Aircraft Wing Design Considerations - Assignment Example

Aircraft Wing Design Considerations Introduction This research paper explores some aspects of aircraft wing design considerations. It starts by describing some common misconceptions that are with lift (as associated with aircraft wings). It then tries to explain how an aerofoil actually generates lift. After this, this research paper tries to answer some questions related to the aerofoil’s shape; why it is rounded and not pointed at the front, why it is thicker at the middle, why it is tapered at the rear and finally, why it is flatter underneath, with a camber making it curve on top. It also explores other possible shapes that might generate lift and those that might possibly not generate lift; a house brick, saucer and soccer, are the few shapes that are explored here. Next, the causes of drag and the main different types of drag are explored and an investigation into what is a boundary layer carried out. Also the wingtip vortices and what possibly causes them is discussed after this. This is followed by the discussion of what entails the most common formulas for lift and drag, what they depend on and gives example uses of the equations. Using the XFLR5 software package plots of CL and CD versus angle for a commonly used NACA 4 series aerofoil of the researcher’s choice is generated and two graphs; CL versus Alpha and CD versus Alpha, are generated. Using these plotted graphs, the angle that gives zero lift and that which gives maximum lift are determined, and if presumably if it flies upon down, the maximum lift it give and at what angle. The drawing of the aerofoil chosen in the above discussion is finally included at the end of the assignment. Discussion 1. Three common misconceptions about lift There are a number of theories that tries to explain the airlift generation phenomenon. However, although some of these theories are popularly found in some websites, encyclopaedias and in a number of books, they are generally regarded to be incorrect. According to NASA, the ‘Equal Transit Theory’ or the ‘Longer Path Theory’ is one among the incorrect theories that are popularly taught in schools and in some aviation colleges. The ‘Longer Path Theory’ states that the shape of the aerofoils; shorter bottom surface and longer upper surface, makes the air molecules to move with a slower speed on the lower surface and faster along the upper surface to meet at the trailing edge, and from Bernoulli’s Principle, the pressure difference created (when air moves faster on the upper surface and slower on the lower surface) across the aerofoil generates lift (NASA 2014). This theory is incorrect since it does not explain why aeroplanes can sometimes fly up-side down (when the longer surface is in this case in the bottom). The second misleading concept is the ‘Skipping Stone Theory’. It states that lift is generated by the simple action-reaction forces; momentum is imparted on the aerofoils when air molecules strike the bottom of the aerofoils (NASA 2014). Although the accuracy of this theory cannot be ruled out completely, it however does not explain most normal flight conditions, for instance, those of airliners at 35, 000 feet at a speed of 500 mph. The third misconception is the Venturi Theory whereby the upper surface of the aerofoil behaves just like the venture nozzle which constricts the airflow. Air flowing through the constriction increases in speed, and from Bernoulli’s Principle, high speed results to low pressure, and the decreased pressure on the surface of the upper surface generates lift (NASA, 2014). The "Venturi" theory is incorrect in that it tries its velocity is based upon an erroneous assumption; airflow at the constriction generates the velocity field. 2. An Explanation on how an Aerofoil really generates lift The lift generation details are really very complex and cannot by any means be simplified. For a gas, its momentum, energy and mass must be simultaneously be maintained in the process. This brings in a great deal of complexity into understanding analysing the aerodynamic problems. For instance, the conservation of mass which produces a change in the gas’s velocity in one direction causes a change in the gas’s velocity in a direction that is perpendicular to the earlier change. This is very dissimilar to the motion of solids which mostly relies on the experiences of physics. Understanding the generation of lift requires a thorough understanding of the Euler Equations (NASA, 2014). An aerofoil produces lift in the following manner. First, the association between pressure and lift force must be well understood. Pressure is described as the change in force for each unit area. It can be denoted as the derivative in the following equation: . Thus, force is considered to be an integral of pressure with regards to area, denoted to as . In order to get the net lift force generated by an airfoil with a specific set of parameters, the force acting down on the upper surface of the aerofoil and the force acting up on the lower surface must be subtracted, causing the integral to be reduced to: , whereby stands for the pressure function that acts upon the lower surface and stands for the pressure function that acts upon the upper surface. Airlift is actually the pressure difference of the pressure function that acts upon the lower surface and the pressure function that acts upon the upper surface (NASA, 2014). 3. The Reasons as to why the Aerofoil is shaped the Way it is. Under this section, the paper explores the reasons as to why the aerofoil is rounded and not pointed at the front, why it is thicker at the middle, why it is tapered at the rear and finally, why it is flatter underneath, with a camber making it curve on top. a. Why is it rounded and not pointed at the front It is difficult for air to turn through a sharp corner, and even a very slight tilt of the aerofoil would greatly disrupt the smooth flowing of air over the aerofoil. This would result to a loss of lift and increased drag. Hence the rounded front in aerofoils is in place to smoothly divide the airflow, even in situations where the aerofoil is tilted upwards or downwards (Babinsky, 2003). b. why is it thicker in the middle A laminar flow aerofoil possesses maximum thickness in the aerofoil’s middle camber line. A negative pressure gradient on the length of the airflow on the aerofoil produces a similar effect as that of reducing the speed. So in this case, with the maximum (thickness) camber occurring in the middle, it is possible to maintain a laminar flow of air over a larger percentage of the aerofoil at higher cruising speed (Harris, 2010). c. Why is it tapered at the rear Were the trailing edge rounded, the higher-pressure airflow along the aerofoil’s lower side can possibly trail through the rounded surface and eventually spill upward to the aerofoil’s lower-pressure region. A tapered at the rear edge prevents the occurrence of this upward spill, since the air cannot make such a sharp turn, while still maintaining the required lift. Instead, it helps the airflow off both the bottom and to surfaces to rejoin smoothly (Lajos, 2002). d. Why is it flatter underneath, with a camber making it curve on top The aerofoil is flatter underneath, with a camber making it curve on top so that the airflow on top of the aerofoil has less space than the airflow underneath the aerofoils.  The airflow on the upper surface must move with a higher speed than the airflow below the aerofoil, due to the required conservation of mass of the gas (air).  As it increases in speed, the air on the upper surface of the aerofoil also loses push and pressure.  And the airflow with the relatively slower underneath the aerofoil maintains much of its pressure, which consequently pushes the aerofoil, and the entire plane, upwards producing a lift. 4. Other Possible Shapes that Might produce Lift a. Will a house brick generate lift? The bricks shape hinders it from generating the required force for it to produce force. Firstly it has sharp corners. From the above discussion it was found that it is difficult for air to turn through a sharp corner of the brick, and this greatly disrupt the smooth flowing of air over the brick. This results to a loss of lift and increased drag; hence a brick won’t generate lift. b. Will a saucer generate lift? A saucer can generate lift since in all aspects it is streamlined in shaped. In all aspects, it shares most of its features with the aerofoil discussed above; its edges can be assumed to be both rounded and tapered at the front and rear respectively, it is thicker in the middle and flatter underneath, with a camber making it curve on top. This helps it to smoothly divide the airflow, maintain a laminar flow of air over a larger percentage of the saucer at higher cruising speed, maintain the required lift and maintain much of its pressure, which consequently pushes the aerofoil, and the entire saucer, upwards producing a lift. c. Will a football generate lift? It should be noted that all that is required to create a lift is to enable the turn of airflow. In the case of a ball, the air molecules stick to the ball’s surface which consequently pulls the surrounding airflow. The external airflow gets pulled towards the spinning ball’s direction. In this case, the net velocity is more than the free stream, creating a pressure difference that generates a lift force. The generated lift force is perpendicular to the direction of the airflow. 5. What causes Drag and what are the main different types of Drag? Drag in aerodynamic is the force that opposes the motion of the aircraft through the air. It is produced at least by each and every part of the airplane; including even the engines. It is produced by the contact and interaction of a solid entity with both a liquid and/or gas; it is never produced by a force field such as the electromagnetic or gravitational fields where an object can impact on another without necessarily being in a physical contact. The generation of drag can be achieved when a solid body is in contact with the fluid (either gas or liquid); no fluid no drag. The difference in speed (velocity) between the fluid and the object generates drag; a relative motion between the fluid and object must exist. Drag is typically a force with a vector quantity; both the magnitude and direction. It acts in an opposite direction to that of the aeroplane. The aeroplane drag has the following components (sources); skin friction that occurs between the body of the aircraft and the air molecules, aerodynamic resistance (form drag) that is caused by change of pressure as the aeroplane moves and the airflows around the aeroplanes body, the induced drag that only occurs in lifted wings of the aeroplane due to the non-uniformity distribution of lift force on the aeroplanes wings, the wave drag is generated when the aeroplane approaches the speed of sound producing shock waves along the body of the plane resulting to loss of total pressure due to static pressure change, and finally the Ram drag generated when free stream air enters the aeroplane (NASA, 2014). 6. What is a boundary layer? The boundary layer can be defined as the layer of the fluid in the direct locality of a bounding surface where the impact of viscosity is noteworthy. In aerodynamics, the boundary layer simplifies the fluid flow equations by dividing the fluid flow into two distinct areas; areas dominated by viscosity inside the boundary layer that creates more of the drag that is usually experienced by the boundary body and the area outside the boundary layer where fluid viscosity is negligible (Pilot Friend, 2014). 7. What are wingtip vortices and what causes them? Wingtip Vortices is the drag created by the spiral air trails off the wings of the aeroplane, and they significantly reduce the motion of the plane by taking some of its motion energy (How Things Fly, 2014). The change in pressure that generates lift also creates a higher-pressure of air underneath the wings which spills over the tip of the wing into the low-pressure area on the upper surface of the wings. The Forward motion of the wing spills spins this upward surge of air into a long tornado-like spiral that usually trails off the tip of the wings. The created vortices reduce air pressure in the whole of the wing’s rear edge which resultantly increases the pressure drag upon the plane (How Things Fly, 2014). 8. What are the common formulas for Lift & Drag? 
What do they depend on? Give example use of the equations a. The Lift Force Formula Whereby; is the Lift Force in Newtons (N) Which depends on: as the Lifting Coefficient as the density of fluid in Kg/m3 as the area of the body in m3 b. The Drag Force Formula Whereby is the drag force That depends on as the darg Coefficient as the density of fluid in Kg/m3 as the area of the body in m3 Example Consider an aircraft travelling at a velocity of 100m/s, possessing a wing area of 20m2, a drag coefficient of 0.06 and a lift coefficient of 0.7. Calculate the lift force and the drag force from this information. Solution The lift force is: The drag force is: 9. Graphs of ; CL versus Alpha and CD versus Alpha 10. From the plots in section question 9; a.  What angle gives zero lift? b.  What angle gives maximum lift? c.  If it flies upside down, what is the maximum lift, and at what angle? This is got from the Cd versus the alpha graph. , at an angle of 16 11. Drawing of the aerofoil you have chosen in section nine Conclusion The aerofoil has almost all the desirable features for aiding in the aircraft wing design considerations; it is rounded and not pointed at the front, it is thicker in the middle and flatter underneath, with a camber making it curve on top and it is tapered at the rear. These features enables the aerofoil to smoothly divide the airflow, even in situations where the aerofoil is tilted upwards or downwards, to maintain a laminar flow of air over a larger percentage of the aerofoil at higher cruising speed, to generate and maintain the required lift. Therefore, once these features are considered, adopted and modified appropriately, they give the best aircraft wing design. Reference List Babinsky, H. 2003. How do wings work? Cambridge, UK: IOP Publishing Ltd Harris, C, D. 2010. NASA Supercritical Airfoils: A Matrix of Family-Related Airfoils, NASA Technical Paper. Hampton, Virginia: Langley Research Centre How Things Fly 2014. Vortex Drag. [Online] (updated 12th May, 2014 ) Available from < http://howthingsfly.si.edu/aerodynamics/vortex-drag> Accessed on 9th Jan, 2015 Lajos, T. 2002. Basics of vehicle aerodynamics. Budapest: Budapest University of Technology and Economics, Department of Fluid Mechanics NASA 2014. Incorrect Theory #1. [Online] (Updated 12th Dec, 2014). Available from Accessed on 8th Jan, 2015 NASA 2014. What is drag? [Online] (Updated 12th Dec, 2014). Available from Accessed on 8th Jan, 2015 NASA 2014. Incorrect Theory #2. [Online] (Updated 12th Dec, 2014). Available from Accessed on 8th Jan, 2015 NASA 2014. Incorrect Theory #3. [Online] (Updated 12th Dec, 2014). Available from Accessed on 8th Jan, 2015 NASA 2014. Getting a ‘Lift’ out of Calculus: Part 1 Activity. [Online] (Updated 12th Dec, 2014). Available from Accessed on 8th Jan, 2015 Pilot Friend 2014. Wing Boundary layer.[Online] (updated 16 June, 2014) Available from < http://www.pilotfriend.com/training/flight_training/aero/boundary.htm> Accessed on 9th Jan, 2015 Read More