How Can We Design the Most Efficient Micro-Wind Turbine Propeller?

[edgar360]

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 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 below).
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 pinwheels due to their looks or anemometers due to their shape)

 

[Mech_Engineer]

Your propellor seems to be on the right track, unfortunately I can't give any useful feedback on the blade shape vs. efficiency. I can tell you that most any blade design will spin with a low enough frictional coefficient on the shaft; if you use super-smooth bearings and low viscosity oil you should be good to go.

 

[Danger]

My concern here is with the 3mph activation speed. That's asking a lot.

 

[edgar360]

It looks like your on the right track.

 

[Aero51]

In this case, more important than blade shape may be blade weight and internal resistance of the motor. Notice that pinwheels are extremely light (as they are made of paper-like products) and have a large blade diameter relative to the hub. A tool which uses blade element theory to design propellers is Javaprop:
<http://www.mh-aerotools.de/airfoils/javaprop.htm>

This tool will provide you with first approximation quality results. This is because the idea behind blade element theory is that each propeller can be divide into...elements! with the thrust generated by each element approximated by:

$T_{i} = CL_{i}(\alpha_{i},M_{i},RE_{i}) / (1/2 \rho V_{i}^2 S_{i})$
Where:
α = angle of attack
M = mach number
RE = reynolds number
CL = lift coefficient as a function of the above three variables
rho = density of air
S = element surface area

The sum of the above equation is the net thrust.