The increasing demand for improved fuel economy along with evermore stringent environmental constraints has resulted in a renewed effort by commercial aircraft manufacturers to design increasingly efficient aircraft (through the reduction of drag).
ARA has considerable experience in the accurate measurement of drag on a range of civil aircraft. We utilise highly accurate instrumentation, such as thermally insensitive strain gauge balances for the measurement of overall model loads, and inclinonometers for the measurement of the model’s attitude to the airflow. These measurements are augmented by pressure measurements from within the model cavity. Transition fixation is achieved through the use of strategically located surface roughness, in the form of ballotini or discrete ‘transition dots’.
A number of visualisation techniques are available to aid in the understanding of the flow physics, and therefore aid in the design for minimised drag:
- Pressure Interpolation
- Oil Flows
- Pressure Sensitive Paint
A Typical Test
During the wind tunnel test a number of aircraft configurations and components are tested in order to determine their effect on the overall model drag through a range of model incidences, roll angles and Mach numbers. Drag is calculated from the aerodynamic loads on the model measured by the internal strain gauge balance. The cavity pressure measurement is used to correct the drag for the internal cavity required to mount the model in the wind tunnel.
The thermally insensitive balances utilised by ARA yield the benefit of a faster, more efficient and accurate test technique compared to wind tunnels utilising less sophisticated balance designs.
An improved understanding of the airflow over the model surface can be achieved very quickly through the use of flow visualisation techniques such as interpolation of the model surface pressures and oil flows. More advanced visualisation methods such as Pressure Sensitive Paint (PSP) and Temperature Sensitive Paint (TSP), which can be used for accurate transition detection are also available.
Accuracy (using a 57.15mm/2.25″ balance).
- Within a test series, drag repeatability is within half a drag count ((△CD = ± 0.00005)
- Between test series, drag repeatability is within a drag count (△CD = ±0.0001)
Transition detection utilising
- Temperature Sensitive Paint visualisation
- Acenaphthene sublimations
Flow visualisation of phenomenon such as shock waves, separation bubbles and vortices utilising
- Oil flows
- Pressure Sensitive Paint (for increased fidelity)