Aerodynamic Design and Experimental Investigation of the Sailplane Wing Tip Devices

This paper describes an experimental set-up for the investigation of wing tip devices developed as part of a study into the velocity and vorticity distributions in the flow field behind winglets, using hot-wire anemometry. In this study, effort was focused on gaining a greater understanding of what happens in the region where the winglet joins the wing. The measurements were performed in the Handley-Page wind tunnel of the Department of Aerospace Engineering at the University of Glasgow. In order to carry out measurements with the hot-wire anemometry system, a new traverse mechanism was designed and manufactured. This traverse mechanism was integrated with the other test instrumentation to create a complete measurement chain. The complete system allows fully automated hot wire measurements to be made over a defined area using programmable test parameters.


Introduction
In the late 1960s, designers began experimenting with wing-tip geometries using'small' vertical extensions to reduce the formation of tip vortices.The winglet concept actually dates back to 1897, when Frederick l-anchester took out a patent on the idea, incorporating it into some of his wing theories [].His wing had nuo 'capping planes' at the end of it, which became knor,vn in the 1920s as 'end plates ', when   Prandd extended his basic lifting line concept [2].
The real break+hrough with winglets was made by Whitcomb.In 1976, Richand Whitcomb, a NASA aerodynamicist,   published a paper that compared a wing with a winglet and the same wingwith a simple extension to increase its span [3]' Whitcomb showed that winglets reduced drag by about 20 percent and increased the wing lift-drag ratio by approximately 9 percent.Induced drag represents 30-40 Percent of the total drag of a transPort plane in cruise condition, so the induced drag reduction has a significant effect on fuel consumption.Whitcomb began a focused investigation into winglet ierodynamics and tested several designs in the wind tunnels at the NASA Langley Research Center' The first industrial application of the winglet concept was in general aviation businessjets.Iior instance, it is claimed that a winglet on a Boeing-747 could significandy reduce fuel burn on long-range flights.Research into the effect of winglets on {irst genlrationjet transport wings showed that they can produce reasonable drag reduction in high lift conditions [4].
Wnglets are now being incorporated into most new com- mercial and military transportjets.Since the 1980s, the most modern high performance sailplanes also have small vertical wing tip extensions.
The first sailplanes to have winglets were the ASW-20F8 GEMINI and NIMBUS 3. Sets of Whitcomb style winglets were fitted to these wings in the late 1980s.Flight tests carried out on these aircraft demonstrated the effect of winglets on high aspect ratio wings [5].
3 How do winglets work The primary effect of the winglet is to control the cross flow in the tip region of the wing in such a way as to reduce induced drag by displacing the vortices ounvard.The air flowing over the winglet, due to the presence of the tip vortex' strikes the winglet at an angle of attack.The winglet, like any wing, produces lift which, in this case, has a component in the forward direction.Thus, the winglet produces thrust (Ftg.l). 4 Winglets for sailplanes Theory and experience have shown that the most emcient sailplane wing is one that is very long and slender.Having a high aspect ratio wing is one way of cutting wing-tip losses.
In essence, the longer wing has the same tip losses but those energy losses will affect a lesser proportion of the total wing.In otherwords, the lift is distributed over the longerwingspan and the trailing vorticity is spread out, dissipating less energy.
Fnrm aconstruction point ofview, a longwing is prone to fle:r and has to be strengthened; this adds weight.The winglet provides ttre effect of an increased aspect ratio without ex- tending the wing-span and so does not increase the wing root bending as much as an actual span er(tension would.The moment arm of the lift from a span extension is approxi- mately one-half of the wing semi-span, whereas the moment arm of the winglet lift is 'mughly only one-half of the vertical winglet span.This small increase does not overload the wing or significantly alter the standand operating limitations.The addition of wingles on sailplane wings also improves the maximum lifVdrag coeffrcient for some l5 m spanJimited FAI sailplane dasses [6].
The induced dragcoefficient is proportional to the square of the lift coefficient hence the reduction in drag also im- proves climbing capability [7], [8].This improvemenr can be used when sailplanes circle in thermal bubbles, the main source of power to stay aloft [9].Achieving a maximum cross country speed during sailplane competitions is anorher important consideration.Hence, the design of the wing- lets must involve the compromise of maximizing the low speed improvement without sacrificing high-speed perfor- mance [0].
The winglet added to an ASW-19 clearly showed that for some speeds the friction drag could orceed the induced drag reduction provided by winglets [ll].A correctly desigrred winglet can, howeve4, be reasonably effective as illustrated in a study using the ASW-20 sailplane [12]. 5Wind tunnel models of winglets The wind tunnel models used in the experiments were real wing tips taken from the wind of a SMCZ sailplane.
The models werre mounted vertically on a base plate that was secured to a rail track mechanism.This mechanism al- lowed the model to be moved backwards and forwards in the wind tunnel working section to change the distance between the model and the hot-wire measuremenr plane.In addition, the base plate was designed to allow the incidence of the model to be changed.During the o<perimental programme four kinds of wing tip were investigated; a wing without a winglet and then three 304C2 sailplane wing tips of different design.The key parzrmerers that defined the winglet designs are shown in Fig.The measurement chain consists of an x-wire sensol a TSI IEA 300 constant temperaturc anemometry system, a personal computel an SMOCI transmitter and a traversing mechanism, as shown in Hg. 4. This figure also shows the model mounting arrangement described prwiously.The X-wire sensor is connected to two channels of the IFA 300 by coaxial cables.The IEA 300,hardware converts the acquired signals fiom the sensors and transmits them to the controlling computer via a BNC adapter block and data acquisition card.
The IEA 300 sofnvare installed on rhe computer then pro- cesses the recorded data.Once the data has been recorded for an entire X traverse, a master program, written in lABView, sends a new instruction ro the SMOCI unit.The SMOCI con- verts the instruction into the signal needed by the stepper motors to move the traverse to its next Y position.When the traverse mechanism reaches this new position, another signal is sent to it and it begins its X traverse.The x-wire sensor con- tinually samples data during this traverse and sends the signal to the computer.This process continues until measurements have been made over the entire measurement grid.The computer controlled Faverse mechanism for probe positioning and data acquisitionwas specifically designed and manufactured for the Present project.The traverse is a mo- torized two-component mechanism that can move the probe to any point within a 850 mmx930 mm grid.In the present series of tests the traverse was mounted behind the test models such that measurements could be made in planes perpendicular to the onset flow.The location of the measure- -..tt plutt.with respect to the test model could be varied using the model mounting tracks described previously.The way in which the traverse is assembled is shown in Ftg. 5   below, and Frg. 6. shows the assembled traverse in the wind tunnel together with a winglet model' The horizontal motion is provided by a large, off-the-shelf linear slide driven by a stePPer motor.Vertical movement is provided by a purpose-built traverse mechanism based on a precision ball screw, which is positioned in front of a linear slide mounted on an aluminium box-section suPPort.The carriage ofthe ball screw is connected to the carriage ofthe linear slide and so, when the ball scrcw is driven by a stePPer moto6 the carriage moves up and down.The incremental resolution of the linear motion is 0.03 mm, Measur€ments of the magnitude and associated direction of the time-dependent velocities behind the winglet models were obtained using a DANTEC 55P61 cross-wire probe con- nected to a TSI IEA-300 three<hannel constant temperature anemometer system.The sensorwires on the probe are 5 mm diameter platinum plated tungpten wires with a lengttr/diameter ratio of 250, which form a measuringvolume of approximately 0.8 mm in diarneter and 0.5 mm in height.The wires are oriented perpendicular to each othe4, corresponding to 45 degrees from the free stream direction, which gives the best angular resolution.An additional temperature probe was used to correct the anemometer output voltages for any variation in ambient flow temperature.For probe calibration, an open jet vertical wind tunnel with a maximum operating velocity of 43 m/s was used.A support allowed the sensors of the X-wire probe to be rotated by 30 degs in the plane of the sensors.Variation of the flow velocity and yaw angle then enabled the coefficients of the effective velociw method to be determined.Fig. ?shows how the hot-wire anemomerry system was integrated into the overall measuremenr sysrem.l0 Investigation grid X-travel: One step of the stepper motor provides 0.03125 mm of linear motion, and the time required for each step is 0.2980 sec.The size of the investigation grid in the x-direc- tionwas 850 mm (FiS.8) or 27200 steps of the stepper moror.
The time required to traverse this distance was, therefore, 8.1055 sec.
At a sampling rate of 2000 Hz, the sampling rime of 8.1055 seconds gives a total of 162ll investigation points per line.To allow for turbulence in the flow, the data is aver- aged in blocks of 48 measurement points corresponding to approximately 2.5 mm of motion.Thus, 339 averaged data values are collected during each X traverse.The length of the investigation grid in the Y-direction was set at 600 mm.This length was divided into steps of 5 mm, giving a total of 12l measurement points in the Y direction.
The total time taken to traverse the entire 850 mmx600 mm grid was just over 20 minutes, and a total of 41019 measurements were obtained for each grid.The scheme of this process is presented in Frg. 8.

I I Experimental results
The flow field behind the wing tip models was measured in three planes (Z/b = 0 .2,7,h= l,Z,lb = 2) for angles of attack c = (-3, 0, 3, 6, 9) degrees.For all tests the free stream velocity Uwas set at 33 m/s (l18.8 km/h) which corresponded to a Reynolds number of Re = 0.8x 106 based on the mean chord of the main wing.Data analysis and graphical presentation were carried out using Tecplot software in the form ofvector plots of velocity distribution and contour line plots of the vorticity component ol* defined in equation I and Frg.9' Frgs.lG-13 present example measured data for four wing tip configurations in the plane Zb = I when the wing was at angle of attack cr=3 degrees and the fiee stream velocity was U-=33 m/s.These figures illustrate the effectiveness of the system in capturing the differences in the flow structures behind the different winglet configurations.For example, wing tip A (Iig. l0), which is basically a standard wing tip with a small vertical extension that projects downwards from the lower surface of the wing, produces an almost classical tip vortex structure where the vorticity is concentrated in a single structure that rolls up slightly inboard of the wing tip.Also visible in this figure is the vorticity in the wake of the main wing.Frg.ll, on the other hand, shows the velocity and vorticity distributions behind a much larger winglet exten- sion.In this case, there are two clear vortex structures; one at the wingleVwing junction and the other at the tip of the winglet.The effect of sweeping this type of winglet back can be observed in lig.12, where measurements on a similar winglet with higher sweepback are presented.In this figure, ETH g@M EilTI Fig. l0: Wing tip model A E@H ct@t E6 the vorticity associated with the wingletAving junction does not appear as a single well defined vortex but rather as a more spread-out region of vorticity.The winglet tip vortex is also less distinct, but it should be remembered that the tip of the winglet will be closer to the measurement plane in this case and so the roll-up may not be as complete.Iinally, the effect of a small upward swept winglet is shown in Ftg. 13.In this case, the vorticity is well distributed and follows the curva- ture of the winglet.It should be noted that these cases are presented merely as an example of the capability of the measuring system.The evolution of the vortex structures downstream of these wing tips is very complex and cannot be inferred from observation of the behaviour in a single measurement section.Experimental instrumentation for investigating of the flow field behind a winglet model has been designed, manu- factured and setup.A newly designed traverse mechanism has made it possible for measurements to be made at high spatial resolution within a large enough area for full size wing tip models to be tested.Results have been presented to demonstrate this capability.An additional feature of the system is that the time required for testing is relatively short.
Fig. l: Forward thrust comPonent development

Fig. 7 :
Fig. 7: Flow chart ofdata acquisition Fig. 9: Vorticity ox component Handley Page wind tunnel is an armospheric low- -speed wind tunnel with a closed return circuit equipped with a rectangular testing-section of dimensions 2.15 m by 1.6 rn and length 3.38 m (FiS.3).The corners have 650 mm fillets that house lamps to provide lighting.Visual access to the working section is provided by 0,84 square metres of plate glass and acrylic windows that permit the model to be viewed from many angles.Several venting slots in the tunnel walls at the test section exit maintain near atmospheric static pres- sure.The nozzle placed in front of the testing section has a contraction ratio of l:4.The power supply is an electric motor that drives a fan 2.3 m in diameter to provide the airflow in the wind tunnel.The tunnel can reach speeds up to 60m/s.
2, and are the wingler airfioil, sweip- back, cant angle, nuist distribution and the ratio of the winglit root chord to the winglet tip chord(taper).The