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Estimating aerofoil lift from flow angle

  • L. W. Traub (a1)

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Estimation of the lift of an aerofoil is one of the fundamental measurements of fluid mechanics. Lift is commonly measured using a load cell or a force balance. Non-intrusive methods to measure lift are usually pressure based. Aerofoils may be pressure tapped where small surface orifices are connected via tubing to a pressure measurement system, either a multi-tube manometre or an electronic system. Both measurement options add cost and complication, especially in an educational setting. Pressure tapping small aerofoils can also be difficult, especially if the models are rapid prototyped (RP). Low model surface resolution (from RP manufacture) and confined geometry complicate model assembly and finishing. Boundary-layer transition caused by poorly implemented tappings (too large a diametre or poorly aligned, i.e. straight aft) can also alter results. Wall pressure tappings may also be used and have the benefit of being non-intrusive. To implement, the test section roof and floor is tapped with a streamwise row of ports that facilitate measurement of the wall pressure signature. Integration of the pressure differential then relates to the lift produced. This measurement methodology still requires a multi-channel pressure acquisition system and modification of the wind tunnel. In Refs 4,5 methods are presented that facilitate calculation of the instantaneous forces acting on a body through flow field measurements determined using particle image velocimetry. However, the required flow field measurements encompass those surrounding the body, and are not a simple point measurement. In Ref. 6 a method is presented to estimate the lift of an aerofoil using two Pitot-static tubes that are used to measure the velocity above and below the aerofoil’s quarter chord. Wall corrections are required to yield an accurate lift estimate.

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1.Barlow, J.W., Rae, A. and Pope, A.Low-Speed Wind Tunnel Testing, 3rd ed. John Wiley & Sons, 1999.
2.Eastman, J., Stack, N., Pinkerton, J. and Robert, M.Airfoil Pressure Distribution Investigation in the Variable Density Wind Tunnel, NACA Rept 353, 1931.
3.Althaus, D.Measurement of Lift and Drag in the Laminar Wind Tunnel, University of Stuttgart, http://www.iag.uni-stuttgart.de/IAG/institut/abteilungen/laminarwindkanal/pdf-dateien/liftdrag2. pdf
4.Lin, J.C. and Rockwell, D.Force identification by vorticity fields: techniques based on flow imaging, J Fluids and Structures, 1996, 10, pp 663668.
5.Noca, F., Shiels, D. and Jeon, D.Measuring instantaneous fluid dynamic forces on bodies, using only velocity fields and their derivatives, J Fluids and Structures, 1997, 11, pp 345350.
6.Traub, L.W.Simple experimental method to estimate the lift of airfoils, J Aircraft, September-October 2008, 45, pp 18101814.
7.Anderson, J.Fundamentals of Aerodynamics, 5th ed, McGraw-Hill, 2010.

Estimating aerofoil lift from flow angle

  • L. W. Traub (a1)

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