Target Function Analysis, Flow Control and Visualization on Flapped Airfoils

of High-performance FAI 18m Sailplanes

Popelka, Lukáš1, Krejčiřík, Petr2, Matějka, Milan3, Jensen, Richard4

1 Ing., Ph.D., Academy of Sciences of the Czech Republic, Dolejškova 5, 182 00 Prague 8, popelka@it.cas.cz

2 Ing., Aeroclub of the Czech Republic, U Mlýna 3, 141 00 Prague 4, pkfly@seznam.cz

3 Ing., Czech Technical University in Prague, Technická 4, 166 07 Prague 6, milan.matejka@fs.cvut.cz

4 Ing., HPH, Ltd., Čáslavská 126, 284 01 Kutná Hora, richardjensen@seznam.cz

Abstrakt: Příspěvek je věnován vyhodnocení cílové funce pomocí metod matematického modelování pro klapkové profily aplikovatelné na křídla větroňů třídy FAI 18m. Do výpočtového postupu byla zahrnuta možnost optimalizace výchylky klapky a řízení přechodu do turbulence. Těmto tématům byla věnována i měření za letu uskutečněná prostřednictvím povrchové vizualizace a tlakových měření. Bylo provedeno úvodní srovnání dynamických vlastností dvou větroňů za podmínek rozvinuté termické konvekce.

Keywords: flapped laminar airfoil, target function, flow control, in-flight measurement

1. Target function analysis Aerodynamic criteria for assessing the performance of flapped airfoils applicable on wings of FAI 18m sailplanes were established by a questionnaire survey. This procedure have been previously used for club and training sailplanes, together with concept of three regimes, concerning low (R1) and increased level of outer stream turbulence (R2) and roughness due to insect (R3), Popelka (2006). CFD methods were used to calculate values of the criteria. Procedure of target function evaluation, enabling to take into account optimum flap deflection was developed and used for five different airfoils, typical for contemporary top-performance sailplanes. Airfoil coordinates were obtained from published sources Althaus & Wortmann (1981), Althaus (1996) or newly designed according to published inviscid pressure distributions,

Horstmann & Quast (1981), Boermans & van Garrel (1997). Coordinates of airfoil HPH_x_n2, being the wing section of new Czech high-performance sailplane HPH304S, were available exclusively for this analysis from the HPH Ltd. company. Flap deflection β of all airfoils were adopted from their application on high-aspect-ratio flapped sailplanes, namely: FX62-K-131: ASW20, FX79-K-144: Ventus b, HQ17: ASW22, DU89-134: ASW27, HPH_x_n2: HPH304S. Technical data of all current top-competition FAI 18m sailplanes were used for definition of reference unballasted and ballasted wing loading m/S and mean airfoil chord cAVG , Tab. 1. For each wing loading and flight regime, 4 airspeeds for circling in thermals, 6 airspeeds for interthermal glide, touch-down landing speed and stall speed were considered. Corresponding lift coefficients and Reynolds numbers were obtained. For sake of simplicity, maximum lift coefficient was set equal for all airfoils1. With use of aerodynamic coefficients, following target function can be defined:

Famous Wortmann FX62-K-131/17 airfoil was used as a reference. Values of the target function, as a measure of fulfilment of aerodynamic requirements were obtained. Results of target function are listed in Tab. 2. Large differences are caused primarily by unadequate flap deflection (airfoil leaves the laminar bucket).

type b [m] S [m2] cAVG [m] (m/S)min [daN] (m/S)max [daN] Antares 18S 18 10,97 0,609 33,7 54,7

ASG 29 18 10,5 0,583 35,2 54,8 ASH 26 18 11,68 0,649 31,7 44,9

DG 808S 18 11,81 0,656 31,0 50,8 HPH 304S 18 11,8 0,656 31,4 50,8

JS 1 18 11,2 0,622 31,3 53,6 LAK 17 18 9,8 0,544 32,1 46,2 LS 10 18 11,45 0,636 31,9 52,4

LS 6-18W 18 11,4 0,633 30,4 46,1 Ventus 2cx 18 11,03 0,613 36,3 54,4 Ventus C 17,6 10,14 0,576 33,1 49,3

0,616 32,5 50,7 Tab. 1 Technical data of FAI 18m flapped sailplanes: wingspan b, wing area S, minimum in-flight and maximum wing loading m/S

1 Maximum lift coefficient cannot be reasonably obtained by CFD methods (since boundary layer encounters broad extent of separation. Detailed discussion is given in Popelka (2006). Coincidentally, all considered airfoils have very similar geometry which governs cLmax (thickness, leading edge shape) and hence no major differences could not be expected. Also importance of stall speed, as given by pilots requirements, is of order 5%, which is not a vital share.

airfoil β [deg] Fm1 [%] F m2 [%] F [%] HPH_x_n2 -12, 0, +12 102.0 107.4 104.4

FX 62-K-131/17 -9, -6, 0, +8 100 100 100 FX 79-K-144/17 -10.9, -4.8, 0, +7.7 96.8 95.5 96.1 HQ 17/14.38 K17 -10.7, -7.7, -0.5, 9.2 92.6 93.9 93.2

DU89-134/14 0, +12.5, +20 90.9 81.7 86.3 Tab. 2 Values of target function F, cAVG = 0.616 m, (m/S)m1 = 32.5 daN, (m/S)m2 = 50.7 daN

Fig. 1 Polars of airfoils as modeled using Xfoil code, in regimes R1 to R3, with flap deflections given by Tab. 2. Wing loading (m/S)m1 = 32.5 daN

Fig. 2 Polars of airfoils as modeled using Xfoil code, in regimes R1 to R3, with flap deflections given

by Tab. 2. Wing loading (m/S)m2 = 50.7 daN To offer better insight into realization of low-drag on analyzed airfoils, polars for both wing loadings are shown on Fig. 1 and 2. Presented methodology has proved to be suitable for wing airfoil selection in preliminary flapped sailplane design, as well as for flap deflection optimization and even flapped airfoil design itself. Airfoils considered in present analysis can be used as initial geometries for further modifications in terms of optimization procedure.

2. In-flight flow control and visualization Possibility of drag coefficient reduction by transition control on the lower surface, while flap setting for circling, was studied. Surface flow visualization and integrating rake pressure measurement for optimum transition control was carried out on a high-performance sailplane HPH304S. No evidence of separation bubble was found on outer part of the wing and its transition to the winglet. Flaperons are equipped by transition control on lower side of the wing in front of their hinge by Zig-zag turbulator. To verify results of calculated optimum location, oil-flow tests were flown on V = 100km/h IAS and integrating rake on V = 85, 100, 120 and 140 km/h IAS. Position of separation bubble while turbulator tape removed and flap setting +2 was determined, Fig. 3. and found in good agreement with calculated data.

Fig. 3 Oil-flow visualization on lower surface of HPH304S wing with turbulator tape removed Xfoil analysis have showed a potential of drag reduction for higher angles of attack and positive flap deflections (circling in thermals) with transition control location xT/c = 0,7 (denoted as CS). Integrating rake was positioned 413 mm from the aircraft plane of symmetry (111 mm from wing root rib). Difference of mean total pressure in the wake pm and undisturbed total pressure p∞ , pRake = p∞ - pm , was measured by pressure transducer. All data were normalized with respect to the minimum reading, ΠRake = pRake / pRake min. Measurements have revealed considerably higher momentum loss in wake with CS configuration compared to both uncontrolled (NC) and factory setting (FT) for 85 km/h IAS, Fig. 4. Similar unacceptable results were obtained for +1 flap setting while 100 km/h IAS, Fig. 5. As a conclusion, there is no apparent need for adaptive transition control, which could have been realized for instance by two rows of pneumatic turbulators. Calculated overall optimum location for turbulator tape (FT) and also appropriate flap deflections for given airspeeds were confirmed experimentally. Values of ΠRake for all flap deflections are shown on Fig. 6.

Fig. 4 Normalised integrating rake pressure difference ΠRake. HPH304S sailplane, Airfoil chord c = 793 mm, V = 85 km/h IAS

Fig. 5 Normalised integrating rake pressure difference ΠRake. HPH304S sailplane, Airfoil chord c = 793 mm, V = 100 km/h IAS

Glide-ratio measurement was performed by GPS method in terms of quantitative comparison with Ventus2C sailplane on four airspeeds using side-by-side flights under the conditions of interthermal glide. Flights on 80, 130, 170 and 200 km/h IAS were accomplished. Examples of acquired data are shown on Fig. 7. Primary objective was to initiate study of dynamic properties of a sailplane in real atmosphere, mainly at the high lift coefficient. Shape of the lift curve in the region of upper limit of low-drag bucket on wing airfoil polar can have effect on sailplane response to the vertical gust, as proposed by Kubrynski (2006). Secondary objective was to give a view of the order of sailplane performance in real in-flight situations.

Fig. 6 Normalised integrating rake pressure difference ΠRake. HPH304S sailplane, Airfoil chord c = 793 mm, V = 100 km/h IAS, factory installed turbulators (FT configuration)

Fig. 7 GPS records of MSL height of side-by-side final glide of HPH304S and Ventus2C sailplanes, 80 km/h and 200 km/h indicated airspeed. Sink-rate in given height VKh, VK0 reduced to h = 0 m and average glide-ratio L/D evaluated. Flown on March 26th 2007

3. Conclusions Procedure of target function evaluation, enabling to take into account optimum flap deflection was developed and used for five different airfoils, typical for contemporary FAI 18m sailplanes. Verified Xfoil code was used to calculate values of the criteria. Surface flow visualization, integrating rake pressure measurement and GPS data recording were implemented for in-flight measurements on two FAI 18m sailplanes. Both numerical and experimental methods were used for investigation of possibility of drag coefficient reduction by transition control. Calculated overall optimum location for turbulator tape and appropriate flap deflections for given airspeeds were confirmed experimentally.

4. Acknowledgement Grant support of GA AS CR No. IAA2076403, No. IAA200760614 is gratefully acknowledged. Work has been also supported by the Ministry of Education, Youth and Sports of the Czech Republic, within project No. 1M06031.

5. Literature Althaus, D., Wortmann, F.X.: Stuttgarter Profilkatalog I

1. vyd. Braunschweig: Vieweg & Sohn Verlagsgesellschaft, 1981. 319 s. ISBN 3-528-08464-2 Althaus, D.: Niedriggeschwindigkeitsprofile

1. vyd. Braunschweig: Vieweg & Sohn Verlagsgesellschaft, 1996. 592 s. ISBN 3-528-03820-9 Boermans, L.M.M., van Garrel, A.: Design and Windtunnel Test Results of a Flapped Laminar Flow

Airfoil for High Performance Sailplane Applications In. Technical Soaring, Vol. 21 (1997), No. 1, p. 11-17. ISSN 07448996

Horstmann, K.H., Quast, A.: Widerstandverminderung durch Blasturbulatoren Forschungsbericht DFVLR-FB 81-33, Braunschweig: DFVLR, 1981. 53 s.

Kubrynski, K.: Aerodynamic Design and Cross-country Flight Performance Analysis of Diana-2 Sailplane. In. Technical Soaring, Vol. 30 (2006), No. 3, p. 79-88. ISSN 07448996

Popelka, L., Matějka, M.: Optimization Criteria and Sailplane Airfoil Design In. Technical Soaring, Vol. 30 (2006), No. 3, p. 74-78. ISSN 07448996