Of Wind Skirts and Winglets
I'm not sure when I noticed the first air skirt on a truck trailer; maybe three years ago. Now, it seems, you don't see many trailers that don't have these devices. In case you're not familiar with them, air or wind skirts are those angled blades mounted under the trailer that run most of the length of it from the tractor to the trailer wheels. They reduce drag by diverting the air from under the trailer, producing more laminar flow along the sides of the trailer.
Drag accounts for most of the energy required to move a vehicle and air skirts reduce drag considerably. One air skirt manufacturer, Freight Wing, claims a fuel economy savings of up to 7 percent. It doesn't sound like much, but it amounts to about 1000 gallons of fuel a year for a typical tractor trailer. At $3.50 per gallon, that ain't exactly chump change.
What's this got to do with airplanes? Maybe nothing, but maybe winglets—especially active winglets—may eventually become as ubiquitous on some airplanes as air skirts are on trucks. Both devices do the same thing: they save fuel by reducing drag. We ran a story earlier this week on a high school team that's proposed an adaptive winglet that they claim would adjust to various phases of flight to yield up to 10 percent in fuel savings. Their work is not even at the proof-of-concept phase yet, but one winglet design that's already out there is Tamarack Aerospace's active winglet. We've done a couple of videos on this and I flew it at the NBAA convention last month in Orlando.
Winglets do for airplanes what air skirts do for trucks, allowing, through reduced drag, the same speed on less fuel or higher speed on the same fuel. For airplanes, they can also increase climb rates measurably, which means that jets get to higher, more fuel-efficient altitudes sooner. As with a trucks, the fuel savings add up over time.
But that's where the truck/airplane analogy ends. It's one thing to bolt a fiberglass skirt under a trailer, quite another to add winglets to a wing with just enough structure to meet certification requirements, but not any excess to carry the additional bending loads imposed by adding a couple of feet to each wing. Tamarack's solution—a clever one—is to make the winglet active with a small, fast-acting flap that dumps the lift in flight phases where it would otherwise overload the wing structure.
What makes this possible, of course, is modern microprocessor technology that's sophisticated and reliable enough to calculate load factors on the fly and activate the winglet flap in fractions of a second. (In flight, this is quite noticeable. While ailerons move deliberately, the winglet flaps snaps to and fro instantaneously.)
Tamarack sees the principle market for its winglet as retrofits to jet aircraft which can benefit from the improved performance and fuel economy. In some airplanes, it only takes a nudge of a few percent to multiply range and performance considerations. Tamarack is making a kit for the Cirrus SR22, but they picked that airplane mainly as a test bed to prove the certification process. The system is too expensive to make much sense for small aircraft.
When you see such developments, it's natural to wonder why someone didn't think of this before. In the case of air skirts on trucks, I can only surmise that no one got around to the wind tunnel work until fuel prices reached a tipping point. After all, air diverters for the tractor roof have been around for decades. As for active winglets, the technology simply didn't exist to execute the idea. Now that it does, we'll see if it has legs.