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Propellers
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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Update: June 2012
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