Understanding Winglets Technology

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overall performance despite arguments to the

overall performance despite arguments to the contrary.

contrary.

Aircraft enginee

Aircraft engineers would have urs would have us believe that aes believe that aerodynamics is a mrodynamics is a mature science. ature science. If that were true,If that were true, then pro and anti-winglet ad

then pro and anti-winglet advocates might no be so strongly oppvocates might no be so strongly opposed. osed. NASA' s Richard T. WhitcombNASA' s Richard T. Whitcomb invented these nearly vertical wingtip extensions in the early 1970s as a means by which wing invented these nearly vertical wingtip extensions in the early 1970s as a means by which wing lift-to-drag performance could be increased.

drag performance could be increased.

Indeed, Whitcomb's research in 1976 indicated that winglets could reduce induced drag by 20 Indeed, Whitcomb's research in 1976 indicated that winglets could reduce induced drag by 20 percent, resulting in about nine percent

percent, resulting in about nine percent better lift-to-drag performance at 0.78 Mach for better lift-to-drag performance at 0.78 Mach for a specific winga specific wing loading.

loading. Whitcomb concludWhitcomb concluded that winglets produed that winglets produced twice the benefit of a wingtip exteced twice the benefit of a wingtip extension with thension with the equivalent are

equivalent area. a. As a result, winglets impoAs a result, winglets imposed much less weight ansed much less weight and drag penalty than incd drag penalty than increasingreasing wingspan.

wingspan. Far from the

Far from the simple wing end plates patented by Lanchester in simple wing end plates patented by Lanchester in 1897, Whitcomb's early 1970s-vintage1897, Whitcomb's early 1970s-vintage winglets were carefully designed airfoils that harnessed the energy of

winglets were carefully designed airfoils that harnessed the energy of the wingtip vortex.the wingtip vortex. Many of the

Many of the original design principloriginal design principles still are es still are used in the used in the latest generation of winglets.latest generation of winglets. Whitcomb has attracted an impressive

Whitcomb has attracted an impressive cadre of business and commuter cadre of business and commuter aircraft manufacturers, as well as aircraft manufacturers, as well as after-market modifiers, in the two decades market modifiers, in the two decades since he first published his winglet since he first published his winglet research

research findings. findings. Canadair,Canadair, Gulfstream and Learjet, along with Gulfstream and Learjet, along with Israel Aircraft, Pilatus and Raytheon, Israel Aircraft, Pilatus and Raytheon, have decided to fit winglets to their have decided to fit winglets to their business

business or or commuter commuter aircraft. aircraft. EachEach

of the these firms cites better wing performance gains than could be achieved by other types of wing of the these firms cites better wing performance gains than could be achieved by other types of wing modification

modifications for s for the same weight and drag the same weight and drag penalties.penalties.

Seattle-based Aviation Partners Inc. has developed winglets for the Gulfstream II that can boost Seattle-based Aviation Partners Inc. has developed winglets for the Gulfstream II that can boost range by as much as seven percent, according to the firm

range by as much as seven percent, according to the firm (October 19(October 1996, page 96, page 96). 96). API claims API claims thatthat its fine-tuned winglets can increase the specific range of many other types of aircraft, including some its fine-tuned winglets can increase the specific range of many other types of aircraft, including some large jet transports such as

large jet transports such as the Airbus 330 and 340, which already have small the Airbus 330 and 340, which already have small winglets as part of theirwinglets as part of their original design.

original design.

But anti-winglet advocates, such as Cessna and Dassault, remain opposed to such non-traditional But anti-winglet advocates, such as Cessna and Dassault, remain opposed to such non-traditional modifications to their plana

modifications to their planar wing designs. r wing designs. They claim that a properly designThey claim that a properly designed wing needs no suched wing needs no such devices and that it

devices and that it offers better performance over a wide offers better performance over a wide range of speed, load and range of speed, load and lift conditions.lift conditions. Traditional aerodynamic design equations and computer codes were written for planar wing forms. Traditional aerodynamic design equations and computer codes were written for planar wing forms. Some of the debate over winglets may result from the relative lack of industry-standard, Some of the debate over winglets may result from the relative lack of industry-standard, three-dimensional, wing design tools that can predict what effect winglets will have on wing performance, dimensional, wing design tools that can predict what effect winglets will have on wing performance, according to Fassi Kafyeke

according to Fassi Kafyeke, Ph.D., Bombardier's manag, Ph.D., Bombardier's manager of advanced aerodynaer of advanced aerodynamics. mics. As a result, aAs a result, a large part of winglet design development still involves time-consuming wind tunnel and flight test large part of winglet design development still involves time-consuming wind tunnel and flight test trials, unless a firm has access to specialized aerodynamic computer software.

trials, unless a firm has access to specialized aerodynamic computer software. The popularity of winglets, though, continue

The popularity of winglets, though, continues to grow. s to grow. This report will focus on why winglets work andThis report will focus on why winglets work and some of the

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TWO

TWO DIMENSIONAL DIMENSIONAL AIRFOILAIRFOIL

Bernoulli taught us that the total pressure of an incompressible fluid is the sum of the static pressure Bernoulli taught us that the total pressure of an incompressible fluid is the sum of the static pressure and the

and the dynamic pdynamic pressure. ressure. The laws The laws of kinetic of kinetic energy genergy govern dynaovern dynamic pressumic pressure. re. It varies It varies with thewith the square of the fluid v

square of the fluid velocity. elocity. As a result, the static pressuAs a result, the static pressure varies inversere varies inversely as a function of totally as a function of total pressure minus the

pressure minus the square of the fluid vesquare of the fluid velocity. locity. Plainly put, if you accPlainly put, if you accelerate the fluid, the statielerate the fluid, the staticc pressure drops.

pressure drops.

If the fluid is air and the means by which the fluid is accelerated is an airfoil, the side on which the If the fluid is air and the means by which the fluid is accelerated is an airfoil, the side on which the fluid travels the

fluid travels the greatest distancgreatest distance will have the will have the highest veloe highest velocity and lowescity and lowest static pressure. t static pressure. TheThe difference in the veloc

difference in the velocity on each side of the airfoil deterity on each side of the airfoil determines the static pressure dimines the static pressure differential. fferential. That isThat is what generates lift.

what generates lift.

The efficiency with which an ai

The efficiency with which an airfoil generates lift is known as the lift coeffirfoil generates lift is known as the lift coefficient. cient. The actual lift force isThe actual lift force is a function of the lift coefficient, air velocity, air density and effective wing area.

a function of the lift coefficient, air velocity, air density and effective wing area.

Varying the airfoil's angle of attack has a direct bearing on lift coefficient because it changes the Varying the airfoil's angle of attack has a direct bearing on lift coefficient because it changes the relative distance the air must travel over the upper and lowe

relative distance the air must travel over the upper and lower surfaces. r surfaces. This results in a change in theThis results in a change in the relative velocities over the two surfaces and thus a change in lift coefficient.

relative velocities over the two surfaces and thus a change in lift coefficient.

Lift coefficient increases almost directly with an increase in angle of attack up to a maximum point. Lift coefficient increases almost directly with an increase in angle of attack up to a maximum point. Any increase in angle of attack beyond that point causes air flow

Any increase in angle of attack beyond that point causes air flow separation on the upper surface andseparation on the upper surface and total loss-of-lift coeffi

total loss-of-lift coefficient. cient. Pilots know this poPilots know this point as the stalling aint as the stalling angle of attack.ngle of attack.

THREE-DIMENSIONAL WING THREE-DIMENSIONAL WING

Designing a win

Designing a wing would be duck-soup sig would be duck-soup simple if it were a two-dimensimple if it were a two-dimensional airfoil. onal airfoil. But a wing has aBut a wing has a finite length that ends at the win

finite length that ends at the wingtip. gtip. The difference in air pressuThe difference in air pressure between the lower anre between the lower and upperd upper surfaces of a wing causes the air to escape around the wingtip, which reduces the available lift.

surfaces of a wing causes the air to escape around the wingtip, which reduces the available lift. The motion of the

The motion of the air rushing around the wingtip coupled with the velocity of air rushing around the wingtip coupled with the velocity of the airflow through whichthe airflow through which the wing is flying causes a vortex to form near the wingtip, as shown in Figure 1. The tip vortices the wing is flying causes a vortex to form near the wingtip, as shown in Figure 1. The tip vortices cause upwash and downwash air currents that alter the direction of the free stream flow around the cause upwash and downwash air currents that alter the direction of the free stream flow around the wing.

wing.

They induce a decrease in the They induce a decrease in the angle of attack of the average angle of attack of the average relative wind flowing around relative wind flowing around the wing.

the wing.

This has two undesirable This has two undesirable byproducts, as shown in byproducts, as shown in Figure 1. First, the wing Figure 1. First, the wing generates lift perpendicular to generates lift perpendicular to the average relative wind. the average relative wind. This diverts the lift vector away This diverts the lift vector away from the desired direction, from the desired direction, which is perpendicu

which is perpendicular to the free stream. lar to the free stream. Induced angle of attack makes it necessary for the wing toInduced angle of attack makes it necessary for the wing to generate more total lift than a

generate more total lift than a theoreticalltheoretically, two-dimensional airfoil to produce the y, two-dimensional airfoil to produce the same effective lift.same effective lift. Second, drag is induced

Second, drag is induced. . Diverting the lift vector causeDiverting the lift vector causes a drag component to be generates a drag component to be generated that isd that is parallel to the free

parallel to the free stream airflow. stream airflow. The drag compoThe drag component varies as the nent varies as the cosine of the angcosine of the angle betweenle between total lift and effective lift vectors, as shown in Figure 2.

total lift and effective lift vectors, as shown in Figure 2. As a tip vortex becomes more in

As a tip vortex becomes more intense, it induces more of a shitense, it induces more of a shift in the average relative wft in the average relative wind. ind. TheThe greater the induced angle of attack, the more effective lift is reduced and induced drag is increased. greater the induced angle of attack, the more effective lift is reduced and induced drag is increased. And, if an aircraft gains weight, its wing has to operate at a higher angle of attack to generate more And, if an aircraft gains weight, its wing has to operate at a higher angle of attack to generate more lift.

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average relative wind over the

average relative wind over the whole wing. whole wing. It follows that if span It follows that if span were infinite, induced drag would were infinite, induced drag would bebe zero because there would be no wingtip; therefore, no tip vortex to create induced drag. In addition, zero because there would be no wingtip; therefore, no tip vortex to create induced drag. In addition, without the tip vortices inducing a shift in the angle of the average relative wind, the wing's lift without the tip vortices inducing a shift in the angle of the average relative wind, the wing's lift coefficient varie

coefficient varies more directly with the wings more directly with the wing's angle of attack relative to the free stream's angle of attack relative to the free stream. . That meansThat means the wing's lift coefficient will increase more sharply with increases in angle of attack, and it will the wing's lift coefficient will increase more sharply with increases in angle of attack, and it will produce its maximum li

produce its maximum lift coefficient at a lower angle of attack. ft coefficient at a lower angle of attack. As a result, a wing with an infinitelAs a result, a wing with an infinitelyy long span that has no tip vortices actually stalls at a lower angle of attack than a short wing that has long span that has no tip vortices actually stalls at a lower angle of attack than a short wing that has intense tip vortices, as shown in Figure 3.

intense tip vortices, as shown in Figure 3.

Induced drag, however, accounts for only about 40 percent of Induced drag, however, accounts for only about 40 percent of anan airplane's

airplane's total total drag. drag. The The rest rest is is parasitic parasitic drag drag andand compressibilit

compressibility drag. y drag. A very A very large, long wing, one large, long wing, one with almost anwith almost an infinite span-to-chord aspect ratio, would have enormous infinite span-to-chord aspect ratio, would have enormous parasitic drag

parasitic drag. . But, if the wing's spanBut, if the wing's span, chord and airfoi, chord and airfoil sectionsl sections are scaled down to an appropriate size, the tip

are scaled down to an appropriate size, the tip vortices still affectvortices still affect only a small percentage of the wing area.

only a small percentage of the wing area.

A high aspect ratio wing can produce almost twice as much lift A high aspect ratio wing can produce almost twice as much lift as a low asp

as a low aspect ratio wing. ect ratio wing. A high aspect raA high aspect ratio wing, theretio wing, therefore,fore, can be about half the size of a low aspect ratio wing and can be about half the size of a low aspect ratio wing and produce the same lift.

produce the same lift.

The desire for high aspect

The desire for high aspect ratio and ratio and optimum aerodynamic efficiencoptimum aerodynamic efficiency must be y must be balanced against otherbalanced against other factors, such as material-strength-to-weight ratios, overall weight of the wing, internal fuel capacity factors, such as material-strength-to-weight ratios, overall weight of the wing, internal fuel capacity and parasitic dra

and parasitic drag. g. For those reasons, the bFor those reasons, the best aspect ratio for the aveest aspect ratio for the average business aircrrage business aircraft wingaft wing might be seven or eight to one.

might be seven or eight to one. For instance, the aspect ratios for the HawkeFor instance, the aspect ratios for the Hawker 800XP, Falcon 2000r 800XP, Falcon 2000 and Beechjet 400A are 7.1,

and Beechjet 400A are 7.1, 7.6 and 7.8, respectively.7.6 and 7.8, respectively. Certain very-lon

Certain very-long-range missions requg-range missions require higher aspect ratio wingire higher aspect ratio wings. s. The Global Express, for examplThe Global Express, for example,e, has an 8.6 aspect ratio.

has an 8.6 aspect ratio. Some aircraft have much lowSome aircraft have much lower aspect ratios because the desier aspect ratios because the designers modifiedgners modified a wing they inherited from an earli

a wing they inherited from an earlier model. er model. The Learjet 35/36 series, for exampThe Learjet 35/36 series, for example, has a 6.2 aspectle, has a 6.2 aspect ratio.

ratio.

Some aircraft have high aspect ratio wings, but the weight they gained during development caused Some aircraft have high aspect ratio wings, but the weight they gained during development caused excessive wing loadin

excessive wing loading. g. The Challenger 600, for examplThe Challenger 600, for example, has an 8.5 aspect ratio, but ended up withe, has an 8.5 aspect ratio, but ended up with a 91.3 pound/square foot wing loading, resulting in excessive drag due to increased angle of attack, a 91.3 pound/square foot wing loading, resulting in excessive drag due to increased angle of attack, higher lift coefficient and stronger tip vortices.

higher lift coefficient and stronger tip vortices. The tip-to-root-chord

The tip-to-root-chord-length taper ratio also h-length taper ratio also has an influence on induas an influence on induced drag. ced drag. Prandtl, an early 20thPrandtl, an early 20th century aerodynamic engineer, found that the lowest induced drag occurred when a wing had an century aerodynamic engineer, found that the lowest induced drag occurred when a wing had an elliptical load di

elliptical load distribution. stribution. This theory had a strong influeThis theory had a strong influence on the wing design of the Supermance on the wing design of the Supermarinerine Spitfire and Lockheed Constellation.

Spitfire and Lockheed Constellation. Such graceful cu

Such graceful curves, though, greatly rves, though, greatly increase manufaincrease manufacturing complexicturing complexity. ty. According to DaAccording to Daniel P.niel P. Raymer, a Rand Corp. consultant and aeronautical engineer, if a trapezoid-shaped wing has a 0.45 Raymer, a Rand Corp. consultant and aeronautical engineer, if a trapezoid-shaped wing has a 0.45 taper ratio, its spanwise loa

taper ratio, its spanwise loading will be very close to the ideal elding will be very close to the ideal elliptical load distribliptical load distribution. ution. This resultsThis results in less-intense tip vortices and lower induced drag.

in less-intense tip vortices and lower induced drag.

WINGLET EFFECTS WINGLET EFFECTS

Stretching wings

Stretching wingspan or increasing apan or increasing aspect ratio certainly respect ratio certainly reduces induced dduces induced drag. rag. Designers, thougDesigners, though,h, have to balance the benefits of less induced drag against the costs of structural weight increases, have to balance the benefits of less induced drag against the costs of structural weight increases, more parasitic drag o

more parasitic drag or cost considerations. r cost considerations. For those reasons, they've oFor those reasons, they've often fitted their aircraft withften fitted their aircraft with winglets duri

winglets during the last two decang the last two decades. des. The trend is increThe trend is increasing.asing.

Winglets work because they efficiently produce aerodynamic side forces that divert the inflow of air Winglets work because they efficiently produce aerodynamic side forces that divert the inflow of air from the tip vortex.

from the tip vortex. That takes a rather sophisticated smaThat takes a rather sophisticated small wing, one that is sized, shapll wing, one that is sized, shaped, cambereded, cambered and canted for a specific application and mounted on the wingtip where it will produce the most and canted for a specific application and mounted on the wingtip where it will produce the most

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Whitcomb optimized his original winglet design for the cruise speed and lift coefficient of a typical jet Whitcomb optimized his original winglet design for the cruise speed and lift coefficient of a typical jet transport.

transport. He fitted the wingHe fitted the wingtip with a compartip with a comparatively small loatively small lower winglet lower winglet located near the wicated near the wingtipngtip leading e

leading edge and a dge and a much largemuch larger upper wir upper winglet farther nglet farther aft. aft. Whitcomb laWhitcomb later concluded ter concluded that thethat the upper/lower winglet combination produced very small reductions in induced drag compared to the upper/lower winglet combination produced very small reductions in induced drag compared to the upper winglet alon

upper winglet alone and that it complicated ground cleae and that it complicated ground clearance problems. rance problems. Very few aircraft, as a result,Very few aircraft, as a result, presently use the upper/ lower win

presently use the upper/ lower winglet combination. glet combination. Only upper winglets have been fitted to businessOnly upper winglets have been fitted to business aircraft, with virtually no exceptions.

aircraft, with virtually no exceptions. Whitcomb's winglets

Whitcomb's winglets were designed for were designed for transonic cruise transonic cruise speeds.

speeds. Jet Jet aircraftaircraft typically cruise at or typically cruise at or above the critical Mach above the critical Mach number, the free stream number, the free stream speed at which local speed at which local airflow over some airflow over some section of the aircraft, section of the aircraft, usually the wing, first usually the wing, first reaches the speed of reaches the speed of sound.

sound. At At the the criticalcritical Mach number, a shock Mach number, a shock wave forms as the air wave forms as the air decelerates to subsonic decelerates to subsonic speed aft of the point of speed aft of the point of maximum chord maximum chord thickness.

thickness.

The shock wave The shock wave intensifies as the intensifies as the supersonic to subsonic supersonic to subsonic speed change becomes speed change becomes more abrupt.

more abrupt.

A strong shock wave A strong shock wave

causes turbulent airflow separation behind it, thereby substantially increasing drag. (Supercritical causes turbulent airflow separation behind it, thereby substantially increasing drag. (Supercritical wings are desig

wings are designed to maintain suned to maintain supersonic airflow opersonic airflow over a large part of the chver a large part of the chord. ord. This moves theThis moves the shock wave aft and weakens its intensity.)

shock wave aft and weakens its intensity.)

Whitcomb positioned the leading edge of the upper winglet at the point of maximum chord thickness Whitcomb positioned the leading edge of the upper winglet at the point of maximum chord thickness at the wingtip.

at the wingtip. The object was to prevenThe object was to prevent the increased velocity ovet the increased velocity over the winglet's inside sur the winglet's inside surface fromrface from boosting the speed of the high velocity air over the forward section of the upper surface of the wing boosting the speed of the high velocity air over the forward section of the upper surface of the wing near the tip.

near the tip. That prevented the winThat prevented the winglet from reinforcing the shglet from reinforcing the shock wave of the wing sectioock wave of the wing section near then near the tip.

tip.

Experimentation indicated that the winglet's trailing edge should be positioned near the wing's trailing Experimentation indicated that the winglet's trailing edge should be positioned near the wing's trailing edge for maximum effectiveness.

edge for maximum effectiveness. The winglet has a

The winglet has a pronounced effect on wing bending moment. pronounced effect on wing bending moment. The winglet produces an inward The winglet produces an inward loadload as it diverts the tip vor

as it diverts the tip vortex. tex. In addition, the wiIn addition, the wingtip section prongtip section produces more lift, whduces more lift, which increaseich increases thes the bending moment.

bending moment. The combination of the two forces greatly incThe combination of the two forces greatly increases the overall bendinreases the overall bending moment ofg moment of the wing, which becomes a major factor in integrating winglets into a wing's design.

the wing, which becomes a major factor in integrating winglets into a wing's design. Increasing the hei

Increasing the height of the winglet produces more oght of the winglet produces more of a decrease in tip vortex. f a decrease in tip vortex. However, the greaterHowever, the greater loads imposed upon the wing result in a compromise between aerodynamic and structural loads imposed upon the wing result in a compromise between aerodynamic and structural engineerin

engineering. g. Balancing the two factors results in a typical winBalancing the two factors results in a typical winglet height of about 10 to 20 percent ofglet height of about 10 to 20 percent of the wing semi-span.

the wing semi-span.

With winglets, the outboard sections of the wing produce more lift, which changes the wing's With winglets, the outboard sections of the wing produce more lift, which changes the wing's

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reduce it from rein

reduce it from reinforcing the shock wforcing the shock wave of the main wiave of the main wing near the tip. ng near the tip. Whitcomb chose an Whitcomb chose an airfoilairfoil with significantly more camber than the wing, but

with significantly more camber than the wing, but with an eight percent thickness-to-chord ratio.with an eight percent thickness-to-chord ratio.

This design provided the best tradeoff in low- and high-speed lift characteristics, along with a This design provided the best tradeoff in low- and high-speed lift characteristics, along with a reasonably

reasonably light weighlight weight structure. t structure. It had a somewIt had a somewhat higher crihat higher critical Mach nutical Mach number than the wmber than the winging section near the tip.

section near the tip.

Whitcomb's winglets were toed out at Whitcomb's winglets were toed out at the root, thus giving them negative the root, thus giving them negative incidence

incidence to the to the free strefree stream. am. However,However, the tip vortex induces a positive angle of the tip vortex induces a positive angle of attack o

attack over the ver the winglet. winglet. The sThe side foide forcerce vector, being perpendicular to the vector, being perpendicular to the average relative wind over the winglet, average relative wind over the winglet, actually provided a slight thrust actually provided a slight thrust component.

component. Kafyeke's Kafyeke's term term for for thisthis forward component is "negative drag." forward component is "negative drag." Trial and error

Trial and error experimentatiexperimentation indicatedon indicated that a four degree toe out was optimum that a four degree toe out was optimum for Whitcomb's tests. The tip vortex is for Whitcomb's tests. The tip vortex is strongest near the w

strongest near the winglet root, thus it ininglet root, thus it induces more angduces more angle of attack at the root than le of attack at the root than the tip. the tip. ThatThat enabled Whitcomb to build a winglet with no geometric twist and achieve the desired, gradual enabled Whitcomb to build a winglet with no geometric twist and achieve the desired, gradual reduction in lift toward the tip of the

reduction in lift toward the tip of the winglet.winglet. Giving the winglet a slig

Giving the winglet a slight cant angle or dihedral alht cant angle or dihedral also improves its aerodynaso improves its aerodynamics. mics. Whitcomb foundWhitcomb found that this reduced the interference at the junction of the winglet and wingtip at transonic speeds and that this reduced the interference at the junction of the winglet and wingtip at transonic speeds and that it pushed the tip vortex outboard, thus furthe

that it pushed the tip vortex outboard, thus further reducing nearby vortex inter reducing nearby vortex intensity. nsity. The optimum cantThe optimum cant angle for Whitcomb's experiment was 15 degrees.

angle for Whitcomb's experiment was 15 degrees.

Whitcomb research showed that carefully designed winglets could improve the lift-to-drag ratio of the Whitcomb research showed that carefully designed winglets could improve the lift-to-drag ratio of the wing by nine percent, compared to four percent for a span increase of equivalent area.

wing by nine percent, compared to four percent for a span increase of equivalent area.

WINGLETS ON BUSINESS AIRCRAFT WINGLETS ON BUSINESS AIRCRAFT

We do not have enough space to cover eac

We do not have enough space to cover each business aircraft wingh business aircraft winglet design. let design. Instead, we'll focus onInstead, we'll focus on a few trendsetters.

a few trendsetters.

Learjet was the first firm

Learjet was the first firm to install winglets on a business aircraft, and the results were impressive.to install winglets on a business aircraft, and the results were impressive. Just over a year after Whitcomb first published his findings, Learjet's chief test pilot, Peter T. Just over a year after Whitcomb first published his findings, Learjet's chief test pilot, Peter T. Reynolds, started test flying a Learjet fitted with the "Longhorn" wing, which was a 20-series Learjet Reynolds, started test flying a Learjet fitted with the "Longhorn" wing, which was a 20-series Learjet wing from which the tip tanks were removed and to which six-foot wing extensions and winglets were wing from which the tip tanks were removed and to which six-foot wing extensions and winglets were added.

added. The Learjet 25 fuselage was mated to the Longhorn wing The Learjet 25 fuselage was mated to the Longhorn wing and the Learjet 28 was created.and the Learjet 28 was created. Without the tip tanks,

Without the tip tanks, fuel capacity shrank by more tfuel capacity shrank by more than 1,450 pounds, however.han 1,450 pounds, however.

To partially regain fuel capacity, Learjet created a second model, To partially regain fuel capacity, Learjet created a second model, the Learjet 29, which had larger fuselage fuel tanks that held the Learjet 29, which had larger fuselage fuel tanks that held almost 700 pounds more fuel, but shrank cabin length by more almost 700 pounds more fuel, but shrank cabin length by more than two feet. The Longhorn wing had nearly 14 percent more than two feet. The Longhorn wing had nearly 14 percent more area than the 20-series wing and a 7.25 aspect ratio compared area than the 20-series wing and a 7.25 aspect ratio compared with 5.46

with 5.46 for the for the original original wing. wing. In additiIn addition, it haon, it had a bd a boost inoost in effective aspect r

effective aspect ratio due to wingatio due to winglets. lets. As a result, LeaAs a result, Learjet 28s andrjet 28s and 29s could cruise comfortably at FL 470 to FL 490 instead of FL 29s could cruise comfortably at FL 470 to FL 490 instead of FL 410 for the Learjet 25. Less induced and parasitic drag, along with 410 for the Learjet 25. Less induced and parasitic drag, along with higher cruise altitudes, enabled the Learjet 28 and 29 to cruise on higher cruise altitudes, enabled the Learjet 28 and 29 to cruise on up to 26 percent less fu

up to 26 percent less fuel at altitude. el at altitude. The fuel bum improvThe fuel bum improvementsements on day-to-day missions were less impressive because of the on day-to-day missions were less impressive because of the thirstthirst of the 20 series turbojet engines during taxi, takeoff, climb and of the 20 series turbojet engines during taxi, takeoff, climb and

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Similar to Whitcomb's design, Learjet canted the winglets 15 degrees and gave them a slight toe out. Similar to Whitcomb's design, Learjet canted the winglets 15 degrees and gave them a slight toe out. Up to the aircraft's 0.81 Mmo maximum operating Mach limit, there was very little flow separation at Up to the aircraft's 0.81 Mmo maximum operating Mach limit, there was very little flow separation at the wingtip-to-winglet junction caused by interference.

the wingtip-to-winglet junction caused by interference. Reynolds found th

Reynolds found that the winglets slighat the winglets slightly increased dihtly increased dihedral effect. edral effect. Without wingleWithout winglets, but in spite ofts, but in spite of the tip tanks, the Learjet 25 has wea

the tip tanks, the Learjet 25 has weakly damped Dutch roll chakly damped Dutch roll characteristics. racteristics. In contrast, the Learjet In contrast, the Learjet 2828 and 29 have very mildly divergent Dutch roll chara

and 29 have very mildly divergent Dutch roll characteristics. cteristics. Reynolds determineReynolds determined that the advancingd that the advancing wing’ s winglet stalled at

wing’ s winglet stalled at six to eight degrees of sideslip, causing an increase in six to eight degrees of sideslip, causing an increase in induced drag and ainduced drag and a "large stabilizing momen

"large stabilizing moment." In other t." In other words, the aircraft yawed back into the direction of the words, the aircraft yawed back into the direction of the slip and theslip and the amplitude of the yaw and roll oscillations slowly increased.

amplitude of the yaw and roll oscillations slowly increased.

The Learjet 25's yaw damper gain was tweaked up and the Learjet 28/29 had no more problems with The Learjet 25's yaw damper gain was tweaked up and the Learjet 28/29 had no more problems with Dutch roll. The Longhorn Learjet's decrease in wing loading and increase in lift coefficient resulting Dutch roll. The Longhorn Learjet's decrease in wing loading and increase in lift coefficient resulting from a higher aspect ratio and winglets broadened the margin between high-speed and low-speed from a higher aspect ratio and winglets broadened the margin between high-speed and low-speed buffet to 65 knots, accordi

buffet to 65 knots, according to Reynolds. ng to Reynolds. This trait made it easy to fly thThis trait made it easy to fly the Learjet 28/29 at Fe Learjet 28/29 at FL 450 toL 450 to 510. More importantly to Learjet's future, the Longhorn wing's improved performance allowed the 510. More importantly to Learjet's future, the Longhorn wing's improved performance allowed the larger and heavier Learjet 50 series and 60 series to use the same wing, thus greatly reducing the larger and heavier Learjet 50 series and 60 series to use the same wing, thus greatly reducing the development cost of these new models.

development cost of these new models.

Even the latest business aircraft equipped with winglets rely heavily upon Whitcomb's research. Even the latest business aircraft equipped with winglets rely heavily upon Whitcomb's research. Bombardier's Global Express, for example, has an 8.6 aspect ratio that is effectively increased by its Bombardier's Global Express, for example, has an 8.6 aspect ratio that is effectively increased by its winglets.

winglets. These are far different from the first-These are far different from the first-generation wigeneration winglets fitted to Canadair Chnglets fitted to Canadair Challengers inallengers in the early 1980s.

the early 1980s. The Global Express wingThe Global Express winglets were designed wilets were designed with a proprietary, three-dimth a proprietary, three-dimensionalensional computer code, under the supervision of Dr.

computer code, under the supervision of Dr. Kafyeke.Kafyeke.

The Global Express' winglets have a more sophisticated, supercritical airfoil section that creates less The Global Express' winglets have a more sophisticated, supercritical airfoil section that creates less of a shock wave.

of a shock wave. That enables the lThat enables the leading edge oeading edge of the winglet root to be mf the winglet root to be moved forward withoved forward withoutout causing interferen

causing interference drag at transonic speedce drag at transonic speeds. s. They also are taller than the CThey also are taller than the Challenger's winghallenger's winglets.lets. However, Whitcomb

However, Whitcomb's influence remains present. 's influence remains present. The Global Express' winglThe Global Express' winglets have almost no twist,ets have almost no twist, approximate

approximately a one-third taper ratio and are toely a one-third taper ratio and are toed out three to four degrees. d out three to four degrees. They're also canted 15They're also canted 15 degrees and have about an eight percent thickness-to-chord ratio.

degrees and have about an eight percent thickness-to-chord ratio. In spite of

In spite of the Global Express' relatively high 8.6 aspect ratio, winglets boost its range performance bythe Global Express' relatively high 8.6 aspect ratio, winglets boost its range performance by as much as four to

as much as four to seven percent.seven percent. Aviation Partners Inc. blended Aviation Partners Inc. blended winglets, designed for winglets, designed for after-market applications, have market applications, have produced

produced similar similar results. results. InIn contrast to most other winglets, contrast to most other winglets, including the original Whitcomb including the original Whitcomb design, API's winglets are joined design, API's winglets are joined to the wingtip in a constant to the wingtip in a constant radius curve, rather than a radius curve, rather than a relatively sharp angle junction. relatively sharp angle junction. The smooth curve, according to The smooth curve, according to API, reduces shock interference API, reduces shock interference between the winglet and wing between the winglet and wing

near the tip, thus allowing the winglet chord to be extended forward of the point of maximum chord near the tip, thus allowing the winglet chord to be extended forward of the point of maximum chord thickness at the tip.

thickness at the tip. Just as important, API uses supercJust as important, API uses supercritical airfoil sectioritical airfoil sections in its winglets that havens in its winglets that have shock waves that are farther aft and we

shock waves that are farther aft and weaker than those of the origiaker than those of the original wing. nal wing. As a result, there is littleAs a result, there is little interference between the two shock waves.

interference between the two shock waves.

API claims that moving the winglet root forward also eliminates problems with small vortices that roll API claims that moving the winglet root forward also eliminates problems with small vortices that roll up from the highly swept wingtip area just ahead of the conventional winglet-towing junction at up from the highly swept wingtip area just ahead of the conventional winglet-towing junction at

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In spite of such performance gains, API's future wingtip modifications may not look at all like small In spite of such performance gains, API's future wingtip modifications may not look at all like small wings.

wings. The firm is developing a spiroiThe firm is developing a spiroid tip modification that virtually eld tip modification that virtually eliminates the concentrated tipiminates the concentrated tip vortex.

vortex. While much more compleWhile much more complex than a winglet, API claims that the spiroid tip can box than a winglet, API claims that the spiroid tip can boost range byost range by as much as 10 percent.

as much as 10 percent.

API is developing the first spiroid tip appl

API is developing the first spiroid tip applications for air transports. ications for air transports. Reduced fuel burn wiReduced fuel burn will allow lowerll allow lower operating cos

operating costs and higher paylots and higher payloads on everyday miads on everyday missions. ssions. This will make them coThis will make them cost-effective forst-effective for commercial ope

commercial operators, according to API. rators, according to API. Spiroid tips won't appSpiroid tips won't appear on business aircraear on business aircraft until sometimeft until sometime in the future.

in the future.

Not all winglets are design

Not all winglets are designed to improve high-speed to improve high-speed cruise efficiency. ed cruise efficiency. On some turboprop aircrOn some turboprop aircraft,aft, such as the Pilatus PC-12 and Raytheon 1900D, their primary purpose is to improve low-speed wing such as the Pilatus PC-12 and Raytheon 1900D, their primary purpose is to improve low-speed wing performance.

performance. At such high lift coefficieAt such high lift coefficients, they function mants, they function mainly as end plainly as end plates to block the hightes to block the high--intensity tip vorte

intensity tip vortex and improve sex and improve section lift coefficiection lift coefficient near the tip. nt near the tip. This can reduce This can reduce stall speed,stall speed, thereby resulting in lower V

thereby resulting in lower V speeds and shorter runway lengths.speeds and shorter runway lengths.

(In the case of the PC-12, adding winglets enabled the aircraft to meet the FAA's 61 KCAS maximum (In the case of the PC-12, adding winglets enabled the aircraft to meet the FAA's 61 KCAS maximum stalling speed requir

stalling speed requirement for single-engine aircement for single-engine aircraft. raft. Pilatus later received a three knot waiPilatus later received a three knot waiver of thatver of that requirement.)

requirement.)

Winglets must be optimized for very spec

Winglets must be optimized for very specific wing lift coefficients and flighific wing lift coefficients and flight regimes. t regimes. They can be fine-They can be fine-tuned for low speed or cruise lift coeffi

tuned for low speed or cruise lift coefficients, but not both. cients, but not both. The aerodynamics of wingThe aerodynamics of winglets designed forlets designed for high-speed cruise have to be carefully fine-tuned because of the effects of compressibility and critical high-speed cruise have to be carefully fine-tuned because of the effects of compressibility and critical Mach number. Choice of airfoil section and flow interference characteristics of low-speed winglets is Mach number. Choice of airfoil section and flow interference characteristics of low-speed winglets is less critical.

less critical.

In the face of such widespread accep

In the face of such widespread acceptance, though, winglet opptance, though, winglet opponents remain skepticalonents remain skeptical. . They claimThey claim that

that winglets add more drag for the same benefit as increwinglets add more drag for the same benefit as increasing wing span anasing wing span and that more span buysd that more span buys better performa

better performance over a wnce over a wide range ide range of lift coefficieof lift coefficients. nts. ""A lot of winglets are put there by marketing A lot of winglets are put there by marketing departments"

departments" said one veteran said one veteran aeronautical engineeaeronautical engineer.r. But winglet advoc

But winglet advocates seem to be prevailiates seem to be prevailing. ng. Winglets are spWinglets are sprouting from an increrouting from an increasing number ofasing number of business and comm

business and commuter aircraft because of demouter aircraft because of demonstrated performance imnstrated performance improvements. provements. There's oneThere's one other reason, according to one well-known modifier and winglet skeptic,

other reason, according to one well-known modifier and winglet skeptic, "It's also because they look "It's also because they look sexy."