How Things Work: Winglets You know those things on the wingtips of airliners that stick straight up? This is why you’re seeing more of them. British Airways Boeing 747 with winglets. (Adrian Pingstone) By George C. Larson Air & Space Magazine | Subscribe September 2001 Winglets reduce wingtip vortices, the twin tornados formed by the difference between the pressure on the upper surface of an airplane's wing and that on the lower surface. High pressure on the lower surface creates a natural airflow that makes its way to the wingtip and curls upward around it. When flow around the wingtips streams out behind the airplane, a vortex is formed. These twisters represent an energy loss and are strong enough to flip airplanes that blunder into them. Winglets produce an especially good performance boost for jets by reducing drag, and that reduction could translate into marginally higher cruise speed. But most operators take advantage of the drag reduction by throttling back to normal speed and pocketing the fuel savings. Several airliners use them. The Airbus A319 and A320 have very small upper and lower winglets. The longer-range twin-engine A330 and four-engine A340 have conventional winglets, as do Boeing 747-400s. Aviation Partners, a Seattle, Washington company, has a new design it calls a "blended" winglet. The Boeing Business Jet (opposite, top), a derivative of the Boeing 737, has a set of the firm's eight-foot winglets with a curving transition from wing to winglet that is characteristic of the company's design. In 1976, shortly after an energy crisis sent fuel prices skyward, Richard Whitcomb, a NASA aerodynamicist, published a paper that compared a wing with a winglet and the same wing with a simple extension to increase its span. As a basis for comparing both devices, the extension and the winglet were sized so that both put an equal structural load on the wing. Whitcomb showed that winglets reduced drag by about 20 percent and offered double the improvement in the wing's lift-to-drag ratio, compared with the simple wing extension. The aspect ratio of a wing is the relationship between its span and its chord-the distance from leading edge to trailing edge. A U-2 has a high aspect ratio; an F-104 has a low one. A wing with high aspect ratio will provide longer range at a given cruise speed than a short, stubby wing because the longer wing is less affected, proportionally, by the energy lost to the wingtip vortex. But long wings are prone to flex and have to be strengthened, which adds weight. Winglets provide the effect of increased aspect ratio without extending the wingspan. One rule of thumb says that for an increase in wing-bending force equal to that of a one-foot increase in span, a wing's structure can support a three-foot winglet that provides the same gain as a two-foot span extension. The airflow around winglets is complicated, and winglets have to be carefully designed and tested for each aircraft. Cant, the angle to which the winglet is bent from the vertical, and toe, the angle at which the winglets' airfoils diverge from the relative wind direction, determine the magnitude and orientation of the lift force generated by the winglet itself. By adjusting these so that the lift force points slightly forward, a designer can produce the equivalent of thrust. A sailboat tacking sharply upwind creates a similar force with its sail while the keel squeezes the boat forward like a pinched watermelon seed. If winglets are so great, why don't all airplanes have them? Because winglets are a tradeoff: In the highly visible case of the 777, an airplane with exceptionally long range, the wings grew so long that folding wingtips were offered to get into tight airport gates. Dave Akiyama, manager of aerodynamics engineering in Boeing product development, points out that designing winglets can be tricky-they have a tendency to flutter, for example. "We find that it really doesn't matter what kind of wingtip device you use-they're all like span," he says. "The devil is in the details. Span extensions are the easiest and least risky." In the past, winglets were more likely to be retrofitted to an existing wing than to be designed in from the start, but now that is beginning to change. Unlike those tailfins on cars, winglets really work.
There are many other benefits to this technology, far beyond the high speed airliners and even fighter aircraft! Have a look! We call them "Tip Fence's" and while they work the exact same way, they also produce some other very useful advantages! Besides maximizing a wings lift by preventing the two air streams around the wing from mixing around the wings tip before they converge and depart the wings trailing edge, they also provide additional yaw stability, help prevent 'Tip Stalls" and tighten up the rudders authority, and they also boost the aileron effectiveness, especially at the very low speeds we typically enjoy flying at! Many of us have gone even further in adding mid span fence's to our wings, this has the benefits of boosting the control surfaces authority further, as well as increasing the lift to weight to speed ratio of the flaps system, as well as tightening the yaw divergence even further, AND, it also allows one to "Hit the brakes" by kicking hard rudder while maintaining wings level flight, these additions provide much needed aerodynamic drag to slow one down quickly, usually called a side slip. I went even further on my build, I also added fence's to my wings joint where it meets the fuselage, further directing and controlling the airflow over the top surface of my wings and pinning it to the flaps which maximizes the amount of lift my system can generate, which for me, lowers the speed at which my wings will actually stall at, far below what an airframe like mine would normally stop flying at! Real World testing has proved this beyond all question, with all the goodies, I went from flaps up power off stall of 26 MPH to a 23 MPH stall, and it never really actually stalls, it naturally drops the nose and immediately starts accelerating and starts flying again! With max landing flaps, my stall happened around 21 MPH before all the tricks, and now, with everything, it stalls at the unbelievably slow speed of 13MPH! I can literally land with the brakes locked and come to a complete stop inside the length of the airframe! Yes, these things really do work that good, and I have full control of the aircraft all the way to the stall at all times! With the addition of leading edge slats, my airplane became stall proof, it's impossible to actually stall, it simply drops it's nose and starts flying again all on it's own! I can shut the engine off and take a nap and it will eventually land it's self all with out me touching the controls! Might not land with the wings level, but it will land! This makes emergency landings a much safer prospect, even if I were forced to put it in the trees, I'm not going to hit very hard, which increases my chances of walking away mostly unharmed! There are also Vortex Generators we all love to add to our planes wings and tail surfaces, these do exactly what the name says, generate small powerful vortexes which increases lift, accelerating the air flow over the top of the wings, and scrubs the wings surface, breaking up the laminar flow and increasing the effectiveness of the flight controls at very low air speeds, with out a penalty in cruise speeds or higher! These can be added to many different parts of the aircraft, usually you see them on the top leading edge of the wings, and the bottom mid span of the horizontal stabilizer as well as ether side of the vertical stabilizer forward of the rudder! I also have them in my flap gaps and the aileron gaps on mine, increasing the air flow and effectiveness of those by quite a bit! It used to be VERY common to see these little Vortex Generators on high performance aircraft, but they do produce drag at very high speeds which despite the advantages they offer, were considered a penalty the operators didn't want to have to pay, so other tech was needed to compensate, but no other tech could match what V.G's provided, and you still do see them on some airframes, most notably on the U.S. Air Force's B-1 Bomber!
I always enjoy watching those great big rubber doughnuts landing at a near walk. Makes landing in the rough a new experience.
