Wind tunnel use - Research Paper Example

?Wind tunnel use Fundamentals a) Explain how lift force is generated include a sketch of a typical cambered aerofoil).? An aerodynamic force is produced by any body which moves inside a fluid. The component of this force which is perpendicular to the direction of motion is called Lift. On the contrary, the force in the direction of motion is called the drag. The Lift force is created according to the hydrostatical theorem of the Archimedes principle. According to this principle, the aircraft will be buoyed up by a force equal to the weight of the air displaced by the aircraft. (Milne-Thomson, 1966) An aerofoil is a perfect example to discuss the creation of the lift and the reasons associated with it. An aerofoil is a non-symmetric object and therefore a non-symmetric flow field is created around that object. This non-symmetric flow field also causes an uneven pressure distribution which would not have happened in case of a symmetric object. In the case of the aero foil, the differences in the length of the upper and the lower surfaces and the shape causes the creation of a pressure gradient which results in the creation of the lift force. Source: www.tpub.com The creation of the lift can further be explained using Bernoulli's equation. In a typical aerofoil, the length at the bottom is shorter than the length at the top. Therefore air at the bottom travels slowly to ensure constant displacements of top and bottom. Bernoulli's equation predicts that higher pressure will develop where velocity is slow and therefore the pressure at the bottom will be higher. Thus the upward force will be greater than the downward thrust resulting in the generation of an upward lift. (Munson, 2008) The horizontal component is labeled induced drag. b)Describe how atmospheric parameters ( temperature, pressure, density) affect the generation of lift and drag as an aircraft gains altitude.? The calculation of the lift has a simple formula which is Where  is the lift coefficient, L is the lift force,  is the fluid density, U is the relative speed between the object and the fluid and A is the cross-sectional area (Munson, Young and Okiishi). This means that the lift coefficient determines the lift force and this coefficient is directly dependent upon the shape of the foil, the fluid properties and the roughness of the surface. Fluid properties include fluid density, viscosity and temperature which contribute to the lift force. The general equation for this is Where Re is the Reynolds Number, Fr is the Froude number, Ma is the Mach number, and  is a measure of surface roughness (Munson, Young and Okiishi) Thus when these external parameters change, the lift coefficient is affected directly which consequentially affects the lift force. Taking temperature to be the first variable, we know that temperature is inversely proportional to density. The aircraft, when it gains altitude, flies at lower temperatures and thus faces an increase in the density of the surrounding air. An increase in air density results in the increase in the weight of the displaced air and thus an increase in the lift force. A contradiction however is the mass of air which is higher near the surface of the earth due to gravity. Thus the density which is dependent upon mass decreases with increasing altitude, resulting in a lower lift force at higher altitudes. When it comes to pressure, it also decreases as the aircraft gains altitude. This is so because the density lowers and the direct effect of this is the reduction in the pressure gradient between opposite surfaces. Again this decrease causes a reduction in the amount of lift force produced. Research a) Describe how drag varies with airspeed and the generation of lift. Drag will always be present on a moving body, even though lift is not produced. What do we call this type of drag and how does shape affect it? As mentioned before, the drag force is the force acting parallel to the direction of motion or the line of symmetry of the object. It normally opposes motion and slows down objects. it is caused by the non-symmetric fluid field of an object creating pressure differences and the general equation for this drag is Where D is the drag force,  is the drag coefficient. It should also be observed from this equation that the drag force will increase with velocity and thus an aircraft moving at a higher speed will encounter a higher resistive thus a higher drag force. a) As lift is generated there is production of induced drag. What is induced drag and how can its affect be seen on an aircraft? There are two basic types of drag forces. The first is due to the shearing stress and the second is related to fluid separation from the solid surface. The shearing force is created due to the effect of fluid viscosity on the solid which causes retardation. This boundary level phenomenon results in the creation of stress near the solid surface and contributes to drag. The second kind of drag force is a resultant of fluid separation from the surface leading to differential pressure creation. To reduce both these effects, aero foils are streamlined to allow for easy, unhindered movement. . A combination of pressure and shearing drag is sometimes referred as the parasite drag (Munson, Young and Okiishi) The aerofoil will move at an angle to the direction of flow of the fluid. This angle is known as the angle of attack and the lift and the drag forces are dependent upon the angle of attack and they are divided into a vertical and the horizontal component. The vertical component is the lift while the horizontal component contributes to the drag. This induced drag has a serious consequence on the aircraft as when the angle of attack or speed is increased to increase the lift force, this parasitical drag also increases hindering the movement of the aircraft. The drag versus velocity curve is often known as a drag polar. The drag polar is often used to link in with aircraft power curves. How are the power required curves and the drag polar linked? The dependence of the drag and the lift coefficient is called the drag polar. Both the drag and the lift coefficients decrease with increasing airspeeds and thus the drag polar and the aircraft power curve can be linked. The following graph illustrates how the changing of the air speed, changes the drag coefficient leading to a change in the aircraft power curve. Describe the operation of supersonic wind tunnels explain why the size of the working area is so small. As Aircraft materials and technology become more advanced the ability of an aircraft to ravel faster continues, how have designers changed their approach to testing new designs to keep pace with the higher operation speeds now possible? Wind tunnels are objects used to experiment and find out the effect of changing parameters on real life occasions. The reason behind the use of these wind tunnels is the optimization of design for example streamlining or reduction of drag. Since wind tunnels present a prototype for real life testing, these supersonic wind tunnels allow for great reduction in cost as real life testing is avoided. We already know that the drag and the lift coefficients are dependent upon dimensionless quantities such as the Reynolds number, it should be ensured that these quantities are similar to real life numbers as much as possible. This is the first step towards the obtaining of accuracy. The reason for keeping the size of the testing area small is the direct reduction in the cost of the model. Since the reduction in size is necessary, other implications resulting from that have to be dealt with. The first is the dependence of the Reynolds number on speed and object dimensions. Since the dimension of the model decrease, in order to ensure that the Reynolds number stays the same, the velocity has to be increased. Thus this increase in velocity results in the wind tunnels being hypersonic or supersonic due to increased speed. In this advanced age, the focus has been on developing lighter, stronger and more durable materials. This has allowed aircraft to be made of lighter, engines more efficient and thus allowing them to travel larger distances. The reduced weight has the advantage of allowing the aircraft to travel at higher altitudes, where there is less drag and increasing the fuel economy as well. Modern day emphasis is laid not only on speed but also on economical functioning. . The reason behind this is simple. Higher speeds means higher drag forces and hence, more fuel costs and pollution. Thus new developments ensure that the speed of the aircraft is kept under the sound barrier. Another development is flying at lower altitudes to fly in denser air thus resulting in more thrust and lift. The use of Morphing wings, different aircraft skin materials and use of high lift systems and actuators are also new developments in the aircraft technology. (C. Thill, 2008) References (n.d.). Retrieved February 1, 2011, from www.tpub.com C. Thill, J. E. (2008). Morphing skins. the aeronautical journal . Milne-Thomson, L. M. (1966). Theoretical aerodynamics. Courier Dover Publications. Munson, B. R. (2008). Fundamentals of Fluid Mechanics. Wiley Publications. Munson, B. R., Young, D. F., & Okiishi, T. H. (2008). Fundamentals of Fluid Mechanics. Wiley Publications. Read More