Wind Tunnel Test of a Paraglider (flexible) Wing Canopy IJMME-IJENS.pdf · International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:10 No:03 7 101803-6464 IJMME-IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:10 No:03 7

101803-6464 IJMME-IJENS © June 2010 IJENS I J E N S

Wind Tunnel Test of a Paraglider (flexible) Wing Canopy

Md. Nizam Uddin and Mohammad Mashud Department of Mechanical Engineering

Khulna University of Engineering & Technology (KUET) Khulna-9203, Bangladesh

E-mail: [email protected]

Abstract-- An inflatable cell model of paragrlider’s wing canopy has been investigated in wind tunnel experiments, which is designed to represent the dynamic behaviors of each cell comprising the canopy. Effects of air-intake of paraglider canopy have also been investigated in this study. The improvement of the shape of air intake and pressure distribution from the standpoints of aerodynamics will contribute to raise flexible wing gliding ratio and safety. To perform the experiment, flexible wing canopy cell model and simplified rigid air intake model had been prepared. Cross-sectional wing profiles and internal surface pressures of the cell model are measured at different angle of attack and Reynolds number, to understand the formation characteristic of paraglider wing canopy model. The cell model is fully inflated at greater than a few degrees angle of attack. In order to understand the lift and drag behaviors, external pressure distributions of the cell model are measured. Index Term-- Paraglider, Air-intake, Inflatable Structures, Flexible Wing.

1. INTRODUCTION Originally paraglider is derived from a parachute. Though it’s originated from parachute, from the first and many researches have been done about the formation method and aerodynamic characteristics of parachute, a research on existing shape of paraglider has not been done. Early 60th NASA has been investigated the aerodynamic characteristics of triangular shape paraglider [1-4] but the existing shape parglider yet done. Due to its low speed handling qualities and versatility of application for precision aerial delivery and recovery of payloads, the paraglider has been used in many areas from leisure to more sophisticated aerial recovery. Recently, it counts upon use from such an advantage as the various spotter plane dexterity in high altitude, recovery of a payload, and substitution of a military dropping parachute. As an example, a Crew-Return Vehicles X-38 of NASA is famous, and it succeeds in fall and a dropping flight experiment. In Europe, the Institute of Flight Research of the German Aerospace Center (DLR) has conducted research to identify the dynamic behavior of a parafoil-load system and to investigate Guidance Navigation and Control (GNC) concepts[6]. Though parglider is originated from parachute, the form maintenance method of a parachute and a paraglider is totally different. The Parachute only uses resisting force which swells and sediment by the wind. A paraglider can maintain form in addition to wing’s inner pressure to lift, which is perpendicular to the wind direction and to maintain form. Therefore, the flight, which can be set horizontally and is maneuverable, is making it the parachute. But the control

characteristics of a paraglider similar to parachute. It is essential that the cross section of the canopy in flight forms a airfoil shape in the presence of air intake part. Therefore, the understanding of the inflated wing characteristics is a first step to develop a high performance canopy. From this stand point we had examined the effects of air intake of a paraglider canopy model and the formation characteristics by measuring the internal pressure and cell profile. In this study, research has been concentrated the attention of the paraglider’s wing formation characteristics and fundamental aerodynamic characteristics of paraglider wing, and the characteristics of air intake, which does not exist on other wings forms even in various research targets. Air intake is the most important to secures wings form, which is located in a front wing tip part, and has great influence on the wings characteristics. Therefore, obviously the improvement of the safety of a paraglider and also the improvement in the flight characteristics are possible by exploring this from an aerodynamic standpoint.

2. GENERAL DESCRIPTION OF PARAGLIDER Paragliding is the simplest form of human flight. A paraglider is a non-motorized, foot-launched inflatable wing. It is easy to transport, easy to launch, and easy to land. A paraglider is very excellent as a sky sports vehicles in recent nature, it will become very compact. Weight is also comparatively low and man can carry it on the back, and it can be moved. The paraglider consists of numbers of flexible wing canopies in parallel with a load suspended beneath it on cables.

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Fig. 1. Paraglider wing. The hole that is opened in each wing front is called air intake, which is formed using ram pressure. The ram pressure obtained from this air intake and a pressure distribution and the tension from a line of the air pressure which works on wings are related to maintenance of the wings under flight. Moreover, there are open holes between each canopy, and the work is carried out to maintain inner pressure stable within wings. Control of such a vehicle is achieved by changing the center of gravity of the paraglider with respect to the wing. Moving the payload to the rear causes as increase in the angle of attack. Turns to the left or right are accomplished by moving the payload left or right with respect to the wing. Paraglider thus far flown have exhibited excellent stability and can be controlled to very high angles of attack. The principle advantage of such a scheme lines in the fact that the structure carries loads primarily in tension and that can be constructed with a minimum of weight. A reasonable weight for a practical single-ply cloth is about 1 kilogram per square meter. Fig. 1 shows a general form of Paraglider.

3. EXPERIMENTAL APARATUS AND PROCEDURE 3.1 Experimental set-up, models description The experiments were conducted in a 400mm×340mm wind tunnel at Khulna University of Engineering & Technology, Mechanical Engineering Department in the Fluid Mechanics Laboratory. A paraglider wing consist a series of same type parallel canopies, so we have studied one wing canopy model to characterize the paraglider wing. For the limitations of our wind tunnel we hade to prepare small size of model. Fig. 2(b) shows one wing canopy model configurations. Model dimensions are, wing chord length 150 mm, wing thickness 22 mm and wing span 150 mm. To reduce the effect of wingtips vortex during external pressure measurement, considerable lager wing span (150mm) model has been made. During the flight period wing ribs acts as a thin rigid body so the ribs can be replaced by any suitable rigid materials instead of inflatable materials for experiment. The rib of the canopy model was made by 2 mm thick acrylic sheet and for each model (canopy) ribs are joined by 4 mm diameter aluminum rod, wing’s upper and lower surface materials are inflatable. Firstly we had selected three different types of inflatable materials, nylon cloth (which is usually used to make paraglider wing) and vinyl sheet with 0.05 and 0.1 mm thickness according to softness. Models which were made by nylon cloth and vinyl sheet 0.1 mm thickness appeared the similar type of leading edge deformation of the wing at different angle of attack that’s

happened due to the materials rigidity for small size model. But the model which was made by vinyl sheet of 0.05 mm thickness acts as like as real paraglider wing and the leading edge deformation changes with angle of attack. Finally, we have selected the thinner vinyl sheet of 0.05 mm thickness for our experiment as inflatable material. The paraglider wing coordinate data received from a maker (FALHARK), and it is used in this research. As of our impression, this wing form is thicker than a common wing and it was thought that the front tip curvature radius was large. It is considered that there is an effect, which raises rigidity by enlarging the thickness of the wing. The rapid lift fall at the time of damping is prevented taking a large curvature radius and it is thought to obtain high lift power coefficient even in low speed.

Fig. 2. (a) Schematic diagram of wind tunnel test section

Fig. 2. (b) Wing canopy model dimensions.

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Fig. 2. (c) Air in-take model As shown in Fig. 2(a), the canopy model is mounted at the center of the test section, whose trailing edge is located about 429 mm downstream of the end of the test section and wing is suspended by the support of the both rib and wind tunnel test section side wall. The Reynolds number Re was defined on the basis of the wing cord length (c) and free stream flow velocity (U∞). Free-stream velocity of the wind tunnel has been changed to examine Reynolds number effects. The

maximum Reynolds number was limited by wind tunnel operating envelope, which was a maximum of about 14 m/s. We believe that this experiment is sufficient to measure the aerodynamic characteristics of paraglider because the maximum speed of paraglider is 5.5m/s to 13.5m/s. All quantities were made dimensionless. The length, pressure and velocity were made dimensionless using wing cord length c, uniform pressure P∞ and uniform velocity U∞. 3.2 Pressure measurement The static pressure along the wing surface was measured by means of digital pressure gauge (model GC 15-611 manufactured by Nagano Keiki, Company Limited). 0.5 mm diameter pressure probes were out-fitted on the upper surfaces of the wing to measure the internal pressure and these

pressure probes were connected with digital pressure gauge by the vinyl tubes. To prevent the time lag difference of each orifice measurement, vinyl tubes of equal length (670 mm) were used to connect the digital pressure gauge to the probes on the surface of the wing.

Fig. 2. (d) Data comparison of vertical probe method andStandard method. Though the best way to measure the external surface pressure of the wing is to drill the pressure probes on the surface of the wing from the inside of that wing, as we are using inflatable canopy cell model, couldn’t get the accurate pressure distributions over the surface of the wing, if we drilled and fitted the pressure probes on the inflatable surface. Because, (1) in that case we must need to connect the pressure orifices to digital pressure gauge by vinyl tubes and we had to have make hole on the rib to come out the vinyl tubes, (2) wing doesn’t inflate freely due to effect of vinyl tubes weight which will pull down the wing surface. For those reasons we adopted another method to measure the external surface pressure. In this method we placed a pressure orifice (probe) normal to the surface of the measuring point, and the pressure orifice is mounted with a mechanical transverse/sliding system which was placed over the test section (Fig. 2(a)), and the pressure orifice is connected with digital pressure gauge by a vinyl tube. Purpose of this mechanical transverse/sliding system is to move the pressure orifice, in this system pressure orifice can be moved to and fro, left and right, and up and

down. As we are using only one pressure orifice and one vinyl tube so we don’t need to think about time lag difference. To get high accuracy, the pressure orifice should be normal to the measuring surface and maintain as low as possible gap between probe and surface. If the probe makes actuate or obtuse angles with surface and/or make the large gap between probe and surface, so the measuring pressure will be affected by the dynamic pressure. To assure the high accuracy of the pressure measurement in our experiment we had maintain the gap between probe and surface as minimum as possible (0.5 to 1.0 mm). Before doing the main experiment firstly we had compare the data of this method for one rigid model with standard method (drilled the pressure orifices on the surface of the model) for the same rigid model which is shown in Fig. 2(d). The pressure variation of this method is ±0.3 Pa with standard method. The pressure measured at the test section inlet was used as reference static and dynamic pressure, thus we could calculate the test section free stream velocity.

4. RESULTS AND DISCUSSION 4.1 Effects of air intake In order to inflate the wing and maintain internal pressure, air intake should be exists in a flexible wing. The existence of air intake is not only important to inflate the wings, but also has the big feature of a flexible wing. Although this intake exists near the wings front tip part which influences the wings characteristic greatly, unfortunately there is no research which has investigated the influence which intake has on the flow of the circumference of wings. Therefore, to investigate the effects of air intake, simplified air intake model was made which shown in Fig.2(c), the flow visualization was investigated over the surface of the open and close air intake model. 4.1.1 Flow visualization The purpose of this visualization experiment is to justify the hot-wire anemometer measurement of air intake model. The flow around the air intake model was visualized with smoke wire method. In this visualization experiment, the Reynolds number of the uniform flow was 20,128. For comparison, the

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open and close model was visualized. In the picture (Fig.4), the bright portion shows where smoke particles are convected by the uniform stream. Thus the dark portions around the object in the pictures correspond to the separated flow regions. Comparison of these two pictures (open and close), the height of the separation zone of open system is higher than that of close system and the same result appears in the hot-wire anemometer experiment. This happens due to the direction of turning flow at the point of separation (at tip edge of the model). Though it’s difficult to identify the exact location of stagnation point due to turbulences, we can gauss the nearest regions of stagnation point from the visualization picture. According to the picture, in close air intake model the stagnation point is slight front of the intake surface boundary. But in open air intake model the stagnation point is inside the intake surface boundary regions. As the stagnation point is different for the open and close system, the direction of turning flow also different and which is shown in Fig.3. For close system, as the stagnation point is slightly in front of the intake surface boundary so the turning flow direction is like as Fig. 3(a) which makes an actuate angle with surface, consequently the height of the separation zone is low. But in open system the stagnation point is inside the model and the

turning flow direction like as Fig. 3(b) which makes an obtuse angle with surface compare to close system, thus it also make the higher separation zone height. From the above discussion, we can also say that the magnitude of main stream flow velocity over the separation region of open system is higher than the close system. As, the height of the separation regions and the main stream flow velocity over the separation region is higher, of open system than close system, so the surface pressure of the open system is lower than that of close system because a fluid particle which moves in the immediate vicinity of the wall in the boundary layer remains under the influence of the same pressure field as that existing outside, because the external pressure is impressed on the boundary layer, which is shown in Fig.6. The length of separation regions couldn’t identify by the visualization experiment due to the lack of smoke all over the surface of the model but in our hot-wire experiment that can be traced out which is about 4.5 times of the air intake model thickness, this result consistent with the previous research of Ota and his co-workers [7]. They have observed the character of leading edge flow at high Reynolds number. They mentioned that the turbulent separated layer reattaches to the plate at a distance of four times the plate thickness from the leading edge.

Fig. 3. Turning flow direction, (a) close model, (b) open model.

Fig. 4. Flow Visualization of air intake model (a) close (b) open. 4.1.2 Pressure distributions The investigation of pressure distribution result is shown in Fig.6 for a range of 0-20 degree angle of attack. In every angle of attack the pressure coefficient of open air intake system is higher than close air intake system, this is also happened due to the change of stagnation point. In open air intake system, as a result of higher magnitude and slope of turning flow velocity at the front tip edge, it causes to develop vortices in right top above of the front tip and creates lower-pressure coefficient than the close air intake system.

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4.2 Wing profile The measured profiles of the inflated cell model are shown in Fig.7 at various angle of attack. It is found that, at 0 degree

angle of attack the inflated surface and rib of the wing are consistence in the same plane, front of the inflated surface is deformed and pushed into the inner side of the cell model, consequently leading edge of the upper and lower surface become closer to each other, as a result air intake size become very smaller and wing couldn’t inflate in proper shape due to lack of insufficient air inside the wing. And the stagnation point of the uniform stream is located on the dented portion of the upper surface. The upper inflated surface also makes a sharp edge because of wing front deformation. Due to this sharp edge, flow is separated from the beginning of the upper inflated surface which is shown in Fig.8(0). Flow also separated at the leading edge of the lower inflated surface and then reattached, because the stagnation region is inside the deformation of the upper surface.

Fig. 7. Wing profile at various angle of attack. As the angle of attack increases from zero to 2 degrees, the inflated wing shape changes dramatically. At 2 degrees angle of attack there is no deformation in the front of the wing even the leading edge of the upper surface advanced slightly and form curvature shape, consequently makes large size of air intake. This implies that the stagnation point moved to a point located ahead of the air intake. At large attack angle greater than two degrees, the overall inflated wing shape is unchanged and only orientation changed. There is no flow separation on the upper inflated surface but there is small separation region

at the leading edge of the lower inflated surface. In this case the inflated wing surface position is upper than the rib of the wing, that is, the wing inflates fully. Further increase angle of attack there is no significant changes of air intake size but reduce the separation region at the leading edge of the lower inflated surface which is shown in Fig.7. From 2 to 30 degree angle of attack, in all cases the inflated surface position is above to the rib of the wing. At 8,10,15 and 20 angle of attack the inflated surface near the trailing edge inflates more than the others attack angle, that is , the inflated surface makes

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large radius of curvature, due to maintain high inner pressure of the wing. For this large radius of curvature of the inflated surface fluid flows smoothly over the surface with small pressure gradient. At 30 degree angle of attack a small portion of the upper inflated leading edge surface shows like flat, there is no pressure gradient at this region. 4.3 Internal pressure distribution The inside pressure of wing is deeply related to wing’s formation of a flexible wing. Internal wing pressure distribution is shown in Fig.9 at different angle of attack and dependency of average internal pressure with angle of attack is shown in Fig. 8. The horizontal abscissa express the non-dimensional distance along the chord length from leading edge of the wing toward trailing edge and vertical abscissa express the non-dimensional pressure. As shown in Fig.9, the inner pressure almost uniform all over the wing at each angle of attack, because a fluid particle which moves in the immediate vicinity of the air intake remains under the influence of the same pressure field as that existing immediate outside of the air intake, because the external pressure at the position of air intake is impressed on the inside fluid particles. It has also shown how the average inner pressure changes with angle of attack. The average inner pressure increases with increasing angle of attack at a certain limit (at 8 degree angle of attack) after that it becomes unchanged. At low angle of attack wing front tip was deformed, consequently front cross-section area increases as a result decrease the air intake size, and the stagnation point appears in the deformed portion of the wing. But at higher angle of attack (few degrees greater than 0) leading edge of the wing doesn’t deformed and the stagnation point located slight front on the air intake. While inner pressure of flexible wing is maintained constant pressure at higher attack angle, flexible wing turns out gently that change of inner pressure contra of attack angle change. By this result, a paraglider can continue maintaining the form of wings also to some disturbance and it is obtained the conclusion that the stable flight is possible even at low attack angle (must be greater than 0).

Fig. 8. Variation of internal pressure with angle of attack.

4.4 External pressure distribution In most practical situation, three-dimensional flow-fields are the rule rather than the exception. The analysis of three-dimensional is a difficult task, and therefore most investigations are concentrating on two-dimensional configurations. As described in the paper by Ruderich and

Fernholz, the flow was nominally two-dimensional if the aspect ratio was larger than 10 where the aspect ratio is formed by tunnel and height of the object. In the present experimental configuration, the ratio of tunnel height and wing height is more than 10, which is sufficient to maintain two-dimensional flow. However, there is a strong three-dimensional flow effect that occur wing tip vortex at the upper surface of the wing. In order to minimize the three-dimensional flow effect during pressure measurement, the pressure orifice was located along the mid-span of the wing. Therefore we assumed the flow around the wing for this experiment is two-dimensional flow.

Fig. 9. Internal and External pressure distributions (angle of attack)

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Fig. 9. Internal and External pressure distributions (angle of attack) The outer surface pressure distribution was measured for various angle of attack and Reynolds number. The results of outer surface pressure distribution are presented as plots of pressure coefficient Cp against fraction of local chord projected to the plan of the leading edge and trailing edge (non-dimensional distance) and are presented in Fig.9. Short

discussions of the pressure data are presented as well as a few comments on the general characteristics of the data. In general, the overall distribution characteristics shown that the large peak pressures occurring near the leading edge of the wing at every angle of attack followed by a large adverse pressure gradient. The stagnation pressure coefficient for every angle of attack is all about similar except 0 degree, because at 0 degree attack angle the leading edge of the wing was deformed but in the other cases the leading edge of the wing doesn’t deformed even the leading edge of the upper surface is slightly advanced from the original position. According to Fig.9, we can also say, there is no effect of Reynolds number on the outer surface pressure distribution of the wing but angle of attack affect the pressure distribution. The change in attack angle not only affects the upper outer surface region, but also the overall flow features around the wing. As described in the NASA technical note by Paul G. Fournier and B. Ann Bell [3], at the low angle of attack for triangular shape rigid paraglider model shows the negative lift values. But in our flexible model gives the positive lift at low angle of attack. At 0 degree angle of attack the pressure distribution characteristics indicate positive lift of the wing, the wing doesn’t inflate properly as like original shape and leading edge was deformed, which is not sufficient to maintain flight. And at 30 degrees angle of attack the value of pressure coefficients (near about 0) is almost unchanged along the upper surface of the wing just after the leading edge. This implies that the main flow is completely separated on the upper surface of the wing from the beginning of the leading edge, that was happening due to increase the pressure gradient in the direction of flow and the boundary layer thickens rapidly. The adverse pressure gradient plus the boundary shear decreases the momentum in the boundary layer and they both act over a sufficient distance they cause the fluid in the boundary layer to come to rest i.e., the retarded fluid particles, cannot, in general penetrate too far into the region of increased pressure owing to their small kinetic energy. Thus, the boundary layer is deflected sideways from the boundary, separates from it and moves into the main stream. At 20 degrees angle of attack the upper surface down stream flow was separated about 20% (chord length) before the trailing edge. Similarly at 15 degrees angle of attack the upper surface down stream flow also separated near the trailing edge. On the other hand, at 2 to 10 degrees angle of attack there is flow separation either upper or lower surface of the wing, i.e., the flow attaches both upper and lower surfaces. But at 8 degree angle of attack shows the better pressure distribution characteristics compare to 2, 6 and 10, implying that the maximum lift.

5. CONCLUSIONS An experimental investigation of simplified rigid air intake model and a paraglider canopy cell model for different angle of attack and different Reynolds number has been conducted. From this investigation, it is clear that the presence of air intake is not only important to inflate the wings, but also has

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the big feature of a flexible wing. It is also found, the wing flexibility is very important to maintain the inner pressure during flight period at different situation and at high angle of attack the main stream flow is separated from the beginning of the leading edge and at low angle of attack the pressure gradient between upper and lower surface is very low, consequently lift is low but at moderate angle of attack (10 degrees) the pressure distribution is satisfactory and get maximum lift. The examination of the pressure force acting on the upper and lower sheets revealed in what aerodynamic mechanism the cell automatically inflates into an airfoil shape and how the inflated cells can bear the buckling forces cased by the suspended weight through the cables in paragliding.

REFERENCES

[1] F. M. Rogallo , J. G. Lowry, D. R. Croom, and R.T. Taylor,” Preliminary Investigation of a Paraglider,” NASA TN D-443,1960.

[2] D. E. Hewes,” Free-Flight Investigation of Radio-Controlled Models with Parawing,” NASA TN D-927,1961.

[3] P.G. Fournier and B.A. Bell,” Low Subsonic Pressure Distributions on Three Rigid Wings Simulating Paragliders With Varied Canopy Curvatures and Leading-Edge Sweep, NASA TN D-983, 1961.

[4] B. Gamse, K. W. Mort, and P. F. Yaggy,” Low-Speed Wind Tunnel Tests of A Large Scale Inflatable Structure Paraglider,” NASA TN D-2859, 1965.

[5] Yamamori and Akira Umemura,” Experimental Investigation of Paraglider Wing canopy Formation” , Journal of JSASS (in print).

[6] G. Hur, and J. Valasek, ”System Identification of Powered Parafoil-Vehicle from Fight Test Data,” AIAA-2003-5539,AIAA Atmospheric Flight Mechanics Conference and Exhibit, Austin, TX ,August 2003.

[7] J.C.Lane and R.I.Loehrke,”Leading Edge Separation From a Blunt Plate at Low Reynolds Number”, Transaction of the ASME, Vol. 102, Dec. 1980.

Documents

Wind Tunnel Test of a Paraglider (flexible) Wing Canopy IJMME-IJENS.pdf · International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:10 No:03 7 101803-6464 IJMME-IJENS