Distributed Measurement of Aerodynamic Loads

A method for accurately measuring the drag associated with a protuberance on an aircraft has been devised and implemented in a wind tunnel using a C-130 model. The C-130 aircraft has been heavily modified for various different configurations and the impact that these modifications have had on the aerodynamic performance of the aircraft has not been well documented. This paper describes a methodology for using a relatively small scale (1/48) model in a wind tunnel to obtain accurate measurements of the aerodynamic drag for various protrusions on the full-scale aircraft. Measurement Theory and Aerodynamic Scaling In order to measure the drag associated with each protuberance, one might choose to measure the drag of the aircraft without the protuberance and then measure the drag with the protuberance using standard wind tunnel techniques and subtract the two measurements to obtain the drag produced by the protuberance. For a 1/48 scale wind tunnel model and a balance with a 25 pound axial force gage, this method for obtaining drag data on a small protuberance results in ___________________________________________________________ † Associate Professor, Aerospace Engineering, Senior Member AIAA ‡ Professor, Aerospace Engineering, Associate Fellow AIAA an effort to measure load differences on the order 0.002 pounds to .015 pounds if one drag count is the required accuracy. Measurements this small are usually buried in measurement uncertainty but in some cases, may be obtained with very limited accuracy. To accurately and reliably measure the drag contribution of each protuberance, another means of measuring the drag associated with each protuberance is needed. Consequently, drag sensors were designed for each protuberance and these individual load cells were mounted inside the fuselage. Each protuberance was attached to an individual sensor and the drag was measured directly. The sensitivity of each sensor was controlled by the design and resulted in measurements that did not depend on the difference of two large numbers. The second substantial practical issue related to accurate measurement of drag on a protuberance in the wind tunnel is the viscous component of the drag. The Reynolds number in the wind tunnel test is quite different from the full scale flight Reynolds number. For the C-130 aircraft, with an air speed of about 250 knots at an altitude of 5000 feet, the full-scale Reynolds number (based on the aircraft reference length) is approximately . In a subsonic tunnel, for a given size model, the only practical way to change the Reynolds number is by varying the tunnel speed, V. In the present test for the1/48th 7 x10 3.2 Copyright @ 2005 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc. , with permission