Results of forebody testing at Defense Research Agency in the United Kingdom
Bihrle Applied Research (Bihrle) has provided wind tunnel consulting services for testing in foreign facilities. The National Aeronautics and Space Administration (NASA) formed a cooperative effort in the early 1990’s with the Defense Research Agency (DRA) in the United Kingdom
for further research on rotary-flow aerodynamics. The purpose of the collaboration was to develop computational fluid dynamics (CFD) methods for predicting Reynolds number effects on fuselage spin-damping characteristics and to correlate measured static and dynamic wind tunnel data to
the CFD results. Bihrle was contracted by NASA to provide the following services:
- Assemble the necessary test equipment and computers
- Produce the data acquisition software for simultaneous force and pressure measurement
- Provide analysis software for the data analysis
- Supply the necessary technical personnel
- Conduct all of the wind tunnel tests for this program
Testing
was performed at the DRA low-speed atmospheric tunnel (13 ft x 9 ft)
in Bedford and the high speed pressurized tunnel (8ft x 6 ft) in Farnborough
England. Two wind tunnels were used for testing to provide the desired
Reynolds number range. The same rotary balance rig was installed and
used for testing in each tunnel. A photo of
the rig with a rectangular ogive model installed in the low speed tunnel
is shown to the left.
Simultaneous force and moment as well as surface pressures were measured for the test models. A six component strain gauge balance was used to measure the forces and moments acting on the model. The surface pressures were measured using eight 32 port pressure transducers modules
manufactured by Pressure System, Inc. (PSI).
The articles selected for testing were circular and rectangular aft bodies with detachable hemispherical and 2.0D ogive forebodies.
A sketch of the circular ogive shape and pressure port locations is shown in figure 2.
In addition to measuring force and moment data with a six component strain gauge balance, each test article was instrumented with surface pressure ports on their fore and aft sections. Simultaneous force and moment and surface pressure measurements were made during the testing. This
allowed for analysis of the aerodynamic flow field and the resulting total forces and moments on the test article to provide an understanding of the complicated characteristics. Bihrle created custom software to use in the analysis of both the force data and surface pressure test
results. The pressure data could be plotted on a shaded surface of the entire object or with the shading turned off showing only the tapped surfaces.
An example is shown in figure 3.
A scale relating the colors to the pressure coefficient values is shown on each plot. The software allows this scale to be modified to produce an appropriate color gradient for optimum clarity.
Another pressure data analysis tool is the isobar contour plot. This is shown in figure 4 where pressure results for the circular ogive shape are compared at two rotation
rates. This allows the engineer to compare to test cases and see changes in the surface pressure distribution. With this method a pressure coefficient value is selected from the slider bar in the upper corner of the plot. By moving the bar up or down surfaces with the selected pressure
are highlighted. This allows the determination of pressure coefficient values in specific areas and quickly shows changes between two configurations that may otherwise not be apparent. As can be seen in this plot, rotation makes the lobes at the selected pressure asymmetric.
Another example of the pressure data plotted for comparison is shown in figure 5. In this case the static circular ogive data for four Reynolds numbers are plotted with
the model shading turned OFF. The model is at an oblique view but the software allows rotation to any angle. Because data were measured simultaneously, a direct comparison can be made between the pressure and force data.
One example of this is between the pressure plots in figure 5 (Wb/2V=0.0) and the force and moment plots in figure 6 (Wb/2V=0.0). In this case the force plots show a large positive yawing moment and negative side force produced at a Reynolds number of 350,000. The pressure data for the same Reynolds
number indicate a large negative pressure on the aft port surface of the test model with very little negative pressure on the forebody and lower negative pressure on the aft starboard side. These tools are very helpful in understanding and relating the model surface pressure
characteristics to the force and moment results.
The test models were designed and constructed by NASA with inputs from Bihrle to insure successful data acquisition during testing. These included the proper location of the pressure modules in the model as well as the documentation and proper connection of the individual surface
ports to the modules.
Bihrle Applied Research was responsible for the acquisition and reduction of all force and moment and pressure data during these tests. Bihrle developed the computer program to simultaneously acquire force and pressure data during the dynamic motion. Because of the dynamic test
motion of the rig this required the transmission of all sensor analog data across sliprings and then be amplified, filtered, and converted to digital signals. These were then converted to forces and moments, and pressures using the appropriate calibration constants and
nondimensionalized with the test article geometry and tunnel flow dynamic pressure as necessary. All reduced data was stored for later analysis.
Bihrle Applied Research can provide a customer with a variety of exclusive services for wind tunnel testing in any type of facility. This includes model construction for both surface pressure and force and moment testing, six-component strain gauge
balance design and construction, data acquisition system design, custom data acquisition and analysis software, technical support personnel, static and dynamic wind tunnel testing, and data utilization in aircraft flight motion simulation. Bihrle can provide complete support for
your next project.
