I had been meaning to do this since getting the full permit but what really motivated me was a vibration which I didn't seem to remember when I first flew it. I put this down to being more familiar and picky with the aircraft the more I flew it - but as you will discover below I was not imagining it after all.
The procedure starts with the top cowling coming off first then two sensors are attached. One is measuring the RPM very accurately and does so by 'seeing' a reflective piece of tape which has been applied to one prop back face. The other sensor measures the vibration in Inches Per Second - IPS. This is mounted to the front of the engine as close to the prop as possible.
The engine is then run up to full static RPM - in this case 2700rpm (with the top cowl off I get 100 more rpm out of the engine as the air is colder going into the air filter).
When full static is achieved the ACES unit is set to measure the vibration which only takes about 5 seconds or so.
The first reading was 1.26 IPS - which is BAD!
Typically they try to get the IPS down to 0.07 or less. Adam the mechanic said the worst they have ever had was 1.6 IPS so mine was quite bad and the ACES machine called for a 45 gram correction weight which is beyond the 30 gram limit.
Another more senior mechanic came out to look at it and suggested to take the prop off and turn it through 180 degrees before fitting it again on the backplate.
So we did that and lo and behold the next reading was only 0.15 - which is nearly a 10 fold reduction in vibration!
I think I must have had the prop this way round when I first installed it - then at the 15hr service I must have put it on the other way. There is no right way as such - theoretically the prop should be the same either way but it must be heavier on one side and this just happens to cancel out the vibration or accentuate it depending on which way round it is fitted.
The next 4 runs were done after putting test weights where the ACES machine told us to. And when the 2nd to last run came up with an IPS of 0.02 Adam was satisfied that we had nailed it. The final weight was then fitted permanently to the backplate and it was just 3.4 grams of correction needed. A final reading after that showed it back up to 0.07 so work stopped there. 0.07 is the figure they work to and Adam explained that the final weight is tricky to get in the right place to retain the IPS we had previously.
Anyway the proof was in the test flight back home - turbine smooth now compared to before.
My wallet is now £300 lighter but this was well worth doing and I would recommend it. Not only is everything subject to less vibration but it is also much more relaxing to fly when things are smoother.
This also explains why I lost the cowling screws on my France trip as the extra vibration must have worked them loose.
|Adam fitting the sensors.|
|Vibration sensor mounted to the front of the engine.|
|Both sensors in place - the yellow sensor taped on the plenum is the RPM sensor.|
|The ACES machine displaying the first reading after switching the prop around. As you can see it calculates a 'solution'. Which is a weight and degrees from datum to fit it.|
|A protractor is then used to figure out where to place the correcting weights.|
|A test correcting weight using one of the spinner holes.|
|The 4 readings after switching the prop around.|