Physics Lab B/C

[Flavorflav]

The rules say that the "blade assembly" have to be within a certain spec. Does that apply to the blades only? or to everything I bring?
I'm thinking of building an interesting rubberband mechanism to go with it, but I to have legs to support my part of my windmill, and i would probably be taller than what the requirement says the "blade assembly" must be.

CD can be modified, you are required to bring a CD (blades should already be mounted on it, no building at the competition)

Rubberbands...? Okay...specs are 25cm-28cm diameter (diameter of blades on the CD).

Why do you have legs? The CD will be mounted on an old CD player motor which in most cases will be clamped to a ring stand-like thing. Maybe I'm reading your post wrong...

No, I think it is pretty clear that he is reading the rules wrong.

[jazzy009]

No, I think it is pretty clear that he is reading the rules wrong.

Agreed, but I didn't want to be so blatant about it
Maybe he read "windmill" and thought of old Dutch windmills...thought he had to make something out of that...while that would be very cultural, this is not an art event.

[diamondplatinum]

Last year, our device was heavy and made out of wood. we got 6th at regionals and 12th? at states. so if you want to use wood, use a light one like balsa or something else...

[andrewwski]

Yes - you want to minimize mass. Mass will directly affect rotational inertia, which you want to minimize.

However, balsa is very inconsistent in terms of density. I find it very hard to balance balsa blades. You need to add mass to do so, but adding it to the correct areas in the right amount can be tricky. Simply massing the blades and adjusting them to the correct mass will not suffice, as the mass of a section relative to its position along the length of the blade makes a difference.

There's a few factors that you want to keep in mind when designing turbine blades.

First, power available can be calculated as follows:

, where P = power available in the wind stream, r=radius of the area the blades sweep, and v=velocity of the air. Thus, increasing the blade length will increase the available power by a squared amount.

However, there are factors playing against you as well, namely rotational inertia and aerodynamic drag. You want the least rotational inertia possible. For the blades, we can use the formula for rotational inertia for a rod. While the blades are not rods, the formula should suffice to show the concept.

, where I=rotational inertia, m=mass, and l=length of the rod (blade). Thus, as you increase the length of the blade, you increase the rotational inertia by a squared function.

The other factor you need to look at is lift. You need the blades to create lift in order to turn. In essence, lift=blade area * net pressure. Thus, as you increase the surface area of the blades, you're increasing lift, but also the rotational inertia and drag.

This plays into the design of the blades. First, you'll notice that wind turbines have blades that are curved - much like an airplane wing. The principle is the same - Bernoulli's Principle - as the velocity increases, the pressure decreases. Thus, the lift formula can be rewritten as follows:

, where L=lift, a=blade area, =density of the fluid (air) and v=airstream speed ( and being the differing pressures on each side of the blade).

All of these are considerations when designing blades. You'll notice that in general, efficient blades will taper, as the airspeed is the same throughout the entire swept area but the rotational inertia increases as you add more mass to the end of the blades. Blades also generally have twist - their pitch decreases as you go along the length of the blade in order to reduce aerodynamic drag on the end of the blades - where it has the most effect. As previously mentioned, the shape of the blades also has an effect.

What you can translate into blades for this event is the bigger question. Most people probably do not have access to devices that would allow them to fabricate a curved, twisted blade. But you may find a way to do so. Other factors - such as reducing rotational inertia, having balanced blades, and proper blade shape (to the extent possible) and length, are thus going to be crucial.

[Cyrus_D]

Okay no means to brag but I was just at the Pembroke Hill Inventational last Sat. (a MO. team) and achived on low 1500 millivolts and on high 2000 millivolts (they had to go one step higher than 200 dcv on the volt meter, it was pretty funny) on their set up, but it didn't look like a standard testing platform. Has anyone else here been able to achieve this? I wish I could bring a camera to take a picture to show you guys the set up, but sorta would get me ranked lower or dq even. I could however take a picture of my blades, but thats top secret .

Edit*- will post evidence of the score sheet that I did achieve those scores if anyone asks.

[guerillagorilla]

Are there any specifics that we should study for in this event, or is ti more of a definitions, concepts sort of thing. The rules sheet is quite vague in this regard.

[overdramatic]

Are you guys attaching the blades themselves to the CD or are you creating a sort of hub to attach to your CD that will hold the blades?

[Spideychief4]

Hello,
I am in Science Olympiad for my first year. I need help on topics in Physical Science Lab, like how to make an efficient wind turbine, for SOINC. Could you give me some links.
Thanks.

[jazzy009]

...

http://soinc.org/physics_lab_b


That site is pretty good. Experiment.

[unknownman]

I have a question. On the sheet that i got, it didn't say what was the minimum diameter. Does anybody know what the minimum diameter for a windmill is?