People are happy that a blade can generate torque in most positions around its circular path because they know that sail boats can manage to sail upwind by "tacking" at an angle to the wind. Everyone accepts that no torque is generated facing directly upwind. People are even more happy that sail boats can sail downwind provided that the wind is traveling faster than the boat. The intuitive understanding breaks down when the blade is travelling downwind and much faster than the wind and yet the mathematicians say that the blade is still generating torque rather than consuming it!
The following diagram shows a top view of this downwind motion scenario. The head wind is produced due to the movement of the blade. The real wind is blowing from bottom to top. When the two winds are vector added to produce the apparent wind, the strength of the wind is less and it is now at a shallow angle to the blade such that the blade is not stalled and will generate lift at right angles to the average direction angle of the apparent wind before and after deflection by the blade. This lift vector can be resolved into two components, one component accelerates the blade forwards and the other component is along the boom connecting the blade to the axis of the turbine.
Why is the lift vector almost at right angles to the apparent wind? The wind is deflected through a small angle by the blade but not very much slowed down because the airfoil has a very low drag. The force to deflect the flow is at right angles to the average angle of the flow before and after deflection. If the deflection angle is small, then this is almost the same as at right angles to the incoming airflow (true angle is tilted backward by a few degrees). The ratio of lift to drag for an airfoil can be between 10 and 100 which is why the drag components do not defeat us until the incoming airflow angle drops below a few degrees. Thus the vertical axis wind turbine generates power at all blade positions except when the blades are nearly aligned with the wind (upwind or downwind).
Now that you know how it works, you will want to build one. Here are some guesses for experimentation.
***UPDATE*** I no longer recommend this laminar flow airfoil due to stall at large angles of attack. Better to use a turbulant flow airfoil such as NACA 0021 which is more forgiving at high angles of attack.
Big commercial eggbeater type turbines may use symmetric airfoils
such as NACA0015 to avoid a pitching moment, but a home built H-rotor
(it looks like an H from a distance) with more rigid blades and booms
can possibly use a cambered airfoil such as NACA 4415. This has a flat
side for easier attachment to the boom and the vector analysis done
shows that using camber or increasing angle of attack will produce
more torque on one half of the cycle than the other but possibly more
overall (See http://home.inreach.com/integener/
for the reasoning behind this.). Also, even if you want a symmetric
profile relative to the wind, the headwind is already actually
slightly curved due to the circular path of the blade and so a
symmetric profile is not
ideal. You will instead need a symmetric profile distorted to fit to a
mean camber line which is a pure circular arc of the same diameter as
the rotor. Here is a table of the %
camber needed for various blade width to diameter ratios.
|Blade chordwidth to rotor
diameter ratio (chord/diameter)
||Needed camber (pure circular
arc) when rotating fast
(0.5/(chord/diameter))*(1-cos(chord/diameter)) *100% (arguments to cos in radians)
Note that these cambers can be significant and are within the range
of conventional cambered airfoils (although standard airfoil camber is
not a pure circular arc). I have my doubts that use of an angled blade
is beneficial since it is already difficult to keep the angle of attack
below stall over the whole cycle and I would need to do a computer
integration round the whole cycle to be convinced.
Meanwhile, I recommend to use a fat symmetric profile such as
NACA 0021 (*** UPDATE *** NACA63-4-021 may have worse stall characteristics than NACA 0021) , with additional pure circular camber (so that it is
symmetric with respect to airflow when rotating fast). The large radius
leading edge on the profile will give a
wide low-drag bucket between Cl of +0.4 and -0.4 and even when
completely stalled and side on to the wind, the large radius leading
edge may deflect the wind to give some starting propulsion. When the
wind is flowing the wrong way (from sharp edge to blunt edge) again the
large "nose" will now contribute drag which is what we want when the
machine is starting up and is below a TSR of 1. The fat thickness
fraction (21%) gives a stiffer blade than the thinner sections such as
Club Cycom's blade design tool is able to produce plots of NACA63-4-021 of any chord width and cambered to a circular arc with camber of 0%, 1.25%, 2.5%, 5% and 10%. Even though the tool is designed for horizontal axis wind turbine design it is possible to vary parameters to get the plot we want. Here is how. Plot section through blade tip and alter lift coefficient until chord is the length that you want it. Adjust angle of attack to get final setting angle of 0 degrees. Ensure that "Draw X as cylinder surface distance" is ticked (true)) and that the airfoil with the right amount of additional circular camber is specified. A Young low drag body 60% laminar flow, 30% thickness could be used for booms and struts. Subscribers will be able to load the scenario darr025.zip to see a plot of NACA63-4-021 pure circular cambered at 2.5% (designated as NACA63-4-021025). *** UPDATE *** use NACA0021025 A screen shot is shown below.
Tip speed ratio will be around 4 or 5 and we don't want the Reynolds number to get too low in low winds so we choose a reasonable chord width.
Again we want to keep Reynolds number high for better attached flow. Few blades means larger chord widths which also mean better strength/load ratio.
Guessing that solidity = 0.1 would be good for an H-rotor TSR in
range 4 or 5. Solidity=Nc/D where N is number of blades, c is chord
width and D is diameter. Centrifugal forces get smaller as the radius
larger so large is good. You will probably need to gear it up to the
generator. If the generator is a dynamo, this can be run as motor to
assist starting. For a 5 m/s wind and a TSR of 4 the rotation is 127
Most designs have a squarish swept area. You can reduce this length if you feel your blades are not light and rigid enough. You may want some struts or wires. Club Cycom's blade design tool will calculate RPM and G-force factors for a given TSR, windspeed and radius. They get very big in high winds.
Foam/fibreglass blade construction for lightness. Unlike the
horizontal axis machines, the centrifugal forces are at right angles to
the blade. Streamline the boom and struts. A double boom, attaching at
0.25 and 0.75 blade length positions would best balance the loads and
resist seesaw type blade movement. For a mono-blade design, you
should still have a balancing boom but with a counter-weight rather
second blade. Motor mode starting might be essential for a mono-blade
however a mono-blade need not be vertical but instead be arranged
slightly helically so that only a small part of the length is in the
dead spot at any one time, the rest of the blade length is generating
torque. In this way it can be self starting.
Vertical axis wind trubines can be hard to stop and can't be yawed
out of the wind. Consider adding a centrifugally operated parachute or
plan for safe auto-destruct in a storm. Note that Cycom has not built
this machine. If you build one and survive, please let us know if it
There are some problems with the above 3 bladed rotor at low
windspeeds of say 5m/s.