Wind Tunnel Test Data Delivery Process


At Calspan, we understand how important it is to receive accurate wind tunnel test data in a timely manner. That is why we continuously review and then deliver immediately after each test run data to a customer-chosen location. This quality assurance process ensures accurate data reduction and includes the maintenance of statistical control charts, conducting periodic check standard models tests and performing tunnel calibrations. Calspan understands the limitation of the wind tunnel test data, the need for empirical corrections and the need to assess its uncertainty.

For each wind tunnel test, Calspan employs its standard data reduction methodology and incorporates each customer’s data reporting requirements. This includes, at a minimum, providing corrected and uncorrected force and moment data from balances, reduced to coefficient form in at least body and stability axes system, including appropriate corrected angular values. Model steady state, base and balance cavity data are reduced to coefficients. To supplement this, Calspan provides real-time aerodynamic coefficient data plots as a function of the primary test variables. Our highly talented engineers work closely with our customers to develop a data reduction format that will meet your exacting requirements.

Your data security is our top priority. Our customers can use our secure website for data transfer. In addition, we can encrypt data as you require. As processed, the model, data and wind tunnel test program details will be classified appropriately. Appropriate markings are placed on all information that contains or reflects data concerning your test program. Classified data processing/protection is also available as Calspan is fully compliant with the JAFAN 6/9 security specification and is certified/approved to support testing at the Secret-SAP level.

Recording media such as CD-ROMs and/or DVD-ROMs containing the final wind tunnel test data and the digital photographs (views) of each test configuration and installation are provided to you, immediately following wind tunnel test completion.

Data Accuracy & Repeatability

As wind tunnel customer accuracy requirements continue to become more stringent, a wind tunnel facility must know, with ever-increasing confidence, wind tunnel data is reliable and data acquired from various measurement sources and obtained at different times (in-test and test-to-test) are of repeatable quality.

Calspan understands the importance of data quality assurance. Calspan has instituted a Data Quality Initiative Program. This program began with requiring periodic Check Standard Model (CSM) and test section calibrations to monitor and substantiate the repeatability of the tunnel flow and test processes. This initiative expanded to include evaluation of strain-gage balance calibration procedures, formulating wind tunnel blockage effects corrections and quantifying volumetric flow angularity in the test section.

Over the years, Calspan has been assessing repeatability in our wind tunnel using not only the repeat runs from many air vehicle programs, but also through testing of our own model standard. Calspan has completed several tests since the CSM was first used in the tunnel in 1996. The test programs simulated typical validation-type/performance testing and included the standard data correction methods (e.g., static tare, sting cavity pressure, flow angularity, buoyancy). The main objective of these programs is to continue to assess the repeatability of the wind tunnel over the operating range of the facility.  For more information on our tunnel, visit this page.

When installed in the wind tunnel, the calibration model has a frontal cross-sectional area to test section cross-sectional area ratio of 0.50%, a wing area to test section cross-sectional area ratio of 4.78% and a wingspan to test section width of 43.8%.

Calspan post-test analysis of the model standard test data, both in-test and test-to-test, has produced excellent data repeatability. This data was obtained from repeat series throughout the entire test program, representing an end-to-end check of the Calspan wind tunnel validating the overall repeatability of our wind tunnel and its processes across the transonic speed regime. The comparisons also demonstrate the data is within desired in-test repeatability criteria and demonstrate the consistency of our test-to-test data.

General tunnel data quality parameters were deduced from model standard test programs. A quantitative assessment of the models 6-component force and moment data and the tunnel’s Mach number variations produced very good results. A summary of the parameters are presented below:

Model Repeatability Parameters

  • CD minimum within ± 0.00016
  • CL/α slope within ± 0.0003/degree
  • α @ CL =0 within ± 0.015°
  • CLM @CL=0 within ± 0.0004

Mach Number Quality

  • 2σ values of ± 0.0015 for MN ≤1.0
  • 2σ values within ± 0.0025 for MN ≥1.0
  • Pause testing MN tolerance ± 0.0005
  • Sweep testing MN tolerance ± 0.003

Test Section Flow

Calspan routinely monitors flow angularity corrections that are applied to the model angle-of-attack and pitching moment coefficient. Calspan determines these corrections by performing an upright (model roll angle = 0°) and an inverted (model roll angle = 180°) run series (easily accomplished using Calspan’s remotely-controlled ±180° roll mechanism). The apparent flow angularity in the test section reveals itself as a shift in the α @ CL=0 and a shift in the CM=0 when the upright and inverted runs are compared. Calspan determines the correction, Δαf (flow angle), to model angle-of-attack using the difference in α between the upright and inverted CN (Coefficient of Normal force) verses α curves of the model. A correction ΔCMf to the model pitching moment coefficient (CM) is also calculated using the difference in CM between the upright and inverted CN vs. CM curves.

Calspan recommends that a flow angularity run series be performed for all test programs, believing that it is more accurate to perform a check using the customer model instead of applying the facility correction from empty test section calibrations. In general, the flow angle is less than ± 0.10° and the DCMf is less than 0.5% of the maximum pitching moment.

Tunnel Turbulence & Noise Levels

Turbulence levels for the testing range of the TWT were last obtained in May 1981 (there have been no major changes made to the tunnel circuit since then). Measurements of fluctuating pressure, total temperature, and mass flux were made in the settling chamber, test section, and diffuser of the tunnel. Test section flow quality, for all operating conditions, was dominated by pressure fluctuations with temperature and vorticity-induced velocity fluctuations being quite small by comparison. Test section temperature fluctuations, Tt’/Ttavg, were reported to be less than 0.02% representing a level below that typically of the upper atmosphere. Values of total temperature fluctuation level were between 0.015 and 0.021%. Mass flux fluctuations /ρuavg were reported less than 0.3% in the test section.