Aircraft Wing Airfoils - Article Example

AIRCRAFT WING AIRFOILS By Student’s name Course code and name Professor’s name University name City, State Date of submission Introduction Airfoils are important inventions in the aircraft industry. Airfoils have found their way into several uses within the aeronautical industry with various adjustments to suite any given nee at a particular time. This article seeks to uncover the misconceptions that are associated with the airfoil and its ability to generate lift. Further, the shape of the airfoil is also covered accordingly with the related equations and their dependents being listed. At the end of this article, the NACA 4418 airfoil is analysed using the XFLR5 software package to generate plots of CL and CD in order to create further knowledge on the topic. Discussion The common explanation for lift is centred on the immersion of an airfoil in streamlines of flowing particles. Those backing this school of thought have indicated that based on the diagram below, where the distance from stagnation point S to the edge T which is referred as the trailing edge is greater at the upper surface as compared to that of the lower surface. The misconception on this theory is quoted as in the distance argument which is not a mandatory condition for generation of lift. This is compared to the schematic analysis of a sail whose lower distance is a complete inverse or even equal in some areas and yet lift is generated. This offers an explanation that the distance theory is wrong and does not therefore achieve the integrity desired as a self explanatory theory (Babinsky, 2003). Figure 1: Smoke streamlines surrounding the airfoil (Babinsky, 2003). It is also argued that fluid particles are bound to meet at the trailing edge T shown in diagram 1 above. Real life observations on why these particles take the same time in order to cover the distance ST are wrong. In an experiment to demonstrate that this is actually a misconception, simultaneously injection of smoke particles in an upstream manner usually generates a line of smoke whose particles scatter below and above the airfoil. The particles on the upper surface reach the trailing edge before those of the lower surface thus the equal time argument does not hold water (Babinsky, 2003). Bernoulli’s principle has also been wrongly applied in coming up with a demonstration of lift in airfoils. Application of high velocity air on a curved paper generates lift and it is believed that the high difference in velocity of air particles flowing in the opposite direction generates differential pressure. This is actually wrong according to Babinsky (2003) who gives an affirmation that the connection between the two sides of the paper cannot be demonstrated using Bernoulli’s principle. In doing this he establishes the argument that if pressure along an airfoil increases then it is definite that the velocity reduces and vices versa. Generating lift in an airfoil is strictly dependent on Newtonian’s laws of motion and Bernoulli’s principle. Newtonian’s law poses the idea of airfoil lift being a reaction force since there must be exertion of force in order for direction to change. The exertion force achieves an equivalent of equal magnitude but in a different direction. This is simply put as; while the airfoil exerts downward force against the air planar, the air exerts force upwardly thus achieving a lift. This argument is derived from Newton’s second law of motion which states that for every action there is an equal reaction force in the opposite direction. Pressure differences have also been used to describe this contentious issue in that the net force is compared to the pressure differences that are demonstrable in equation (1) below. A slight pressure difference on the upper surface of an airfoil poses a reaction on the underside pressure which eventually triggers lift in a bid to generate a balance between the two (Langley Flying School, Inc., 2013). (1) Where R = curvature’s radius. ρ = density v = velocity It is also known that supersonic speeds at the top surface on an airfoil concede flow turning at the bottom. In this explanation, the air molecules are said to bounce on each other thus contributing to the lift that is experienced on an airfoil. Another theory that is attached to the Newtonian laws of motion with regard to airfoil lift is the angle of attack. The angle of attack occurs between the oncoming air and the foil thus increasing this angle translates to deflection through a larger angle and an increased lift. Therefore the angle of attack is proportional to the lift achieved in an airfoil (Langley Flying School, Inc., 2013). Bernoulli’s principle has also been used in explaining the phenomenon of airfoil lift. The relationship between speed and pressure are incorporated within one equation to demonstrate the pressure imbalance and the expected speed differences. For any airfoil to generate lift, the pressure difference between the lower and the upper section. The mathematical relationship is that speed at all points of an airfoil can be used to calculate the pressure parameters and vice versa. The discrepancy with Bernoulli’s equation is that there is no explanation as to why there is a higher velocity of air flow on the upper side of the wing than the bottom side. The law of conservation of mass however tries to explain it in that the there is a steady flow of fluid. The air is assumed to be incompressible thus a stream-tube becomes narrower based on the area of action between the streamlines. Since the air on the upper surface flows at a higher velocity it elicits a pressure difference which causes an aerodynamic force that flows downstream with lift as the perpendicular component (Langley Flying School, Inc., 2013). Airfoils are designed in streamlined shapes since they have to drag a very small force in order to realize a lift. As an aircraft moves at a high speed using the mechanical propulsion components, the pressure above the wing becomes massive than that of the lower side. This leads to an upward force which is a resultant between the differential pressures. An airfoil has to be cambered i.e. curved or inclined to the direction of airflow in order to give a lift force when subjected to pressure changes. This also encourages viscosity which leads to the formation of a vortex which is responsible of the lifting conditions (Nakamura, 1999). The conditions for an object to cause lift are clearly outlined as cambered and relatively inclined to the direction of flow of air. Therefore block shapes such as house brick walls cannot create any lift since it is not streamlined. A saucer, an oval component and a dome may generate a lift because they are streamlined and they fulfil the conditions required for lift to occur. Figure 2: Oval component. Drag is generated from the viscosity force or resistance of a fluid. This force opposes an aircraft motion through the air just like friction opposes movement of objects on a surface. This force acts perpendicular to the lift force. Drag force includes parasitic or induced drag among others. Induced drag is caused b y differential pressure between the upper and lower surfaces of an airfoil. This is due to the vortex being realised at the tip of an aircraft’s wing. On the other side, parasitic drag is the force that opposes motion when air is displaced due to friction of the airfoil surface. Form drag is an example of parasitic drag which is caused by airflow separation at the surface of the airfoil. Friction drag is also associated with the interference that generates laminar flow although it is gradually eliminated as the lift persists. A boundary layer is closely associated with these forces in that the boundary layer is the point at which the turbulent and laminar flows are separated. This principle has been applied in coming up with the small airfoils that are placed perpendicularly to the upper wing surface so as the airfoil can meet the angle of attack at the laminar flow (Payne, 2006). Wingtip vortices are elicited when lift is induced on an aircraft wing thereby leaving trails of rotating air. This also occurs in other areas of the aircraft where lift varies due to pressure differences. The lower surface of an airfoil allows air to flow around the wing tip towards the upper wing surface resulting to a horizontal tornado as the high velocity air escapes from the tip. These can only be reduced by varying the aspect ratio of the wings to achieve an elliptical shape (Efa, n.d.). The formula associated with lift is shown in (2) below. (2) This depends on the density of the medium through to which the airfoil is subjected through, the square of the velocity, surface area of the air foil, the compressibility or viscosity of the medium, the airfoil’s angle of inclination and lastly the shape of the air foil (Benson, 2010). The drag equation is stated as shown in (3) below. (3) The drag equation depends on the square of velocity, density of medium, viscosity and compressibility, inclination of the airfoil and the size or shape. These variables are all contained in the coefficient of drag Cd which is contained in the equation above (Benson, 2010). Figure 3: A graph of Cd Vs Alpha. Figure 4: A graph of Cl Vs Alpha. Figure 5: A drawing of naca4418 airfoil. Conclusion The modes of operation of the airfoil are discussed in the above arguments basing all the arguments on factual matter obtained from various sources. The fact that not just any shape can generate lift proves a point as the aeronautical engineers come up with calculations and analysis such as XFLR5 software which is used to analyze the NACA 4418 airfoil shown above from figure 3 – 5 as directed by the assignment questions. List of References Babinsky, H. (2003) 'How do wings work?', Physics Education, vol. 38, no. 497-503. Benson, T. (2010) The Drag Equation, 10 August, [Online], Available: http://www.grc.nasa.gov/WWW/k-12/airplane/drageq.html [03 December 2013]. Benson, T. (2010) The Lift Equation, 28 July, [Online], Available: http://www.grc.nasa.gov/WWW/k-12/airplane/lifteq.html [01 December 2013]. Efa (n.d) Component parts of the aeroplane, ExecutiveFlight. Langley Flying School, Inc. (2013) Langley Flying School, Inc., [Online], Available: http://www.langleyflyingschool.com [01 December 2013]. Nakamura, M. (1999) Airfoil, [Online], Available: http://web.mit.edu/2.972/www/reports/airfoil/airfoil.html [01 December 2013]. Payne, C.M. (2006) Principles of Naval Weapon Systems, Annapolis: Naval Institute Press. Read More