Science Buddies: "Ask an Expert"

I'm doing a variation to the project wind power -Is bigger better?- June 13,2008. My question is if I'm testing a 3 blade turbine, and a 4 blade turbine with different wind orientation sources would my variable be the different wind source orientations, or the different blade number? What would my control be?
I will be testing the amount of energy produced, as work done, by lifting a load and taking measurements.
I set up my project as follows- im using 2 carton containers one for each turbine, wooden skewers, thread, paper clips(this is the load) to be lifted.
I made 2 different blades from paper- 4 blades, and three blades. My wind source, hair dryer, will be positioned at 15, degrees, 45 degrees, 60 degrees, 90 degrees, 120 etc.

Hello and welcome to the forums!

It seems like the variable you're testing is the number of blades. You are comparing their results versus the direction of the wind. Your control will be defined by your hypothesis. For instance, you might state that "I believe the 4 blade model will be better than the 3 blade model." In this case, the 3 blade model is the "control" (loosely speaking.)

One of the things I look for in this type of experiment (as a judge) is whether the student controlled for mass and blade area. In other words, say you find the 4 blade works better. However, the 4 blade model presents (possibly) 33% more blade area to the wind. Also, it may be 33% more massive, which might slow it down. Have you controlled for this?

Also consider a 2 blade model - please search the web for information about the number of blades vs. efficiency - there's a lot of information out there. Let us know how it works out for you!

Keith

Dear Keith,
Thank you for the speedy response to my question. I'm not sure how to control for mass and blade area. Im using the same kind paper for both blades. My 3 bladed pinwheel measures 7 3/8 in by 7 3/8 by 7 3/8, my 4 bladed pinwheel measures 5 inches all around. Thank you

hi keith, I have redesigned my blades so that the 4 blade and three blade are both cut 6 by 6 by 6 using the same paper. Does this mean then that the swept area is constant?

Hello again:

I was assuming you were using separate, discrete blades, which would have a width and length (WxL=area), but it would be different with a pinwheel. I'm not sure what this means:

My 3 bladed pinwheel measures 7 3/8 in by 7 3/8 by 7 3/8

- I found 2 different ways to make a pinwheel out of a square sheet of paper, http://www.enchantedlearning.com/crafts/pinwheel/ and http://www.pinwheelsforpeace.com/pinwheeltemplate.html - in either case, there are 4 blades. With a pinwheel, I guess the problem is that the area is not a nice rectangular pattern that's easy to measure? Can you unfold the pinwheel and divide the area up into geometrical shapes you can measure (such as triangles and squares) and then add up all the shapes? You would want to then add up all the blades - for instance, the 3 blade one is the blade area times 3, the 4 blade times 4. (The same goes for mass.) HTH.

Keith

Hi keith, I'm not sure what to do at this point. I still dont understand the control and variable for this project. I will give the details of how I made the paper pinwheels. The three bladed paper pinwheel template I used is from www.mste.uiuc.edu/projects/wind (appendix d ). The 4 bladed pinwheel is from there also, and is on the page template for a blank pinwheel.

When I determine the area of the blades,yes they are different. As for the mass I'm not sure what you mean? So how does this help me determine the control and variable? thank you

Hi:

I looked up your reference, and the 2 triangles had these measurements: 3 blades, each side is 20 cm, 4 blades, each side is 12 cm. Using those measurements, the 3 bladed pinwheel has an area of 173.2 cm2, the 4 bladed 144 cm2. If you use the same weight paper to make each, then the 3 bladed one has about 20% more area and mass. You could shrink it by 20% and make one with a side of 16.6 cm, and that way have pinwheels of the same mass.

Pinwheel blades are rather complicated geometries, so it's hard to know what the blade area is. I would say you're safe to use pinwheels of similar masses for comparison.

From your previous post, I thought you were comparing the number of blades. That would be the variable. See the previous post for my comments on the control.

Keith