Darius... how does it work?

[voltimorte]

I have 1 HAWT generator up and I understand the way it works. But I really need to

build VAWT's because the wind changes very frequently and sometimes doesn't blow strong enough to get the HAWT moving. I fly RC planes and copters but I have no idea of what makes the Darius design turn in the wind so I can design an appropriate airfoil. I've seen some comments on using dual camber profiles but that would seem to give equal forces to both sides as air passes through. I would think you would want a flatter side on the out side and a rounded side on the inside.

I sure would appreciate a short answer on how airflow around the airfoil forces the blade to turn. I am planning to make the blades out of 10 ft pieces of wood re-enforced by fiberglass and carbon.

Thanks in advance for any help.

[vawtman]

If the airfoil is curved for the rotation the turbine starts easily.Short has i can get.

[Ungrounded Lightning Rod]

Short answer:

• The rotor is a symmetrical airfoil with the chord at right angles to the radius.
  
• An incoming airstream at a range of angles of attack will "attach" to the airfoil at the rounded leading edge and leave at the sharp trailing edge.
  
• The air leaves straight back along the chord.  If it also came in parallel to the chord you just get drag.  But for other angles of attack you've also bent its path, changing its momentum and applying a reaction thrust to the blade.
  
• The change in momentum is transferred to the blade, appearing as "lift" on the curving front portion.  For a significant range of angles of attack the lift force exceeds the drag force and you get net mechanical power.
  
• Split this lift into radial and circumferential components:  The radial component stresses the blade against its structure and the circumferential component pulls it forward.
  
• The range of angles of attack where the air attaches is small and the range where you get net gain is still smaller.  So when the rotor is stopped there are only a narrow range of positions where it will find thrust to attempt to start it.  Thus Darrieus rotors are usually not self-starting.
  
• But the airfoil "flies in the APPARENT wind", the vector sum of the real wind with the wind from its own motion.  Once it's spun up - to a tip speed significantly above the wind speed - it "sees" the wind as coming from a direction that oscillates around the direction of its chord.  The air stays attached and the rotor gets net power from the wind through most of its rotation, far more than it loses when the blades are running nearly straight into or away from the wind and it experiences more drag than forward lift.
  
  

The downside to Darrieus designs is that they are somewhat lacy and hard to support against the radial forces, the radial forces cycle in direction with every turn (fatiguing the material), and the rotation is very fast so the centripital forces are also very strong.  This tends to lead to mills that come apart violently in a storm after a few years, vibrate a lot, and otherwise are hard to make reliable.

Upside is a Darrieus (like a HAWT) can get pretty close to the Betz limit.  But a Sandia Savonius gets to about 2/3 of Betz, has a tip speed somewhat less than the wind rather than maybe 6 or 7 times as fast, and is MUCH easier to make big and strong so scaling it up by 50% to match a Savonius might be easier.  Thus that, or structurally stronger hybrid designs such as the Lenz, are more popular with some of the WAWT-fans on this board.

[Ungrounded Lightning Rod]

... when the rotor is stopped there are only a narrow range of positions where it will find thrust to attempt to start it.  Thus Darrieus rotors are usually not self-starting.

This applies to the straight-up-and-down blades.  The ones that spiral will have a thrust-producing region somewhere on the blade at all times and thus self-start a lot better.

[Ungrounded Lightning Rod]

(as vawtman pointed out)

Helical blades also reduce the vibration issue considerably.  Do it right and the torque is independent of the angle of the rotor versus the wind, rather than cycling with position.  Also:  Though the forces on the blade do vary they do that by having strong regions that move up or down the blade smoothly rather than having a strong cyclic force on the whole blade at the same time.  And three blades have the forces at a given height tending to average out, where with two blades they'd cycle in unison.

[voltimorte]

Thanks for the technical lecture... now I got a better picture of what I need to do.

A symmetrical airfoil is doable but a big pain in the butt to make on the scale I was thinking for a test. I may have to go sit under the apple tree and re-think this.

[vawtman]

Here's a pic of the profile i use on my Giromill

[voltimorte]

vawtman

Thanks for the pic of the profiles. Do I assume the notches are where you have supports?

bruce

[vawtman]

Hi Bruce

 The top one is for an 8ft blade and the bottom one is for a 4ft.On the 8ft the slots support a 1x4x8 and a 1x2x8 pretty simple.The 4 profiles are cut from one 1x6x8.The spar is then wrapped with trimcoil after things are glued and screwed.

 Mark

[voltimorte]

Thanks Mark..

I might have a go at this when the weather starts to get wet and I m stuck indoors.

bruce