I have a problem with rotor design philosophy

[rotornuts]

Whenever I post to discussion boards I'm compelled to add the standard I may be nuts disclaimer so there it is.

When describing the interaction between rotor and wind an airplane wing a propeller, or a helicopter rotor are often cited as examples. The leap is then made to wind turbine rotor and although I acknowledge most people recognize something of a difference, I can't help but feel an error is being made. Rotor design philosophy changes the apparent wind direction according to the rotor speed and this seems the result of an oversight that a windmill blade is being driven by the wind. the applied force is the wind and it escapes me to understand how the windmill blade could rotate faster than the wind speed in an efficient manner. Intuition tells me that tip speeds in excess of wind speed are the result of the inner portions of the blade doing the work at higher wind speed leading to drag and poor efficiencies. I've scoured the net for answers but can't accept this.

any thoughts?

[stop4stuff]

hi

take a look at the basic blade desing page here;

http://www.windmission.dk/workshop/BasicBladeDesign/bladedesignleft.html

and particularly this page showing the vleocities in the rotor plane;

http://www.windmission.dk/workshop/BasicBladeDesign/velocities.html

The way I understand blades is;

The back portion of the blade sees a higher wind velocity than the front and that higher wind velocity is responsible for the additional rotor speed due to lift gained from the backside of the blade in the direction the blade is traveling...

...pretty much like a sailboat tacking across the wind can go faster than if just being blown along by the wind.

Someone tell me if i got it wrong.

paul

[silverbug2004]

rather than thinking that the wind is pushing the blades, think in terms of the shape of the blade creating a lower pressure area which sucks the blade in a forward motion.

[JF]

Dear Rotornuts and other friends

As others have mentioned kindly study the above suggested files to be found on designer and millwright Claus Nybroe's site:

http://www.windmission.dk

However also consult the very comprehensive and good download file to be found on the same site - concerning the design theory, construction and operation of a modern BONUS wind turbine.

http://www.windmission.dk/workshop/specialissue.html

With best wishes and greetings - JF

[skravlinge]

The lift (the force from low pressure on the backside)  will not  be from the beginning. it is drag which start the blades. When the lift has force the wind is very much faster close to the airfoils than  the normal wind speed. This result in a higher speed than the normal wind. A darrieus windmill works only on lift, and can normally not start by itself. A Savonius  use  only drag as main force, the reason it is good in slow winds but is not very well in higher winds.

On plane drag is the resist to that the driving propeller have to overcome.

it moves the plane forwards, but it will not fly until condition of lift is present on the wings, so much it can lift the plane. An autogiro must start like a plane but can land more like a helicopter, as it has rotating wings and the lift decrease with the speed and the angle of rotors. Helicopter propellers are wings  and as they move themselfves, lift can be to move it upwards, a plane has to move  forward to reach the same result.

Windmills not fly away we hope.

[scoraigwind]

I have to disagree with the statement that the drag force is useful for horizontal axis wind turbines.  As far as I understand it, this is a misconception.

Lift is defined as the force acting at right angles to the local wind direction (wind and 'headwind' due to motion).  Drag is defined as the force parallel to the local wind vector.  This local wind is usually refered to as apparent wind.

When the rotor is stationary, the wind vector is parallel to the shaft of the machine (if correctly positioned).  Drag will push the blades straight back (yes and not at any sort of angle).  This doesn't come from any analysis of flow or pressure it just comes from the definition of the drag force.  This force cannot propel the blade in any way since the blade cannot move back - only sideways.  The lift force is sideways but very small, because the blades are almost flat-on to the wind, and deeply stalled.

As the rotor begins to move it experiences a headwind that rotates its apparent wind to an angle with the plane of rotation that is less then 90 degrees. The angle reduces with increasing speed and especially toward the tip where the speed is highest.  Now the drag force is totally counterproductive.  The lift force mainly produces a bending force/thrust force pushing the blade back (and, by reaction, the wind forward, slowing it).  However there is a small part fo the lift force that actually drives the blade.  This has to be greater than the drag force if we are to get any power.

High speed machines do not have any sort of monopoly over lift.  Farm pumping windmills also use lift (they have to by definition of the word!)  The big difference is that the high speed machines have much better lift/drag ratio due to their more sophisticated airfoil shape.  They have to or they could not run so fast.

As for the original question - yes it seems wrong intuitively, but so do many thigns that work.  and it certainly does work.   This is a billion dollar industry worldwide and it produces some of the cheapest eletricity on the planet.  So they must be doing sometjhing right.  And all the big machines have blade tips that move many times (6-8 times) faster than the wind.  Count the revolutions and do the sums yourself.

[rotornuts]

 O.K. I was rolling around in bed trying to fall asleep and considering the problem of rotor vs. wind speed when the answer dawned on me, leaving me feeling somewhat silly. Although my methodoligy for reconciling that problem leaves me still considering the appearant or resultant wind direction. I read the bonus paper and have seen the same info from other sources with regards to blade areodynamics (although nowhere nearly as well explained) but I still remain somewhat confounded by the headwind effect. Perhaps I should go to bed again.

[skravlinge]

If you read the analogy with the areoplan I defined drag as something resist, and the lift something lifting the plane up ,not added a force in the direction of or  opposite to the drag force.

Drag can be used to move a rotor if it is of such type.

Without drag there is no lift  so if you want to eliminate drag, no lift can start to be created. If we talk about propelled type HAWT I agree fully with you, I do not know if you misunderstood my not fully English. I did not mean drag moves (in the rotating direction) the propeller, but it needs drag to produce the lift at start, and lift is lift in  practical when it is so strong it moves the rotor, until that drag is a force helping the propeller. On a darrius you can add drag until it breaks, no lift will come, A darrius need help to produce so much lift so it can spin up fully by it self.

[Mentally Moribund]

Ok, I didn't have a problem with my rotor design except whenI glued it all together.  The next day I realized I put the wrong side facing forward!  How much do you you guys think this will effect my performance?

[troy]

If you have your blades in backwards (flat surface downwind) your performance will be BAD.

And yes, the key to understanding the lift performace of a windmill rotor is to understand the apparent wind direction and speed as perceived by the blade.  If the blade tip is trabvelling at 100 mph (easy, often much faster on a 10' rotor) then it must be experiencing approximately a 100 mph "headwind" as it sees the wind. This is true even if the true windspeed is only 10 mph.  

Really, it's pretty easy vector math using basic trig to discover the resultant or perceived windspeed from the "real" wind combined with the "artificial" wind generated by the rotation of the blade itself.

That's why good blades MUST have twist, with the steep part of the blade at the root and the shallow, or almost no pitch portion of the blade out at the tips.  The root rotates at a very slow rate in absolute terms of mph, thus very little "artificial" headwind generated, thus much different perceived wind speed and direction at the root vs the tip of the blade.

Good luck and have fun!

troy

[hvirtane]

I think that in practice it is easiest to think

in the terms of air pressures.  

The curved side of the blade will force

the air to travel faster on that side

of the blade than the flat side.

That faster air travel will cause a lower

pressure on that side. That will cause

the lifting force.

The lifting forces with the blades

work basically the same

way as the forces of the sails

of a sailing boat.

The faster the blade is moving the smaller

the pitch angle should be. You can

understand this idea easily if you'll

draw some pictures with the real

wind speed vector, the apparent wind speed

vector and the blade speed vector.

- Hannu