The defense contractor McDonnell Aircraft won a contract to design, build, and test a prototype giromill from the Energy Research & Development Administration in the mid-1970s.

The contract and McDonnell Aircraft’s participation were part of the infamous “top-down” development model favored by US government researchers and the aerospace community. After all, this model had worked to put an American on the moon, why wouldn’t it work with something as prosaic as a wind turbine.

McDonnell won its bid with a giromill, or articulating, straight-bladed Vertical Axis Wind Turbine. To the uninitiated—even today—the design appears novel, though Georges Darrieus had patented the concept with other forms of what we now call “Darrieus” wind turbines in the 1920s and it probably wasn’t original with him either.

Unlike Darrieus turbines that use fixed pitch blades that are rigidly attached to the torque tube, giromills or cycloturbines varied the angle of attack of the blades as the rotor revolved. The angle of attack was controlled by a pitch linkage coupled to a wind vane either mechanically as in the Pinson Cycloturbine or electrically as in McDonnell’s version.

If my memory serves me correctly, McDonnell’s giromill was rated at 40 kW. It was not a small machine. The rotor was 42 feet x 58 feet or in more common units, 12.8 meters tall by 17.7 meters in diameter. Thus, the rotor swept 227 square meters. The tower was about 70 feet tall or 21 meters.

With a standard power rating of 200 W/m² McDonnell’s giromill would be rated at 45 kW. Consequently, McDonnell’s turbine and in fact all others in the research program were conservatively rated.

At about the same time, designers were taking the same approach in other countries, notably Denmark. And in the US, both Mike Bergey of later Bergey Windpower fame,  and Herman Drees of Pinson were also experimenting with the same concept.

However, McDonnell’s attempt stood out from the others for its over-the-top hype and Chutzpah. Not that it was the last developer of this “new” concept to be bitten by the “We’ve beaten Betz” bug.

According to McDonnell’s statements of the day, they had “proven in wind tunnel tests” that their giromill had beaten the Betz limit. In the 1970s as now, this was big news. In theory this meant that it would perform better and, hence, be more cost effective than conventional wind turbines.

Alas, it was not to be. Though it was long ago and I don’t remember the details now, but it became clear that the turbine didn’t perform as advertised. It turns out in hindsight that the engineers made a fundamental error during their “wind tunnel tests”. They forgot, or didn’t know, about the effects of the wind tunnel’s wall on a model that was too big for the chamber. In effect the wind tunnel acted as a giant funnel giving erroneous results which the McDonnell engineers gleefully reported.

This story may be apocryphal but for whatever reason the prototype—along with all the others in the program—never was commercialized.

Since then others have continued to try to commercialize giromills, but to date all have failed. As with most giromills the struts, stays, and attachments points all add power-robbing drag to the rotor. None have lived up to their expectations.

McDonnell 40-kW Giromill wind system: Phase I. Design and analysis. Volume I. Executive summary

McDonnell 40-kW Giromill Wind System. Phase II. Fabrication and test

McDonnell 40-kW Giromill Overview. SERI/TP-35-263, 6046045