Flanger is a very common modulation effect with a distinctive airplane engine-like sound, that we have listen to a lot of songs. It can be used at every instrument like guitars (Rush - The Spirit Of Radio, Van Halen - Unchained), synths and sometimes in portions of basses or even drums. It is a nice tool for experimentation. I generally avoid to use it in very bass frequencies as the resulting fluctuation could destroy a mix. I also try to keep optimal levels when use a flanger as it would very easily introduce digital clipping of high intensities.
Its sound derives from notches and peaks within the frequency region when we sum 2 identical signals and one of it is delayed by a modulated and always changing time length period. The way we change this duration is a crucial parameter for those frequency sweeps and their speed. Actually this is the infamous comb filter which we can also find in many synthesizers as a distinctive type of filter, Serum synth is a great example.
A typical flanger uses short delay values, this is the best setting for designing a nice flanging sound. While those peaks and notches travels slowly within the spectrum we get this unique timbral element. Those delay values are additionally varying with the usage of an LFO. Very often the signal is fed back into the signal, producing more resonant effects. The latter is controlled with the feedback knob we usually find in a common flanger.
Arturia Flanger BL-20
Other settings we can find in a typical flanger is the wet/dry knob and the manipulation adjustments of the modulation sources which can be an LFO or an envelope follower. Those could be the rate, the phase or even the shape of the modulation sources,
The flanger can be easily drive the sound to more metallic timbres or even some distorted ugliness.