Balancing a radial?

[stevehuckss396]

Is there a rule of thumb for figuring out the weight of the bob weight for balancing a 9 cylinder radial. On a conventional V8 i think it's the half the reciprocating weight and all the rotating weight equals the weight of the bob weight. Any idea what it is for a radial?

[nj111]

Pretty sure that is covered in the book Aircraft Engine Design by Joseph Liston  (Circa 1942)  -a most interesting reference book.  I shall dig out my copy and report back!
Nick

[AVTUR]

From memory I think the primary balance of a radial engine with an odd number of cylinders is "perfect". I may be wrong but I cannot find my college notes from over 50 years ago. I remember it as a simple problem but the internet and text books make it quite complex.

If the crankpin is crowded with conrod bearings, the cylinders arranged in a helix, there will be a rocking couple. This can be removed by using master and slave conrods but the master rod will be considerably larger than the slave rods and introduces an out of balance force similar to a single cylinder engine (similar?).

AVTUR

Thinking a little bit more. Most of the mass of the master rod is around the crankpin, producing an out of balance force as a simple rotating mass on the shaft. This can be removed by bob weights.

[Vixen]

My notes relating to Bristol radial engines say

"Bristol employed the crankshaft balancing strategy of the balance weights being equal to half the reciprocating mass plus the whole of the rotating mass.
The weight of both ends of the Master rod and each Articulated rod was measured accurately by suspending each rod horizontally between two precision scales. In this way, the effective weight of the big end and little ends could be determined and added to either the rotating or reciprocating sum. "

Stay safe

Mike

[stevehuckss396]

I had someone point me to a referance on the internet that said the same thing

weight to be hung from the crank throw is equal to the rotating weight plus .508 of the reciprocating weight.

Thanks for helping me out.