Measuring on a Grounded Aeroplane
In the early days of PULSE, a team from Brüel & Kjær had the opportunity of performing a ground test on a Boeing 767. The left engine of the aeroplane had undergone a scheduled overhaul. The engine was reassembled and verified in a test cell. Everything seemed fine. The engine was mounted on the aeroplane and the aeroplane was ready for operation.
However, there was a problem. At cruising speed, a constant alarm from the monitoring system indicated excessive vibration in the engine. During climbing, where the engine runs at a higher RPM to provide the thrust needed for the climb, the alarm disappeared indicating that everything was again all right. This was, of course, unacceptable. The plane was taken out of operation and subjected to a ground test to clarify whether there really was a vibration problem or whether the alarm was due to a malfunction of the monitoring system.
The task was pretty straightforward. Measure the vibration of the1st Order of the engine’s Gas Generator (GG) as a function of GG RPM from idle to full thrust. Piece of cake we thought, but it turned out that it was impossible to get a tacho signal from the GG. We could, however, get access to a tacho from an alternator driven by the High Pressure Turbine (HP), not that it could be used for tracking the GG, but it would at least show where we were during the run-up.
We got to the plane at nine in the morning and by noon we had mounted an accelerometer next to the monitoring accelerometer and taped up cables from the engine along the wing and fuselage into the cabin where our PULSE system was placed.
In the afternoon we got a time slot for the test and we taxied the aeroplane into the field toa location where we could get the plane up against the wind to prevent generator surge during the test.
We had planned 3 run-ups, but during the first one we got a furious call from the tower, “Shut that lawnmower down right now!” Not that we were cutting the grass, but we were, literally, moving the lawn with roots and soil when approaching full thrust. We had to relocate the plane to a spot where we found solid paving behind the engine and had three successful runs. The conclusion? Despite the absence of the GG tacho, we were able to show that the 1stOrder GG was excessive and that there really was a vibration problem.
PULSE Smart Start
That day was, without doubt, an exciting day, but we would really like to perform that same test again today. With Smart Start we could setup the test quickly and easily and, using the Auto tracker to give us the GG RPM, we could accomplish what we set out for – the vibration of the 1st order of the GG!
The plane is long gone but, fortunately, we kept a data record (PULSE Type 7701) of the session, so rather than dream we will redo the test with PULSE Version 12.
We launch PULSE/New Project and PULSE comes up with the Smart Start template as shown in Fig.1. The template comprises some Tasks of which ‘Analyses’, ‘3D’ and ‘White Board’ are the most important ones. To save space, the content of Fig.1 has been modified a little so that it shows the most important parts of ‘Analyses’ and ‘3D’ – the two Tasks that we can use to set up the complete measurement.
Performing the Analysis
When performing a troubleshooting task such as this one, we need a system that can interactively take us through some simple analysis steps to provide an overview, and then onto more dedicated analyses that will allow us to conclude more specifically on the problem. In this case, we want to start with a simple FFT and then move on with Order Tracking for amore specific analysis of the actual cause of the problem!
The ‘Analyses’ Task
We open the recorder file and the ‘Hardware Setup’ table now appears as it would in the online measurement. The rows in the table represent the available channels. In the “Analysis” columns we can select any of the available analyzers. Firstly, we select an FFT on the ‘Acc1’ signal. We right-click in the FFT cell and set the properties of the FFT analyzer to a 400 Hz Span and, as we want to perform a run-up, we set the ‘No. of Avg.’ to 1.
Analysing the ‘Run up’ in 3D View
We want a 3D colour contour of the vibrationspectrum of the GG and an RPM profile of theHP Turbine as a function of time. So in the‘Multi-Buffer Selection Table’ we check theFFT Analyzer and the Tachometer to deliverresults to the ‘3D-time’ buffer to get a colourcontour of the vibration spectrum of the GG.We select the update rate to be 1 second andthe length to cover the run-up session whichlasts around 4.5 minutes. We are now ready tomeasure, and press ‘Start’.
The analysis is now running and we go to the ‘White Board’ Task to organise our displays and results. In this, all results are shown in a tree view. We simply double-click on the ‘3D’Autospectrum of Acc 1 and get the display shown in Fig. 2. The name ‘White Board’ indicates that whatever is put onto the screen stays there when you leave the task to go to another. The displayed colour contour clearly shows the vibration related to the orders of GG as well as the HP. Inspection of Fig. 2 shows that the 1st Order vibration is higher at cruising RPM than at climbing RPM so there is a1st Order GG problem and the plane needs to remain grounded until the problem has been identified.
Digging Deeper – Applying ‘Autotracker’ and ‘Order Analysis’
In PULSE 12 we have the Auto tracker with which we can estimate the GG RPM from the vibration signal. In turn, we can use this RPM as reference for an Order Analysis of the GG vibration signal.
So we return to the ‘Analyses’ Task (Fig. 1),and select Auto tracker and Order as further analyses of the Acc 1 signal. On inspection of the 1st order of the GG in the colour contour(Fig. 2), we find that the run-up ranges from idle at 14.5 Hz = 870 RPM to around 3300 RPM at full thrust and that the max. acceleration during the run is 50 RPM/s. We enter these values into the properties of the Auto tracker. We right-click on the Order Analyzer and set the Order Span to 40 to cover the GG blade passing frequency.
We then proceed to the ‘3D’ Task and check-in the Auto tracker and the Order Analyzer to contribute to the ‘3D-time’ buffer. We then check the Order Analyzer to deliver ‘Pre-Slices’. Via the ‘Pre-Slices’ cell we get to the properties and specify that we want the Order Analyzer to extract the 1st Order of the vibration of the GG, which is the result we seek. We press ‘Start’ and return to the ‘White board’ to inspect the results.
We already saw that the alarm was due to areal vibration problem (which turned out to be a problem with mounting). We hope that, with this example, we have illustrated how easy it is to set up a rather complex multi-analysis with Smart Start. All of it done from three tasks with a few mouse clicks. You can argue that it’s only a single-channel measurement! Yes, but to extend the analyses to cover as many channels as your front-end configuration supports, couldn’t be easier. Using Excel-like methods, that is, Copy, Paste and Drag ’n’ Fill, you simply extend analyses to cover the signals you want. System analysis? Check the ‘Ref’ cell in ‘Hardware Setup’ (Fig.1) and you get all the appropriate cross-functions. Fast and easy.