Spiroid Winglets – Sculpting Air to Increase Efficiency

It might not seem logical that adding additional structure to an aircraft’s wing could reduce drag and enable it to slip through the air more efficiently. If we allowed a rowboat’s oar to drag through the water or installed a roof rack onto a car, we would create more drag, thereby decreasing speed and efficiency.

However, when aerospace engineering intersects with fluid dynamics, adding winglets to the tips of an aircraft’s wing effectively reduces drag and enhances performance. One of the most fascinating winglet designs, the spiroid, has a captivating story to tell.

Aerodynamically, a winglet is little more than an increase in the span of the wing. When a wing generates lift, a small, tornado-like vortex spills off each tip, creating drag. By extending the wing and tapering the wingtip, these vortices are controlled, are far less powerful, and create a fraction of the drag compared to the originals. They are one of the few examples where adding weight to the structure enables the aircraft to carry more than the weight added.

Winglets first appeared on civil aircraft in the late 1970s on the Gates Learjet 28. As the concept became refined and gained popularity, winglets eventually evolved from the exception to the rule. Today, virtually every new jet aircraft type incorporates them, and with one exception, they’ve all taken on the same form of a simple, upturned tip.

The most notable exception? The spiroid wingtip, created by winglet manufacturer Aviation Partners in the early 1990s.

Unlike traditional winglets, the spiroid is an entirely closed loop. The rationale behind the design is that without a defined tip to the wing (or winglet), no vortices can form, thus eliminating tip vortex drag entirely. Instead, airflow progressing outward along the length of the wing is dissipated cleanly.

Curious to explore how the concept performs in practice, Aviation Partners installed the first set of spiroids on a Gulfstream II in 1993. The company touted a 10% reduction in cruise fuel consumption compared to the standard wing. Previous tests showed a fuel reduction of 7% using traditional blended winglets, suggesting an additional 3% benefit of the spiroid tips.
 

Of course, any additional structure added to an airframe has to perform well throughout the flight envelope, and spiroids are no exception. While it’s important to design them to be as small and as light as possible, a structure that’s too flexible can distort under load. This can introduce potentially catastrophic risks, such as flutter and separation.

The development of structurally and aerodynamically complex wingtips is, therefore, something to be conducted thoroughly and methodically. With significant experience with various other designs, Aviation Partners applied their testing processes to the spiroids and further refined them using computational fluid dynamics (CFD). In 2010, the company installed the new, redesigned spiroids on a Falcon 50 owned by the company’s founder, Joe Clark and debuted them in Oshkosh, Wisconsin, at the Experimental Aircraft Association’s annual AirVenture fly-in.