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LIPX3 wrote:

bhavjain wrote:Hey Guys,

Just wondering which specific fan (a link would help!) would be sufficient to be used on a 2kg hovercraft. Not able to find one... Looking at past threads, I've come across these options:

1. http://www.digikey.com/product-search/e ... 70-1126-ND
2. http://www.digikey.com/product-detail/e ... ND/2560506
3. http://www.digikey.com/product-detail/e ... ND/4840565

Also, aren't these are brushless? If so, which of these is allowed?

Thanks.

These fans do have brushless motors. There was a rule clarification that says you are allowed to use brushless motors if they are in a computer fan, https://www.soinc.org/official_rules_clarif . The first fan you linked would not hover. 2 kg * 9.8 m/s^2 / (0.15 m * 0.25 m) = 522.666667 pascals, and that times 4/3 equals 696.888889 pascals (The fan is 12v, but you can only use 9v batteries or less.) If you had a perfect design, you would need a 12v fan with 697 pascals of static pressure. The second fan probably wouldn't hover, as it would have an output of about 764.25 pascals of static pressure with a 9v battery, which is pretty close to 697 pascals with a perfect design. The third fan probably would hover, as it would output about 975 pascals of static pressure with a 9v battery.

Thanks! Is the third fan legal? Are there much better options out there for hovering 2kg?

What are some ideas for a skirt/hover mechanism? The full bag bottom has too much friction which I dislike and a fixed rectangular frame with a ballast of air doesn't hover more than a fraction of a millimeter

windu34 wrote:What are some ideas for a skirt/hover mechanism? The full bag bottom has too much friction which I dislike and a fixed rectangular frame with a ballast of air doesn't hover more than a fraction of a millimeter

What do you mean it has too much friction? Is this an objective measurement you've made? Plenty of designs with full bottom bags work.

windu34 wrote:What are some ideas for a skirt/hover mechanism? The full bag bottom has too much friction which I dislike and a fixed rectangular frame with a ballast of air doesn't hover more than a fraction of a millimeter

It doesn't matter how much it hovers, 10 meters or a millimeter has the same friction.

chalker wrote:

windu34 wrote:What are some ideas for a skirt/hover mechanism? The full bag bottom has too much friction which I dislike and a fixed rectangular frame with a ballast of air doesn't hover more than a fraction of a millimeter

What do you mean it has too much friction? Is this an objective measurement you've made? Plenty of designs with full bottom bags work.

The coefficient of friction of the surface will vary at each competition therefore messing up my time score. Sure full bottom bags work, but IMO, not the best design for when you start maxing out the mass as the effects of an unknown friction force increase linearly

LIPX3 wrote:

windu34 wrote:What are some ideas for a skirt/hover mechanism? The full bag bottom has too much friction which I dislike and a fixed rectangular frame with a ballast of air doesn't hover more than a fraction of a millimeter

It doesn't matter how much it hovers, 10 meters or a millimeter has the same friction.

Yes Im aware, but a miniscule hoverheight could lead to disqualification if the supervisor deems the hover height insignificant

windu34 wrote: The coefficient of friction of the surface will vary at each competition therefore messing up my time score. Sure full bottom bags work, but IMO, not the best design for when you start maxing out the mass as the effects of an unknown friction force increase linearly

I'm going to go out on a limb here a bit and say that while there might be some slight factors that impact your time score, in general, friction isn't going to be impacted by surface area. Friction can be really counter-intuitive to think about sometime. The key formula is: F=uN (Yes, Physics is FUN sometimes;), where F is the force due to friction (parallel to the surface), u is the coefficient of friction (a constant based upon the surface materials involved), and N is the normal force.

What this means is that for a given weight vehicle, the friction that will be experienced is the same whether there is a full bottom bag or just an edge skirt in contact with the surface. The key is that in both cases, you don't want ANY contact with the surface. You want to design your vehicle such that there is a full air cushion completely separating the vehicle from the surface.

chalker wrote:

windu34 wrote: The coefficient of friction of the surface will vary at each competition therefore messing up my time score. Sure full bottom bags work, but IMO, not the best design for when you start maxing out the mass as the effects of an unknown friction force increase linearly

I'm going to go out on a limb here a bit and say that while there might be some slight factors that impact your time score, in general, friction isn't going to be impacted by surface area. Friction can be really counter-intuitive to think about sometime. The key formula is: F=uN (Yes, Physics is FUN sometimes;), where F is the force due to friction (parallel to the surface), u is the coefficient of friction (a constant based upon the surface materials involved), and N is the normal force.

What this means is that for a given weight vehicle, the friction that will be experienced is the same whether there is a full bottom bag or just an edge skirt in contact with the surface. The key is that in both cases, you don't want ANY contact with the surface. You want to design your vehicle such that there is a full air cushion completely separating the vehicle from the surface.

Yes, and when my craft is properly balanced, it is completely frictionless (as far as I can tell, I'm sure there is a tiny amount somewhere that I cant discern). A full bottom bag doesn't even give you a chance at that. I understand the allure: it is much easier to design and build and perhaps, it should even be advertised as the basis of one's first craft, but in reality, I doubt a full bottom bag will yield as good results as that of a near-frictionless craft in respect to repeatability.

Do the hovercraft skirts, when not inflated, have to be less than 19.5 cm x 30 cm? Or are those the maximum dimensions for the skirt once inflated?

Mil291 wrote:Do the hovercraft skirts, when not inflated, have to be less than 19.5 cm x 30 cm? Or are those the maximum dimensions for the skirt once inflated?

I think those are the dimensions for when everything is completely inflated (rules 4.b and 4.f)