Physics Lab B/C

[amerikestrel]

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?

Here's a site I just found... I don't have time to read it thoroughly, but it might answer your question.

http://www.flyingturtle.org/energy/wind-enrgy.html

[amerikestrel]

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?

Here's a site I just found... I don't have time to read it thoroughly, but it might answer your question.

http://www.flyingturtle.org/energy/wind-enrgy.html

Edit: I just read it, and I'm pretty sure that it doesn't help to answer your question. It is interesting, though...

Edit 2: Sorry, I accidently click on quote, not edit... And for some reason I can't delete this post.

[icyfire]

by going through the blades i mean that there is a wind between each blade on the Cd. picutre a CD with blades, and in the space between each blade there is wind. wouldn't this stop the blades

[andrewwski]

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?

It's obtained through calculating a number known as the Betz limit. We can calculate it as follows.

Let's call the speed behind the blades and the speed in front of the blades . The formula for the fraction of total power available in the wind that the turbine will capture, , is:



Let's replace the ratio with a single variable, which we will call .



Now we want to find at which value of that is the greatest. We can do this by some very simple calculus - taking the derivative of and setting it equal to 0 to find the maximum.







So we see that the derivative of the function is equal to 0 at 1/3 and -1. If we look at the function, it is positive on both sides of -1, indicating a plateau but no turning point. This is irrelevant anyway, as the wind speed can't be negative without it blowing in the other direction - which would obviously be counterproductive. At 1/3, the graph goes from positive to negative, so we know that that is the maximum of the original function.

Thus, or . So the ideal wind speed in front of the blades is three times that of the speed behind the blades, to achieve the maximum possible efficiency.

We can take it a step further and plug our number back in to calculate the maximum efficiency of a wind turbine:





So the maximum efficiency, or power captured versus power available in the wind stream, is 59.3%.

by going through the blades i mean that there is a wind between each blade on the Cd. picutre a CD with blades, and in the space between each blade there is wind. wouldn't this stop the blades

I'm not sure I'm getting what you're saying. You have a hub made from a CD with blades extending from it. Obviously there will be air between the blades. Yes, this will create drag that hinders rotation - but the only way to eliminate that completely is to create the turbine in a vacuum - which would be totally useless.

You're going to have some wind that isn't captured by the blades. As shown before, you want the wind speed behind the blades to be a third of what it is in front of them.

[Flavorflav]

Thank you. That was a very thorough reply.

[unknownman]

I am looking for a very simple blade assembly so that i can test out my motor. Does anyone have an idea?

[Flavorflav]

I am looking for a very simple blade assembly so that i can test out my motor. Does anyone have an idea?

You can get a model airplane propeller at a hobby store for a couple of bucks. You will need to put a standoff between the propeller and the cd and it won't be very efficient, but it will let you test your generator setup and you can make it in a few minutes.

[Greg Doe]

Here's a little more information on what sort of prop to get to test your generator. When you go to the hobby shop you will find props for fuel, gas, electric, and (if you are lucky) rubber power. They will vary in length from a few inches to 2 feet. If they have props for rubber power airplanes, look for one about 8 to 10 inches long. If they don't have any rubber props look for an electric prop in the 6 to 8 inch diameter. Stay away from the props for fuel or gas engines, they are way too heavy.
Last year I made 4 generators for my students. They were all from portable (3 volt) CD players.
I found that for most of them I could pry off the black plastic "drive device" on the end of the motor shaft. Unfortunately on one motor it was so tight I damaged motor. I mention this because it might be easier to mount the model airplane propeller on the motor shaft, than on the CD driver. If you find a rubber propeller the hole in it might have to be inlarged. If you have to use an electric prop than the hole might have to be sleeved down to match the motor shaft. Take your motor-generator with you when you go to the hobby shop, so you can compare your shaft to the props.
Another suggestion: I found one CD player in the school Lost And Found, I found another at the Goodwill Store, I found more at the Flee Market. I also found one at a Pawn Shop, but thew don't fool with them any more, because there is no money in them. The most I paid was $5.00.
The last suggestion is ask questions at the hobby shop if you need help. Electric model airplane flying
is booming, so there is a lot of electric motor and propeller knowledge out there. Physics Lab B/C
is the same application in reverse. Good Luck
Greg Doe
Smyrna, TN

[georgexu316]

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...

I'm assuming that your asking why my "blade assembly" have legs and not why I have legs? lol.
It has legs so it can stand up for itself. Don't ask me why, I just feel better if it does.

[jazzy009]

I'm assuming that your asking why my "blade assembly" have legs and not why I have legs? lol.
It has legs so it can stand up for itself. Don't ask me why, I just feel better if it does.

So...your "legs" are just for testing right?