Hi everyone!
I would be so happy if someone could help my little brother and I answer a question for a science project that we have researched ineffectively for hours. We hope to design the most efficient propeller for a micro-wind turbine. The purpose of this propeller is to naturally rotate when held in the air by a walking person. Thus, disregarding wind, a mere 3mph (walking speed) oncoming air velocity or less must instigate rotation. Below are our design assumptions for such a propeller:
- Propeller between 2”-4” in diameter.
- Designed for easy rotational start rather than fast rotation once turning.
- Thus, large surface area on blades, accepting the additional weight.
- Blades pitched very shallow, and blades as thin as possible.
- Propeller mounted on thinnest bar possible (1/16”) more efficiency.
How correct are we about the above? We think the best propeller design would be very similar to that of the HY mini wind-turbine charger (pictures).
The HY propeller is designed to generate power at a 9mph velocity, whereas ours only needs to rotate freely but at a 3mph velocity. What changes need to be made to the HY propeller design in order to customize it for our purpose? A difference in blade shape or pitch? Please be as descriptive as possible or even better, draw it! We’re determined to win this science contest! (we are not interested in a pinwheel)
Replies
The hardest concepts to grasp in the area of fluid dynamics are the effects of Reynolds Number (Re) scaling. The Re range of operation is the reason airplane propellers (high Re) are shaped quite differently from boat propellers (low Re), for example.
http://en.wikipedia.org/wiki/Reynolds_number
While I have not done much research in low Re aerodynamics (your application is about as low as Re can get in air), the fundamentals suggest very thin, flat "cambered plate" airfoils with wide chords are the solution. In short, the optimum design would probably look like an electric trolling (fishing) boat propeller. Or, dare I say, very much like the pinwheel in which you claim no interest. :-)
Maybe I misunderstood the question though. As far as I know thicker airfoils make more lift eg. ZenithCH701, and high aspect ratio wings are better for slower flight. and sailplanes dont necessarily have thick airfoils, they do have high aspect ratio wings.
I did a search a while ago "propeller aerodynamics free books" and found a pdf about wind turbines, but the math is way over my head and doesn't really mean anything to me, and I couldn't find it just now after a quick search, that was a while ago and back then I was doing alot of searching. (It was the large industrial grade type of wind turbine that are installed on hilltops)
It seems to me that the wind turbines usually have very high aspect ratio airfoils, like sailplanes have high aspect ratios are better for slower flight, but like david said the little handheld anemometers have the same design you are using, aerodynamics can get pretty intense and I would imagine the anemometer manufactures did a very lengthy and professional research and developement to get the final design the way it is. so I think You are definately on the right track.
Many small hand-held anemometers ( wind speed meters) use a spinning "propeller" to measure speeds as low as walking speed/s, but they don't generate power.
here's a google image search of the sorts of thing I mean: https://www.google.com.au/search?q=wind+speed+measurement+anemomete...