(The eighth in a series of articles on cars that “stretch the rules”)
Several years ago Maximum Velocity introduced a “Propeller Car Kit”, which is a car with propulsion assist from a ducted fan.
Propeller Car Kit from Maximum Velocity
The kit is typically used as a “crowd pleaser”, but there is an occasional race where propulsion (other than gravity) is allowed.
Last year we introduced an upgraded ducted fan, which provided the kit with additional speed. But can the Propeller Car go faster? Certainly! But to understand why, let me share a little technical information.
First, a ducted fan requires a specific voltage and amperage to run at its top RPM (revolutions per minute). The fan is typically used on RC planes, supplied with power by R/C-type, rechargeable batteries, typically lithium polymer (LiPo) batteries. LiPo batteries are very light weight, and readily supply the required voltage and amperage to run the fan at its top RPM.
However, LiPo batteries are somewhat expensive, require special charging equipment, and can explode if improperly used. Alternate types of RC batteries have considerable weight, which doesn’t work well with a propulsion-assisted car.
So, to stay on the safe (and less expensive) side, we use a standard, 9 Volt alkaline battery with the Propeller Car. This battery will supply the required voltage, but will not supply enough amperage to run the fan at top RPM.(1)
A NOVEL SOLUTION
So, how can we get more amperage without resorting to LiPo batteries? That question was answered by Rod Shampine. He came up with a novel way to get more amperage to the fan while still using an alkaline battery. His solution was to use a pair of capacitors with a high amperage rating.(2) The capacitors store up power from the battery, and then at the flip of a switch, release the stored power quickly to the fan. So the fan has ample voltage and amperage to run at top RPM for a brief period of time (until the capacitors are discharged).
Rod Shampine’s Car
Rod left the battery off the car, made it very light weight, extended the length, and used needle axles. His car was very fast, but he found out quickly that propulsion-assisted cars need some structure and solid axles, as his car took a beating (note the bent front axles). As you can see in the photo, to charge the capacitors between runs, Rod applied a battery using wire clips.
To simplify matters, I decided to keep the battery on the car and modify the Propeller Car Kit to accept a pair of 5 volt capacitors. The capacitors were acquired from Mouser Electronics (part #504- PM-5R0V305-R) for just over $14 each.
I decided to eliminate the toggle (kill) switch) and make full use of the front contact switch. In the kit, the front contact switch is a SPDT (Single Pole, Double Throw – basically two switches in one), but only half of switch is used. In the capacitor version of the kit, when the switch is not in contact with the track’s starting pin, the capacitors discharge to the fan and the battery is disconnected. After the capacitors are fully discharged, the car is fully inactive. When the car is placed on the track and the switch is depressed, the capacitors are disconnected from the fan, and the battery is connected to the capacitors. As long as the car sits at the starting gate for at least five seconds, the capacitors will be fully charged for the run.
By removing the toggle switch, room was made to recess the capacitors into the body of the car. I milled a pocket completely through the car, and then glued a 1/16 inch thick piece of basswood to the bottom of the car to support the capacitors.
Capacitor Propeller Car – Top View
Capacitor Propeller Car – Side View
The wiring of the Capacitor Propeller Car is a bit more complicated than on the standard version. Note in the wiring diagram that the capacitors are connected in series. This doubles the voltage capacity to 10 Volts, so that the 9 Volt battery will work.
Capacitor Propeller Car Wiring Diagram
On our 32 foot aluminum track, a standard Propeller Car with a new battery crosses the finish line in 1.89 seconds. The Capacitor Propeller Car crosses the line in 1.73 seconds. Not only is the capacitor version faster, but an added benefit is that it turns itself off, since the fan quits turning after the capacitors fully discharge (in about ten seconds).
Just for fun, I removed the battery and battery holder, and taped down the connecting wires. This considerably reduced the weight of the car. At the starting gate I charged the capacitors with the battery, and then let her rip. The car was much faster at 1.52 seconds.
However, there was a disadvantage. The car was so light that the spring on the switch wanted to push the car up the hill and start the fan. By being very careful, I could make it work, but if the car was any lighter, it would not have staged properly. Rod ran into the same problem on his car. He had to set a lead weight against the back of the car for proper staging (can be seen in the photo). When the car took off, the lead weight was left behind.
A LITTLE CAUTION
Using capacitors with a 9 volt battery is generally less risky than using LiPo batteries. However, there is some risk, so if you are not comfortable with basic electronic wiring, then this project is probably not for you.
1. Soldering is required. Wear safety glasses and be very careful to avoid getting burned.
2. Capacitors do not react nicely if they are wired backwards (they can rupture). Be careful with the polarity. On the capacitors specified in the article, the long wire lead is positive, while the short wire lead is negative. The risk increases if you choose to leave the battery off of the car and perform manual charging. An inexperienced person could easily connect the battery backwards. 3. Do not charge the capacitors for an extended period of time. When not in use, allow the capacitors to discharge.
4. Do not touch the leads of the capacitors after they are charged. They will deliver a much stronger shock than a 9 Volt battery. For safety, apply electrical tape over the capacitor’s connections after the wiring is complete.
(1) One way to picture voltage and amperage is by thinking of voltage as water pressure and amperage as the water flow. Even with a lot of pressure (Voltage), if the pipe is small, the flow of water (amperage) is restricted. Alkaline batteries essentially limit the flow of electricity (amperage).
(2) To continue the analogy, think of a capacitor like a water tank with a large release valve. Even with a small flow of water into the tank (amperage), as long as the pressure is high enough (voltage), the tank (capacitor) will fill up and build up pressure. When the release valve is opened, most of the water inside will quickly rush out.
Editor’s Note: If you are interested in one of these cars, but don’t want to do it yourself, Maximum Velocity offers a kit with all of the soldering completed. It has several improvements beyond what is described in this article. You can find the kit
Current Propeller Car Kit
From Pinewood Derby Times Volume 10, Issue 5
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