Aircraft Radio Bares All

There is a certain charm to older electronics gear. Heavy metal chassis and obviously hand-wired harness can be a work of art even if they would be economically impractical for most modern gear. Watching [msylvain59’s] tear down of a Collins 51R VOR receiver is a good example of that. The construction looks so solid.

If you aren’t familiar with VOR, it stands for VHF omnidirectional range and allows airplanes to tune into a fixed ground-based beacon and determine its heading in relation to the beacon. In some cases, it can also calculate distance.

Years ago, VOR antennas actually rotated rapidly, but today it is more likely to be an array of antennas switched electrically. The radio beam has several encoding schemes, including an FM reference signal that allows an airplane receiver — like the 51R — to determine the heading to the beacon. If you know the heading to two beacons, you can figure out where you are. Even with a single beacon, you can hold course towards it or away from it if you maintain a steady heading.

A receiver like the Collins would drive a Course Deviation Indicator (CDI), a Horizontal Situation Indicator (HSI), and a Radio Magnetic Indicator (RMI) on the plane’s cockpit and might also provide input to an autopilot. Compared to modern gear, the components look huge.

Understanding where an aircraft is has always been a big deal, and there are many ways to deal with it. Transponders are one part of the equation. We see a lot of hacks relating to ADS-B, too.

9 thoughts on “Aircraft Radio Bares All

  1. Beautiful equipment, made in an era long gone.

    There is sooo much that could be discussed in this video, like how the crystal synthesiser operated, allowing a range of frequencies to be synthesised to 0.05Mhz between 108 to 117.95 Mhz. Or how the indicators in the cabin were driven using AC signals that varied in phase….

    So much more to see than somebody being amazed that they used Siemens capacitors.

  2. >Even with a single beacon, you can hold course towards it or away from it if you maintain a steady heading.

    The distance to the beacon can be obtained by going sideways to the beacon and observing the angle at two points. You’re flying along the bottom of a triangle and the two angles point up the sides towards the point of the triangle. The rest is simple trigonometry, and the distance can be calculated continuously by knowing your ground speed and the rate of change of the beacon angle relative to your heading. You can always calculate your distance even with a single beacon as long as you’re not going directly at or away from it.

    An alternative method is to steer the plane 90 degrees from the beacon and then keep flying in a circle so the beacon stays at 90 degrees. Your rate of turning will be proportional to the diameter of the circle.

    Either way, if you also have a magnetic compass or any other way to tell absolute heading, you can place yourself on the map and plot a course anywhere you like. Even if you only have a very rough idea of where is north, you can already plot a circular course to your destination – all you need to do is guess whether clockwise or anti-clockwise is shorter.

    1. Basically, suppose you’re coming in from the south and you see the beacon on your right. You don’t know your exact heading, you’re just flying “north”. You know your destination is northwest on the other side of the beacon.

      Therefore you pick the circle around the beacon that goes through your destination point and then just maintain a constant heading relative to the beacon so you’ll miss it from the left. This means you’re on a spiraling path into the beacon and as the spiral winds tighter, your gyroscope will tell you that you’re turning faster and faster, therefore getting closer and closer. At some point you will intersect with the circle that goes to your destination, so you change your heading to 90 degrees and fly the circle to where you wanted to go.

    2. I believe it was very common practice to pair VOR with a DME (Distance Measuring Equipment) system. With DME the aircraft would sent a radio pulse which would be received by the ground station at the airfield. The ground station would send back a radio pulse reply. The elapsed time between the aircraft sending out the initial pulse and receiving the reply pulse is proportional to distance.

    3. The only problem is that the word ‘simple’ and ‘trigonometry’ don’t fit together in the same sentence in some people’s heads. That being said, I get what you’re saying and thanks for sharing.

  3. The VOR can also be used to relay information from a flight services station. This can be done to help increase the range that the FSS can transmit. Been a long time since I did that in primary flight training though.

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