MagLev C

[iwonder]

I was worried about this when I first saw the rules. In order to take 15s, the car requires a very low force. However, if this force is less that the force of static friction between the car and the side rails, it will be impossible for the car to move, without a push-start from some sort of larger force. I don't know how this could be remedied, without sonehow decreasing the friction between the car and siderails.
As to your secod question, nothing official but I interpreted it as a complete shield encasing the whole fan, so probably some sorr of mesh or screen.

Life gets interesting. I interpreted it as a ducted fan. So definitely will need official word

As to the static friction, one solution is maybe to use lighter grease in your bearings (or light oil, if you can and aren't already) to reduce the static friction. Another solution might be to increase the kinetic friction to slow the vehicle down, so you can use a faster speed.

Also, how consistent is your time? I was thinking that a rheostat would vary in time if it got bumped or heated up. I've used higher powered things like that in the past and have little respect for a resistor where you can get oil and dirt on windings and touch to change the resistance(unless you somehow found a sealed one :O )

[chalker]

Also, regarding rule 3h (I know Chalker.... rules clarification), what is your opinion of the following blurb:
"Propellers....... must be shielded from direct contact such that the event supervisor is not able to make contact with the propeller with a standard 1/4" dowel."

Does this merely imply a ducted-fan design? If so, then you could stick a dowell inside the duct to contact the prop easily. Do you think this means that some sort of screen needs to be employed at the ends of the duct to prevent any contact with the prop whatsoever?
Any input is appreciated.

Unofficially as always: we used that specific language for a reason. We don't like to proscribe specific designs if we don't have to. In this situation, we were looking at the safety aspects of people's fingers getting cut by propellers. Safety is hard to objectively measure sometimes, so in this situation we devised a simple test that event supervisors can use to check for this safety aspect. Thus, yes a screen of some sort is probably going to be needed, but it doesn't imply a ducted-fan design. Think of the standard small room fans - they have wire cages around the entire blades to prevent contact with them and aren't ducted-fans.

Also, regarding the slow speeds / static friction issue - give us a little credit. We did some testing and selected the variable ranges we did on purpose in order to make the event accessible to everyone, yet still challenging at the 'elite' levels. Of course it's not going to be a slam dunk to get a solution that works at the extreme edges of the rules.

[TyBlood13]

The new rules keep me from modifying my maglev from last year, but that's probably for the best. Took 4:46 to complete a test run last year at regionals. Still got 3rd place though.

[Flavorflav]

Finally, I don't want to be difficult, but a 3000mAh LiPo means effectively nothing for overall performance, that's just how much charge the battery can carry. What really matters (or mattered) in this event is how quickly it can discharge and how evenly it can discharge. Those ratings are the "C" ratings on a battery. A tiny 100mAh battery rated at 70C would outperform a huge 3000mAh battery rated at 15C in a short race.

I don't want to be difficult either, but it was my understanding that the C rate was a factor of how quickly a battery could discharge itself, with 1 C meaning it could discharge completely in 1 hour, 2C in 1/2 hour etc. If that is right, then your 3000mAh battery would have to output a current of 45 amps to get a 15C rating, while the little one would only put out 7 amps. Am I wrong in my understanding of C rate?

[chalker7]

Finally, I don't want to be difficult, but a 3000mAh LiPo means effectively nothing for overall performance, that's just how much charge the battery can carry. What really matters (or mattered) in this event is how quickly it can discharge and how evenly it can discharge. Those ratings are the "C" ratings on a battery. A tiny 100mAh battery rated at 70C would outperform a huge 3000mAh battery rated at 15C in a short race.

I don't want to be difficult either, but it was my understanding that the C rate was a factor of how quickly a battery could discharge itself, with 1 C meaning it could discharge completely in 1 hour, 2C in 1/2 hour etc. If that is right, then your 3000mAh battery would have to output a current of 45 amps to get a 15C rating, while the little one would only put out 7 amps. Am I wrong in my understanding of C rate?

Yes, you are sort of correct. I semi-arbitrarily chose numbers without checking the math. C rating isn't how long it takes a battery to discharge relative to hours, but how quickly it can discharge (or charge, for that matter) relative to its capacity. So yes, my comparison is flawed. A 100mAh 70C battery has an effective discharge rate of 7000A/h, while a 3000mAh 15C battery has a rate of 45000A/h. Obviously the second one can dump more current through a motor, turning it faster. More to the point, if you have a big battery with a really small discharge rate it will underperform a little one with a huge discharge rate (provided the math works out.)

All that said, my original point stands. Capacity isn't the only metric you need to pay attention to; other factors always apply in engineering. In this example discharge rate is what we're focusing on, but I could easily see mass, size, cost, availability, voltage or a number of other variables coming into play.

[Flavorflav]

Right, I was not questioning your central point, just checking my own understanding of C rate. I would add that, even more to the point, you probably don't even want a high discharge rate this year. As someone else pointed out, any decent vehicle from last year could have easily made a run in less than five seconds on the bare floor. Such high currents do not seem likely to be helpful this year.

Back to C rate for a second, I don't think I follow your Amps/hour examples. Since an Amp is a Coulomb per second and battery capacity should be a matter of Coulombs, shouldn't discharge rate be in simple Amps?

[chalker7]

Right, I was not questioning your central point, just checking my own understanding of C rate. I would add that, even more to the point, you probably don't even want a high discharge rate this year. As someone else pointed out, any decent vehicle from last year could have easily made a run in less than five seconds on the bare floor. Such high currents do not seem likely to be helpful this year.

Back to C rate for a second, I don't think I follow your Amps/hour examples. Since an Amp is a Coulomb per second and battery capacity should be a matter of Coulombs, shouldn't discharge rate be in simple Amps?

Yeah.....it is just amps, but you can also think of it as the integral and how much cummulative current is running over a specified amount of time, getting back to the capacity. I was also off on the mA/A issue as well. Obviously these batteries aren't discharging at 45000A. 45A is still plenty to kill you though, so be careful! There are some LiPos out there that can discharge quickly enough to start a car, even though they fit in the palm of your hand.
And yes, I don't think high discharge rates are necessarily important this year. What you want is consistency and reliability. One way to get there is use a battery that discharges extremely quickly (thereby without much room for variation) and then control the flow another way using high quality materials (that is, not rely on the variable internal resistance of a battery) but I'm not sure if that's the right solution or even a particularly good one.

[Jdogg]

Right, I was not questioning your central point, just checking my own understanding of C rate. I would add that, even more to the point, you probably don't even want a high discharge rate this year. As someone else pointed out, any decent vehicle from last year could have easily made a run in less than five seconds on the bare floor. Such high currents do not seem likely to be helpful this year.

Back to C rate for a second, I don't think I follow your Amps/hour examples. Since an Amp is a Coulomb per second and battery capacity should be a matter of Coulombs, shouldn't discharge rate be in simple Amps?

Yeah.....it is just amps, but you can also think of it as the integral and how much cummulative current is running over a specified amount of time, getting back to the capacity. I was also off on the mA/A issue as well. Obviously these batteries aren't discharging at 45000A. 45A is still plenty to kill you though, so be careful! There are some LiPos out there that can discharge quickly enough to start a car, even though they fit in the palm of your hand.
And yes, I don't think high discharge rates are necessarily important this year. What you want is consistency and reliability. One way to get there is use a battery that discharges extremely quickly (thereby without much room for variation) and then control the flow another way using high quality materials (that is, not rely on the variable internal resistance of a battery) but I'm not sure if that's the right solution or even a particularly good one.

To determine the maximum discharge rate of a li-po battery you take the capacity of the battery multiplied by the ___C number on the battery. As the C number stands for the maximum continuous discharge of the cells capacity. So a battery that's 1000mah with a 40C rating can discharge 40 amps continuously (not that it will if you have a load across it) while a 20000mah battery with a 2C rating can only discharge up to 40 amps continuously as well.
This really is just defining how much internal resistance the battery has.

[JimY]

This may be a little early in the game, but we have built and are testing an early (and nowhere near final) 2kg prototype that is controlled via rheostat and have noticed a peculiarity that is making long runs (around the 15s mark) very difficult to control.
It seems that, even with friction nearly out of the picture (our car has bearings at the corners to reduce friction with the track), the speed at which the prop needs to rotate to provide the longer run times requires a push to even get it moving at all. Its really weird. We have a "15s" mark on the hand-written dial of the rheostat. When we set it to that mark and turn the car on, the prop moves but the car doesn't. Then, if you merely touch the rear of the car (surely mondo illegal), it starts moving at the prescribed speed. If you then set the prop to the lowest speed that will move the car with no help, it completes the run in around 9 seconds. We thought we'd have this one knocked out quickly so it would be "on to the Mission possible and Scrambler events". Oops!
Just curious if anybody else has seen this.

I was worried about this when I first saw the rules. In order to take 15s, the car requires a very low force. However, if this force is less that the force of static friction between the car and the side rails, it will be impossible for the car to move, without a push-start from some sort of larger force. I don't know how this could be remedied, without sonehow decreasing the friction between the car and siderails.
As to your secod question, nothing official but I interpreted it as a complete shield encasing the whole fan, so probably some sorr of mesh or screen.

Considering rule 4c regarding side rail magnets being allowed, I'd get rid of the bearings, add magnets to the sides of the car and track, and get rid of the friction. Then do the calibration work.

I've now experienced the issue of a 2kg car wanting to hug one side or the other, and have found no good way to get it to do otherwise. Side rail magnets only seem to cause the car to ride higher or lower. We'll be going with bearings and hoping to not experience the same friction issues at longer target times.

[iwonder]

A cheaper solution might be to increase kinetic friction so you can just run the propeller faster to get to 15 seconds. Assuming you can still make 5 it's probably easier than adding 2 more sets of magnets and making a wider track.