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Top Fuel Cars - Improved Propeller Car

(The eighth in a series of articles on cars that "stretch the rules")

Two 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

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)

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.

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.

Read More at: Pinewood Derby Times Volume 10, Issue 5

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