Force measurement techniques in short duration hypersonic facilities
Keywords: force measurement
hypersonic
aerodynamics
free-flight

In this thesis, the free-flight force measurement technique is commissioned, validated and implemented with increasing complexity in geometries within the University of Oxford High Density Tunnel, a short-duration, heated Ludwieg Tube. The motivation of the work was to improve the current capability of force measurement techniques through the free-flight methodology, allowing for the measurement of high quality, flight representative forces with lower uncertainties than established force measurement techniques. Rather than rigidly mount a sub-scale model through a sting and measure forces through a load cell, the free-flight technique allows the model to move in six degrees of freedom during the test time. This allows for forces and moments to be measured that are not impacted by the mounting structure but require non-intrusive methods to measure the model's kinematics. In this work, kinematics are measured through a 3 degree of freedom image tracking algorithm with sub-pixel accuracy and through accelerometers mounted internally to the model on an on-board data acquisition system.
This thesis introduces the static free-flight technique, whereby the aerodynamic pitching moment is minimised by matching the location of centre of gravity to centre of pressure, reducing the static margin of the model. This technique limits the dynamic influence on the measured aerodynamic coefficients and allows for only the static derivatives to be measured. This technique was conducted with a 7 degree half-angle cone, a simple geometry in which the measured coefficients can be validated against a numerical panel method code, giving confidence in the methodology. The tests were conducted at a Mach 7 condition representative of a hypersonic vehicle’s trajectory at 35 km altitude. Non-intrusive image tracking and on-board inertial measurement units were used to determine accelerations to calculate the aerodynamic forces acting on the cone. Results for lift, drag and pitching moment coefficients were obtained over a range of angles of attack and compared with predictions from a hypersonic panel method code. The experimental and numerical data sets agreed well over the range of angles of attack with the experimental uncertainties remaining below 3.1 \% for all coefficients.
Following validation of the static free-flight technique, tests were conducted to directly compare free-flight against force balance techniques. The force balance was calibrated using two methods; a static calibration and by dynamically calibrating the balance by generating a global impulse response function of the model and using the stress wave deconvolution methodology. The experimental model was a blunted 7 degree half-angle cone and experiments were conducted at a Mach 5 test condition which provided sufficient dynamic pressure to generate aerodynamic forces suitable for the measurement range of the force balance. Results for lift, drag and pitching moment coefficients were obtained over a range of angles of attack and compared with predictions from a hypersonic panel method code. Agreement between the independent force techniques and numerical data sets was good over the range of angles of attack. Maximum uncertainties were shown to be +/- 0.56 N and + /- 0.44 N for free-flight for lift and drag respectively and + /- 1.59 N and + /- 1.27 N for the dynamically calibrated force balance.
Finally, after showing that the free-flight methodology in the High Density Tunnel resulted in lower uncertainties than other force measurement techniques, it was applied to a 7 degree half-angle cone with fins, at a flight-representative Mach 7 condition. The fins resulted in a centre of pressure that was a function of angle of attack. Longitudinal static force and moment coefficients were measured at the Mach 7 condition with the experimental data agreeing well with numerical predictions. Forces were obtained using both image tracking at 76,000 fps as well as the direct measurement of accelerations using an on-board inertial measurement unit. Uncertainties remained below + /- 0.0075 for lift coefficient and + /- 0.0051 for drag coefficient. The fins were also angled to allow the model to roll in free-flight so that roll coefficient could also be determined.