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Name: Melissa and Victoria
Status: Student
Grade: 9-12
Location: N/A
Country: United States
Date: February 2007


Question:
We are studying propellers, and trying to determine the most efficient propeller designs. For our project, we altered two aspects of the propeller, the chord length (width of the blade) and the diameter.

We used three different propellers for the two experiments, and calculated which ones generated the most voltage. Our results showed us that the wider the blade and the longer the blade the better. We have done research, but we need to elaborate on our analysis of the experiment, and have trouble understanding the results.



Replies:
Melissa and Victoria,

My first comment is to ask how "voltage" has anything to do with propeller design! It would be helpful for you to explain more fully what you are doing in this experiment. First, with reference to your puzzling comment on voltage, are you assuming water or air flowing over a propeller, and the propeller is driving a generator? If so, you need to be aware that voltage is not power. Merely spinning a generator faster will usually generate a higher voltage, but that just means the propeller is spinning faster. It does not necessarily mean it is producing more power.

You state that by increasing the blade width and the blade length (thus the blade area), the propeller works "better". This actually seems intuitive, since going the other way, gradually reducing blade area will eventually result in no blade at all, and no power output.

I note also that you mention you are "calculating" the results. Something as highly complex as fluid flow interacting with a propeller is extremely difficult to calculate. Generally, it is necessary to use highly computer-intensive Finite Element Analysis (FEA) software to get accurate results. If you are using more simple methods than that, I would suggest your calculations may not be giving you useful results. You are absolutely right to question purely calculated results of something so complex. Too many engineers have been led down the proverbial garden path by blindly trusting calculations without doing a "sanity check" on them, or trying (as you are clearly trying to do here) to get a "gut feel" for the basic mechanisms involved.

Regards,

Bob Wilson.



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