Measuring changes in wing shape in a wind tunnel

The problem

Tests on a compliant wing that changed shape depending on the air flow over it required that the changes in shape be determined as the wind tunnel testing progressed. The changes had to be measured without influencing the air flow. At the same time the strains within the special core material that supported the wing skins had to be measured to check that it was behaving in the expected way (negative Poisson's ratio).

The measurements were part of an undergraduate project, and the students had no prior experience of instrumentation to obtain hard to acquire data in a wind tunnel environment.

The Solution

The truss-like structure of the special auxetic (negative Poisson's ratio) core material can clearly be seen in the picture. Targets were marked at the nodes and could be used to track both the global shape changes and the node to node strains. This allowed the response to changes in angle of attack and windspeed etc., to be determined so that information on shape could be directly coupled to lift and drag information. At the same time the strains within a unit cell of the core were measured to demonstrate that the properties of the core material exhibited the expected auxetic behaviour with positive strains both along and across the core to give a negative Poisson's ratio.

Advantages of the Video Gauge

Instrumentation within a wind tunnel environment is always challenging because of the requirement to avoid introducing anything into the wind tunnel that changes the airflow in unpredicted ways. Whilst instruments such as LVDTs could be used to measure the deflections they would have had to be reset for each change in angle of attack and the user would have to know in advance where the optimum measuring points were sited. With the Video Gauge there was no interference with the wind tunnel environment and setting up for changes in wind tunnel conditions was minimal. Equally, the measurement of the strains required the node to node strains to be measured rather than the strain in the ligaments between the nodes. Strain gauges could capture the ligament strains but special purpose extensometry would have to be designed and built to capture the node to node strains. The Video Gauge allowed all the necessary data to be captured with no additional effort, by unskilled users, without extensive training, and within tight time constraints.