Top Fuel Cars: Propeller Car

(The third in a series of articles on cars that ‘stretch the rules’ – this is an updated version of an article that first appeared in Volume 3, Issue 5, November 26, 2003)
Many organizations give parents and siblings an opportunity to race by sponsoring an open competition race. Typically the rules for these races are somewhat relaxed, allowing more innovation and creativity in car design.
In today’s article I would like to first share some typical ways to ‘open-up’ the rules for this type of race. Then I would like to explore a more radical way to add excitement to your derby event.
Here are some common ways to allow more flexibility in open competitions:
1. Wheelbase – If the organization event mandates the use of a standard wheelbase (distance between the front and back axles) then allow the wheelbase to be modified as desired, as long as the overall length specification is maintained.
2. Alternate axles – Instead of requiring the use of the organization standard axles, allow the use of after-market ‘speed axles’.
3. Wheel shape – Typically, standard rules allow minimal wheel modification (e.g., only a light sanding). Try removing this restriction, allowing any plastic wheel, as well as any wheel treatment including narrowed, grooved tread surface, and/or overall weight reduction.
4. Length – Try changing the maximum length from the typical seven inches to twelve inches (or one inch less than the distance from the starting pin to the back edge of the track).
5. Weight – Consider changing the maximum weight to one or more pounds (with standard wheels and axles, I don’t recommend exceeding 1 pound).
Wouldn’t it be fun to add a whole new dimension to your open competition? I’m thinking power!
Okay, I know what you’re thinking. Someone will strap a rocket engine on the car, and the car will damage the track, a person, or the building! But of course, restrictions would be necessary to minimize any risk. So, I suggest the following rules.
Rules for a Radical Open Competition
(Adjust to fit your track, and leave an open lane between cars)
– 11 inches maximum length
– 5 inches maximum width
– No limit on height (remove the electronic finish line)
– 3 inches maximum inside wheel base
– 3/8 inch minimum track clearance
– 16 ounces maximum weight
Power Sources
In addition to gravity power, other power sources may be used as long as they do not pose a risk to the spectators or to the track.
1. Allowed power sources include, but are not limited to electrical, mechanical, and pneumatic.
2. Power sources which use combustion of any type are explicitly disallowed. This includes combustion engines, rocket engines, and explosives.
Wheels, Axles and Body
A pinewood derby kit may be used, but it is not required. Any material can be used for the car as long as the finished product meets the specifications. However, the wheels must be made of a material that will not damage the track. Metal, ceramic, glass, or other hard substances may not be used for wheels. Allowable materials include plastic, rubber, and wood.
Race Procedure
The owner of each car will stage the car on the starting line, and pick the car up at the end of the run. Cars may be adjusted between races as long as the adjustment does not delay the race. However, cars may not be lubricated during the race.
Possible Designs
I have heard of all sorts of ideas for adding power to pinewood derby cars. Listed below are a few that have merit.
Powered Rear Axle
– Spring wind-up mechanism
– Rubber-band powered
– Mouse-trap powered
– Electric motor powered (R/C type)
– Weight powered (suspended weight drops and turns rear axle)
All of these ideas can be made to work. But two problems must be worked out:
1. How to apply power when the starting pin drops, or
2. If power is applied before the pin drops, how to apply the maximum torque without the wheels spinning-out.
Non-powered Rear Axle
The design ideas below eliminate problem number two above, but problem number one must still be resolved.
– Compressed CO2 or Air (acts like a ‘jet’ engine)
– Propeller powered (push or pull)
I have had some experience with propeller power, so I would like to share the design of three cars that I have built. The first one was built in 2002, the second in 2005, and the third in 2007. I have also included photos of a few propeller cars submitted by readers.
First, here are some principles that I have learned through experience.
1. Maximize thrust through higher RPM motors, steeper blade pitch, and larger props.
2. Maximize acceleration by minimizing weight. This mainly means lighter weight batteries.
3. The longer the track, the greater the speed (and the larger the advantage over conventional cars).
My First Propeller Car
Please don’t laugh too hard; I severely over-engineered this car. But I can say the car has never had to be repaired! This car smoked the competition when it raced in 2002, and it easily beats any standard pinewood derby car (2.7 seconds on a track measuring 33 feet, 3 inches from the starting pin to the finish line – measured with a stop watch as it doesn’t fit under the finish line bridge).

First Car: Front View

First Car – Rear View
Here is a parts list and some notes on implementation.
– Motor: 12 VDC motor, RF-370CA by Mabuchi Motors. This is a VCR motor, but other motors would work as well. I drove the motor at 18V for more power. This would eventually burn out the motor, but the on-time is so short that the motor doesn’t get a chance to overheat. This motor is available at:
Jameco’s part number is 238473

Motor Diagram
– Motor Mounting: The two screw holes on the front of the motor are for mounting. The required ISO screws can be purchased at a hardware store. I used a piece of thin stainless steel (hardware store item), drilled holes for the mounting screws, shaft, and assembly bolts (see front view picture).
– Propeller: I used a plastic prop intended for rubber band powered airplanes. The hobby store I visited sold them in packs of three. The prop is about 5 inches long, and I had to trim the tips a bit for clearance. The shaft hole was too small, so I drilled it out to fit snuggly on the motor shaft. I then used epoxy to glue it in place. I believe this type of propeller works better than an R/C airplane propeller. R/C propellers are heavier, and the blade angle is smaller, providing less thrust.
– Infrastructure: I had access to an old erector set, but you can use any light metal strapping. It does need to be securely mounted as some torque is generated. Make sure there is clearance for the propeller. I had to remove some metal from the erector material. If you look carefully at the front view, you can see where it was removed.
– Motor Shroud: Quaker Oats oatmeal container. I put this on for two reasons: (1) To keep hands from touching the blades (it hurts, but doesn’t cut if you touch the spinning blades), and (2) To keep the blade from breaking if the car rolled over – if the blade comes off, the race if over.
– Motor Alignment: The motor does need to be pointing as straight forward as possible. Add/remove small washers where the stainless steel strap fastens to the infrastructure to adjust alignment.
– Starting Pin Switch: I used a contact switch (part #275-016 at Radio Shack). It is normally on. When the car rests against the starting pin, the weight of the car closes the switch turning the motor off. Thus, when the pin drops, away it goes.
– Kill Switch: If you look carefully at the front view you will see a small toggle switch (part #275-624 at Radio Shack) that is used to turn the motor off when not in use. Just make sure to turn it on at the starting gate!
– Batteries and Holders: Two standard 9V batteries. The battery holders are Radio Shack part #275-326.
– Wiring: Light gage electronics wire. Positive side of the battery goes to the Kill Switch, then to the Starting Pin Switch, then to the motor. The negative wire goes to the motor. Batteries are wired in series.
– Wood: Pine, 1-3/4 inches wide. The car is 10 inches long, but it could be shortened.
– Wheels/axles: Standard BSA issue. Axle holes are drilled with a drill press. All four wheels touch the ground.
My Second Car
After seeing the propeller cars built by others, I decided to build another car. This one would be less bulky, but would use the same basic components. The main differences are the use of Outlaw Wheels, the full 5 inch long propeller (did not need to be shortened), and significant weight reduction.

Second Car: Front View

Second Car – Rear View
With the reduced weight the performance improved significantly, completing the same distance as prop car 1 in 2.2 seconds, 1/2 second faster than my first car (again measured with a stopwatch). But remember the reason for the oatmeal container on the first car? Sure enough, on the second run, the car tipped over on the braking section, and the propeller flew off. So the lesson is that if you run this type of car, keep the finish line area clear of people (to avoid an eye injury), and have someone stationed at the end to catch the car and turn off the motor.
My Third Car
I decided I needed to build a car that fit within the standard dimensions, so that it would go under the finish line bridge. After a reader posted some information on a derby forum, I decided to do this with a ducted fan from an R/C ‘Jet’ plane (purchased from Hobby Lobby). To get a high enough RPM, I used a 7.2 V Speed 300 compatible motor that was intended for an R/C boat. The motor is powered at 9 volts. The two batteries are wired in parallel to provide enough current for the motor.

Third Car: Front View

Third Car – Rear View
This car is loud and fast, but not quite as fast as the second car. It runs the same course in 2.3 seconds. I believe the speed could be improved with a smooth ring around the intake (like on a real jet engine). According to the documentation that came with the duct fan, this can improve thrust by as much as 25 percent.
Here are a few propeller cars sent to me by readers.

Keith Gosselin – Like my car, this one has a starting pin contact switch and a kill switch. Hanging the propeller (tri-blade in this case) off the back of the car permits the use of a longer propeller.

Mark Pugsley – A similar design using a rubber band airplane propeller (and liberal use of rubber bands in the infrastructure).

Unknown – A dual prop model that uses an R/C type battery and control. It appears to be quite heavy, so I am not sure how will it performed. (This photo was sent to me by another participant in the race. If this is your car, please let me know so that I can give you credit).

Unknown – (If this is your car, please let me know so that I can give you credit).
Maybe this all seems a bit ridiculous to you. But I guarantee that if you have a group with a lot of handy parents, an open competition race with power assist is a blast and a certain crowd pleaser!
From Pinewood Derby Times Volume 6, Issue 15
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