Lots of smaller modern aircraft use 28Vdc for their main power distribution. They use a 28Vdc battery system and a 28V engine-driven alternator (puts out six-phase rectified DC, battery acts as a filter, so the bus ripple is only a few mV). This is just like an automotive 14Vdc charging system, except it uses a 12cell battery instead of a 6cell battery. Both my Piper and Cessna are older models which use 14Vdc alternator systems (actually derived from automotive systems).
The alternator ripple varies with engine speed, but is typically between a few kHz up to about 10kHz. If you create ground-loops in the audio/avionics suite, you hear the alternator whine, and it is very annoying. Most factory aircraft have alternator whine in headphone audio; they don't seem to understand "single point grounding".
Only large turbine-powered Biz Jets (Falcon) and Commercial Airliners use 400Hz three-phase AC primary power.