PB-4 Propeller & Rotor Balancer

User Interface Manual

Smart Avionics Ltd.www.smartavionics.com

Revision: 1.7

Android is a trademark of Google Inc.

iOS is a trademark of Apple Inc.


Table of Contents

1. Introduction
1. Accessing the PB-4 user interface
1.1. Accessing the PB-4 user interface (AP mode)
1.2. Accessing the PB-4 user interface (STA mode)
2. Initial screen
3. Jobs
4. Menu button
5. Connect button
2. Polar Tab - Propeller Balancing
1. Polar chart
1.1. Capture button
2. Points list
3. Option buttons
3.1. Print Polar Chart menu
3.2. What Next? button
3.3. Show Waveform button
3.4. Polar Options button
4. Polar details dialog
3. Polar Tab - Rotor Balancing
1. Polar chart
1.1. Capture button
2. Points list
3. Option buttons
3.1. Print Polar Chart menu
3.2. What Next? button
3.3. Show Waveform button
3.4. Polar Options button
4. Polar details dialog
5. Edit move line dialog
5.1. Defining move line nodes
4. Spectrum Tab
1. Spectrum display
1.1. Capture button
2. Lines and spectra lists
3. Option buttons
3.1. Print Spectrum button
3.2. Spectrum Options button
5. Data Tab
6. Options Page
1. Change to another job
2. Create a new job
3. Rename the current job
4. Delete the current job
5. Save / restore PB-4 data
6. PB-4 configuration
6.1. Set access point details
6.2. PB-4 hardware configuration
6.2.1. Set the battery level to 100%
6.2.2. Selecting the tachometer input
6.2.3. Accelerometer axis calibration
6.3. Get / set configuration values
6.4. Firmware upload
7. Upload UI files
8. Set PB-4 IP address
7. Propeller Balancing
1. Minimising Other Sources of Vibration
2. Propeller mass imbalance
3. Static Propeller Balancing
4. Dynamic Propeller Balancing
5. Taking a Vibration Reading
6. Balancing Procedure
7. After Balancing
8. Troubleshooting
8.1. The displayed RPM is erratic or wrong
8.2. The current polar point position varies widely
8.3. Adding weight does not reduce the vibration level
8. Rotor Balancing
1. Causes Of Rotor Vibration
2. Rotor Mass Balancing
2.1. Defining move lines
2.2. Using the move lines to balance
A. Configuration values
Index

List of Figures

1.1. Initial screen
2.1. Polar tab - propeller balancing (with options visible)
2.2. Polar chart - propeller balancing
2.3. Point details dialog - propeller mode
3.1. Polar tab - rotor balancing (with options visible)
3.2. Polar chart - rotor balancing
3.3. Point details dialog - rotor mode
3.4. Edit move line dialog
4.1. Spectrum tab (with options visible)
4.2. Rotor spectrum with line info box
5.1. Data tab (with options visible)
6.1. Options page (when connected to PB-4)
7.1. Sine wave
8.1. Rotor spectrum with large × 2 peak

Chapter 1. Introduction

This manual describes the PB-4's browser based user interface and how to use it to balance a propeller or rotor. Please refer to the PB-4 Hardware Manual (pb4_hw_manual.pdf) for information regarding the PB-4 hardware and how it is installed prior to carrying out a balancing job.

For simplicity, this manual refers to the user interface as a tablet but it could be any device that supports a modern web browser and it talks about clicking buttons. Obviously, if the device has a touch screen then you will be touching or tapping rather than clicking.

1. Accessing the PB-4 user interface

Safari users please note that the PB-4 is not compatible with Safari's Private Mode.

Accessing the PB-4 user interface from your tablet's browser requires the PB-4 and the tablet to be using the same Wi-Fi network.

1.1. Accessing the PB-4 user interface (AP mode)

When the PB-4 is in AP mode, the Wi-Fi network name (SSID) will be PB4-XX where XX are two letters/digits. You will need to tell your tablet to use that Wi-Fi network (via its Wi-Fi configuration). At the airfield, the tablet will probably use the PB-4 Wi-Fi automatically because your normal Wi-Fi is not available. If not, you will have to tell the tablet to use the PB-4 Wi-Fi manually.

Once the tablet is using the PB-4 network, you can load the user interface page into the browser. If your tablet supports MDNS, you can simply use pb4.local as the host name in the browser. Apple devices (mac/ipad/iphone) and most Linux systems have MDNS, most Windows systems do not. If not, use the IP address 172.21.21.1. Having accessed the PB-4 user interface page, just add a bookmark for that page in your browser called something like "PB-4 AP Mode (at the airfield)".

1.2. Accessing the PB-4 user interface (STA mode)

When the PB-4 is in STA mode, the Wi-Fi network name (SSID) will be whatever you normally use at the current location. The PB-4 will need to be taught that SSID and the associated password. The PB-4 supports WPS for automatically obtaining the SSID and password from the access point which is quick and easy if the access point also supports this feature. Alternatively, you can manually enter the credentials by starting the PB-4 in AP mode, loading the user interface into your browser (as described above), connecting to the PB-4 (click on red button) and then going to Menu / Options / PB-4 Configuration / Set Access Point Details.

Once the PB-4 is able to connect to the local network, you can load the user interface page into the tablet's browser. Again, if MDNS is supported by the tablet, the host name pb4.local can be used. Otherwise, you need to find out the IP address that has been assigned to the PB-4 by the access point. Follow these steps:

  1. Turn on the PB-4 in STA mode (press the on/off switch until LED 2 goes green). When you release the switch, LED 2 will go off and then after a few seconds will flash green slowly if the PB-4 has successfully managed to log into the access point.

  2. Turn the PB-4 off (press switch until the LEDs go out).

  3. Turn the PB-4 on again but this time start it in AP mode (press on/off only until LED 1 comes on).

  4. Access the PB-4 user interface page with your browser as described in the previous section, connect to the PB-4 (click on red button) and then go to Menu / About. The about box will contain a line that says STA Mode IP Address: XX.XX.XX.XX. The address that is shown is the IP address that has been assigned to the PB-4 by the access point, note it down.

Having found out the PB-4's IP address when in STA mode, you can use that address to access the user interface page and then add a bookmark for that page in your browser called something like "PB-4 STA Mode (at home)".

2. Initial screen

When you load the PB-4 user interface into your browser, after showing the splash screen for a few seconds it will show an initial screen, like Figure 1.1, “Initial screen”

Figure 1.1. Initial screen

Initial screen

This shows a polar chart and a table of captured points (both empty as no points have been captured yet). At the top of the screen, are the Menu and Connect buttons and the name of the current job.

Most of the screen is used to display the PB-4 data. At the top of the region are three tabs (Polar, Spectrum & Data) which you use to select the data you wish to see.

Polar

The polar tab shows a polar chart, a table of captured points and some buttons. For a detailed description of using the polar tab see Chapter 2, Polar Tab - Propeller Balancing (propeller balancing) and Chapter 3, Polar Tab - Rotor Balancing (rotor balancing).

SpectrumThe spectrum tab shows a spectral display, a table of captured spectra, a table of spectral lines and some buttons. See Chapter 4, Spectrum Tab for a detailed description of the spectrum tab.
DataThe data tab shows a table that displays the current vibration data (RPM, IPS, DEG) as text and some buttons. See Chapter 5, Data Tab for a detailed description of the data tab.

The PB-4 user interface is "responsive" - the layout changes depending on the resolution and orientation of the user interface's screen. It should be usable on many different user interface devices. For in-cockpit use, a 7" form factor tablet is ideal. For lower resolution devices, you may find that changing the orientation to landscape is beneficial.

3. Jobs

The PB-4 supports the notion of a "job".

  • A job is simply a named collection of data (points and spectra) associated with a single balancing job.

  • There is always a current job. The name of the current job is displayed alongside the connect button. When data is captured, it will be stored in the current job.

  • Each job is named and may be loaded into the user interface to become the current job. The points and spectra captured for that job can then be viewed.

  • When new, the PB-4 contains a single job, called "default".

  • The default job is always available. If you delete it, it is recreated.

  • Jobs can be created, deleted and renamed.

  • All of the job data is stored on an SD card in the PB-4.

  • Jobs can be exported and imported so you can share job data with other PB-4 operators.

4. Menu button

Clicking the menu button pops up a menu with these items:

Options

Clicking this will toggle the visibility of the options page. When the options page is being displayed, to return to the tabs page you can either click Menu / Options again or press the browser's back button.

Fullscreen

If your browser supports displaying a web page fullscreen, clicking this will toggle fullscreen mode. To exit fullscreen mode, either click Menu / Fullscreen again or press the Esc key (if your device has a keyboard!)

Help

Clicking this will display a simple web page with links to the PB-4 documentation.

About

Clicking this will display the About dialog. Some of the information in the dialog is only available when the PB-4 is connected.

5. Connect button

The connect button toggles the user interface - PB-4 connection state. The current state is indicated by the colour of the button:

red

When the button is red, the user interface is not connected to the PB-4. The browser can display the data for the current job that has already been captured but no new data can be displayed or another job viewed until the connection has been made to the PB-4.

green

When the button is green, the user interface is connected to the PB-4 and the text on the button is updated with the PB-4's battery capacity remaining[a]. New data can be displayed and other jobs can be accessed via the options page.

orange

The button is orange while the connection to the PB-4 is being established and also when the battery is down to the last 20-30 minutes of run time to remind you that you will need to change the batteries soon[b].

[a] The battery capacity remaining is estimated from the time the PB-4 has been running since the batteries were last changed. It assumes the batteries were fully charged to their rated capacity when they were installed.

[b] This low battery warning is based on the battery voltage and is an accurate indication that the batteries are almost flat.

If the browser page is reloaded from the PB-4 while connected or the browser is closed, the connection will be dropped.

The PB-4 will turn itself off automatically after 30 minutes of not being connected.

Chapter 2. Polar Tab - Propeller Balancing

The polar tab shows the polar chart we use when balancing. Some features of this tab differ depending on whether we are balancing a propeller or a rotor. Here we see the polar tab displaying the results of a propeller balancing job.

Figure 2.1. Polar tab - propeller balancing (with options visible)

Polar tab - propeller balancing (with options visible)

1. Polar chart

The polar chart shows the points that were captured during the balancing process. The chart may be panned and zoomed[1]. If the PB-4 is connected and the Update checkbox is checked, the chart will be updated in real-time to show the current balance condition.

The current point is simply the most recent point sent from the PB-4 when the display is being updated in real-time or the point that has been selected in the points list when the display is not being updated in real-time.

Each point is plotted as a circle, the colour coding of the plotted points are:

WhiteCaptured point.
BlueStart point.
YellowThe current point.

Figure 2.2. Polar chart - propeller balancing

Polar chart - propeller balancing

The chart shows some text in yellow boxes that describes the current point:

The top-left box contains the tacho channel name (A or B) and the current point's propeller RPM.

The top-right box contains the accelerometer channel name and the current point's IPS and DEG values.

The bottom-right box contains the date & time the current point was measured.

The bottom-left box is only visible if there is a start point defined for the axis. It shows a solution, i.e. what adjustment needs to be made to the balance weight to improve the balance. The adjustment has the form ANGLE × SCALE. ANGLE is signed; a positive value means move the weight forward (in the direction of propeller rotation) that many degrees and a negative value means move the weight backward (opposite to the direction of propeller rotation). SCALE is a multiplier that specifies the required change in weight; e.g. a value of 1.2 means increase the weight by 20%, a value of 0.5 means halve the weight. So in the chart above, the solution is saying move the weight backwards 4° and keep the amount the same.

1.1. Capture button

Below the polar chart is the Capture button. Clicking on this will capture the next polar point produced by the PB-4. All axes (X, Y and also X2, Y2 if the second accelerometer is in use) are captured together irrespective of which axis is currently being displayed.

2. Points list

To the right of the polar chart are the Update and Show All check boxes and the points list.

  • The Update checkbox controls whether the polar chart is automatically updated to show the latest polar point(s) returned by the PB-4 when it is connected. If not checked, the last displayed data is preserved. So if you want to pause the display, un-check this checkbox.

  • The Show All checkbox decides whether hidden points are listed in the points list or not. When checked, all points are listed, when un-checked, hidden points are not displayed. Also, when this checkbox is checked, the point index number column is highlighted and clicking on a point's index number will toggle its hidden state.

  • The points list contains one row for each point that has been captured. The points are listed in reverse capture order, the most recently captured point being at the top of the list. Clicking on the Time header reverses the order of the list. Clicking on a row will make the selected point the current point and it will be displayed with a yellow background. Clicking that row again will display the point details dialog for the selected point.

    Each row in the table shows the data for a single point (time of capture, RPM, IPS, DEG, the amounts and locations of the balance weights that were present when that point was captured and the point's notes). The weight info has the form AMOUNT @ SITE where AMOUNT is a number ≥ 0 and SITE is a value from 1 to the number of weight sites.

3. Option buttons

To the right of the points list are a drop down menu and some buttons:

3.1. Print Polar Chart menu

This gives you the choice of either printing just the chart or the chart and a table of points. It will create a new page in your browser which you can then print using the normal browser print page facility (e.g. type Ctrl-P).

3.2. What Next? button

Clicking this will open a dialog that suggests your next action. When you are doing your first propeller balances, you may find this useful.

3.3. Show Waveform button

Clicking this will open a dialog that shows the current acceleration waveform.

3.4. Polar Options button

Clicking this will toggle the display of the polar options box, the options in the box are:

  • The Propeller/Rotor Mode button pops up a menu that let's you choose the polar mode for the current job.

  • Drop down menu, Axis 1 lets you choose the axis to be displayed in the points list and polar chart.

  • The Point Smoothing text input box is used to specify the number of readings that are averaged together to produce a single polar point. A low value will allow more variation in the polar points and the display will react more quickly to input changes (i.e. changes of RPM). A large value will tend to smooth out variations in the polar points but will make the display slower to react to input changes so you will need to wait longer for the point to settle in the averaged position. The value should be between 1 and 50, the default is 5.

  • The Weight Sites text input box is used to specify the number of locations that balance weights may be attached to. For a typical propeller balancing job, this would be the number of fasteners that hold the spinner to its backplate. It cannot be less than 3.

  • The PSRU Ratio text input box is used to enter the ratio of the engine to propeller step down gearbox. If your engine has a PSRU and you know the ratio, enter it here. If you don't know the ratio or the engine doesn't have a PSRU, enter 1. When applicable, using a value other than 1 has two benefits:

    1. For certain gearbox ratios, the PB-4 can produce more accurate polar data (IPS, DEG) values.

    2. Knowing the PSRU ratio allows the software to calculate the engine RPM from the propeller RPM and this information can be used to help identify the source of peaks that may occur in the spectral display.

4. Polar details dialog

This dialog displays the detailed information for the selected point along with some text input boxes and buttons as shown in Figure 2.3, “Point details dialog - propeller mode”. The buttons are:

  • The Hide This Point button will make the current point invisible, i.e. it will not be displayed in the points list or shown on the polar chart. If the Show All checkbox is checked, the point will be displayed in the points list. When a point has been hidden, this button becomes the Show This Point button and clicking on it will make the point visible again.

  • The Use As Start Point button designates the current point to be a start point. A start point is simply a point that was captured before any balance weights were attached, it represents the propeller balance starting condition. Once a point has been chosen as a start point, the button becomes the Don't Use As Start Point button and clicking on it makes the current point no longer to be considered a start point.

  • If the point was captured with acceleration data (see the With Waveform Data option in Chapter 5, Data Tab), the Show/Hide Waveform button will be visible and clicking that will toggle the display of the waveform.

The text input boxes are:

  • The Notes text input box is used to enter any notes that you wish to record for the point.

  • Two rows contain text input boxes that are used to enter the amount and location (site number) of the balance weights that were present when this point was captured.

    The weight amounts can be in real weight units (e.g. grams or ounces!) or simply numbers of washers.

    This information has to be entered by you after the point has been captured. Once you have done that, the lines below will show the change required to improve the balance. The new weight amounts displayed are shown rounded to the nearest integer and also as (more) exact values.

Figure 2.3. Point details dialog - propeller mode

Point details dialog - propeller mode



[1] Pinch-zooming is supported for touch screens and mouse-wheel zooming for laptops, etc.

Chapter 3. Polar Tab - Rotor Balancing

The polar tab shows the polar chart we use when balancing. Some features of this tab differ depending on whether we are balancing a propeller or a rotor. Here we see the polar tab displaying the results of a rotor balancing job.

Figure 3.1. Polar tab - rotor balancing (with options visible)

Polar tab - rotor balancing (with options visible)

1. Polar chart

The polar chart shows the points that were captured during the balancing process. The chart may be panned and zoomed[2]. If the PB-4 is connected and the Update checkbox is checked, the chart will be updated in real-time to show the current balance condition.

The chart may show move lines. Each move line is defined by the positions of two nodes which are drawn as filled circles containing the node's value.

The current point is simply the most recent point sent from the PB-4 when the display is being updated in real-time or the point that has been selected in the points list when the display is not being updated in real-time.

Each point is plotted as a circle, the colour coding of the plotted points are:

WhitePoint captured from axis 1.
GreyPoint captured from axis 2.
YellowThe current axis 1 point.
GoldThe current axis 2 point.

Figure 3.2. Polar chart - rotor balancing

Polar chart - rotor balancing

The chart shows some text in yellow (or gold) boxes that describes the current point:

The top-left box contains the tacho channel name (A or B) and the current point's rotor RPM.

The top-right box contains the accelerometer channel name and the current point's IPS and DEG values.

The bottom-right box contains the date & time the current point was measured.

The boxes in the bottom-left are only visible if there are active move lines defined for this axis. One box is shown for each active move line. They show the adjustments required to improve the balance. So in the chart above, the adjustments required are add 2 washers for the move line named X span and -60 degrees of movement for the move line named X chord.

1.1. Capture button

Below the polar chart is the Capture button. Clicking on this will capture the next polar point produced by the PB-4. All axes (X, Y and also X2, Y2 if the second accelerometer is in use) are captured together irrespective of which axis is currently being displayed.

2. Points list

To the right of the polar chart are the Update and Show All check boxes and the points list.

  • The Update checkbox controls whether the polar chart is automatically updated to show the latest polar point(s) returned by the PB-4 when it is connected. If not checked, the last displayed data is preserved. So if you want to pause the display, un-check this checkbox.

  • The Show All checkbox decides whether hidden points are listed in the points list or not. When checked, all points are listed, when un-checked, hidden points are not displayed. Also, when this checkbox is checked, the point index number column is highlighted and clicking on a point's index number will toggle its hidden state.

  • The points list contains one row for each point that has been captured. The rows that correspond to axis 1 have a white background and the rows that correspond to axis 2 have a grey background. The points are listed in reverse capture order, the most recently captured point being at the top of the list. Clicking on the Time header reverses the order of the list. Clicking on a row will make the selected point the current point and it will be displayed with a yellow (axis 1) or gold (axis 2) background. Clicking that row again will display the point details dialog for the selected point.

    Each row in the table shows the data for a single point (time of capture, RPM, IPS, DEG and the point's notes).

3. Option buttons

To the right of the points list are a drop down menu and some buttons:

3.1. Print Polar Chart menu

This gives you the choice of either printing just the chart or the chart and a table of points. It will create a new page in your browser which you can then print using the normal browser print page facility (e.g. type Ctrl-P).

3.2. What Next? button

Clicking this will open a dialog that suggests your next action. When you are doing your first rotor balances, you may find this useful.

3.3. Show Waveform button

Clicking this will open a dialog that shows the current acceleration waveform.

3.4. Polar Options button

Clicking this will toggle the display of the polar options box, the options in the box are:

  • The Propeller/Rotor Mode button pops up a menu that let's you choose the polar mode for the current job.

  • Drop down menus, Axis 1 and Axis 2 let you choose the axes to be displayed in the points list and polar chart.

  • The Point Smoothing text input box is used to specify the number of readings that are averaged together to produce a single polar point. A low value will allow more variation in the polar points and the display will react more quickly to input changes (i.e. changes of RPM). A large value will tend to smooth out variations in the polar points but will make the display slower to react to input changes so you will need to wait longer for the point to settle in the averaged position. The value should be between 1 and 50, the default is 5.

  • The Add Move Line button creates a new move line for the axis currently selected as Axis 1. The edit move line dialog will open to allow you to define the move line's parameters. The new move line is always called A Move Line (it can be renamed).

  • The Edit Move Line button pops up a menu of the existing move lines defined for the axis currently selected as Axis 1. Select the move line you wish to edit and the edit move line dialog will open.

4. Polar details dialog

This dialog shows the detailed information for the selected point along with a text input box and some buttons as shown in Figure 3.3, “Point details dialog - rotor mode”. The buttons are:

  • The Hide This Point button will make the current point invisible, i.e. it will not be displayed in the points list or shown on the polar chart. If the Show All checkbox is checked, the point will be displayed in the points list. When a point has been hidden, this button becomes the Show This Point button and clicking on it will make the point visible again.

  • If the point was captured with acceleration data (see the With Waveform Data option in Chapter 5, Data Tab), the Show/Hide Waveform button will be visible and clicking that will toggle the display of the waveform.

Figure 3.3. Point details dialog - rotor mode

Point details dialog - rotor mode

The Notes text input box is used to enter any notes that you wish to record for the point.

Following that is the Balance Solution. This is the same information as is shown in the bottom left-hand corner of the polar chart when this point is selected. It tells you what adjustments need to be made to improve the balance.

5. Edit move line dialog

The edit move line dialog is where you define your move lines. It shows the polar chart so you can immediately see the effects of the changes you make.

Normally, the move lines (and their nodes) are drawn on top of the polar points. This ensures that the node values are not obscured by any points. Obviously, this can mean that some points are hidden behind the move line nodes and so to make it possible to see those hidden points, while the chart is being panned, the points are drawn on top of the move lines.

Figure 3.4. Edit move line dialog

Edit move line dialog

On the right hand side of the dialog are various text entry boxes and a couple of checkboxes:

NameEnter here the name of the move line (the software doesn't require it to be unique but you will get confused if it isn't!)
Colour

The colour used to draw the move line.

[Note]Note

What you will get when you click on this depends on the browser you are using. Some browsers will present a colour chooser widget when you click on this. If you're not so lucky it will probably get the chance to enter the colour as a number like #0b00ff (that's the value for the blue move line shown above).

SolidityThe percentage solidity of the move line (20, 50, 80 or 100%).
UnitsThe units of adjustment for this move line. This is just text, it doesn't feature in any calculations.
Step

When the adjustment is calculated, the resulting value will be a multiple of this value. So if you are using washers for the units of adjustment and you only have washers of one size, then a step of 1 would be appropriate but if you also had some washers that weighed half as much, then a step of 0.5 would be appropriate because then the suggested amount of adjustment would be to the nearest 1/2 washer.

VisibleThis needs to be checked to see the move line on the polar chart.
Active

This needs to be checked for the software to calculate an adjustment for this move line.

[Note]Note

A maximum of three move lines may be active per accelerometer axis at any one time. If you activate more than three, a solution cannot be computed.

5.1. Defining move line nodes

To define a move line, two nodes on the polar chart (called Node 1 and Node 2) that the line passes through need to be specified. The nodes are drawn on the chart as coloured blobs that are slightly larger than the points. For each node we can enter the following information:

PointIf you want the node to be located exactly at the position of a captured point, enter the point's index number here and the following fields (Deg and IPS) will be taken from the chosen point.
DegThe angular position of the move line node.
IPSThe magnitude of vibration at the move line node.
Value

The adjustment that was in place at the time that the point(s) were captured that define the position of this node.

In Figure 3.4, “Edit move line dialog” above, Node 1 is defined using the point that was captured when the chord balance mechanism had been adjusted by 180° and Node 2 is defined using the point that was captured when the chord balance mechanism had been adjusted by -180°.

At the bottom of the dialog is the Delete This Move Line button, you will be prompted to confirm the action before the move line is deleted.



[2] Pinch-zooming is supported for touch screens and mouse-wheel zooming for laptops, etc.

Chapter 4. Spectrum Tab

The spectrum tab displays the vibration spectrum. Vibration level (IPS) is plotted against RPM. The displayed spectrum may either be the live data coming from the PB-4 or a previously captured spectrum. The spectrum is plotted as a curve by joining the tops of the spectral lines[3]. Each spectral line provides the IPS value for a specific RPM (e.g. 100, 200, 300, etc.). The spectrum may be panned and zoomed[4].

The spectrum display is extremely useful for helping to diagnose vibration problems as it allows you to determine how much vibration is occurring at each of the frequencies of interest (prop/rotor frequency, blade pass frequency[5], engine crank and half-crank frequencies, etc.)

Here we see the spectrum tab displaying a previously captured spectrum.

Figure 4.1. Spectrum tab (with options visible)

Spectrum tab (with options visible)

The spectrum tab contains (from left to right, top to bottom):

1. Spectrum display

The spectrum display shows vibration level (in IPS) plotted against RPM. If the PB-4 is connected and the Update checkbox is checked, the display will be continuously updated to show the latest spectrum received from the PB-4. The Axis 1 menu (see below) controls which axis the displayed spectrum is sourced from.

In the top right-hand corner are shown:

  • Date and time the spectrum was captured.

  • The tachometer channel used (A or B) and the rotor/propeller RPM (if known).

  • The accelerometer channel used (X, Y, X2 or Y2) and the vibration level[6] at the rotor/propeller RPM (if that RPM is known).

Figure 4.2. Rotor spectrum with line info box

Rotor spectrum with line info box

If the rotor/propeller RPM is known, the region of the spectrum around that RPM is displayed in red and if any line labels have been defined, they will be displayed in a box above their respective lines. Line labels x1 and x2 are shown in the figures.

The bottom 1/3 of the display is the scroll zone. When zoomed into the display, the spectrum may be scrolled left or right by clicking and moving in the scroll zone. On computers with a keyboard and a mouse you can also scroll using shift-mousewheel.

Clicking above the scroll zone displays a cursor line and an info box which contains information about the spectral line below the cursor. The information displayed is the line's vibration level (IPS), frequency (RPM) and ratio of the line's RPM and the propeller/rotor RPM.

Clicking in the middle 1/3 of the display moves the cursor line to the position of the click. Clicking in the top 1/3 of the display moves the cursor one line left or right depending on which side of the cursor you clicked.

1.1. Capture button

Below the spectrum display is the Capture button. Clicking on this will capture the next spectrum produced by the PB-4. All axes (X, Y and also X2, Y2 if the second accelerometer is in use) are captured together irrespective of which axis is currently being displayed.

2. Lines and spectra lists

To the right of the spectrum display are the Update and Show All checkboxes and the lines and spectra lists.

  • The Update checkbox controls whether the spectrum display (and spectra list) are automatically updated to show the latest spectrum returned by the PB-4 when it is connected. If not checked, the last displayed data is preserved. So if you want to pause the display, un-check this checkbox.

  • The Show All checkbox decides whether hidden spectra are listed in the spectra list or not. When checked, all spectra are listed, when un-checked, hidden spectra are not displayed. Also, when this checkbox is checked, the spectrum index number column is highlighted and clicking on a spectrum's index number will toggle its hidden state.

  • The lines list contains one row for each spectral line in the currently displayed spectrum. Each row displays the frequency of the line and the amount of vibration at that frequency (IPS). Clicking on the FREQ header at the top of the list toggles how the frequency is displayed in the list, RPM or as the ratio of the line's RPM to the rotor/propeller RPM.

    If the propeller/rotor RPM is known, the row in the list that corresponds to that RPM is shown in red. Clicking on an entry in the list will move the cursor to that line. If the user interface device screen is very narrow, the lines list is not displayed.

  • The spectra list contains one row for each spectrum that has been captured. The rows that correspond to axis 1 have a white background and the rows that correspond to axis 2 have a grey background. The spectra are listed in reverse capture order, the most recently captured spectrum being at the top of the list. Clicking on the Time header reverses the order of the list. Clicking on a row will display the selected spectrum. Clicking that row again will display the spectrum details dialog for the selected spectrum where you can enter a note for the spectrum if you wish.

    If at the time a given spectrum was captured, the propeller/rotor RPM was known, it will be shown to the right of the spectrum's capture time.

    If a note has been entered for a spectrum, it will be displayed in the list. If the spectrum doesn't have a note entered but a point that was captured in the 10 seconds before the spectrum was captured does have a note, then that note will be shown in square brackets [like this!]. The idea here is that you can enter notes for polar points and those notes will then be shown in both the polar point and spectrum lists.

3. Option buttons

To the right of the spectra list are some buttons:

3.1. Print Spectrum button

Clicking this button will create a new page in your browser showing the currently selected spectrum and some textual information. If the currently selected spectrum shows line labels, a table detailing the line labels will be included. You can then print this new window using the normal browser print page facility (e.g. type Ctrl-P).

3.2. Spectrum Options button

The Spectrum Options button toggles the display of the spectrum options box (visible above), the options are:

  • Drop down menus, Axis 1 and Axis 2 let you choose the axes to be included in the spectra list.

  • The Range button pops ups a menu that lets you choose the display's full scale value. Setting it to Auto will automatically scale the display so that the largest peak almost reaches the top. Setting it to one of the fixed values makes it easier to compare spectra.

  • The RPM / Line button pops up a menu from which you can select the width (in RPM) of each spectral line. The available widths are 10, 20, 50 and 100. The trade off here is overall frequency range displayed versus resolution. Also, the narrower the width of each line, the longer it takes to capture each spectrum. So using 100 RPM per line gives the fastest update rate and the widest range of frequencies displayed with the lowest resolution. Using 10 RPM per line would give the slowest update rate and narrowest range of frequencies displayed but the greatest resolution.

    Note that once a spectrum has been captured, the width is fixed. You can't capture a spectrum with one width and later display it with another width.

  • The FFT Window button pops up a menu from which you can select the type of FFT window to use.The available window types are:

    None

    No window is used so the display will show artifacts due to the spectral leakage that occurs when a periodic waveform is processed by a discrete Fourier transform.

    Hann

    The Hann window reduces the spectral leakage and produces a display with fewer artifacts and better frequency resolution. This is the default window type.

    Flat Top

    The Flat Top window reduces the spectral leakage and produces the most accurate magnitudes.

    Note that once a spectrum has been captured, the window type is fixed. You can't capture a spectrum with one window type and later display it with another window type.

  • The Line Labels checkbox enables the display of line labels. Line labels are simply short labels that are displayed above the line they refer to. They allow the user to quickly identify the lines of particular interest. The labels are associated with a given spectral line by specifying their tacho RPM multiplier. A multiplier value of 1 will position the label above the line that is 1 × the current tacho RPM. A multiplier value of 2 will position the label above the line that is 2 × the current tacho RPM and so on. The multiplier does not have to be an integer value, so values like 2.5 are fine.

  • The Add button pops up a dialog that lets you input the line label text (ideally just a few characters, no spaces allowed), an optional description and the tacho RPM multiplier (a positive number).

  • The Edit button pops up a menu of existing line labels so you can choose the one you wish to edit. The edit dialog lets you alter the label, description and tacho RPM multiplier. If the label is cleared or the multiplier is cleared or set to zero, the label is deleted.



[3] The spectral lines are not actually shown in the display.

[4] Pinch-zooming is supported for touch screens and mouse-wheel zooming for laptops, etc.

[5] Blade pass frequency is the prop/rotor frequency × number of blades.

[6] The displayed vibration level originates from the current polar point and may differ from the magnitude of the spectral line(s) that are closest to the actual RPM.

Chapter 5. Data Tab

The data tab displays the current vibration data (RPM, IPS, DEG) in a text form. Here we see the data tab displaying data for the X and Y axes.

Figure 5.1. Data tab (with options visible)

Data tab (with options visible)

If the Capture Points and/or Capture Spectra checkboxes are enabled (see below), clicking within the box that surrounds the displayed values will initiate a capture. The box will flash green to indicate that the capture was successful. It will flash red if the PB-4 is not connected or neither of the capture checkboxes are enabled.

The Update checkbox controls whether the data display is automatically updated to show the latest data returned by the PB-4 when it is connected. If not checked, the last displayed data is preserved. So if you want to pause the display, un-check this checkbox.

The Data Options button toggles the display of the data options box (visible above), the options are:

  • Drop down menus, Axis 1 and Axis 2 let you choose the axes to display.

  • The Capture Points checkbox enables the capture of points when the data box is clicked. It also controls whether points will be captured when the PB-4's offline capture button is pressed.

  • The With Waveform Data checkbox enables the capturing of acceleration waveform data when a polar point is captured. The waveform data is not required for the balancing to be carried out.

  • The Capture Spectrum checkbox enables the capture of spectra when the data box is clicked. It also controls whether spectra will be captured when the PB-4's offline capture button is pressed.

  • The Enable PB-4 Offline Capture checkbox enables the PB-4's hardware capture button that can be used to initiate a capture when the user interface is not connected. This checkbox must be checked along with either/both of the Capture Points and Capture Spectra checkboxes for offline capture to occur.

Chapter 6. Options Page

The options page contains large buttons that, when clicked, expand the selected section to show the controls for that option. Most of the options are only available when the user interface is connected to the PB-4.

Figure 6.1. Options page (when connected to PB-4)

Options page (when connected to PB-4)

1. Change to another job

The section expands to show a list of jobs known to the PB-4, click on one to load it into the browser so it becomes the current job.

2. Create a new job

The section expands to show 3 controls:

  • A text input box into which you enter the name of the new job - it must not be the same as an existing job name.

  • A drop-down list of existing job names - if you select one, the new job will be a copy of the selected job. This provides an easy means to create a new job with a similar configuration to an existing job.

  • A Create Job button that creates the job and loads it into the browser.

3. Rename the current job

The section expands to show a text input box into which you enter the new name for the current job - it must not be the same as an existing job name. Press enter to action the rename.

4. Delete the current job

The section expands to show 3 buttons:

  • A Delete Job Completely button that deletes all the data and configuration information for the current job.

  • A Delete Only Points button that deletes all of the points for the current job.

  • A Delete Only Spectra button that deletes all of the spectra for the current job.

Pressing one of those buttons will ask you to confirm your action. Think twice here because the deleted data cannot be un-deleted. However, the data stored on the PB-4 can be exported (before you delete it, not after!) and subsequently imported again. So it is possible to back up your data to your computer.

5. Save / restore PB-4 data

[Note]Note

Due to limitations in iOS, it is not possible to save/restore data to/from an ipad or iphone.

The section expands to show controls to save and restore PB-4 data.

A drop-down list of known jobs is provided so you can select the jobs whose data is to be saved. Multiple jobs can be saved at the same time. When the jobs to be saved have been selected, press the Save Selected Jobs To File button.

Pressing the Select PB-4 Archive To Restore button will pop-up a file chooser that lets you select the PB-4 data archive that you wish to load onto the PB-4. Because all job names must be unique, if the archive file contains any jobs with the same name as existing jobs, the name of the new job will have a # character appended.

6. PB-4 configuration

This section expands to show several sub-sections:

6.1. Set access point details

For the PB-4 to be able to access an access point in STA mode, it needs to know the access point's network name (SSID) and password. If the access point supports Wi-Fi Protected Setup (WPS), these credentials can easily be obtained by following the procedure detailed in the PB-4 hardware manual.

If the access point doesn't support WPS or the procedure fails for some reason, you can manually enter the details in here in the two text input boxes labelled Access Point SSID and Access Point Password.

6.2. PB-4 hardware configuration

This sub-section contains controls for configuring various aspects of the PB-4 hardware.

6.2.1. Set the battery level to 100%

Normally, when the exhausted batteries are replaced with a fresh set, the PB-4 detects that they have been changed and resets the battery level to 100%. If it fails to detect the changed batteries, pressing the New Batteries Installed button will reset the battery level to 100%.

6.2.2. Selecting the tachometer input

The PB-4 supports two tachometer inputs (A and B), clicking the Select Tacho Input button lets you choose which tacho input will supply the tacho signal. Input A corresponds to the 3 pin socket tacho input and input B corresponds to the jack socket tacho input. Unless you have a second tacho connected through the jack socket this should always be set to A.

6.2.3. Accelerometer axis calibration

Clicking the Select Axis To Calibrate button pops up a menu of accelerometer axis names (X, Y, X2 and Y2). Select one of those names to start the accelerometer axis calibration procedure. As the accelerometers are sensitive to gravity, we can use the force of gravity (1 G) to calibrate the accelerometer using the following steps:

  1. Measure the output from the accelerometer with the axis being calibrated pointing straight upwards (+1 G applied).

  2. Invert the accelerometer so that the axis being calibrated is now pointing downwards and measure the output again (-1 G applied).

  3. Subtract the second value from the first and divide by 2 and that gives the accelerometer output for 1 G of acceleration which can be saved as the calibration value.

The user interface takes you through these 3 steps. At each step, a dialog box appears telling you what to do. It's very easy.

6.3. Get / set configuration values

Various aspects of the PB-4's behaviour can be configured via a set of configuration values.

  • To read the current value, enter the value's name in the Name text input box followed by the enter or accept key and the value will be fetched from the PB-4 and stored in the Value text input box.

  • To set a value, enter it's name in the Name text input box and the new value in the Value text input box.

See Appendix A, Configuration values for a list of configuration value names and their default values.

6.4. Firmware upload

[Note]Note

Due to limitations in iOS, it is not possible to load firmware files from an ipad or iphone.

This section expands to show two buttons:

  • Pressing the Select PB-4 CPU Firmware File To Upload pops up a file chooser that lets you select the file containing the new PB-4 CPU firmware you wish to install. Select the required file and the contents will be uploaded to the PB-4 and installed.

  • Pressing the Select PB-4 Wi-FI Firmware File To Upload pops up a file chooser that lets you select the file containing the new PB-4 Wi-Fi firmware you wish to install. Select the required file and the contents will be uploaded to the PB-4 and installed.

Please see the PB-4 hardware manual for more information regarding the uploading of firmware.

7. Upload UI files

[Note]Note

Due to limitations in iOS, it is not possible to load user interface files from an ipad or iphone.

The section expands to show two buttons and a checkbox:

  • Pressing the Select PB-4 UI Zip File To Upload pops up a file chooser that lets you select the zip file containing the new PB-4 UI files you wish to install. Select the required file and the contents will be uploaded to the PB-4 and installed.

  • Pressing the Select File To Upload button pops up a file chooser that lets you select a single file to be uploaded and stored on the PB-4 web server. Once uploaded, the file will be accessible through your browser.

    [Note]Note

    You can use this facility to update individual files such as this manual. For example, you may download the latest version of pb4_ui_manual.pdf from the Smart Avionics website and then use this button to install it onto the PB-4.

  • The Enable UI File Caching checkbox enables the caching of the UI files in the browser's local storage. By storing those files locally in the browser, it greatly speeds up the loading of the UI web page.

8. Set PB-4 IP address

This section expands to show a text input box into which you can enter the IP address (network address) that will be used to connect the browser to the PB-4 when you click on the connect button. You do not normally need to do this because the browser knows the PB-4's IP address because that's where the user interface web files (HTML, etc.) you are currently viewing were loaded from. The only time you would enter an IP address here is when you are loading the user interface web pages from a different server than the PB-4.

Chapter 7. Propeller Balancing

1. Minimising Other Sources of Vibration

Propeller mass imbalance can be a major source of vibration. However, there are other sources of vibration as well. To minimise the overall vibration level and to make the dynamic balancing process more effective, all other sources of vibration must be minimised before dynamic balancing is carried out.

[Important]Important

Unless the engine is running smoothly, there is little point in trying to balance the propeller. Carburettor imbalance, dirty plugs, loose engine mounts and general wear and tear are just some of the reasons why the engine could be producing excess vibration.

Propellers with an adjustable blade pitch will produce a lot of vibration if all the blades are not set to the same pitch. This is critical: if a blade's pitch differs from its neighbours' by even a fraction of a degree, it will produce vibration that appears to be caused by mass imbalance but cannot actually be removed by mass balancing.

[Important]Important

Before attempting to dynamically balance a variable pitch propeller (either ground adjustable or in-flight adjustable), confirm that the blades' pitch are equal to within the tolerance specified by the propeller's manufacturer (typically, 0.25°).

2. Propeller mass imbalance

A major source of propeller vibration is propeller mass imbalance. When an object rotates around an axis, if the mass of the object is not uniformly distributed around that axis, a force (the centripetal force) will be generated and will cause vibration. As the magnitude of the force is proportional to the square of the rotational velocity, at high RPMs (high rotational velocity) even a small mass imbalance in a propeller will generate an appreciable amount of force (and hence vibration). This vibration can be measured by mounting a sensor on the engine as close to the propeller as possible. Conventionally, the magnitude of a propeller's vibration is reported as a peak velocity in units of Inches Per Second (IPS).

3. Static Propeller Balancing

A propeller can be statically balanced in the workshop using a static balancing tool. This often involves suspending the propeller from its central axis. If the propeller is statically balanced, the blades should be level[7]. If one blade is heavier than the others (or its centre of mass is further from the centre of the propeller), it will dip towards the floor. If this occurs, weight can be added to the hub on the opposite side of the central axis to the dipping blade to bring the propeller level.

All propellers should be manufactured with blades that have equal mass (and mass distribution) and so a new propeller should not require static balancing. Propellers that have suffered damage to the blades (stone chips or tip abrasion) may well benefit from being statically balanced.

While statically balancing a propeller is worthwhile, the best results will be obtained if the propeller is dynamically balanced together with the spinner.

4. Dynamic Propeller Balancing

Dynamic propeller balancing involves measuring the actual rotational vibration generated at a realistic propeller RPM and then adding weights to the propeller hub or spinner backplate to minimise the measured vibration level. Because the balancing operation is carried out with the propeller and spinner attached to the engine, the best possible solution is obtained.

The vibration is measured using a sensor known as an accelerometer. The accelerometer is securely attached to the engine as close to the propeller as possible and it measures the acceleration of the front of the engine in one direction (normal to the propeller shaft). If the propeller is out of balance, as the centre of mass rotates around the axis of rotation, the resulting centripetal force tries to pull the propeller (along with the spinner and engine) towards the centre of mass. This rotating imbalance force acts on the mass of the engine/propeller combination and accelerates it. It is this acceleration that is measured by the accelerometer.

Figure 7.1. Sine wave

Sine wave

If the accelerometer was very selective and measured only the vibration caused by the rotating out-of-balance propeller, the signal it produced for one rotation of the propeller would look like a sine wave as shown here. In reality, the measured acceleration waveform is much more complex than a simple sine wave. This is mainly because of the vibration generated by the engine and also the turbulence generated by the rotating propeller blades. The dynamics of the engine mountings also affect the waveform.

The balancer's processing unit digitises the measured acceleration waveform and uses the resulting numbers to calculate the magnitude of the vibration signal. This magnitude is reported as a peak velocity in units of Inches Per Second (IPS).

The accelerometer senses the magnitude of the vibration but more information is required to carry out the balancing process. This is because it is not sufficient to know just the magnitude of the vibration signal. It is also necessary to know the phase of that signal. The phase of the signal is the relationship of the signal waveform to the angular position of the propeller. Given the phase information, it is possible to determine where the weight is required to be added to reduce the vibration. By detecting when one particular propeller blade passes an optical sensor, the balancer can measure and report the phase of the vibration waveform. The optical sensor also works as a tachometer to measure the propeller RPM.

5. Taking a Vibration Reading

[Warning]Warning

Propellers can kill.

Make sure that the engine ignition is switched off before touching the propeller.

Always assume that the engine could fire when the propeller is being moved.

Make sure that the aircraft is securely chocked or tied down while carrying out the balancing process.

Please see the PB-4 Hardware Manual for a description of the PB-3 sensors and how they are attached to the aircraft.

For maximum accuracy, the dynamic balancing process should only be carried out in light winds. Ideally, the wind should be less than 5 kts. The aircraft should be positioned so that it is pointing into any wind.

[Important]Important

Before you start the engine, check that the Polar Options are all set appropriately.

Make sure the engine is thoroughly warm before taking any readings.

Firstly, you need to decide what propeller RPM you are going to use while balancing. For a typical propeller, an RPM in the range 1500-2000 is often a good choice. Using the polar tab, observe the movement of the current polar point circle. If you think it is necessary, adjust the throttle to find the RPM that minimises the variation in the current point's position. When you adjust the throttle it will take a few seconds for the point to settle down enough to allow you to capture a reading.

[Important]Important

Once you have determined the best RPM to use, it is important to use the same RPM for each balancing run so that you get consistent results.

With a steady RPM being reported by the balancer, press the Capture button to capture a polar point. The point's data will be entered into the table of polar points for the current job.

Capture a few points – if the propeller is obviously out of balance, the points will be clustered together some distance away from the centre of the chart.

Now stop the engine and double-check that the ignition is switched off.

If the vibration level is already 0.15 IPS or less, the propeller can be considered reasonably well balanced – in ideal conditions, the balancer is capable of balancing a propeller down to about 0.03 IPS so you may wish to continue the process to achieve a better result. If you want to improve the balance, you must carry out the balancing procedure as described in the next section.

[Tip]Tip

To reduce battery drain in the PB-4 and the tablet, it is best to disconnect the user interface from the PB-4 when you are not actually capturing points or viewing the live data. When you are ready to capture a new point, just reconnect.

6. Balancing Procedure

[Important]Important

It is the operator's responsibility to ensure that any procedures or guidelines that have been issued by the manufacturers of the propeller, engine or aircraft or some other agency (e.g. the FAA/CAA/LAA/BMAA), that specify how the propeller is to be balanced, are adhered to.

The following instructions describe the balancing process from the point of view of operating the balancer and determining where balance weights are to be attached. The exact detail of how balance weights are attached to the hub or spinner backplate is beyond the scope of this document. If you have any doubt, please consult your inspector/engineer.

[Note]Note

Weight sites are numbered from 1 to the number of sites. Considering the direction of normal propeller rotation, site 1 will pass a fixed point (e.g. the tacho) before site number 2, and so on.

The position and the mass of the required balance weight(s) is determined as follows:

  1. With no balance weights attached to the propeller[8], capture one or more points. Choose one of those points to be the start point by displaying the point details dialog (click on the point's entry in the point list to highlight it and then click again to open the dialog). Click on the Use As Start Point button.

  2. Now attach a trial weight. The angular position of the weight is not important at this stage so just attach it at site 1. For a typical composite propeller, a few AN970 washers would be a reasonable initial trial weight. So, having checked the ignition is switched off, securely attach the trial weight using an approved method.

  3. With the trial weight in place, capture a few more points. If the propeller is still well out of balance, all of the points should be located close together on the chart. If the propeller is only slightly out of balance or there is an appreciable amount of vibration from other sources, the locations will tend to differ.

    To improve the balance, select one of the latest points and display its details. Now you must enter the amount of weight that was present when you captured that point and the position (site number) of where it was located. The amount value can be specified in any units (grams, ounces, washers, etc.) To begin with, you only have weight attached at a single location (the trial weight) so in one row enter the amount (e.g. 3 for 3 washers) and the location (e.g. 1). An amount of zero means that no weight was attached.

    In general, you will end up needing to distribute the weight between two sites and so the dialog box shows two rows, each row being used to enter the details of a single weight. Once the details of the attached weight(s) have been entered, the dialog will tell you how those weights should be altered (amount and location) to improve the balance.

    Follow the suggested adjustments to the position and amount of weight accordingly and capture some more points to see what difference that has made.

    Depending on the quality of the captured points and how closely you follow the instructions, you may well have to repeat this step a few times to get the best result.

    [Note]Note

    The PB-4 does not actually say that the propeller is balanced or not; it's left up to the operator to decide when to stop the process based on the IPS reduction achieved.

7. After Balancing

When the balancing has been completed, double-check that all balance weights are securely attached. If you have been using temporary weights to carry out the balancing, they should be replaced with permanent weights whose mass and position are such that they have the same effect as the temporary weights. If in doubt, recheck the balance once the permanent weights are installed.

Remove the sensors and the tape from the propeller blade. Make an entry in the appropriate log book to record the vibration level achieved and the RPM used.

8. Troubleshooting

This section provides answers to common problems that can arise when balancing.

8.1. The displayed RPM is erratic or wrong

The RPM could be unsteady for the following reasons (most likely first):

  • The optical sensor is not pointing at the reflective tape or the tape is not parallel with the sensor's block. It may help to add one or more further strips of reflective tape to increase the width of the reflective area.

  • The optical sensor is either too close or too far away from the propeller.

  • The angle of incidence of the light beam on the propeller blade is outside the acceptable range.

  • The sensor is being confused by extra reflections from shiny propeller blades. Rotate the sensor to increase the angle of incidence of the light beam on the propeller blade. You can also try adding non-reflective tape to the other blades in the same position as the reflective tape.

  • The engine RPM really is changing!

8.2. The current polar point position varies widely

This indicates that the vibration waveform is not consistent from one propeller revolution to the next. This implies that some (perhaps most) of the measured vibration is out of phase with the propeller. Any of the following problems will cause the position of the polar point to vary:

  • The incorrect PSRU ratio is being used.

  • You are using the wrong accelerometer axis.

  • The RPM is not being reliably detected. Unless the RPM is correct, the IPS and DEG and their associated standard deviations are meaningless.

  • The wind is gusting. At low vibration levels, just a few knots of wind can make it appreciably harder to obtain reliable data.

  • The engine is not running smoothly or the engine mounts are in poor condition.

  • You are using an RPM that is triggering an airframe resonance.

  • The accelerometer is not securely mounted or the sensing axis is not pointing at the propeller's axis of rotation or the sensing axis is parallel with the direction of movement of the engine's pistons.

  • The spinner is wobbling.

  • The engine cowling is not fitted to minimise the effects of turbulence.

8.3. Adding weight does not reduce the vibration level

If the balancer shows a significant level of vibration with a small standard deviation and adding weight doesn't appear to reduce the level or substantially change the angle, any of the following could be true:

  • The incorrect PSRU ratio is being used.

  • You are using the wrong accelerometer axis.

  • The propeller is suffering from pitch imbalance (the blades don't have equal pitch). Pitch imbalance has to be corrected before the propeller can be dynamically balanced.

  • The amount of weight being added is too small to have an effect. The heavier the propeller/spinner, the more weight has to be added to reduce a given level of vibration.

  • The weight is not being added at the correct position.



[7] The propeller will be statically balanced when the mass of each blade multiplied by the distance from the blade's centre of mass to the centre of the propeller is the same for all the blades.

[8] Apart from any weights that were added during static balancing – don't remove those.

Chapter 8. Rotor Balancing

This chapter aims to provide an overview of the rotor balancing process. Although the emphasis is on gyroplane rotor balancing, it is also largely applicable to helicopter rotors.

Please see the PB-4 Hardware Manual for details of how the sensors are attached to the aircraft.

1. Causes Of Rotor Vibration

Mass imbalance is one of the causes of rotor vibration and the PB-4 system can help you balance the rotor to eliminate that vibration. Other vibrations may also be present. In particular, gyroplanes often suffer strong vibration at twice the rotor frequency. This × 2 (times 2) vibration is often much larger than the vibration due to imbalance and so even when the rotor has been mass balanced satisfactorily, vibration will still be noticeable to the occupants.

Figure 8.1. Rotor spectrum with large × 2 peak

Rotor spectrum with large × 2 peak

The above spectrum is from a gyroplane rotor that shows the very large × 2 vibration still present after the rotor was balanced. You can see that the vibration at the rotor RPM (367) was only 0.08 IPS but the × 2 vibration at 750 RPM was 3.98 IPS, that's nearly 50 times larger!

Unlike, rotor mass imbalance, the × 2 vibration cannot be suppressed by simply adding weights. It is fundamental to the geometry of a (2 bladed) rotor system and the physical properties of its components (blades, hub, mast, etc.) The × 2 vibration can be caused by a variety of reasons. Some possibilities are:

  • The rotor teeter height is wrong.

  • The rotor blades are exhibiting in-plane resonance which is excited twice per revolution when the blades are perpendicular to the machine's direction of flight.

  • When the blades are perpendicular to the direction of flight more drag is produced.

Furthermore, the amount of × 2 vibration being produced is influenced by the flight conditions (rotor speed, airspeed, etc.) At this time, the PB-4 system can only report the magnitude of the × 2 vibration and it does not offer any suggestions as to how the level of vibration can be reduced. Perhaps in the future when the causes of the × 2 vibration are better understood, it will be able to make helpful suggestions.

2. Rotor Mass Balancing

When a propeller is balanced, it is normally the case that the balance weight can be added almost anywhere on spinner backplate and so both the angular position and amount can be varied as required. By contrast, gyroplane rotors are mass balanced by making adjustments in one or two fixed axes[9]:

Spanwise

The spanwise balance of a gyroplane rotor can be adjusted by adding weight to one rotor blade tip or the other.

Chordwise

The chordwise balance of a gyroplane rotor is most often adjusted by shifting the mass of the rotor blades along the teeter pivot. Exactly how this is done depends on the mechanical design of the particular rotor system but it could involve changing shims or adjusting a threaded component (nut, barrel, etc.) The amount of movement required is often small (a fraction of a mm).

What you need to know to balance the rotor is how much adjustment (magnitude and sense) is required for each of these axes. You can determine that information using a polar chart that has been augmented with move lines. A move line is literally a line drawn on a polar chart that shows the direction a polar point would move when a particular adjustment is made to the system. For a gyroplane rotor, two move lines are required, one for the spanwise balance and another for the chordwise balance.

[Important]Important

Before you start capturing any data, please ensure that the current job has the correct Polar Options.

It is strongly recommended that you use the PB-4's offline capture button to capture the data while flying as this obviates the need to interact with the tablet while flying, it doesn't even need the tablet to be in the aircraft.

If you do wish to use the tablet to capture the points, consider using the data tab as that just requires you to click on the data table box so it's easier to do while flying. Also, if the machine has an open cockpit, take precautions to avoid the tablet disappearing aft through the propeller! If possible, attach it firmly to a knee pad or to the aircraft structure.

2.1. Defining move lines

It is straightforward to define a move line. All you have to do is capture two or more polar points that correspond to known conditions and enter what those conditions were.

For example, to define the spanwise move line you would add weight to one of the rotor blade tips, capture a few polar points, move the weight to the other rotor blade tip, capture a few more points, remove the weight again (to return the rotor to the original condition) and, finally, define the move line.

The process can be repeated to define the chordwise move line. This time the adjustments would involve shifting the rotor mass on the teeter pivot. Once both move lines are defined, you can balance the rotor.

[Important]Important

To ensure that the move lines are good, follow this advice:

  • Only make one adjustment at a time.

  • Always fly the aircraft at the same speed, weight and rotor RPM when capturing the data.

  • Fly smoothly and avoid turbulence.

  • Capture multiple points (3, say) to avoid being fooled by a rogue point.

  • Make the adjustment large enough to produce a noticeable effect. If you are unsure how big an adjustment to make, start small and keep doubling the adjustment until it produces at least 0.5 IPS (preferably 1 IPS) of movement on the polar chart compared to the original condition.

[Note]Note

Move lines are only valid for a given flight condition (airspeed, AUW, rotor RPM, trim position, etc.) and sensor configuration. If you move the sensors or, say, invert an accelerometer axis, the move lines will no longer valid.

2.2. Using the move lines to balance

With the move lines defined, fly again in the original configuration (no weights added or rotor shifting) and capture a few polar points. If you now select one of those points in the point list the point's details dialog will show the suggested adjustments required to improve the balance. Carry out the suggested adjustment and capture some more points. Repeat as required.

Although multiple adjustments may be suggested, it is recommended that you only make one adjustment at a time and then capture further polar points for comparison. It is also recommended that you make use of the feature that allows you to add a note (some text) to a point so that you can track the adjustments you make.



[9] Don't confuse these axes with the accelerometer axes, they are different.

Appendix A. Configuration values

Here are the names and default values for the PB-4 configuration values. Unless advised to by Smart Avionics, do not alter any of these values.

NameDefault ValueNotes
accel-adc-counts-per-g-0382 
accel-adc-counts-per-g-1382 
accel-adc-counts-per-g-2382 
accel-adc-counts-per-g-3382 
balancer-cycles255 
balancer-min-cycles4 
balancer-max-points-to-average5 
balancer-polar-mode0 
fft-window-type1Hann
fft-rpm-per-line50 
idle-timeout30seconds
mcu-flash-crc  
tacho-channel0 
battery-mah1900 
capture-points-file  
capture-spectra-file  
capture-button-mask  
wifi-chan1 
wifi-ap-ssid  
wifi-ap-password  
wifi-enable-mdns1 
wifi-last-sta-ip-addr  
wifi-mac-addr  
wifi-fw-version  
wifi-ssid  
wifi-udp-broadcast-period1000mS
wifi-udp-broadcast-port55555 

Index

B

balance
solution, Polar details dialog
balancing
procedure, Balancing Procedure
batteries
low warning, Connect button
new, Set the battery level to 100%
blade pass frequency, Spectrum Tab

C

capture
button, Capture button, Capture button, Capture button
polar point, Taking a Vibration Reading
capture points
checkbox, Data Tab
capture spectrum
checkbox, Data Tab
centripetal force, Propeller mass imbalance, Dynamic Propeller Balancing
configuration values
accessing, Get / set configuration values
connect
button, Connect button
current point, Polar chart, Polar chart

D

data options, Data Tab
data tab, Data Tab

E

edit move line
button, Polar Options button
dialog, Edit move line dialog
enable PB-4 offline capture
checkbox, Data Tab

F

FFT
window, Spectrum Options button
firmware
upload, Firmware upload
fullscreen mode, Menu button

H

help page, Menu button

I

Inches Per Second, Propeller mass imbalance
IP address
set, Set PB-4 IP address

J

job, Jobs
change to another, Change to another job
create new, Create a new job
default, Jobs
delete, Delete the current job
rename, Rename the current job

O

options page, Options Page

R

range
spectrum display, Spectrum Options button
responsive
user interface, Initial screen
RPM
choice, Taking a Vibration Reading
erratic, The displayed RPM is erratic or wrong
per spectral line, Spectrum Options button