Hopup, Audioguru and hevans1944, Thank you very much for your prompt replies and guidance. Much appreciated!
It's too bad that you found Electronics Point this late for the 2018 Science Olympiad competition, but given that it appears your school and team made it to the 34th Annual Science Olympiad National Tournament, to be hosted on May 18-19, 2018, by Colorado State University in Ft. Collins, Colorado, maybe there is still enough time to provide some help.
For those EP members who are unfamiliar with the the Hovercraft event at Science Olympiad, it involves a student-constructed air-cushion-levitated hovercraft, propelled by an electric motor-driven fan down a level "track" between two side rails (typically 2x4 wood framing studs) over a specified distance. Usually the fan used for levitating the vehicle is separate from the fan used to propel it down the track. A vertical dowel rod, fixed to the front of the vehicle, is used to "trip" photo-sensors at the beginning and the end of each test run.
Teams have an eight minute "window" in which to conduct as many "runs" as possible. One objective of the competition is to complete the run between "start" and "finish" in a specified length of time: fifteen seconds. Points are awarded based on how close the vehicle completes its "run" within the allotted fifteen seconds. Points are also awarded for the vehicle moving the most mass, in the form of rolled penny rolls (up to 16 rolls). There is also a written test and submitted documentation judgement, but these are not factors that need to be addressed here.
The main variables in the vehicle competition are propeller force, vehicle friction, and vehicle weight. The usual problem is the propeller force is too large, the vehicle friction is too small, and the vehicle weight is too light. The result is the hovercraft traverses the 165cm distance in a time much less than the targeted time of fifteen seconds.
Of the three variables available for the students to adjust (and record for the documentation part of their score), only the number of penny rolls is easily manipulated. The vehicle friction is vanishingly small because of the air levitation, but it could possibly be increased by using adjustable, spring-loaded, "pressure pads" mounted outboard of the vehicle and pressing against the side rails. However, the usual construction strategy is to minimize friction contact with the guide rails and maintain sufficient clearance to allow the hovercraft to move down the track. See this
Science Olympiad video clip for what happens when too much clearance is allowed.
The final variable, propeller force, can seemingly be adjusted by simply varying the propeller motor speed. With permanent-magnet brushed DC motors this is not difficult, but with the brushless DC motors used in all computer fans and most equipment cooling fans, it is very difficult to vary the fan speed. The embedded brushless motor electronics tries to maintain a constant fan speed in the presence of a varying supply voltage.
If you are committed to using a brushless fan motor, it is strongly suggested that you become familiar, by performing measurements, with its operating characteristics as a function of applied voltage, resulting load current, and output torque or propeller speed. It would also be wise to do the same measurements on a brushed DC motor for comparison. If it is possible at this late date to use a brushed DC motor, then
@Audioguru's suggestion in post #3 above is a good way to vary the speed of that type of motor
for this competition. It is a terribly inefficient way to control a brushed DC motor that delivers a significant amount of power, but it does get the job done.
An alternative to controlling the pusher propeller speed is to control the volume of air that is moved. With a shrouded fan, this is easily accomplished by placing a baffle in front of the fan to restrict the air flow. A simple cardboard baffle, sliding into and held by a suitable holder frame, could be used. The students would cut holes of various sizes and numbers in multiple cardboard baffles to experiment with different air-flow rates. If this method is chosen, take plenty of blank and extra cardboard baffles to the competition to "fine tune" the air flow rate. A hand-held paper punch can be used to add extra holes for increased air flow, and ordinary transparent adhesive tape or masking tape can be used to cover up existing holes for decreased air flow.
The Hovercraft competition is primarily NOT about electronics, as evidenced by the following rule that limits what you can do electrically or electronically:
g. Electrical components shall be limited to batteries, wires, motors, switches, resistors, potentiometers, capacitors, mechanical relays, fans, and blowers. Brushless motors and integrated circuits are not permitted unless they are an integral part of or embedded into commercially available fans used for cooling electronics or computers.
The Hovercraft competition appears to be aimed at demonstrating understanding and proficiency in basic physics. Below is a picture of a work-in-progress hovercraft. Note the metal "2x4s" used for the guide rails.
For a short video clip of the Hovercraft competition,
visit this link.
Hop
