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geniusjohn5 wrote:The rules state that the craft must be impounded in a 40x40x40 cm box? Can the dimensions of the box be any less or does it have to be exactly 40 cm?

ISTM that's not quite what the rules say.

2.c say "Bonus points are given for vehicles impounded in a box." So added points if in A box, size not specified.

3.a says "The vehicle may be made of any material and have any mass but must fit into a 40.0 cm x 40.0 cm x 40.0 cm box when levitated with any inflated skirt" So the craft has to be smaller than 40x40x40 when running. ESs will/should have a box or rig to test for oversize craft.

What type of battery would be best? 9v? 9v NiMH rechargeable? 8.4 battery pack rechargeable? 7.2 volt battery pack? LiPo batteries? And why?

geniusjohn5 wrote:What type of battery would be best? 9v? 9v NiMH rechargeable? 8.4 battery pack rechargeable? 7.2 volt battery pack? LiPo batteries? And why?

Conventially, LiPo's would typically be the best, for their weight, consistency in power (comparatively), among other things. However, note that they would NOT be allowed this year, according to rule 3f. This year, my team is using NiMH's in lieu of LiPo's. Definitely not ideal, but it's what we found to be the best as a replacement.

Tesel wrote:

dmis wrote:Does anyone have any thoughts about the following problem from an invitational last Saturday? Neither I nor my physics teacher had any idea:

22. The mass flow rate through a cylindrical pipe of cross sectional area A =0.500 m2 is 1500. kg/sec. What is the pressure drop over a distance of 10.0 m?

It should be 0. It's basically a Bernoulli equation problem, with no change in area/velocity or height, so there should not be a change in pressure. I assume the question was incorrectly worded.

dmis wrote:

dmis wrote: 22. The mass flow rate through a cylindrical pipe of cross sectional area A =0.500 m2 is 1500. kg/sec. What is the pressure drop over a distance of 10.0 m?

@nicholasmaurer, did the es ever provide you an answer to this problem?

Quick update for everyone: per the ES, the correct answer is 0.0 Pa. The original answer key incorrectly recorded the answer as 10.0 Pa.

nicholasmaurer wrote:

Tesel wrote:

dmis wrote:Does anyone have any thoughts about the following problem from an invitational last Saturday? Neither I nor my physics teacher had any idea:

22. The mass flow rate through a cylindrical pipe of cross sectional area A =0.500 m2 is 1500. kg/sec. What is the pressure drop over a distance of 10.0 m?

It should be 0. It's basically a Bernoulli equation problem, with no change in area/velocity or height, so there should not be a change in pressure. I assume the question was incorrectly worded.

dmis wrote:

dmis wrote: 22. The mass flow rate through a cylindrical pipe of cross sectional area A =0.500 m2 is 1500. kg/sec. What is the pressure drop over a distance of 10.0 m?

@nicholasmaurer, did the es ever provide you an answer to this problem?

Quick update for everyone: per the ES, the correct answer is 0.0 Pa. The original answer key incorrectly recorded the answer as 10.0 Pa.

Thanks again for the clarification. That makes much more sense.

geniusjohn5 wrote:What type of battery would be best? 9v? 9v NiMH rechargeable? 8.4 battery pack rechargeable? 7.2 volt battery pack? LiPo batteries? And why?

9 volt rechargeable is not a good choice in my opinion. Slow to discharge and not very long lasting on a charge.
7 NiMH in a 7 AA place (not 8) gives a slightly higher initial voltage and has a very good total mah. The commercial battery pack is nearly identical in performance, but weighs about twice as much.

Had planned to get a 7 AAA battery holder so it would weigh even less , but nobody wanted to try it. This would still have several times more power than a rechargeable 9 volt battery.

retired1 wrote:

geniusjohn5 wrote:What type of battery would be best? 9v? 9v NiMH rechargeable? 8.4 battery pack rechargeable? 7.2 volt battery pack? LiPo batteries? And why?

9 volt rechargeable is not a good choice in my opinion. Slow to discharge and not very long lasting on a charge.
7 NiMH in a 7 AA place (not 8) gives a slightly higher initial voltage and has a very good total mah. The commercial battery pack is nearly identical in performance, but weighs about twice as much.

Had planned to get a 7 AAA battery holder so it would weigh even less , but nobody wanted to try it. This would still have several times more power than a rechargeable 9 volt battery.

Yea good advice. Essentially, just look for the ratio of capacity:weight of a NimH pack that satisfies you (the only reason I am not giving a specific recommendation is that it depends upon your hover). It is a good investment, and is pretty much the best option.

Also, LiPos are banned, just so you know (pretty sure a gazillion ppl would have already pointed that out- but just throwing it out there one more time)

antoine_ego wrote:

geniusjohn5 wrote:If I connected two 9v batteries in parallel, would that have any effect on the speed of the fan/motor? I'm trying to get my hovercraft to move forward but it is unable to

Honestly, I'd just invest in a nice and powerful 8.4 NiMH pack. These can be overcharged up to 10.2V, which is legal since the 9.0V limit is dependent upon the label. If you can't get your craft to move forward, the battery is most likely not at fault. You probably need to redesign your skirt to reduce friction, or get a more powerful back fan. A better back fan is honestly the best solution.

I think you need to be careful with terminology here to avoid misleading others.

In all normal battery charging, a slightly higher than nominal voltage is applied. This is normal charging. For an 8.4V NiMH pack, an open circuit voltage after a charge is about 9.8V, which drops very quickly when in use.

Overcharging is not normal charging, can cause overheating and is against policy.

Furthermore, there is no point in overcharging as a standard 8.4V pack is sufficient to power most fans encountered in the context of Hovercraft.

copper.hat wrote:
Overcharging is not normal charging, can cause overheating and is against policy.

Furthermore, there is no point in overcharging as a standard 8.4V pack is sufficient to power most fans encountered in the context of Hovercraft.

It is not against policy, since the batteries must be less than 9.0V by the labeled voltage, not the actual. The extra volt at the beginning does give an added boost at the beginning of a run.

antoine_ego wrote:

copper.hat wrote:
Overcharging is not normal charging, can cause overheating and is against policy.

Furthermore, there is no point in overcharging as a standard 8.4V pack is sufficient to power most fans encountered in the context of Hovercraft.

It is not against policy, since the batteries must be less than 9.0V by the labeled voltage, not the actual. The extra volt at the beginning does give an added boost at the beginning of a run.

The concept of overcharging in this context is really just normal charging. Charging an 8.4 pack to 9V or slightly more is normal, as NimH cells always charge NORMALLY to the range of 1.4-1.6V at full capacity. So, its not really overcharging either way.