The rotor I have built for this can be used for different kinds of airborne wind energy systems. Hence I have decided publish its design and to give it its own name:
MAR-1 / My Airborne Rotor 1
MAR-1 is open source hardware in accordance with the OSHW definition. Feel free to study, make or modify it or its design.
There will be two blog posts. This first one is about the MAR design. The second one will be about how to build a MAR.
Full disclosure: I am an engineer and maker - BUT I have worked in IT for the last quarter of a century hence: If you find any errors – I am sure there are some - or if you have suggestions PLEASE let me know!
Since the tips of a turbine make most of the power the inner part of the blades is mostly obsolete – especially if you do not need it as a structural component to keep the blades in the air or to transmit a force to a hub. You can think of a tip-rotor as an implementation of the "dancing kites" concept - with the kites being connected by a thin strut instead of a control algorithm :)
Number of blades
The smaller the number of rotor blades, the faster a wind turbine must rotate to extract the maximum power from the wind. I want to keep the rounds per minute (rpm) as low as possible – while avoiding gearing for the generator.
My 5-phase Falco generator reaches 150W at 120rpm – so we will work with 120rpm as the nominal speed:
For a small rotor this will lead to a small Tip Speed Ratio (TSR). If we use four blades we can achieve a decent efficiency (cp) with a TSR of 3:
Source FAO IRRIGATION AND DRAINAGE PAPER 43, Water lifting, by P.L. Fraenkel
With 2 rounds per second (rps / equals 120rpm) and a nominal wind speed of 10m/s we will need a rotor radius of 2.4m to reach that Tips Speed Ratio of 3:
With a short chord (6”) and a slow blade we will need an airfoil that works at low Reynolds numbers. I wanted to use the flyingfoam.com service for my blade cores. They can cut any airfoil from the UIUC Airfoil Coordinates Database. In that database I found and selected the FX 63-137 airfoil:
It has been specifically designed for low Reynolds number applications on small wind turbines by the UIUC Applied Aerodynamics group as part of their Low-Speed Airfoil Test program.
MAR-1 design parameters:
- 10 m/s Wind Speed
- 4 blades
- 120 rpm
- Rotor Radius: 2.4m
- FX 63-137 airfoil
The next post will show you how to build your own :)