CFD ANALYSIS OF AIRCRAFT WING FLAPS Mrs.S.Vandaarkuzhali
Transcription
CFD ANALYSIS OF AIRCRAFT WING FLAPS Mrs.S.Vandaarkuzhali
Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” CFD ANALYSIS OF AIRCRAFT WING FLAPS 1 Mrs.S.Vandaarkuzhali, 2R.Manikandan, 1 AssoProfessor,2PG Students Department Of Mechanical Engineering, Mailam Engineering College, Mailam. E-Mail: 1shan-kuzhali82@yahoo.com,2 manirmech27@gmail.com ABSTRACT A Flap is the simplest trailing edge device which can be used as a high lift device for low speed applications like micro air vehicles, gliders, wind turbines etc. The Gurney Flap is named after American aero dynamist Dan Gurney who introduced it in the form of a vertical tab attached to the trailing edge of an ordinary aerofoil. This modification makes the flap capable of producing higher lift force at lower velocities.This paper is based on the ―CFD ANALYSIS OF AIRCRAFT WING FLAPS‖ and from the results of the experiment an empirical relation for the optimum flap height has been proposed. And also‖Comparison Between NACA palf tuohtiw dna htiw liofria 0015‖.The paper contains a vivid description of the hysteresis effects of the flow on the flap. The paper also mentions the advantages, disadvantages and applications of the flap. aerofoil geometry, which is the camber of 1.INTRODUCTION High lift devices are one of the most aerofoil. The second kind of devices work important aerodynamic devices attached to on the principle of energizing the boundary aircrafts and other flying machines. As the layer. A gurney flap is a typical and simple name indicates these are intended to produce high lift device which works on the principle higher lift force than conventional wings or of changing the effective camber of the aerofoil. Generally two types of high lift airfoil. devices are used in practice. The first type The Gurney flap is a vertical tab works on the principle of increasing the added to the trailing edge on the pressure 54 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” side of a wing. Car racer Dan Gurney is The first application of the flap was credited as the inventor in the early 1970s, in 1971, after Gurney retired from driving although no patent could be granted. The and began managing his own racing team Gurney flap is a simple device, consisting of full-time. His driver, Bobby Unser, had been a short strip, on the order of 1–5% of airfoil testing a new Gurney designed car at chord in height, fitted perpendicular to the Phoenix International Raceway, and was pressure surface or the chord-line along the unhappy with the car's performance on the trailing edge of a wing.The most common track. Gurney needed to do something to application of this device is in racing-car restore his driver's confidence before the spoilers, where it is used to increase the race, and recalled experiments conducted in down-force increase in lift and a slight the 1950s by certain racing teams with reduction in drag. Larger lift increments "spoilers" affixed to the rear of the were observed for greater flap heights, but bodywork to cancel lift. (At that level of the development, the spoilers were not thought drag increased noticeably beyond heights of approximately 2%C. of as potential performance enhancers— merely devices to cancel out destabilizing 2.HISTORY OF GURNEY FLAPS and potentially deadly aerodynamic lift.) by Gurney decided to try adding a "spoiler" to automobile racing icon Dan Gurney, was a the trailing edge of the rear wing. The right-angle piece of sheet metal, rigidly device was fabricated and fitted in under an fixed to the top trailing edge of the rear wing hour, but Unser's test laps with the modified on his open wheel racing cars of the early wing turned in equally poor times. When 1970s. The device was installed pointing Unser was able to speak to Gurney in upwards to increase down force generated confidence, he disclosed that the lap times by the wing, improving traction. He field with the new wing were slowed because it tested it and found it allowed a car to was now producing so much down force that negotiate turns at higher speed, while also the car was under steering. All that was achieving higher speed in the straight needed was to balance this by adding sections of the track. additional down force in front. The original application, 55 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” Unser realized the value of this 2.1 GURNEY FLAP breakthrough immediately and wanted to ―Gurney‖ is a device the teams use to adjust conceal it from the competition, including the downforce generated by the wing. his brother Al. Not wanting to call attention to the devices, Gurney left them out in the ―Gurney‖, also called a ―Gurney flap‖ or open. To conceal his true intent, Gurney ―wickerbill,‖ is a trailing edge flap. deceived inquisitive competitors by telling Airplanes use flaps for increased lift during them the blunted trailing edge was intended take off and landing. to prevent injury and damage when pushing the car by hand. Some copied the design, and some of them even ―improved‖ it by pointing the flap downwards, which actually hurt performance. Gurney was able to use the device in racing for several years before its true purpose became known. Later, he discussed FIG- 1 Gurney flap with trailing edge of his ideas with aerodynamicist and wing wing. designer The Gurney Flap (or wickerbill) is a small Bob Liebeck of Douglas Aircraft Company. flat tab projecting from the trailing edge of a Liebeck tested the device, which he later wing. Typically it is set at a right angle to named the ―Gurney flap,‖ and confirmed the pressure side surface of the airfoil and Gurney’s field test results using a 1.25% projects 1% to 2% of the wing chord This chord flap on a Newman symmetric airfoil. trailing edge device can improve the His 1976 AIAA paper (76-406) ―On the performance of a simple airfoil to nearly the design of subsonic airfoils for high lift‖ same level as a complex high-performance introduced the concept to the aerodynamics design. community. 56 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” 3.FLEXIBLE EXTENDED TRAILING EDGE A Biologically-Inspired Concept FIG 2.-Gurney flaps airfoil. The device operates by increasing pressure on the pressure side, decreasing pressure on the suction side, and helping the FIG 4-Example for flexible extended boundary layer flow stay attached all the trailing edge. way to the trailing edge on the suction side 3.1THEORY OF OPERATION of the airfoil. Common applications occur in auto racing, helicopter horizontal stabilizers, The Gurney flap increases and aircraft where high lift is essential, such the maximum lift coefficient (CL,max), decreases as banner-towing airplanes. the angle of attack for zero lift (α0), and increases the nosedown pitching moment (CM), which is consistent with an increase in camber of the airfoil. It also typically increases the drag coefficient (Cd), especially at low angles of attack, although for thick airfoils, a reduction in drag has been reported. A net benefit in overall lift to FIG 3.-Trailing edge flow field for an airfoil drag ratio is possible if the flap is sized with Gurney flap . appropriately based on the boundary layer thickness 57 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” wings to generate the necessary lift force ,at lower velocities. More over the application of high lift devices reduces the stalling speed of the aircraft. Stalling speed of the aircraft is the minimum speed required to produce the necessary lift, so that the aircraft is in FIG 5- Wing Mounted in Test Section equilibrium. A reduced stalling speed makes the aircraft to land, take off or even fly at The Gurney flap increases lift by altering the low speed. Kutta condition at the trailing edge. The wake behind the flap is a pair of counter- 3.3.HELICOPTER APPLICATION rotating vortices that are alternately shed in Gurney flaps have found wide a von Kármán vortex street. In addition to these spanwise vortices shed behind the flap, application on helicopter horizontal chordwise vortices shed from in front of the stabilizers, because they operate over a very flap become important at high angles of wide range of both positive and negative attack.The increased pressure on the lower angles of attack. At one extreme, in a high- surface ahead of the flap means the upper powered climb, the negative angle of attack surface suction can be reduced while of the horizontal stabilizer can be as high as produced lift. -25°; at the other extreme, in autorotation, it may be +15°. As a result, at least half of all 3.2NEED FOR HIGH LIFT DEVICE modern helicopters built in the West have them in one form or another. From the basic principles of aerodynamics, the lift force produced by an aerofoil is directly proportional to the velocity of flow. For an aircraft when landing or take off, the velocity is desirable to be lower to reduce the length of runway required .But for this some additional high lift devices has to be incorporated in the 58 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” low angles of attack. The double gurney flap reduces the control input required to transition from hover to forward flight. 3.4PRESSURE DISTRIBUTION OVER THE GURNEY FLAPS The device basically operates by increasing pressure on the pressure side of the wing, decreasing pressure on the suction side, and helping the boundary layer flow T stay attached all the way to the trailing edge FIG 6- Gurney flaps in Holicopter for on the suction side of the airfoil. At the same time, a long wake downstream of the flap horizontal stabilizer. containing a pair of counter-rotating vortices can delay or eliminate the flow separation he Gurney flap was first applied to near the trailing edge on the upper surface the Sikorsky S-76B variant,when flight (aircraft wing) or lower surface (racing car testing revealed the horizontal stabilizer wing). Correspondingly, the total suction on from the original S-76 did not provide the airfoil is increased. sufficient lift. Engineers fitted a Gurney flap to the NACA 2412 inverted airfoil to resolve the problem without redesigning the stabilizer from scratch. A Gurney flap was also fitted to the Bell JetRanger to correct an angle of incidence problem in the design that was too difficult to correct directly. The Eurocopter AS355 TwinStar FIG 6- Effect of Gurney flaps. helicopter uses a double Gurney flap that projects from both surfaces of the vertical stabilizer. This is used to correct a problem 3.5.DIMENSIONS OF FLAPS with lift reversal in thick airfoil sections at 59 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” The experiment on the flap was (1) Mass is conserved , (2) Newton’s second law ( force = mass conducted in a low speed open jet wind * acceleration ), tunnel. Open jet wind tunnel was preferred (3) because of the ease of taking measurements Energy is conserved. These fundamental physical from it. The test section was a 0.457m principles can be expressed in terms of basic square section and was 1.2m long.The mathematical equations, which in their most velocity range for the air in tunnel was general form are either integral equations or ranging from 4m/s to 15m/s.The aerofoil partial differential equations. Computational was made of balsa wood and its surface was fluid dynamics is the art of replacing the polished and coated with water proof paint integrals or the partial derivatives ( as the The aerofoil was rectangular in plan. case may be ) in these equations with Following were the important dimensions of discredited algebraic forms, which in turn the aerofoil. are solved to obtain numbers for the flow ¢-Span=0.457m ¢-Chord=0.154m ¢ -Maximum thickness =10mm field values at discrete points in time and/or space. The end product of CFD is indeed a collection of numbers, in contrast to a closed-from analytical solution. However, in at 15% chord the long run, the objective of most ¢-Maximum thickness to chord engineering ratio=0.065 analyses, closed form or otherwise, is a quantitative description of the problem, i.e., numbers. The instrument The experimental setup was incorporated which has allowed the practical growth of with a pyramidal balance with digital read CFD is the high-speed digital computer. out to measure the forces acting on the flap CFD accurately. solutions generally require the repetitive manipulation of many thousands, even millions, of numbers, a task that is 4.INTRODUCTION TO CFD humanly impossible without the aid of a To answer this question, we note that computer. Therefore, advances in CFD, and the physical aspects of any fluid flow are its application to problems of more and governed by three fundamental principles: more 60 detail and sophistication, are NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” intimately related to advances in computer the flow field in a reasonable time. As a hardware, particularly in regard to storage result of these factors, computational fluid and execution speed. This is why the dynamics is established industrial design strongest force driving the development of tool, helping to reduce design timescales and new supercomputers is coming from the improve CFD community. engineering world. CFD provides the cost- the process throughout the effective and accurate alternative to scale 4.1 History of CFD model testing, with variations on the Computers have been used to solve simulation fluid flow problems for many years. being performed quickly, offering obvious advantages. Numerous programs have been written to 4.2 Application of CFD solve either specific problems, or specific classes of problems, or specific classes of CFD is the analysis of systems problem. From the mid-1970’s the complex involving fluid flow, heat transfer and mathematics required to generalize the associated phenomena such as chemical algorithms began to be understood, and reaction by means of computer based general were simulation. This technique is powerful and developed. These began to appear in the spans a wide range of industrial and non- early 1980’s and required what were then industrial application areas. Some examples very powerful computers, as well as an in- are: purpose CFD solves depth knowledge of fluid dynamics, and Aerospace Automobile and Engine Industrial Manufacturing Naval Architecture Civil Engineering models mean that the process of creating a Environment Application CFD model and analyzing the result is much Health and Safety large amounts of time to set up simulations. Consequently CFD was a tool used almost exclusively in research. Recent advances in computing power, together with powerful graphics and interactive 3-D manipulation of 4.3 Methodology of CFD less labour-intensive, reducing the time and there the cost. Advanced solvers contain CFD codes are structured around the algorithms which enables robust solution of numerical algorithms that can tackle fluid 61 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” flow problem. All codes contain three up. Remember Bernoulli The slower air on elements: top is at a higher pressure and presses down on the wing surface. The force a wing Pre-processor Solver Post-processor produces depends on the airfoil shape, the area of the wing, and the square of its speed through the air. 4.4 Pre-processor: It consists the input of a flow problem to a CFD program by a user. The user activities at the pre-processing stage involve. Definition of the geometry of the On the second picture is a racing car wing region of interest: the computational at a high angle of attack. At high angles of domain attack, air is unable to follow the contour of Grid generation the lower wing surface and can detach Selection of the physical (stall), lowering the efficiency (downforce) and of chemical phenomena that need to be the wing and adding drag. modelled Definition of fluid properties Specification of appropriate boundary conditions. A small lip on the trailing edge, The first picture shows a racing car shown in the third picture, causes a lower wing which generates downforce or negative pressure just behind it which sucks the lower lift as it moves through the air. The air has flow back up to the wing surface. The to accelerate to go around the lower side of Gurney flap causes some extra drag, but the the wing and loses pressure when it speeds 62 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” wing can be run at a higher angle of attack There are mainly three distinct and produces more downforce. streams of numerical solution techniques; finite element finite difference and spectral Designers can only use limited methods. In outline the numerical methods amount of the wing on a racecar because of that from the basis of solver perform rules limiting the number and dimensions of wings. Side pods and tires get in the way and they just can't be left out 5.COMPARISION WITH AND WITHOUT GURNEY FLAP 5.1 WITHOUT GURNEY FLAP: 5.2 LIFT AND DRAG FOR WITHOUT GURNEY FLAP a)DRAG FORCE VECTOR: 63 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” zone name Pressure (N) Upper side of 8823.7261 force Viscous Total force (N) Total coefficient 2827.6292 11651.355 0.58711792 2827.6292 11975.162 0.60343472 5608.9031 23626.518 1.1905526 force(N) the airfoil Lower side of 9193.8884 the airfoil Net 18017.615 b)LIFT FORCE VECTOR: zone name Pressure force Viscous force(N) Total force (N) Total coefficient (N) Upper side of 468804.26 6.4666261 468810.73 23.623619 -0.71912061 -468656.44 -23.615844 5.7475055 154.28458 0.007774481 the airfoil Lower side of -468655.72 the airfoil Net 148.53707 64 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” 5.3 WITH GURNEY FLAP 5.4 LIFT AND DRAG FOR WITH GURNEY FLAP a) DRAG FORCE VECTOR: zone name Pressure (N) Gurney flap 75014.62 force Viscous Total force (N) force(N) Total coefficient 33.325835 75047.946 3.7817054 3375.5778 -25474.154 -1.283656 2163.026 21308.691 1.0737561 5571.9296 70882.482 3.5718055 portion Upper side of -28849.732 the airfoil Lower side of 19145.665 the airfoil Net 65310.553 65 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” b.)LIFT FORCE VECTOR: zone name Pressure force Viscous (N) Total coefficient -26.104107 -15950.99 -0.80377875 56.97324 1094942.1 55.17471 -1.4946828 189102.24 9.5289615 29.37445 1268093.4 63.899893 force(N) flap -15924.886 Gurney Total force (N) portion Upper side of 1094885.2 the airfoil Lower side of 189103.74 the airfoil 1268064 Net 5.5. AIRFOIL DATA velocity=180m/s airfoil =0015 ACAN angle of attck is zero Application to helicopter rotors . If a Gurney flap can be incorporated on a helicopter rotor successfully, the dna speed of the rotor can be reduced to 4 % of produce the same lift. chord 6.CONCLUSION Application to delta wing aircrafts Active Gurney flaps for race cars The analysis carried out on the chord at the For race cars the speed will be trailing edge of the airfoil with NACA varying throughout the track. So the airfoil with and without flap 0015.The above optimum height of the flap keeps on figures are comparisons between naca changing. By the special material of distributed at the pressure in upper and Gurney flap it can be rendered as an lower surface have equal pressure shown in active one that is a flap which is figure. The pressure distribution in the upper capable of changing the height surface less one unlike shown in figure and according to the real time velocity of angle of attack is zero. 66 NCETIME – 2k13 Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13” car and thus keeping the optimum height for all the time. REFERENCE 1. L.Brown and A.Filippone (2003),Aerofoil at low speeds with Gurney Flaps, The Aeronautical Journal, No.2800, pages 539 to 546 2. L J Clancy, Aerodynamics, Longman Group, 1996 Edition 3.http://aerodyn.org/HighLift/gurney.html 4. http://www.allamericanracers.com 5.http://www.as.go.dlr.de/Transsonium. 6. http://www.cfd.tu-berlin.des 67 NCETIME – 2k13