Design and Validation of a Force Measurement System for the Analysis of Airfoil Lift and Drag Characteristics
Manning, Niall (2010) Design and Validation of a Force Measurement System for the Analysis of Airfoil Lift and Drag Characteristics. Master of Engineering thesis, Dublin City University.
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One of the aims of this research was to reduce the flow separation and therefore the pressure drag generated by a wing section, through the adding of surface modifications. The modifications consisted of roughening the surface of polystyrene airfoils using a sand blasting technique, which resulted in a dimpled surface. This idea originated from the design of a golf ball, as the dimples are known to delay flow separation which allows the ball to travel further. In relation to wing design, there has been very little research available that either proved or dismissed the theory of using a roughened airfoil surface to reduce the pressure drag created by flow separation. Therefore three types of airfoils were tested, the NACA 0006, 0024 and 4424 profiles. Different levels of surface roughness, created by the dimples, were applied to various portions and sides of the airfoils. These were in turn tested on the three airfoil types, including a series of tests with a split flap configuration and the use of end-plates to reduce the induced drag component. The wind tunnel testing was conducted up to a flow velocity of 23m/s, which led to a maximum Re of 321,227 and 588,141 for the single and split flap airfoil testing respectively.
The overall research was split into two phases, where in each phase a force measurement system was designed in conjunction with a series of experimental tests. Therefore, during Phase 2 of the research, a new external strain gauge balance (System 2) was developed specifically for the DCU wind tunnel and with the nature of the testing in mind. This followed on from the previous design (System 1) which had certain measurement errors and design inadequacies. Aside from the improved mechanical design of System 2, strain gauge amplifier circuits and a data acquisition system were also developed. The resulting set-up was very sensitive and stable, which was necessary for the low drag and lift forces involved, with a sensitivity of 0.71N/V corresponding to a minimum load response of approximately 0.0007N, with respect to the slightly more sensitive drag measurement. Therefore, the design of the force measurement system was the main aim of this research.
As a result, it was necessary to validate the design with extensive experimental testing, which formed the secondary aim of the research. The resulting dimples patterns tested promoted less flow separation in particular tests, however these dimples were also found to have a negative effect on lift generation. An important finding also was that in some of the tests the dimple patterns created an increase in the lift force. In addition, large aerodynamic instabilities were observed in tests where dimples were applied to the frontal sections (<0.33c point) of the airfoils, on their low pressure side. By adding a high level of surface roughness (high Ra) via the dimples, to the middle and last 1/3rd sections on the low pressure (flow separation) side of a NACA 0024 airfoil, from 10° angle of attack onwards the drag force generated compared to the baseline (unmodified smooth airfoil) decreased. However, the lift force also decreased from approximately 8° onwards and at 20° there was a drag decrease of 11.15% and a lift decrease of 8.46%. This therefore led to an overall marginal increase in the aerodynamic efficiency of 3.13%. Overall, the effects varied depending on the airfoil test configuration, the surface modification and the angle of attack. A smoke flow visualisation system was also developed and photographs were captured to provide a further analysis of some of the experimental resul
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