Feedback on Sencenbaugh's O2 Delight: Windmill Blade Design

A reader in Romayor, Texas questions the efficiency of the windmill blade design Jim Sencenbaugh used in his O2 Delight windmill. Jim Sencenbaugh explains his decision and comments on current and future refinements he's planning.

| July/August 1973

  • pix2258a
    Top illustration shows how the lift vector of an airfoil positioned with its curved face to the wind impedes windmill rotation. Bottom illustration shows how the lift vector of an airfoil positioned with it's flat face to the wind aids rotation.
  • pix2258b
    Illustrations showing shape of pressure envelope around an airfoil and its relationship to effective pitch angle.

  • pix2258a
  • pix2258b

I think that the blades an the Sencenbaugh 02 Powered Delight are mounted with the curvature of the airfoil in the wrong direction. My illustrations are intended to demonstrate that if the blades are mounted as shown in the article, the lift vector of the airfoil—which is perpendicular to a chord drawn between the leading edge and the trailing edge—will have a component opposite to the direction of rotation. The purpose of the fan is to extract energy from the wind by turning its movement into rotational motion of a shaft. Since rotation is what you're after, it seems counterproductive to have the lift of the airfoil pulling against that very action.

If blades are mounted with the curved side away from the wind (that is, the wind coming from the top of the page in my sketches), the lift vector has a component working in the direction of rotation. I wish I could explain that a little less pedantically and a little more poetically, but I said it as clearly as I could. Take a look at some store-bought wind chargers and windmills and see if I'm not right.

author unknown
Romayor, Tex.

The drawing in MOTHER EARTH NEWS is a correct and accurate representation of how the blades were originally built. It was my intention initially to attempt to make the blades self-limiting ... hence the configuration as illustrated. The effective pitch angle was set at 15-16 degrees from the chord line, which provided good low-speed response and sufficient rpm for charge.

Since the article was written and the plans drawn, we have been using a new prop made of laminated birch-ply with the airfoil (retaining the "Clark Y") on the conventional side and with a twisted effective pitch (15 at root to 3 degrees at tip) along its length. The performance, naturally, is greatly improved. I've been sending out supplementary plans (at no extra charge to the buyer) along with instructions on how to rewind the existing alternator to obtain charge at lower speed. With the new prop and rewound alternator we are now obtaining charge in winds of 5 to 6 mph as compared to the 8 to 10 mph wind previously required. Therefore, I'm highly recommending these additional improvements to people now building a unit.

Also, the following modifications to the present plans were made:


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