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Winglets pay- both in the short-term and the long-term. Watching the aviation ideas that come and go- and return from a mix of engineers- designers and dreamers often makes me wonder where we’d be if they had all succeeded.The unducted fan concept recently returned- for example; or the concept for super-efficient laminar-flow wings with the flow optimized by the suction of air through millions of pin-head-sized holes in the upper surface. Aviation inventors ...

Dave Higdon   |   1st August 2009
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Dave Higdon Dave Higdon

Dave Higdon writes about aviation from his base in Wichita Kansas. During three decades in...
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Winging It To Higher Efficiency
Winglets pay- both in the short-term and the long-term.

Watching the aviation ideas that come and go- and return from a mix of engineers- designers and dreamers often makes me wonder where we’d be if they had all succeeded.

The unducted fan concept recently returned- for example; or the concept for super-efficient laminar-flow wings with the flow optimized by the suction of air through millions of pin-head-sized holes in the upper surface. Aviation inventors unrelentingly keep pressure on the performance envelop- chasing gains in speed and fuel efficiency- and climb and runway performance.

Often the gains cross lines; sometimes the improvement leaves the pilot with flexibility in the aircraft’s application. Essentially the goals of the said designers- engineers and dreamers are very similar: to develop technology that enables an aircraft to cruise faster on the prior fuel flow- or to stick with the old flight-plan speed while burning less. Basically- the aim is the same - to save time and/or money.

These two benefits form the core of one particular improvement- one that owners increasing choose to embrace for those very same reasons: time – which almost always saves money. For a long time- up until just a few years ago- this particular popular retrofit held such low sway with planemakers that few opted to incorporate them into their designs. Too many attempts failed to pass the test of demonstrable- documentable results.

Some actually hurt aircraft performance – demonstrably- to their misfortune. Gaining little respect- often denigrated as little more than oversize “letter openers-” the winds began to shift in the 1990s for winglets.

Breakthroughs in design – and in documenting the benefits of them – helped the once-belittled wingtip treatment to establish new legitimacy and respect. Advances in manufacturing and blending metals and composites made crafting strong- light examples cost-efficient. A new star in aviation technology emerged.

That OEM aircraft manufacturers increasingly employ winglets on new or newly updated aircraft designs may stand as the best testament to their demonstrated value- right behind the embrace of several major airlines which decided to upgrade hundreds of airplanes with model-specific winglets – looking for the fast-payoff cost savings in annual per-aircraft fuel spending.

As it almost always is with the public at-large- few performance-boosting options make it into widespread use without demonstrated value- and winglets today evidently clear that bar.

Even casual observers of bird flight notice the long “V” formation large migratory birds fly when traversing the continent. The drag reduction induced by this formation flying helps ease the workload of each successive bird behind the leader. By rotating the lead-bird slot regularly- a skein of geese can manage long- demanding flights as the birds change feeding grounds each spring and fall.

The formation reduces drag by reducing the force of the air vortices that spill off each wingtip with each downward beat. This aviary inspired many aspects in the ingenuity of our aviation community.

The sights and delights of tracking feathered flyers in humanity transformed into a quest for flight for the sake of the quest. So it was with the V-formation. Answering the “Why?” of birds flying that way helped inform efforts to seek solutions to the same phenomenon in man-made wings: tip vortices and the drag they induce.

Think of a soft- gauzy soda straw- spinning rapidly… so soft- but spinning so violently that it knocks you off your feet. When viewed coming off the tips of flying aircraft- where the pressure drop occurring condenses airborne moisture briefly into fog- tip vortices appear pretty and elusive- stretching behind the airplane and softly flexing while floating until they vanish back into airborne moisture. But you certainly wouldn’t want to feel one – particularly by flying another airplane into a tip-generated vortex!

In that instance- the previously benign meteorological phenomenon takes on the nature of a monster- violently tossing an airplane- possibly inverting it against the pilot’s control inputs; a horizontal tornado- if you will. And once it’s over- it seems like a dream that it happened – possibly a nightmare. This is called “Wake Turbulence” – but it originates out of the wing-tip vortices. All that force comes from the escape of high-pressure air underneath a wing moving span-wise to escape off the tip.

Where the high-pressure air spills upward- low pressure across the upper wing helps spin the air into a long- spinning- horizontal tube of atmosphere. The lower the altitude- the heavier the airplane- the slower the speed of the airplane- the more forceful the vortices act when coming off the tips.

But the wake-turbulence impact on other airplanes- while notable and worth acting to avoid- isn’t at the foundation of efforts to reduce those drag sources. In their creation- tip vortices also act to hold back the airplane – they create drag on the aircraft which requires power and fuel to counter. Reducing the drag induced by tip vortices: that’s the Holy Grail.

Where only one Holy Grail exists in legend- in aviation several exist serving devotees of several aircraft types. But the benefits of attracting buyers remain the same: speed- fuel- time and value.

Student pilots learn early that in steady state flight- aircraft fly with four forces equalized: Lift balances out gravity; and thrust- or power- balances out drag. Reduce the drag in this equation- and the airplane should fly faster on the same power used before. Reduce drag- and you can choose to reduce power to fly the old speed – and use less fuel in the process.

This collection of benefits and choice underpin the appeal of today’s modern winglet products: Imagine a typical business jet flying 250 to 350 hours a year – five to seven hours weekly. Now imagine flying those same 250 hours for the fuel costs of 230 hours – at several hundred dollars an hour in fuel alone- the operator stands to save several thousand dollars per year - year after year- with almost no increase in maintenance.

As a bonus- the airplane climbs to cruise altitude a little quicker- contributing to the overall fuel savings- and needs a little less power to maintain descent speeds – adding- again- to the fuel-consumption reduction. Claims of three percent to five percent are nominal; and one retrofit winglet source has demonstrated improvements as high as seven percent.

Of course- the bigger the airplane in any example- the larger the dollar payback per drag-reduction point. So it’s no wonder that Continental and Southwest Airlines opted to install winglets on their airliners. When the airplane in question flies thousands of hours annually- the cost savings can repay the investment quickly and start putting those savings directly back in the pocket of the airline.

By reducing the force of the high- and low-pressure air masses meeting at the tips- winglets lower the force of the tip vortices which- in turn- reduces the drag forces on the airplane. That drag reduction shows up when a winglet-equipped airplane flies at its normal cruise speed while showing lower numbers on the fuel-flow meter because the airplane needs less power to balance the total drag.

Needless to say that saving money is never a drag for any aircraft owner or operator. That’s why Learjet years ago adopted winglets as a design feature- one that remains today on all current-production Learjets. Boeing gave the 747-400 winglets- Beechcraft gave the King Air 350 its own version – and even some high-performance piston singles sport wing tips with an upsweep that aids their aerodynamics.

These days a vibrant aftermarket industry has grown up around these enhancements. Regrettably- fitting up a favorite flying machine with a suitable drag-reducing winglet isn’t as easy as calling the shop with an order- though. As with most of aviation- where power and structures are concerned- winglets must be proven and certified for a specific airframe under the Supplemental Type Certificate process.

As with many elements of aviation ignored or neglected by the OEM community- aftermarket entrepreneurs have stepped up to meet the challenge for several designs. So let’s take a look at today’s prominent players in winglet technologies.

Aviation Partners Inc. (API) in Seattle stands as arguably the first consistently successful purveyor of winglets for airliners and general aviation hardware. So successful is API’s winglets for the Boeing Business Jet that the company entered into a joint venture with The Boeing Company dubbed Aviation Partners Boeing in 1999.

API’s products enhance a growing number of aircraft designs- but the 737 model leads the pack- and is available as a retrofit or as factory-installed. In addition to Continental and Southwest- Alaska Airlines is converting its 737 fleet- and the winglets are standard on all versions of the 737-based Boeing Business Jet. API also makes its blended winglets for Boeing 757 and 767 airliners. In the business aviation world- APIs winglets also work their magic on the Hawker 800 series mid-cabin jets- the Gulfstream II (penetrating more than 70 percent of the fleet).

The STC & EASA for the Dassault Falcon 2000 series was received in 2009- and Blended Winglets are in the works for the 900 and Falcon 50. The appeal is simple: Blended Winglets work- reducing drag to an extent that it enhances climb- cruise and descent performance to the benefit of the operators. Blended Winglets are also eco-friendly- saving fuel and CO2 emissions.

The blended winglet design first introduced in 1992 to the Gulfstream II effectively smoothes the pressure transition between high- and low-pressure boundaries and effectively lowers the force of the vortices- in tangent reducing the drag by 5-7%.

In 2001 blended winglets were announced for the Hawker 800 series- the tip giving the Hawker 800 an additional 30 minutes of flight- 180 miles more range- an 18-knot speed gain and an initial cruise-altitude capability 2-000 feet higher than before- according to API founder Joe Clark.

And the High-Mach Blended Winglets API designed for the Falcon 2000 series are specifically tailored to produce optimum results at high Mach numbers. Now standard on Dassault’s Falcon 2000LX- the winglets make the airplane five percent faster on less fuel over comparable distances- and reduce climb-time dramatically – particularly in hot-and-high conditions.

Across the product line- API’s winglets are tailored to mesh with the aircraft structure to form a strong- clean- enhanced wing- with appropriate lighting considerations integral to the tip’s construction and ready for fitting. API’s winglets hold the potential to pay back their costs- with the pay-back period pegged to hours flown- which is why airlines find them so irresistible.

Further- the company has another design it is testing; one far more contemporary in appearance called the Spiroid Winglet that could potentially produce even more benefits than the Blended Winglet.

More information from www.aviationpartners.com

A relative newcomer to the field- Winglet Technology- LLC in Wichita is focusing its efforts on developing an enhancement to a single specific model with great performance already – and great potential for improvement.

Winglet Technology (WT) has the benefit of recognition by- and partnership with the manufacturer of the jet it chose: Cessna and its fastest-in-class Citation X. In test flights since late 2007- Winglet Technology’s ‘elliptical winglet’ takes a different approach to achieving the same goal – reducing the force of- and drag from- wingtip vortices.

Winglet Technology had the attention of Airbus Industries- which tested WT’s and its own in-house winglet designs on the A320- the single-aisle airliner that serves as the foundation aircraft for the Airbus Corporate Jet (ACJ). Ultimately- Airbus decided against winglets altogether citing a claim that the weight of the tips would negate any aerodynamic benefits – a conclusion at odds with other examples. Nonetheless- the Citation X project proceeds.

The wingtip maker cites these targets for its efforts:

• An at-altitude cruise-speed gain of 15 knots;
• Hot-and-high performance improved to accept temperatures up to 4°C higher or carrying 1-200 pounds more payload;
• A range increase of up to 150 nautical miles;
• Improved climb performance- with the ability to climb directly to FL450; and
• Fuel-consumption reduction of four-five percent for long-range missions.

More information from www.winglet-technology.com

Based in Everett- Washington- BLR Aerospace makes- and markets modifications for a wide cross-section of aircraft. Among the company’s most-recent innovations are winglet enhancements for the Hawker Beechcraft King Air line. Designed first for the 200- and 300-series King Air models- a version of the company’s winglet design also exists for the world’s most-popular propjet- the venerable 90-series King Air.

Years ago- Beech Aircraft became convinced of the value of winglets and created a special design for the company’s largest King Air- the 350; more recently- Hawker Beech has opted to make winglets a standard on one of its new Hawker jets.

BLR designed a tip specifically for the King Airs that brings benefits to slower- lower-cruising turboprops. The modification not only adds the winglet- it also slightly increases wing span- providing a bit more lifting area while reducing drag.

From company information- its King Air winglets provide the following benefits:
• A cruise-speed gain of about three to five knots;
• Fuel consumption reduced by about four percent;
• Climb rate improved by 300 fpm to 400 fpm;
• Improvements in handling- particularly lower in the speed envelop- such as in the terminal area and on approaches when the pilot is most-likely to be hand-flying the aircraft.

More information from http://www.nwlink.com/~blrweb/index.html

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