The Oracle @ Delphi #8 10/5/2006 - Through the looking glass
"An airplane should run smoothly?" But, really, what is smooth? I?ve owned my plane for several years now; I?ve made trips from Canada to Mexico. If you had asked me if my plane was smooth, I would have answered, ?Yup, Sure, It?s Smooooth.? Then I had my propeller dynamically balanced.
What really sparked my interest in having my prop dynamically balanced was the ability to glean some insight as to the internal state of my engine. I?ve spent lots of time looking at how to really understand and optimize what my engine is doing. I?ve got my own compression tester which I use as a trending tool. I?ve replaced the factory fuel injectors with Gami injectors, to balance the fuel to air ratio in my cylinders. I?ve also installed a JPI engine monitor to allow lean of peak operations.. But I realized that a vibration analysis was a critical tool to gain insight as to what my engine was doing when it was really running. What else didn?t I know? There is always something else to know with flying.
I had just replaced my engine mounts (under supervision), other than the engine hoist, it was a piece of cake job, when I made an appointment with Jeremy MacDonald from Norcal Aero Services, to come out and take a look at my beautiful bird. I knew that he wasn?t going to find anything. My airplane was in tip-top shape with an engine a little more that half way to TBO. I was hoping that I wouldn?t even have to pay the additional fee for the weights and the sign-off.
After helping Jeremy place the two accelerometers on my engine (one in front and one in back) and the silver tape on the prop, we cleared the area and I started my engine. After running it for a few moments he had me shut it down. I?m sitting there in the cockpit and he comes over to me in a great English accent and says ?This plane has one of the largest mass imbalances that I have ever seen.? A little stunned, I looked at the graph that he had on the screen. And there it was, a spike that looked like it was as tall as Mount Everest. I immediately started to add up this new cost. After a few deep breaths Jeremy told me that most of the imbalance, 1.02 IPS (Inch Per Second), was coming from the front sensor and .54 coming from the rear. This problem was likely a prop problem. We spent some time looking closely at the propeller and discovered that the spinner was poorly made and was way off center. This off center spinner at 2400 RPM was putting a massive strain on the engine bearings, engine mounts, aircraft instruments, and the pilot. And I never knew!
A few test runs later, Jeremy had fashioned a counter weight out of some approved AN parts, and we ran another test. Jeremy had reduced the IPS from 1.02 IPS (about 128 LBS of side load) to .08 IPS (about 9 LBS) on the front and from .54 IPS (about 60 LBS) to .25 (about 36) on the rear. A few days later on our way to Washington, my wife said that she noticed a huge difference in how the plane was handling. The comment was followed by the two questions that every pilot?s spouse asks when something has changed in an aircraft: what did I do, and how much did it cost? I just smiled, having been reminded that you will never know what you will find until you look