Most instruments work together in a system. The gyroscopic, pitot-static, electrical, and vacuum systems are all examples.
Ask Explorers: What is a system? (The word “system” describes how different parts of a plane interact to form a complex whole. For example, the pitot system involves static air, ram air, altimeter, airspeed indicator, and vertical speed indicator, and sometimes includes an alternate static system. A plugged static or ram port would cause faulty indications in multiple instruments.)
Discuss airplane systems and how an airplane’s instruments interact to create a system. You can also discuss redundancy and engineering constraints. (For example, there could be backups for all systems, but that might make the plane too heavy to fly.)
Discuss static ports and pitot tubes and how they work, as well as ram air versus static air and how the difference between the two helps determine altitude, airspeed, and vertical speed indicators. Discuss alternate static and why it works. Ask: What do their purposes have to do with their placement on the plane? What engineering constraints dictate their placement? What would happen if the pitot tube were clogged? What if the static port were clogged? (Static pressure measures outside air pressure and ram air measures moving air. The difference between the two pressures can determine altitude and airspeed. A blocked pitot tube will affect the airspeed indicator, with the indicator showing an increase with increased altitude and a decrease with decreased altitude. A blocked static port is more serious, which is why many planes have an alternate static source. The altimeter may indicate a higher than actual altitude, which means you might be too close to the ground or other obstacles. The vertical speed indicator will momentarily indicate a climb, then will settle back to the initial indication. The airspeed indicator will indicate greater than normal airspeed, which will cause the pilot to slow down, risking a stall.)
Discuss how a gyroscope works and how it works with the heading indicator and attitude indicator. Discuss the principles of a gyroscope (rigidity in space and precession) and Newton’s First Law. If a gyroscope is available, use it to demonstrate the two principles.
Set a gyroscope in motion. As you tilt the gyroscope, have participants imagine that the airframe is tilting and notice how the inside wheel does not tilt. In an attitude indicator, the model plane is on the part of the gyroscope that stays upright, and as the plane around the wheel tilts, the model plane gives an indication of the plane’s attitude with respect to the artificial horizon.
Wikipedia has an excellent explanation of how a gyroscope works, along with relevant animations. See http://en.wikipedia.org/wiki/Gyroscope.
Briefly mention other gyroscopic instruments, including the turn and slip indicator and the heading indicator.
Explain to participants that airplane instruments can be powered in one of several ways: mechanically, electrically, via lasers, or via solid state. On smaller aircraft, they’re usually powered by the airplane’s vacuum system. The vacuum system draws air in through a filter assembly, which then moves through turbines in the instruments, where it causes the gyros to spin at high speed. The air then continues on to the engine-driven vacuum pump, where it is expelled. A relief valve prevents the pressure from getting too high. Another, more expensive option used in many larger aircraft is electrically powered instruments. Despite the expense, some smaller aircraft are now employing these electrically powered gyroscopic systems as well. If desired, students can research laser and solid state technologies.
Discuss engineering constraints and opportunities and why some ideas are better for certain situations and certain aircraft.