Forces That Affect the Movement of an Aircraft and the Aerodynamic Behavior - Term Paper Example

Abstract This report gives the result of a group investigation on how aerofoil reacts in the confines on wind tunnel. The results were used as a model how aerofoils of aircraft behave when the aircraft is in flight. The experiment involved measurement of lift force, drag force, pitching moment, velocity and temperature. There results indicated that an increase in angle of attack and flip angle results to increase in lift force, drag force and pitching moment but results to reduction in wind velocity. Table of Contents Nomenclature iv Introduction 1 Drag force 1 Aerodynamic forces acting upon an aircraft during a turn 2 Flaps and ailerons 3 Methodology 4 Results 7 Lift force versus angle of attack 7 Drag force versus angle of attack 7 Variation of pitching moment 8 Variation of wind speed 9 Temperature variations 10 Equations used to calculate variables 10 Discussion 11 Conclusion 13 References 14 Nomenclature CL= Lift coefficient (No units) V= Velocity (m/s) CD= Drag coefficient (No units) l=Length of wing/aerofoil (m) Cm= Pitching Moment coefficient (No units) S=Area of wing/aerofoil (m²) ρ= Density (air at 21° =1.27Kg/m^3) s=Distance along contour line (m) c=Chord of wing or aerofoil A= Area (m²) D=Drag (N) L=Lift (N) Mp=Pitching Moment (N.m) Introduction In order for an aircraft to fly there should be enough upward that will keep the weight of the aircraft in the air. While spacecrafts and missiles work on different principles the force for lifting an aircraft is supplied by the air in surrounding it. There is a misconception that generation of lift comes from there being a difference in speed the air immediate to the wing and that in the general atmosphere (Anderson, 2004). This is not absolutely true owing to the fact that it is possible to have airstream of differing speed but the pressure prevailing in the two streams to be of the same magnitude. Lift is caused by change in air that causes change in pressure between the atmosphere and the immediate surroundings of the aerofoils (Clancy, 1975). The difference in pressure on the upper side of the wing and the surrounding atmosphere is not enough to cause lift. It is necessary that the pressure in the lower region of the aerofoil being higher so that pressure difference can be created that will bring about lift. The shape of aerofoil plays an important role in the generation of the lift. In general the upper part of an aerofoil is found to be more curved than the lower side. As a result of this shape there is generation n of high pressure at the lower side than on the top of the aerofoil and this brings about the lift. The design of the aerofoil is such that it tappers at the rear end so that there is reduction in turbulence level and thus eliminating the destruction that could come as a result of the turbulence. Drag force Drag is an important force that results from the air resisting the movement of the aircraft motion. Parasitic drag is purely because of the movement of the aircraft in air and it consists of skin friction and form drag. The shape and size of the aerofoil will dictate the level of form drag with aerofoils large cross sections having proportionally large form drag while thin ones will have lower drag force. Induced drag force is created on purpose so as to the desired lift is achieved. It is necessary to have high induced drag force when the aircraft is moving with low speed. This is normally achieved by having a higher angle of attack. With an increase in the aircraft speed there is reduction in the angle of attack thus bringing down the induced drag. Aerodynamic forces acting upon an aircraft during a turn The forces acting on an aircraft changes when a turn is being made and banking provides the turning force of an aircraft whereby when the aircraft banks the lift acts inwards to the centre of the circle of the turn in addition to the normal lift. In the normal straight line motion of an aircraft the lift and the weight are the tow forces that can be visualized to act on a plane. When the plane is banked the lift does not act in opposite direction with weight but it starts acting in the direction of banking. The basic law of motion is that any object will either remain motion or will remain at rest unless compelled to do otherwise by some external force. The aircraft being considered to obey this law would require some side force to effect turning and this force in aircraft results from banking. Banking splits the lift force into two: the normal upward and the new component toward the centre of the curve. The horizontal force causes the pulling of the aircraft such that it deviates from the straight line motion and effectively making it to make a turn. In the turning process there is generation of centrifugal force which acts in the opposite direction of the inward force. Unlike in cars and marine vessels where there is steering in effecting turning in aircraft banking is the available option. Banking will automatically affect turning in an aircraft provided the aircraft does not slip in the inner side of the turn. In order to have good control of the aircraft it is important to bear in mind that the aircraft will attempt to turn when the wings are not in a level position. Flaps and ailerons The aerofoils can be split into two parts, the flaps and aileron, with both playing a role in taking off with a high lift and ensuring low lift when landing. The flaps and aileron also play a role in the primary control system when the aircraft is flight. Even though the flap and aileron are located adjacent to each other, there play different roles in flying of the aircraft. The aileron takes control of the surface of the aircraft, with its location being on the lateral axis and thus it can be said to control rolling motion. The flaps have similar design to that of aileron with the difference being their smaller size. In order for there to be an increase in lift, the flaps are to apply pressure on both the upper and the lower side so as for the lift to be realized. The flaps and ailerons have the commonality of functioning on the same theory (Anderson, 2005). In the flight mode the ailerons are in opposition to each other whereby the motion of aileron on the left in specific direction brings about the motion of the right aileron in the opposing direction. In addition the operation of the flaps is done simultaneously meaning that the motion of the right flap goes together with that of the left flap (Anderson 2005). Incorporation of slots has been found to result to account for 63% increase in the lift that is generated. The finding triggered Handley Page venture into combining the flap and slat so as to come up with the slotted flap that was then tested in a wind tunnel (Anderson, 1997). The flap is aimed at increasing the lift that can be created by the aerofoil of an aircraft. Research findings has shown that operating slotted flaps on thick aerofoils provided high lifts than that in then aerofoils. With this research first being in early days of World War 1, there has been rising evolution in aerofoil designs with specific requirements being needed with regards to the widths of the aerofoils (Anderson, 1997) Methodology Observation was the specific method was utilized so as to collect reliable data whereby there was close observation of all the results and then the results being recorded. In overall the experiment took both quantitative approach with slight bias towards quantitative approach where by the test was run several times with there being a change in angle of attack and the flap angle but there was no repetition of the same set up in observing time taken in completing the experiment. This test gave a good insight on how aerofoils behave aerodynamically. The aerofoils were fitted with trailing edge flap with the angle of the flap and the attack angle being changed constantly. The test is important in order to control the altitude of an aircraft the angles of aero flap are usually varied. Several methods were used in retrieving some important data in the wind tunnel test. First there was to be a though examination on what was to be achieved through the experiment and having knowledge on what was to be varied and what was to be kept constant. There was single data collection because there was no running of a number of tests under the same conditions and thus it was important that readings were to be gotten right on the one chance given as to have the desired accurate results. The test was carried out through splitting into groups and the collected information was shared by the groups. Each group had five members who were assigned the tasks of noting down the five variables lift, drag, pitching moment, temperature and wind speed sym. The first measurements involved the angle of attack being set at set at 0 then progressed slowly by 4⁰, such that the angles of attack used were 0⁰,4⁰,8⁰,12⁰, 16⁰ and 20⁰. The wind tunnel speed was to be kept constant at 35m/s even though the actual was observed to vary increase in the angle of attack and also high flap angle had lower wind speed. The room temperature was assumed to be constant at 21.4⁰C. First the test flap angle was set at 0⁰ and the five variables were recorded at an angle of attack of 0⁰, and then the angles of attack were increased to 4⁰, 8⁰, 12⁰, 16⁰ and 20⁰. After competing test at a flap angle of 0⁰ the angle was increased in steps of 100 to flap angles of 100, 200, 300 and 400. The flap adjustment were done manually and on obtaining the results each group had had to share their result with other groups this being an illustration the need of having good communication skills in carrying out the tunnel test. Poor communication among the groups there would be processed and this would be passed forward to other groups and this would result to inaccuracies in the experiment. Even through assigning tasks to groups means higher risk in collecting wrong results, this was a good idea in terms of saving time that was used in the analysis of the data. This was also good as groups were ready to receive a briefing from the lecturer after the test. The aim of any data collection and processing system is ensuring that there is successful collection of the data; the data is then processed in desired fashion followed by recording in a form suitable for storage and presentation. In relation to acquisition of data in carrying out this test, the data was first collected then processed into the desired attack angle and flap angle. There was recording of the drag, pitching moment and the lift. This resulted in acquisition of data that was quite reliable with the process being short with no subsequent processing of data being involved. A further analysis check that was carried out involved simple verbal appraisal of test results, owing to the fact that no major errors were involved with there being no need for complex theoretical analysis of errors. The apparatus that were used in the wind tunnel test was a simple computer that displayed a graphical interface that was used in starting and stopping the data acquisition process. Even though the test was able to provide a clear computer reading, the process of acquiring data did not translate to the simplicity of the starting and stopping function. In the process of undertaking the test there was continuous fluctuation in the data even after setting the angle of attack and flap angles. This means that there was still some variation in the results and thus calling for some improvement to be made so as to have a stop and play function included for measurement of the desired variables. This would definitely make a more practical and efficient method that would help in obtaining well organized results with high reliability. Results Lift force versus angle of attack From figure one it can be seen the general trend is that there is an increase in lift force as the angle of attack is increased from 00 to 200 the increase being almost linear. The results also show that the lift force increases with as the flap angle is increased from 00 to 400. Drag force versus angle of attack From figure 2 it can be seen that just like for lift force general trend is that there is a linear increase in drag force as the angle of attack is increased from 00 to 200 but unlike in the former the drag force is given in a positive sense. The results also show that the drag force increases with as the flap angle is increased from 00 to 400. Variation of pitching moment The pitching moment is given in negative sense and it is seen to increase as the angle of attack is increased. The pitching moment also is seen to increase as the flap angle is increased (though in the negative sense). Variation of wind speed From figure 3 it can be seen that unlike the other variables the wind speed is seed to reduce as the angle of attack is increased. The increase in flap angle from 00 to 400 also is seen to bring about reduction in wind speed in the tunnel. Temperature variations From fig 5 below it can be seen that the temperature does not exhibit any particular pattern. Equations used to calculate variables The graphs above show a various amount of data which was collected and calculated using a range of equations. Coefficients:( Piercy N.A.V.-1955) CL=L/1/2*ρ*(v) ²* S, CD=D/1/2*ρ*(v) ²*S, Cm=M/1/2*ρ*(v) ²* Sc, Lift to Drag Ratio:L/D=Ratio of Lift and Drag Discussion The results in fig 1 and fig 3 show negative values as a result of the layout of the attachment rig of the wind tunnels. The aerofoil was attached in an upside down manner resulting to the lift and pitch moment being negative as the lift force affect the upward movement while the pitching moment has effect on the centre of the aircraft. Suppose the model was not placed in an upside down all the results would have been positive where by the lift and pitching moment would have not been affected to be negatives. The lift and drag force have been seen to increase with an increase in the angle of attack which is in tandem with the lift versus angle of attack rule that states increasing the angle of attack will bring about an increase bin lift” (NASA, 2013). In fig 1 it is seen that an increase in angle of attack resulted to an increase in the lift produced. In the normal function of an aircraft, there would be no malfunction below 200 while the flap angle can be varied from 00 to 400. In relation to NACA 23012, it is allowable to beyond an angle attack of 20°. The lift increase is accompanied by an increase in the drag force but this does not show direct proportional relationship owing to the fact that the aerofoil can bring about an increase in the drag force, through increasing the amount of turbulent flow occurring along the aerofoil. Through change of flap angle, there is disruption in laminar flow, resulting to the flow becoming turbulent. The time that the phenomenon could occur was not established, but still it was a factor that could have some contribution to the fluctuation of the ratio between lift and drag. The shape of the aerofoil has effect on the magnitude of the frictional drag which as a result would bring about increased turbulent flow. This is likely to have effect on the results through increase in the magnitude of the frictional drag and reduction in the magnitude of lift. In figure 3 there is demonstration of a steady increase process, whereby there was increase in the moment starting from 0° to 20°. In order to have the pitching moment increased in the real life scenario the nose of the aircraft would be required to face upwards. In summary an increase in angle of attack was accompanied by a proportional increase in pitching moment. With the pitch moment being derived from the lift force and the drag force it was predicted that there would be an increase in both the two variables with an increase in angle of attack and this was proved correct as seen from the results. The wind tunnel speed was seen to reduce with an increase in angle of attack as well as the flap angle increase. This could be attributed to there being increased resistance as a result of turbulence. In order to insure the test is more accurate in the future, it would be sensible to carry out the same test three times at the same angle of attack and flap angle. Due to the fact that the results were constantly fluctuating between different numbers it is beneficial to repeat the process and then on the third time it would allow the experimenters to write down the result that was a mean of the three. By doing so it would change the data obtained to multi-sample data, as it would allow a repeat of the experiment and also outline errors made on the first or second attempt. Conclusion In general, this experiment has brought about understanding of the forces that affects the movement of an aircraft and the aerodynamic behaviour displayed by aerofoils when subjected to different attack angles. By the test being undertaken by flaps and using open tunnel wind there has been successful simulation of the behaviour. The decrease in the coefficient forces is as a result of stalling of an aircraft. This is as a result of air going over wind starts forming a vortex on passing over the top wing bring about excessive drag. When such event occurs during a flight it is normally referred to as stalling angle of attack. Having improvement in experimental procedures can bring about reduction in errors thus having high validity and reliability and this would result to having improvement in the result collected. By in the experiment it has been seen that when the angle of attack and flap angles are increased, this would result to significant increase in lift, drag and pitching moment. If there would be ideal situation where the friction drag is negligible, then to get the required lift it would only require having a small angle of attack and flap angles. As the speed over the aerofoils increase, turbulent flow would bring about increased frictional drag with a certain reduction in lift which is likely to result to stalling in the aircraft. In order to prove this phenomenon , there would be need to have more experimental data collection so as to identify the point at which laminar flow changes to turbulent flow along the aerofoil and ascertain if the flow of air would have significant effect on the different types of drag and lift forces. References Read More