Hovercraft B/C

[black]

As usual, my opinion, but if you wind up a spring beforehand, you are giving the vehicle additional potential energy before the run that doesn't come from the batteries. I'm not sure how mass is an energy source, because you aren't extracting energy from the mass itself.

I'm saying that all mass is a "source" of energy, even if you're not using it. You could even argue that you are using it if at any point any mass gets closer to the center of the Earth. Of course, that's impossible to prohibit; however, since that's allowed, then it's entirely possible that a spring-wound timer would be allowed (as long as the extracted energy isn't going towards propelling or levitating the device).

[antoine_ego]

As usual, my opinion, but if you wind up a spring beforehand, you are giving the vehicle additional potential energy before the run that doesn't come from the batteries. I'm not sure how mass is an energy source, because you aren't extracting energy from the mass itself.

I'm saying that all mass is a "source" of energy, even if you're not using it. You could even argue that you are using it if at any point any mass gets closer to the center of the Earth. Of course, that's impossible to prohibit; however, since that's allowed, then it's entirely possible that a spring-wound timer would be allowed (as long as the extracted energy isn't going towards propelling or levitating the device).

But the craft starts off at rest, meaning not off the ground, so all gravitational potential energy comes from the power produced by the fans and in turn the battery. So, the initial energy is all supplied by the battery.

[fxsandiego]

Another new coach here.

I am wondering, if two fans have the same static pressure spec but different diameters. Would the diameter make a difference, ie, the one with bigger diameter performs better or vise versa?

I saw in last year's posts some people were using fans rated at 900Pa+. Does the fan need to be that powerful?

[cuber]

Just wondering, would a mechanical timing relay be alowed? specifically this one: http://www.qualitymobilevideo.com/bu508 ... eLw-WhSzD4?
Rule 3g says mechanical relays are permited.

[DaiDiimon]

The rules say "Brushless motors and integrated circuits are not permitted unless they are an integral part of or embedded into commercially available fans used for cooling electronics or computers".
On this fan, https://www.digikey.com/product-detail/ ... ND/3078695, the label straight up says "DC Brushless". Is this what they mean by "commercially available fans", meaning the fan is legal? Is it just standalone brushless motors that are banned?

I am wondering, if two fans have the same static pressure spec but different diameters. Would the diameter make a difference, ie, the one with bigger diameter performs better or vise versa?

I was also wondering the same thing as fxsandiego. My thoughts were that it wouldn't be diameter dependent. The question boils down to if P=F/A and P is the static pressure of the fan, is A the area of the fan or the area of the applied surface (in this case the hovercraft's base/skirt).
However, I've heard people say it is indeed the fan's area that matters and I can see the argument why.

[fxsandiego]

I think that is allowed. only stand-alone brushless motors (the ones that have three leads) are banned.

BTW, the fan in your link has a static pressure rating at 625 Pa. Do we need such powerful fans? Let's say, the whole craft, including 16 penny rolls weighs 2.5kg ( 2kg of penny rolls + 0.5 kg of craft itself) and its area is 40cm x 40cm. The required pressure is only 153Pa. Even considering running a 12V fan at 9V and other losses, it seems to me a 350Pa fan should be enough to lift the craft. Am I missing something?

[antoine_ego]

I think that is allowed. only stand-alone brushless motors (the ones that have three leads) are banned.

BTW, the fan in your link has a static pressure rating at 625 Pa. Do we need such powerful fans? Let's say, the whole craft, including 16 penny rolls weighs 2.5kg ( 2kg of penny rolls + 0.5 kg of craft itself) and its area is 40cm x 40cm. The required pressure is only 153Pa. Even considering running a 12V fan at 9V and other losses, it seems to me a 350Pa fan should be enough to lift the craft. Am I missing something?

Remember that the 628 Pa value is given at 12V, which is above the constraints of the event. Second, it is given assuming zero cubic feet per minute of airflow. So, the values given do not reflect the actual power of the fan. The actual power that you get will be lower. You can look at the graph n the datasheet for that particular fan.

Also, you probably won't be going to maximum size of the craft because it's always good to have a little bit of leeway when it comes to size, so even the theoretical static pressure will be greater.

Odds are, from my experience, you're going to need 900+Pa to hold everything.

[kinghong1970]

Oh joy!

dc motors that sound like jet engines + little curious kids (Div B)...

"DO NOT PUT YOUR FINGER IN THE MEAT GRINDER!" they do a lot of damage!

hahaha

[fxsandiego]

Like this, https://www.digikey.com/product-detail/ ... ND/3283580?

Current rating is 3.2A. Need a really big battery to run it. The self weight of the craft is going up.

[ftf841]

Like this, https://www.digikey.com/product-detail/ ... ND/3283580?

Current rating is 3.2A. Need a really big battery to run it. The self weight of the craft is going up.

A single 8.4v NiMh should be enough to power that and a blower fan at a similar current. The fan that I and many other teams used last year was https://www.digikey.com/product-detail/ ... ND/2560506