Introduction: Ducted Fan Pinewood Derby Car
When my place of work (Adobe) announced a Pinewood Derby competition I decided to participate in the outlaw/unlimited category. A good number of years ago I made a car using a propeller & electric motor re-purposed from an RC glider, and this time around I decided to do something similar but pull in some experience I've gained working with quad-copters and embedded systems. While not a full blow-by-blow of the entire build process, I wanted to show some of the steps that go into building a electric ducted fan (EDF) powered pinewood derby car. The basic components of this build included:
- Stock BSA Pinewood Derby Block & Wheels
- 4" x 4" x 12" block of balsa
- Fiberglass cloth
- Dr. Mad 40A ESC
- Dr. Mad 50mm EDF (650g thrust)
- 1300 3 Cell mAh Lithium Polymer Battery
- Arduino Pro Micro 5V
- 2x LEDs
- 2x 220 Ohm Resistors
- 1x Console-mount Button
- 1x "Ping" Parralax Ultrasonic Rangefinder
Misc: Solder, wood putty, shrink wrap tubing, epoxy resin & hardener, spray paint, superglue
Tools: Lathe, drill press, soldering iron, dremel rotary tool,
A lot of this I had on hand, but starting out fresh you'd probably be looking at around $120-150 in equipment. If you have the time & patience to ship from China, you can get most of this at hobbyking.com.
Step 1: The Nacelle
The nacelle is the part of the engine that the air goes around and through. The shape of the nacelle matters pretty significantly when designing an airplane, but in my case I was mostly just going for the look. I failed to get pictures of the block of balsa before being shaped (sorry). Fortunately I had access to a small lathe which made the process pretty quick. I started by finding the profile of a jet engine, and printing it which I used as a guideline. I put the block on the lathe using a 4-screw chuck and using a curved chisel to get a rough-round shape with about a 2 1/2" diameter. Considering how soft balsa is this happened really, really fast. I then used some medium-grain sandpaper to bring it down to the appropriate, jet-engine shape.
I needed the interior diameter to match the shroud (cylinder) as closely as possible, and 2" circular saw turns out to be almost exactly the 52mm that I needed to fit it in. Since The end of the nacelle tapered down to less than 2" and I didn't want to destroy the shape by using the same size hole the entire way through, I cut the nacelle in half. I put the 2" hole through the front where the fan was to be mounted and a smaller 1 1/2" hole through the back, hand-sanding them to taper in the middle. As I was drilling through the balsa the spring-loaded clamp I was using to hold it in place actually split the then-hollowed nacelle into 2 half-pipe looking pieces, which the drill press sent flying. A bit of superglue however made the pieces stick nicely back together.
After re-attaching the front and back of the nacelle and painting it, I realized it was going to need more strength to withstand the vibration of the EDF and any smashing it might do. Off came the paint, and on went a layer of fiberglass, and paint back on top of the fiberglass. After this was finished it took a bit of sanding to remove some epoxy around the mouth of the nacelle, but the EDF slipped in perfectly. I drilled a hole in the bottom to pull the ESC connection wires through. The friction alone would probably have been sufficient to keep the EDF from shooting out, but I put a few dabs of superglue on it as it slid in to reduce any chance.
Step 2: Body & Electronics Compartment
Between the bulk of the battery and the ESC alone I knew I was going to need more space. I epoxied on a few 3"x1"x1" blocks of balsa onto the sides of the block between the axle lines and sanded them to a contour. I then used a drill press and Dremel tool (I really, really need to get a router) to create a fair bit of open space in the front and bottom from the top, and through the middle on the bottom. The front I completely took out and epoxied the ultrasonic rangefinder to the front, and added another block in front of it. The board of the rangefinder was just wider than the block, so as I was sanding it down I actually ended up taking off a bit of the edges, exposing some of the traces. Fortunately I didn't break any of them. I knew a chunk of the top was going to be exposed to view, but needed space to run wires to the EDF and the microcontroller which would be in the back of the car, so I completely removed a strip of the top of the block.
I also drilled a hole in the back for the "Arm" button and brake light LEDs.
Step 3: Embedding the ESC
The speed controller was going to be completely embedded in the front of the car. I slid it in from the bottom, and made sure that there was sufficient room for the battery to be up there alongside it. After trimming & pulling the wires to the right spot I used some JB Weld Wood Putty and covered it up. Once the wood putty was cured, I sanded it down to be flush with the block.
*Note: ESCs can get pretty warm if run for an extended amount of time. Since most runs for this car are only going to be 3-5 seconds, I could get away with the lack of heat dissipation.
Step 4: Bench Testing & Installing the Arduino
I had tested most of the components individually, but to get an idea of how the whole thing was going to work I did some bench testing. Using a prototyping breadboard I laid out how everything was going to connect together. Without getting into too much detail, the brakelight LEDs are each on their own timer which when disarmed flash back and forth. Prior to starting this, you need to use an ESC calibrate routine to set the maximum and minimum pulse on the throttle. When not armed the arduino keeps the ESC throttle cut. Upon being pushed for 3 seconds, the "ARM" button in the back the arduino starts polling the rangefinder. If the distance read is infinite (0), it goes full speed ahead. If the distance is non-0, it scales back depending on how much room there is. If the distance is read is less than 25cm, it cuts the motor completely. If while armed the "ARM" button is pushed for any length of time the motor is cut.
Please feel free to contact me if you would like the code for this.
To allow for modification of the arduino code after being built, I drilled an additional hole to permit access to the micro USB port. After soldering in the rangefinder, LED, ESC, and button connections, I slide all the pieces into place. I then wood puttied it up and sanded it down, same as the ESC in the front.
Step 5: Finishing It Up
The last bit was just connecting the ESC in the body to the EDF in the nacelle, epoxying it in place, and using (yet more) wood putty to smooth it together. I'm not entirely happy with the paint job, the body could have used a good amount of more sanding & filling but eventually you just have to decide that a project has taken enough time.
Threw some wheels on it and it was good to go!
Step 6: The Race
Although the propulsion provided by the EDF disqualified the car from even the "Limited Unlimited" class, the trackmaster allowed the car on the track. A number of very well built cars were getting 3.0-3.1 seconds on the track, the EDF was pushing the car down in 2.1 seconds, without having spent hardly any time on the wheels or alignment at all. It's amazing what just an extra 600g of thrust will do for you!
Thanks to Adobe for organizing such an awesome event!
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