Question

Strain gauges are placed at two locations on a wing that is being tested in a wind tunnel. The first gauge is near the fixed base of the wing and the other gauge is near the free wing tip. Two tests are performed: (a) the wing vibration is recorded for low velocity laminar flow and (b) for high velocity turbulent flow. For each of these tests, discuss the following: (i) Describe and compare the time histories you would expect to see recorded by each strain gauge; (ii) Discuss the type of cross-correlation you would expect between X 1 and X 2, the locations of the gauges.

Answer 1

1. Analogue instrument meter with analogue display ("needles"). Until the later decades the most common basic type.^[3]
2. Digital instrument meter with analogue display. A screen that shows an "analogue meter", commonly used in modern aircraft cockpits, and some hospital equipment etc.
3. Digital instrument meter with digital display. Only numbers are shown at a digital display.
4. Analogue instrument meter with digital display. Only numbers are displayed, but through a mechanical or electro-mechanical display (today very rare but has existed for clocks, certain Doppler meters and informational screens at all kinds of stations and airports)

The two first basic types are usually easier for the human eyes and brain to interpret, especially if many instrument meters must be read simultaneously. The other two types are only displaying digits, which are more complex for humans to read and interpret. The ultimate example is cockpit instrumentation in aircraft. The flight instruments cannot display figures only, hence even in the most modern "glass-cockpits" where almost all instruments are displayed at screens, few figures are visible. Instead the screens display analogue meters.