[Radio Wrangler]

Avionics gear is usually built to a very high density and a lot of things require forced air cooling. In small aircraft this amounts to an outside pick-up scoop piped into a distribution piping bus with something along the way to (try to) remove water droplets. In bigger aircraft you'll find high pressure blowers.

Without forced air cooling some pieces of gear will cook themselves. They are required to be tested with cooling air failed to ensure they do not get into a dangerous condition. Some of the fancy new glass panel units produce a surprising amount of heat from all the processing power behind them. It's not just transmitters that go on the cooling air bus. lab testing of military avionics needs not just power, but also cooling air.

So cooling air movers for aviation are wanted to be very reliable. Ccentrifugal switches are avoided. That motor will be capacitor-run and designed for high slippage, acting as a torque generator... rather like those Papst motors driving the spools on a Revox.

At high altitude, the cooling capability of air diminishes a lot and fan revs go up. What's needed 'up there' becomes a tornado at ground level. At altitude the voltage at which air gaps break down also fall and you need to take precautions against outbreaks of St Elmo's fire inside equipment.

In pressurised cabins, things aren't actually any easier. In normal service the inside pressure is reduced as the plane climbs. Typical cabins being rated to 8psi pressure over what's outside. On descent, the overpressure lags the outside air pressure so the passengers don't get quite such fierce ear-popping, and the rate of change of stress on the structure is managed. Like in the compressor thread, there need to be facilities to remove condensate. However, the avionics have to be designed to live without such coddling. They get tested to prove they operate correctly if pressurisation fails, and also for sudden decompression. When they suddenly have to grab oxygen masks and dive, the pilots want the radios and instruments working. They will be too worried about what the structural failure was. Those little fans have to perform.

Some planes run (relatively) constant speed generators making 400Hz 3-phase. Some smaller planes run a more car-like system with an alternator and rectifier charging a 12 or 24v battery. There may be an inverter making 400Hz for any instruments and equipment needing it. This could be either 3 phase or single phase. The inverter could be either a rotary machine, or a static inverter done by electronics.

So there are parallel markets for 400Hz stuff in both single and 3 phase forms. So there are two versions of that little fan.

With care, a lot of avionics gear can be made to pass all the altitude tests without needing forced-air cooling. This is seen as a very big plus point by people choosing equipment. As an equipment designer, not having wet air and muck forced in has its attractions, but you have to arrange enough conduction cooling to handle 40,000 ft or so.

No wonder that little fan shifts so much air at sea level?

David