Building a Cooling iPad Mini Kneeboard

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Building a Cooling iPad Mini Kneeboard

By Tom Nery, EAA Lifetime 1022753

Throughout the evolution of aviation, technical advances in non-related fields have provided direct benefit to emerging aircraft and the economy of flight, frequently with unforeseen side effects. Tablet computers are an excellent example of both the benefit and impact. No longer do we have to drive to our local pilot shop every 28 days to update our charts: instead they are delivered and installed automatically to our devices. One of the side effects, besides the disappearance of the local pilot shop, is our reliance on the device to operate reliably in flight. I experienced this on my 2019 trip to AirVenture Oshkosh when my tablet overheated and shut down. Luckily, the shutdown occurred en route and not on an approach, but being a belt and suspenders kind of pilot, I was pleased to have a second tablet in my flight bag.

Christmas 2019, I received (actually, I bought myself, as I find it the best way to get what I want) a 3D printer. The 3D printer is another technology I expect, that as it matures, will continue to positively impact experimental and even certified aviation. Initially, I used it to print pre-designed models found on websites such as Thingiverse. I even found there are aviation related models available, but many require tweaking to meet my specific needs.

Thanks to EAA and our SOLIDWORKS benefit, I had the tools to modify the designs to meet those needs. Using a flame retardant Polycarbonate filament, I modified a Zulu controller mount, air vents and even created a tie-down action camera mount from scratch.

This article represents my most complex design to date — a force air-cooled kneeboard for the iPad mini. It is my hope that the force air cooling will help prevent future shutdowns due to temperature, but it will need a full summer to test before declaring success.

The design goals for this kneeboard were:

  1. Forced air cooling to the back of the iPad mini
  2. Charge the iPad mini while powering the fans
  3. Minimize the size of the kneeboard
  4. Comfortable and secure to wear
  5. Economical to build

This product meets all of the design goals.

  1. Cooling is provided by two 40-mm square, 10-mm high axial fans with cooling air ducted to improve efficiency. Each fan moves 2.7 cubic feet of air per minute and is low noise.
  2. Power for the fans and iPad charging are satisfied with a single USB connection.
  3. Kneeboard measures 8.25 inches x 5.6 inches x 0.8 inches. This is only 0.25 inches wider and longer than an iPad mini and about 0.5 inches higher.
  4. Comfort and security is provided by a two-inch flexible strap.
  5. Entire device can be built for less than $30.00.

SOLIDWORKS Design

The kneeboard was designed utilizing the educational version of SOLIDWORKS, available to each of us as members of EAA. While not a proficient SOLIDWORKS user, the online tutorials and YouTube videos provided enough knowledge to complete this complex design.

Figure 1: SOLIDWORKS Design Assembly

The project consists of four unique parts: Kneepad top, kneepad bottom, extractor, and strap holder. Once the design is completed, each part is saved as an .STL file. An STL file is a stereolithography computer-aided design file that will be used to generate a 3D print.

Introduction to 3D printing

3D printing is an additive process where very thin layers of plastic are printed on top of each other to make three dimensional prints. For this project, each layer is 0.16 mm (0.0063 inch) thick, which the highest quality output for my hobbyist 3D printer.

Figure 2: 3D Printer

To print the part, saved as an STL file, it must be sliced into 0.16 mm layers. This is accomplished by software which is normally included with the 3D printer or downloadable for free. Besides slicing the three-dimensional object, the slicing software also configures the 3D printer for the type of plastic being printed as well as user print preferences and printer-specific commands.

3D Printing Materials

There are a variety of materials available to consumer-grade 3D printers. The materials are sold in the form of a 1.75 mm filament that is wrapped on a hub, as shown at the top of Figure 2. The two types of materials considered for this project are:

PLA: One of the most-used desktop printing filament. PLA is the easiest plastic to print, requires no special hardware and produces high quality prints. It is not flexible and will shatter if hit with a hammer. PLA is also flammable. Another downside of PLA is that it deforms at a low temperature, with a glass transition temperature of about 125°F. PLA, therefore, could deform if left in direct sunlight.

PETG: Also a common desktop printing filament, PETG is likewise easy to print. Its layers bond very strongly together which sometimes makes PETG hard to remove from the printer bed, not to mention removal of support structures. PETG prints tend to have slightly more surface defects that PLA. The upside of PETG is that it has a higher glass transition temperature, 165°F. The higher temperature may make it more suitable for printing items left in sunlight. PETG is less flammable and depending on brand will not sustain fire.

The kneeboard shown was printed with the following print preferences:

  • Material: PLA
  • Layer height: 0.16 mm
  • Top Solid Layers: 3
  • Bottom Solid Layers: 3
  • Outline/Wall Layers: 2
  • Infill: 35%
  • Generate Support Material: 30%

Although PETG may be a better choice, I opted to try PLA, remembering not to leave it in direct sunlight when I leave the cockpit.

To further understand how these preferences affect the print, refer to Figure 2, Layer 52 of Slicer Output. Three layers will be printed for tops and bottoms of the printed piece. Two lines will be printed for each inside and outside wall. Any area between the walls will be filled using an infill of 35 percent. Lastly, support material will be generated wherever printing needs to be done and if there is no printed material below.

Figure 3: Layer 52 of Slicer Output

Building the Kneeboard

To print this kneeboard, you will need a 3D printer with at least a 250 mm x 200 mm print bed.

Parts List:

Figure 4: USB Cable Wiring

Step 1: USB Charging Cable, Fan Wiring

As shown in Figure 4, the USB cable will be modified to power the fans, in addition to providing charging power to the iPad. To create the cable:

  1. Cut the USB cable to a length of 9.5 inches from the lighting connector.
  2. Remove 3 inches of sheath from both sides of the cut cable and secure the cloth wrap with electrical tape. This should leave 3 inches of the four conductors available for connection.
  3. Strip away 1/4 inch of insulation from each of the wires.
  4. Using a voltmeter, plug the USB connector into a USB port and verify the 5 volt and ground lines (in the case of my cable, the black wire was ground and the red wire was 5 volts).
  5. Place the heat shrink tubing on the wires that are neither 5 volts or ground (white and grey in Figure 4).
  6. Re-join with solder the same color to its counterpart on cut cables.
  7. Move the heat shrink tubing over the soldered connection and apply heat to shrink.
  8. Cut one fan cable to a length of 5 inches.
  9. Cut one fan cable to a length of 2 inches.
  10. Split both fan cables, separating the red from the black for a length of 3/4 inch.
  11. On the 6-inch cable that was removed in step 9, split the red and black wires for a length of 1.5 inches.
  12. Strip each fan wire 1/4 inch.
  13. Slip heat shrink tubing over each of the 2 wires on the 6-inch cable.
  14. Join the 6-inch cable and two fan cable red wires together with solder.
  15. Join the 6-inch cable and two fan cable black wires together with solder.
  16. Move the heat shrink tubing over the soldered connections and apply heat to shrink.
  17. Cut the connector from the fan assembly, just created.
  18. Split both fan cables, separating the Red from the black for a length of 3/4-inch.
  19. Strip each fan wire 1/4-inch.
  20. Place heat-shrink tubing on each of the 5 volt and ground USB wires
  21. Connect using solder the black fan wire to the two USB ground wires identified in step 4, making sure to join both cut cables.
  22. Move the heat shrink tubing over the soldered connections and apply heat to shrink.
  23. Connect the red fan wire to the two 5-volt USB wires identified in step 4, making sure to join both cut cables.
  24. Move the heat shrink tubing over the soldered connections and apply heat to shrink.
  25. Verify that each of the USB cable wires are connected to the same color wire as on the lightning connector side of the cable (red to red, black to black, etc.). Failure to ensure this connection could damage the iPad.
  26. Verify that the fans are correctly cabled by plugging the USB connector into a charging socket and confirming that both fans are powered.
  27. Once the fan power is verified, insert the lighting connector into the iPad and verify that the iPad is charging.

Step 2: Install fans and wiring into the kneeboard

Install the fans, routing the wires as shown in Figure 5. M2 self-tapping screws should be used to attach the fans to their mounts.

Figure 5: Fan Installation

Step 3: Attach Kneeboard Strap and Strap Holders to the Bottom

Position the Velcro strap, with hooks facing the kneeboard bottom and the plastic buckle arranged such that it covers the fan grates. Attach the two strap holders to the kneeboard bottom utilizing M3 screws and nuts. Adjust the strap so that the end of the plastic buckle is 3 inches from its closest strap holder and tighten in place as shown in Figure 6.

Figure 6: Strap Attachment

Make sure that the ejector can slide into the storage slot on the side of the top fully and is not blocked by the strap screws.

Step 4: Attach Kneeboard Bottom to Kneeboard Top

Slide the iPad mini into the slot on the kneeboard top and rotate the lightning connector so that it plugs into the iPad. Invert the kneeboard top and ensure that the customized fan cable is inserted into the slots provided, all connecting wiring is in the housing area. Once done, use six M2 self-tapping screws to attach the kneeboard bottom to kneeboard top.

Conclusion

Figure 7: Final Product

Once the steps are completed, you should end up with a kneeboard that will safely secure the iPad onto your leg. Should you need to remove the iPad, detach the ejector from the storage slot on the side and use it to push out the iPad by inserting it into the slot on the lightning connector side of the kneeboard. If instead of a kneeboard mount you wish to secure the tablet to the yoke, leave the strap holders off and make certain you have a yoke mount that can handle the 7/8-inch depth of the kneeboard.

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