Increasing Efficiency at the NTF by Optimizing Model AoA Positioning

Abstract

The National Transonic Facility (NTF) at NASA Langley Research Center (LaRC) is a national resource for aeronautical research and development. The government, military and private industries rely on the capability of this facility for realistic flight data. Reducing the operation costs and keeping the NTF affordable is essential for aeronautics research. The cost concern here is the liquid nitrogen (LN 2 ) and how much is used in the process of setting the model pitch. Tests were conducted at the NTF in January 2005 to reduce lag in the angle of attack (AoA) data used by the model pitch control system. Lag in both the filter and the digital voltmeter (DVM) contribute to long set times between data points. Reducing these lags was investigated and is discussed here. I IntroductionThe current LN 2 cost is around $90 per ton and NTF can use a ton in about 4 seconds when operating at high Reynolds number conditions. In NTF's effort to reduce cost, a test was undertaken to determine how to reduce the average range of time between data points which is currently 15s -18s. A substantial amount of money can be saved by reducing this time. For example, in a recent test the customer required 5000 data points. If a 4 second per data point reduction could be achieved then that would equate to a savings of roughly $450,000. Putnam1 and Kegelman 2 take a global look at the productivity improvement needs at the NTF in the mid to late 1990s but this test focused on reducing costly nitrogen consumption through reducing lag in the control system. Specifically it concentrated on two aspects of the AoA signal being provided to the control system. The first area is filtering the signal dynamics and the second deals with removing the integrating DVM from the system. Both of these systems create lag that could be reduced but need to be studied to ensure there is not an unacceptable loss in data quality.NTF typically uses an active 4 pole Bessel low-pass filter with the cut-off frequency set to 0.4 Hz. This smoothes the dynamic accelerometer signal to an acceptable level but it induces a lag of about 0.7s (at a 2 deg/s slew rate). As the cut-off frequency is increased the lag will decrease but the dynamics will have more influence on the AoA reading. Doing this has the potential of reducing 0.5s of lag out of the system as long as the data quality remains within acceptable levels.The integrating DVM, by its nature, introduces lag into the system. Integrating at 10 power line cycles coupled with the overhead associated with processing the readings creates about 0.25s to 0.33s of lag. Removing them from the system would have many advantages, it would: eliminate this lag, not require 1 Aerospace Technologist, Aeronautics Systems Engineering Branch, MS 238, Senior Member.