Prop Balancing


There was a time in my modeling career when I meticulously balanced all my props to the milligram. Then a friend rescued me from that drudgery by demonstrating that for props used on single-cylinder motors, careful prop balancing was largely wasted effort. This guy -- Don Garry, of Cocoa, Florida -- used deliberately UNbalanced propellers to enhance the performance of some of his engines.

The fact is that it is IMPOSSIBLE to dynamically balance a single-cylinder engine. The best that can be done in that line is a compromise. Generally that takes the form of counterbalancing the crankshaft such that it would sit motionless on a "frictionless balance stand" with ALL the weight of the lower half of the conrod plus HALF the weight of the piston, wristpin, and UPPER half of the conrod -- suspended from the crankpin.

The idea is to transfer half of the up-and-down unbalance of a one-banger motor into side-to-side vibration. The "dynamic resultant" of that SHOULD reduce the total vibratory effects by about a third. However, the realm of dynamics isn't quite that simple. When my friend Herb Wahl was developing his "replica" of the Bunch "Tiger Aero" engine, he discovered to his amazement that the engine ran smoothest with no shaft counterbalancing at all! Yet John Brodbeck Sr. of K&B found that an extra lead-alloy counterweight (driven from the rear of the crankpin) made the Greenhead R/C .45 a far smoother runner.

Why the variability? Seems that nothing in any mechanical system is truly rigid. Everything can distort, compress, and/or vibrate like a tuning fork if given the opportunity. In model engines SOMETIMES these distortions, compressions, and/or vibrations can prove beneficial; canceling out some or all of the dynamic imbalance effects.

This is what Don Garry found he could take advantage of. By installing a purposely-out-of-balance prop on some of his engines, with the heavier blade pointing "down" while the crankpin was at TDC, Don obtained noticeably smoother running -- and enhanced performance!

One more negative aspect of today's sophisticated prop balancing devices is this: Every dagnab one is a static balance machine. Would you want your car wheels balanced that way? DYNAMIC balance is all that counts -- and there's NO WAY you can adjust that conveniently on a single-cylinder model engine. Trial and error under operating conditions is the only way to go there.

What I do instead of using a "precision prop balancer" for my own model flying is this. Whenever I install a new prop (after removing all its sharp edges and smoothing its leading edge and tips into a neat radius) I slip a loose-fitting steel dowel through the hub hole and use that rough-and-ready "balancing tool" to detect a noticeably heavy blade. Usually I don't find one. But when I do, I install the prop anyway, being careful to put the heavy blade on the BOTTOM when the piston's at the TOP. So far I've had no regrets -- and I might mention that I'm somewhat more alert to engine vibration than most model flyers, because I've been totally deaf since 1959. The only way I can adjust my engines for optimum performance is by "feel".

Note in the preceding paragraph the use of the word "optimum" to refer to "performance". That's because I NEVER try for "peak" power output. But being deaf sometimes causes me an odd engine-running problem that's hard to solve. That is tactfully preventing some well-wishing modeler who knows I can't hear, from rushing over when I'm about ready to fly and leaning out my needle setting. He obviously thinks I can't tell the mixture's on the rich side -- and is blissfully unaware of the benefits of running model motors that way.