 IPS using a Dynavibe Classic and accurately drilling a single hole.

My point being that drilling a hole to mount a weight is not overly difficult, but does require accurate measurements.

If I could do it over again, I would have pre-drilled eight to ten holes in the bulkhead before assembly and prop installation. That way I could have precisely measured their location. I could have specified these locations in the Dynavibe and saved a step or two in the balancing process.

Steve, I think I will do the same. I believe Lycoming starter rings have their holes at 30 degree intervals and that always worked well for balance weights. Thanks!

Randy, I?d check with Van?s to make sure there are no structural issues, don?t think there would be. Also, I?d deburr and chamfer the holes thoroughly to avoid cracking.

When I balance a Rotax or Continental and trying different solutions as the balancer is acquiring data, I mount the suggested weight on the screws that hold the spinner to the rear bulkhead. When an acceptable solution is found, that weight gets transferred and mounted by screws and nuts to the rear bulkhead. Due to a small amount of material removed during the drilling and a slightly shorter arm from the prop flange center, a little bit more weight will be required to achieve the same results. It’s not complicated at all. If y’all are concerned with making the balance process easier/better, don’t worry with it. The Rotax/Sensenich combo balances very well. I personally don’t see any need to drill the bulkhead during the build.

I wouldn?t even consider drilling a hole without deburring/chamfering appropriately! ;-)

A cure searching for a problem?
 

A few things to consider.....

Completing a dynamic prop balance is not something done on a regular basis like an oil change. Once done, it may be many years before someone would have to consider doing it again.

The spinner bulkheads on RV's are made from a rather soft aluminum. Regardless of how well the holes are drilled and deburred, I have some concern about adding a bunch of un-needed holes to a rotating component that is subjected to a lot of vibration. Adding a bunch of holes for balancing convenience is not something I would recommend.

If you are building an RV-12 to be an E-LSA, this is not something that would be an approved modification prior to certification.

Scott,

When placing the temporary weights on the outside of the spinner, what moment arm, specific distance, do you use?

We have an ASES balancing system.
It learns the balance influence of your temp weights (installed on the outside of the spinner) while you are going through the balancing process so you don't even have to tell it what the radius position of them is.
Once you resolve to a final solution it calculates the adjustment in mass for you once you tell it what the spinner diam. is and what offset inward on the spinner bulkhead you want to use.

Since the spinner is part of the rotating assembly that you are trying to balance, it has to be in place during the process. Removing the spinner each time you had to install temp weights would be a huge pain. Another reason that I think pre-drilled holes would not be of a lot of value (and may be a detriment as already mentioned).

If someone is using a balancer that doesn't calculate the offset for you, it is relatively simple math and can still be done.

Does Van's or someone else close by the PDX metro area offer the dynamic balancing that Scott has referenced?