Hovercraft B/C

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Re: Hovercraft B/C

Postby PandaDevs » September 17th, 2016, 8:05 am

Is this event considered a trial event, and is subject to change at any time?
Simply said, this event can be removed and previous work done will constitute as useless.
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Re: Hovercraft B/C

Postby HandsFreeCookieDunk » September 17th, 2016, 11:00 am

Is this event considered a trial event, and is subject to change at any time?
Simply said, this event can be removed and previous work done will constitute as useless.
No, this is not a trial event. That being said, I believe every competition site has the right to remove any event.

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Re: Hovercraft B/C

Postby retired1 » September 17th, 2016, 3:12 pm

While not common at the state level, they may drop an event and substitute another. For some poorly explained reason, Florida is doing it this year.
I do not like it as it puts FL teams behind other states in experience for this event at Nationals.
It is somewhat common for regionals to substitute as they may not be able to support a given event.

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Re: Hovercraft B/C

Postby windu34 » September 17th, 2016, 4:23 pm

So as far as the definition of levitation goes, I feel I may be given a construction violation for what I have. I'm levitating barely .5mm off the ground, but as far as performance goes, its pretty good and very frictionless. Do I need to build a skirt system so that I levitate higher or do you think ES will be okay with it? (I know all competitions are different, etc)
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Re: Hovercraft B/C

Postby chalker » September 17th, 2016, 4:55 pm

So as far as the definition of levitation goes, I feel I may be given a construction violation for what I have. I'm levitating barely .5mm off the ground, but as far as performance goes, its pretty good and very frictionless. Do I need to build a skirt system so that I levitate higher or do you think ES will be okay with it? (I know all competitions are different, etc)
The rules clearly define how to demonstrate levitation. Does your device rise when pushed down? Is the rise readily apparent to an objective observer? If so, it meets the criteria.

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Re: Hovercraft B/C

Postby windu34 » September 17th, 2016, 5:03 pm

So as far as the definition of levitation goes, I feel I may be given a construction violation for what I have. I'm levitating barely .5mm off the ground, but as far as performance goes, its pretty good and very frictionless. Do I need to build a skirt system so that I levitate higher or do you think ES will be okay with it? (I know all competitions are different, etc)
The rules clearly define how to demonstrate levitation. Does your device rise when pushed down? Is the rise readily apparent to an objective observer? If so, it meets the criteria.
I don't think it is readily apparent. Can you point to examples of skirts competitors could build? Most of the youtube videos use garbage bags, but I don't think that would be durable for a craft of high mass
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Re: Hovercraft B/C

Postby chalker » September 17th, 2016, 8:43 pm

I don't think it is readily apparent. Can you point to examples of skirts competitors could build? Most of the youtube videos use garbage bags, but I don't think that would be durable for a craft of high mass
Why do you say that? I don't understand what relationship mass has to the durability of the skirt.

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Re: Hovercraft B/C

Postby windu34 » September 17th, 2016, 10:37 pm

I don't think it is readily apparent. Can you point to examples of skirts competitors could build? Most of the youtube videos use garbage bags, but I don't think that would be durable for a craft of high mass
Why do you say that? I don't understand what relationship mass has to the durability of the skirt.
crafts with more mass require the skirt to be stronger to provide a force to oppose the air pressure inside the skirt. The skirt will also drag more on a c raft of more mass because it will sit lower than the same craft of less mass
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Re: Hovercraft B/C

Postby chalker » September 18th, 2016, 7:24 am

crafts with more mass require the skirt to be stronger to provide a force to oppose the air pressure inside the skirt. The skirt will also drag more on a c raft of more mass because it will sit lower than the same craft of less mass
I see where you are going with this, but I think your gut feeling is being too cautious. I did a quick Google search on tensile strength of garbage bags and found this cool science project report: http://www.selah.k12.wa.us/SOAR/SciProj ... l_data.pdf Note that in all the situations, the PSI listed was over a thousand PSI (and in many situations multiple thousands of PSI). 1000 PSI is approximately 7000 kPa in SI units

PSI can be hard to get a gut feeling for, since it relates force to area. The max hovercraft mass allowed is 2kg, which is ~20 N in standard Earth gravity. 1 Pascal is 1 Newton / square meter. Thus, working in reverse, 20 N / 7000 kPa = 0.28 * 10-7 m^2, or about a quarter of a square millimeter. Said another way, you'd have to concentrate your entire vehicle on a spot the size of a pencil point in order to break a typical garbage bag.

But in reality, the skirt you will use is at least dozens of square centimeter in size. Thus the force on any given spot is going to be a very small fraction of whats needed to tear the plastic.

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Re: Hovercraft B/C

Postby retired1 » September 18th, 2016, 9:51 am

PVC comes in many different thicknesses and I doubt that 2 kg is going to burst any pvc bag. With hundreds of practice runs, I can see a hole being abraded in the bag.
With a full bottom bag, and not just a skirt, I can somewhat see this happening. With a bag skirt, the blown air is taking up the bulk of the weight and the friction, so I do not see it happening there.
Initial trials had the bag getting a bit stickey when there was barely enough air to keep it levitated. Switched material, construction and design, and the problem went away. Better control of the air probably had a lot to do with it.
This is with a 2kg craft and skirt bags.


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