Physics Lab B/C

[andrewwski]

The wording on that as pretty confusing as written. They did release a clarification:

http://soinc.org/official_rules_clarif_2009

[Krissy_bubbles09]

Hmmm I'm a little confused-- could someone help? Is the CD provided at the competition and we have to attach our blades to it, or do we bring the blades already attached to a CD? o_o

you have to attach your blade to your own CD and then bring the whole thing to the competition. You then hook the CD blade to the wind machine there.

[Krissy_bubbles09]

does anyone have an idea about the the written test part is going to be about?

[AlphaTauri]

The rules say it includes "basic info and definitions about energy, work, heat, and heat transfer", "general info about renewable energy", "energy conservation practices", and "relationships and equations used to determine heat loss/gain, specific heat, and heat transfer". Keep in mind, this is just for Div B; there's more formulas and stuff for Div C.

[icyfire]

Has anyone come up with a blade design that works fairly well? I heard earlier on this forum mylar blades, but when i searched them online i didnt get any results...
Would it be better to put a set of shorter blades further from the CD hole and extended out more out a set of longer blades that are placed closer to the center CD's hole bu extend the same length out?
We are new to this event, and we were wondering if there were any good plastics or materials that make a good blade.

ive heard of a lot of teams cutting down their CD's to make a better page. would this be better?

Of course, the weight would be less, but the main thing is i think that with surface of the CD the power of the fan could hit the suface of the CD then be pushed off towards the blade, but when you cut that area off the CD, you not only "lose" some of the fan power but the wind going through the blades could stop the blades from spinning?

[andrewwski]

Has anyone come up with a blade design that works fairly well? I heard earlier on this forum mylar blades, but when i searched them online i didnt get any results...
Would it be better to put a set of shorter blades further from the CD hole and extended out more out a set of longer blades that are placed closer to the center CD's hole bu extend the same length out?
We are new to this event, and we were wondering if there were any good plastics or materials that make a good blade.

If you're asking whether or not to make shorter blades extended further from the hub or longer blades that start at the hub, definitely go for the latter. Having extended blades is going to be very inefficient - the rotational inertia becomes greater as the length (distance from the point of rotation) increases, by a square function. Thus, the portion of the blades closest to the hub is the most efficient - it, in theory, captures the same amount of wind power, but is influenced by rotational inertia and aerodynamic drag the least.

While removing mass on the inside of the blades would decrease rotational inertia, it would not be a great decrease. You want to decrease it from the outer ends of the blades - of which there is a minimum length. I would experiment with different blade lengths that fit within the specs. It's only 3 or 4 cm of a difference - but there will probably still be a measurable difference.

ive heard of a lot of teams cutting down their CD's to make a better page. would this be better?

Of course, the weight would be less, but the main thing is i think that with surface of the CD the power of the fan could hit the suface of the CD then be pushed off towards the blade, but when you cut that area off the CD, you not only "lose" some of the fan power but the wind going through the blades could stop the blades from spinning?

By all means, yes, cut down your CD to just the center hole and enough area to anchor your blades. By doing so, you're increasing the actual length of the blades - and as mentioned, the portion of the blade closest to the hub is the most efficient. Plus, you're decreasing mass, and thus rotational inertia, making the turbine easier to spin.

[Flavorflav]

ive heard of a lot of teams cutting down their CD's to make a better page. would this be better?

Of course, the weight would be less, but the main thing is i think that with surface of the CD the power of the fan could hit the suface of the CD then be pushed off towards the blade, but when you cut that area off the CD, you not only "lose" some of the fan power but the wind going through the blades could stop the blades from spinning?

It is the wind going through the blades that makes the blades spin. If it can't, they won't.

[amerikestrel]

ive heard of a lot of teams cutting down their CD's to make a better page. would this be better?

Of course, the weight would be less, but the main thing is i think that with surface of the CD the power of the fan could hit the suface of the CD then be pushed off towards the blade, but when you cut that area off the CD, you not only "lose" some of the fan power but the wind going through the blades could stop the blades from spinning?

It is the wind going through the blades that makes the blades spin. If it can't, they won't.

Wouldn't the wind going through the blades make them spin more? Unless it's going in the other direction... I don't know.

[andrewwski]

The blades aren't spinning without wind going through them, period. The wind is the only thing that will cause the blades to turn.

You only have one wind source (the fan).

In theory, you want the wind speed before the blades to be three times greater than the wind speed leaving (behind) the blades.

[Flavorflav]

In theory, you want the wind speed before the blades to be three times greater than the wind speed leaving (behind) the blades.

How do you arrive at that number?