WRT light aircraft: Early aircraft were usually fitted with 'dynamos' which generate DC and don't need any rectification; they have a separate voltage regulator usually mounted on the firewall. Although generating DC there is still some 'noise' in the system and capacitors are fitted to reduce problems with radios.
Later aircraft were fitted with alternators which have an internal bridge diode rectifying system to produce DC and unlike a car have a separate voltage regulator, again usually mounted on the firewall. A capacitor is fitted between the alternator output and ground this is to smooth out any remaining AC and to stop radio interference. This is often not enough and a 'choke' is also fitted in the output line.
A capacitor passes AC and blocks DC is 'wot' I was taught.