ABOUT WINGLETS - Ultraligero.Net

ABOUT WINGLETS
by Mark D. Maughmer
Over the past ten years, from initially being able to do little to improve
overall sailplane performance, winglets have developed to such an
extent that few gliders leave the factories without them. They are now a
familiar sight to nearly every soaring pilot. Few, however, really
understand what winglets do.
Understanding Drag
A winglet’s main purpose is to improve performance by reducing drag. To understand
how this is done, it is first necessary to understand the distinction between profile drag
and induced drag.
Profile drag is a consequence of the viscosity, or stickiness, of the air moving along the
surface of the airfoil, as well as due to pressure drag (pressure forces acting over the front
of a body not being balanced by those acting over its rear). As a wing moves through
viscous air, it pulls some of the air along with it, and leaves some of this air in motion.
Clearly, it takes energy to set air in motion. The transfer of this energy from the wing to
the air is profile drag.
Profile drag depends on, among other things, the amount of surface exposed to the air
(the wetted area), the shape of the airfoil, and its angle of attack. Profile drag is
proportional to the airspeed squared. Readers interested in a more thorough explanation
of these concepts are directed to refs. 1 and 2.
To measure an airfoil’s profile drag in a wind tunnel, a constant-chord wing section is
made to span the width of the wind-tunnel test section. In this way, the airflow is not free
to come around the wing tips. There is thus no flow in the spanwise direction -- the wing
section behaves as if it belonged to a wing of infinite span.
Induced drag is the drag that is a consequence of producing lift by a finite wing. If a
wing is producing lift, there must be higher pressure on the underside of the wing than on
the upper side. Thus, there is a flow around the wingtip from the high-pressure air on the
underside of the wing to the low-pressure air on the upper side (fig. 1). In other words,
there is spanwise flow on the finite wing that was not present on the infinite wing (fig.
2). This spanwise flow is felt all along the trailing edge as the flow leaving the upper
surface moves inward while that on the lower surface moves outward. As these opposing
flows meet at the trailing edge, they give rise to a swirling motion that, within a short
distance downstream, is concentrated into the two well-known tip vortices. Clearly, the
generation of tip vortices requires energy. The transfer of this energy from the wing to the
air is induced drag.

This process can be idealized as a “horseshoe” vortex system (fig. 3). As a consequence
of producing lift, “an equal and opposite reaction” must occur -- air must be given a
downward velocity, or downwash. With this downwash comes spanwise flow, tip
vortices, and induced drag. The goal is to minimize this drag by minimizing the amount
of energy used in producing the required downwash -- to reduce the energy that is
“wasted” in creating unnecessary spanwise flow and in the rolling up of the tip
vortices.
In observing the flowfield around the wing in Fig.2, it should be clear that the greater the
span, the less the tip effect is felt on the inboard portions of the wing. That is, the greater
the span, the more “two-dimensional like” will be the rest of the wing and, consequently,
the less its induced drag. As the span approaches infinity, the downwash and induced
drag approach zero. Likewise, if the wing is not producing lift, there will be no
downwash and thus no induced drag.
It is found that the induced drag is a function of the inverse of the square of the airspeed--
it is smallest at high speeds and increases as the aircraft slows down. It also depends on
the weight squared divided by the span squared, (W/b) 2 , how much weight each foot of
wing is asked to support. Thus, it increases with the square of the aircraft weight and
decreases with the inverse of the span squared.
Induced drag also depends on the wing design itself -- how efficiently it produces lift. As
a reference point, the most efficient planar wing (a wing with no dihedral or a winglet) is
one that has an elliptical loading (greatest at the root and decreasing toward the tip,
following the equation of an ellipse). Typical planar wings are slightly less efficient,
while non-planar geometries can be somewhat better than the elliptical case.
Controlling Induced Drag
It has been known for over a century that an endplate at the tip of a finite wing can reduce
spanwise flow and induced drag. Unfortunately, to be effective at this, the endplate must
be so large that the increase in skin friction drag due to excessive wetted area far
outweighs the reduction in induced drag.
A winglet provides a way to do better. 3 Rather than being a simple “fence,” it carries an
aerodynamic load. The idea is to produce a flowfield that interacts with that of the main
wing to reduce the amount of spanwise flow. That is, the spanwise induced velocities
from the winglet oppose and thereby cancel those generated by the main wing.
This effect has been measured experimentally (Fig. 4). Here it is observed that the
spanwise flow has been largely eliminated by the presence of the winglet. In essence, the
winglet diffuses or spreads out the influence of the tip vortex such that the downwash,
and thereby the induced drag, is reduced. In this way, the winglet acts like an endplate in
reducing the spanwise flow but, by carrying the proper aerodynamic loading, it

accomplishes this with much less wetted area. Nevertheless, recalling the penalty of
profile drag with increasing airspeeds, the designer’s goal is to gain the most reduction in
induced drag for the smallest increase in profile drag.
The Winglet Design Process
My involvement began over a decade ago when I was asked by Peter Masak to help in
the design of winglets for the then-current crop of 15-meter racing sailplanes. Early
design procedures were based on the idea of a crossover point -- a breakeven airspeed
below which winglets improves performance by reducing induced drag and above which
their extra wetted area adds enough profile drag that performance is lower. Our first
successful winglets for sailplanes were guided by this notion. A trial-and-error approach
was employed that eventually led to some significant improvements. 4 In 1989, one of
these designs was adopted by Schempp-Hirth as the “factory winglet” for the Ventus. In
retrospect, with the understanding that has come since, it seems that this process, while
systematic and logical, was accompanied with a great deal of luck. It now seems
somewhat remarkable that with the tool then at hand, we were able to come up with a
design that worked so well.
In spite of some success, I was somewhat frustrated by the lack of tools then available to
analyze or design winglets. Thus, along with a succession of excellent students, a
research effort was begun at Penn State to better this situation. In 1994, a collaborative
research arrangement with M&H Soaring (Monty Sullivan and Heinz Weissenbueller) in
Elmira, New York was begun. Their close proximity to Penn State, along with their
acceptance that it would not be a trivial matter to fabricate and flight test the number of
trials needed to develop and validate sound design methods, resulted in a fruitful and
enjoyable cooperation that continues still.
As our ability to predict the induced drag for a given wing geometry improved, 5 so did
the ability to predict the effects on sailplane performance due to small changes in
geometry. With this, it became possible to design winglets that much more closely
achieved the intended results.
Some rules-of-thumb were established. First, whether it be with up-turned tips or
winglets, it can be beneficial to go “out-of-plane.” Second, while a great deal of work
has been directed toward achieving the minimum induced drag, 6 our experience is that
driving a winglet toward this optimum penalizes the profile drag far more than it benefits
the reduction in induced drag. The design goal is clearly to minimize the overall drag,
not just the induced drag.
The cross-country performance of a sailplane can now be predicted with enough accuracy
to determine whether small changes in winglet geometry are beneficial or not. To do
this, straight flight and turning speed polars are calculated, including the influence of
variations in the spanwise lift distribution over the speed range, profile drag of the

aerodynamic surfaces as they depend on Reynolds number, flap deflections, and trim
drag. 7 Optimum flap settings over the speed range are also computed. These results are
then used to predict average cross-country speeds in given weather conditions. After the
optimum bank angles are determined for a range of thermal strengths, sizes, and lift
profiles, a MacCready climb/glide analysis shows the average cross-country speed of the
glider as a function of thermal strength. So rather than design the winglet to simply not
hurt the lift-to-drag ratio below a certain airspeed, the winglet can be tailored to give the
best cross-country performance over a wide range of operating conditions.
Performance Gains: A Case Study
To see the performance increases that are possible with winglets, the predicted speed
polars for the Schempp-Hirth Discus 2, with and without winglets, ballasted and
unballasted, are shown in Fig. 5. Although gains are demonstrated, they are difficult to
assess using the polars shown. Thus, the data are replotted in terms of L/D verses
velocity in Fig. 6. In addition to demonstrating the gains from carrying water ballast at
higher cruising speeds, the benefit of winglets can now be seen. To get an even better
idea of the gains in L/D, in Fig.7 these data are again replotted in terms of the percentage
increase in L/D relative to the unballasted and ballasted glider without winglets. Note
how this winglet's crossover point occurs at airspeeds that are above the maximum
allowable -- there are no allowable flight conditions in this case for which the winglets
penalize performance. Although a slight overall gain could be achieved by tailoring the
winglet more for climb, this would result in relatively large penalties at high speeds.
While the percentage gain in L/D does not appear to be very great, it is significant that it
comes without any penalty at higher speeds.
The effect of winglets on the percentage change in average cross-country speed relative
to that of the baseline aircraft is presented in Fig. 8. The winglets improve the crosscountry
performance for all the thermals considered, that is, for thermals having a 500’
radius and strengths of up to 12 kts. As expected, the performance gain is very
significant for weak thermals -- having winglets allows for some climb rate, whereas
without them it is minimal or zero. As the thermal strengths increase, the benefits
decrease; however, for this glider winglets do not hurt cross-country speed even for
average thermal strengths of more than 12 kts.
The point at which full water ballast becomes beneficial is indicated by the crossing of
the unballasted and ballasted curves at an average thermal strength of about 8 kts,
corresponding to a climb rate with full ballast that is predicted to be about 5.2 kts. In this
case, “full ballast” corresponds to a wingloading of 10.6 lbs/ft 2 rather than the 9.0 lbs/ft 2
allowed by U.S. Standard Class rules. As indicated, ballast causes a reduction in average
cross-country speed for average thermal strengths of less than 8 kts. For thermal
strengths greater than this, winglets improve the cross-country speed, but only by a halfpercent
or so. The glider with winglets, however, can profitably carry ballast in slightly
weaker conditions than can the glider without winglets.

Other Issues
From the experience of designing winglets for a variety of sailplanes (as well as for a few
non-sailplane applications), it seems that all wings can be improved with winglets,
although the better the original wing from an induced drag standpoint, the smaller the
gain possible with winglets (and the more difficult is the design process).
It is sometimes heard that winglets were tried on a certain glider and did not work. What
this really says is that a particular poor design did not work. As an example of how
critical some of the design issues are, the effect of toe (incidence) angles on the Discus 2
winglet design is presented in Fig. 9. Obviously, a small deviation from the optimum can
cause the winglet to become a speed brake. Furthermore, each glider must have winglets
specifically designed for it -- rules of thumb can be disastrous. From personal
experience, there is no doubt that it is much easier to make a glider worse with winglets
than it is to make it better!
Winglets sometimes can fix problems of the original wing. For example, in the case of a
flapped glider, it is important that the flaps/ailerons extend to the wingtip. Otherwise,
when the flaps are deflected upward for high-speed cruise, the tips are loaded far more
than they should be. Although only a small portion of the wing is influenced, it can result
in a very significant induced drag increase. In these cases, cutting the tip back to the
aileron in order to mount the winglet has resulted in unexpected gains, especially at high
speeds.
By understanding of how winglets achieve induced drag reduction, it also becomes clear
how they can produce other performance and handling gains. In particular, it has been
found that winglets improve the flow in the tip region and thereby improve the
effectiveness of the ailerons. This is in part due to the local angle of attack in the vicinity
of the ailerons being reduced less by the reduced downwash velocities, and by the
reduction of spanwise flow helping to keep the ailerons effective. One of the benefits of
greater control effectiveness is that smaller aileron deflections are required for a given
rolling moment. This means less drag for a given roll rate and a higher maximum roll
rate. Likewise, woolen tufts attached to glider wings have shown that much of the flow
over the inside tip during turning flight is separated, which is nearly eliminated by the
presence of a winglet. Winglets also benefit safety -- ailerons remain effective much
deeper into a stall than before.
The improvement in handling qualities are very succinctly described by Werner Meuser,
the current 15-Meter Class World Champion, in a message sent to me by the Schempp-
Hirth factory describing his first impressions of the new Ventus 2ax winglets. “…..very
impressed by the handling change. He reported the glider got more gentle and harmless
at low speeds and felt very ‘clean’ close to stall speed., which seems to have decreased
remarkably. Even in steep circles, there was no tendency to stall or ‘misbehave’. Before
anything at the wing started to separate, he felt the tailplane couldn’t handle the angle of
attack anymore. Allover, very positive impressions….”

Conclusions
Although performance gains achieved with winglets are only a few percent at moderate
thermal strengths, such small differences can be important in determining the outcome of
many cross-country flights and contests. For example, in the 1999 U.S. Open Class
Nationals, just 68 points separated the first six places. This difference amounted to less
than 1.5% -- far less than the performance advantage that can be achieved using welldesigned
winglets.
Since their shaky introduction many years ago, the acceptance of winglets is now
widespread. At the World Championships in Uvalde, Texas in 1991, of 105 competing
gliders, 19 used winglets. At the most recent championships in South Africa, essentially
every glider entered had winglets or some type of tip treatment.
Thus, after more than a decade of winglets being applied to sailplanes, it is clear that the
benefits are far-reaching. If properly designed such that the profile drag penalty is of
little consequence over the range of airspeeds at which the glider is flown, then there
seems to be no reason whatsoever not to take advantage of the performance and handling
qualities benefits that winglets offer.
Finally, although some of the spinning characteristics of gliders with winglets have been
explored, testing has not been extensive. The anecdotal evidence indicates that gliders
with winglets are more reluctant to spin, but once they do, the altitude required for
recovery is somewhat greater. Given that many glider fatalities are a consequence of
stall/spin accidents during approach, at altitudes from which safe recovery is not possible,
a question worth pondering is whether even the most basic training gliders might benefit
from the installation of winglets.
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References
1Thomas,
F., Fundamentals of Sailplane Design, Translated by Judah Milgram, College Park
Press, MD, 1999.
2
Falk, T.J. and Matteson, F. H., “Sailplane Aerodynamics,” American Soaring Handbook,
Soaring Society of America, 1971.
3
Whitcomb, R.T., “A Design Approach and Selected Wind-Tunnel Result at High Subsonic
Speed for Wing-Tip Mounted Winglets,” NASA TN D-8260, July 1976.
4
Masak, P.C., �Design of Winglets for Sailplanes,� Soaring, June 1993, pp. 21-27.
5
Mortara, K.W. and Maughmer, M.D., “A Method for the Prediction of Induced Drag for Planar
and Non-Planar Wings,” AIAA Paper 93-3420, Aug. 1993.
6
Munk, M.M., “Minimum Induced Drag of Aerofoils,” NACA Technical Report No. 121, 1921.
7 Maughmer, M.D. and Kunz, P.J., �Sailplane Winglet Design,� Technical Soaring, Vol.
XXll, No. 4, Oct. 1998, pp. 116-123.

Fig. 1 Higher pressure air on the wing lower surface flowing around
wingtip to upper surface.
Fig. 2 Spanwise flow on a finite wing - solid lines, upper surface;
dashed lines, lower surface.

Fig. 3 Idealized “horseshoe” vortex system.
Fig. 4 Experimentally determined flowfield crossflow velocity vectors
behind model with and without winglets. 6

V S (kts)
V (kts)
0
0
40 60 80 100 120
2
4
6
8
Discus 2, 685 lbs
Discus 2 WL, 685 lbs
Discus 2, 1060 lbs
Discus 2 WL, 1060 lbs
Fig. 5 Predicted straight flight polars of unballasted and ballasted
Discus 2, with and without winglets.
L/D
50
40
30
20
10
0
Discus 2, 685 lbs
Discus 2 WL, 685 lbs
Discus 2, 1060 lbs
Discus 2 WL, 1060 lbs
40 60 80 100 120
V (kts)
Fig. 6 Comparison of predicted lift-to-drag ratios for unballasted
and ballasted Discus 2, with and without winglets.

∆∆L/D %
%
3
2
1
0
-1
Discus 2 WL, 685 lbs
Discus 2 WL, 1160 lbs
40 60 80 100 120
V (kts)
Fig. 7 Percentage gain in predicted lift-to-drag ratios due to
winglets for unballasted and ballasted Discus 2.
∆V CC %
5
4
3
2
1
0
-1
Baseline: Discus 2, 685 lbs
Discus 2 WL, 685 lbs
Discus 2, 1160 lbs
Discus 2 WL, 1160 lbs
2 4 6 8 10
Thermal Strength (kts)
Fig. 8 Percentage gain in predicted average cross-country speed
due to winglets and ballast relative to unballasted Discus 2 (685 lbs)
without winglets.

∆V CC %
5
4
3
2
1
0
-1
Baseline: Discus 2, 685 lbs
Winglet Toe Angles
(From zero-lift angle)
-3 deg
1 deg
5 deg
2 4 6 8 10
Thermal Strength (kts)
Fig. 9 Percentage change in predicted average cross-country speed
as it depends on winglet toe angle for an unballasted Discus 2. Toe
angles are measured relative to the zero-lift angle of attack.

WINGLET DESIGN
FOR SAILPLANES
Peter Masak
IN THE ONGOING QUEST for higher performance
sailplanes, winglets have provided a
means for improving the performance with
only a modest price per L/D point gain. Winglets
act to reduce induced drag and act to
control the crossflow in the tip region of the
wings in such a way as to improve the handling
characteristics at the same time.
By introducing a vertical cambered surface
at the tip, the downwash field behind the wing
is spread horizontally by several inches. Since
the induced drag is inversely proportional to
the effective width of this downwash field, the
winglet therefore acts to reduce induced drag
by displacing the vortices outward. Presumably
the greatest effect would be obtained by
introducing a high lift large surface winglet
which would displace more air outward and
alter the circulation pattern in a more significant
way. However, the design of winglets
involves the compromise of maximizing the
low speed improvement without sacrificing
high speed performance. Pilots will not fly
with winglets if they perceive any deterioration
of high speed performance.
BACKGROUND
First use of winglets
Winglets for modern aircraft were first proposed
by Dr. Richard Whitcomb, at NASA
Langley in the mid–1970’s. At that time, wind
tunnel models and subsequent full size flight
tests on a Boeing 707 commercial jetliner demonstrated
a significant reduction in total drag
at high lift coefficients.
After the publication of the design philosophy,
numerous researchers in industry tackled
winglet design with varying degrees of
success. Most tried to use potential flow methods
for predicting tip inflow angles and surface
pressure distributions, however given the
nature of the flow field at the tip, this has lead
many investigators to the wrong conclusions.
Potential flow analysis seems to steer the designer
in the direction of excessively large
winglets, while experimental data suggests
that large winglets pay a greater–than–predicted
penalty in high speed performance.
Since potential flow methods cannot accurately
predict the vortex roll–up at the tip, or
the influence of secondary flows on the boundary
layer, these methods have not provided
the complete picture of the effect of winglets
on performance. Also, potential flow methods
do not show the significant influence of the
effect of the fore–aft position of the winglets.
Experience with sailplanes
In sailplane racing circles, winglets were tried
and then dropped by a number of university
flying groups (Darmstadt, Braunschweig), and
6
the French manufacturer Centrair. The overriding
concern repeatedly expressed by racing
pilots was that the winglets, although they
were known to provide a significant gain at
low speed, would detract from performance
at the high speed cruise condition, with a
resulting net loss or perhaps no achieved gain
in overall performance.
This concern is justified since winglets act to
reduce both induced drag and drag due to
crossflow at the tip; however, at high speed
neither of these effects are large and thus
there is some speed at which the overall surface
friction drag of the winglet exceeds the
induced/interference drag reduction provided
by the winglet. The graphs below show this
effect with large winglets added to an ASW–
19 at Braunschweig. Clearly the key is to provide
a minimum drag surface which does not
stall at circling speeds.
Prompted by interest from Dr. David Marsden
at the University of Alberta, and my own successful
experience a decade ago with a homebuilt
HP–18, the challenge was struck to
sink rate – m/s
0
L/D
0.4
0.8
1.2
1.6
2.0
40
35
30
25
20
speed polars
60 80 100 120 km/h 140 160
lift – drag polars
Figure 2 — Influence of winglets on the performance of an ASW–19
design an efficient pair of winglets for a Nimbus
III for the World championships in 1989
at Wiener Neustadt, Austria.
Marsden had proposed using an unusual double
element winglet on the Nimbus III (emulating
the primary wing feathers of a soaring
bird) which was inspired by a successful version
on Marsden’s DG–200. His experiments
had shown that he was obtaining a significant
improvement in lift capability of a tip section
fitted with winglets.
Experiments with dual winglets
The initial promise of dual winglets on the
Nimbus III tips did not prove out in either
flight tests or wind tunnel tests. Although a
gain in lift was measured, the interference
drag of the two lifting surfaces caused the
airflow across the rear winglet to be separated
at even modest lift coefficients. This resulted
in the winglet not being effective at
either high or low flight speeds. At speeds
below 55 knots, the rear winglet would experience
massive separation (seen with tufts);
and at speeds higher than that, the winglet
with winglets
without winglets
free flight 2/92

friction drag due to the highly cambered airfoils
was so high as to cause an overall loss.
Second Iteration
The narrow tip chord of the Nimbus III (9 in)
forced an abnormally low chord for the dual
winglets (3–4 in). The resulting low Reynold’s
number of the winglet elements probably contributed
to the separation problem and high
drag. Thus it was evident that this design
could be improved by going back to the conventional
single element winglet. (An airfoil’s
Reynold’s number is related to its size — all
else being equal, a small airfoil does not
“work” as well as a large one. The R e of a
typical sailplane wing is 1,000,000. ed.)
DESIGN OPTIMIZATION
Apart from the selection of a winglet airfoil,
there were five key parameters that had to be
chosen to optimize the design:
• Cant angle • Twist distribution
• Sweepback • Taper ratio
• Ratio of winglet root chord to sailplane
tip chord
Cant angle
The selection of cant angle evolved from an
unusual consideration specific to sailplanes:
the narrow and highly flexible wings provide
for a wingtip angle in flight which can
approach 30 degrees on some sailplanes
when flying with water ballast. A more common
angle for modern 15 metre ships is 7–12
degrees.
On winglets that are nominally set to a cant
angle of 0 degrees (at right angles to the
wing), as the wing deflects, the winglet generates
a sideload in flight which has a component
oriented downward. This is a self
defeating situation, since the winglet is generating
additional drag by contributing to the
weight of the aircraft. Thus a more reasonable
approach is to set the winglets at least
at a cant angle on the ground of 0 degrees
plus the in–flight local tip deflection angle.
Sweepback
The selection of the sweepback angle was
based on experimental observations. It was
first believed that the sweepback angle for
the winglet should be equal to that for the
main wing (0 degrees), however experience
proves otherwise. If a vertical winglet with no
sweepback is built, it will be observed that
the root of the winglet will stall first and that
the tip will remain flying.
The optimum situation from an aerodynamic
standpoint is to have the aerodynamic loading
such that the entire winglet surface stalls
uniformly. This can be achieved by sweeping
back the winglet, which will increase the loading
on the tip. Because of the rapid variation
in angle of attack of the winglet as a function
of height, a large degree of sweepback is
required to load the tip correctly. For our winglets,
a 30 degree leading edge sweep angle
was used to achieve this effect.
Ratio of winglet root chord to sailplane tip chord
It would seem that the winglet might ideally
be designed as an extension of the wing, and
thus the optimum winglet would be a smooth
transition of the wing from horizontal to vertical.
Experiments suggest otherwise.
If the root chord of the winglet is equal to the
tip chord of the wing, then the inflow angle at
the tip will be less than when the winglet is a
smaller fraction of the tip chord. The result
will be that at high speed, the inflow angle
may not be sufficient so as to prevent separation
of the airflow from the outer (lower) surface
of the winglet. Since other considerations
require that a toe–out angle be set (about
–3 degrees), it is desirable to allow some vortex
induced flow to wrap around the wingtip
and provide a positive angle of attack for the
winglet at all flight speeds.
For the various winglets fabricated, the following
ratios of root chord of the winglet to tip
chord of the wing were used:
• DG–600 0.60 • Discus 0.70
• Ventus 0.57 • Nimbus III 0.95
• ASW–20 0.50
The choice of the root chord of the winglet is
also constrained by the nominal tip chord of
the wing, and by considering Reynold’s number
effects. Too small a winglet chord can
result in extensive laminar separation and high
drag. For the Nimbus III and Discus winglets,
the small nominal tip chords force the winglet
geometry to be smaller than would be desirable
from a Reynold’s number consideration.
Twist distribution
The twist distribution on a winglet is normally
selected so as to provide a uniform load distribution
across the winglet span. Since the
inflow angle is higher at the base, the winglet
is twisted to higher angles of attack toward
the tip. This is opposite to the general design
methodology for wings, which normally have
washout (either geometric or aerodynamic)
so as to decrease the angle of attack towards
the tips.
The determination of optimum twist for our
winglets was made by iterating experimentally.
When flight tested, the first set of winglets
fabricated stalled at the root first with a
progressive stall developing upwards towards
the winglet tip. By twisting the winglet to increase
the angle of attack at the tip, the entire
surface of the winglet could be made to
stall simultaneously. Two degrees of twist from
root to tip proved to be optimum.
The second benefit of positive twist on the
winglet is that the high speed performance is
enhanced — there is less likelihood of developing
separation on the outer surface of the
winglet at low inflow angles (high speed =
low coefficient of lift, C l).
Taper ratio
The effect of taper ratio on inflow angles and
the resulting optimum twist distribution was
analyzed theoretically by K.H. Horstmann in
his PhD thesis. It was shown that as taper
ratio increases, the optimum twist distribution
for the winglet varies more linearly from root
to tip. From a construction standpoint it is
also easier and more accurate to build a winglet
with a linear change in twist angle along
the winglet span. This favours a winglet with a
larger tip chord. We also want to try to maximize
the tip chord so as to maximize the
Reynold’s number. Accordingly, a ratio of tip
to root chord of 0.6 was selected.
Toe–out
The determination of toe–out was based on
the simple consideration that we were trying
to maximize the speed at which no further
benefit is gained from the winglet, and thus
select an angle of attack (α) setting for the
winglet that will minimize the high speed drag.
Considering the C l–vs–α prediction for the
PSU–90–125 winglet airfoil, an angle of attack
of –3 degrees corresponds to a C l of 0. Given
the fact that even at high speed there is a
small inflow component at the tip, the winglet
will actually be generating a slightly positive
lift, even with the –3 degree root toe–out. Calculations
show that when the wing is operating
at a nominal lift coefficient of 1.0 (which
corresponds to the circling lift coefficient), the
lift coefficient of the winglet is 0.6 at the root
and reduces to zero at the tip.
WINGLET AIRFOIL
The winglet airfoil was designed with the following
criteria in mind:
• to minimize drag at low C l conditions
• to design the winglet airfoil to be tolerant
of low R e
• to maximize tolerance to negative α
These design requirements are different than
for a conventional sailplane airfoil. The resulting
custom airfoil designed by Dr. Maughmer
and Mr. Selig of Pennsylvania State University
is shown in the figure below. Dr. Maughmer
described the airfoil design philosophy
as follows:
“The airfoil has the traditional undercamber
removed from the lower surface trailing edge
area, which minimizes the tendency to form
detrimental laminar separation bubbles at low
or negative angles of attack. At the price of a
little C lmax, which isn’t important for a winglet
anyway, the drag is lower than other sailplane
airfoils everywhere up to C l = 0.85, as well as
0 50 percent of chord 100
PSU–90–125 winglet airfoil
2/92 free flight 7

at negative Cl’s, so that sideslips and horizontal
gusts can be tolerated. The corners of
the laminar bucket have been rounded to
avoid unstable yawing moments that would
be generated otherwise if the sailplane yawed
to angles exceeding those corresponding to
the sharp corners of the traditional Wortmann
sailplane airfoils. Finally, the airfoil was designed
to avoid laminar separation bubbles
down to R e = 350,000.”
WING AERODYNAMICS
The change in the lift distribution of a wing
with and without winglets is shown below. The
boundary condition at the wingtip of the main
wing no longer requires that the lift taper to
zero at the tip. The assumed lift distribution
for a wing with a winglet is assumed to terminate
at an imaginary point equal to unfolding
the vertical winglet in the horizontal plane. As
a result the outer portion of the wing carries a
higher load than it does without the winglet.
Recent calculations on sailplanes with double
trapezoidal planforms such as the ASW–
20 or LS–6 suggest that this outer tip loading
is more efficient from the standpoint of induced
drag.
Secondly, the additional lift capability of the
main wing means that the Clmax of the overall
wing is increased and the sailplane’s circling
performance will be enhanced.
Structural Loading
One of the key advantages of winglets is that
they provide a performance increase while
only fractionally increasing the root bending
moment on the spar compared to a span extension.
Whereas the moment arm of a span
extension is one–half the semi–span of the
wing (about 7.5 metres), the moment arm of a
winglet is only equal to approximately one–
half the vertical span (0.3 m) plus the deflected
wing elevation at the tip. For sailplanes
which are certified with tip extensions, one
can be assured that the winglet will not overload
the wing and all standard operating limitations
will apply (Ventus, ASW–20, DG–600).
8
moment arm
of winglet lift
span loading without winglet
FINAL DESIGN
span loading with winglet
Lift distribution on a wing with and without winglet
The final choice of design parameters is reflected
in the design of the Ventus and ASW–
20 winglets, which have been highly successful
in competition. The ASW–20 winglet went
through two iterations and the Ventus, three,
before it was concluded that the design had
reached a high level of refinement.
FLIGHT TEST RESULTS
Competition Results
The response of pilots flying with winglets in
competition has been very positive overall.
Certainly one of the measures of the success
of the design is the fact that pilots after a
period of evaluation have chosen to fly with
the winglets. At the 1991 World contest in
Uvalde, Texas, ten pilots chose to fly with our
winglets – 8 Ventus, 1 ASW–20B, and 1 Nimbus
III. At the end of the contest, a Ventus
flying with our winglets had won four of twelve
contest days and on the fastest day of the
contest, the top five places in the 15 metre
class went to sailplanes flying with our winglets.
Additionally the trophy for the highest
speed achieved overall went to Jan Anderson
of Denmark, flying a Ventus with our winglets
(his speed also exceeded the highest
achieved in the Open Class). Two weeks prior,
at the 15 metre Nationals in Hobbs, New
Mexico, Reinhard Schramme from Germany
established an unofficial record of sorts by
flying his Ventus–C around a closed course
of greater than 500 km with an average speed
of 171 km/h (he would have won were it not
for a photo penalty).
Bruno Gantenbrink and Hermann Hajek of
Germany chose to retrofit winglets to their
Ventus–C’s and were delighted with the handling
and performance qualities that they observed.
Mr. Hajek noted as a particular advantage
the improvement in his ability to maintain
constant bank angle and speed with a
full load of water. With winglets the effective
dihedral is increased and the sailplane can
be banked steeper while retaining control.
moment arm of lift from span extension
additional lift from wing
in presence of winglet
The dolphining performance is naturally improved
with the winglets since they act to
reduce induced drag while pulling positive
‘g’, and several pilots have perceived their
sailplanes to have improved glide performance
even at high cruising speeds in strong
weather.
Flight Test Data
These positive results are confirmed by flight
tests based on three high tows with each sailplane
type which show the following performance
gains as measured by the two–glider
comparison technique.
ASW–20 flight test data:
(pilots –Striedieck, Seymour)
speed duration Δ with Δ
winglets ft/min
50 mi/h 5 min + 30 ft 6
65 mi/h 5 min + 7 ft 1.5
80 mi/h 2 min + 10 ft 5
100 mi/h 2 min 0 0
Ventus flight test data:
(pilots –Mockler, Masak)
speed (knots) flap Δ with
winglets
40 dry, 53 wet +2 9.1 ft/min
50 dry, 66 wet 0 9.0 ft/min
60 dry, 79 wet 0 9.8 ft/min
84 dry, 110 wet –2 3.3 ft/min
Maximum performance gains
with Masak winglets
sailplane winglet airfoil L/D gain
ASW–20 NASA Van Dam 2.1
Discus PSU–90–125 2.5
Ventus PSU–90–125 3.5
CONCLUSIONS
The overall performance gains measured in
free flight on sailplanes retrofitted with winglets
are impressive and are supported by positive
contest results. Handling qualities are improved
in all cases, including improvement in
roll rate and roll authority at high lift conditions.
The performance measurements have shown
a higher gain in performance than would otherwise
be predicted by conventional theory.
It is believed that major benefits are derived
from inhibiting the secondary flow that contaminates
the boundary layer near the tip region.
Prediction of this phenomenon requires
computational power out of my grasp, and
the present designs have been developed
via experimentation and in–flight testing.
By August 1991, there were over forty–five
sailplanes in the world flying with winglets
designed and fabricated by the author. No
negative reports or dangerous incidents (ie.
flutter) of any kind have been reported. As a
result of the positive service experience,
Transport Canada have recently issued a supplementary
type certificate for flight with winglets
on the Ventus model, using JAR–22 as a
basis for compliance. •
A bibliography is on page 13
free flight 2/92

Do winglets work?
Steve Smith
from Pacific Soaring Council West Wind
W
ELL, in a word, yes. But don’t they
hurt high speed performance? Not
necessarily. It seems I’m often discussing
winglets with glider pilots. So I’d like to try
to provide some technical framework for
understanding what winglets do.
Sources of Drag First, in order to understand
winglets, you need to understand drag.
Airplanes have three primary sources of
drag. The first source is often called parasite
drag or profile drag, and this has to do
with the skin friction created by airflow over
the aircraft surface. The second source is
called induced drag, which is a result of
generating lift with a finite wing span –
an infinite wing would be nice, but it
won’t fit in your trailer! The third drag
source is caused by compressibility effects
on aircraft that fly nearly as fast as
the speed of sound, or faster. Except for
John McMaster’s Altostratus, we don’t
need to worry about compressibility
drag. The primary effect of winglets is to
reduce the induced drag.
Parasite drag is naturally affected by the
amount of wetted surface area. It also
depends on whether the boundary layer
is laminar or turbulent – but that’s another
story. For now, you need to know
that parasite drag increases in proportion
to the square of the airspeed. This
turns out to be sort of universal – most
aerodynamic forces increase in proportion
to the square of the velocity, because the
ability of the air to produce forces is related
to the kinetic energy in the flow:
Dparasitic = kμV2 Induced drag is a bit more complicated. A
finite wing ends with a wingtip, where the
higher pressure air under the wing can leak
around the end and fill the low pressure
area on top of the wing. This flow around
the tip forms a vortex that trails off downstream.
The flow around the tip also reduces
the lift in the area near the tip by
tending to equalize the low pressure above
the wing. The vortex contains energy in the
form of the swirling flow velocity. We call
the force required to pull the wing along to
produce these tip vortices “induced drag”.
The mechanism through which the wing
“feels” the presence of the tip vortices is the
downward velocity induced on the wing by
the vortices. It is as if the wing is flying in a
self-generated region of sink.
This concept is very oversimplified – a more
realistic explanation requires a fair bit of
math and physics. What really happens is
that vorticity is shed all along the trailing
16
50
40
30
20
10
edge, not just at the tip. The distribution of
lift along the span of the wing determines
how much vorticity is shed along the trailing
edge. It can be proven that for a planar
wing (no winglets), the induced drag is the
smallest when the spanwise distribution of
lift is shaped like an ellipse. This lift distribution
produces the vorticity distribution
with the minimum energy. In steady flight,
induced drag varies in proportion to the
square of the weight, and inversely with the
square of the wingspan and velocity:
Dinduced = kμ(W/bV) 2
If the aircraft is heavier, it needs more lift,
and so produces more induced drag. If the
lift is distributed over a longer wingspan,
Typical drag curve for a sailplane in steady flight
drag (lbs)
stall
total
induced
parasitic
airspeed (kts) 30 60 90 120
the trailing vorticity is spread out more as
well, dissipating less energy. If the aircraft
flies faster, it produces the same lift with
less angle of attack, less disturbance to the
flow, and creates weaker vorticity in the
trailing wake.
For a given aircraft weight, the total drag is
the combination of the parasite drag and
the induced drag. Looking at the above
diagram, you can see that a minimum drag
point occurs where the parasite drag and
the induced drag are equal. At lower speeds,
the parasite drag is small, but the induced
drag increases very fast. At higher speeds,
parasite drag increases but induced drag
becomes small. This trade-off between parasite
drag and induced drag is what makes
the design of winglets interesting.
How do winglets reduce induced drag?
Adding a winglet to a wing has a similar
effect to adding wing span. By providing
more length of trailing edge, the vorticity is
spread out more for the same total lift, so
the energy loss is less. The detailed interactions
between the wing and winglet are a
bit different than a simple span extension,
but the effect is similar. In both cases, the
induced downwash is reduced. A well designed
winglet is equivalent to about half
its height in span increase. At the same time,
the winglet adds much less additional structural
load to the wing than a tip extension
does. Detailed studies of the combined structural
and aerodynamic effects of winglets
on transport aircraft show that they are not
quite equal in overall performance to a simple
span extension. Current conventional
wisdom states that winglets should only be
used in cases where there is some limiting
constraint on wingspan. Applying these results
to sailplane design would indicate that
winglets should not be used on Open class
sailplanes, but should be used on 15 metre
and Standard class sailplanes.
What about high speed performance?
Looking at the figure, you can see that induced
drag becomes unimportant at high
speeds, whereas the parasite drag becomes
dominant. A crossover point occurs where
the induced drag benefit of the winglet is
outweighed by the increase in parasite drag.
Here’s a realistic example. Suppose a winglet
is installed that reduces the induced
drag by 10% and adds 1% to the parasite
drag. At the speed for best L/D,
where induced drag and parasite drag
are equal, the net improvement would
be 4.5% (.5 x .1 – .5 x .01 = .045). This
amounts to about 6 ft/min for a typical
15 metre sailplane. At a speed of 1.73
times the best L/D speed, parasite drag
is 90% of the total, and induced drag
only 10%. At this speed, the net improvement
is almost zero (.1 x .1 – .9 x
.01 = .001). For a sailplane with a best
L/D speed of 60 knots, the theoretical
crossover speed for these winglets is 104
knots. Above this speed, these winglets
degrade performance.
But overall cross-country performance
is a balance between the low and high speed
performance. Classical MacCready theory
indicates that 50% of the time is spent cruising
and 50% climbing. In this case, the
break-even speed would occur where the
disadvantage at high speed equals the advantage
at low speed. Because the actual
drag is much higher at cruise, we can’t compare
on a percentage basis. The comparison
must be made based on actual sink
rate. Since half the time is spent cruising,
the break-even cruise speed occurs where
the increased sink rate equals the reduced
sink rate at low speed. In other words, how
fast do you need to fly so that the sink rate
with winglets is 6 ft/min greater than without
winglets? For the example used here,
this occurs at 2.3 x best L/D speed or 138
knots. It’s pretty rare that your MacCready
directed speed to fly would be this fast!
You might point out that as soaring conditions
become stronger, the MacCready
model doesn’t apply: the fraction of time
spent circling becomes much smaller. But
that doesn’t necessarily mean that the time
spent flying slow (near best L/D) also becomes
small. Efficient use of cloud streets
still dictates flying slowly in good lift. So,
free flight 4/97

suppose you never fly slower than 70 knots.
At this speed, the winglets improve your
sink rate by almost 4 ft/min. You would
need to fly 118 knots in order for the winglet
penalty to be 4 ft/min, negating the benefit.
About the only situation where soaring
speed is consistently high enough that winglets
would actually hurt overall is ridge running.
Even in ridge soaring, there may be
long gaps to cross where the benefit of the
winglets would offset any cruise penalty.
Can the same argument be applied
to tip extensions?
Well, that depends on the structural limitations
on the sailplane. First of all, for
the same improvement in induced drag, a
shorter span extension will be required
(about half, right?) but the tip extension has
more wetted area, so more parasite drag.
This added area is needed to prevent the tip
extension from stalling at low speed. The
reason winglets don’t need the same area
to prevent stalling will be explained later.
Anyway, a tip extension equivalent to the
winglet example might improve induced
drag 11%, but add 2% in parasite drag. At
the best L/D speed: .5 x .11 – .5 x .02 =
.045 (once again). But there is a crucial
assumption hidden in these examples. The
comparison is made at constant weight. If
you install your tip extensions, are you allowed
to ballast the sailplane to the same
weight? If so, then the example is still valid.
Now compare the performance of this tip
extension at 1.73 times the best L/D speed,
where parasite drag is 90% of the total, and
we find: .1 x .11 – .9 x .02 = -.007. So,
now the tip extension that appeared to be
equivalent at low speed degrades high speed
performance 0.7% at the speed where the
winglets still provide a 0.1% benefit. One
way to explain this is to say that the tip
extension reduced the wing loading. What
is really happening is that the parasite drag
was increased for the same weight. What if
you must reduce the gross weight when
you install the tip extensions? In that case,
the tip extensions hurt even more. This
also illustrates why high wing loading is
so important for Open class sailplanes.
The results here depend on many assumptions,
but they do challenge the conventional
wisdom that winglets are not as good
as tip extensions. One major difference
between sailplanes and transport aircraft is
the range of speeds over which they perform.
Transport aircraft adjust their cruising
altitude so that they cruise only slightly faster
than the best L/D speed, but sailplanes are
expected to perform well at almost twice
the best L/D speed.
What about stall? I mentioned that tip
extensions are prone to tip stall, but winglets
are not. Two effects come into play
here. First is that fact that as you scale down
an airfoil, the critical angle of attack for
stall is reduced. This is called a “Reynolds
number effect”. In essence, the basic character
of the flow is affected by the size of
the wing. To achieve the desired elliptical
lift distribution, you would like to make the
tip chord very small, but if the chord is too
small, it will be prone to stall early. So,
now you want to put a tip extension on the
wing, and you still try to achieve that elliptical
lift distribution, but the tip chord must
not get too small. So, you maintain more
surface area and compensate by reducing
the airfoil camber or twisting the wing
slightly to reduce the tip angle of attack.
The added wetted surface area increases
the parasite drag. The second effect explains
why winglets can have such a small chord
(and therefore smaller wetted area) without
stalling. As the sailplane slows down and
the angle of attack increases to maintain
the lift equal to the weight, the tip extension
experiences the same angle of attack
increase, but a winglet does not. The flow
angle experienced by the winglet is determined
by the strength and distribution of
the trailing vorticity, which is indirectly influenced
by the increased angle of attack.
The net result is that the effective increase
in angle of attack for the winglet is much
less than the increase in angle of attack on
the wing. So, the lift doesn’t build up as fast
on the winglet and the wing stalls first. In
practise, this effect is exploited to reduce
the wetted area of the winglet as much as
possible to the point where, ideally, the
wing and winglet would stall at about the
same time.
Other good things about winglets
Aside from the performance improvement
offered by winglets, there are other benefits.
The most notable of these are the
increase in dihedral, increase in aileron
effectiveness, and the reduction of adverse
yaw. The increase in effective dihedral improves
handling in thermals. There is less
need for “top stick” to prevent a spiral dive.
KR-03a “Krosno”
the trainer we’ve all been
waiting for
Ed Hollestelle, 2371 Dundas St. London ON N5V 1R4
(519) 461-1464 phone & fax
103002.722@compuserve.com
The impression is that the aircraft “grooves”
better in a turn. The increase in aileron effectiveness
and the reduction in adverse yaw
both come from the lift of the winglet when
the aileron is deflected. When the aileron is
deflected, there is less “tip loss” of the added
lift. There is much less of an increase in the
tip vortex strength, again because the vorticity
is spread out along the longer trailing
edge, and the tip is further away. As a result,
adverse yaw may be eliminated. For
heavily ballasted sailplanes, the increased
control and safety offered by the winglets
may be a big advantage, regardless of any
improvement in glide performance.
Other bad things about winglets
One disadvantage that is not often discussed
is the reduction in flutter speed. Classical
flutter occurs when the natural frequency
in bending and the natural frequency in
torsion get too close together. The torsion
frequency is always somewhat higher than
the bending frequency. By adding weight
above the plane of the wing, the torsional
moment of inertia is increased, which reduces
the torsion frequency of the wing.
Of course, tip extensions also reduce flutter
speed. Both can be compensated for by
clever addition of balance weights to the
wing, but this is a complex problem requiring
sophisticated analysis.
Conclusion I hope I’ve answered more
questions than I’ve raised. I’m happy to discuss
winglets in more detail with anyone,
feel free to contact me by email at
scsmith@mail.arc.nasa.gov ❖
Steve Smith is a Senior Aerospace Engineer
at the NASA Ames Research Center. A full
discussion of winglet design concepts can
be found in “free flight” 2/92 p6.
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