Non-Traditional Blade Shape

[MattM]

I'm curious if anyone has produced non-airfoil blade designs for their windmills.  It seems airfoils are designed to produce lift which would add stress to the blades.  By using shapes from mother nature, like the shape of the bird body, wouldn't you retain your same results with less twisting?  The general shape of the bird's body helps it maintain a level position in flight with some lift but it is more optimized to minimal drag.  And that seems to be ideal, to have minimal drag as the rotor rotates.  Has anyone used such a shape for their blades with a generator attached?



If I understand this correctly, this would be sort of a Reflex camber.

[Adriaan Kragten]

A problem with all airfoils is that the drag/lift ratio depends very much on the Reynolds number and that below a Reynolds value of about 10^5 all common airfoils become bad. The wings of birds have to do with this same effect and therefore wings of large birds look different as wings of small birds and especially wings of insects. A way to make that an airfoil performs also well at low Reynolds values is to make the boundary layer turbulent. This can be done by adding a thin wire in front of the airfoil nose or by making the nose sharp. The turbulence has a mixing effect and this prevents stalling at low angles of attack. A turbulent boundary layer normally has more drag than a laminar boundary layer but the drag is much lower than for a stalling airfoil. Certain bird wings can change the shape or the roughness of outside such that turbulence is created and this makes that the wings also work at low flying speeds. These kind of effects are also used to minimize the drag of swimming and skating suits and of golf balls.

[SparWeb]

Hi Matt,

There are modified leading edges to airfoils - but it goes in a different direction than you might think:



These are "Slats" added to the leading edge of aircraft wings.  They permit much higher angles of attack and help boost the lift coefficient for landing.
However, they add drag.  For normal flight, they need to be sucked back in, to make the wing as smooth as possible. 
They also add complication to the construction of the wing - just like they would for construction of a wind turbine blade.

Keep in mind that an efficient "anything" does not need to look crazy.  Wings, helicopter blades, wind turbine blades, insects, seeds from maple trees... 
It only needs to be optimized for its normal working condition.  You only design for extremes if the machine has to work in the extremes. 
Things to optimize are like best lift divided by lowest drag (L/D ratio) or some other proportion that balances performance and cost.

[MattM]

But doesn't our Reynold's number for a HAWT needs to be realized in two dimensions?  The resistance of airflow is not only in the plane of the blade, there is a second movement of air that is perpendicular to the path of the blade.  An aircraft wing is trying to equalize lift with gravitational forces whereas the windmill blade is not concerned whatsoever about lift.  Modern airliners have increasingly moved to supercritical airfoils because they can generate good lift with as small as a 3 degree angle of attack.  But since all lift is drag, I'd logically conclude that creation of lift on the blade is at the expense of energy that could otherwise be pushing the blade.  The bird shape is a reflex camber shaping which actually has poor lift qualities and is more of a cruise profile.  Reflex curves self trim, which means it would resist blade twist.  It also results in an increase of drag over a conventional camber when at an increasing angle of attack because there is a corresponding larger increase of the effective leading edge drag.  But that self trimming property means it is going to resist any increase in the angle of attack in order to get back into its horizontal position.  The main reason I'd think the bird shape would be ideal is that it doesn't impede air moving below the blade, allowing that second direction of airflow to effect one side for turning the rotor and do minimal interaction with the back side of the.  Conventional airfoils are going to interact with that leading edge which should slow down the air transiting from the front to the back of the blade.  Conventional airfoils of the same camber height with any pitch would have a higher drag profile on the black of the blade.  I'm not trying to say I'm right, you're wrong because this is purely my own personal speculation.  I'm interested in seeing if anyone has explored this avenue yet.  Splitting a board on a table saw would be easy enough, but my router is a little small to make this shape.

SparWeb-

Fowler flaps are about the nicest form of flap when you want to increase lift with the least amount of drag.  But AFAIK we really don't want lift in the case of a HAWT or am I missing something?

[Adriaan Kragten]

But doesn't our Reynold's number for a HAWT needs to be realized in two dimensions?  The resistance of airflow is not only in the plane of the blade, there is a second movement of air that is perpendicular to the path of the blade. 

The relative wind speed W is the vectorial sum of the speed of the blade U and the axial speed of the wind through the rotor plane (which is 2/3 V for the maximum Cp). The angle phi is the angle in between the relative wind speed W and the rotor plane. The lift L is perpendicular to W and the drag D is in the direction of W (see public report KD 35 figure 3.2). Both L and D can be resolved into components in the rotor plane and components perpendicular to the rotor plane. The nett component in the rotor plane in the direction of U (see figure 4.4) gives the driving force Fu (see formula 4.13) and this force multiplied with the speed gives the power. The nett component perpendicular to the rotor plane give the rotor thrust T (see formula 4.14). As W is used for the determination of the Cd/Cl ratio, the axial speed through the rotor plane is already taken into account. So there is no second movement which adds more lift or drag. The Reynolds number is also determined for W (see derivation of formula 5.5 at page 30 of KD 35). The influence of the axial flow is given by the factor 4/9 in formula 5.5.

[SparWeb]

Hi Matt,
I've seen this happen before - you start trying to chase each air molecule along its path and figure out where it's going to go next...  it's easy to get down in the weeds since the development of airfoil shapes is such a deep deep subject.  There's a lot to read about the subject, but it's not often prioritized in a way that helps keep perspective.

Instead, ask a simple question from personal experience - what makes it possible to gently throw a frisbee, a playing card, a bowler hat, a boomerang, or a paper airplane and hit the far wall of the room?
Surely the shape isn't the biggest factor because a boomerang doesn't look anything like a hat.
Your shape looks like the cross-section of a paper plate, and I'm sure a paper plate can fly across a gymnasium and hit the far wall if you wanted it to.

Anchoring what you learn with the facts you know and that make sense in personal experience helps prevent misunderstanding and confusion.  That way you can avoid being misled by stuff like "Reflex curves self trim, which means it would resist blade twist." which isn't true, doesn't make sense in a wind turbine context, and hints that you're reading stuff from airplane model builders who may be confused too.

You're definitely right, that fowler flaps have no place in wind turbines, just like slat's don't either.

Here's another avenue to investigate, hopefully that will help focus attention on what's most important: where is the flow attachment point on the leading edge?  If the LE is round, the attachment point can change as the angle of attack changes.  If the LE is sharp, there's only 1 spot for it to attach to.  Look at the profile you proposed again, with this in mind - it's the second case.  What are the implications of that?  How would it behave at negative angle of attack?  At high positive angle of attack?  Would the same conclusions apply to a WT blade made from a plate of steel? 

[MattM]

Actually I was reading a paper section linked in an aviation forum.  The discussion was about Boeing and the possible ways to improve the BWB design.  The reflex camber was in the paper and they were talking about moving wings forward and other changes.  Back in the forum the B-2 shape was brought up.


The participants are way above my head with the aircraft designs, but since of what they talk about does make sense when it applies to certain content.

[SparWeb]

Those two bodies aren't related.
The B2 bomber doesn't flap its wings to generate thrust, nor does it change its wing dihedral in a turn.
Sure, an aviation forum will make sense in its own context, but wind turbines are a different context.

A much better direction for improving WT performance is drag reduction.  Surface finish and build quality are strong drivers there, and these details are where the home-builder of a WT will reap the greatest reward.