PB-4 Propeller Balancing Tutorial

Everyone is familiar with the idea of balancing an object so that it may rotate smoothly without wobbling. Every time we get a new tyre fitted to a car wheel, the technician will balance the wheel using a dynamic balancing machine which detects the vibration caused by the imbalance, does some calculations and then indicates where the small weights need to be added to the rim of the wheel to improve the balance. The tyre technician makes adjustments to the state of balance by adding and removing weights. They are fortunate in that there is normally no constraints as to where the weights can be added to the wheel rim. All 360° of the rim is available. By comparison, when a propeller is balanced, the weights can often only be added at certain fixed locations, e.g. the spinner fasteners or existing tapped holes in the engine flywheel. Because of this constraint that the weights can only be attached at certain locations, when the propeller has been balanced, the final weight configuration normally involves two weights attached at adjacent locations, one either side of the ideal location.

The PB-4 Viewer & Simulator is an online resource that lets you view previously saved PB-4 job data (polar points, spectra). The simulation capability generates polar points without needing a PB-4 or any sensors or even a real vibration source and provides an ideal training environment for practising propeller or rotor balancing. At the moment, the simulator is limited to the X and Y axes only and it does not produce spectra or waveform data, just basic polar points. The simulated polar points are “perfect” in that for a given balance condition and environment they do not vary like real polar points do. Furthermore, the simulation is quite simplistic in its modelling of the effects of adjustments on the position of polar points and so will always produce a reasonable result if the adjustments are set appropriately. The intended purpose of the simulator is to give people a means of practising the balancing procedure rather than providing a totally realistic simulation.

Start a modern web browser (e.g. Firefox, Chrome, Safari) and load the viewer/simulator from www.smartavionics.com/pb4/viewer/viewer.html. After the page has loaded and the splash screen has gone you should see a page like this.

Figure 1. Viewer Initial Page

The polar points list is initially empty apart from the header row. Note the two distinctive header row items WT1 and WT2.

By default, the job name is “default” and the job is set to propeller mode. You can easily change the job name from default to something more meaningful, just click on the name and you can enter a new name.

Click Polar Options and you will see these options with their default values.

Figure 2. Polar Options

The Weight Sites field contains the number of weight sites that your propeller provides. It has to be greater than 3.

Now we can start the simulator which is done via the main menu (top left of page). Choose Simulate (0.45 IPS @ 78°). This always simulates those particular IPS/Deg values. It's also possible to simulate a random IPS/Deg combination. The polar chart should now be displaying “live” polar points with the X point on 0.45 IPS @ 78°. It should look like the following. Note how the RPM is constant at 1800 (a typical propeller RPM) and the time is incrementing. The point is static because the simulation does not deliberately vary the polar point values (or the RPM). Obviously, when balancing a real propeller, the position of the polar point will be varying along with the RPM due to engine and airframe vibration, gusts of wind, etc.

Figure 3. Initial Polar Chart

Capturing points is easy, just click on the large Capture button below the polar chart. If the button is disabled it means that the simulation is not running (it can be stopped and started from the main menu). Press the capture button and you will see an X point added to the polar point list and plotted on the chart. By default, the most recently captured point is at the top of the list. When you click on a row in the list, that point is highlighted in the list and on the polar chart and the yellow info boxes in the corners of the polar chart show the values from the selected point. Clicking on the highlighted row again will open a dialog that shows the point's details (more on that topic later). While a point is selected in the list, the chart is not being updated. Chart update is controlled by the Update checkbox just above and to the right of the polar chart which is cleared when a polar point is selected in the list. Just click the Update checkbox to re-enable chart updates.

Figure 4. X Point Selected

With the initial condition of our propeller captured, we can explore the effect of making changes to the weight configuration. Because this is a simulator and not a real installation we just enter the changes rather than physically altering the amounts and positions of the weights. Click on WT1 or WT2 in the polar point list header and the Set Weight Amounts And Positions dialog appears.

Figure 5. Set Weight Amounts And Positions Dialog

Enter an amount, e.g. 3, into the WT1 amount field and close the dialog and you will see that the polar point has moved on the chart (if it hasn't moved, the points are not being updated, make sure the Update checkbox top right of the polar chart is checked). That movement of the polar point is the result of the effect that adding weight to a weight site has. Now open that dialog again and alter the location (site number) of the weight and see what effect that has. A few things to note about the effect are:

Remembering that the goal of the propeller balancing “game” is to get the polar points as close as possible to the centre of chart, we need to determine the weight configuration (amounts and locations) that will yield the best result. You could attempt to do this by trial and error but, fortunately, the PB-4 software helps us there by calculating a solution based on initial condition and the positions of points captured after weight has been added.

2.3.1. Defining The Start Point

We must first nominate one of our initial points (points that were captured before any weight was attached) to be the start point. To do this click on the point's line in the polar point list twice so that the point details dialog appears.

Figure 6. Polar Point Details Dialog

Click on Use As Start Point to nominate this point to be the start point and close the dialog. The point will now be coloured blue to distinguish it from other points.

2.3.2. Add A Trial Weight

Now add a trial weight by clicking on WT1 or WT1 in the point list header and enter, say 3, into the WT1 amount field. The location of the weight is not important so you can just leave it as 1.

Important

If this was a real balance session rather than a simulation, it would be important that the weight amount and location that you enter match the real weight amount and location that you have used.

Close the dialog and check that the polar point has moved a reasonable distance away from its initial position. You should be aiming for a movement of at least 1/4 of the distance from the initial point to the centre of the chart. Capture one or more points to get a screen like this.

Figure 7. Trial Weight Added

You can see that installing 3 units of weight at site one has moved the point North.

2.3.3. Improving The Balance

Now click twice on the points list row for the new point to display its point details and you will see that there is additional information displayed.

Figure 8. Trial Weight Point Details

There are two additional sections in the dialog:

Current Weight Configuration

The weight configuration that was in place when the point was captured is shown using 4 input fields (just like you see when clicking on WT1 or WT2 in the point list header). The primary reason those values can be altered here is because (being human) it's possible to forget to update the PB-4 weight configuration after physically changing the weights on the aircraft and capturing some points and so, in that situation, the captured points would have the old configuration values rather than the new values. So you can change a point's values here rather than having to capture new points.

Balance Solution

At the bottom of the dialog is the balance solution which has been calculated using the position of the start point, the position of the current point and the current point's weight configuration. The solution tells you what the weight configuration should be changed to for the balance to be improved. Although we initially attached just a single trial weight at one location, the solution will nearly always require two weights and they will be located at adjacent sites. The required weight values are specified to the nearest 1/2 weight unit and also as more exact values. Below the solution are buttons that update the weight configuration to either of those solutions (just like you had clicked on WT1 or WT2 and typed the values yourself). Obviously, when balancing a real propeller you would also need to change the real weight configuration to match.

Once the suggested solution has been implemented, the polar points should be closer to the centre of the chart. If the result is not as good as you would like, capture further points and repeat the process to refine the weight configuration.

Figure 9. Balance Solution

The simulation should always produce a good balance given a suitable weight configuration. The quality of balance you can achieve on a real propeller depends on many factors. Here are some factors that will have an influence on the accuracy of the polar points (listed in no particular order):