TB20 Vibration Analysis
This is an attempt to analyse the sources that contribute to cockpit vibration levels in the Socata TB20 aircraft.
It is a topic which has had owners (of all aircraft types) puzzled, often leading them to chase around specialist companies which can do dynamic balancing of propellers or crankshafts. Dynamic prop balancing does make a very noticeable difference; what is unclear is why doing it in flight is claimed to deliver a better result.
Procedure
A Monitran MTN/1800 accelerometer was attached to an exposed part of the metal airframe, inside the cockpit, with the attachment point (one of the RHS rudder pedal supports) chosen to have a direct connection to the main airframe structure.
The accelerometer outputs 1 volt per G. It requires a constant current supply (around 5mA) with an open circuit voltage of 18V and this was provided with a custom built interface box.
A Tektronix TDS2004B portable digital oscilloscope was used to display the data as a Fourier Transform (FFT) which shows the frequency spectrum. This instrument can capture screen images to a USB FLASH stick, as sequentially numbered TIFF files. These images are reproduced here unaltered. Note that the vertical scale is in db i.e. logarithmic.
The oscilloscope was powered from the aircraft 28V cigar lighter socket, using a 28V to 240V inverter. The data was taken during cruise (except the idle data which was on the ground) at about 140kt and 5000ft.
Frequencies of Interest
An engine running at say 2400rpm has a crankshaft rotation frequency of 40 revs per second and any out of balance component on this would be expected to generate a line at 40Hz.
The camshaft rotates at half the crankshaft frequency i.e. 20Hz.
The 3-blade propeller would be expected to show an out of balance line at 40Hz, and any aerodynamic (slipstream impact) effects on the airframe would be expected at the 3rd harmonic i.e. 120Hz.
Exhaust effects (three combustion events per engine revolution) would be expected at the 3rd harmonic i.e. 120Hz.
It is important to note that nothing inside the engine rotates at 3x the crankshaft speed so any 3rd harmonic cannot be a simple mechanical imbalance.
Data
Fig 1 Engine idling at 1200rpm, leaned to 2.8GPH. This shows a small contribution at the 20Hz crankshaft frequency and the major peak is at the 3rd harmonic i.e. prop blade or exhaust pulse rate.
Fig 2 Full power climb, 2575 rpm. The peak is equally at the 3rd harmonic and the 6th harmonic. The fundamental barely features:
Fig 3 Economy cruise, 23"/2400rpm/11GPH, very slightly lean of peak. The fundamental and the 6th harmonic are buried in noise and the 3rd harmonic dominates. The other peak is at 280Hz which is at 7 times the crank frequency (bizzare).
Fig 4 The following is under same conditions as above but with the mixture enriched to 14GPH which is well rich of peak. The power output in this condition is higher, so the prop blade pitch is coarser. The rich mixture feeds the engine with excess fuel and eliminates any cylinder power balance issues. The fundamental is still nonexistent, the 3rd harmonic dominates, and the rest is similar, so uneven power delivery is not an issue in this experiment.
Fig 5 The following is at 23"/2500rpm/11GPH which is again slightly lean of peak. Apart from the slightly higher revs, the conditions are identical to Fig 3. We now have a new dominant peak at the 6th harmonic. The engine sounds noticeably smoother, which is not suprising considering how much cleaner the area around 250Hz (6th harmonic) is.
Conclusion
The most obvious thing is what is not present! The contribution from the fundamental frequency (the crankshaft and propeller speed) is negligible, except during idle when the rest of the noise is much lower. This is a most suprising result. However, this is with an engine which has been rebuilt (SB569 crank swap) by a highly reputable U.S. engine shop which dynamically balanced the crankshaft and matched up the piston and conrod weights. Moreover, the prop had just been overhauled by a very careful individual known to me personally and was then dynamically balanced on the engine. The dynamic balance was checked with accelerometers on both the front and the back of the engine and both were below 0.1 IPS (inches per second) which would be a good figure for a turbine engine. My regret is that I did not have this equipment before all this work was done; for example, when the prop was only statically balanced there was a very noticeable vibration at 1200rpm.
The second interesting outcome is that the higher harmonics dominate. The 3rd harmonic in particular is either the prop slipstream (which rotates at 3x the crank speed and this rotating airflow impacts the airframe all around), or the exhaust gases, or some engine vibration related to the exhaust event frequency.
There is also a noticeable half-crank-speed (20Hz at 2400rpm) component - it is not obvious where this could come from. Only the camshaft rotates at this speed.
A brief spectrum analysis taken from the engine mounted accelerometers (no graphic available) also shows strong contributions from the higher harmonics, suggesting that this higher order airframe vibration could well be coming from the engine and, since nothing rotates at 3x the crank speed, this must be due to the combustion event.
The other very noticeable thing is that there is a lot of energy right across the spectrum, from nearly zero (probably about 10Hz) all the way to the end of the scale which is 500Hz. This is most likely aerodynamic noise. Clearly this type of light aircraft would benefit from some heavy sound absorbing material, but the weight penalty would likely be huge and anyway it cannot be applied to the large windows.
Last edited 25th July 2008
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