Hovercraft B/C

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windu34
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Re: Hovercraft B/C

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

chalker wrote:
windu34 wrote: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

windu34 wrote: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

chalker wrote:
windu34 wrote: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

windu34 wrote: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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Re: Hovercraft B/C

Postby kinghong1970 » September 19th, 2016, 3:22 pm

so just read on the rules revision...

Brushless motors and integrated circuits are not permitted, unless they are an integral part of and are imbedded into a commerically available electronic or computer cooling fan. (9/12/16)


so in such a case where a commercially available fan is a brushless, it is allowed?
noticing some pc fans and it comes with 3 wires on all of the more standard pc fans out there...

oh, and slight typo on the "commercially" vs "commerically" as you have on the website.

thanks

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Re: Hovercraft B/C

Postby SPP SciO » September 19th, 2016, 3:54 pm

Regarding skirts, a student mentioned disposable shower caps, with the elastic band, and using it similar to a fitted sheet on a mattress. Interesting idea - but will it work! This calls for science.
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Re: Hovercraft B/C

Postby chalker » September 19th, 2016, 4:57 pm

kinghong1970 wrote:so just read on the rules revision...

Brushless motors and integrated circuits are not permitted, unless they are an integral part of and are imbedded into a commerically available electronic or computer cooling fan. (9/12/16)


so in such a case where a commercially available fan is a brushless, it is allowed?
noticing some pc fans and it comes with 3 wires on all of the more standard pc fans out there...

oh, and slight typo on the "commercially" vs "commerically" as you have on the website.

thanks


Standard caveat that this is not the place for official statements....

Generally, the 3 wire connectors have an extra wire that serves as a tachometer signal that measures how fast the fan is spinning. Regardless, yes, brushless fans that are designed for pc cooling would fall into the allowed category.

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Re: Hovercraft B/C

Postby chalker » September 19th, 2016, 4:59 pm

SPP SciO wrote:Regarding skirts, a student mentioned disposable shower caps, with the elastic band, and using it similar to a fitted sheet on a mattress. Interesting idea - but will it work! This calls for science.


I doubt the elastic band will provide enough of a seal to keep air from leaking out... but it never hurts to try it!

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Re: Hovercraft B/C

Postby retired1 » September 20th, 2016, 9:05 am

I agree with Chalker, but a strip of duct tape can solve the leak problem. Not beautiful, but effective.

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Re: Hovercraft B/C

Postby Flavorflav » September 23rd, 2016, 10:00 am

What was the reasoning behind specifying the width of the beams and the width of the test base? Neither would seem to have any impact on how teams prepare for the event, so as far as I can tell it just establishes a hoop for supervisors to jump through. I can't use angle iron, for example, and running the event on a lab bench might well be out of spec. Is there some way that these measurements might affect competitors that I haven't thought of?

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Re: Hovercraft B/C

Postby chalker » September 23rd, 2016, 3:15 pm

Flavorflav wrote:What was the reasoning behind specifying the width of the beams and the width of the test base? Neither would seem to have any impact on how teams prepare for the event, so as far as I can tell it just establishes a hoop for supervisors to jump through. I can't use angle iron, for example, and running the event on a lab bench might well be out of spec. Is there some way that these measurements might affect competitors that I haven't thought of?


Primarily to help the novice event supervisor who wouldn't know where to start or to think through all the aspects of it. If you look at the draft rules from last year, we specified a standard 8' table and 2x4s. We abstracted that out a bit to provide more flexibility by talking about beams and surfaces (which is where the general dimension came from).

The easiest route is to just get 2 metal 2x studs and a standard classroom table. If we just specified a width of the track, some novice event supervisors would like use thin pieces of wood, which will bend and warp. Likewise, many lab bench tops are too narrow to both have the track and standard iron support stands for the lasers / photogates.

Angle iron is going to be a lot more expensive, but in reality is probably an acceptable solution if you want to use it. As event supervisor you have some discretion and can notify the teams in advance about such a minor tweak. As long as you have the same conditions for all teams you should be fine.

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Re: Hovercraft B/C

Postby sridevi » September 26th, 2016, 4:44 pm

To get a good score do you need to have a fast Time Score and a small Mass score?

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Re: Hovercraft B/C

Postby windu34 » September 26th, 2016, 5:01 pm

sridevi wrote:To get a good score do you need to have a fast Time Score and a small Mass score?

Read the rules. Larger the mass and more accurate the time, better the score.
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Re: Hovercraft B/C

Postby Unome » September 26th, 2016, 5:59 pm

sridevi wrote:To get a good score do you need to have a fast Time Score and a small Mass score?

To elaborate on the time score: the goal is for your device's time to be as close as possible to the target time, hence you will need to design and test your device so that you can modify it for all of the allowable target times (see the rules).
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