The NoDrag Flap Wind Turbine Blade Mod
Blade Cross Section with the Approaching Relative Wind at a TSR = 5
For example, a blade tangential speed of 150 mph and a wind speed of 30 mph
Now look at what is happening. The attack angle is so small that even with a wide blade the approaching airflow sees ahead of it or "faces" only a blade dimension that is equal to "1" in the diagram. To widen the blade for more airflow to face it would take a much larger blade chord dimension with little effective gain in blade faced by the airflow. The idea herein is to keep blades narrow and yet provide plenty of airflow deflection as if the blades were wider.
But if another blade element were to be added beneath the trailing edge as is shown above, much more effective blade width would be gained with little cost or increase in actual blade width. The blade dimension that is faced now would be equal to "2" in the diagram, which is 57.5% larger than "1".
This also makes the blade effectively 57.5% wider in encountering the wind without making the blade actually wider in this case. More wind is deflected and greater power is produced. Included in this website are evidences from testing and practice of successful implementation with improved performance of second element mods such as this to blades.
That May Bear on Wind Energy
Drawing From Airfoil Theory vs. Photo Of Actual Streamlines
Drawing From Wind Blade Patent Application vs. Photo Of Actual Streamlines
Added to this is the missing fact that the airflow velocity is not only increased above the airfoil near the leading edge as indicated but is increased also beneath the airfoil near the trailling edge, which causes a lowered pressure there as well.
Corrective measures are sometimes being considered. The vortex generators introduced on the downwind sides of blades have the purpose of increasing the Coanda Effect. However, experience has shown a relatively small effect gained. A possible solution is offered by a second blade element. The element may be a thin flap added under the trailing edge as superimposed in the photo - as is - on the left.
In the image on the right, a better view of the potential effect of this second element is gained. An approximate conversion of the image to the Earth Frame of Reference is obtained by means of skewing. The flow attack angle ahead of the stagnation point then is fixed at minus 90 degrees beneath the airfoil to eliminate the blade tangential velocity vector. It becomes quite clear that the additional element beneath the airfoil moves to the rear providing more effective blade width as shown. If the flow streamlines were adjusted to suit, a better deflection of the entire trailing flow downward and closer to the blade chord line would be seen. Greater airflow deflection, of course, means more power production.
Answers such as this are needed. Small one kilowatt turbines using double element blade concepts have been a remarkable success. Shows to go (!).
The theory which sets the flow lines to follow the airfoil surfaces closely and leave the trailing edge smoothly following its direction is found in early aviation theory publications¹ (page segment copied in "fair use" on the left for review purposes).
What is happening here is that a "clockwise flow circulation" has been postulated around the airfoil profile. The strength of this circulation, termed "gamma" and notated by the Greek letter Γ, is being determined by how well the flow thereby adjusted deflects sufficiently to leave the trailing edge smoothly. This has been given a name: "The Kutta-Joukowsky Condition".
The value of gamma found thereby is seen as the last line in the analysis. But the underlying assumptions have made this analysis superficial and unwarranted. The flow has momentum that overrides any circulation present. The actual value of this parameter can not be found in this manner. In fact, the actual value has never been adequately studied and found as is presently known. There is some deflection occurring but how much is a more complex problem than this. Suffice it to say that this logic has been given more credence than is justified. The evidence from practice now demonstrates that the "Kutta-Joukowsky Condition" is not a valid deflection theory hypothesis. Wind turbine blade design is better approached without it.
And yes, small wind turbines can sometimes readily and quickly cover gaps in the theories. Small is beautiful (!).
¹ From page 52 of Theory of Wing Sections by Abbott and von Doenhoff, 1959, ISBN 486-60586-8, Dover Publications Inc., Mineola, NY.