|Entry #||Driver Name||Car Name||Mass
|15||22||David Farless||Cold Confusion||325||9.75||3.75||9.75||2.50||5.41|
Balloon car designer David Farless writes:
The idea was to apply a jet force on a moment arm to rotate a wheel. The two balloons are attached at each end of a short, hollow axle at the center of two tin pie plates for wheels. Air from the balloons feeds into a square tube plenum chamber located between the wheels and whose length is slightly less than the diameter of the wheels. Brass tubes are used at the ends of the plenum to direct air flow perpendicular to the radius of the wheels. Air from the two balloons goes through the axle, into the plenum, and exits through the tubes at each end of the plenum, causing the wheels to rotate and roll along the ground.
One problem with this design is that, because the wheels are rolling on the ground and are not free to rotate about the axle, air from one end of the plenum (the end nearest to the ground) is not very useful in causing the wheel to rotate, and, in fact, is working against the desired rotation when the jet is near the bottom of the wheel.
To relieve this problem, the plenum chamber incorporates a unique ball valve system to close off the air to each jet when it is below the level of the axel. Marbles are placed in the plenum, free to run along tracks formed by piano wire. The marbles will always roll to the lower end of the plenum and block the flow of air into the jet tube at that end.
Since the marbles are always at one end of the plenum tube, the wheels are always off-balance with the weight of the marbles acting to help rotation for part of a revolution and acting to hinder rotation in another part of the revolution. By releasing the vehicle at the start with a particular orientation, this effect could be used to set the wheel in motion, aiding the jets in getting the inertia going.
The plenum was built a la model airplane construction. The square tube was built as a grid of balsa wood sticks and then covered with heat-shrink, self-stick, iron-on plastic material called "Monokote". (Clear monocote was used so that the action of the marbles in the valve could be observed.) Model plywood (1/32" sheet) was used at the axel and the plenum ends to provide re-inforced holes for penetration of the tubes. The plywood at the ends of the plenum also provided support for the four piano wire tracks used to guide the marbles as they rolled from end to end.
The first trade-off was wheel diameter. Larger for a large moment arm, smaller for less mass. I used 9-in tin pie-plates because they were available and fit well with the 9-in balloons. I think the size was o.k., but they are too heavy.
The second trade was jet outlet size. Larger for more thrust, smaller for longer duration. I used a brass tube with about 3/16" ID as the smallest that would turn my heavy wheels.
The third trade was the number of marbles to use in the plenum valve. More would provide a faster operating valve (less distance for the marbles to travel) and would more nearly balance the wheel. Fewer provides less mass and the unbalance has an advantage for getting started. I used 3 when 9 would have filled the plenum length.
Tin pie plates are too heavy! Paper plates would probably be strong enough, or large lightening holes could be formed in the tin plates (a tough process to do neatly).
The marbles I used in the ball valve plenum were neither round nor smooth! I bought them at the drug store. They are NOT glass as I expected, but made from some hard synthetic material, which chips and deforms. They did not provide a good seal against the brass tube jet inlets.
WHAT FUN !!!