The Medium is the Message |
1. Control and Safety in the Air |
The term Avionics stands for aviation electronics. Even from the earliest days of flight, electrical and electronic circuits became key components in making sure that aeroplanes flew safely. Radios ensured that aeroplanes could contact aerodromes easily, and became a key component for navigation.
All small private aircraft and many commercial machines are still controlled by mechanical linkages. The control surfaces are connected to the pilot's controls by wires and rods. Old-fashioned but effective. However with larger machines the controls can be very heavy to operate and hydraulic operation has been widely used.
Automatic pilots were produced to take the work-load off pilots. These electronic devices continuously monitor the way the aeroplane is flying as regards the direction, height, and whether the aeroplane is climbing or descending.
In this picture you can see the array of instruments needed by pilots. To the right you can see the avionics stack.
As electronics advanced, it was natural that computers were installed into aeroplanes to help with flight. The Airbus was the first commercial airliner to be controlled entirely with fly-by-wire technology. The pilot selects a range of flight behaviour from the computer, which controls the control surfaces. If the pilot wants to fly the plane manually, he can do so with a very small joystick. But this enters commands into the computer. The computer will prevent the pilot doing risky manoeuvres.
The only thing a computer cannot do is to land the plane. It has been tried. The plane flared to land, but carried on about 10 metres above the runway into some trees at the end.
The Eurofighter is a military aircraft that uses fly-by-wire. It needs to; it is highly unstable in flight and no pilot could control it. The computer makes it incredibly easy to fly, as easy as a light plane.
The computer collects data from a large number of sensors and processes them to enable it to react to flight situations. It also displays data to the pilot.
Some modern aircraft use fly-by-light technology, in which data are passed from sensors to the computer as pulses of light along optical fibres.
Most sensors are potentiometers of one sort or another. The resistance changes according to the temperature, pressure, etc. We will look at the sensor that detects the positions of control surfaces, such as flaps.
This is a typical potentiometer circuit. In a linear potentiometer, the output voltage depends on the position of the slider. When the slider is at the bottom, the voltage is zero. When it's half-way up, the voltage is 50 % of the battery voltage. When it's at the top, the voltage is 100 % of the battery voltage.
This assumes that the voltmeter is perfect, having an infinite input resistance. Modern digital voltmeters have a very high input resistance, so they can be counted as perfect.
If the voltmeter has a low resistance, we treat the voltmeter as being a perfect voltmeter in parallel with a resistor whose resistance is that of the voltmeter.
We then treat this circuit as:
We solve this problem in two steps:
The big brute is an Antonov 225, a huge transport aircraft. Alongside is a Cessna 172, a typical private aeroplane.
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