Pinewood Derby Stories and Photos from Maximum Velocity
Propeller Car Evolution
By Randy Davis
(The third in a series of articles originally published November 26, 2003 and December 1, 2010)
Many organizations sponsor open competition races, some of which allow the use of powered cars. Whether your organization holds this type of race, or you are looking for a "crowd pleaser", the propeller car is a fairly simple way to get awesome speed from a pinewood derby car.
Propeller cars have come a long way since the first "Erector Set" version I built in 2001.
Figures 1 and 2 - Original Propeller Car
The Erector Set car is indestructible, but it carried too much weight to be fast.
Through various iterations, the propeller car was optimized with the version we sold from 2008 to 2012.
Figure 3 - Production Propeller Car
This car had fewer parts, was light-weight, and used a compact ducted fan to produce crowd-pleasing speed.
But in the spirit of continuous improvement, more speed had to be found. This was accomplished in a novel way by Rod Shampine, and documented in Volume 10, Issue 5. Instead of feeding the motor directly with a 9 volt battery, Rod used capacitors to feed power to the motor. This allowed the motor to run at a higher RPM (the battery limited the amps available to the motor), thus producing more speed.
My first version of the capacitor car is shown in Figures 4 and 5.
Figures 4 and 5 - Original Capacitor Car
The car is similar to Rod's design (but a bit more robust).(1) When the front 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 long enough, the capacitors will be fully charged for the run.
PROPELLER CAR II
After some consideration, a few improvements to the capacitor car become obvious. First, removing the battery from the car reduces weight and increases the speed.(2) Next, the wiring and capacitors could be mounted under the car for a cleaner look. Finally, the ducted fan was mounted at the back (more for looks than speed). This
upgraded capacitor car design is now available as a kit from Maximum Velocity in our "Propeller Car II" kit (see Figure 6).(3)
Figure 6 - Propeller Car II
The new Propeller Car is much faster than the previous version, is lighter weight, and has a sleeker design. The car is charged with a 9 volt battery using a power jack and plug (provided with the kit). The kit requires just a few common tools to assemble - no soldering is required.
PROPELLER CAR II PERFORMANCE
On a 32 foot track, the standard propeller car crosses the finish line in about 1.9 seconds. The Propeller Car II crosses the same distance in about 1.6 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. The links below have videos of the Propeller Car II racing against the original Propeller Car and a standard pinewood derby car. I think you will be able to figure out which is which!