Justin72835

For a science competition I have to build a small, compact hovercraft that is about the same size as a shoe box. The goal of the project is to make the device move across a track and reach a certain distance in a specific amount of time. These values are different for each competition too. For example, the target distance and target time may be 1.7 meters in 15 seconds for one competition, and 0.9 meters in 27 seconds for the next competition. Because of this, I need to be able to alter the speed of the hovercraft to meet the requirements. However, I ran into some technical problems and I was wondering if anyone could provide a solution.




The hovercraft works by using two fans. The first fan directs air downwards, which supplys lift to the vehicle and decreasing the friction between the device and the floor. The second fan directs air toward the back of the device, which pushes the device foward.




Here's where we run into some problems. For any object, the force of friction is always greater when the object is still than when the object is in motion. This is why it may be difficult to get a sofa moving by pushing it, but once you get it moving it becomes a lot easier. The same is true for the hovercraft. Once I set the hovercraft down on the track, it doesn't move because friction is holding in place. However, if I give the device a slight push, it travels along the track without a problem.




I need to find a way to give the device this "slight push" without actually touching it (pushing yhe device is a clear violation of rules). So, What I am looking for is a way to give the hovercraft a "boost" of speed for maybe 1 or 2 seconds just so that it's able to overcome the friction and start moving. This could be done by increasing the voltage slightly for that small amount of time and then decreasing immediately after. I am unable to to make the vehicle run at full power right from the start as the battery drains extremely quickly and I need to be able to run the device multiple times at the same competition (you aren't allowed to charge in between runs). Also, I can't decrease the voltage because you aren't allowed to touch or interfere with device while it's in motion. I know you could possibly accomplish this with a microcontroller but the use of integrated circuits are not allowed in this competition. I was thinking I could use a timer or a capacitor of some sort but I have very limited knowledge on what they do or how to use them. How would I go about doing this?

mfr02

When the lift fan is fully working and there is no direct contact between vehicle and ground (air exiting the skirt all round), there ain't no friction. Inertia, yes, friction, no. That is the basic idea of hovercraft. Your main problem will be keeping it still if the ground is not perfectly level*, but if it does not balance properly, you will get the problem that you have described. A factor that you might not have considered is that a drive fan, invariably high on the device, must exert a twist in the direction of thrust, causing the front to dip. Maybe ducting it to be low would help, a bit like Cockerell did when he invented the first one, although he was bleeding pressure off from the lift fan.
Having a resistor in series with the motor will slow it, having a large value charged electrolytic capacitor** will briefly give full voltage to the motor, but to charge it you will need the switch between the junction of the resistor and capacitor. Battery - resistor, capacitor, ground line. Junction of R and C, switch, motor, ground.
If the speeds and distances are as suggested, you are going to need really really still air as well as a very level track. And a pair of good big air rudders in the pro washto keep it pointing the right way.

*model hovercraft - very good at "finding" the low spot on an uneven floor in still air.
** About 1000mF per amp of fan motor drive current should give about 1 second of boost.